Photophysical and Electroluminescence Characteristics of Polyﬂuorene Derivatives with Triphenylamine

Article Photophysical and Electroluminescence Characteristics of Polyﬂuorene Derivatives with Triphenylamine

Qiang Zhang 1, Po-I. Wang 1 , Guang Liang Ong 2 , Shen Hoong Tan 2, Zhong Wei Tan 2, Yew Han Hii 2, Yee Lin Wong 2, Khee Sang Cheah 2 , Seong Ling Yap 3, Teng Sian Ong 2 , Teck Yong Tou 2, Chen Hon Nee 2,* , Der Jang Liaw 1,* and Seong Shan Yap 2 1 Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan; [email protected] (Q.Z.); [email protected] (P.-I.W.) 2 Faculty of Engineering, Multimedia University, Persiaran Multimedia, Cyberjaya 63100, Malaysia; [email protected] (G.L.O.); [email protected] (S.H.T.); [email protected] (Z.W.T.); [email protected] (Y.H.H.); [email protected] (Y.L.W.); [email protected] (K.S.C.); [email protected] (T.S.O.); [email protected] (T.Y.T.); [email protected] (S.S.Y.) 3 Department of Physics, University of Malaya, Lembah Pantai, Kuala Lumpur 50603, Malaysia; [email protected] * Correspondence: [email protected] (C.H.N.); [email protected] (D.J.L.)

Received: 7 March 2019; Accepted: 30 March 2019; Published: 9 May 2019

Abstract: In this work, polymers of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-triphenylamine] with side chains containing: pyrene (C1), diphenyl (C2), naphthalene (C3), and isopropyl (C6) structures were synthesized via a Suzuki coupling reaction. The structures were verified using NMR and cyclic voltammetry measurements provide the HOMO and LUMO of the polymers. The polymer with pyrene (C1) and naphthalene (C3) produced photoluminescence in the green while the polymer with the side chain containing diphenyl (C2) and isopropyl (C6) produce dual emission peaks of blue-green photoluminescence (PL). In order to examine the electroluminescence properties of the polymers, the solutions were spin-coated onto patterned ITO anode, dried, and subsequently coated with an Al cathode layer to form pristine single layer polymer LEDs. The results are compared to a standard PFO sample. The electroluminescence spectra resemble the PL spectra for C1 and C3. The devices of C2, C3, and C6 exhibit voltage-dependent EL. An additional red emission peak was detected for C2 and C6, resulting in spectra with peaks at 435 nm, 490 nm, and 625 nm. The effects of the side chains on the spectral characteristics of the polymer are discussed.

Keywords: semiconducting polymer; electroluminescence; polyﬂuorene; triphenylamine; PLED; Suzuki coupling; OLED

1. Introduction Conjugated polymers are attractive because of the combined advantage of flexibility and conductivity for electronics and various forthcoming applications [1–6]. Since 1990, electroluminescence (EL) has been observed from a variety of conjugated polymers [7]. It was first shown that multiple emission colors can be obtained using blending polymers with different emission and charge-transport characteristics [8]. Polyfluorene (PF) is one of the promising polymers for both organic solar cell and light emitting diodes because of the large bandgap that enable blue fluorescence [9]. The capability makes polyfluorene the first family that emits in the whole visible spectra, from blue to red [10]. Single emission was typically obtained from each polymer, but the findings infer that multiple emissions may be achieved via modification and proper design of the polymers. The introduction of a copolymer

Polymers 2019, 11, 840; doi:10.3390/polym11050840 www.mdpi.com/journal/polymers Polymers 2019, 11, 840 2 of 11 Polymers 2018, 10, x FOR PEER REVIEW 2 of 11 increasesor side chain the tunability into polyfluorene, of the emission for example, spectra further of the increases polymer, the showing tunability that of multicolor the emission emission spectra fromof the a polymer,single polymer showing component that multicolor is possible emission [11]. fromRed-green-blue a single polymer emission component from a single is possible polymer [11]. wasRed-green-blue achieved recently emission from from a star-shaped a single polymer polymer was [12], achieved where recently PF provided from a emission star-shaped in the polymer blue, and [12], modificationwhere PF provided in the core emission produced in the green blue, emission, and modification and finally, in the the core addition produced of a dopant green emission, contributed and thefinally, red theemission. addition The of a dopantdopant contributedcan consist the of red meta emission.l complexes, The dopant dye, canor quantum consist of metaldots complexes,that were incorporateddye, or quantum into the dots polymer that were [11–14]. incorporated In order intoto stabilize the polymer the blue [11 emission–14]. In order from PF, to stabilize triphenylamine the blue (TPA)emission has from been PF, used triphenylamine with PF. A TPA-based (TPA) has beenpolymer used al withso exhibits PF. A TPA-based good hole-transporting polymer also exhibits properties good forhole-transporting organic electronics properties [15]. forPF organic derivatives electronics bearing [15 ].TPA PF derivativeshave been bearing shown TPAto result have beenin a shown two- π dimensionalto result in aconjugated two-dimensional system conjugatedthat suppress system π-stacking/aggregation, that suppress -stacking improves/aggregation, hole injection, improves and facilitateshole injection, intramolecular and facilitates energy intramolecular transfer in energy single- transfer and indouble-layer single- and double-layerOLEDs [16]. OLEDs PF-based [16]. copolymersPF-based copolymers containing containing TPA in the TPA side in thechain side impr chainove improve the thermal the thermal and morphological and morphological stability stability and actand as act an as antioxidant, an antioxidant, thus thus stabilizing stabilizing the theblue blue emission emission from from poly(9,9-di-n-octylfluorenyl-2,7-diyl) poly(9,9-di-n-octylfluorenyl-2,7-diyl) PFOPFO [17]. [17]. The The presence presence of of a a triphenylamine-based triphenylamine-based hydrazone hydrazone comonomer comonomer with with PF PF also also improved improved the the devicedevice performance performance as as compared compared to to those those with with a a PF PF homopolymer homopolymer [18]. [18]. InIn this this work, work, we we synthesized synthesized and and investigated investigated the the spectral spectral characteristics characteristics of of four four polymers polymers constitutingconstituting a PFOPFO andand TPA TPA backbone backbone of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-triphenylamine]of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-triphenylamine] with withside side chains chains containing containing either: either: pyrene pyrene (C1), (C1), diphenyl diphenyl (C2), (C2), naphthalenenaphthalene (C3),(C3), or isopropyl isopropyl (C6). (C6). TheThe eeffectsffects of of di ffdifferenterent side chainsside chains on the photophysicalon the photophysical and electroluminescence and electroluminescence spectral characteristics spectral characteristicsare compared andare compared discussed. and Polymers discussed. C1 and Polymers C3 exhibited C1 and rather C3 exhibited different emissionrather different characteristics emission as characteristicscompared to C2 as and compared C6 when to opticallyC2 and C6 excited. when Theoptically difference excited. can The also difference be observed can when also thebe observed polymers whenare fabricated the polymers into singleare fabricated layer OLEDs into single with the layer structure OLEDs of with ITO the/polymer structure/Al. of ITO/polymer/Al. 2. Experimental 2. Experimental 2.1. Polymer Synthesis 2.1. Polymer Synthesis The dibromo monomers of M1, M2, M3, and M6 were synthesized via Buchwald–Hartwig aminationThe dibromo as shown monomers in Figure 1of. SuzukiM1, M2, coupling M3, and polymerization M6 were synthesized yielded the via polymer Buchwald–Hartwig with M1, M2, aminationM3, and M6 as inshown Figure in2. Figure The synthesis 1. Suzuki processes coupling are polymerization described in the yielded Supplementary the polymer Information with M1, and M2, in M3,our and previous M6 in work Figure [15 2.,19 The,20 ].synthesis The details processes of C6 are are given described in the in supplementary the Supplementary data (Figures Information S1 and and S2), inwhile our theprevious data for work C1, [15,19,20]. C2, and C3 The are details derived of from C6 are our given previous in the work. supplementary The structures data were (Figures verified S1 andusing S2), NMR while (Figure the data S3) for and C1, cyclic C2, voltammetryand C3 are der measurementsived from our (Figure previous S4) work. to provide The structures the HOMO were and verifiedLUMO ofusing the polymersNMR (Figure as shown S3) and in Tablecyclic1. voltammetry measurements (Figure S4) to provide the HOMO and LUMO of the polymers as shown in Table 1.

N N N

R = H3C CH3 M2 M1 M3 M6

Figure 1. The Buchwald–Hartwig amination process was used to produce M1, M2, M3, and M6.

PolymersPolymers2019 2018, ,11 10,, 840 x FOR PEER REVIEW 3 ofof 1111

N N

C H C H C H C H 8 17 8 17 n 8 17 8 17 n N N C1 C2 C3

N N

C H C H C H C H 8 17 8 17 n 8 17 8 17 n N H C CH C6 C2 3 3

Figure 2. Polymers C1, C2, C3, and C6 are synthesized via the Suzuki coupling method.

Table 1. Electrochemical Properties of the Conjugated Polymers C1, C2, C3, and C6.

Figure 2. Polymers C1, C2, C3, anda C6 are synthesizeda via the Suzuki couplingb method. Polymer HOMO (eV) LUMO (eV) Eg (eV) C1c 4.96 2.20 2.76 Table 1. Electrochemical Properties− of the Conjugated− Polymers C1, C2, C3, and C6. C2c 5.03 2.19 2.84 Polymer −HOMO a (eV) LUMO− a (eV) Eg b (eV) C3c 4.86 2.08 2.78 C1c − −4.96 −−2.20 2.76 C6 C2c 5.16−5.03 −2.192.26 2.84 2.90 − − a c ox Fc b Calculated using the equations:C3 HOMO = (−E4.86onset Eonset) 4.8−2.08 and LUMO = 2.78HOMO + band gap. Calculated − − − abs c from the UV absorption spectrumC6 of the polymer−5.16 films using the equation:−2.26 band gap2.90 (eV) = 1240/λonset. Data of C2 deriveda from our previous work [15], and C1, C3 from references [19,20]. b Calculated using the equations: HOMO = −(𝐸 −𝐸) − 4.8 and LUMO = HOMO + band gap. Calculated from the UV absorption spectrum of the polymer films using the equation: band gap (eV) 2.2. OLED Fabrication c = 1240/𝜆. Data of C2 derived from our previous work [15], and C1, C3 from references [19,20]. The synthesized polymers in the form of powders were first dissolved in chloroform to obtain di2.2.fferent OLED concentrations Fabrication in a glove box. As a comparison, commercial poly(9,9-di-n-octylfluorenyl- 2,7-diyl)The synthesized (PFO) (Ossila, polymers Sheffi eld,in the UK) form was of powder also useds were and first dissolveddissolved in in chloroform the same to solvent. obtain Thedifferent concentrations concentrations of the in polymera glove box. solution As a werecomparison, kept at commercial 5–6 mg/mL, poly(9,9-di-n-octylfluorenyl- except for C1, which had a2,7-diyl) lower solubility, (PFO) (Ossila, thus a Sheffield, concentration U.K.) of was 2 mg al/mLso used was used.and dissolved In order toin fabricate the same OLED solvent. devices The with the polymers, ITO-coated glass (2.5 cm 2.5 cm) with a resistivity of 2 10 4 Ω cm were etched concentrations of the polymer solution were× kept at 5–6 mg/mL, except for× C1,− which· had a lower withsolubility, HBr into thus 2 mma concentration strips to define of 2 the mg/mL anode. was After used. wet In cleaning order to and fabricate UV ozone OLED cleaning devices for with 10 min, the polymerpolymers, solutions ITO-coated were glass spin-cast (2.5 ontocm × the2.5 ITOcm) substrateswith a resistivity at 2000 rpmof 2 × and 10−4annealed Ω·cm were at 60etched◦C for with 30 min.HBr Subsequently, an Al layer with thickness 100 nm was deposited by using a thermal evaporator into 2 mm strips to define the anode. After≈ wet cleaning and UV ozone cleaning for 10 min, polymer (Edwardssolutions Autowere 306,spin-cast Edwards, onto the Burgess ITO Hill,substrates UK) asat the2000 cathode. rpm and The annealed resultant at thicknesses60 °C for 30 of min. the polymer layer of C2, C3, C6, and PFO were 50–80 nm, while C1 was 30 nm. The thicknesses of Subsequently, an Al layer with thickness ≈≈100 nm was deposited by ≈using a thermal evaporator (Edwards Auto 306, Edwards, Burgess Hill, U.K) as the cathode. The resultant thicknesses of the

Polymers 20182019,, 1011,, x 840 FOR PEER REVIEW 4 4of of 11 11 Polymers 2018, 10, x FOR PEER REVIEW 4 of 11 polymer layer of C2, C3, C6, and PFO were ≈50–80 nm, while C1 was ≈30 nm. The thicknesses of layerslayerspolymer were were layer measured measured of C2, C3,by by using usingC6, and a a profilometer profilometerPFO were ≈ 50–80(Mahr (Mahr nm, Perthometer Perthometer while C1 S2, S2,was Mahr, Mahr, ≈30 nm. Göttingen, Göttingen, The thicknesses Germany). Germany). of Thelayers absorbanceabsorbance were measured (Abs) (Abs) and byand photoluminescenceusing photoluminescence a profilometer (PL) (Mahr(PL) spectra spectra Perthometer of the of polymer the S2, polymer Mahr, filmsprepared Göttingen,films prepared from Germany). solution from solutionwithThe diabsorbanceff erentwith different concentrations (Abs) concentrationsand photoluminescence were measured were meas using (PL)ured a UV-vis spectra using NIR aof UV-vis spectrometerthe polymer NIR spectrometer (Avantes,films prepared Apeldoorn, (Avantes, from Apeldoorn,Thesolution Netherlands), with Netherlands), different and excitedconcentrations and excited with 368 withwere nm 368 lightmeas nm toured lig obtain htusing to theobtain a PLUV-vis spectra.the PLNIR spectra. Thespectrometer single-layer The single-layer (Avantes, device deviceofApeldoorn, ITO of/polymer ITO/polymer/Al Netherlands),/Al is shown is and shown in excited Figure in Figure with3. The 3.368 The I–V nm I–V characteristicslig characteristicsht to obtain werethe were PL measured measuredspectra. usingThe using single-layer a a source source measurementdevice of ITO/polymer/Al unit unit (Keithley is shown236) 236) and and in Figurethe electrol electroluminescence 3. Theuminescence I–V characteristics (EL) (EL) spectra spectra were measured were were captured using ausing source a spectrometerspectrometermeasurement (200–1100 unit (Keithley nm) (Avantes 236) and AvaSpec-2048L,AvaSpe the electrolc-2048L,uminescence Avantes,Avantes, Apeldoorn,Apeldoorn,(EL) spectra TheNetherland). were Netherlands). captured using a spectrometer (200–1100 nm) (Avantes AvaSpec-2048L, Avantes, Apeldoorn, Netherland).

Figure 3. Schematic of the side view and top view of the polymer OLED device structure. Figure 3. Schematic of the side view and top view of the polymer OLED device structure. Figure 3. Schematic of the side view and top view of the polymer OLED device structure. 3. Results Results and and Discussion Discussion 3. Results and Discussion 3.1. Absorption Absorption and and Photoluminescence Photoluminescence Spectra Spectra 3.1. Absorption and Photoluminescence Spectra The absorption absorption spectra of all the the polymer polymer films ﬁlms were were compared compared to to the the absorption absorption of of a a commercial commercial PFO sample The absorption in Figure spectra 44.. AllAll theofthe all polymerspolymers the polymer absorbedabsorbed films were fromfrom compared ≈300300 nm nm to to 420–450 420–450the absorption nm nm depending of a commercial on on the ≈ polymerPFO sample samples. in Figure The The maximum4. maximum All the polymers absorption absorption absorbed peak peak for for from PFO PFO ≈ 300was nm obtained to 420–450 at ≈383383 nm nm nmdepending because because ofon of the the ≈ π−ππpolymerπ** transition. samples. When When The themaximum the backbone backbone absorption was was modified modiﬁed peak forwith with PFO TPA TPA was in in obtainedC1, C1, C2, C2, C3, at and≈ 383 C6, nm the because absorption of the − peaksπ−π* transition.were broadened. When theIn In addition,backbone addition, thewas the main mainmodified peak peak withat at 380–390 380–390 TPA in nm nmC1, was wasC2, red-shiftedC3, red-shifted and C6, for forthe C1 C1absorption and and C3. C3. Additionalpeaks were peaks broadened. at 320–348 In addition, nm were thedetected main for peak C1 C1, at, where where 380–390 it was nm reported reportedwas red-shifted to be due due for to to C1the the andpyrene pyrene C3. structurestructureAdditional in peaksthe side at chain 320–348 [21]. [21 nm]. C3 C3 were with with detected a a naphthal naphthalene forene C1 ,structure structurewhere it was in the the reported side side chain to behad had due a a lower lowerto the peak peakpyrene at at ≈structure310310 nm. nm. Thein The the maximum maximum side chain absorption absorption [21]. C3 with peaks peaks a naphthal of of C2 C2 and andene C6 C6structure were were close close in the to to side that that chain of of a a PFO. had a Another lower peak peak at ≈ was≈310 detected nm. The at maximum ≈300300 nm, nm, which whichabsorption has has been beenpeaks reported reported of C2 andfor for the theC6 triphenylamine triphenylaminewere close to that unit unit of in ina the thePFO. polymer polymer Another [15,22]. [15 peak,22]. ≈ ItItwas was was detected clearly clearly distinguishable distinguishableat ≈300 nm, which for hasC2 C2 andbeen and C6 C6 reported with with a a lessfor less therigid rigid triphenylamine side side chain, chain, but but unit was was in not not the well well polymer resolved resolved [15,22]. for for C1It was and clearly C3. distinguishable for C2 and C6 with a less rigid side chain, but was not well resolved for C1 and C3.

PFO (383 nm) 1.0 C1PFO (394 (383 nm) nm) 1.0 C2C1 (384(394 nm)nm) C3C2 (385(384 nm)nm) 0.8 C3 (385 nm) 0.8 C6 (382 nm) C6 (382 nm) 0.6

0.6

0.4 0.4

Normalized Intensity (a.u.) Intensity Normalized 0.2 Normalized Intensity (a.u.) Intensity Normalized 0.2

0.0 0.0250 300 350 400 450 500 550 600 250 300 350Wavelength 400 450 (nm) 500 550 600 Wavelength (nm) Figure 4. Normalized absorbance of C1, C2, C3, and C6 as compared to a PFO ﬁlm (black). The maximum Figure 4. Normalized absorbance of C1, C2, C3, and C6 as compared to a PFO film (black). The absorption wavelengths for each polymer are given in brackets. maximumFigure 4. Normalizedabsorption wavelengths absorbance forof C1,each C2, polymer C3, and are C6 given as comparedin brackets. to a PFO film (black). The maximum absorption wavelengths for each polymer are given in brackets.

Polymers 2019, 11, 840 5 of 11

PolymersPolymers 20182018,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 55 ofof 1111 The photoluminescence spectra of the polymer ﬁlms are shown in Figure5. C1 and C3 ﬂuoresce in the greenTheThe photoluminescencephotoluminescence at 513 and 490 nm, spectraspectra while ofof C2 thethe and polymerpolymer C6 emit filmfilm atss areare435 shownshown nm when inin FigureFigure excited 5.5. C1C1 by andand alight C3C3 fluorescefluoresce source of ≈ ≈ 368inin nm. thethe Thus, greengreen polymer atat ≈≈513513 andand C1 490 with490 nm,nm, a pyrene whilewhile C2 sideC2 andand chain C6C6 emit hademit the atat ≈≈ largest435435 nmnm Stokes whenwhen excitedexcited shift of byby 123 aa nm, lightlight followed sourcesource ofof by C3,368368 C6, nm.nm. and Thus,Thus, C2. The polymerpolymer green C1C1 emission withwith aa pyrenepyrene coincided sideside with chainchain those hadhad thethe obtained largestlargest inStokesStokes pyrene-doped shiftshift ofof 123123 PFO nm,nm, followed structures,followed by C3, C6, and C2. The green emission coincided with those obtained in pyrene-doped PFO whereby theC3, emissionC6, and atC2. 515The nm green was emission directly relatedcoincide tod the with pyrene those concentration obtained in ofpyrene-doped the structures PFO [21 ]. structures, where the≈ emission at ≈515 nm was directly related to the pyrene concentration of the Thus,structures, the presence where of the pyrene emission in the at side ≈515 chain nm inwas C1 directly played related a main to role the in pyrene the green concentration emission observed of the structures [21]. Thus, the presence of pyrene in the side chain in C1 played a main role in the green in thisstructures work. It[21]. is notedThus, thatthe presence the spectra of pyrene of C3 was in the rather side broad, chain in spanning C1 played from a main 400–700 role nm.in the PL green peaks emission observed in this work. It is noted that the spectra of C3 was rather broad, spanning from of C2emission and C6 observed were close in tothis those work. of PFO,It is noted but the that spectra the spectra were broadenedof C3 was rather as compared broad, tospanning the PL offrom PFO 400–700400–700 nm.nm. PLPL peakspeaks ofof C2C2 andand C6C6 werewere closeclose toto thosethose ofof PFO,PFO, butbut thethe spectraspectra werewere broadenedbroadened asas where the PL peaks lay at 435 nm, 464 nm, and 493 nm. comparedcompared toto thethe PLPL ofof PFOPFO wherewhere thethe PLPL peakspeaks laylay atat 435435 nm,nm, 464464 nm,nm, andand 493493 nm.nm.

1.0 C6C6 C3C3 C1C1 PFOPFO 1.0 C2C2 PFO PFO C1C1 C2C2 0.80.8 C3C3 C6C6

0.60.6

0.40.4

Normalized Intensity (a.u.) Intensity Normalized 0.2 Normalized Intensity (a.u.) Intensity Normalized 0.2

0.00.0 400400 450 450 500 500 550 550 600 600 650 650 700 700 750 750 Wavelength (nm) Wavelength (nm) Figure 5. Normalized PL of the polymer ﬁlms excited using a 368 nm light source. FigureFigure 5.5. NormalizedNormalized PLPL ofof thethe polymerpolymer filmsfilms excitedexcited usingusing aa 368368 nmnm lightlight source.source. 3.2. Electroluminescence Characteristics 3.2.3.2. ElectroluminescenceElectroluminescence CharacteristicsCharacteristics All the single-layer devices produced electroluminescence. The current–voltage characteristics of the single-layerAllAll thethe single-layersingle-layer devices of devicesdevices C1, C2, producedproduced C3, C6, and electrolumelectrolum PFO areinescence.inescence. shown in TheThe Figure current–voltagecurrent–voltage6. C1 and C3 characteristicscharacteristics operated at a lowerofof thethe voltage single-layersingle-layer of 5–10 devicesdevices V, while ofof C1, theC1, operatingC2,C2, C3,C3, C6,C6, voltage andand PFPF forOO areare C2, shownshown C6, and inin PFO FigureFigure were 6.6. C1C110–20 andand C3C3 V. Theoperatedoperated emission atat ≈ ≈ intensityaa lowerlower from voltagevoltage C1 was ofof ≈≈ the5–105–10 lowest V,V, whilewhile among thethe alloperatingoperating the samples voltagevoltage due forfor to C2,C2, the C6,C6, lower andand concentration PFOPFO werewere ≈≈10–2010–20 and thickness V.V. TheThe emission intensity from C1 was the lowest among all the samples due to the lower concentration and of theemission sample intensity in this study. from C1 C2, was C6, the and lowest PFO alsoamong exhibited all the samples highercurrent due to the injections lower concentration as compared and to C1 thickness of the sample in this study. C2, C6, and PFO also exhibited higher current injections as andthickness C3. The ELof the spectra sample of the in devicesthis study. are C2, shown C6, inand Figure PFO7 also. The exhibited EL spectra higher of C1 current and C3 injections consisted as of compared to C1 and C3. The EL spectra of the devices are shown in Figure 7. The EL spectra of C1 a broadcompared peak centeredto C1 and at C3.510 The nm EL andspectra 490 of nm, the respectively, devices are shown close to in their Figure PL 7. spectra. The EL Thespectra EL spectraof C1 and C3 consisted of a broad≈ peak centered at ≈510 nm and 490 nm, respectively, close to their PL fromand C3 C3 spanned consisted across of a thebroad visible peak region centered from at 400–700≈510 nm nm. and 490 nm, respectively, close to their PL spectra.spectra. TheThe ELEL spectraspectra fromfrom C3C3 spannedspanned acrossacross thethe visiblevisible regionregion fromfrom 400–700400–700 nm.nm.

0.0120.012

C1 C3 C6 0.0100.010 C1 C3 C6 C2C2 0.0080.008

0.0060.006 PFOPFO Current (A) Current (A) 0.0040.004

0.0020.002

0.0000.000 00 2 2 4 4 6 6 8 8 10 10 12 12 14 14 16 16 18 18 20 20 Voltage (V) Voltage (V) Figure 6. I–V curves for the single-layer device ITO/polymer/Al with PFO, C1, C2, C3, and C6. FigureFigure 6.6. I–VI–V curvescurves forfor thethe single-layersingle-layer devicedevice ITO/ITO/polymer/Alpolymer/Al withwith PFO,PFO, C1,C1, C2,C2, C3,C3, andand C6.C6.

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700 20000 (a) (b) 490 nm C1 C2 600 510 nm 6 V 10 V 7 V 12 V 15000 500 8 V 14 V 9 V 435 nm 16 V 400 18 V 20 V 10000 300 Intensity (a.u.)

Intensity (a.u) 200 5000 100 626 nm

0 0 400 450 500 550 600 650 700 750 400 450 500 550 600 650 700 750 Wavelength (nm) Wavelength (nm)

(c) (d) 488 nm 1400 490 nm C3 6000 439 nm C6 5 V 8 V 1200 6 V 12 V 5000 7 V 14 V 1000 8 V 16 V 4000 800 3000 600 Intensity (a.u.) Intensity(a.u.) 2000 628 nm 400

200 1000

0 400 450 500 550 600 650 700 750 400 450 500 550 600 650 700 750 Wavelength (nm) Wavelength (nm) 20000 (e) 434 nm PFO 12V 14V 15000 16V 491 nm 18V 520 nm 20V

10000 Intensity (a.u.) Intensity

5000

0 400 450 500 550 600 650 700 750 Wavelength (nm) FigureFigure 7. 7.EL EL spectraspectra of of ( a(a))C1, C1, ((bb))C2, C2, ( c)C3,) C3, ( (dd)C6) C6 as as compared compared to to (e (e)PFO) PFO at at different diﬀerent applied applied voltages. voltages.

TwoTwo emissions emissions peaks peaks were were observed observed for bothfor both C2 and C2 C6and at 430C6 nmat 430 and nm 495 and nm, while495 nm, an additionalwhile an peakadditional at 520 nmpeak was at 520 detected nm was for detected PFO. The for peak PFO. at 520The nmpeak at at the 520 green nm at emission the green band emission may be band caused may by oxidationbe caused e ffbyects oxidation [17]. In effects the report, [17]. theIn the effects report, were the stabilized effects were by the stabilized presence by of the TPA presence in the polymer.of TPA Thein the overall polymer. spectra The width overall was spectra broader width than was the broader PL spectra than for the these PL spectra polymers. for these The intensity polymers. of The PFO increasedintensity withof PFO applied increased voltage with consistently applied withoutvoltage significantconsistently change without in the significant spectral shape.change However, in the forspectral C2 and shape. C6, theHowever, increase for of C2 the and applied C6, the voltage increase induced of the applied changes voltage to the induced spectral changes shape. Forto the C2, thespectral ratio ofshape. the intensity For C2, the at 435 ratio nm of to the 495 intensity nm increased at 435 nm from to when 495 nm the increased voltage was from increased when the from voltage 10 V towas 20 V.increased Another from peak 10 at V 626 to 20 nm V. emerged Another whenpeak at the 626 applied nm emerged voltage when was 18the V. applied For C6, voltage the ratio was of 18 the peakV. For at 439C6, nmthe ratio to 488 of nm the changed peak at 439 from nm 1:0.8 to 488 to 1:1.1, nm changed and another from peak 1:0.8 at to 628 1:1.1, nm and occurred another above peak 14 at V.

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ThePolymers devices 2018 of, 10 C2, x FOR and PEER C6appeared REVIEW blue-white at higher voltage. The detailed spectral characteristics7 of 11 of each polymer are plotted in Figure S5. 628 nm occurred above 14 V. The devices of C2 and C6 appeared blue-white at higher voltage. The 3.3.detailed Discussion spectral characteristics of each polymer are plotted in Figure S5.

3.3.In Discussion order to investigate the emission spectral characteristics of the polymers, the absorbance and emission spectra of the polymers were compared to those of a PFO, as seen in Figure8. For all In order to investigate the emission spectral characteristics of the polymers, the absorbance and the polymers, the main absorption peak of the polymers remained within 380–390 nm, unlike those emission spectra of the polymers were compared to those of a PFO, as seen in Figure 8. For all the that have been obtained with TPA connected through a vinylene bridge to PFO, which absorbed at polymers, the main absorption peak of the polymers remained within 380–390 nm, unlike those that 360 nm [16]. However, as a result of peak shifting and broadening, the following observation was ≈ have been obtained with TPA connected through a vinylene bridge to PFO, which absorbed at ≈360 recorded. The spectral overlapping of the absorption and PL peak for C1 and C3 caused Forster nm [16]. However, as a result of peak shifting and broadening, the following observation was energy transfer where PL peaks at 430 nm and 450 nm from PFO structures were absent (Figure8a,c). recorded. The spectral overlapping≈ of the absorption and PL peak for C1 and C3 caused Forster Theenergy absorbance transfer peak where of C1PL broadenedpeaks at ≈430 and nm shifted, and 450 covering nm from up toPFO ~480 structures nm, thus were absorbing absent emissions(Figure <4808a,c). nm The from absorbance the PFO structure. peak of C1 This broadened resulted and in a largeshifted, Stokes covering shift, up and to PL~480 occurred nm, thus only absorbing at 515 nm. The absorbance peak of C3 broadened to 450 nm where emission at <450 nm would be re-absorbed emissions <480 nm from the PFO structure.≈ This resulted in a large Stokes shift, and PL occurred only andat left515 withnm. The some absorbance residue emissionpeak of C3 at broadened 450 nm. Thisto ≈450 resulted nm where in a emission PL peak at at <450 490 nm nm. would The samebe eﬀre-absorbedects were observed and left with in another some residue report emission where PFO at 450 was nm. blended This resulted with in PFM a PL (with peak PFOat 490 and nm. TPA The in thesame backbone) effects were [23]. observed In the report, in another deep report blue where emission PFO was was convertedblended with into PFM a longer (with PFO wavelength and TPA as thein amount the backbone) of PFM [23]. increased. In the report, C2 and deep C6 blue were emission less aﬀ ectedwas converted by the energy into a transferlonger wavelength because of as less overlappingthe amount (Figure of PFM8b,d). increased. The higher C2 and amount C6 were of overlappingless affected betweenby the energy the absorption transfer because of the polymerof less andoverlapping the emission (Figure of a PFO8b,d). structure The higher led amount to a larger of overlapping red shift in between the order the of ab pyrenesorption (C1) of >thenaphthalene polymer (C3)and> theisopropyl emission (C6) of a >PFOdiphenyl structure (C2). led to The a larger larger red side shift chain in the of order pyrene of pyrene (C1) and (C1) naphthalene > naphthalene (C3) induced(C3) > broadeningisopropyl (C6) of the> diphenyl absorption (C2). spectra The larger and resultedside chain in anof pyrene emission (C1) toward and naphthalene the green region. (C3) induced broadening of the absorption spectra and resulted in an emission toward the green region.

(a) (b) C1 C2 1.0 1.0 Abs Abs PL PL

0.8 0.8

0.6 0.6

0.4 PFO 0.4 PL PFO PL

0.2 PFO (a.u.) Intensity Normalized 0.2 PFO NomalizedIntensity (a.u.) ABS Abs

0.0 0.0 250 300 350 400 450 500 550 600 650 700 750 250 300 350 400 450 500 550 600 650 700 750 Wavelength (nm) Wavelength (nm)

0.6 0.6

0.4 0.4 PFO PL PFO PFO Normalized Intensity(a.u.) 0.2 ABS (a.u.) Intensity Normalized 0.2 ABS PFO PL

0.0 0.0 250 300 350 400 450 500 550 600 650 700 750 250 300 350 400 450 500 550 600 650 700 750 Wavelength (nm) Wavelength (nm)

FigureFigure 8. 8.Absorbance Absorbance and and PLPL ofof the polymer films ﬁlms (a ()a )C1, C1, (b ()b C2,) C2, (c) ( cC3,) C3, and and (d) (C6d) C6as compared as compared to a to a PFOPFO ﬁlmfilm (black(black dotted line). line).

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At higher operation voltage, devices of a single polymer of C2 or C6 exhibited interesting EL with multiple peaks that spanned from the blue to red regions. The emission at 435 nm were assigned to ≈ the excitonic emission, while the peak at 490 nm could be due to the excimers based on the reported polyfluorene derivatives [24]. At high applied voltage, the EL spectra were different from the PL spectra due to the complex charge transport and recombination that could be involved (Figure S5). The distinct difference in PL and EL peaks has been observed in some polymers with PFO and/or TPA structures. A large red-shift in the EL peak was detected for polymers with a TPA structure (PBPITP) [25]. The PL peak was detected at 470 nm, but EL occurred at 618 nm, indicating a different ≈ recombination mechanism of charge carriers for PL and EL. In another report, polymers with PFO in the backbone and a TPA side chain were studied [17]. The TPA side chain was able to stabilize the blue emission from the polymer. However, a small EL peak at 630–650 nm was detected in the high voltage range. The origin of the additional peak in the EL spectra can be contributed by a few mechanisms. First, a keto defect can form as a result of photooxidation or thermal oxidation. However, it normally induced an additional peak at 2.2–2.3 eV (539–564 nm) for PF [26]. More importantly, TPA is electroactive where it can be excited by high applied voltage [27]. A TPA electromer or excited-state complex could be formed between molecules or subunits carrying a charge [22]. In our previous report, both C2 and C3 were shown to have fully reversible electrochromic properties [15,20]. C2 switched to green and blue, which may account for the increase of the 490 nm peak in Figure7b. However, the red EL peak from an OLED has not been observed before. The characteristics may be related to electron removal at the N atom from the TPA unit in the main chain, which was observed in a polymer with only a PFO-TPA main chain (referred to as M2 in the report) [28]. When tested for its electrochromic properties, PFO-TPA thin film switches from a transmissive neutral state (pale yellow) to a fully oxidized state (red). We performed a measurement on the electrochromic properties of C6, the results indicated a colour transition to red upon an applied voltage of 1.2 V. Finally, the red emission may also be due to a delayed fluorescence process. The occurrence of a delayed fluorescence has been observed in amino endcapped poly[9,9-bis(2-ethylhexy1-8-luorine-2,7-diyl] where the amino consisted of TPA units. Low temperature PL measurements at 15 K revealed that the red emission occurred as a delayed fluorescence from triplet excitons [29]. Delayed fluorescence or phosphorescence is always red-shifted to occur at a longer wavelength. Optical excitation produce singlet excited states, while some triplet states can only be formed via intersystem crossing from the singlet excited state at room temperature. Thus, triplet luminescence can hardly be detected in PL measurements at room temperature. However, in electrical excitation, charge injection occurred to form either singlet or triplet states in addition to the intersystem crossing from singlet to triplet states [30]. Thus, the overall probability of radiative recombination to produce phosphorescence is increased in electrical excitation. Such a process requires sufficiently high carrier injection at higher operating voltage. The EL spectra of each polymer is converted to the CIE coordinate and overlaid onto the approximate color regions on a CIE 1931 x,y chromaticity diagram (Figure9). The emission from C1 lay within the green region, while C3 emitted at almost the center of the chart, and there was a shift as the voltage increased. The emissions from C2 and C6 were at the edge of the blue-violet region. C2 emitted in the greenish-blue region, but as the voltage increased, the resultant emission shifted when the intensity of the 490 nm peak increased. For the devices based on C6, the emissions in the blue-shifted towards the center region with increasing voltage at high concentration, largely due to the appearance of red emissions at 628 nm. Polymers 2019, 11, 840 9 of 11 Polymers 2018, 10, x FOR PEER REVIEW 9 of 11

PFO C1 C2 C3 C6

Figure 9. Chromaticity of C1, C2, C3, C6, and PFO devices. The increase of applied voltage (indicated Figure 9. Chromaticity of C1, C2, C3, C6, and PFO devices. The increase of applied voltage (indicated by the black arrow) shifts the emission region. by the black arrow) shifts the emission region. 4. Conclusions 4. Conclusion A PF derivative with TPA exhibited interesting electroluminescence properties. Polymers with the sameA PF back derivative bone of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-triphenylamine]with TPA exhibited interesting electroluminescence properties. with diff Polymerserent side with chains thewere same synthesized, back bone and of theirpoly[(9,9-dioctylfluorenyl-2,7-diyl)-co-triphenylamine] PL and EL as a single layer OLED were obtained. The with diff erentdifferent side side chains chainsgreatly were influenced synthesized, the electroluminescence and their PL and EL of as the a polymers.single layer The OLED polymers were obtained. with a larger The sidechain different of sidepyrene chains (C1) greatly and naphthaleneinfluenced the structure electroluminesce (C3) suppressednce of the bluepolymers. emission The for polymers PFO because with a of larger Forster sidechainenergy transferof pyrene and (C1) resulted and naphthalene in a green emission. structure Polymer (C3) suppressed C2 and C6 the exhibited blue emission multiple for EL PFO peaks becauseat 435 of nm,Forster 490 energy nm, and transfer 625 nm. and The resulted ratios ofin thea green peaks emission. were dependent Polymer onC2 theand applied C6 exhibited voltage. ≈ multipleThe results EL peaks suggest at ≈ tunability435 nm, 490 of thenm, emission and 625 bynm. varying The ratios the appliedof the peaks voltage. were The dependent occurrence on ofthe the appliedred peak voltage. at 625 The nm results observed suggest in EL tunability but not PLof spectrathe emission suggest by thatvarying the recombinationthe applied voltage. mechanisms The ≈ occurrenceare complex. of the The red origin peak of theat peak≈625 maynm observed be related in to theEL electroactivebut not PL naturespectra of suggest TPA units that or the from recombinationemission from mechanisms triplet states, are which complex. are dependent The origin on of thethe appliedpeak may electric be related field. to the electroactive nature of TPA units or from emission from triplet states, which are dependent on the applied electric field.Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/11/5/840/s1: Figure S1. Structure of M6 monomer, Figure S2. Structure of conjugate polymer C6, Figure S3. NMR Spectra of C6 (a) 1H NMR (b) 13C NMR, Figure S4. Cyclic voltammogram of the conjugated polymer C6 films on ITO-coated Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1: Figure S1. Structure ofox glass in acetonitrile solutions containing 0.1 M TBAP under argon atmosphere; the scan rate was 100 mV/s. Eonset M6of monomer, C6 were 0.76VFigure via S2. calculation. Structure of The conjugat HOMOe polymer for C6 was C6, Figure5.16 eV. S3. The NMR HOMO Spectra of of C6 C6 was (a) calculated 1H NMR ( usingb) 13C the ≈ NMR,empirical Figure formula S4. Cyclic HOMO voltammogram= (Eox ofE Fcthe conjugated) 4.8, Figure polymer S5. Absorbance, C6 films on PL, ITO-coated and EL spectra glass in of acetonitrile (a) C1, (b) C2, − onset − onset − solutions(c) C3, (containingd) C6, and 0.1 (e) PFO.M TBAP under argon atmosphere; the scan rate was 100 mV/s. E of C6 were 0.76V via calculation. The HOMO for C6 was ≈5.16 eV. The HOMO of C6 was calculated using the empirical formula Author Contributions: Q.Z. and P.-I.W. synthesized and characterized the polymers. G.L.O., S.H.T., Z.W.T., HOMOY.H.H., = − Y.L.W.,(𝐸 −𝐸 K.S.C. and) − 4.8, T.S.O. Figure fabricated, S5. Absorbance, characterized PL, and the EL devices spectra and of ( analyzeda) C1, (b) theC2, results.(c) C3, (d S.L.Y.,) C6, and T.Y.T., (e)C.H.N., PFO. D.J.L. and S.S.Y supervised and designed the experiment. S.L.Y., C.H.N., S.S.Y and D.J.L. co-wrote the paper. Author Contributions: Q.Z. and P.W. synthesized and characterized the polymers. G.L.O., S.H.T., Z.W.T., Y.H.H., Y.L.W., K.S.C. and T.S.O. fabricated, characterized the devices and analyzed the results. S.L.Y., T.Y.T., C.H.N., D.J.L. and S.S.Y supervised and designed the experiment. S.L.Y., C.H.N., S.S.Y and D.J.L. co-wrote the paper.

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Acknowledgments: The authors would like to thank the Ministry of Science and Technology of Taiwan (MOST) for the financial support for this work. We also acknowledge the support from Ministry of Higher Education Malaysia (MMUE 160023) and the Penang State Government (MMUE 150001). Conflicts of Interest: The authors declare no conflict of interest.

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