Polyfluorene-Based Semiconductors Combined with Various Periodic

Progress in Polymer Science 37 (2012) 1192–1264

Contents lists available at SciVerse ScienceDirect

Progress in Polymer Science

j ournal homepage: www.elsevier.com/locate/ppolysci

Polyﬂuorene-based semiconductors combined with various periodic

table elements for organic electronics

∗

Ling-Hai Xie, Cheng-Rong Yin, Wen-Yong Lai, Qu-Li Fan, Wei Huang

Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts &

Telecommunications (NUPT), Nanjing 210046, China

a r t i c l e i n f o a b s t r a c t

Article history: Polyﬂuorenes have emerged as versatile semiconducting materials with applications in

Received 12 April 2011

various polymer optoelectronic devices, such as light-emitting devices, lasers, solar cells,

Received in revised form 8 February 2012

memories, ﬁeld-effect transistors and sensors. Organic syntheses and polymerizations

Accepted 10 February 2012

allow for the powerful introduction of various periodic table elements and their build-

Available online 16 February 2012

ing blocks into ␲-conjugated polymers to meet the requirements of organic devices. In

this review, a soccer-team-like framework with 11 nodes is initially proposed to illus-

Keywords:

trate the structure–property relationships at three levels: chain structures, thin ﬁlms

␲-Conjugated polymers

and devices. Second, the modelling of hydrocarbon polyﬂuorenes (CPFs) is summarized

Band-gap engineering

Light-emitting diodes within the framework of a four-element design principle, in which we have highlighted

Photovoltaic cell polymorphic poly(9,9-dialkylﬂuorene)s with unique supramolecular interactions, various

Field-effect transistors hydrocarbon-based monomers with different electronic structures, functional bulky groups

Memories

with steric hindrance effects and ladder-type, kinked, hyperbranched and dendritic confor-

mations. Finally, the detailed electronic structure designs of main-chain-type heteroatomic

copolyﬂuorenes (HPFs) and metallopolyﬂuorenes (MPFs) are described in the third and

fourth sections. Supramolecular, nano and soft semiconductors are the future of polyﬂuo-

Contents

1. Introduction ...... 1194

1.1. Background and scope ...... 1194

1.2. Basic knowledge and principles ...... 1195

1.2.1. Performance and stability of polymer devices ...... 1196

1.2.2. Optoelectronic property and morphology of semiconducting polymer ﬁlms...... 1197

1.2.3. Four-element design of semiconducting polymers ...... 1199

2. Hydrocarbon polyﬂuorenes (CPFs) ...... 1202

2.1. Poly(9,9-dialkylﬂuorene)s (PDAFs) ...... 1202

␲

2.2. Polyﬂuorenes with hydrocarbon-based -conjugated monomers ...... 1204

2.3. Polyﬂuorenes substituted with various bulky groups ...... 1206

2.4. Fused and ladder-type polyﬂuorenes ...... 1207

2.5. Polyﬂuorenes with kinked conformations...... 1208

2.6. Hyperbranched and dendritic polyﬂuorenes ...... 1209

∗

Corresponding author. Tel.: +86 25 8586 6008; fax: +86 25 8586 6999.

E-mail address: [email protected] (W. Huang).

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1193

3. ␲-Conjugated heteroatomic copolyﬂuorenes (HPFs) ...... 1210

3.1. Copolyﬂuorenes containing heterocycles in 16th group ...... 1210

3.1.1. Copolyﬂuorenes containing oxygen heterocycles ...... 1211

3.1.2. Copolyﬂuorenes containing sulphur heterocycles...... 1212

3.1.3. Copolyﬂuorenes containing selenium heterocycles ...... 1217

3.2. Copolyﬂuorenes containing heterocycles in 15th group ...... 1217

3.2.1. Copolyﬂuorenes containing nitrogen heterocycles ...... 1218

3.2.2. Copolyﬂuorenes containing phosphorus heterocycles ...... 1229

3.3. Copolyﬂuorenes containing heterocycles in 14th group ...... 1231

3.4. Copolyﬂuorenes containing heterocycles in 13th group ...... 1232

4. Metallopolyﬂuorenes (MPFs) ...... 1233

4.1. Copolyﬂuorenes containing main-group metals ...... 1234

4.2. Copolyﬂuorenes containing transition metals...... 1235

4.2.1. Copolyﬂuorenes containing Zn(II) complexes ...... 1235

4.2.2. Copolyﬂuorenes containing Pt(II) complexes...... 1237

4.2.3. Copolyﬂuorenes containing Ir(III) complexes ...... 1237

4.2.4. Copolyﬂuorenes containing Hg, Fe, Ru, Os, Re, or Zr complexes ...... 1242

4.3. Copolyﬂuorenes containing rare-earth metals ...... 1245

4.3.1. Outlook ...... 1248

Acknowledgements ...... 1248

References ...... 1248

Nomenclature

MLCT metal-to-ligand charge transfer

AFM atomic force microscopy MPFs metallopolyﬂuorenes

BT benzothiadiazole NIR near-infrared region

BHJ bulk heterojunction NTSC National Television System Committee

Cz carbazole OXD 1,3,4-oxadiazole

CD circular dichroism PC71BM phenyl-C71-butyric acid methyl ester

CIE Commission Internationale de l’Éclairage PCBM (6,6)-phenyl C61-butyric acid methyl ester

CV cyclic voltammetry PCE power conversion efﬁciencies

CPFs hydrocarbon polyﬂuorenes PEDOT:PSS poly(3,4-ethylenedioxythiophene):

D–A donor–acceptor poly(styrenesulphonate)

DRAM dynamic random-access memory PF6 poly(9,9-dihexylﬂuorene)

DSC differential scanning calorimetry PF2/6 poly(9,9-di(2-ethylhexyl)ﬂuorene)

DTBT 4,7-di-2-thienyl-2,1,3-benzothiadiazole PFO poly(2,7-(9,9-dioctylﬂuorene))

DTTP 5,7-dithien-2-yl-thieno[3,4-b]pyrazine PFs polyﬂuorenes

EA electronic afﬁnity PL photoluminescence

EL electroluminescence PLEDs polymer light-emitting devices

ET energy transfer PSCs polymer solar cells

EQE external quantum efﬁciency QE quantum efﬁciency

FET ﬁeld-effect transistor RGB red, green and blue

FF ﬁll factor SAM self-assembled monolayer

FRET Förster resonance energy transfer SAXS small-angle X-ray scattering

HOMO highest occupied molecular orbital SEM scanning electron microscope

HPFs heteroatomic copolyﬂuorenes TEM transmission electron microscopy

LUMO lowest unoccupied molecular orbital TFTs thin-ﬁlm transistors

IP ionization potential TGA thermogravimetric analysis

ISC intersystem crossing TOF time-of-ﬂight

ITO indium tin oxide UPS ultraviolet photoelectron spectrum

I–V–L current–voltage–luminance WOLEDs white organic light-emitting devices

LC liquid crystal UV–vis ultraviolet-visible

LE luminance efﬁciency WORM write-once/read-many-times

LECs light-emitting electrochemical cells WRER write-read-erase-reread

LEPs light-emitting polymers XRD X-ray diffraction

1194 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

1. Introduction groups [59,60], including those of Bazan [61–63], Huang

[64–69], Liu [70,71] and others [72–74]. Furthermore,

1.1. Background and scope ionic-functionalized PFs have become next-generation

electron-injection or electron-extraction layers for poly-

Since poly(p-phenylenevinylene) (PPV)-based polymer mer devices such as light-emitting electrochemical cells

light-emitting diodes (PLEDs) were reported by Friend (LECs), PLEDs and solar cells. Cao and coworkers [75–79],

in 1990 [1], polymer semiconductors and devices [2] Bazan and coworkers [80–82], and other groups [83–85]

have attracted scientiﬁc and industrial interest as plastic have performed elegant research in this respect.

electronic candidates for the advancement of informa- Recently, copolyﬂuorenes with wide-absorption ranges

tion technology and the resolution of energy issues. have become attractive due to their potential applications

This reﬂects several advantages offered by polymer-based in bulk heterojunction (BHJ) polymer solar cells (PSCs)

electonics over their silicon-based counterparts, includ- by the Inganas et al. [86] and Chen and Cao [87]. High-

ing light weight, low cost, large area and ﬂexibility mobility PFs have also been applied in polymer ﬁeld-effect

[3,4]. To date, polymer semiconductors have been exten- transistors (FETs) by several groups [88–91]. For exam-

sively and intensively investigated [5,6]; these systems ple, Sirringhaus et al. reported active-matrix displays made

␲

include -conjugated poly(p-phenylenevinylenes) (PPVs) using printed polymer thin-ﬁlm transistors (TFTs) of PFs

[7], polyﬂuorenes (PFs, Fig. 1) [8,9], polythiophenes (PThs) [92]. In addition, PFs have been also used to achieve poly-

[10], poly(p-phenylene-ethynylenes) (PPEs) [11], and ␲- mer memories by Ouisse et al. [93], Kang and coworkers

stacked poly(N-vinylcarbazoles) (PVKs) [12,13]. Among [94–95], and Zhao et al. [96]. PFs have been isolated as

these semiconductors, PFs exhibit wide-band-gaps (Eg) outstanding ␲-conjugated polymers for plastic electronics.

between 2.8 and 3.5 eV, excellent thermal stability, high Diversity is one distinct advantage that polymer semi-

photoluminescence (PL) quantum efﬁciency, and good conductors hold over inorganic semiconductors. Indeed,

charge-transport properties [14–17], which make them their band gaps and electronic structures may be ﬂex-

promising blue light-emitting polymers (LEPs) for appli- ibly tailored via organic synthesis and polymerization

cations in PLEDs [18,19]. techniques to meet the requirements of optoelectronic

Liquid–crystalline PFs have been utilized to achieve ori- devices. Within this context, many periodic table elements

ented electroluminescence (EL) by Grell and Neher and and numerous molecular segments have been successfully

coworkers [20,21]. PFs are also potential candidates for use introduced into the main-chain backbones, ends or side-

in lasers and have been demonstrated by several groups, chains of polymer semiconductors [97].

such as Heeger and coworkers [22,23], Heliotis et al. [24] Chemists have developed many polymer semiconduc-

and others [25,26]. However, one key issue regarding the tors with uniquely distributed electronic structures to

use of PFs in optoelectronics is overcoming the low-energy effectively break through the bottleneck in plastic elec-

emission bands at 2.2–2.3 eV (also called g-bands); these tronics. However, the relationships between numerous

may be caused by either aggregates/excimers or ketone building blocks and polymer devices are complex. Elements

defects [27–31], and remain controversial. Many efforts are likely to change, not only the electronic structures

have been made to design and synthesize stable and efﬁ- of conjugated polymer chains dramatically varied, but

cient blue PF-based LEPs by the Müllen group [32,33], also steric hindrance, conformational topology and non-

Carter and coworkers [34], Miller and coworkers [35,36], covalent interaction that all play key roles in determining

Chen and coworkers [37], Jen and coworkers [38] and polymer morphologies and optoelectronic properties. In

the Huang group [39–41]. High-efﬁciency PFs, which act this review, we focus on PFs as models of semiconduct-

as hosts that harvest energy for dyes and are produced ing polymers to summarize the general and detailed roles

by feasible copolymerization techniques, are an alterna- of periodic table elements and their building blocks, except

tive to achieve high-efﬁciency green or red LEPs [42–46]. for the radioactive elements of period 7 and the inert ele-

According to similar principles, white PLEDs have been ments of group 18 in the periodic table, which distinguishes

realized through the control of incomplete energy trans- this review from most reviews of ␲-conjugated polymers,

fer by the Wang group [47–51], Cao group [52–54], and including a few reviews on PF semiconductors published

others [57]. Further widening the energy band gap of PFs is in the last 10 years by Grimsdale and Müllen [98], Ler-

a challenge that involves obtaining high triplet energy for clerc and coworkers [57], Neher [21], Scherf and List [9] and

high-efﬁciency electrophosphorescent PLEDs [55–58]. others [86,99]. Nevertheless, it is useful to establish a gen-

Water-soluble PFs have become another important eral guideline on how to design semiconducting polymers

branch for the application of chemical and biosensors for plastic electronics. It should be noted that this review

and imaging due to their high ﬂuorescence quantum efﬁ- mainly focuses on the incorporation of elements and their

ciency; these materials have been explored by several molecular segments incorporated into PF main chain due to

Fig. 1. Poly(9,9-dioctylﬂuorene-2,7-diyl) (CPF-01b, PF8 or PFO).

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1195

Fig. 2. (a) The device structure of a typical two-terminal sandwich-type polymer, (b) a soccer-team-like framework with 11 nodes indicating concepts that

must be addressed at the levels of polymer chains, semiconducting ﬁlms and devices.

their simple and desirable models for structure–property aggregates rather than device structures. For polymer

relationships. Most PFs with pendant or side-chain groups devices, one major research task is to improve device

are regrettably omitted due to the limited scope of this performances; the stability and lifetime of devices are crit-

review. ical aspects regarding their commercialization. Polymer

This review is composed of five sections. The first devices generally adopt sandwich-type configurations

section begins with a general introduction of PFs, fol- composed of electrodes, active polymer semiconductor

lowed by the basic framework of polymer semiconductors layers and various interfaces, and their performances

and devices. In the second section, which discusses ␲- are closely related to the optoelectronic properties and

conjugated hydrocarbon PFs (CPFs), we take carbon as an morphologies of polymer ﬁlms, which are regarded as

example to illustrate the four-element design of PFs. In statistical aggregations of numerous semiconducting

third and fourth sections, the structure–property relation- polymer chains.

ships regarding other element effects in PFs are discussed, The basic knowledge and philosophy regarding poly-

␲

including a discussion of -conjugated heteroatomic PFs mer semiconductors and devices are simply illustrated in

␲

(HPFs) and -conjugated metallopolyﬂuorenes (MPFs). Fig. 2. Within this framework, plastic electronics research

Finally, a brief summary and outlook are presented in the includes 11 key nodes and involves three conceptual

ﬁfth section. levels: chain structures, ﬁlms and devices. Multilayered

structures and interface engineering are conventional

1.2. Basic knowledge and principles tools to improve device performance and lifetime at the

device level. Host–guest doping and/or self-assembly

According to Moore’s Law, one driving force of plastic techniques are alternatives to create high-performance

electronics research is the fact that polymer semicon- polymer devices at the ﬁlm level. Organic synthesis and

ductors are favourable for miniaturisation of devices copolymerization are the distinguishing “bottom-up” tech-

because their functionalities stem from molecules and/or niques at the molecular level to design chain structures of

1196 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

polymeric semiconductors, including the electronic struc- To date, one great challenge that has persisted is how

ture, steric hindrance, conformation and topology as well to realize electrically pumped polymer lasers. Polymer

as supramolecular interaction. The four-element design control devices mainly include sensors and actuators,

of polymer chains is dramatically different from that of which may be used in environment monitor systems and

inorganic semiconductors. A more detailed description of robotics, respectively [112–114].

this framework is described as follows. Polymer transistors have been applied in the drive

arrays of active-matrix displays (electrophoretic display

1.2.1. Performance and stability of polymer devices pixels) [115,116], sensors [117], information storage

Polymer devices may be roughly divided into energy devices and other applications [118–120]. They have

devices, information devices and control devices by func- also been applied in the organic integrated circuits of

tion and may be categorised into two-terminal devices, radio-frequency identiﬁcation tags [121]. There are two

three-terminal devices and multi-terminal devices in types of polymer transistors: planar and vertical tran-

terms of their physical structures. PSCs represent a poten- sistors [122,123]. The performance of each is mainly

tial alternative to fossil energy sources [100]. It remains determined by several important parameters, such as

challenging to improve PCEs beyond 10% for commer- ﬁeld-effect mobility, ON/OFF ratio, threshold voltage,

cialization purposes [101]. To date, Yang and coworkers and sub-threshold slope. In this area, the main issues

have reported the highest PCEs of polymer/fullerene BHJ faced by chemists are improving the mobility of n-type

solar cells, with [6,6]-phenyl-C61-butyric acid methyl and p-type devices with high on/off ratios. Currently,

ester (PCBM) as an electron acceptor, to reach up to 7.4% the charge mobilities of polymer transistors may exceed

[102–105]. The record efﬁciency of organic solar cells has 1.95 cm /(V s), as reported by Bronstein et al. [124]. Note

been updated by Mitsusbishi Co., which has reported an that the introduction of molecular functionality into

efﬁciency of up to 9.2% [106]. polymer devices is an important strategy in addressing

Information devices include PLEDs for ﬂexible large- the challenges to manipulate desirable and complex

area displays, polymer memories for information storage, current–voltage curves beyond the limit of Moore’s law. To

polymer solid-state lasers and photodetectors for sig- date, PFs have been used in the various devices mentioned

nal processing. PLEDs have been considered as potential above. The details regarding the introduction of their basic

inexpensive, energy-efﬁcient alternatives to liquid–crystal device structures and working principles have been omit-

displays [107,108] since the ﬁrst report by Burroughes ted, though they may be found in related reviews on PLEDs

et al. [1]. Besides the luminescence efﬁciency of devices, [5], PSCs [125–132], polymer lasers [23,133], polymer

colour purities are key issues for PLEDs for display appli- memories [134], and polymer transistors [135–139].

cations as they are generally composed of three primary The commercialization of polymer devices is hampered

colours red, green and blue (RGB). Primary red, green, by their performance and lifespan. Device behaviour is

blue and white emission have Commission Internationale dominated by the energy level features, which may be

de l’Éclairage (CIE) coordinates of (0.630, 0.340), (0.310, optimized by altering the conﬁguration of the electrodes

0.595), (0.155, 0.070), and (0.3217, 0.3290), respectively, [140], multilayered semiconducting layers and modiﬁed

according to the standards from National Television Sys- interfaces [141]. For example, the use of active metal elec-

tem Committee (NTSC). The colour temperature and the trodes or the introduction of charge-transport layers can

colour-rendering index are two key characteristics besides balance electrons from cathodes and holes from anodes,

the CIE for white PLEDs for applications as solid-state light- to achieve the efﬁcient exciton combination in PLEDs,

ing sources [109]. To update, the highest reported power resulting in high current efﬁciency [142,143]. The per-

efﬁciencies of white PLED are still lagging behind the white formances of polymer transistors, lasers and memories

organic light-emitting devices (WOLEDs) that reach up to are still undesirable; therefore only PLEDs and PSCs have

100 lm/W [110]. involved a focus on the evaluation and improvement of

Polymer memory devices are an alternative or supple- device lifetimes. Device encapsulation techniques featur-

mentary technology to conventional devices [92]. They are ing a polymer/inorganic hybrid multilayer barrier such as

divided into write-once read-many-times (WORM) mem- TiOx can greatly improve the storage stability and oper-

ories, dynamic random-access memories (DRAMs), and ating lifetime of polymer devices. Device lifespan is also

ﬂash memories according to their current–voltage curves. closely related to the stability of polymer semiconductors.

These resistance-type memory devices are either volatile Polymer semiconductors are more susceptible to degrada-

or non-volatile. Among them, WORM memory devices tion by exposure to oxygen and water than their inorganic

exhibit the impressive feature of being non-erasable counterparts. It is a challenge to design these materials with

and are considered typical non-volatile memories [111]. high thermal, electrochemical and functional stability. For

DRAMs are volatile memory devices that lose stored data PLEDs, the factors that limit device lifetime in display and

when the power supply is removed. Flash memory is a type lighting applications are primarily spectral stability and

of non-volatile memory, which may be electrically erased colour purity, which principally depend on morphological

and reprogrammed. The ON/OFF current ratio and write- and environmental stability [144]. In particular, blue LEPs

read-erase-read (WRER) and/or programme cycles are are lagging far behind green and red components. Excitons

␲

two key parameters used to evaluate device performances located at vulnerable -conjugated segments by exposure

and lifetimes. For polymer lasers, a low-energy threshold to adsorbed water and oxygen result in the deterioration

is desired [23]. Slope efﬁciency and beam quality factor of device performance. The thermal decomposition tem-

are two other parameters used to assess performance. peratures (Td) and the glass transition temperature (Tg)

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1197

are crucial properties of amorphous polymer semiconduc- the defect of phase separation exists [155,156]. With the

tors in designing LEPs with long lifetimes; Td and Tg may rapid progress in nanotechnology, nanostructure-based

be determined by thermogravimetric analysis (TGA), and polymer ﬁlms are becoming new-generation materials

differential scanning calorimetry (DSC), respectively. for plastic electronics [157,158]. Self-assembly techniques

For PFs, ketone defects may cause spectra instabil- allow for controlling polymer ordered superstructures. PSC

ity [9,145]. One strategy to alleviate degradation is to performance depends heavily on the morphology of the

induce efﬁcient charge separation and shift the zone of functional ﬁlms used in PSC design [159]. Semiconduct-

exciton generation to robust regions between trapped elec- ing polymer ﬁlms are considered to statistical and average

trons and holes via the copolymerization or grafting of ensembles of numerous single-chained semiconducting

appropriate units. Huang reported a method to introduce polymers via complex condensation from solution to the

antioxidant-hindered amine light-stabilizers into conju- solid state. Generally, materials based on single-chained

gated polymers to protect against degradation [146–148]. polymers and condensed polymers exhibit different opto-

The high standard of the stability that has been set for poly- electronic properties due to intrachain bending, torsion and

mer semiconductors must be met to realize polymer lasers, kinking or interchain aggregates. The relationship between

which require large electrical currents. optoelectronic properties and morphology is discussed in

PSCs face the more serious challenge of enhanc- detail as follows:

ing their environmental stability and achieving longer

operating lifetime than PLEDs due to their ease of photo- 1.2.2.1. Optoelectronic properties. Ex situ optoelectronic

oxidation in sunlight-irradiated air [149]. Hauch and characterization of semiconducting polymer ﬁlms without

coworkers reported that poly(3-hexylthiophene-2,5-diyl) electrodes provide the basis for screening ␲-conjugated

(P3HT):PCBM bulk-heterojunction modules have an out- polymers suitable for high-performance devices. For exam-

door lifetime of more than 1 year [150,151]. Inverted ple, high PL quantum efﬁciencies of polymer ﬁlms are

devices dramatically improve stability via the spin-coating generally necessary to obtain higher external quantum

of a PEDOT:PSS layer above the active layers as the top efﬁciencies of EL in devices, although the EL efﬁciency

buffer layer. The lifetimes of low-efﬁciency PSCs reach is also determined by many other factors. The optoelec-

values over 20,000 h [152]. For polymer transistors and tronic properties of semiconducting polymer ﬁlms include

solar cells, the sensitivity to oxidative doping is closely electronic and photophysical properties, which directly

related to the oxidation potential (IP) of the active poly- reﬂect the behaviour of basic particles, such as carriers

mer ﬁlms. Therefore, one of the strategies used to resist and excitons. Several semiconducting parameters, such as

air oxidation is to lower the highest occupied molecular the HOMO, lowest unoccupied molecular orbital (LUMO),

orbital (HOMO) of the polymer ﬁlms while retaining the band gap and charge mobility, are used to character-

self-organization properties of the ﬁlms. For n-type high- ize polymer ﬁlms. Generally, the absolute values of the

mobility polymers, air stability is much more difﬁcult to energy of the HOMO and the eigenvalue of the LUMO are

achieve. A high electronic afﬁnity (EA) greatly beneﬁts approximately equivalent to the IP and EA, respectively,

the stabilization of electron carriers, and increasing the according to Koopman theorem. Accordingly, these are

hydrophobicity of polymers helps these materials repel measured by cyclic voltammetry (CV) to determine elec-

moisture and thereby increase their environmental stabil- trochemical turn-on oxidation and reduction potentials.

ity and enhance device stability. To summarize, developing The HOMO energy level can also be calculated by ultra-

high-performance devices with long operating and envi- violet photo-electron spectroscopy (UPS). Semiconducting

ronmental lifetimes is the ﬁrst objective in the structural properties may be determined by different methods. The

design of polymer semiconductors and their devices. electrical band gap (E ) is associated with the dif-

g electr

ference between the EA and IP. The optical band gap

1.2.2. Optoelectronic property and morphology of (Eg opt) is determined from the absorption-edge wave-

semiconducting polymer ﬁlms length ( ) (E = 1240/ ). The exciton binding

edge, abs g edge

One clear advantage of polymer devices is their low cost energy may be estimated as the difference between E

g electr

of fabrication with respect to that of their inorganic coun- and Eg opt. Polymer ﬁlms with large exciton binding ener-

terparts. To date, several solution-processable techniques gies, such as PFO (∼30 meV) are suitable for use in EL

have been demonstrated, including inkjet printing, gravure devices, while polymers with small binding energies pro-

printing, micro-contact printing, roll-to-roll printing, and mote exciton dissociation in PSCs. The emission colour

other techniques [153]. For single-layered polymer devices of light-emitting polymer ﬁlms depends on the energy

with sandwich-type conﬁgurations, it is essential to design gap between the LUMO and HOMO, with the visible-light

and characterize the semiconducting polymer ﬁlms to spectrum (380–780 nm) corresponding to 1.5–3.2 eV. For

achieve stable, high-performance devices. Generally, the photovoltaic polymer ﬁlms, the open-circuit voltage (Voc)

typical thicknesses of semiconducting polymer ﬁlms range is related to the energy difference between the LUMO

from 10 to 200 nm in polymer devices. Host–guest doping of the acceptor (PCBM) and the HOMO of the donor

systems [154] or self-assembly techniques of polymer ﬁlms (for low-band-gap ␲-conjugated polymers) [160–163]. The

are two effective methods used to tune ﬁlm properties and HOMO–LUMO band-gap energy is closely related to the

achieve high device performance at the nanometre scale. absorption range of PSCs and the mobility of ﬁeld-effect

For PLEDs, red or white EL may be achieved by blending transistors. The charge-carrier mobility of polymer ﬁlms

host matrices with various dye dopants based on energy may be measured by either time-of-ﬂight (TOF) methods,

transfer and tunable charge-transport behaviour, although time-resolved microwave conductivity, or by analysing

1198 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

diode or transistor conﬁgurations [164]. Carrier type and most p-type conjugated polymers, high LUMO energy lev-

concentration could be determined by exploiting the Hall els should be reduced to improve electron injection at the

Effect. Other related transport parameters, such as density cathodes. Consequently, to obtain high-efﬁciency PLEDs,

of states, reorganization energy, and transfer integration, bipolar conjugated polymers are desirable candidates for

could be investigated by ﬁrst-principle theoretical calcula- high-efﬁciency devices in single-layer devices due to their

tions using the Gaussian software package. High-mobility balanced hole- and electron-transport abilities [170]. Gra-

polymers are required for polymer transistors used in inte- dient charge injection and energy transfer have become

grated circuits. For high-mobility polymer ﬁlms, n-type an effective method to further improve the performance

polymers are relatively rare because most polymers exhibit of PLEDs [171]. For PSCs, electrical currents may be gener-

p-type behaviour. Most conjugated polymers exhibit faster ated by light absorption according to the photovoltaic effect

hole transport than electron transport [165]. For mem- and exciton diffusion and dissociation to generate carri-

ories, charge mobility of polymers need not extremely ers, transport carriers, and collect carriers into electrodes.

high. Stability and trap depth of charge-transfer complexes Low-band gaps and high charge mobilities play key roles in

determine their type: WORM, DRAM or ﬂash memories improving the PCE of BHJ solar cells. For polymer lasing, the

that may be created. active material must exhibit strong stimulated emission

Steady-state or time-resolved electronic absorption and under optical or electrical excitation. Polymer semicon-

emission spectra can measure several many molecular ductors are generally four-level energy systems that afford

optical parameters, including electronic absorption peaks, the possibility to realize population inversion, which is a

molar extinction coefﬁcient and emission bands, Stokes prerequisite for lasing applications. Several distinct prop-

shift, and other photophysical parameters. Conformational erties of polymers such as high luminescence efﬁciency,

changes may be monitored in UV spectra, such as the rise high chromophore density, large cross section for the stim-

of the planar ˇ chains of poly(9,9-dioctylﬂuorene) (PFO) ulated emission, and low ampliﬁed spontaneous emission

together with the generation of new peaks. The effec- (ASE) threshold characterize a good laser material with a

tive conjugation lengths of polymers can be estimated large gain coefﬁcient.

by examining the plot of absorption energy (or some

proportional quantity) versus the quantity 1/n, where n 1.2.2.2. Morphology. Active polymer ﬁlms are complex

represents the oligomeric length [166,167]. An extended matrices of polymer channels that feature carrier par-

range of light absorption with a very large molar extinc- ticles according to the channel-wave/particle theory. In

tion coefﬁcient enhances the photocurrents and PCE of polymer semiconductors, channels and carriers are two

PSCs. With respect to PL spectra, the colour purities and interplaying factors that are critical in determining device

CIE coordinates of PLEDs are closely related to the emis- performance and functionality. The electron states of

sion peak and full-width at half-maximum values. A high single-chained polymers in the solid state are much more

QE is favourable for the ampliﬁed spontaneous emission complex than those in solution due to interchain aggrega-

of a laser, improves the detection sensitivity of sensors tion and other conformational processes. Exerting control

and enhances the external quantum efﬁciency (EQE) of over morphology is an effective strategy to tailor the car-

PLEDs. Fluorescence and phosphorescence exhibit dra- rier and exciton transport behaviour of polymer ﬁlms,

matic photophysical properties, the latter of which has which leads to optimized device performance. Normally,

the advantage of exhibiting almost 100% internal quan- phase morphology may be characterized using atomic force

tum efﬁciency (IQE). Luminescent longevity may be used microscopy (AFM), scanning electron microscopy (SEM),

to distinguish singlet from triplet excitons. Triplet energy transmission electron microscopy (TEM), small-angle X-

levels are important parameters for electrophosphorescent ray scattering (SAXS) and other analytic methods. The

host–guest materials. Förster resonance energy transfer polymorphism of conjugated polymer ﬁlms makes the

(FRET) may be effectively harnessed in designing white relationship more complex, in which several condensed

PLEDs, ﬂuorescent sensors and other devices. Charge trap- phases of the same polymer, including amorphous states,

ping plays a key role in electrophosphorescent PLEDs in semi-crystalline phases, liquid–crystal phases and poly-

addition to energy transfer, in which there are large dif- crystal phases, may be obtained by altering the external

ferences in the triplet energies between host and guest preparation conditions, such as temperature, solvent type,

materials [168,169]. concentration, etc. PF materials are good examples of mate-

These electronic and optical properties are fundamen- rials that exhibit extensive polymorphism.

tal in determining whether a polymer semiconductor is Charge mobility and other important electronic param-

suitable for a certain device. In practice, any single device eters are highly sensitive to phase morphology [172].

has a certain set of required optoelectronic properties Self-assembly affords powerful approaches to tailor nanos-

that must be met by the active polymer ﬁlms that are tructures. The hierarchical architectures of active polymer

used in its design. For PLEDs, under an applied voltage, ﬁlms also strongly inﬂuence their photophysical behaviour.

charge carriers undergo carrier-charge injection, trans- For example, the exciton binding energy of well-oriented

port and combination processes to emit light; in PSCs, polyﬂuorene ﬁlms is about 10 times smaller than that of

these processes are controlled to harness solar energy. disordered ﬁlms [173,174]. Morphology-dependent energy

For light-emitting polymer films, device efficiency is sen- transfer within PF thin films has been investigated by

sitive to the position of the exciton recombination zone, Khan et al. [175]. Microstructures dramatically change

which is determined by the injection and transport abil- the optoelectronic properties and device performance. The

ity of charge carriers from the anode and cathode. For modulation of excitation energy transfer by controlled

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1199

self-assembly allows for slow energy migration and par-

tial energy transfer to realize RGB-based white emission,

as reported by Vijayakumar et al. [176]. Generally, there

are speciﬁc requirements that must be met by active

polymer ﬁlms to produce high-performance polymer

devices. Amorphous phases with high Tg are favourable

for PLEDs, nano-scale phases for polymer solar cells,

long-range ordered liquid crystal or polycrystal phase for

high-mobility polymer transistors [90], and phase trans-

formation for non-volatile bistable memories. For PSCs,

Heeger and coworkers introduced the BHJ concept to over-

come the limitation of exciton separation and the length

of exciton diffusion and effectively enhance the photo-

electrical conversion efﬁciency of solar cell devices [177].

In polymer BHJ solar cells, interpenetrating p–n networks

with a desired domain size of 10 nm are easily formed

by blending ␲-conjugated polymers (p-type) with soluble

fullerenes (n-type) via the driving force of phase separation.

Although the limitation of the exciton separation has been

overcome using the BHJ concept, it is still challenging to

control charge collection kinetics by controlling morphol-

ogy.

1.2.3. Four-element design of semiconducting polymers

From the chemical structure point of view, polymer

semiconductors are essentially charge-transport ␲-orbital

channels, which are mainly divided into ␲-conjugated

polymers and ␲-stacked polymers. Organic synthesis

Fig. 3. (a) Electronic structure design principle, (b) the three basic types

affords ﬂexible tools to control charge-carriers behaviour

of energy level diagrams of p–n (D–A) semiconducting copolymers.

through the combination and reorganization of ␲-orbitals,

resulting in dramatically different optoelectronic proper-

ties as well as morphologies for semiconducting polymer

ﬁlms applied in various devices. The conceptual frame- HOMO, LUMO and band gap, which has led to the

work established by physical organic chemistry offers optimization of device performance [179]. Frontier molec-

useful templates guiding the principles and strategies of ular orbital theory is suitable for the conceptualization

polymer semiconductor design. Semiconducting polymer of not only organic reactions, but also organic opto-

chains are effectively tuned by means of four fundamen- electronic materials [180,181]. Electron-donating groups

tal nodes, including electronic structure, steric hindrance, and electron-withdrawing groups have been introduced

conformation and topology, and supramolecular inter- into polymer ﬁlms through substitution, copolymeriza-

action, which have been proposed by Xie–Huang group tion, end-capping and other techniques. Speciﬁc electronic

[178]. The introduction of periodic table elements into structure groups may be selected according to doping

polymer chains is used primarily to tailor the electronic or blending host–guest experiments. Charge transfer or

structures of polymers due to the different electroneg- energy transfer make alteration of the electronic behaviour

␲

ativities and dipoles of molecular segments, directly of -conjugated polymers dramatically. The Huang group

resulting in dramatic alteration of the optoelectronic prop- has proposed a novel p–n semiconducting copolymeriza-

erties of polymer ﬁlms. Correspondingly, supramolecular tion strategy for polymer-band-gap engineering [170], in

interactions alter the morphology of polymer ﬁlms. The which there are three basic types of electronic energy

four-element design of ␲-conjugated polymers allows diagrams that describe conjugated polymers containing

for stable and high-performance polymer devices to be donors or acceptors, as shown in Fig. 3.

designed at the molecular scale, an approach that is not For type I, there is seldom any energy level overlap

readily feasible for the design of inorganic semiconduc- between donors and acceptors. In this case, ground-

tors. In addition, it has become increasingly important state electron transfer occurs due to the negative

to investigate molecular electronics because they help free-energy driving force. The bulk conductivity of con-

not only to clarify device behaviour of polymer ﬁlms, ducting polymers may be enhanced by chemical doping.

but also to explore new functionalities of polymer Using this technique, hole or electron carrier injection

ﬁlms. have been effectively enhanced by either tetraﬂuo-

rotetracyanoquinodimethane [182,183] and molybdenum

1.2.3.1. Electronic structure design. The introduction of tris-[1,2-bis(triﬂuoromethyl)ethane-1,2-dithiolene] as p-

molecular segments with various polar or electronic effects dopants [184,185] or cobaltocene and decamethylcobal-

provides an effective method to tune the optoelectronic tocene as n-dopants [186,187]. For type II, there is a partial

properties of semiconducting polymer ﬁlms, such as the overlap of energy levels between donors and acceptors. In

1200 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

this case, hybridization after copolymerization generate a of acceptors may be employed to tune the bistable

new HOMO located at the donor and a new LUMO located at switching and stable charge-transfer state, resulting in

the acceptor, thus resulting in a lower band gap [188,189]. WORMs, DRAMs or ﬂash memories [204–206]. Utilization

+ −

↔

Furthermore, mesomerism (D–A D = A ) leads to the of excited-state intramolecular proton transfer or twisted

improvement in a double-bond character between the D intramolecular charge transfer to develop next-generation

and A units, thereby facilitating charge transfer. In this polymer devices presents a current research challenge

system, the improvement in the quinoid states or de- [207,208].

aromaticity occur after the hybridization between donors

and acceptors. As a result, these types of D–A conju- 1.2.3.2. Steric hindrance design. Steric hindrance design

gated copolymers exhibit a strong ground-state dipole due provides a tool used to improve device performance by

to intramolecular charge transfer. Photo-induced electron modifying the spatial arrangement of active molecular

transfer is readily observed in the excited state. The Huang bricks, morphology and the ﬁnely optoelectronic proper-

group has demonstrated that the emission colour and ties of conjugated polymers. In contrast with electronic

redox properties of type II p–n light-emitting copolymers structure design, most of the sterically hindered groups do

may be effectively tuned to achieve RGB emission in PLEDs not alter the HOMO and LUMO energy level signiﬁcantly

␲

by balancing the charge carriers from the anode and cath- when it is introduced into -conjugated polymer chains.

ode [165,190–193]. The Jenekhe group showed that type There are two basic types of hindrance effects: intra-

II ␲-conjugated oligomers and copolymers exhibit excel- and interchain steric hindrances. The former can serve to

lent ambipolar charge-transport behaviour and low-band twist dihedral angles between the adjacent monomer units,

gaps [194,539,669]. The incorporation of electron–hole- while the latter can suppress interchain aggregation. The

transport units into PFs also helps to balance charge-carrier softest sterically hindered groups are various alkyl side-

populations and achieve good spectral stability and excel- chains and pendant groups, which can alter interchain

lent thermal stability [38,195,196]. The reason lies in supramolecular interactions [209]. Rigid bulky groups are

the alternation of charge-transfer state on the polymer favourable for producing high Tg and colour stability

chains, reducing the probability of excited-state trapping in amorphous LEPs [27,210]. The Xie–Huang group has

in undesirable defect states. Currently, D–A conjugated demonstrated that spirobiﬂuorenes and 9-phenylﬂuorenyl

copolymers offer the most effective tool with which to moieties are outstanding sterically hindered groups that

develop low-band-gap materials with wide light absorp- may be used in conjugated polymers and stacked polymers

tion that can improve the efﬁciency of polymer solar cells [211–219]. The supramolecular steric hindrance effect is

[102]. Ling and coworkers have developed polymer DRAMs a new design technique that may be used to balance the

using this type of copolymer by controlling the stability of colour of light emitted and current efﬁciency [220–222].

charge-transfer states [197]. For type III, there is a close Swager and coworkers have developed iptycene-type ster-

relationship between the donors and acceptors. When ically hindered groups [223]. Dendron substitution is

the type III energy structure between molecular segments an extreme example of steric hindrance that exploits

occurs, it has been proven that conjugated copolymers the self-shield and self-packed effect of polymer chains

facilitate FRET more easily than charge transfer. The type [32–34,224]. It should be noted that the substitution of

III energy structure is the basis of the Annette effect used pendant groups often produces binary or multiple func-

to harvest energy for photosynthesis. If efﬁcient energy tionalities [38,225]. Chen and coworkers have designed a

transfer from a wide-gap segment to a narrow gap site series of conjugated PFs with gradient electronic structures

occurs, another prerequisite condition is that exciton trans- to achieve stable colour-puriﬁed high-efﬁciency blue LEPs

fer and trapping between the chromophoric segments [226], in which side and/ or pendant chains serve as both

must be much faster process than radiative and nonradia- electronic structural groups and steric hindered groups.

tive decay. Copolymers of the type III structure are very The ␲-substituent groups are multifunctional to design

useful in designing red and white LEPs. Burn et al. reported sterically hindered donors or acceptors, which represents

the use of exciton conﬁnement in random copolymers to a powerful tool for electroluminescent materials design.

improve the EQE of PLEDs [198], in which efﬁcient energy Wang and coworkers have reported a series of blue-, green-

transfer and exciton trapping occur from segments with and red-light-emitting PFs containing dyes with high PL

wider energy gaps to heterocycle sites with narrow band efﬁciency [227–230].

gaps. Oligoﬂuorenes have been chosen to harvest energy to

porphyrin dyes for high-efﬁciency red PLEDs [414]. More- 1.2.3.3. Conformation and topology design. Beyond steric

over, exciton migration and trapping by highly emissive hindrance, conformation and topology determine the basic

chromophoric segments offer a tool with which to tune carrier-transport channels in polymers, which are impor-

colour emissions through the use of copolymers [199]. tant to the design of polymer semiconductors for use in

Colour tuning in blends containing perylene copolymers high-performance devices. Linear ␲-conjugated polymers

has been reported by Tasch et al. [200]. Cao and cowork- might be considered one-dimensional carrier-transport

ers have used intrachain energy-transfer conjugated channels with low-dimensional characteristics. In gen-

polymers to develop high-efﬁciency red-light-emitting eral, linear and planar conformations afford high charge

polymers [201,202]. Attaching dye-containing copolyﬂu- mobilities in conjugated polymers. Their degree of electron

orenes to side-chains has become one practical method delocalization is determined by the effective conjugation

of developing commercialized white-light-emitting mate- length, which may be extended by the planarization of

rials [57,109,203]. In polymer memories, the trap depth conformations, resulting in low-band gaps, high charge

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1201

mobilities and red-shifted emissions [231]. Ladder-type morphologies of polymers at the highest levels of concep-

␲

-conjugated polymers containing unique linear confor- tualization. In this context, supramolecular analysis could

mation possess high backbone rigidity, which can be further clarify the roles of supramolecular interactions

characterized by the associated Stokes shifts, beneﬁting in solution, active polymer ﬁlms and devices. Among the

the improvement of charge mobility [232]. The substi- many supramolecular weak interactions that exist, ␲–␲

tuted isomerization of linear monomers allows for the stacking interactions play a unique role in organic elec-

construction of zigzagged or kinked conformations with tronics due to their basic effect on the electron-hopping

shorter effective conjugation lengths than those of their channels [251,252]. The edge-on or face-on arrangements

␲

linear counterparts [233,234]. The effective conjugation of -segments and intermolecular self-organization dra-

lengths of ␲-conjugation-interrupted polymers may be matically inﬂuence the mobility of polymer transistors

further reduced by linking through tetrahedral sp car- [253]. Previously, we proposed the study of supramolec-

bons [235], which generates an amorphous phase with ular semiconductors to explore device functionalities

high spectral and thermal stability that is well suited and performance [13]. Intramolecular supramolecular

for applications violet-light or electrophosphorescent host systems will be a promising tool to explore memory-effect

materials. Beyond the linear and kinked conformations, polymers [245]. Such ␲-stacked polymers as PVK have

other complex polymer arrangements, such as multi- also become important semiconducting polymers, besides

␲

armed macromolecules, hyperbranched polymers and -conjugated polymers [254–256]. The supramolecular

dendrimers [236], have also been developed [237–239]. functionalization of polymer semiconductors allows for

Three-dimensional ␲-conjugated polymers show many the development of various new organic electronic sys-

porous superstructures that are suitable for applications tems. Conjugated polyelectrolytes with supramolecular

in either biosensors or the detection of explosives, such as ionic side-chains, such as the quaternization of the amino

2,4,6-trinitrotoluene [240]. Dendrimer core–shell systems group, are one form of water-soluble ﬂuorescent poly-

are important scaffold of FRET that suppress luminescence mers used in biosensors [257–259]. A recent important

quenching processes and achieve high QE [236]. Macrocy- development in PFs was the development of ionic PFs and

cles are favourable for the construction of many complex their application as electron-injection layers in PLEDs and

conformations and topologies. Macrocyclic PFs have been electron-extraction layers in polymer solar cells. Bazan

synthesized by the Müllen group [241]. They represent an and coworkers, Cao and coworkers, and Park et al. have

important kind of molecular segment that can be used performed elegant research in this ﬁeld [75–77,80,83,260].

to further construct catenane and rotaxane complexes. Furthermore, conjugated homopolymer amphiphiles

Polyretaxane, an example of a molecule that exhibits a are an alternative to construct well-deﬁned, hierar-

novel conformation, has also been applied as a molec- chically self-assembled architectures. Schenning and

ular wire. Encapsulation can also improve the stability Meijer proposed that supramolecular electronics is an

of polymer chains against thermal and/or photooxidation intermediate ﬁeld between molecular electronics and

to enhance the colour purity of PFs [242–244]. Further- organic-ﬁlm electronics [261]. Following the develop-

more, controlling conformational changes is useful in the ment of side-chain supramolecular conjugated polymers

design of memory-effect polymers [245]. Interrupted con- and stacked polymers, main-chain-type supramolecular

jugated polymers exhibit many conformational isomers, polymers with dynamic linkages of noncovalent bonds

which may become a type of memory-effect polymer. were demonstrated to be fantastic semiconducting poly-

In addition, rod-coil conjugated polymers have two dif- mers [262–264]. Researchers have introduced various

ferent block segments, one of which is unique to the hydrogen bonds into conjugated systems to systemically

conformational design, exhibiting versatile supramolecu- investigate the ﬁne control that may be exerted over

lar assembly behaviours [246,247]. More recently, rod–rod self-assembled behaviour and morphology-dependent

copolymers have also been explored to investigate their optoelectronic properties [265]. Wurthner et al. reported

self-assembly behaviours [248]. the development of supramolecular p–n heterojunctions

[266].

1.2.3.4. Supramolecular interaction design. Along with To summarize, the performance as well as stability

the molecular characteristics of organic semiconductors, of polymer devices may be controlled on a large scale or

supramolecular features provide another opportu- ﬁnely manipulated via self-assembly techniques and/or the

nity to explore device performance and functionality, organic synthesis of polymer chains. As illustrated in Fig. 2,

which clearly distinguishes organic semiconductors from the four elements electronic structure, steric hindrance,

their inorganic counterparts [249]. In general, the self- conformation and topology as well as supramolecular

organization of semiconducting conjugated polymers interaction have been combined with supplementary

during solution processes results in a complex relationship elements to deeply and clearly understand the relation-

between polymer ﬁlms and devices. Supramolecular inter- ships that exist among the three conceptual levels of

␲

actions between polymer chains intrinsically determine -conjugated polymers: chain structures, thin ﬁlms and

the various morphology phases adopted by active polymer devices. It should be noted that the four-element principle

ﬁlms, resulting in the alteration of the current–voltage is an ideal model. Each node may affect other elements, as

characteristics of devices [250]. Supramolecular interac- shown in the framework. When one periodic table atom

tions and noncovalent bands that serve as molecular-scale with an electronegativity value dramatically different from

interfaces determine the self-assembly behaviour, molec- that of carbon is incorporated into polymer semiconduc-

ular nanostructures, phase separations, and domain tors, it will mainly affect the electronic structures of the

1202 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Fig. 4. Polyﬂuorenes containing various periodic table elements.

polymers, followed by their optoelectronic properties. In way of end-capping functional groups and constructing

fact, this incorporation will also alter the steric hindrance, block copolymers [272–274]. In this section, following

conformation, topology, and even supramolecular interac- the framework of the four-element design principle of

tions exhibited by the polymers. Herein, we will focus on semiconducting polymer design, we ﬁrst discuss poly(9,9-

PFs as a model semiconducting polymers to demonstrate dialkylﬂuorene)s (PDAFs) to illustrate the relationship

in detail the effect of various elements with different elec- between morphology and optoelectronic properties, in

tronegativity and their molecular segments with different which the inﬂuence of alkyl-substitution and supramolec-

electron-donating or accepting ability on polymer ﬁlms ular ␲–␲ interactions as well as external conditions on

and devices (Fig. 4). The intrinsic electronic effects of device performance will be addressed. Second, hydrocar-

elements and their molecular segments on optoelectronic bon copolyﬂuorenes will be introduced to illustrate the

properties and device performance will be highlighted. basic principle of electronic structure design for band-gap

engineering, followed by the steric hindrance design of

bulky groups. Conformation and topology design includes

2. Hydrocarbon polyﬂuorenes (CPFs)

fused and ladder-type, kinked, hyperbranched and den-

dritic CPFs.

Molecular segments consisting of carbon and hydro-

gen atoms are abundant due to the catenation of their

characteristics and ﬂexible bonding features, which facil- 2.1. Poly(9,9-dialkylﬂuorene)s (PDAFs)

itate the construction of different electronic and steric

effects. Many CPFs have been reported since the oxida- PFs are regarded as the simplest regular step-

tive polymerization of PFs was demonstrated using ferric ladder-type poly(para-phenylene)s (PPPs), in which

chloride in 1989 by Fukuda et al. [267–269]. Since then, two phenyl rings are locked into a plane via the C-9

transition-metal-catalysed polymerizations, such as those carbon of the ﬂuorene units. PDAFs have good solubil-

performed by such as Suzuki, Yamamoto, Stille and others, ity owing to the introduction of alkyl chains, including

have proved to be more effective methods for synthesiz- poly(9,9-dihexylﬂuorene) (PF6) (CPF-01a) (Scheme 1),

ing high-molecular-weight PFs [6,19,270,271]. Recently, poly(9,9-di-n-octylﬂuorene) (PF8 or PFO) (CPF-01b), and

Wang and coworkers and the McCullough group reported poly(9,9-di(2-ethylhexyl)ﬂuorene) (PF2/6) (CPF-01c).

a chain-growth mechanism to synthesize PFO using the PDAFs possess relatively large band gaps, which are suit-

Grignard metathesis method (GRIM), which offers a able for blue-light-emitting materials. PF8 is a basic model

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1203

Scheme 1. Poly(9,9-dialkylﬂuorene)s (PDAFs).

of the structure–property relationships exhibited by semi- or the presence of excimers as being responsible for the

conducting polymers [9]. The HOMO and LUMO energy low-energy emission band [27,292–294]; other reports

− −

levels of PF8 are estimated to be 5.80 and 2.12 eV, suggested that the low-energy emission band might be

respectively, according to CV measurements [275]. PF8 attributed to on-chain chemical ketone defects [9,29,295],

has the difference between the E (3.1 eV) and E interchain ketone-based excimers or hydroxy-terminated

g electr g opt

(2.95 eV) with an exciton binding energy of approximately oxidation at the interfaces of devices [296]. Nevertheless,

30 eV [275,276]. Our group previously reported that PF6 the thermal stress of a device tends to form keto-type

− possess HOMO and LUMO energy levels of 5.50 and defects at the 9-position of PF units by exposure to oxygen

−

2.37 eV (E = 3.13 eV, E = 2.86 eV), respectively and water, which is accompanied by the appearance of the

g electr g opt

[277]. With respect to carrier transport, PF8 exhibits excel- undesirable green emission band. The design of stable PFs

lent nondispersive hole transport, with a mobility of up to without g-bands has become a key objective for organic

× −4 2

3 10 cm /(V s), as measured by the TOF method [278]. and polymer chemists.

Recently, a PF8-based PLEDs using MoO3 ohmic contact PDAFs exhibited complex phase behaviour and numer-

layers was shown to allow for the direct determination ous morphologies that depended delicately on aver-

−5 2

of the zero-ﬁeld hole mobility (1.3 10 cm /(V s)) from age molecular weight [297,298], side-chain architecture

the space-charge-limited current (SCLC) regime [279]. [299,300] and ﬁlm preparation conditions [301–303]. One

PDAFs are promising light-emitting polymer materials thorny issue in this ﬁeld is the relationship between poly-

due to their PL efﬁciency of more than 80% in solution morphs and optoelectronic properties along with the need

and approximately 50% in solid ﬁlm without heavy-atom to clarify the origin of the g-band [304]. In terms of charge

effects [280]. Their triplet energy level is approximately transport, Redecker et al. have demonstrated that relatively

−3 2

2.15 eV [281], which is high enough to act as a host for high charge-carrier mobilities (up to 8.5 × 10 cm /(V s))

red electrophosphorescent complexes, but not suitable for can be achieved when PF8 was oriented in its nematic

◦

green or blue guests. The inﬂuence of the chain length phase (T > 160 C) [305]. The amorphous phase [306],

and side-chains of PDAFs on optical properties has been liquid–crystal phase, crystalline alpha (␣) and alpha’ phase

studied. Miller investigated that the effective conjugation [307], beta ( ) phase, and gel phase of PF8 have been

lengths of the PDAFs is ca. 10 monomers [282]. Single- demonstrated by several groups using various microscopy

layer blue PLEDs based on PF6 exhibit a maximum emission techniques and theoretical simulations [308], such as X-ray

wavelength of 470 nm at 10 V and at room temperature, diffraction (XRD), near-ﬁeld scanning optical microscopy

as reported by Ohmori et al. [18]. In fact, PDAFs exhibited [309] confocal laser spectroscopy [310], NMR spectroscopy

poor electron-transport abilities [283]. Grice et al. reported [311], and Monte Carlo simulations [312].

a PF8-based double-layer blue PLED with a brightness of Pioneering works on the thermotropic liquid–crystal

600 cd/m at a bias voltage of 20 V, emission peaks at (LC) behaviour of PF8 were reported by Grell et al. [313].

436 nm, and luminance efﬁciency (LE) of 0.25 cd/A [284]. According to Bradley’s ﬁndings, a nematic (N) mesophase

PDAFs showed excellent thermal stability but a low of PF8 exists when it is heated to its melting temper-

◦ ◦

Tg of 75 C and limited spectral stability in PLEDs. Low- ature of 170 C [313–315]. Ueda and coworkers found

energy green emission bands (also called long-wavelength that a single-crystal-like thin ﬁlm of PF8 may be created

emission bands, or g-bands) of 2.2–2.3 eV (520–530 nm) by a friction-transfer technique with subsequent thermal

were clearly observed under high or long-term operating treatments [316]. PF8 is known to be crystalline. Su and

voltages [144,285]. Pei and Yang [19] and Kreyenschmidt coworkers investigated the chain-packing behaviour of

et al. [286] ﬁrst reported the issue of the low-energy green crystalline PF8 featuring eight polymer chains and a space

emission bands of PDAFs in EL devices. These bands in group P212121 [317]. They also performed cold crystal-

PDAFs are manifested as poor colour purity and limited lization and conducted a Gibbs-Thomson analysis of PF8

lifetime in PLEDs. To clarify and identify the origin of [318–321]. Brinkmann developed oriented PF8 by direc-

this long-wavelength emission, various techniques have tional epitaxial crystallization in a 1,3,5-trichlorobenzene

been used to characterize ﬁlms annealed in an air or N2 solvent [322].

atmosphere, including X-ray photoelectron spectroscopy Unlike the ␣ phase, the ˇ phase of PF8 has a pla-

◦

(XPS) [287], Fourier transform infrared (FTIR) spectroscopy nar conformation with a dihedral angle of 180 between

[288], matrix-assisted laser desorption/ionization time- repeating ﬂuorene units and exhibits remarkable pho-

of-ﬂight mass (MALDI-TOF-MS) spectrometry [145], tophysical properties. The ˇ phase can be induced by

steady-state UV–vis absorption and PL spectra, and time- annealing or solvent-vapour exposure [304]. Morgado and

resolved PL measurements [289]. However, the origin of Charas reported PF8 based PLEDs with pure ˇ-phase emis-

this low-energy emission band is still the subject of debate sion and higher colour stability upon increasing the driving

[290,291]. Some evidence supports interchain aggregation voltage [323]. Lu et al. created a self-dopant form of the ˇ

1204 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

ˇ phase of PF8 to develop efﬁcient and stable pure blue elec- to create PF-based nanoparticles with amorphous and

troluminescent devices [324]. The -phase of PF8 has been phases controlled by solvent type [350–352]. Polymer gels

proved to help improve the cross sections of organic lasers, represent another important phase, which is featureed as

as illustrated by Rothe et al. [325] and Anni and coworker physical cross-linked networks that can be explored to

[326,327]. investigate the self-assembly mechanisms or new applica-

Different substitutions of alkyl side-chain signiﬁcantly tion of semiconducting polymers. Several studies on PF gels

impact the conformation, phase behaviour and self- have been reported [353] since PF8/toluene gel was ﬁrst

assembly of the conjugated PF backbone [209,299,328]. synthesized by Grell et al. [354]. Knaapila et al. noted that

Monkman and coworkers studied the inﬂuence of alkyl- PFn (6-9)/methylcyclohexane solutions became viscous or

◦

chain length on ˇ-phase formation in PFs [328,329]. They gel-like when the solutions were cooled to −25 C [355].

ˇ ˇ

concluded that the -phase formation is controlled by the Chen and coworkers observed the -phase PF8 gels in

side-chain interactions in methylcyclohexane solution: PF8 toluene and methylcyclohexane solvents [356,357]. Lin and

is more easily controlled than PF6, PF9, and PF10. Su and coworkers successfully prepared PF8/1,2-dichloroethane

coworkers reported four different phases of PF6, which gels at room temperature to utilize their stimulus-response

had previously received little attention compared to its feature to develop soft polymer semiconductors sensor

homologues PF8 and PF2/6 [330]. PF2/6, as a stiff heli- and actuator applications [292]. PFO/1,2-dichloroethane

cal polymer, possesses better solubility in organic solvents gel exhibits up to 50% ˇ phase and unique photolumines-

than PF8 because of its branched alkyl groups. Knoll and cence peaks at 470, 500 and 550 nm, which are red-shifted

coworkers were the ﬁrst to synthesize LC-phase PF2/6 with respect to the three emission bands at 419, 443 and

and investigated its blue polarized EL [20]. Lieser et al. 472 nm in solution and the feacturing peaks at 446, 473, and

identiﬁed axially oriented PF2/6 with hexagonal unit cells 500 nm in the ˇ-phase, assigned to the 0–0, 0–1 and 0–2

[331]. Knaapila et al. reported biaxially aligned PF2/6 with vibrational transitions. Moreover, xerogels, which exhibit

cylindrically isotropic orientation in a hexagonal phase by porous morphology, can be used in gas sensor applications

grazing-incidence XRD [332–335]. Winokur and cowork- and are promising organic porous semiconductors and soft

ers utilized polarized optical absorption spectroscopy, semiconductors.

near-edge X-ray absorption ﬁne structure spectroscopy PDAFs, as typical PFs, exhibit excellent wide-band-gap

(NEXAFS), and grazing-incidence X-ray diffraction (GIXRD) semiconducting properties in blue electroluminescent or

to investigate PF2/6 [336]. They found that the top host materials. However, their low-energy green emission

and bottom surfaces exhibited appreciable planar, uni- bands (g-bands) hinder their commercialization. The stud-

axial alignment, while the ﬁlm interior showed a higher ies on PDAFs’ polymorphs have been performed not only to

proportion of tilted chains after thermal cycling. These uncover the origin of these g-bands, but also to extend their

inhomogeneities are likely to inﬂuence technologically applications in lasers, sensors, and other photonic devices.

important optical and transport properties. PF2/6 has a hole

−4 2

mobility of 4.4 × 10 cm /(V s) in the nematic liquid phase, 2.2. Polyﬂuorenes with hydrocarbon-based

which is lower than that of PF8 in the same phase [20,313]. -conjugated monomers

Neher and coworkers ﬁrst demonstrated the circularly

polarized EL emanating from LC chiral PFs, poly[2,7-(9,9- Several unsaturated molecular segments of sp- or sp -

di((S)-3,7-dimethyloctyl)ﬂuorene)] (CPF-01d) with large hybridised carbons, including ethylene, acetylene and

gCPEL values of −0.25 [337]. Chen and coworkers reported arenes, have been copolymerized into the main chains of

chiral CPF-01d and CPF-01e in exploring the effect of chain PFs. Their electron delocalization abilities are reﬂected by

length and side-chain on thermotropic and optical proper- the band gaps of homopolymers, such as 1.5 eV for trans-

ties using circular dichroism (CD) [338]. The self-assembly polyacetylene (t-PA), 1.7 eV for polydiacetylene (PDA),

and morphology characterization of chiral PFs have also 2.5 eV for PPV, 3.0 eV for PPP and 3.2 eV for PFs. Most

been investigated by Meskers and coworkers [339–342] hydrocarbon-based PFs exhibit type II energy character-

through temperature-dependent CD and dynamic light istics, in which the orbital reorganization of polymer

scattering. They have found that the ˇ phase is an interme- chains occurs via hybridization, resulting in the alter-

diate phase in the formation of chiral PF aggregates with ation of band gaps. Triple bonds have a low-energy

3S,7-dimethyloctyl side-chains. Oriol and coworkers fab- barrier to the intramolecular rotation of single bonds and

ricated a mirror-less cholesteric liquid–crystal laser using a strong tendency for interchain ␲–␲ stacking interac-

a chiral oligoﬂuorene dye-doped liquid–crystal resonator tion to form aggregates and/or excimers [358,359]. With

[343]. Recently, Fujiki and coworkers utilized solvent chi- respect to PDAFs, the introduction of double bonds facil-

rality transfer using enantiomeric pairs of limonene and itates the delocalization of electrons and thus produces

a-pinene, allowing for the successful production of opti- a red-shifted emission, as reported by Jin et al., Suh

cally active poly(9,9-di-n-decylﬂuorene) (PF10) aggregates et al. and others [360–362]. Poly(ﬂuorenevinylenes) (CPF-

with CD and circularly polarized luminescence (CPL) prop- 02, Scheme 2) exhibited green emission (max = 505 nm)

erties [344]. [361]. Compared with the behaviour observed for PF8,

Nanostructures [345,346] of PDAFs are suitable for the introduction of vinylene units into the ﬂuorene back-

applications in lasing and optical switching systems [347]. bone could enhance the HOMO energy level, consequently

Redmond and coworkers demonstrated microcavity effects matching better with the work function of the anode elec-

and optically pumped lasing using nanowires of ˇ-phase trode and resulting in a reduced turn-on voltage in PLEDs.

PF8 [348,349]. McNeil reported a reprecipitation method The maximum luminescence and LE of CPF-02a-based

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1205

␲

Scheme 2. Polyﬂuorenes with hydrocarbon-based -conjugated monomers.

PLEDs are approximately 870 cd/m at 10 V and 0.16 cd/A, 3.25 eV, relative to the absorption peak of CPF-06b [27].

respectively [360]. Kreyenschmidt et al. [286] reported For a ﬂuorene–benzene copolymer CPF-06a, Charas et al.

conjugated cis- and trans-stilbene-containing copolyﬂu- [369,370] reported a high ﬂuorescence quantum yield of

orenes CPF-03 with PL spectra ( max = 430 nm) between approximately 100% in both cyclohexane and chloroform

those of PF6 (max = 420 nm) and PFVs (max = 505 nm) in solutions. PLEDs based on CPF-06a have been shown to

thin ﬁlms. The conjugation lengths of statistical copoly- exhibit high EL efﬁciency. The triplet energy of CPF-06a

mers containing both meta- and para-divinylenephenylene was reported to be identical to that of PF2/6 [371]. Longer

CPF-04 can be further extended, with a PL emission of alkyl side-chains on the phenylene rings could enhance

475 nm, as reported by Kim et al. [363]. Ethynylene-based the spectral stability of PFs [372]. To cover the entire

copolyﬂuorenes (CPF-05) have an optical band gap of visible-light spectrum, several large acenes with intrinsi-

◦

2.76 eV (449 nm) in chloroform and a low Tg of 65 C; cally high mobilities have also been incorporated into PFs.

these polymers were synthesized by Zhu and coworkers The introduction of anthracenes unit into PFs generates

to investigate their third-order nonlinear optical response large dihedral angles between the anthracene units and

[364,365]. Phenylene is a typical arene. The backbone ﬂuorene, in which strongly twisted conformation may sup-

structural modiﬁcation of PDAFs (i.e., alternately insert- press excimer formation. Miller and coworkers prepared

ing substituted phenylene units) offers an effective way of the random copolymer CPF-07 from dihexylﬂuorene using

optimizing the emission spectral quality of ﬂuorene-based 9,10-substituted anthracene by the Yamamoto method;

blue light-emitting polymers and suppressing excimer the polymer showed stable blue emission ( max = 455 nm),

formation in polymers. Huang group synthesized CPF- even after prolonged annealing [35]. Poly(anthracene-alt-

∼

06b c showing much better thermal spectral stability than ﬂuorene)s (CPF-08) with the 2,6-positions of anthracene

PDAFs [366–368]. Modifying the backbone structure of exhibited blue-shifted absorption and red-shifted PL, with

PDAFs by alternately inserting phenylene units also left a maximum peak at 460 nm, with respect to the spec-

the electronic structure of the polymers unaffected. CPF- tra of PF8 [373]. Perylene, a low-band-gap chromophore,

␲

was incorporated randomly into a -conjugated PFs main

06b exhibited an absorption peak at 371 nm, with an Eg opt

of 2.92 eV. In terms of the steric hindrance of the hexyl chain, which served as an efﬁcient exciton energy trap

chains, which decreases the coplanarity between the adja- upon the excitation of the PF segments. The PL emission of

cent ﬂuorene and phenylene units, the absorption peak polymer CPF-09 was centred at 540 nm without blue emis-

sion, indicating efﬁcient exciton migration from ﬂuorene of CPF-06c was blue-shifted to 324 nm, with an Eg opt of

1206 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

∼

segments to perylene [199]. Copolyﬂuorenes CPF-10 11 (CPF-15b) [378]. The presence of the rigid spiro-bis(2,2-

containing tetracene and pentacene units allow for fur- diphenylvinyl)ﬂuorene side chain on the polymer could not

ther red-shift emission [374]. They show emission from only efﬁciently suppress interchain interactions but also

the acene units: green ( max = 520 nm) emission for CPF- utilize energy transferred from the PF main chain, resulting

10 and red ( max = 625 nm) emission for CPF-11. In these in a stable and efﬁcient blue EL. PLEDs composed of CPF-

polymers, the torsion between the acene and ﬂuorene 15 exhibit a voltage-independent and stable blue emission

units suppresses interchain aggregation, particularly in the with CIE coordinates (0.15, 0.17), low turn-on voltage

pentacene-containing PF. PLEDs showed maximum lumi- of 4.6 V, and a maximum brightness of 3137 cd/m . The

2 2

nances of up to 5400 cd/m for CPF-10 and 850 cd/m spirobiﬂuorene-functionalized ladder-type PF CPF-16 was

for CPF-11. The incorporation of an azulene group into reported by Wang et al. [379]. CPF-16 emitted blue light

the backbone of PF signiﬁcantly lowered the band gap ( max = 455 nm), with a high solution PL quantum yield

of such conjugated polymers. The band gap was reduced (94%), and exhibited excellent thermal oxidative stability

from 2.86 eV for PF6 to approximately 1.60 eV for the with no aggregation tendency in the solid state. Bo et al. also

ﬂuorene- and azulene-based copolymers CPF-12. Azulene- reported soluble poly(spiroﬂuroene) with highly stable

containing conjugated polymers exhibited interesting blue emission [380]. Kim synthesized a spiroanthracene-

electrochemical behaviour and could potentially used as conﬁgured polyindenoﬂuorenes derivative CPF-17 [381].

electrochromic materials [375]. In summary, hydrocarbon- The stable PL spectra of the resulting material after ther-

based molecular segments are able to tune the emission mal treatment and photoirradiation are suitable for laser

and mobility of PFs via the delocalized conjugation of applications. With respect to that of CPF-16, the slightly

unsaturated carbons with different dipole moments. red-shifted emission peak of CPF-17 at 445 nm (blue-light-

emitting material) enhanced the emission’s perceived

2.3. Polyﬂuorenes substituted with various bulky groups intensity and its colour purity. The Merck-Covion Company

has disclosed the chemical structures of these blue-, green-

With respect to PPP and other conjugated polymers, and red-light-emitting poly(spiroﬂuorenes). This family of

an obvious advantage of PFs is the remote substituted materials exhibits the best comprehensive performance

effects at the 9-position of ﬂuorene, in which the dihe- and is the only practically useful group of polyﬂuorene

dral angle of the ﬂuorene units remains nearly unaffected, semiconductors [382].

thus retaining good electron delocalization. With the Diarylﬂuorene is another popular bulky group, featur-

exception of alkylidenes, in most cases uniquely sub- ing a simple synthetic route compared to spirobiﬂuorenes.

stituted hydrocarbon-based groups serve as sources of It should be noted that the improved stability of poly(9,9-

steric hindrance [376]. Alkylidenes, through ␲-substitution diarylﬂuorene)s is also attributed to the more robust struc-

at the 9-position, can not only increase conjugation ture of aryl substituents than the alkyl counterparts at the

lengths but also adopt a coplanar conformation rela- 9-position of ﬂuorene against oxidation. Kwon and cowork-

tive to the polymer backbone, thus facilitating cofacial ers [383] introduced bulky 9,9-dialkylﬂuorene substituents

aggregation. Solution-processable polymer transistors into the C-9 position of ﬂuorene units to effectively shield

composed of polyalkylideneﬂuorene CPF-13 (Scheme 3) polymer CPF-18 from the formation of aggregates and sup-

with a nematic mesophase liquid-crystalline phase exhib- press interchain interactions, resulting in efﬁcient blue

−3 2

ited ﬁeld-effect mobilities of up to 2 × 10 cm /(V s), EL. The emission colour of laddered PFs in the ﬁlm state

4 6

on/off ratios of 10 –10 and excellent environmental strongly depends on the substituents that occur at the

stability. 9-postion of ﬂuorene. Unlike alkyl-substituted polyinde-

The introduction of bulky groups into the 9-position noﬂuorenes, blue-emitting CPF-19b exhibits more stable

of ﬂuorene is an effective way to enhance the thermal blue emission in the solid state and in PLEDs fabricated

and morphological stability of ﬂuorene against interchain by Jacob et al. [384]. The long-wavelength emission due

aggregates. An excellent type of bulky groups is the spirob- to ketone defects in laddered PFs may be overcome by

iﬂuorene building block with two biphenyls connected via the introduction of aryl substituents. However, Guo et al.

a tetrahedral carbon for use in amorphous LEPs [377]. [385] reported that aryl-substituted CPF-20 displays dra-

Beyond PFs, poly(spiroﬂuorene) and its derivatives are matically red-shifted emission, with a maximum PL peak

useful in practical applications because of their good ther- at 494 nm, in the thin-ﬁlm state with respect to the spec-

mal stability, high electroluminescence efﬁciency and long tra of alkyl-substituted polymers (max = 463 nm). Thus,

device lifetime. Yu et al. were the ﬁrst to report spiro- the PLEDs with the conﬁguration ITO/PEDOT:PSS/CPF-20

◦

functionalized PFs [210]. The Tg of CPF-14 is 105 C, which (EML)/Ca (10 nm)/Al exhibited green emission with a max-

represents a signiﬁcant improvement compared to the Tg imum peak at 520 nm and CIE coordinates of (0.27, 0.49) at a

◦

of PF8 (75 C). The higher Tg is favourable for improv- turn-on voltage of 3.8 V and maximum LE, EQE, and bright-

ing the spectral stability of PFs, which protect EL devices ness of 1.73 cd/A, 0.60%, and 4531 cd/m (22 V), respec-

from long-wavelength emission (low-energy green emis- tively. This result is probably due to other supramolecular

sion) during operation. CPF-14 also exhibits more stable effects besides steric hindrance effects. Iptycenes are

and narrower emission spectra compared to the spec- another type of excellent bulky groups. Swager reported an

tra of PDAFs. These results indicate that spiro-structure iptycene-containing PF CPF-21 [223,386]. CPF-21 showed

modiﬁcation is a promising approach to improve colour a maximum absorption and emission peak at 335 and

purity and spectra stability. Shu and coworkers have pre- 391 nm, respectively, which was blue-shifted with respect

pared a spiobis(2,2-diphenylvinyl) ﬂuorene-containing PFs to that of PF8 owing to the large intrachain dihedral

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1207

Scheme 3. Polyﬂuorenes substituted with various bulky groups.

angles between ﬂuorene and iptycene induced by steric the rigid backbone of the CPF-22, which intrinsically

effects. inhibits the formation of keto defects and thereby sta-

bilizes the blue emission of the devices. A monolayer

2.4. Fused and ladder-type polyﬂuorenes PLED based on CPF-22 showed excellent deep-blue emis-

sion (CIE coordinates: x = 0.17, y = 0.12) and a maximum

The ladderization of PFs is useful to modify chain LE of 0.70 cd/A at 180 mA/cm . Suh and coworkers also

conformations to tailor optoelectronic properties and mor- investigated cyclopenta[def]phenanthrene and vinylene

phologies. Ladder-type PFs exhibit robust and planar copolymers CPF-23. Quenching and broadening effects

structures and tend to crystallize, which are favourable were clearly observed in the ﬂuorescence spectra of CPF-23

for their applications in polymer transistors and lasers. in polar solvent, and the solid state showed that inter-

A detailed summary of this behaviour has been reported chain interactions in fused cyclopenta[def]phenanthrene

by Grimsdale and Müllen [98]. Ladderization can restrict are strong. Recently, a fused-ﬂuorene analog, 5,10-

the conformational motion and improve the chain rigid- dihydroindeno[2,1-a]indene, was developed by the same

ity of polymers, resulting in increased QE for PL and EL. group. The fused polymer CPF-24 exhibits greater stability

Cyclopenta[def]phenanthrene can be regarded as a con- than PPV, suggesting its potential application in PLEDs and

ventional ﬂuorene fused with an extra double bond at solar cells [389].

the 4,5-position of fluorenes. Suh and coworkers [387,388] Indenofluorene, as a key fused-fluorene analog, exhibits

synthesized poly(cyclopenta[def]phenanthrene)s (CPF- planar conjugated p-terphenyl structures. Setayesh et al.

22) (Scheme 4) as stable and efﬁcient EL materials. determined the effective conjugation length of stepladder

poly(tetraalkylindenoﬂuorenes) (CPF-25) to be 6–7 units

The Eg opt of CPF-22 was approximately 2.98 eV, which

is slightly wider than that of PF2/6 (2.91 eV). CPF-22 by the extrapolation of energy versus 1/n (n = number of

exhibited excellent colour stability compared to typical indenoﬂuorene units) [390,391]. The conjugation exten-

PDAFs materials. No distinct long-wavelength emission sion resulted in a red-shifted emission spectra with respect

was observed, even after the prolonged operation of to the spectra of PDAFs. CPF-25 exhibits thermotropic

◦

the PLEDs in air. This was attributed to the nature of LC behaviour at high temperatures (250–300 C). The

1208 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

different alkyl substituents in PIFs clearly change the phase 430 nm for polyindenoﬂuorenes to 450 nm for ladder-type

transition behaviour and solid-state ﬂuorescence. CPF-25b, PFs. The rigidity of the polymer-hindered deformation in

with branched ethylhexyl side-chains, exhibits blue emis- going from the ground to the excited state resulted in PL

sion ( max = 428 nm), while CPF-25a, with straight octyl quantum yields of up to 90% for CPF-28a. CPF-28a has blue

side-chains, emits green in thin ﬁlms. Zhao and coworkers emission (max = 450–460 nm) but exhibits unstable spec-

investigated the structural variations, defects, and aggre- tra. The blue band was observed to completely disappear,

gation of CPF-25 on the absorption and emission spectra and the yellow band rapidly appeared upon annealing at

◦

using the multimode Brownian oscillator model [392]. The 150 C. This was attributed to the formation of excimers

␲

indenoﬂuorene–anthracene copolymer CPF-26, which fea- from aggregates formed by the -stacking of the poly-

tures the energy structure of exciton conﬁnement, exhib- mer chains. As a result, PLEDs using LPPPs showed yellow

ited a stable blue EL colour ( max = 445 nm). The UV and PL EL. The stability of the emission of fully ladder-type PFs

spectra of CPF-26 were bathochromically shifted relative to can be substantially enhanced by the replacement of the

the spectra PF-co-anthracene ( abs = 380 nm, em = 425 nm) hydrogen at the methane bridges with a methyl group

due to the extensive conjugation length of fused ﬂuo- to produce Me-LPPP (CPF-28b). Compared to the spec-

rene in the solution [36]. Wang and coworkers [385,393] tra of LPPPs, the red-shifted aggregate emission spectrum

synthesized the naphthylene-modiﬁed indenoﬂuorene of Me-LPPP is weaker, and the emission of Me-LPPP did

homopolymer CPF-27. Larger dihedral angles emanat- not change upon annealing. PLEDs using Me-LPPP pro-

ing from the steric hindrance of binaphthyl connection duced blue-green emission with EL efﬁciencies of up to 4%

between repeating units beneﬁtted the amorphous states. [394–397].

CPF-27-based monolayer devices emit at a maximum of

461 nm with CIE coordinates of (0.19, 0.26), independent 2.5. Polyﬂuorenes with kinked conformations

of the driving voltage, and a maximum LE of 1.40 cd/A.

The device performance may be further improved by the Kinked conformations of PFs may not only increase

incorporation of ﬂuorene-based copolymers. the sizes of band gaps to aid the design of electrophos-

Ladder-type polymers generally exhibit smaller Stokes phorescent host materials, but may also be favourable for

shifts and ﬁne vibronic structure owing to their rigid back- the amorphous state of stable blue materials. Generally,

␲

bone. Compared to polyphenylenes, these materials show -conjugated poly(2,7-ﬂuorene)s exhibit linear confor-

efﬁcient red-shifted PL to produce blue or blue-green emis- mations. The 3,6-linkage of ﬂuorene segments offers an

sion due to the increase in chain rigidity. This ﬂuorescence effective way of constructing kinked conjugated poly-

emission wavelength increases from 420 nm for PFs to mers [398–400]. With respect to the 2,7-linking patterns

Scheme 4. Fused and ladder-type polyﬂuorenes.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1209

of PFs, poly(3,6-ﬂuorene)s show wider band gaps and 2.6. Hyperbranched and dendritic polyﬂuorenes

higher triplet energies as well as higher Tg. Mo et al.

[400] reported soluble poly(3,6-ﬂuorene)s (CPF-29a) as Hyperbranched and dendritic PFs have great advan-

violet-blue electroluminescent polymers with an emission tages over their linear counterparts in terms of their

−

maximum at 347 nm, HOMO level of 6.05 eV and opti- weak intermolecular interactions, low degree of crystal-

cal band gap of 3.6 eV. Wide-band-gap poly(3,6-ﬂuorene) lization and high solubility. Highly branched and globular

(CPF-29b) (Eg opt = 3.65 eV) has also been exploited as a blue features are favourable for enhancing the efﬁciencies of

or green host material owing to its high triplet energy level light-emitting polymers and also make good amorphous

at approximately 2.6 eV, which is much higher than that of ﬁlms. Li Bo [406] introduced phenyl branching units into

poly(2,7-ﬂuorene)s (2.15–2.3 eV) reported by Cao’s group PFs to construct hyperbranched polymer structures that

[401]. PLEDs with the conﬁguration ITO/PEDOT/FIrpic:CPF- effectively suppress aggregation and excimer formation.

29b/Ba/Al have an LE of 0.4 cd/A. Bradforth and coworkers The hyperbranched polymers CPF-34 obtained (Scheme 6)

[402] synthesized a series of 2,7-ﬂuorene- and 3,6- are efﬁcient blue-light-emitting materials, exhibiting very

ﬂuorene-based copolymers CPF-30 to understand their good luminescent stabilities without green-blue emis-

◦

effective conjugation length and photophysical aspects. In sion, even after annealing at 200 C for 30 min in air.

these copolymers, the incorporation of increasing fractions Hyperbranched architectures can increase the dihedral

of 3,6-ﬂuorene units caused blue-shifted UV absorptions angle of molecular segments to obtain a large Stokes shift

and distinct blue emissions. The IP increased linearly with respect to their linear counterparts. Hyperbranched

with the 3,6-content, and the effective conjugation length poly(ﬂuorenevinylene)s CPF-35 show hypsochromically

became shorter. Crayston and coworkers investigated the electronic absorption spectra at 372 nm with respect to

effect of introducing meta linkages to interrupt the con- the spectrum of their linear counterpart at 430 nm [407].

jugation in ﬂuorene-phenylene alternating copolymers However, the PL spectra of CPF-35 show peaks at ca.

CPF-31 [403]. The main result was that the introduction 467–468 nm, similar to their linear analogs due to effec-

of meta linkages helped suppress the undesirably long- tive photoexcited energy transfer. An EL device featuring

wavelength emission in both annealed PL and EL. end-capped hyperbranched poly(ﬂuorenevinylene) CPF-

Alternatively, kinked conformations may be created by 35 as the emitting layer exhibited a maximum luminance

␲ 2

-conjugation-interrupted molecular segments, which can of 2180 cd/m and maximum LE of 0.89 cd/A. Recently, a

also limit the effective conjugation lengths of the poly- series of soluble truxene-based hyperbranched ladder-type

mer backbone. Vak et al. reported a spirobiﬂuorene-based PFs CPF-36 with pure blue-light emission ranging from

polymer CPF-32 with a conjugation-interrupted conforma- 417 to 465 nm as well as well-resolved absorption and

tion [404]. Perpendicular spirobiﬂuorenes segments can emission spectra and small Stokes shifts were reported

not only prevent the ␲-stacking of the polymer backbone, [408]. Carter and coworkers [409,410] have incorporated

but also shorten the effective conjugation length, resulting branching tetrafunctional spiro-structure into PFs (CPF-37,

in an improvement in both thermal and colour stabili- CPF-38). The resulting amorphous cross-linking materials

ties. CPF-32 also serves as an effective host matrix that are suitable for incorporation into multilayer PLED devices

can transfer its excitation energy to a perylene dopant, with LE values of 1.8 cd/A at a brightness of 10,000 cd/m

yielding an efﬁcient blue-light-emitting layer. Meyer and for double-layer devices.

coworkers reported the synthesis of multiblock copoly- In summary, CPFs exhibit high QE for PL and EL in PLEDs.

mers consisting of alternating 9,9-dihexylﬂuorene and The control over the morphology of PDAFs has expanded

methylene segments [405]. The lengths of the ﬂuorene seg- their application in lasers, optical switches, polarized lumi-

ments dramatically impact the Tg due to their different nescence and other applications. The emission colour

rigidities of the polymer chains. Poly(9,9-dihexylﬂuorene- and electronic structures of PDAFs could be tailored

mb-methylene)s (CPF-33) offer another excellent model to via the introduction of vinylene, ethynylene, phenylene,

investigate the relationship between photophysical prop- acenes, and other arenes. Spiro-structure modiﬁcation is

erties and morphology (Scheme 5). a promising effective steric hindrance approach to solve

Scheme 5. Polyﬂuorenes with kinked conformations.

1210 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 6. Hyperbranched and dendritic polyﬂuorenes.

the problem of colour impurity and poor spectral stability contents heteroatoms into PFs [37,417–420]. (iii) Copoly-

of PFs. Poly(spiroﬂuorenes) are long-lasting blue electrolu- merization has been used to integrate relatively high

minescent materials with high-energy amorphous states. contents of heteroatomic segments into PFs. It should be

␲

The use of -interrupted and kinked conformations is noted that random or statistical copolymerization offers a

an effective method to shorten the conjugation lengths more ﬂexible tool to introduce various contents of donors

for the design of wide-band-gap host materials and UV or acceptors than alternating and/or diblock copolymer-

LEPs. However, it is still challenging to realize white elec- ization. The alternating copolymers offer a more regular

troluminescent materials using PFs without incorporating conjugated backbone with a high crystallization or orien-

other element-based molecular segments. Furthermore, tation tendency. (iv) High-content heteroatomic PFs may

the relatively low charge mobility of PFs also limits their be created by the homo-polymerization of heteroatom-

widespread application in polymer transistors and solar based ﬂuorene monomers. Donors or acceptors can also be

cells. introduced into either the side-chains or main chains of

PFs. Halogens are introduced only into PFs through sub-

stitution due to their electronic conﬁguration. Fluoride

3. ␲-Conjugated heteroatomic copolyﬂuorenes

(HPFs) and triﬂuoride methyl groups have been incorporated into

phenyl or vinyl groups to hinder excimer and exciplex for-

mation, to reduce the barrier for electron injection, or to

Many heteroatomic segments with different polar

tune the emission colours [421–423]. It should be noted

effects are either electron donors or electron acceptors,

that heteroatomic molecular segments in side-chains often

depending on their electronegativity and orbital hybrid

change both the electronic structure and steric hindrance

[411], affording a versatile toolbox to design the electronic

of polymers, even inducing a conformational change and

structure of PFs, and thereby meet the requirements of

supramolecular interactions; this interesting aspect will

electronic devices. The incorporation of heteroatoms and

not be discussed due to the limited scope of this review,

their molecular segments into PFs have extended their

but related studies [27,40,424–428] may be consulted for

applications in PLEDs, polymer transistors, solar cells and

further details. In this section, we mainly focus on various

memories. Heterocycle-based p–n copolyﬂuorenes with

heterocycle-based p–n copolyﬂuorenes incorporated into

different electronic structures offer a powerful tool to

main chains with an emphasis on band-gap engineering,

control intrachain charge-transfer and/or energy-transfer

structure–property relationships and the applications of

processes [165,170,191,412]. To deeply understand the

these polymer systems. This section has been subdivided

detailed electronic structure design of PFs, the content and

into three subsections and features separate discussions on

position of donors or acceptors in PFs should be clariﬁed.

PFs containing heterocycles in the 16th, 15th, 14th and 13th

Different contents of donors or acceptors, ranging from

groups.

extremely low to very high, could be easily introduced

into PFs by several modern polymerization techniques.

(i) PFs with extremely low traces of heteroatoms may be 3.1. Copolyﬂuorenes containing heterocycles in 16th

achieved by the incorporation of one atom into multi- group

armed [413–415] and/or three-dimensional dendrimer

frameworks [416]. (ii) The end-capping of linear and hyper- Oxygen, sulphur, and selenium have been incorporated

branched polymers offers another way of introducing low into PFs in the 16th group. Five-cycled furan, thiophene,

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1211

Scheme 7. Copolyﬂuorenes containing oxygen heterocycles.

and selenophene are popular electron donors in PFs. The side-groups was reported by Yang and coworkers [432].

most impressive donor is thiophene, owing to its small HPF-02 shows good thermal and optical stability. A

dihedral angle, favourable for the design of high-mobility HPF-02-based EL device exhibited stable blue emission

conjugated PFs. Its electronic structures are easily adjusted ( max = 434 nm) and good colour purity (full-width half-

by substitution or oxidation. In this section, we will discuss maximum 59 nm), although the maximum brightness and

the effects of oxygen, sulphur, and selenium on the various luminance efﬁciency were not high. The oxygen-containing

optoelectronic properties and parameters of copolyﬂuo- six-membered cycle PM is a D–A type molecule segment in

renes, such as the intrinsic HOMO, LUMO and band gap. which CN groups can effectively lower the LUMO energy

level of the molecules by increasing the electron afﬁnity

3.1.1. Copolyﬂuorenes containing oxygen heterocycles and extending the emission to longer wavelengths. Sat-

With respect to useful alkyloxy substituents, only a lim- urated red emission at 641–662 nm was achieved by the

ited number of oxygen heterocycles, such as furan, are introduction of PM units into a PF backbone by Peng et al.

used in plastic electronics due to their limited stability. [433]. Due to the incorporation of the electron-deﬁcient PM

∼

All oxygen heterocycles that have been incorporated into moiety, the PM-based PFs HPF-03 04 showed very high

copolyfluorenes, including five-membered cycles such as electron affinities, indicating that they may be promising

furan/benzofuran, six-membered cycles such as 2-pyran- candidates for electron-transport or hole-blocking materi-

4-ylidenemalononitrile (PM), and seven-membered cycles als in PLEDs. HPF-03-based double-layer PLEDs can emit

such as 5,7-dihydrodibenz[c,e]oxepin (DBO), have been red light, with EQEs of 0.21–0.38%. Tian and coworkers uti-

summarized. Due to the electron-donating property of lized HPF-04 as a donor blended with PCBM as an acceptor

furan, ﬂuorene–furan-based electroactive and lumines- in BHJ solar cells [434].

cent copolymers HPF-01 (Scheme 7) have a signiﬁcantly DBO is an oxygen heterocycle featuring a dramati-

lower band gap of 2.6 eV with respect to that of PF8 cally twisted seven-membered ring via nonconjugated

[411,429]. Benzofuran derivatives not only have excel- linkages between oxygen and phenyl rings. Wang et al.

lent hole transport and high ﬂuorescent efﬁciency, but are [435] conducted theoretical studies on the absorption and

also more chemically stable blue chromophores than the emission properties of the ﬂuorene and DBO-based

furan counterparts [430,431]. A benzofuran and ﬂuorene- conjugated polymers poly(2,7-ﬂuorene-alt-co-5,7-

based alternating copolymer HPF-02 bearing oxadiazole dihydrodibenz[c,e]oxepin) (HPF-05a). The results show

1212 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

that for PFDBO the IP and EA are higher and the band gap section. Heterocyclic arenes containing both sulphur and

is larger than the gaps of PFs; moreover, the dramatically nitrogen will be discussed in the next section.

twisted structure of DBO results in reduced conjugation

in the chain, leading to a blue shift in both the maximum 3.1.2.1. Thiophene-based copolyﬂuorenes. The develop-

absorption and emission wavelengths relative to those of ment of multiple applications of PTs as conductors,

PFs. Following this approach, Cacialli and coworkers [436] semiconductors and other electrode materials accord-

incorporated DBO into PFs to reduce the close packing ingly promote the utilization of thiophene units [440].

between chains and shorten the conjugation length by Five-membered thiophenes have been introduced into

increasing the inter-ring twist angle to suppress green the backbone of PFs for intensive investigation [441]. A

emission. Ma and coworkers introduced DBO into the main series of ﬂuorene–thiophene-based alternating copoly-

chains of PFs to obtain wide-band-gap semiconducting mers HPF-06∼08 (Scheme 8) were studied with respect

polymers with violet emission [437]. Polymer HPF-05b to their luminescent, triplet-state, and electrochemical

emitted in the ultraviolet and blue regions. The torsion properties as well as their self-organization behaviour by

◦

angle of approximately 40 for the biphenyl unit in HPF- several groups [369–371,411,442–444]. Nonsubstituted

05b induced an absorption spectral blue shift of 0.18 eV ﬂuorene-alt-thiophene copolymers were reported to

with respect to the absorption spectrum of PF6. Ketone exhibit green emission in ﬁlms, such as 480 nm for

has also been incorporated into PFs to study the origin of HPF-06a, 531 nm for HPF-06b and 476 nm for HPF-06c.

long-wavelength green bands centred at 530 nm. In fact, Compared to ﬂuorene- and phenylene-based copolymers

except for furan, dibenzofuran, pyrane, and DBO, there are that emit in the blue region, HPF-06 showed a red-shifted

very limited kinds of oxygen heterocycles that have been emission spectrum and lower-energy band gap due to its

incorporated into PFs. higher planarity and longer effective conjugation length.

For thiophene-based polymers, one of the most striking

features is their easy and wide electronic tunability by side-

3.1.2. Copolyﬂuorenes containing sulphur heterocycles chain modiﬁcation. Different structural regioregularities

Thiophene and its derivatives are basic sulphur- arising from side-chain substitutions in thiophene units

containing heterocyclic arenes. Thiophene-based light- offer additional opportunities for ﬁne-tuning the electronic

emitting materials usually have low quantum efﬁciency properties of PFs. Therefore, to investigate the substituent

due to the nonradiative processes known also as the effect on the optoelectronic, thermal and electrochem-

heavy-atom effect; for example, PTs in solid ﬁlms exhibit ical properties of ﬂuorene-alt-thiophene copolymers, a

a low QE of 1–3% for PL. However, thiophene rings are series of soluble, alternating conjugated copolymers com-

more favourable for electron delocalization than phenyl posed of ﬂuorene and mono- or disubstituted thiophene

rings due to their lower aromatic stabilization energy. moieties were reported. Previously, our group [441,445]

The small dihedral angle and stronger intramolecular synthesized and investigated monosubstituted ﬂuorene-

supramolecular coupling interactions between thiophenes alt-thiophene copolymers HPF-07b, which showed bluish

endows the materials with high mobility, allowing for green emission in solution and in ﬁlms. We found that

their potential application in polymer transistors and the monosubstitution of a decyl chain on the one ˇ-

solar cells [438]. S–S supramolecular interactions in position of the thiophene ring in HPF-07b produces a

electron-rich thiophenes also beneﬁt crystallization. Elec- blue-shifted emission spectrum with respect to the spec-

tronic structures of thiophene may be easily converted tra of nonsubstituted ﬂuorene-alt-thiophene copolymers.

through substitution to afford more sulphur-containing Subsequently, Su and coworkers also performed a study on

molecular segments with different electron-donating several monosubstituted ﬂuorene-alt-thiophene copoly-

or -accepting properties [439]. To date, there have mers HPF-07a,c–e with different substituents, including

been many reports on various thiophene-based electron the electron-donating groups of hexyl/hexyloxymethyl

donors and acceptors, such as 3,4-ethylenedioxythiophene, group and the electron-withdrawing groups of hexyl

thiophene-S,S-dioxide, cyclopentadithiophene, thienoth- carboxylate/cyano groups on the one -position of the thio-

iophene, which offer more ﬂexible building blocks to phene ring [446]. They found that the steric effects of the

facilitate tuning the electronic structure of PFs. In this sec- bulkier substituents, such as hexyl/hexyloxymethyl/hexyl

tion, we will summarize various thiophene building blocks carboxylate groups, dominate the electronic effects such

∼

and their fused arene-based PFs. Sulphur-containing build- that HPF-07a and HPF-07c d exhibit a higher band gap

ing blocks incorporated into PFs will be summarized in this than that of HPF-06a, while small substituents, such as

Scheme 8. Thiophene-based copolyﬂuorenes.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1213

Scheme 9. Bithiophene/terthiophene-based copolyﬂuorenes.

the cyano group, can effectively reduce the band gap and spectrum of HPF-09a, mainly due to the decrease in the

enhance the QE for PL due to electronic effects. Among effective conjugation length of the polymer main chain

HPF-07a,c–e, only HPF-07e exhibited red-shifted elec- caused by the enhanced steric effects of the decyl chain on

tronic spectra, with a quantum yield 1.6 times greater than the thiophene ring. This result is the same as the results

that of nonsubstituted ﬂuorene-alt-thiophene copolymers. obtained after comparing nonsubstituted with substituted

It has also been conﬁrmed by our group and the Leclerc thiophene- and ﬂuorene-based copolymers. HPF-11 and

group that the introduction of disubstituted thiophene HPF-12, which exhibit different structural regioregularity,

moieties into copolyﬂuorenes can restrict the rotation of show similar optical and electrochemical properties. How-

thiophene and ﬂuorene segments due to the steric effects ever, HPF-11 shows higher Tg and better environmental

induced by the two substituents in thiophene units, lead- stability than HPF-12. This indicates that the structural

ing to polymers HPF-08 with larger band gaps, blue-shifted regioregularity of head-to-head or tail-to-tail conforma-

emission spectra and greater spectral stability compared to tions between the two substituted thiophene rings does

those of nonsubstituted and monosubstituted ﬂuorene-alt- not affect the optical or electrochemical properties of the

thiophene copolymers (HPF-06∼07) [314]. resulting polymers, but does affect the thermal stability

to some extent. PLEDs made from F8T2 show a green

3.1.2.2. Bithiophene/terthiophene-based copolyﬂuorenes. emission at 545 nm, which is red-shifted 65 nm relative to

Compared with thiophene, bithiophene and terthio- the emission wavelength of HPF-06b-based devices.

phene are more useful comonomers with longer Among these ﬂuorene and bithiophene copolymers,

conjugation lengths. They have been introduced into F8T2 and F6T2 (HPF-09a), have become exception-

PFs, polyindenoﬂuorenes, and polyﬂuorenones for ally promising materials for polymer transistors and

the structure–performance relationship, high-molibity solar cells [91,449,450]. F8T2 shows good hole-transport

polymer transistors and solar cells. properties [451] and excellent thermotropic liquid crys-

Fluorene- and bithiophene/terthiophene-based tallinity [452,453]. The hole mobilities of F8T2 with its

light-emitting conjugated copolymers HPF-09∼12 polymer chains aligned in the LC phase reach up to

(Scheme 9) were reported by our group and Morgado 0.01–0.02 cm /(V s) [91]. A comparable transistor prepared

[371,441,443,445,447,448]. Due to the incorporation of by inkjet printing with high-temperature-treated polymer

more thiophene units into the repeat unit, HPF-09∼12 F8T2 as a semiconductor was reported by Sirringhaus et al.

usually show increased conjugation lengths and red- to give relatively high charge mobility of 0.02 cm /(V s)

shifted emission spectra compared with HPF-06∼07; for with an on/off ratio of 10 [88]. Furthermore, F8T2 forms an

example, F8T2 (HPF-09b) in the ﬁlm state showed an ordered conformation resembling the -phase conforma-

emission peak at 548 nm, which was red-shifted 17 nm tion of PF8 [454]. Owing to the attractive characteristics

with respect to the spectrum of HPF-06b. In the case of the discussed above, F8T2 has been used by Plastic Logic to

substituted bithiophene- and ﬂuorene-based copolymers, fabricate arrays of 4800 TFTs with 50 dpi, which are used

∼

HPF-11 12 in ﬁlms (490 nm for HPF-11, 493 nm for as backplanes for active-matrix displays [455]. Moreover,

HPF-12) showed a blue-shift emission with respect to the F8T2 shows promising results in BHJ solar cells with PCBM,

1214 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 10. Multinary thiophene-based copolyﬂuorenes.

−4 2

× which can achieve an efﬁciency of 2.14% with an open- similar electron and hole mobilities of 2 10 cm /(V s)

∼ circuit voltage (Voc) of 0.99 V and short-circuit current and Ion/Ioff ratios ( 10 ).

2 ◦

(Jsc) of 4.24 mA/cm after annealing the device at 70 C for

20 min [456]. Another analog of F8T2, F6T2, was explored 3.1.2.3. Multinary thiophene-based copolyﬂuorenes. The

for potential application in BHJ solar cells [457]. F6T2 shows introduction of other molecular segments into thiophene-

−5 2

a hole mobility of 8.4 10 cm /(V s), which is lower than based copolyfluorenes is a flexible alternative to exert fine

F8T2; this may be due to unaligned polymer chains of F6T2. control over electronic structures. Multinary thiophene-

Although the hole mobility of F6T2 is not high, BHJ solar based copolyﬂuorene HPF-17 (Scheme 10) was reported to

cells that blend crystalline P6T2 with PCBM as the active exhibit a different triplet-state absorption at 640 nm with

layer show a promising PCE of 2.4% and Voc of 0.9 V, which respect to corresponding bithiophene/terthiophene-based

are comparable to the values exhibited by F8T2- and PCBM- copolyﬂuorenes, which show triplet-state absorption in

based solar cells. the 730–760 nm region [371,443]. Chen and cowork-

Bithiophene and terthiophene have also been utilized ers incorporated 2-phenyl-2-cyanovinyl together with

for copolymerization with ﬂuorenone and indenoﬂuo- thiophene into PFs to obtain random copolymer HPF-18

∼

rene derivatives to afford polymers HPF-13 16, which [462]. Each thiophene core in this polymer is attached

exhibit attractive properties for solar cell and poly- to two 2-phenyl-2-cyanovinyl groups, which could

mer transistor application. Demadrille and coworkers reduce the content of heavy atoms (sulphur) to enhance

have conceptually designed and synthesized a regioreg- the polymer’s emission performance. Due to the over-

ular ﬂuorenone-bithiophene-based alternating copolymer whelming energy transfer from ﬂuorene segments to

HPF-13 for plastic solar cell applications [458,459]. How- 2-phenyl-2-cyanovinyl chromophores, HPF-18-based EL

ever, due to their weak absorption in the visible-light devices exhibit yellowish-green emission with a max-

range, the HPF-13-based devices showed poor external imum brightness of 5230 cd/m and LE of 0.65 cd/A.

PCE. Leclere and coworkers [460] reported indenoﬂu- White-light-emitting devices with the CIE coordinates

orene and bithiophene/terthiophene-based alternating (0.26, 0.32) were realized by blending HPF-18 with PF8

∼

copolymers HPF-14 15. Due to its more extended conju- (w/w = 10/1). Tian and coworkers [434] simultaneously

gation in terthiophene, HPF-15 exhibited green emission incorporated thiophene and PM into PFs to construct the

with a PL maximum at 554 nm, which was red-shifted D–A copolymer HPF-19. The stronger electron-donating

compared to that of HPF-14 ( max = 529 nm) and PIFs ability of thiophene in HPF-19 improved the effective

( max = 432 nm) [461]. AFM analysis of the thin-ﬁlm conjugation length along the polymer backbone, which

∼

morphologies of HPF-14 15 show that HPF-15 fea- resulted in an increase in electronic delocalization, and

turing terthiophene units displays well-deﬁned ﬁbrillar consequently, a higher HOMO energy level compared with

morphology resulting from highly regular dense pack- that of HPF-08a was attained. HPF-19-based solar cells

␲ ␲

ing due to strong – interchain interactions, while showed an improvement in their short-circuit currents

HPF-14 featuring bithiophene units cannot form dense compared with those of HPF-08a-based cells due to the

␲

-stacked structures. Therefore, the polymer transistors better miscibility between HPF-19 and PCBM.

−4 2

using HPF-15 showed mobilities of 1.1 × 10 cm /(V s),

nearly one order of magnitude higher than the cor- 3.1.2.4. 3,4-ethylenedioxythiophene/thiophene-S,S-dioxide-

−5 2

responding devices using HPF-14 [1.5 10 cm /(V s)]. based copolyﬂuorenes. The alkyloxyl substitution or

Indenoﬂuorenebis(dicyanovinylene), a derivative of inde- oxidation of thiophenes can effectively convert the mate-

noﬂuorene, was copolymerized with bithiophene units to rials from being electron donators to being electron

achieve the ﬁrst air-stable ambipolar polymer transistor acceptors. Various substituted thiophenes have been

[89]. The HPF-16-based polymer transistors were observed incorporated into PFs to adjust the HOMO or LUMO

to be ambipolar under ambient conditions and exhibited energy levels, as well as the emitting colour of PFs.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1215

Scheme 11. 3,4-ethylenedioxythiophene/thiophene-S,S-dioxide-based copolyﬂuorenes.

Ethylenedioxythiophene, an electron-donating thiophene Under the same conditions, F8TT-based transistor devices

−3 2

derivative with double oxygen at the 3,4 positions of thio- exhibited an improved mobility of 1.1 × 10 cm /(V s),

phene, was incorporated into PFs by the Lévesque group to which is three times higher than that of F8T2 devices

∼ −3 2

obtain HPF-20 21 (Scheme 11). Electrochemical studies (0.4 10 cm /(V s)). The performance of F8TT-based TFTs

indicate that the introduction of ethylenedioxythiophene would be further improved by alignment to achieve

could raise the HOMO energy levels of PFs [314,463]. molecular-level orientation. F8TT and F6TT (HPF-25b) pre-

Thiophene-S,S-dioxide, an electron-accepting thiophene sented signiﬁcantly blue-shifted UV–vis and PL maxima

derivative, could be incorporated into PFs to lower the compared with bithiophene-based polymer F8T2 and F6T2,

LUMO energy levels and adjust the emission colour. mainly due to the signiﬁcantly shorter effective conju-

The polymer HPF-22 emits in the orange region of the gation length along the polymer backbones; moreover,

visible spectrum (max = 604 nm), which is red-shifted F8TT showed UV–vis and PL maxima of 448 and 495 nm

with respect to HPF-06c (max = 476 nm). However, due to in ﬁlms, both blue-shifted with respect to the spectra of

the detrimental effect of the S,S-dioxide functionalization F8T2 in ﬁlms ( abs = 458 nm, em = 511 nm). F8TT-based

of the thiophene unit, HPF-22 was observed to exhibit PLEDs exhibiting a pure green emission with CIE coordi-

lower ﬂuorescence efﬁciency compared with HPF-06c nates (0.29, 0.63) and a low turn-on voltage of 3.3 V showed

[369,370,443,464]. Based on HPF-22, two alternating greater efﬁciency than PF8- and F8T2-based devices. Addi-

PF copolymers, HPF-23b and HPF-24, with thiophene- tionally, F6TT-based PLEDs exhibited pure green emission

thiophene-S,S-dioxide-thiophene existing as a single with a maximum brightness of 6000 cd/m and LE of

segment, were synthesized and investigated by Beaupre 2.7 cd/A.

and Leclerc [42]. In the solid state, HPF-23b presented The incorporation of a ˇ-alkyl chain into thieno[3,2-

red-orange emission with a maximum at 610 nm, and b]thiophene unit of F6TT led to blue-shifted maximum

HPF-24 showed a red emission with a maximum at wavelengths in absorption and emission spectra due to

666 nm, both of which were red-shifted compared to steric effects. The HPF-26-based device obtained exhib-

HPF-22. Furthermore, quantum-chemical studies showed ited very good spectral stability and robust emission,

that the LUMO energies of HPF-23a were approximately with only 21% decay in luminance after being driven for

1.4 eV lower than those of PFs, suggesting a signiﬁcant 64 h at a constant current of 1 mA. The dithieno[3,2-b]

improvement in the electron-accepting and -transport thiophene- and ﬂuorene-based polymer HPF-27 exhib-

abilities of PFs [465]. ited morphological characteristics similar to those of

F8TT; however, polymer transistors based on HPF-27

3.1.2.5. Fused thiophene-based copolyﬂuorenes. Fused-ring showed relatively low performance due to the poor qual-

thiophenes with high degrees of planarity and rigidity of the ﬁlms, which arose from their low solubility.

ity foster ␲–␲ stacking interactions and intermolecular As an isomer of thieno[3,2-b]thiophene, cross-conjugated

ordering, facilitating a more crystalline phase as well as thieno[2,3-b]thiophene may be incorporated into poly-

a more ordered liquid-crystal phase than PFs and F8T2 mer backbones to suppress electron delocalization relative

and providing a new opportunity to develop materials to the fully conjugated thieno[3,2-b]thiophene isomer,

for polymer transistors with higher carrier mobilities. resulting in a blue-shifted emission. The thieno[2,3-

Thienothiophene is one type of fused thiophene. Alternat- b]thiophene-containing alternating copolymer HPF-28

ing copolymers composed of ﬂuorene and nonsubstituted showed an emission peak at 417 nm, which was signiﬁ-

or alkyl-substituted thieno[3,2-b]thiophene/thieno[2,3- cantly blue-shifted compared to that of F8TT containing

b]thiophene moieties were synthesized for polymer thieno[3,2-b]thiophene group. HPF-28-based PLEDs exhib-

transistor and PLED applications [90,466–468]. The intro- ited deep-blue EL emission at 410 nm [469].

duction of rigid fused thieno[3,2-b]thiophene into PFs Another important fused-ring thiophene derivative is

resulted in the polymer F8TT (HPF-25a) (Scheme 12), cyclopentadithiophene [470]. This type of fused-ring thio-

which showed higher crystallinity, a more ordered mor- phene derivative can lower the reorganization energy and

phology and better TFT device performance than F8T2. consequently strongly affect the charge-carrier mobility

1216 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 12. Thienothiophene/cyclopentadithiophene-based copolyﬂuorenes.

of organic semiconductors, making this type of com- as high-efﬁciency solar cell sensitisers. The replacement

pound a potential candidate for use in organic electronics. of the ﬂuorene unit in ﬂuorene-cyclopentadithiophene

Cyclopentadithiophene- and ﬂuorene-based copolymers copolymers with indenoﬂuorene units yielded polymers

∼

have been investigated as semiconductor layers in polymer HPF-35 37, which exhibit better solubility due to the pres-

transistors. Due to a disordered amorphous structure in the ence of four hexyl groups.

solid state, cyclopentadithiophene alternating copolymers Another sulphur-containing heterocyclic aromatic

HPF-29a and HPF-30 exhibited p-type semiconductor hydrocarbon (thieno-acene) is dibenzothiophene, which

−6

behaviour, with ﬁeld-effect mobilities of 5 × 10 and can also be easily converted to electron-withdrawing

−7 2

10 cm /(V s), respectively, which are lower than those molecular segments: dibenzothiophene-S,S-dioxide build-

of F8T2 and F8TT. The effects of substituents of cyclopen- ing blocks. The study of HPF 38∼39 shows that the

tadithiophene in ﬂuorene or indenoﬂuorene alternating introduction of dibenzothiophene into PFs can suppress

copolymers HPF-29b,31∼36 on photophysical properties aggregation and can improve the emission colour purity

have been investigated by Wei-Fang Su [471,472]. Com- and that the incorporation of dibenzothiophene-S,S-

pared with nonsubstituted HPF-29b, HPF-31∼32, which dioxide with high fluorescence efficiency, electron affinity

features electron-donating non-␲-substituents (ethylene- energy and electron-transport properties into PFs can

dioxy and propylenedioxy bridges the 3,3-positions of the enhance intrachain charge transfer (ICT), which beneﬁts

thiophene groups), displayed high ﬂuorescence quantum spectral stability by suppressing the formation of exci-

yields and red-shifted absorption. Meanwhile, HPF-33∼34, tons on ﬂuorene units [473,474]. Moreover, Cao et al.

␲

which feature electron-withdrawing -substituents (car- conﬁrmed that the dibenzothiophene-S,S-dioxide unit

bonyl and dicyanoethenyl), are weakly ﬂuorescent and could lower the LUMO energy level as well as balance the

exhibited blue-shifted absorption due to band splitting injection and transport of both electrons and holes in the

and low ﬂuorescence yields. The presence of electronic polymers, thereby improving device efﬁciencies. Several

␲ ∼

coupling between the -substituent and the conjugated saturated blue-light-emitting PFs HPF-40 41 containing

polymer backbone, which resembles a phenylene-derived dibenzothiophene-S,S-dioxide units were synthesized and

conjugated system, results in good charge separation; this studied. The devices based on HPF-40 and HPF-41 showed

would allow the polymer to be used in such applications an EQE of 3.8% and a LE of 4.6 cd/A with CIE coordinates

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1217

of (0.15, 0.12) and an EQE of 3.9% and a LE of 3.1 cd/A demonstrating efﬁcient blue-to-green-light emission all

with CIE coordinates of (0.16, 0.07), respectively [475,476] showed increased conjugation lengths and red-shifted

(Scheme 13). emission spectra compared with those of thiophene- and

ﬂuorene-based polymers. By aligning the polymer chains

in the LC phase, F8T2, a ﬂuorene–bithiophene polymer

3.1.3. Copolyﬂuorenes containing selenium heterocycles 2

with a high hole mobility of 0.01–0.02 cm /(V s), could

Selenophene is a homologue of thiophene but differs

also be employed in BHJ solar cells to achieve a PCE of

from thiophene with respect to its aromaticity and the

2.14%. Although the use of indenoﬂuorene derivative units

heavy-atom effect [358]. Selenophene/biselenophene was

instead of ﬂuorene units in bithiophene/terthiophene- and

incorporated into light-emitting PFs to examine their effect

ﬂuorene-based systems could not improve the charge

on the PL and EL in PLEDs [477] or on charge-carrier trans-

mobility, it provided an opportunity to achieve air-stable

port in polymer transistors [478] by different groups. In

ambipolar transistor polymers. As an electron-donating

comparison with the very well-studied poly(ﬂuorene-co-

thiophene derivative, ethylenedioxythiophene could raise

thiophene), polymers HPF-42 (Scheme 14) and HPF-43

the HOMO energy levels of PFs, and thiophene-S,S-

showed a red-shifted PL due to the electron-donating

dioxide, as an electron-accepting thiophene derivative,

properties of selenium. However, the device performance

could increase the electron afﬁnity and decrease the

of the selenophene-containing PF8 copolymer is much

HOMO energy levels of PFs. Under the same measurement

lower than that of corresponding PF8-thiophene copoly-

conditions, thienothiophene-based polymer transistors

mers. The introduction of the selenophene moiety into

exhibited improved mobilities with respect to the mobil-

the LC polymer system results in better transistor per-

ity of F8T2 due to the enhanced intermolecular ordering

formance than that of P8T2. Solution-processed polymer

and ␲–␲ stacking interactions afforded by their planar

transistors based on HPF-43 with a thermotropic liquid-

and rigid molecular structure. However, copolymers based

crystalline phase constructed using a bottom-contact

on ﬂuorene and another fused-ring thiophene deriva-

geometry were found to exhibit an excellent hole mobil-

2 tive, cyclopentadithiophene, exhibited lower ﬁeld-effect

ity of 0.012 cm /(V s) and a low threshold voltage of −4 V

mobilities than F8T2 due to their disordered amorphous

with respect to their sulphur analog; this constitutes one

structure in the solid state. Further improvements in

of the best performances exhibited by solution-processable

the performance of TFTs based on F8TT should be pos-

polyﬂuorene-based transistors. However, no Te-based con-

sible by optimizing the fabrication conditions and/or

jugated PFs were reported.

the alignment technique used to achieve molecular-level

In summary, oxygen-containing cyclic molecular seg-

orientation. Dibenzothiophene and derivatives building

ments, such as furan/benzofuran, were introduced into

blocks can effectively suppress aggregation phenom-

PFs to decrease their band gaps and improve their emis-

ena in PFs, resulting in the improvement of the

sion colour purity. Moreover, saturated red emission

colour purity of blue-emitting PFs. The selenophene-

was achieved through the introduction of electron-

and ﬂuorene-based LC polymer systems exhibit bet-

deﬁcient oxygen-containing PM moieties into PFs, and

ter polymer transistor performance than F8T2, with

DBO, another oxygen heterocycle featuring a dramatically 2

an excellent hole mobility of 0.012 cm /(V s). Moreover,

twisted seven-membered ring, was incorporated into PFs

multinary thiophene-based copolyﬂuorenes have been

to obtain wide-band-gap semiconducting polymers with

developed to realize multiple functionalizations of PFs and

violet emission. Additionally, ﬁve-membered thiophenes,

devices.

as basic sulphur-containing building blocks, were copoly-

merized with ﬂuorene to produce green emission. The

side-chains of thiophene rings in ﬂuorene-alt-thiophene 3.2. Copolyﬂuorenes containing heterocycles in 15th

copolymers could be modiﬁed to ﬁne-tune the electronic group

properties of the material and thereby increase the band

gap and blue shift of emission due to the steric hindrance Heteroatoms in the nitrogen group include nitrogen

effect of substituents. Bithiophene/terthiophene-based PFs (N), phosphorus (P), arsenic (As) and antimony (Sb), each

Scheme 13. Dibenzothiophene-based copolyﬂuorenes.

1218 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 14. Copolyﬂuorenes containing selenium heterocycles.

of which possesses ﬁve electrons in its outermost shell. studies involving the ﬂuorene- and 2,7-carbazole-based

As the fundamental element of this group, nitrogen has blue-light-emitting polymer HPF-47 [277].

a more ﬂexible valence-electron pattern with which to Soluble and thermally stable ﬂuorene- and Cz-based

construct many molecular segments with different dipole ladder-type polymers HPF-48∼52 have been reported

and electronic structures. The carbazole (Cz) group, which [482,483]. The polymers HPF-48∼49 showed red-shifted

∼

contains sp nitrogen, is a typical and useful molecular emission spectra compared to those of HPF-44 46 due

segment with excellent hole-transport ability. In contrast, to their more planar conformations. The semi-ladder

pyridine and bipyridinyl, which feature sp nitrogen, are polymer HPF-50, which features multiform constituents,

important electron acceptors that exhibit coordination was reported to inhibit planar conformations, leading to

characteristics and are sensitive to the surrounding pH the aggregation behaviour of HPF-44∼46, better colour

levels. These aspects promote the synthesis and charac- stability and higher efﬁciency [484]. Single-layer PLEDs

terization of N-containing conjugated polymers. Owing using HPF-50 as the active layer showed stable pure

to their strong coordination ability, PFs containing hete- blue emission (max = 447 nm, CIE coordinates: 0.15, 0.05)

rocycles from the 15th group not only yield many D–A with a maximum luminescence of 5500 cd/m . Arylated

conjugated systems with gradient electronic structures but ladder-type PFs, HPF-51, exhibited better stability against

also form a basic platform on which to introduce metallic oxidation than HPF-50 [485]. Both HPF-51a and HPF-

elements. With the combination of oxygen groups, many 51b exhibited good EL properties in initial PLED tests,

multi-heteroatom aromatic cycles have also been intro- with a luminance of 700–900 cd/m at a bias of 10 V.

duced into conjugated PFs to more ﬂexibly control their A well-ordered Cz and ladderized pentaphenylene with

optoelectronic properties, morphology and stability; such diketone-bridge-based soluble conjugated p–n copolymer

materials include the electron-donating phenothiazine and HPF-52 was synthesized and characterized by Müllen

electron-deﬁcient 1,3,4-oxadiazole (OXD), thiadiazole (TZ) [486]. The copolymer obtained exhibited efﬁcient energy

and benzothiadiazole (BT). In this section, PFs contain- and charge transfer and macroscopic organization between

ing heterocycles from the 15th group are reviewed. Many Cz and ladderized pentaphenylene with diketone moieties

nitrogen-based aromatic heterocycles, including electron- in the solid state, which suggests that these polymers are

donating Cz and electron-accepting pyridine/bipyridinyl, good candidates as photoactive materials in solar cells.

quinoline/quinoxaline, have been incorporated into the PFs

used in PLEDs, lasers, sensors, and solar cells as well as low-

3.2.1.2. Pyridine/bipyridine-based copolyﬂuorenes. Pyri-

band-gap conjugated polymers and stimulus-responsive

dine, which contains electron-deﬁcient imine nitrogen,

conjugated polymers.

is both luminescent and oxidative stable and has thus

attracted interest as a potentially useful material in elec-

3.2.1. Copolyﬂuorenes containing nitrogen heterocycles tronic and photonic devices. Pyridine units with different

3.2.1.1. Carbazole-based copolyﬂuorenes. Cz units with connectivities (2,5- or 2,6- or 3,5-linkage) were incorpo-

different connectivities (3,6- or 2,7-linkage) were rated into the polymer backbone to improve the material’s

incorporated into the backbones of PFs to improve electron afﬁnity and oxidation stability [487]. Our group

the hole-transport ability of these polymers. 3,6-carbazole was the ﬁrst to synthesize alternating-2,5-pyridine-based

and fluorene with different substituent-based random copolyfluorenes, poly[2,7-(9,9-dihexylfluorene)-co-alt-

and alternating copolymers (HPF-44∼46) (Scheme 15) 2,5-pyridine] (HPF-53a) (Scheme 16) [277]. Another

exhibited elevated HOMO energy levels relative to those 2,5-pyridine- and ﬂuorene-based analog, poly[2,7-(9,9-

of PF6 or PF8 and thus facilitated hole injection into the dioctylﬂuorene)-co-alt-2,5-pyridine] (HPF-53b), was

copolymers from an ITO anode [479–481]. Therefore, reported by Vanderzande [488]. Compared to PF6 or PF8,

these polymers could be used as the hole transport as HPF-53a and HPF-53b showed lower LUMO energy levels

− −

well as the light-emitting layer in blue PLEDs. HPF-44∼46 ( 2.83 eV for HPF-53a and 2.60 eV for HPF-53b) due to

exhibited blue emission upon excitation and showed the electron-deﬁcient pyridine moiety in the main chain,

greater stability than PF6 or PF8 owing to the “kinked” which allowed for easy electron injection from a cathode.

linkage of 3,6-carbazole, which suppressed aggregation Both the absorption and emission spectra of HPF-53a and

and improved the thermal/luminescence stability of PFs. HPF-53b show a slight red shift relative to the spectra of

Multiple-layered PLEDs based on HPF-44 as a hole trans- PF6 or PF8; speciﬁcally, HPF-53b in the solid state exhibits

port and emissive layer and Alq3 as an electron-transport a UV absorption peak at 380 nm, deep-blue PL emission

layer demonstrated better performance than PFs-based at 430 and 450 nm, and greenish EL emission at 500 nm.

devices. Moreover, HPF-46a–c-based devices emitted The PL maxima of the alternating copolymers HPF-53a,

∼

nearly blue light with EL peaks at 445, 435 and 444 nm, 54 55 occurred at 430, 420, and 407 nm, respectively,

respectively. Accordingly, our group performed some indicating that the para-linkage (2,5-linkage) in the PFs

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1219

Scheme 15. Carbazole-based copolyﬂuorenes.

is more favourable for electron delocalization than the with a brightness of 2010 cd/m and EQE of 0.1%,

meta-linkage (3,5- or 2,6-linkage) [487]. while HPF-57-based PLEDs exhibited pale-blue emission

Electron-donating bithiophene was introduced into ( max = 493 nm) with a lower brightness and EQE mainly

HPF-53b [489]. Polymer HPF-56 showed UV–vis absorp- due to the larger energy barrier to electron injection from

tion and PL emission peaks at 449 and 573 nm, respectively, a cathode.

which were red-shifted compared to those of HPF-54b. Conjugated polymers with high electron afﬁnities

HPF-57 with 2,6-pyridine units as electron acceptors was can also be constructed using 2,2 -bipyridine moieties

also synthesized by Lee to compare with HPF-58. The opti- as building blocks. Our group [68] synthesized three

cal, electrochemical, and EL properties of these polymers conjugated polymers composed of 9,9-dioctylﬂuorene

arising from the different connectivities of their pyridine and 2,2 -bipyridine, which were alternatively linked by

units were compared. The UV–vis absorption and PL emis- a C C single bond (HPF-58), vinylene bond (HPF-59), or

sion peaks of HPF-57 in the solid state were observed to ethynylene bond (HPF-60). HPF-58 exhibited an absorp-

occur at 398 and 490 nm, respectively, which were blue- tion maximum at 383 nm, with a PL spectrum peak at

shifted with respect to those of HPF-56; this is attributed 425 nm in the solid state. By contrast, HPF-59 and HPF-60

to the kinked linkages of 2,6-pyridine, which reduce the showed a clear spectral red shift in both the absorption and

␲

-conjugation length of HPF-57. Electrochemical studies emission spectra due to the insertion of the vinylene or

show that HPF-56 has a smaller E (3.08 eV) than

g electr ethynylene unit, which increased the effective conjugation

HPF-57 (3.49 eV). The LUMO level of HPF-57 was found length. All three polymers were responsive to a wide vari-

− −

to be 2.20 eV higher than that of HPF-56 ( 2.43 eV), ety of transition-metal ions, exhibiting a spectral red shift

indicating that HPF-56 has a smaller energy barrier to elec- in absorption and ﬂuorescence quenching due to the chela-

tron injection from a cathode. The PLEDs fabricated using tion between the bipyridine and the metal ions. Another

HPF-56 exhibited pale-orange emission ( max = 580 nm) bipyridine- and ﬂuorene-based random copolymer

1220 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 16. Pyridine/bipyridine-based copolyﬂuorenes.

HPF-61 showed pure blue PL, with a ﬂuorescence quan- produced green EL emission, with an emission peak at

tum yield of more than 0.6 in dilute solutions, as reported 544 nm. Moreover, due to the electron-withdrawing abil-

by Ma [490]. The IP of these polymers decreased signiﬁ- ity of quinoline, HPF-62 showed good electron-transport

cantly with the increase in the bipyridine content of the properties with a low LUMO level of −3.21 eV. HPF-62 was

polymer backbone, and a LUMO level of 2.43 eV for HPF- used as an electron-transport polymer by the Kim group to

61 (x = 0.5) was achieved, making these polymers good improve the efﬁciency of PLEDs [492]. Meanwhile, vinylene

candidates as electron-injection and -transport materials was introduced into the backbone of HPF-62 to facilitate

in EL devices. the delocalization of electrons and to extend the conjuga-

tion length of the polymer; the emission spectra of polymer

3.2.1.3. Quinoline/quinoxaline/pyrido[3,4-b]pyrazine- HPF-63 in the solid state ( max = 494 and 534 nm) was

based copolyﬂuorenes. Other electron-withdrawing red-shifted, and the band gap was reduced relative to the

fused-pyridine molecular segments, such as quinoline, spectra and band gap of HPF-62 [493]. The authors also

quinoxaline and pyrido[3,4-b]pyrazine, have also been reported quinoline-containing PF derivatives HPF-64 with

−

incorporated into PFs to improve their electron-injection a low LUMO level of 3.24 eV [494].

and -transport ability as well as to tune their morphology. Compared to quinoline, planar and rigid quinoxa-

In an attempt to combine the excellent thermal stabil- line have a similar chemical structure but one more N

ity of polyquinolines with the light-emitting properties of atom and enhanced electron deﬁciency. Sterically hin-

PFs, PF6 with a quinoline moiety in its main chain was pre- dered quinoxaline-based substituents were introduced

pared by Friedländer quinoline synthesis [491]. Compared into the ﬂuorene moiety to enhance the processability

with typical PFs, polymer HPF-62 (Scheme 17) showed blue and amorphism of the copolymer. The quinoxaline-

∼

ﬂuorescence in dilute solutions (max = 434 nm) and green ﬂuorene-based alternating copolymers HPF-65 66, which

ﬂuorescence in the solid state (max = 446 and 480 nm) due contained linear rod-like 6,6-bis(3-phenylquinoxaline)

to interchain excimer formation. A HPF-62-based device units in the main chain, emitted blue light with emission

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1221

Scheme 17. Quinoline/quinoxaline/pyrido[3,4-b]pyrazine-based copolyﬂuorenes.

1222 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 18. Phenanthridine/phenanthrolines-based copolyﬂuorenes.

maxima at 455–460 nm and poor CIE coordinates optical response to the protonation and deprotonation of

−

[494,495]. HPF-65a showed a lower LUMO level of 3.37 eV the phenanthridine rings. The low LUMO energy levels,

−

compared with that of HPF-62a due to the increased elec- 2.94 eV for HPF-79a and −3.03 eV for HPF-79b, were

tron deﬁciency of the quinoxaline ring. Subsequently, a attributed to the high electron-withdrawing ability of these

2,3-bis-(p-phenylene)quinoxaline-ﬂuorene-based poly- polymers. Polymer HPF-80b was conﬁrmed to show much

mer HPF-67 in which the PF backbone was kinked by higher sensitivity to metal ions than dihexylﬂuorenes and

incorporated quinoxaline units exhibited an improved 2,2 -bipyridine-based polymers.

probability of retaining the blue emission, with deep-blue

∼

CIE coordinates relative to those of HPF-65 66 [496]. Lin-

3.2.1.5. Porphyrin-based copolyﬂuorenes. Porphyrin is

ear linkages of benzene between quinoxaline and ﬂuorene

an aromatic heterocyclic macrocycle composed of four

∼

result in the polymers HPF-68 71 with more efﬁcient con- 2 3

modiﬁed pyrrole subunits with both sp and sp nitrogen;

jugation than that in HPF-65∼67. And HPF-68∼70 yielded

it is abundant in red blood cells and plants, in which it

red-shifted emission spectra compared with those of

aids in biosynthesis and photosynthesis. The IP (5.4 eV) of

∼

HPF-65 66. HPF-68, HPF-69, HPF-70 and HPF-71 showed

tetraphenylporphyrin (TPP) is 0.4 eV lower than that of PF8

green emission at the peaks of 493, 492, 524 and 527 nm

(5.8 eV). As a result, TPP was ﬁrst doped into PFs as a hole

in the solid state, respectively [497–499]. HPF-71-based

trap to balance the electron–hole transport, monitored by

OLEDs emitted pure green light at wavelength of approx-

TOF measurements by Campbell et al. [171]. The TPP moi-

imately 520 nm with the CIE chromaticity coordinates

eties were introduced into the main chain of PF to extend

of (x = 0.32, y = 0.59). 5,8-dithienylquinoxaline ring was

the spectral coverage of the copolymers and to make the

copolymerized with ﬂuorene for use in near-infrared LEDs

most of the solar spectrum. The copolymers obtained

and solar cell applications. Furthermore, emission was fur-

had a strong absorption over 600 nm and nearly covered

ther extended in three ﬂuorene-5,8-dithienylquinoxaline

the entire visible spectrum between 400 and 700 nm.

copolymers HPF-71, which showed red emission with an

Novel PF copolymers HPF-81 (Scheme 19) containing

emission peak at 560 nm for copolymer HPF-72a, 635 nm

porphyrin and thiophene moieties that exhibited broad

for HPF-72b and 640 nm for HPF-72c [500]. Fluorene- and

absorption and efﬁcient charge transfer and separation

5,8-dithienylquinoxaline-based alternating D–A copoly-

were reported [513]. The band gaps of these polymers

∼ ∼

mers HPF-73 75 with Eg opt values of 1.9 1.94 eV were

were observed to be low (between 1.96 and 2.03 eV). Thus,

applied in PSCs [501–503]. HPF-73-based PSCs displayed

they might be good candidates as photoactive materials in

PCE values of 3.7%, while HPF-74- and HPF-75-based

solar cells. Another porphyrin-based LC multi-component

devices displayed lower PCE values of 1.75% and 1.1%,

copolyﬂuorene HPF-82 were reported to give broadband

respectively. A random quinoxaline-containing PF material

white EL emission covering the visible region from 400 to

HPF-76 with 30% 5,8-dithienylquinoxaline had a slightly

700 nm [514]. In this polymer, ﬂuorene functions as both a

larger E (2.14 eV), and a HPF-76-based PSC device

g electr

blue-emitting unit and the host due to its large band gap,

showed a lower PCE of 1.18% [504].

and oligo(phenylenevinylene) and porphyrin function as

Pyrido[3,4-b]pyrazine is another electron acceptor that

green- and red-emitting units, respectively. Single-layer

is known for its use in near-infrared PLEDs [505,506].

PLEDs featuring HPF-82 emitted white light, with CIE

Jenekhe reported two pyrido[3,4-b]pyrazine and ﬂuorene-

coordinates (0.29, 0.30) and a maximum brightness of

based polymers HPF-77∼78 [507]. HPF-78 gave a broad, 2

443 cd/m .

featureless and near-infrared emission band centred at

611 nm in a chloroform solution and at 692 nm in a thin

ﬁlm. 3.2.1.6. Polyatomic heterocycle-based copolyﬂuorenes.

Although many electron-accepting moieties, includ-

ing quinoline, quinoxaline, 2,1,3-benzothiadiazole, and

␲

3.2.1.4. Phenanthridine/phenanthrolines-based copolyﬂuo- pyridazine, have been explored in -conjugated D–A

renes. Phenanthridine and phenanthrolines, fused aro- copolymer systems, the strength of the ICT in most of

matic compounds with an electron-withdrawing nitrogen these copolymers is not strong enough to extend the

atom or two nitrogen atoms in their scaffolds, were intro- absorption bands into the near-infrared spectrum, which

duced into PFs to construct polymer-based chemosensors, limits their application. Polyatomic heterocycles offer more

+ n+

which responded well to H or M with changes in UV–vis ﬂexible molecular segments with unique electronic struc-

and PL spectra [508–512]. The sythesized phenanthridine- tures. Some electron-withdrawing heterocyclic molecular

containing polymers HPF-79 (Scheme 18) generated high segments with both nitrogen and oxygen/sulphur, such

quantum yields in solution and showed a reversible as oxadiazole, thiazolothiazole, 2,1,3-benzothiadiazole,

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1223

Scheme 19. Porphyrin-based copolyﬂuorenes.

−3 2

have been incorporated into PF derivatives to change an electron mobility of 10 cm /(V s) and a hole-transport

−3 2

␲

the emission colour or generate low-band-gap p–n - mobility of 2 × 10 cm /(V s) [515,516]. HPF-83a also

conjugated polymers, affording efﬁcient models to ﬁnely exhibited a relatively high EA of 3.3 eV, a large IP of 5.9 eV,

control energy transfer and charge transfer as well as and its LUMO localized on the BT units [517,518]. HPF-83a

improve the electron-injection and -transport properties has been used extensively as an electron-transport mate-

of PF derivatives. rial in blend PLEDs [519] and photovoltaic cells and as an

To explore the building blocks of strong ICT for ambipolar material in light-emitting polymer transistors

their application in red PLEDs and BHJ polymer solar [520]. The stronger electron afﬁnity of BT relative to that

cells, it is necessary to further extend the absorp- of quinoxaline has been conﬁrmed by Chen and coworkers

␲

tion of the p–n -conjugated polymers to near-infrared by comparing their respective LUMO levels. A red shift in

bands. The incorporation of oxygen, sulphur or sele- the PL of a ﬂuorene- and BT-based alternating copolymer

nium into electron-withdrawing nitrogen-based hete- HPF-83b ﬁlm (max = 540 nm) was observed relative to the

rocycles can further ﬁne-tune the HOMO and LUMO PL of a ﬂuorene- and quinoxaline-based copolymer HPF-68

energy levels. Several molecular segments, such as (max = 493 nm), and the LUMO level of HPF-83b (−3.14 eV)

oxadiazole (OXD), benzothiadiazole (BT), thieno[3,4- was observed to be smaller than that of HPF-68 (−2.65 eV)

b]pyrazine (TP), [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ), [497]. The green-light-emitting dioctylﬂuorene- and

2,1,3-benzoselenadiazole (BSeD), thiazolothiazole (TZ), BT-based random copolymer HPF-84a was reported by

bithienopyridines (BTP), and pyrrolo[3,4-c]pyrrole-1,4- Wang to emit green-yellow light (max = 537 nm) with CIE

dione, have been copolymerized into the main chains of coordinates of (x = 0.34, y = 0.59). The author subsequently

PFs. Among these segments, BT and its derivative, 4,7- inserted two phenylene moieties into the backbone of

di-2-thienyl-2,1,3-benzothiadiazole (DTBT), are the most HPF-84a to reduce the effective conjugation length in

popular molecular segments that have been introduced the vicinity of the BT unit; this resulted in the polymer

into PFs to achieve pure green, red and white emission or HPF-85, which showed a slight blue shift in its PL emission

low-band-gap materials for solar cell applications. Here, we ( max = 521 nm) [521]. HPF-85-based PLEDs emitted pure

provide a systematic overview of ﬂuorene- and BT-based green light with CIE coordinates of (0.29, 0.63) and exhib-

copolymers. ited a turn-on voltage of 4.2 V, LE of 5.96 cd/A and power

By incorporating the strongly electron-accepting BT efﬁciency of 2.21 lm/W. To circumvent the poor hole mobil-

unit into the homopolymer PF8, the resulting copolymer ity of HPF-84, Jen and coworkers incorporated thiophene

HPF-83a (Scheme 20) possesses higher electron afﬁnities into the copolymer backbone of HPF-84b for ﬁne-tuning

and preferable electron-transport properties compared the polymer’s charge-transport properties. The copolymers

to those of PF8. The 9,9-dioctylﬂuorene- and BT-based HPF-86∼88 exhibited strong green emission at approx-

alternating copolymer HPF-83a, which is an efﬁcient imately 540 nm, attributed to either the charge transfer

green-light emitter with PL efﬁciencies of 50–60% in solid between electron-rich segments and electron-deﬁcient

ﬁlms, was reported to exhibit ambipolar properties with BT-containing segments of the polymers or FRET between

1224 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 20. Benzothiadiazole-based copolyﬂuorenes.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1225

different polymer chains. The QE was up to 55% for PL absorption band of HPF-91b were obtained by replacing

∼

for HPF-86 88. A double-layer device using HPF-88 as the ﬂuorene unit of HPF-91b with the indenoﬂuorene unit.

the emissive layer exhibited a low turn-on voltage of Photophysical studies revealed a low-band-gap of ∼1.9 eV

3.4 V, a very high EQE of 6.0%, and a high brightness of for HPF-91a, which could harvest the broad solar spectrum

59,400 cd/cm [522]. from 300 to 650 nm. HPF-91a showed a hole mobility of

−3 2

BT units were introduced into the main chains of the 1.1 × 10 cm /(V s). The best solar cell performance, with

ladder-type ﬂuorene-based polymer CPF-22 to change the a Voc of 0.77 V, a Jsc of 5.50 mA/cm and a PCE of 1.70%,

polymer’s colour and its electron-transport properties, as was achieved for HPF-94a and a PC71BM-based PSC. The

well as to improve the recombination efﬁciency of holes copolymers HPF-94b-c and HPF-95 obtained exhibited

and electrons [523]. Polymer ﬁlms of HPF-89 emitted ﬁeld-effect mobilities as high as 0.011 cm /(V s). Without

green light, with maximum peaks at 510–535 nm, and extended optimization, a high PCE of 3.67% and a high

−

their LUMO energy levels were low ( 3.33 ∼ −3.50 eV), Voc of 1.06 V were achieved using a HPF-95/PC61BM blend

attributed to the energy transfer from CPF-22 to BT. BT under ambient conditions. A high PCE of 4.5% was also

units were introduced into the CPF-22 backbone to induce achieved by a PSC with an active layer containing 20 wt%

a red-shifted colour and increase the electron afﬁnities of HPF-95 and 80 wt% PC71BM.

the copolymers. The best device featured HPF-89 (with Indenoﬂuorene, BT, and DTBT derivatives were used as

30 mol% BT in the feed), which showed the highest LE of blue-, green-, and red-light-emitting structures, respec-

1.25 cd/A, which was higher than that of CPF-22 (0.7 cd/A). tively, to construct a new, complex white-light-emitting

DTBT, a derivative of BT, is a low-band-gap copolymer HPF-96 by controlling their respective feed

donor–acceptor–donor (DAD) segment comonomer ratios [53,532]. The EL device using this polymer showed a

widely used to create red PLEDs. Moreover, it is an maximum brightness of 4088 cd/m at 8 V with CIE coordi-

important molecular segment for BHJ solar cells due to nates of (0.34, 0.32).

its extended absorption spectrum [86]. Saturated red By introducing DTBT into the PCPPs, the Hongsuk Suh

PLEDs have been created with random ﬂuorene-DTBT group synthesized a low-band-gap (2.0 eV) polymer HPF-

copolymers HPF-90. Due to exciton trapping in narrow- 97 for PSCs in which the CPP unit was combined with a

band-gap DTBT sites, the energy transfer between ﬂuorene DTBT unit to create a larger difference between the donor

and DTBT in HPF-90 was observed to be more efﬁcient HOMO level and the acceptor LUMO level of the copolymer,

∼

than that between ﬂuorene and BT in HPF-83 85. The which could be an essential for obtaining an enhanced Voc

exciton emission of HPF-83∼85 is centred on the DTBT in PSCs [533]. HPF-97 emitted deep-red ﬂuorescence in a

chromophore and gives rise to saturated red emission THF solution, with maximum peaks at 621 nm, suggesting

with CIE coordinates (0.70, 0.30); meanwhile, polymers that there was very efﬁcient energy transfer from the PCPPs

HPF-83∼85 all produce green emission [524]. Several unit to the DTBT-cored unit. BHJ solar cells based on the

DTBT-ﬂuorene-based alternating polymers HPF-91 with blends of polymers with PC71BM produced a PCE of 1.00%.

different side-chains on the ﬂuorene units have been TQ is a fused-BT derivative with a strong electron-

designed and combined with fullerenes to create BHJ accepting ability. The ﬂuorene- and TQ-based polymers

materials for PSCs. HPF-91a, which features a band gap of HPF-98 (Scheme 21) [534] and HPF-99 [535–537] exhib-

1.96 eV, is commonly used to study certain unique charac- ited optical band gaps of 1.40 and 1.21 eV, respectively,

teristics [86,525]. PSCs based on HPF-91c, which features which are lower than those of corresponding BT and ﬂu-

two different side-chains on the ﬂuorene unit, show PCEs orene copolymers, owing to the signiﬁcant ICT between

of 2.4% [526,527]. The PCE of a PSC constructed using the the ﬂuorene donor and TQ acceptor. HPF-99 has an

DTBT-ﬂuorene-based alternating copolymer HPF-91d, absorption tail that extends beyond 900 nm, which may

which features two ethylhexyl side-chains on the ﬂuorene be improved by LC processing to exhibit typical p-

unit, has been reported to be 2.67% [528]. PCEs of 4.2% channel characteristics and showed a transistor mobility

have been obtained for the HPF-91e-based PSC [529]. as high as 0.03 cm /(V s); this material may be used

Neither ternary random copolymerization with added in photovoltaic devices. The investigation of the per-

thiophene segments HPF-92 nor alternating polymers formance of HPF-98∼99-based PSCs should be further

using alkyl pyrrole-BT-pyrrole HPF-93 showed any excel- developed.

lent properties for solar cell applications due to their TP, which is a stronger acceptor and possesses more

disordered morphology and limited absorption wave- planar backbone compared to quinoxaline, has been

length [527]. shown to be an excellent precursor for the production of

The indenoﬂuorene moiety is expected to function as low-band-gap conjugated polymers. The ﬂuorene- and TP-

an efﬁcient electron-rich unit in ICT-type low-band-gap based alternating and random copolymers HPF-100∼101

copolymers for PSCs. The indenoﬂuorene was used as (Scheme 22) have been reported [497]. Although the

an electron donor unit in these copolymers to provide electron-accepting strength of BT is lower than that of

a deeper HOMO level and to create polymer solar cells TP, the smaller band gap of HPF-100 compared to that of

with higher open-circuit voltages, while DTBT was chosen HPF-83 may be attributed to the backbone planarity. TP

as an electron acceptor unit to ﬁne-tune the electronic with a ﬁve-member thiophene ring results in a smaller

band gaps of polymers to afford a better light-harvesting torsional angle with ﬂuorene than that of BT with a

ability. Polymers HPF-94a-c and HPF-95 [530,531] were six-member phenylene ring; this allows for efﬁcient ICT

synthesized. HPF-94a-c and HPF-95 with enhanced and and results in a smaller band gap. A ﬁlm of HPF-100 was

bathochromically shifted absorption bands relative to the observed to produce red emission with an emission peak

1226 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 21. Thieno[3,4-b]pyrazine-based copolyﬂuorenes.

Scheme 22. Thiadiazolo[3,4-g]quinoxaline-based copolyﬂuorenes.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1227

Scheme 23. Benzoselenadiazole-based copolyﬂuorenes.

of 674 nm, which was red-shifted by 134 nm relative to response of HPF-102-based PSCs was extended to 750 nm,

the emission of HPF-83. although the device performance was not fully optimized.

5,7-dithien-2-yl-thieno[3,4-b]pyrazine (DTTP) is an iso- A PLED with an EQE of 1.62% was obtained by utilising HPF-

mer of DTBT with sp nitrogen and sulphur but has a 108-based blend-type emitters. A HPF-108:PCBM-based

relatively stronger electron-accepting ability. Therefore, PSC showed a PCE of 0.91%.

∼

a series of ﬂuorene-DTTP-based polymers HPF-102 105 TZ and BTP as well as OXD are other sulphur/oxygen and

were reported to show lower band gaps compared with nitrogen heteroatom-containing segments that have rela-

those of DTBT-based PFs HPF-91, resulting in an extended tively weaker electron-withdrawing abilities than BT, TP,

∼

absorption spectrum [86,528,536,538,539]. HPF-103 105 TQ and their derivatives. A study of the TZ/BTP/OXD- and

has the same low-band gap of 1.50 eV, and the band gap was ﬂuorene-based copolymers HPF-110∼117 (Scheme 24)

observed to be 1.81 eV for HPF-105a and 1.71 eV for HPF- showed that the introduction of TZ, BTP or OXD into PFs and

∼

99b. However, HPF-96 99-based PSCs exhibited the PCEs PIFs could improve the electron-accepting properties of

of less than 2% and are lower than that of HPF-91. A similar the resulting polymers, leading to efﬁcient light-emitting

result was obtained by comparing the indenoﬂuorene- and materials with good performance in LEDs [494,542–546].

DTTP-based copolymer HPF-100 with the indenoﬂuorene- The studies show that HPF-110∼117 exhibited reduction

and DTBT-based copolymer HPF-94a [530]. HPF-106 has a potentials lower than those corresponding to ﬂuorene and

band gap that is 1.6 eV lower than that of HPF-94a (1.9 eV); indenoﬂuorene homopolymers, indicating much improved

however, a HPF-106-based PSC using PC71BM as an accep- electron-accepting properties. A HPF-110-based device

tor showed a PCE 0.45% lower than that of a HPF-94a-based was reported to emit bright blue light with an emission

PSC (1.71%). maximum at 480 nm and a QE of 0.44%, which is three

BSeD is a BT analog that contains nitrogen and sele- times greater than that of the PF8 homopolymer. Blue-

nium. The Se atom, which is much larger in size and less green colour emission with low turn-on voltages were

electronegative than the S atom, may exert a special effect realized for HPF-111∼112-based PLEDs. A HPF-116-based

on the optoelectronic properties of PF copolymers with Se- device showed green emission with CIE coordinates of

containing heterocycles. Cao and coworkers [201,540,541] (0.25, 0.38). The OXD- and ﬂuorene-based conjugated

developed a series of random BSeD-based PFs for use in polymer HPF-117 was applied in the development of a

LEDs and photovoltaic devices. It has been found that memory device. The HPF-117-based device exhibited low

incorporating BSeD into the PF main chain resulted in reading, writing and erasing voltages and a high ON/OFF

a signiﬁcant red shift relative to the spectrum of its current ratio up to 10 , in which both the ON and OFF

sulphur analog. The ﬂuorene- and BSeD-based copolymer states were stable up to 10 read cycles at a read voltage

−

HPF-107 (Scheme 23) has an EL of 580 nm, signiﬁcantly of 1.0 V [197]. Although pyrrolo[3,4-c]pyrrole-1,4-dione

red-shifted relative to the green emission of ﬂuorene- units possess a stronger electron-withdrawing ability

␲ ␲

BT copolymers due to the narrower – * gap of the than TZ/BTP/OXD and can greatly lower the LUMO levels

BSeD unit. Devices composed of such copolymers emited of PFs, the broad EL colour range of the copolymers

orange-red light with emission peaks at approximately obtained from these units is undesirable. The EL colour of

570–600 nm and reach a maximum EQE of 1.0%. 4,7- the pyrrolo[3,4-c]pyrrole-1,4-dione- and ﬂuorene-based

di(2 -selenophenyl)-2,1,3-benzoselenadiazole (SeBSe), random copolymer HPF-118 varies from orange to red,

4,7-di-2-thienyl-2,1,3-benzoselen-adiazole (DBSe), which corresponding to CIE coordinates ranging from (0.52, 0.46)

has a narrower band gap than BSeD, was incorporated into to (0.62, 0.37), which are undesirable [547,548].

PFs to obtain new low-band-gap deep-red EL polymers Phenothiazine, an electron-donating heterocycle con-

∼

HPF-108 109 for PLEDs and PSCs. The EL emission peak taining sulphur and nitrogen heteroatoms, has been

∼

of the devices fabricated from HPF-108 109 was shifted explored as a stronger electron donor for the syn-

to the near-infrared region 727–790 nm. The spectral thesis of conjugated polymers. Phenothiazine has been

1228 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 24. Thiazolothiazole/bithienopyridines/pyrrolo[3,4-c]pyrrole-1,4-dione-based copolyﬂuorenes.

incorporated into PFs to impede ␲-stacking aggregation ﬂuorenone moieties in HPF-120 were found to act as emis-

and intermolecular excimer formation, improving the sive exciton traps on the terpolymer chains, leading to new

charge mobility of copolymers or to construct D–A copoly- blue-green (475–485 nm) and green (520–525 nm) emis-

mers. The polymer HPF-119 (Scheme 25) has a low IP of sion bands in addition to the blue (415 nm) emission of

5.1 eV, which suggests potential for use as a hole-transport the ﬂuorene segments. Green to yellow EL was achieved

semiconductor in organic devices. HPF-119-based OLEDs in terpolymer light-emitting devices, with luminances of

show greenish-blue electroluminescence, with a low 1900–8970 cd/m and a LE of 0.5–3.5 cd/A, which varied

luminance of 320 cd/m [549]. Electron-donating phenoth- with the copolymer composition. HPF-121 has a higher

iazine could also be utilized to construct D–A copolymers HOMO energy level than HPF-19. The highest PCE value

featuring ﬂuorene, which are specialized to improve of 0.52% was obtained from a device based on HPF-121,

the effective conjugation length of polymer backbones. which exhibited the strongest electron-donating ability,

∼

HPF-120 121 are similar copolymers [434,550]. The 13 times greater than that of the device based on HPF-19

electron-donating phenothiazine and electron-accepting (0.04%).

Scheme 25. Phenothiazine-based copolyﬂuorenes.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1229

Scheme 26. Copolyﬂuorenes containing phosphorus heterocycles.

3.2.2. Copolyﬂuorenes containing phosphorus Wu and coworkers successfully demonstrated the advan-

heterocycles tages of the electronic features of incorporated phosphorus

Unlike abundant nitrogen heterocycles, there are centres through a series of phosphole-based OLED devices

more limited phosphorous heterocycles. With respect to [553–555]. Electron-donating phosphole [556,557] and

nitrogen systems, phosphorus heterocycles with three cyclotriphosphazene [558] have also been introduced into

important characteristics offer some advantages in design- PFs. Phosphorus-containing PFs in particular have stimu-

ing optoelectronic materials: (1) trivalent phosphorus(III) lated great interest among materials scientists. However,

may be easily converted to pentavalent phosphorus(V), (2) reports of phosphorus-containing conjugated PFs are lim-

phosphorus heterocycles have a relatively strong afﬁnity ited. Morisaki et al. [556,557] prepared several types of 2,5-

for oxygen and sulphur, and (3) phosphorus heterocycles substituted phosphorus-containing conjugated PFs, such as

feature a vacant d-orbital, which leads to transition-metal HPF-122 (Scheme 26). Polymer HPF-122a was observed to

coordination. The versatile reactivity and electronic nature be an efﬁcient emitter of green light, and HPF-122b showed

of phosphorus offer many opportunities for the develop- a peak at 435 nm in the visible blue region. Phosphaﬂu-

ment of new materials with novel properties [551,552]. orene was used as a new building block by our group to

1230 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

construct solution-processable phosphaﬂuorene-ﬂuorene In summary, many nitrogen-based molecular segments

∼

conjugated copolymers HPF-123 124 with strong blue have been introduced in PFs to construct as well as to

and single-layer white EL [559,560]. The incorporation of alter the morphology and stability of various low-band-gap

phosphaﬂuorene into PF8 resulted in the red shift of the semiconducting polymers, expanding their applications

absorption peaks in the solid state and in dilute THF solu- from PLEDs to BHJ solar cells and transistors. The addition

tion, suggesting that the conjugated length increased after of electron-rich Cz units, including not only 3,6-carbazole

phosphaﬂuorene incorporation. The LUMO and HOMO but also 2,7-carbazole, into PFs could raise the HOMO

−

of HPF-123 were calculated to be – 5.87 and 2.19 eV, energy levels of polymers, thus facilitating hole injection

respectively, suggesting that the incorporation of phos- into copolymers from ITO anodes and resulting in the

phaﬂuorene into PF8 resulted in a reduced HOMO, reduced application of copolymers as hole-transport as well as

LUMO, and unaltered energy gap; this further indicates that light-emitting layers in blue LEDs. Meanwhile, the kinked

the electron-injection and -transport ability of the copoly- structure of 3,6-carbazole could suppress aggregation and

mer HPF-123 improved. By comparing the photophysical improve the thermal stability and luminescence of blue-

properties of HPF-123 and HPF-124, we concluded that light-emitting PFs. Fluorene- and carbazole-based arylated

the oxidation of the phosphorus centre resulted in the red semi-ladder-type copolymers overcome the disadvantages

shift of the absorption and emission peaks and a bandgap of fully ladder-type or alkylated semi-ladder-type copoly-

reduction. High PL efﬁciency was observed for these two mers, namely to easily aggregate and are susceptible to

polymers, with QE of 53.1% and 52.4%. oxidation, resulting in enhanced spectrum stability and

Phosphole may be easily modiﬁed through oxidation luminescence efﬁciency. The combination of electron-

or complexation reactions via Lewis acids or transition- accepting pyridine/bipyridine with the ﬂuorene moiety

metal complexes to ﬁne-tune the optoelectronic properties has resulted in reduced LUMO energies and consequently

and morphology of polymers. Dithienophosphole (DTP) is a greatly facilitated the electron-transport process. Further-

molecular segment that joins two thiophene subunits and a more, bipyridine- and ﬂuorene-based copolymers may be

central phosphole moiety via anellation. Baumgartner and used to detect metal ions. Quinoxaline-based PFs show

coworkers [561–563] introduced modiﬁed DTP into PFs. lower LUMO energy levels than quinoline-based PFs due to

Cationic DTP-conjugated polyelectrolytes with ionic cen- their increased electron deﬁciency, while quinoline-based

tres as part of the backbone exhibit strong green emission, PFs exhibit LUMO energy level that are lower than those of

even in solid ﬁlms, which are obviously different from other pyridine/bipyridine-based PFs. Phenanthridine as well as

polyelectrolyte pendants with ionic centres on side-chains phenanthrolines have been introduced into PFs to produce

and are promising materials for multilayer optoelec- polymer-based chemosensors, which respond well to H or

tronic devices. The emission and absorption maxima M with changes in UV–vis and PL spectra. The addition of

of polymer HPF-125 were strongly red-shifted com- TPP to PFs could produce copolymers with low-band gaps

pared with those of the corresponding PF6 ( abs = 388 nm; and absorption spectra that extend beyond 600 nm, useful

em = 445 nm) and related ﬂuorene copolymers containing for solar cells or to realize white EL emission.

2,2 -bithienyl (F6T2) ( abs = 398 nm; em = 483 and 520 nm) Full-colour emission may be achieved by the incorpo-

or dithienosilole (HPF-29b) ( abs = 386 nm; em = 486 and ration of different acceptors with one more heteroatoms

515 nm) moieties due to the signiﬁcantly reduced LUMO besides nitrogen, such as BT and TQ, into PFs. BT units

level of DTP monomers in the polymer. Researchers also possessing stronger electron afﬁnities than quinoxaline

reported a series of low-band-gap DTP copolymers (HPF- have been introduced into the main chains of PFs or PIFs to

126∼129) that featured DTP building blocks for potential realize strong green emission and relatively higher device

application in BHJ solar cells. The three copolymers HPF- efﬁciency. Among BT- and ﬂuorene-based copolymers, one

∼

126a c exhibited a yellow-orange emission in solution at polymer exhibited an attractive device efﬁciency, with a

em = 545 nm (HPF-126a) em = 550 nm (HPF-126b), and low turn-on voltage of 3.4 V, high EQE of 6.0%, maximum

em = 546 nm (HPF-126c). The PL quantum yields are 0.49 LE of 28.6 cd/A, and high brightness of 59,400 cd/cm .

for HPF-126a, 0.24 for HPF-126b, and 0.48 for HPF-126c. DTBT-based low-band-gap PFs/PIFs were reported to

The copolymer HPF-127, which contains the EDOT unit, produce red emission, the highest ﬁeld-effect mobility

showed a quantum yield of 0.60, higher than that of HPF- of which reached 0.011 cm /(V s). The highest PCE of BHJ

126. On the other hand, the BT-based HPF-122 showed solar cells based on DTBT and ﬂuorene-based copolymers

signiﬁcantly shifted photophysics when compared with reached up to 4.5%. Furthermore, indenoﬂuorene, BT,

HPF-127. The presence of the thienyl-capped BT units led and DTBT derivatives have been used as blue, green, and

to intense dark-red ﬂuorescence emission at 658 nm with a red-light-emitting structures, respectively, to construct

PL quantum yield of 0.46. In these polymers, the copolymer a new, complex white-light-emitting copolymer. TQ-

HPF-128 was found to not only exhibit a suitable band gap and ﬂuorene-based copolymers exhibit optical band gaps

for applications in solar cell (solution: 2.0 eV; solid state: that are generally lower than those of corresponding

1.7 eV) but also showed good solubility as well as good BT- and ﬂuorene copolymers due to the signiﬁcant ICT

electron-transfer properties in the presence of fullerenes. between ﬂuorene donors and TQ acceptors; one ﬂuorene-

However, no device data have been reported. Recently, the and TQ-based copolymer exhibited typical p-channel

cyclotriphosphazene-ﬂuorene copolymers HPF-129 poly- characteristics and a high-polymer transistor mobility of

merized via the ether linkage have also been designed as 0.03 cm /(V s). TP- and ﬂuorene-based copolymers exhibit

stable blue-light-emitting hybrid organic–inorganic poly- optical band gaps that are generally lower than those

mers [558]. of corresponding BT and ﬂuorene copolymers, which

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1231

may be attributed to the backbone planarity. TP units create hybrid materials. However, there are no reports

with ﬁve-member thiophene rings resulting in smaller of organoarsenic-, organoantimony- or organobismuth-

torsional angles with ﬂuorene than those of BT units containing conjugated PFs.

with six-member phenylene rings, which assists efﬁcient

intramolecular charge transfer and results in a smaller

3.3. Copolyﬂuorenes containing heterocycles in 14th

band gap, produce red emission. Due to the relatively group

stronger electron-accepting ability of DTTP with respect

to that of DTBT, DTTP-based PFs/PIFs show smaller band Silicon and germanium are elements in the same

gaps than DTBT-based PFs/PIFs, resulting in the expansion nonmetal group as carbon. Siloles and germoles are basic

of absorption. However, it has been reported that the PCE heterocyclic building blocks incorporated into conjugated

* *

of DTTP-based PFs/PIFs solar cells are lower than those PFs. Silole precursors possess ␴ –␲ conjugation with

of DTBT-based PFs/PIFs. It has also been observed that electron-deﬁcient properties. Siloles have been conﬁrmed

incorporating selenium-containing heterocycles into PFs to be non-dispersive and air-stable electron-transport

can generate red emission that red-shifted relative to materials with green emission (aggregation-induced

sulphur-containing PFs. emission). They have been applied in PLEDs, PSCs and

TZ and BTP as well as OXD have electron-accepting FETs. Siloles are narrow-band-gap materials that afford

abilities that are weaker than those of BT, TP, TQ, DTBT, copolymers with various electron conﬁgurations and opto-

and DTTP but similar to those of quinoline or quinox- electronic properties. A series of linear silole-containing

aline. The incorporation of TZ and BTP as well as OXD random or alternating copolymers HPF-130 (Scheme 27),

into PFs might induce effects similar to those generated HPF-131 and HPF-132 that comprise a terﬂuorene and a

by quinoline or quinoxaline, which improve electron- 2,5-silole ring have been reported by Cao and coworkers

accepting properties, leading to efﬁcient light-emitting [564–566]. Copolymers HPF-130 and HPF-131 exhibited

materials that show good performance in LEDs. One OXD- green EL emission bands at 540 and 530 nm, respectively,

and ﬂuorene-based conjugated polymer was applied in a with high absolute PL quantum yields in the ﬁlm state.

memory device that exhibited low reading, writing and When the alternating copolymer HPF-130 and PFO were

erasing voltages and a high ON/OFF current ratio up to used as the blend-type emissive layer in a device, a

10 . Phosphorus-based PFs have also been utilized to maximum EQE of 1.99% was realized. Red-light-emitting

Scheme 27. Silole/silafluorene/germafluorene-based copolyfluorenes.

1232 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

PLEDs with emission wavelengths as long as 600 nm, and Chujo via Sonogashira coupling polymerization [577].

PSCs with a PCE of 2.01%, and FETs with a hole mobility HPF-137 exhibited ﬂuorescence characteristic of pyraz-

−5 2

of 4.5 10 cm /(V s) were achieved using the silole- abole, with a peak at 406 nm and a ﬂuorescence quantum

containing polymers HPF-132. The authors also observed yield of 53% in chloroform. This result indicate that

only a broad absorption band for HPF-132, which indicated either energy transfer or charge transfer occurs between

a mixed and TST-dominated electronic conﬁguration. ﬂuorene and pyrazaboles. However, poor expansion of

␲

Dibenzosilole is also known as silaﬂuorene. The incor- -conjugation through the backbone should be expected

poration of dibenzosilole into PFs can improve the spectral in these polymers because of the poor electronic inter-

␲

stability because the vulnerable C-9 carbon in PF has actions between the -orbitals of the pyrazole moiety

been replaced with a silicon heteroatom [567,568]. The exhibit poor electron-relay characteristics. The incorpora-

incorporation of a 3,6-silaﬂuorene unit can suppress the tion of organoboron compounds into conjugated polymers

long-wavelength emission and improve the efﬁciency and makes them anion-sensing materials because of their

colour purity [398,399]. Cao et al. [569] have reported strong afﬁnity towards inorganic anions, such as ﬂuoride.

poly(3,6-silafluorene-co-2,7-fluorene) (HPF-133) with an The dialkylfluorene–dibenzoborol-based random conju-

EQE of 3.34% and an LE of 2.02 cd/A at a brightness of gated polymer HPF-138, which has been reported by Scherf

326 cd/m with a surprisingly narrow FWHM (19 nm) and and coworkers [578], is a typical example of a mate-

− −

CIE coordinates of (0.16, 0.07), which almost match the rial that can distinguish between F /CN (ﬂuorescence

− −

NTSC standard blue pixel coordinates of (0.14, 0.08). Our quenching) and Br /Cl (no response) and therefore has

group synthesized a series of soluble blue-light-emitting potential applications in the detection of toxic gases and

conjugated random and alternating copolymers HPF-134 their decomposition products (e.g., sarin vs. tabun). BOD-

derived from 9,9-dioctylﬂuorene and 3,6-dimethoxy-2,7- IPY dyes possess desirable chemical and photostability

silafluorene [570]. Methoxyl substituents in this polymer attributes, high absorption coefficients and high fluores-

exhibit an obvious steric hindrance effect that not only cence quantum yields. Liu and coworkers [579] have

impacts the polymerization degree, but also results in reported that ﬂuorescent conjugated copolymers HPF-

a blue-shift of PL emission. Dithienosilole with a silole 139, which comprise alternating ﬂuorene and BODIPY in

ring fused with two thiophene rings is another unique the main chain with an orange emission and a ﬂuores-

electron-deﬁcient silole derivative with a low-lying LUMO cent quantum yield as high as 85% in methylene chloride

− −

energy level [571]. It has been copolymerized with ﬂu- solution, have sensitive ﬂuorescent responses to F /CN

orene to overcome the drawback of weak ﬂuorescent through their multivalent interactions. Carborane cages

emission of the dithienosilole-based homopolymer due (C2B10H12) are icosahedral cluster compounds that con-

to effective FRET from the high-energy ﬂuorene seg- sist of 10 boron atoms and 2 carbon atoms and exhibit

ments to the lower-energy dithienosilole segments [572]. three-dimensional aromaticity and electron-withdrawing

Dithienosilole and ﬂuorene random copolymer HPF-135 properties. The introduction of the carborane moiety into

exhibited a bright green emission with the brightness the main chain of the ␲-conjugated polymer extended

as high as 25,900 cd/m and a maximum EQE of 1.64% the conjugation length along the polymer main chain due

in a double-layer device. Germole and dibenzogermole to the aromatic character of carborane. The incorpora-

(germaﬂuorene) exhibit properties associated with their tion of m-, p- and o-carborane into PFs (HPF-140∼142)

low-lying LUMO levels that are similar to those of silole has been investigated by Chujo and Carter and cowork-

and dibenzosilole, respectively [573,574]. Our group [575] ers [580–582]. The ability to delocalize the ␲-electrons is

was the ﬁrst to report the introduction of germaﬂuo- very limited in m-carborane-based ␲-conjugated PFs HPF-

rene into a random ﬂuorene copolymer. Polymer HPF-136 140 ( max (chloroform) = 386 and 410 nm, solution = 0.11).

exhibited an efﬁcient blue-light emission under ultravi- These m-carborane-based ␲-conjugated polymers possess

olet irradiation, and a single-layer EL device in which low-lying LUMO levels, which facilitate their application as

it was incorporated produced a maximum brightness of an electron-transport material and as n-type organic semi-

2630 cd/m at 7.8 V. The copolymer can also serve as a conductors. HPF-141 with p-carborane in the backbone

yellow host material for phosphorescent metal complexes showed a linear conformation without disruption to its lin-

with a maximum brightness of 15,600 cd/m and a QE of ear rigid-rod behaviour; it was a blue-emitting material

8.5%. Recently, Mario Leclerc presented the ﬁrst utilization with an emission peak at 400 nm and exhibited excel-

of such copolymers in polymer transistors and PSCs [576]. lent thermal properties. The incorporation of o-carborane

into PFs contributed to the conjugation of the system and

3.4. Copolyﬂuorenes containing heterocycles in 13th gave rise to a new lower-energy emission in the solution

group state. The HPF-142 polymers in the solid state emit sta-

ble pure-green radiation centred at approximately 520 nm.

Boron heterocycles with empty ␲-orbital have been Therefore, the incorporation of carborane into the back-

incorporated into backbones of conjugated PFs to produce bone of PFs provides an important route to new materials

emissive materials and sensors. Until now, pyrazabole, for sensors and light-emitting polymers.

dibenzoborole, 4-diﬂuoro-4-bora-3a,4a-diaza-s-indacene In summary, heteroatoms such as ﬂuoride, oxygen, sul-

(BODIPY) and carborane have been successfully incorpo- phur, selenium, nitrogen, phosphorus, silicon, germanium,

rated into PFs. Pyrazaboles are a novel class of stable and boron have been incorporated into p–n-type PFs, which

boron heterocycles. A pyrazabole-based copolyﬂuorene further expands the applications of PFs in PLEDs, poly-

HPF-137 (Scheme 28) was synthesized by Matsumoto mer solar cells and transistors as well as memory devices.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1233

Scheme 28. Boron-based copolyﬂuorenes.

PFs that contain substituted main-group nonmetal ele- Random copolyﬂuorenes with low acceptor contents prove

ments clearly provide an ideal model for investigations to be excellent amorphous red-light-emitting materials,

of the effects of molecular segments on the optoelec- and alternating copolymers are favourable for crystalliza-

tronic properties and morphology of the PFs. Different tion, which results in the formation of charge-gathering

heteroatoms can greatly inﬂuence the polymer band gap channels in solar cells. Ternary copolymerization offers

and the electronic properties. Copolymerization with dif- ﬂexible frameworks for white PLEDs. Heteroatoms may be

ferent electronegative heteroatoms represent the latest incorporated into the side-chains to further adjust elec-

development in PF-based red, green and blue (RGB) materi- tronic structures.

als for light-emitting diodes [583]. The linear and coplanar

alternating copolyﬂuorenes exhibit higher hole or electron 4. Metallopolyﬂuorenes (MPFs)

mobilities than the homopolyﬂuorenes. Flexible polymer-

ization and copolymerizations to control the content and Inorganic metal elements are dramatically different

position of heteroatoms have resulted in ﬁnely adjusted from nometal elements in many respects, such as elec-

electronic states, electron distributions and morphologies. tronegativity, reactivity, stability, interatomic interactions

1234 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

and optoelectronic properties. In addition to conventional bipyridine and terpyridine as well as crown ethers [599].

functionalities and catalytic properties, metal elements For example, porphyrin complexes facilitate the introduc-

exhibit abundant optical, electrical and magnetic proper- tion of various metal elements, such as Co, Ni, Fe, Mn, and

ties as well as self-assembling properties. Metal-containing Zn. One example of the incorporation of metal nanocrystals

polymers (metallopolymers), as hybrid materials that do into PFs is the PF8/CdS hybrid for stable blue PLEDs reported

not exhibit severe phase separation with respect to blend- by Chou [600]. Electropolymerization is one of methods to

ing systems, exhibit impressive properties that differ from synthesize the metal-containing conjugated polymers that

those of their individual organic and inorganic components were developed early [601]. Recently, metallopolymers are

␲

[584,585]. -Conjugated metallopolymers combine the usually prepared by Suzuki-type or Yamamoto-type poly-

solution-processing advantages of polymers with the opto- merization of conjugated monomers and metal complex

electronic functionality provided by the presence of metal monomers that belong to direct polymerization methods.

␲

centres. Linear -conjugated backbones offer channels The other method involves the utilization of a post-

to exchange and communicate electrons, and the hyper- coordination strategy in which the ligand-containing PFs

branched matrix can act as antenna for harvesting energy are synthesized ﬁrst, followed by the metal coordina-

[586,587]. Supramolecular ␲-conjugated polymers with tion process. More recently, another strategy has emerged

␲

dynamic features and -conjugated polymers networks that involves coordination polymerization of metal ionics

could be easily organised by metal ionic cross-linking pro- with functionalized double ligands, such as terpyridine-

cesses [588,589]. Metal-containing conjugated-polymer based supramolecular polymers. In this section, various

hybrids provide an opportunity for the construction of var- ␲-conjugated MPFs that contain main-group metals, tran-

ious functional materials and devices, such as chemical sition metals, and rare-earth metals are discussed. Metals

and biosensors as well as electromagnetic wave detec- and their molecular segments incorporated into PFs via

tors [590]. The incorporation of metal elements into more complicated methods are summarized. The effects of

␲-conjugated PFs through copolymerization is a useful metal elements on the optoelectronic properties and device

strategy for the continuous and multi-scale control of elec- performance are also discussed.

tronic structure, morphology and stability of PF ﬁlms as

well as for device energy diagrams for chemists. PFs have 4.1. Copolyﬂuorenes containing main-group metals

been easily tailored with molecular segments to improve

their carrier-injection and transport behaviour [591]. The literature contains very few reports of ␲-conjugated

Fluorene-based metallopolymers have been explored in PFs with main-group metal elements despite their prospec-

such areas as LEDs, solar cells, memory devices, and sensors tive functionalities; for example, bismuth is the most

[592]. appropriate metal for radioactive imaging [602], tin metal

Two kinds of ﬂuorene-based conjugated metallopoly- is well suited for nonlinear optical materials [603,604] and

mers exist: main-chain organometallic polymers [593] active materials for OLEDs [605], organolead and nanoma-

␲

and -conjugated PFs pendent with an organometallic terials are well suited for photodetectors and photovoltaic

side-group. Several kinds of buildling blocks, such as cells [606–609], and organogallium is an appropriate

organometallic molecular segments through carbon-metal choice for organic electronic materials [610,611]. Until

covalent (C–M) bonds,coordination complexes and clus- now, gallium (Ga), indium (In), thallium (Tl), tin (Sn),

ters via coordination bonding and nano-architectures via lead (Pb) and bismuth (Bi) have been introduced into

weak reversible supramolecular interactions, have been conjugated polymers through stable C–M bonding at

incorporated into conjugated PFs. Although a covalent room temperature. Bi-containing conjugated polymers

bond is the strongest linkage, some metal complexes with moderate bluish-green photoluminescence in solu-

can not be introduced into conjugated polymers through tion were ﬁrst reported by Chujo and coworkers [612]. Choi

C–M organometallic bonds because of their sensitivity to and coworkers have reported Sn-containing ␲-interrupted

air atmosphere. Coordination bonding has been used as and conjugated PFs MPF-01 (Scheme 29) in main chains,

alternative key linkage for conjugated metallopolymers, which exhibit stable and strong blue-green PL with high QE

such as metallocenes [594,595], which is especially use- [613,614]. They also demonstrated blue-green lasing action

ful for the incorporation of rare-earth elements with larger by picosecond laser pulse excitation in a micro-ring cav-

atomic radii and f-orbital-based complexes into conju- ity or in cylindrical microcavities. In addition, alkali metals

gated polymers. Some typical coordination-complex-based and alkaline-earth metals with unstable C–M bonds could

molecular segments include Schiff basic M(salen)-type be introduced into conjugated polymers pendent with

complexes [596], phthalocyanines, porphyrins [597–598], crown ether and special bioactive molecules through weak

Scheme 29. Copolyﬂuorenes containing main-group metals.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1235

coordination. Aluminium(III) bis(8-quinolinolate) (AlQ3) is radioactive decay from the triplet state. In this section,

the most popular green molecule and has recently been transition metals have been divided into early transition-

incorporated into the backbone of PFs by Anzenbacher, metal systems [622] and late-transition-metal systems to

who obtained coordination polymer MPF-02 [615]. Fem- allow summaries of their properties and functionalities.

tosecond transient spectroscopy has been used to study In late-transition-metal systems, several important met-

ultrafast energy transfer from oligoﬂuorene to quinolino- als, including zinc, mercury, copper [599], gold [623–626],

late moieties in MPF-02, which was found to proceed at platinum, iridium, iron, ruthenium and osmium, have

11 −1

a rate of 10 s . Spin-coating devices based on coor- been incorporated into PFs. However, only few exam-

dination polymer MPF-02 showed bright-yellow EL with ples have been reported for early transition-metal

a maximum brightness of 6000 cd/m , a turn-on volt- systems.

age of 6 V and a maximum EQE of 1.2%. Alkaline-earth

ions also induce a conformation change and conjugation 4.2.1. Copolyﬂuorenes containing Zn(II) complexes

enhancement as well as ﬂuorescence change, which pro- Zinc(II) features relatively simple coordination modes

vided an excellent platform for the study of selectively and, as a result, forms complexes with simple struc-

metal-ion-mediated carbohydrate–carbohydrate interac- tures. Therefore, the structures of its metallopolymer,

tions. Huang’s group [616] has reported PFs substituted especially its coordinate supramolecular metallopolymer,

with carboxylic acid sodium salt in which the interchain are controllable, which makes the study of the rela-

interactions were suppressed due to electrostatic repul- tionship between their structure and properties feasible.

10 2

sion [617]. Ion–polymer interactions also offer a method Zn(II) with an electron conﬁguration of [Ar]3d 4s has

for obtaining ordered self-assembling aggregates. Han and been introduced into PFs via either the Suzuki coupling

coworkers [588] have synthesized a series of ﬂuores- reaction of macrocyclic Zn(II) complexes and a metallo-

cent conjugated PFs with pendant lactopyranosyl ligands. porphyrin unit or the metal coordination polymerization

The aggregation of d-lactosyl-bearing PFs derived from of chelating bipyridyl, terpyridyl and zinc-salen units to

Ca -mediated complex formation led to ﬂuorescence construct luminescent metal-containing polymers with

quenching. Calcium-induced aggregation was confirmed well-defined structures. Ligand-containing PFs offer a flex-

using dynamic light scattering to determine the corre- ible platform for the detection of transition metals by

sponding hydrodynamic diameters. To this point, tin (Sn), luminescence quenching or by enhancing or changing

aluminium (Al), sodium (Na), and calcium (Ca) are the only the wavelength. In comparison with the conventional

main-group metal elements that have been successfully molecular-based ﬂuorescent chemosensors, conjugated

introduced into conjugated PFs. polymer-based chemosensors have shown enhanced sen-

sitivity through the ampliﬁcation of sensory signals

4.2. Copolyﬂuorenes containing transition metals [627,628]. Bipyridine-based ligands, which have the ability

to coordinate to a large number of metal ions, are attractive

The reactivity, colour, and optoelectronic properties of recognition receptors, especially for transition-metal ions.

PFs with incorporated transition metals were different Our group [68] was the ﬁrst report a series of alternating

from those with incorporated main-group metals due to 2,2 -bipyridine and ﬂuorene copolymers and their appli-

the additional d-orbital. A variety of stable organometallic cation in the sensing of metal ions, especially Zn ions.

compounds or metal-coordinate compounds could easily The results provide clear guidance for the molecular design

be introduced into conjugated polymers to form hybrid of conjugated polymer-based chemosensors through the

materials. According to complex-ﬁeld theory, transition- use of 2,2 -bipyridine as the recognition site: a C–C sin-

metal complexes exhibit abundant colour because of the gle bond linker between conjugated units (compared with

strong spin–orbit coupling interactions between transition vinylene and ethynylene linkers) and lower backbone

metals and ligands. Transition-metal-based conjugated rigidity give higher response sensitivity. After the publi-

polymers have been applied in the ﬁelds of EL devices, cation of our work, Ma and coworkers investigated metal

chemical sensors, smart windows and memory devices ionochromic effects of conjugated polymers for metal-

[585,618,619]. Forrest and Thompson and coworkers have ion sensing [512,629]. Zinc tetraphenylporphyrin (ZnTPP)

developed high-efﬁciency PLEDs that utilize triplet exciton is a stable Zn complex with red emission similar to

3/4 of platinum complexes to challenge the theoretical efﬁ- that of TPP. Li and coworkers have synthesized ZnTPP-

ciency of 25% based on ﬂuorescence [620]. This approach ﬂuorene hyperbranched copolymers MPF-03 (Scheme 30)

allows the possibility of realizing commercial full-colour with different ZnTPP contents [630]. The PLED based on

display devices and white solid-lighting sources. This MPF-03 with a ZnTPP content of 5.50% exhibited pure

design concept has been extended into conjugated poly- red emission with CIE coordinates (x = 0.64, y = 0.30) and

mers as an example of integration of host–guest systems. a maximum luminance of 740 cd/m . Zinc-terpyridines

Numerous similar works have been subsequently reported and zinc-salen also offer facile polymerization procedures

by some groups, such as Huang, Cao, and Paul [416]. through ligand–metal coordination under mild conditions

Recently, in consideration of the longer lifetime of an exci- without a metal catalyst to yield new kinds of supramolec-

ton, which is favourable for charge separation in solar ular coordination polymers. 2,2 ,6 ,2 -Terpyridine ligands

cells, Wong reported the use of metallopolymers as photo- with a high binding afﬁnity towards transition-metal ions

␲ ␲

voltaic active materials [621]. The incorporation of heavy due to d –p * back bonding of the metal to the pyridine

atoms such as Ir, Pt and Ru into the polymer back- rings and due to the chelate effect have been utilized to

bone promotes efﬁcient intersystem crossing that enables construct terpyridyl Zn(II) PFs by Che and coworkers [631].

1236 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 30. Copolyﬂuorenes containing Zn(II) complexes.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1237

Zn-terpyridyl- and ﬂuorene-based self-assembled EL poly- with a maximum LE of 6 cd/A [640]. The longer life-

−

mer MPF-04a has a LUMO energy of 3.22 eV and a HOMO time of triplet excitons in metallo-polymers may lead to

−

energy level of 6.15 eV. A device prepared from MPF- increased sensitivity to trace analytes. Two copolymers

∼

04a exhibited a yellow emission with an emission peak MPF-12 13 that contain ﬂuorene and a cyclometalated

at 572 nm, a maximum LE of 1.1 cd/A and a luminance square-planar Pt(II) complex with excited-state lifetimes

of 2380 cd/m at 13 V. Similar metallopolymers MPF- of approximately 14 ␮s have been reported by Swager and

04b have been prepared by Schubert’s group [632]. The coworkers [641]. These conjugated materials exhibit an

incorporation of terpyridyl Zn(II) moieties into different increased sensitivity to dissolved oxygen because of the

main-chain structures gave different emission wavelength longer excited-state lifetime, which demonstrates their

that ranged from violet to yellow. The large red shift of potential use as chemosensing materials.

more than 30 nm of the EL with respect to the PL spectra has A series of soluble and thermally stable Pt(II) polyyne

∼

been conﬁrmed by the evaluation of device performance. PFs (MPF-14 18) have been synthesized and inves-

To improve the charge-transport ability, Lin and cowork- tigated by Wong and coworkers for phosphorescence

ers [633–635] incorporated Cz and OXD as side-chains photophysics and photovoltaic solar cells [642–648].

into ﬂuorene/ethynylene-based terpyridyl Zn(II) metal- In this polyyne system, the S1 singlet state is delo-

∼

lopolymers MPF-05 07. Compared with metallopolymers calized over several repeat units, but the T1 triplet

that contain alkyl pendants, the quantum yields were state is strongly localized. The efﬁciency of the triplet

greatly enhanced by the introduction of OXD pendants, emission increases with increasing the band gap. Blue-

but were reduced by the introduction of Cz pendants. light-emitting Pt-containing polyynes may be obtained

EL devices based on the light-emitting metallopolymers by the hybridization of sp -silanes and germanes as

MPF-06a,07b as emitters showed green EL emissions conjugation interrupters to limit the effective conjuga-

∼

( 550 nm) with turn-on voltages of 6.0–6.5 V, maximum tion length, which boosts the phosphorescence decay

LEs of 1.05–1.35 cd/A (at 100 mA/cm ), and maximum rates essential for light-energy harvesting from the triplet

luminances of 2313–3550 cd/m (at ∼15 V). Schubert and excited state. Low-band-gap Pt(II) polyyne polymers with

coworkers used this self-assembly procedure to prepare a 9-dicyanomethyleneﬂuorene spacer have been achieved

a statistically random supramolecular copolymer MPF-08 with a band gap of 1.58 eV [649]. Wong and cowork-

that features an energy transfer from the donor to the ers also studied the effect of acetylenic chain length on

acceptor [636]. Peng et al. reported a series of soluble bis(N- the tuning of the functional properties and the effect

alkylsalicylaldiminato) Zn(II) complex-based supramolec- of oligothienyl chain length on the tuning of the solar

ular polymer MPF-09 through ligand–metal coordination cell performance in ﬂuorene-bridged polyplatinynes. From

under mild conditions [637]. The MPF-09 polymer ﬁlms MPF-14 to MPF-15, with an extension of the acetylenic

obtained emitted strong green PL with relatively high unit, lower triplet energy accompanied by a decreased

quantum efﬁciencies of 51%. The LUMO and HOMO energy in the triplet quantum yield and lifetime was real-

levels were estimated to be −3.23 and −6.14 eV, respec- ized [646]. A series of strongly visible-light absorbing

tively. An MPF-09-based device exhibited green-light polyplatinynes that contained oligothienyl–ﬂuorene ring

emission with a brightness of 2760 cd/m and a maximum hybrids (MPF-16) were synthesized and characterized.

LE of 2.3 cd/A at a current density of 10.64 mA/cm . The photovoltaic behaviour of MPF-16 depended signiﬁ-

cantly on the number of thienyl rings along the polymer

4.2.2. Copolyﬂuorenes containing Pt(II) complexes chain, and some of these PSCs exhibited high PCEs of

Similar to Zn(II) complexes, platinum(II) complexes also up to 2.9% and a peak EQE as high as 83% under

have simple planar coordinate structures. However, unlike AM1.5 simulated solar illumination [647]. Recently, PtP

the d Zn(II) complexes, which undergo only intraligand (MPF-19)/amphiphilic behenic acid/europium-substituted

charge transfer (ILCT), Pt(II) complexes have been studied polyoxometalate Langmuir–Blodgett ﬁlms have been pre-

intensively because of their metal-to-ligand charge trans- pared with interesting near-white emission spectra that

fer (MLCT). Platinum has been introduced into PF through result from the dual-emissive nature of the mixed MPF-

the formation of C–Pt bonds with the salen, TPP, and 19/POM blends and electrical response at a tunnelling

polyyne for PLEDs and solar cells. Later porphyrin–Pt(II) current of up to ±100 nA when scanned at −1 and 7 V [650].

was used as a red-emitting material to demonstrate the

spin-forbidden emission that originates from the partially 4.2.3. Copolyﬂuorenes containing Ir(III) complexes

allowed triplet excited state (phosphorescence) [620,638], Nearly all of the side-chain-type complexed copoly-

conjugated polymers MPF-10 (Scheme 31) that contain mers use a pendent ˇ-diketone as the anchor site for three

soluble TPP–Pt(II) complexes with a characteristic MLCT reasons: (i) alkyl ˇ-diketones do not inﬂuence the opti-

emission of 684 nm at room temperature were synthe- cal properties of the complexes; (ii) ˇ-diketones may be

sized for deep-red-coloured electrophosphorescent LEPs bonded to the conjugated main chain through alkyl groups

[639]. The highest EQE was 1.95%, with an emission without the addition heteroatoms and avoiding the for-

peak at 684 nm at a current density of 1.42 mA/cm mation of oxygen- and moisture-sensitive benzyl groups;

for an MPF-10 based device. Pt(II)–salen-based statistical and (iii) OOˆ ligands are more active than CNˆ ligands in the

ﬂuorene-type copolymers MPF-11 have also been synthe- coordinate reaction, which supports the high yield of the

sized for electrophosphorescence. The poor performance complexed monomers.

of PLEDs prepared from this material due to aggregate In contrast to Pt(II) complexes with planar confor-

quenching was improved by blending it with PF2/6 matrix mations, which leads to self-assembly behaviour and

1238 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 31. Copolyﬂuorenes containing Pt(II) complexes.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1239

luminescence quenching due to Pt–Pt interactions [651], functions as the blue-emissive units, whereas BT and the

cyclometalated Ir(III) complexes with signiﬁcantly shorter Ir(III) complex function as green- and red-emitting units,

emission lifetimes and more intense phosphorescence QEs respectively. White-light emission was achieved when

at room temperature relative to those of other heavy-metal the contents of BT and Ir(III) complex in the polymer

complexes have been considered as prime candidates for were adjusted. The devices exhibited a maximum LE of

phosphorescent materials [652–654]. The incorporation 6.1 cd/A at a current density of 2.2 mA/cm and a maxi-

of electrophosphorescent Ir(III) complexes into solution- mum luminance of 10,110 cd/m at a current density of

processable conjugated polymers offers an important 345 mA/cm . White-light-emitting polymers MPF-28 have

method for the development of high-efﬁciency RGB and also been reported [659]. White-emitting PLEDs with an LE

white PLEDs as well as solar cells. PFs with Ir(III) complexes of 4.49 cd/A and a maximum power efﬁciency of 2.35 lm/W

may be divided into two classes: PFs with neutral Ir(III) at 6.0 V were realized by tuning the content of orange

complexes and those with charged Ir(III) complexes in the phosphorescent Ir(III) complex in polymer MPF-28a.

main chain or in the side chain, respectively. Both CNˆ and OOˆ ligands may be embedded in the

Initially, Chen et al. ﬁrst reported high-efﬁciency red- main chain to form the main-chain-type Ir(III)-complexed

light emission from PFs grafted with cyclometalated Ir(III) PFs. Sandee et al. were the ﬁrst to report main-chain-

∼

complexes with a diketone pendant ligand attached to type bis-cyclo-metalated Ir(III) complex PFs MPF-29 30

the 9-carbon position of ﬂuorene [43]. They found that (Scheme 33), which exhibited an EQE of 1.5% and an emis-

almost complete intra- and intermolecular energy trans- sion peak at 610 nm [55]. MPF-29∼30, in which the triplet

fer from the host ﬂuorene segments to the guest Ir(III) energy level have been better matched between the Ir(III)

complexes was achieved, compared with the incom- complex and the ﬂuorene energy levels, may be used in

plete transfer in the corresponding blend system. The efﬁcient red-phosphorescent PLEDs. Subsequently, Cao and

incorporation of charge-transport Cz into polymers can coworkers [660,661] designed and synthesized polymer

signiﬁcantly increase energy transfer from the host to MPF-31∼33 by a method similar to that of MPF-30 and also

Ir(III) complexes to improve the efﬁciency and lower the achieved highly efﬁcient saturated red-phosphorescent

turn-on voltage. The MPF-20-based polymer (Scheme 32) PLEDs. The emission colour may be tuned easily over the

based device exhibited a high EQE of 1.59% and an LE entire visible region via simple modiﬁcations to the chem-

of 2.8 cd/A at 7.0 V with luminance of 65 cd/m and an ical structures of the ligands. Moreover, the best device

emission peak at 610 nm. Poly(ﬂuorene-co-carbazole), as performance was an EQE of 6.5% at a current density of

a host for the Ir(III) guest, beneﬁts the hole-transport 38 mA/cm with an emission peak at 630 nm and a lumi-

process on the polymer/ITO interface, acting as charge trap- nance of 926 cd/m .

ping site and suppressing back-energy transfer from the The most important criterion for service as a phospho-

triplet state of Ir(III) complexes to the backbone. MPF-21- rescent host is that the ET of host materials should be above

23 with a ﬂuorene-alt-carbazole backbone grafted with that of the dopant to satisfy exothermic energy transfer.

cyclometalated Ir(III) complexes have been synthesized. This factor is crucial for achieving high phosphorescence

An electrophosphorescent red-light-emitting device based efﬁciency in the host–guest system. The high-efﬁciency

on MPF-21a, which exhibited a QE of 4.9% and an LE of energy transfer from the PFs to the Ir(III) complex in the

4.0 cd/A with 240 cd/m at a bias voltage of 7.7 V and a peak polymer main chain via an efﬁcient intramolecular energy

emission at 610 nm, was achieved by Yang and cowork- transfer would be expected in polymers that contain Cz

ers [655,656]. The polymers MPF-24-25 were investigated in the main chain. The incorporation of other units, such

by Evans et al. to elucidate the inﬂuence of the length of as Py or thiophene, into the backbone of PFs that con-

the side chain on the energy transfer from the host to the tain Ir(III) complex could reduce the effective conjugation,

Ir(III) complex [56]. The appropriate conformation may be which would increase the energy level of the polymers

helpful for Dexter-type triplet energy transfer. Long alkyl [662]. Polymers MPF-34∼35 were obtained. Quenching of

side-chains are likely to be helpful in the suppression of the phosphor was reduced due to the increased triplet

the back transfer of triplet excitons from the red phospho- energy of the conjugated backbone by the incorporation

rescent Ir(III) complex to the PFs backbone, which would of a 3,4-linked thienyl group.

result in higher PL and EL efﬁciencies. Subsequently, red- White-light emission was realized by the introduc-

light-emitting devices using Ir(III) complex that contained tion of a red-light-emitting Ir(III) complex and BT into

PFs MPF-26 pendent with amino-alkyl groups allowed the the main chain of PFs [663]. A WPLED based on polymer

use of Al or Au as a high-work-function metal as the cath- MPF-36 showed a maximum EQE of 3.7% and a maxi-

2 2

ode, which exhibited a maximal luminance of 1032 cd/m mum LE of 3.9 cd/A at a current density of 1.6 mA/cm

with a QE of 3.70% and an LEE of 1.64 cd/A [657]. with CIE coordinates of (0.33, 0.34). The maximum lumi-

Because of their excellent luminescent properties, PFs nance of 4180 cd/m was achieved at a current density

not only serve as host matrices but can also be used as the of 268 mA/cm with CIE coordinates of (0.31, 0.32).

blue-emitting unit in the construction of white-light emit- White-light-emitting devices from phosphorescent single-

ters through light doping of green or red emitters. As one of polymer MPF-37 systems based on blue-light-emitting

the highlights, Ir(III) complexes were introduced into the ﬂuorene, green-light-emitting naphthalimide, and red-

side chain of a single polymer with three individual emis- light-emitting Ir(III) complex monomers has been reported

sion species to realize white-light emission while avoiding [664]. A device based on MPF-37 emitted white light with

the drawbacks of polymer-blend or small-molecule-doped CIE coordinates of (0.33, 0.34) and exhibited a maximum

2 2

polymer systems [52,658]. In polymer MPF-27, ﬂuorene luminance of 300 cd/m at a current density of 2900 A/m .

1240 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 32. Copolyﬂuorenes containing neutral Ir(III) complexes in the side-chain.

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1241

Scheme 33. Copolyﬂuorenes containing neutral Ir(III) complexes in the main-chain with CNˆ ligand.

1242 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Beyond PLEDs, Holdcroft has introduced triplet-forming Yang and coworkers [675] have synthesized a series of

Ir(III) complexes into PF-based polymers blended with an ﬂuorene-co-carbazole copolymers with charged Ir(III)

electron acceptor, which considerably enhances external complexes in the side chain (MPF-53). A device based on

quantum efﬁciencies (EQE) of the devices approximately MPF-53 exhibited excellent performance, with a maxi-

a factor of 10 with respect to the Ir-free precursor, from mum EQE of 7.3% and a maximum LE of 6.9 cd/A with a

2 2

1.1% to 10.3%, resulting in the power conversion efﬁciency luminance of 138 cd/m at a current density of 1.9 mA/cm .

improving from 0.002 for precursor to 0.07% for Ir-based

polyﬂuorene MPF-38 due to the formation of the triplet 4.2.4. Copolyﬂuorenes containing Hg, Fe, Ru, Os, Re, or Zr

state [665]. complexes

Another way to incorporate Ir(III) complexes into the For late-transition-metal blocks, other examples of PFs

main chain of PFs was realized by Suzuki polycondensation that incorporate heavy-metal complexes, including mer-

␤

of ﬂuorene segments and a -diketone ligand chelated cury (Hg), gold (Au), and iron (Fe) complexes for switching

with an Ir(III) chloride-bridged dimer. Yang and coworkers memories, ruthenium (Ru) complexes for phosphores-

[666–669] synthesized a series of random copolymers cence and solar cells, osmium (Os) complexes for white

MPF-39–48 (Scheme 34) that contained a ˇ-diketonate PLEDs, and copper (Cu) complexes for sensing detec-

ancillary ligand in the main chain that incorporated a tion, have been reported. Similar to the behaviour of

cyclometalated Ir(III) complex through either copoly- Pt(II) polyynes, isoelectronic mercury and Au polyynes

merization techniques or an end-capping procedure. A also exhibited linear-chain conformations with exten-

saturated red-emitting PLED based on MPF-40 with an sive electronic conjugation. Conjugated PFs that contain

emission peak at 628 nm, a maximum EQE of 0.6% at a cur- Hg(II) or Au(I) have been systematically synthesized and

rent density of 38.5 mA/cm and a maximum luminance of investigated by Wong and coworkers, although limited

541 cd/m at 15.8 V was achieved. They also investigated details were reported with respect to device perfor-

the effects of homo-fluorene, fluorene-alt-carbazole, mance [623,624,676]. They reported the first examples

fluorene-alt-spirobifluorene, or fluorene-alt-oxadiazole on of soluble well-defined Hg(II) polyyne polymers MPF-54

device performance. As a result, a maximum EQE of 2.93% (Scheme 36) with 9,9-dialkylﬂuorene groups, which could

with an emission peak at 629 nm was attained from the render (␲␲*) phosphorescence through efﬁcient ISC by

copolymer MPF-46 with a ﬂuorene-alt-oxadiazole back- ligation to the Hg(II) centre [677]. The insertion of differ-

bone due to its relatively high ET (2.32 eV). For copolymer ent peripheral substituents on the central ﬂuorene spacer

MPF-48, both ﬂuorenone and Ir(III) complex were incorpo- units allowed ﬂexible adjustments to their electronic and

rated into the polymer backbone. The device based on MPF- optical properties. The energy of the S1 excited state

48 emitted white light with coordinates of (0.32, 0.45), a decreased from polyyne MPF-54a to butadiynyl PF MPF-55

maximum LE of 5.5 cd/A, and a maximum luminance of [646]. The cardo-type ﬂuorene-interrupted Hg(II) polyyne

1015 cd/m [670]. The single-chain white-light-emitting MPF-56 with a limited effective conjugation length has

poly(ﬂuorene-co-carbazole) copolymer MPF-49 that been reported by the same group to give a more efﬁcient

contained a novel Ir(III) complex with a ˇ-diketonate blue-phosphorescence emission [626]. Related theoretical

as the red-emission unit has been reported [671,672]. A works on the electronic characteristics and substitution

device based on polymer MPF-49 emitted white light com- effects of PFs that contain Hg(II) or Au(I) have been reported

posed of blue and red emissions with CIE coordinates of by Feng and coworkers [625,678,679].

(0.31, 0.32), close to the standard for white-light emission. Ferrocene, as a typical iron organometallic com-

Compared with neutral complexes, charged Ir(III) compound, exhibits stable and reversible redox properties

plexes have numerous features that may make them one and has potential applications in the control of PL and

of the best candidates for lighting and display applications, voltage–current characteristics [594,680,681]. The spec-

such as the utilization of inert metal electrodes and the troscopic, redox and thermal behaviour of conjugated

low power consumption of the devices based on Ir(III) copolyﬂuorene MPF-57 with a pendant ferrocene moi-

complexes. Our group [673,674] was the ﬁrst to synthesize ety have been reported [682,683]. Ferrocene has been

a series of PF derivatives (MPF-50∼52) (Scheme 35) with incorporated into PFs for application in non-volatile ﬂash

charged red-light-emitting Ir(III) complexes in the back- memory devices. Lee and coworkers [684] demonstrated

bones, in which the phosphorescent chromophores were that ferrocene-containing PFs MPF-58 exhibit electrically

molecularly dispersed within the composite material. All bistable I–V characteristics with threshold voltages (Vth) for

± of the chelating polymers displayed good thermal stability, both the on and off states in the range of 2 V and an on/off

redox reversibility and ﬁlm-forming properties. We found ratio of 10 .

that the chelating polymer MPF-50 possesses almost the Ru(II) complexes have also been incorporated into PFs

same HOMO energy level as that of PF8, whereas the for electrophosphorescent LEPs and solar cells. Wei and

LUMO energy level is slightly lower than that of PF8. These coworkers compared the PL of PFs with a Ru(II) complex

results suggest that the chelating polymers have better (MPF-59) with its precursor poly(ﬂuorene-co-phenylene)

electron-injection and transport properties due to the and found that the QE for PL of blue-light-emitting MPF-59

incorporation of charged Ir(III) complexes into the main was improved signiﬁcantly because of the efﬁcient energy

chain of the polymers. Saturated red-light emission may transfer from the PF chain to the metal complex [685]. The

be realized by the selection of appropriate ligands. PLEDs incorporation of trithiocyanato Ru(II) complexes into con-

using MPF-51 as the light-emitting layers were fabricated, jugated PFs can also extend the absorption band to longer

and saturated red electrophosphorescence was achieved. wavelengths and enhance the photosensitivity in this

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1243

Scheme 34. Copolyﬂuorenes containing neutral Ir(III) complexes in the main-chain with ␤-diketone ligand.

1244 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 35. Copolyﬂuorenes containing charged Ir(III) complexes.

region, which facilitates the generation of charge carriers. green and white emissions [690,691]. The incorporation of

Fluorene-based polymer MPF-60 functionalized with pen- a charge-transfer ligand with a low absorption energy to

dant Ru(II) terpyridine trithiocyanato complexes exhibited the Re(I) led to a nonemissive complex with a low-band

a very broad absorption band that spanned from 400 to gap, which is favourable for photovoltaic processes. Chan

␲

750 nm due to the presence of a -conjugated system and and coworkers developed PFs with a pendent low-band-

the Ru(II) complexes [686]. Under simulated AM1.5 solar- gap Re(I) complex (MPF-65) as a photosensitiser for solar

light illumination, BHJ photovoltaic cells based on MPF-60 cells [692]. Under AM1.5 illumination, a device based on

2 2

exhibited a Jsc of 1.60 mA/cm , a Voc of 0.18 V, and a PCE of MPF-65:PCBM (1:4) exhibited a Jsc of 0.13 A/cm and a Voc

0.08%. of 920 mV, which resulted in a PCE of 0.03%.

Divalent Os(II) complexes exhibited desirable shorter In addition, some MPFs have been reported to behave

triplet emission lifetimes and lower oxidation potentials. as reactive intermediates rather than as semiconducting

Chou and coworkers [687] introduced Os(II) complexes polymers. Characterizations of their optoelectronic prop-

into PFs to construct polymer MPF-61 as a light-emitting erties, morphologies and stabilities have been limited. For

material. A highly efﬁcient red PLED has been realized with example, zirconacyclopentadienes are useful precursors

an EQE of 18.0%, a maximum brightness of 38,000 cd/m , to a wide range of organic molecules, including dienes,

and an emission peak at 618 nm. arenes, cyclopentadienes, thiophenes, thiophene oxides,

Among the early transition metals, rhenium (Re) is germoles, phospholes, and bismoles via electrophilic sub-

the only one that has been incorporated into PFs. Elec- stitution of the metal centre. Tilley and coworkers [693]

trophosphorescent (bipyridine)Re(CO)3Cl complexes were reported a medium-zirconium-containing PF (MPF-66)

␲

ﬁrst introduced into -conjugated ﬂuorene-co-bipyridine with a high number-average molecular weight poly(p-

copolymers by Ma and coworkers [688]. However, these ﬂuorenylenedienylene) after demetalation with benzoic

∼

chelating copolymers MPF-62 63 (Scheme 37) are not acid, Mn = 24,400 (apparent by SEC). It should be noted

suitable for application in LEPs because of their lower PL that to date, no reports of PFs that contain cadmium,

␲

and EL efﬁciencies, which stems from the extensive - silver, copper, cobalt, rhodium, nickel(II), palladium, man-

conjugation-induced charge-carrier separation of localized ganese, technetium, chromium, molybdenum, tungsten,

excitons. Cao and coworkers found that aminoalkyl-based vanadium, niobium, tantalum, titanium, or hafnium have

Re(I)-based PFs prove an effective electron-transport layer appeared in the literature, although some small molecules

in multilayer PLED devices [689]. Lee and coworkers devel- have been reported [619]. However, in view of the

oped an post-coordination method to incorporate Re(I) excellent optoelectronic properties of metal-containing

chromophores into the pyridine- or quinoline-end-capped compounds such as copper phthalocyanine (CuPc) for hole-

PFs (MPF-64), resulting in the electrophosphorescent transport materials [283], TiO2 for dye-sensitised solar

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1245

Scheme 36. Copolyﬂuorenes containing Hg, Fe, Ru, Os complexes.

cells, and MoO3 for hole dopant and hole-injection lay- bands (FWHM < 10 nm), large Stokes shifts (>150 nm), and

∼

ers, PFs that contain such transition-metal compounds high quantum yields ( 100%). Therefore, rare-earth-metal

could be expected to be promising metallopolymers complexes have become one of the most widely used mate-

for organic electronic devices such as solar cells and rials in inorganic semiconducting devices. Recently, pure

memories. red, green, blue, white, and infrared ELs from the respec-

3+ 3+ 3+ 3+ 3+ 3+ 3+

tive complexes of Sc , Y , Eu , Tb , Tm , Dy , and Er

4.3. Copolyﬂuorenes containing rare-earth metals have been reported [610,694–698]. Rare-earth-containing

PFs are ideal energy-transfer systems for application in

In contrast to transition metals, rare-earth metals pos- polymer devices. In this aspect, our group was the ﬁrst to

sess atomic structures with valence electrons in f-orbitals, synthesize a series of poly(ﬂuorene-co-benzene) copoly-

which determine their unique photophysical and elec- mers grafted with Eu(III) complexes via the chelated

tronic behaviour as well as their magnetic properties. interaction of carboxyl groups [699]. The PL spectra of

Emissions from rare-earth ions are expected to be sharp copolymer MPF-67 (Scheme 38) consisted of two emission

and narrow, and rare-earth compounds are excellent bands, one in the 350–550 nm region and a second sharp

chromophores that exhibit intense emissions with long luminescence with a dominant emission at approximately

−2 −6 *

␲ → ␲

excited-state lifetimes (10 –10 s), narrow emission em = 612 nm, which correspond to the transitions

1246 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Scheme 37. Copolyﬂuorenes containing Re, Zr complexes.

5 7

of the ﬂuorene moieties and the D0 → F2 transition of memory in a sandwich structure of Al/polymer/ITO. In

the Eu(III) ions, respectively. In copolymer ﬁlms cast from these active polymers, the ﬂuorene moiety served as the

solutions, the emission from the ﬂuorene moieties could be backbone and as the electron donor, whereas the Eu(III)

suppressed, and the absorbed excitation energy was effec- complex served as the electron acceptor. The MPF-67b-

tively transferred to the Eu(III) complexes in the copolymer. based WORM electronic memory device showed a high

Effective energy transfer was conﬁrmed in solid ﬁlms ON/OFF current ratio up to 10 , stable ON states and OFF

rather than in solutions when Eu(III) complexes were incor- states up to 10 read cycles at a read voltage of 1 V, and

porated into the copolyﬂuorenes. A nearly monochromatic stability of up to 10 years at a constant voltage stress of

red emission was detected under UV excitation at room 1 V. Copolymer MPF-67e also exhibited WORM-type mem-

temperature. Subsequently, our group designed and syn- ory characteristics, with two conductivity states with an

thesized photocrosslinkable PFs with rare-earth complexes on/off current ratio as high as 10 and stable ON and OFF

via comonomer bipyridine to fabricate multilayer PLEDs states with read cycles up to 10 at a read voltage of 1 V

[700]. Polymer MPF-68 has been crosslinked photochemi- [701].

cally by the addition of a photoinitiator to yield an absolute Copolyﬂuorenes with sensitive ligands exhibit respon-

insoluble network. PF networks MPF-69 have also been sive functionalities upon the introduction of rare-earth-

obtained using Eu(III) complexes as cross-linking agents metal ions, which results in the alternation or quenching

[617]. of absorption and emission. This behaviour is potentially

Rare-earth metals and their complexes exhibit vari- useful in lanthanide-metal-ion sensors and in tuning the

ous functionalities, such as luminescence, reversible ligand emission wavelength of PLEDs. Crayston et al. found that

interactions, and charge trapping, that suit them for poten- Eu(III) and Tb(III) ions chelated with dibenzoylmethane

tial applications in electrically bistable memories and (dbm) or acetylacetonate (acac) ligands may be effectively

sensors as well as organic EL materials. For example, rare- coordinated with ﬂuorene and 1,10-phenanthroline-based

earth Eu(III) complexes act as traps when hole or electron copolymers [511]. A control experiment showed that intra-

carriers go through the conjugated main chain, where the chain energy transfers from both Tb(III) ions and Eu(III)

trap depth determines the type of polymer memory. Kang ions to PFs are ineffective in dilute solutions. In the

and coworkers [204–206] were the ﬁrst to demonstrate thin-ﬁlm state, Eu-polymer MPF-70a can make smooth

a conjugated copolymer MPF-67b of 9,9-dialkylﬂuorene energy transfer from the polymer backbone to metal-

5 7

→

and Eu-complexed benzoate for application in WORM centred Eu f states ( D0 F2) with a narrow-line-width

L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264 1247

Scheme 38. Copolyﬂuorenes containing rare-earth metals.

red emission that results from interchain aggregates. diagnostics [702]. The new agent exhibited a higher relax-

However, Tb-containing polymer MPF-70b exhibited only ivity and a more pronounced enhancement than did the

backbone-based green emission. Recently, gadolinium(III)- (NMG)2–Gd–DTPA widely used for clinical diagnosis. How-

containing PFs (MPF-71) have been reported by Wang and ever, the drawback of this new agent is its cytotoxicity to

coworkers in an attempt to develop potential contrasting the cell and its lack of biodegradability, which restricts its

agents for nuclear magnetic resonance imaging in clinical in vivo applications.

1248 L.-H. Xie et al. / Progress in Polymer Science 37 (2012) 1192–1264

Metal/carbon hybrids facilitate not only the control of 4.3.1. Outlook

optoelectronic properties via the electronegativity of metal Multi-component polymer semiconductors are poten-

atoms or dipole of their complexes, but also the alternation tially useful in the optimization of device performance

of the morphologies through the nonlinear conforma- and stability as well as in the cutting fabrication costs

tions or unique noncovalent bonding. Various metal-based [703]. Single-molecular polymer white-light, photovoltaic

building blocks that range from atoms, to organometal- materials or memory systems will likely be realized

lic or metal complexes, to clusters/nanoparticles have through the precise control of polymer chains. Multi-

been incorporated into PFs through ﬂexible synthetic level design principles will direct the high-performance

approaches, different from those for heteroatoms. Out- or multifunctional polyﬂuorenes. Supramolecular self-

standing Ir(III)-containing PFs are high efﬁcient RGB assembly and two-dimensional nanostructures of PFs will

and white electrophosphorescent materials that suitable become another powerful tool to tailor and control the

for commercialized PLEDs. Sn-containing PFs have been detailed performance of devices in the context of the

applied to optical-pump lasers, and Eu-containing PFs have rapid progress in nanotechnology. The combination of

been applied to WORM-type memory devices. Blue PFs organic synthesis and self-assembly allows for a new gen-

that contain unique ligands have illustrated the detec- eration of highly ordered polymer nano-semiconductors

tion capacity to sensor various metal ions. No reports for polymer nanostructured thin-ﬁlm devices. They are

have appeared on photovoltaic solar cells prepared from promising tools to integrate various photonic, electronic,

PFs that contain rare-earth metals. The limited synthetic magnetic, mechanistic, and other stimuli-responsive ele-

routes of metallopolymers are hindering the investigation ments and to extend the functionality of molecular

of the structure–property relationships and the exploration sensors and actuators, molecular computing, molecular

of optoelectronic devices, although numorous reports on machines and robotics. In consideration of the coming

blending metal-based building blocks with PF matrices in “consciousness age” (also called “awareness age”) marked

the application of hybrid polymer devices. by robotics, plastic mechano-electronics or mechano-

In conclusion, we have proposed a soccer-team-like optoelectronics, soft polymer semiconductors will be one

framework with 11 nodes to describe the framework potential direction, in which PFs-based organic/polymer

between periodic table elements and devices. In this mechano-semiconductors are promising and challenging.

framework, four elements have been abstracted: electronic This review hopefully provides a useful template for the

␲

structure, steric hindrance, conformation and topology, development of other -conjugated polymers.

and supramolecular interaction. Meanwhile, polyﬂuorenes

that contain various elements via covalent or coordi-

Acknowledgements

nate bonds have been summarized into three categories:

hydrocarbon PFs (CPFs), main-chain-type heteroatomic PFs

We thank the “973” project (2009CB930600), NNSFC

(HPFs) and metallopolyﬂuorenes (MPFs). The electronic

(Grants 20704023, 60876010, 60706017, and 20774043),

structures of polyﬂuorenes are obviously changed by the

the Key Project of Chinese Ministry of Education (No.

introduction of various heteroatomic non-metals, basic

104246, 208050, 707032), the NSF of the Education

metals and metalloids, alkali and alkaline metals, tran-

Committee of Jiangsu Province (Grants 08KJB510013,

sition metals and rare-earth metals. HPFs facilitate the

BK2008053, SJ209003, TJ209035, SBK20080618,

behaviour of FRET or charge transfer, which created a

SBK200910193), and the Nanjing Science and Tech-

family of high-efﬁciency red-electroluminescent materials

nology Center (200907003). The authors acknowledge Dr

for full-colour displays, high-mobility materials for poly-

Rui Zhu with University of California at Los Angeles, Dr.

mer transistors and low-bandgap and wide-absorption

Hui Xu with Heilongjiang University, Dr Zhipei Qin with

materials for polymer solar cells. PFs have been widely

University of Waterloo, Dr Qiang Zhao, Dr Runfeng Chen,

applied beyond PLEDs because of the introduction of elec-

Dr Juqing Liu and Dr Xiaoya Hou with Nanjing University of

tronegative elements into their structures. MPFs exhibit

Posts & Telecommunication for their helpful discussions.

the heavy-metal advantage of 100% quantum efﬁciency

via spin–orbital coupling interactions to give potentially

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