Structure and Electronic Properties

Molecules for plastic electronics: structure and electronic properties

Beatrice Saglio

Dipartimento di Chimica, Materiali e Ing. Chimica “G. Natta”

Politecnico di Milano

e-mail: [email protected] tel. 02.2399.3226 SMART MATERIAL

INPUT OUTPUT ¤ INPUT: ¤ OUTPUT Light Electrical Voltage Light Environment Optical Pressure Mechanical Heat Electrical

ü The change of properties in the bulk reflects a modification at the molecular level. ü The output has to be easily detected ü Reversibility to feature a working function. ü Fatigue resistance → device lifetime. INPUT OUTPUT

Sensors/Biosensors Electrical Mechanical

Piezoelectric materials Smart material modification upon stimulus

hν

S S S S Smart material modification upon stimulus

F F Δn = 0.127 O F

O O

C3H7 Fundamentals of molecular design of materials for molecular electronics/organic optoelectronics

ü Fundamental ingredients: polarizable electrons

ü Molecular design → main units → substituents (EA, ED)

ü Lego-chemistry as a tool = tuning of properties

ü solubility → material characterization processing

ü L stability → device lifetime Fundamental requirement: polarizable electrons

ü basics on the C=C double bond

ü delocalized chemical bond: conjugation

ü aromatic rings: benzene benzenoid fused rings heterocycles

ü substituents: alkyl chains electron-donors e electron-acceptors

ü Organic compounds and solubility

ü Aggregation phenomena

ü (Self-)Assembly Organic functional materials: basic ingredients

ü σ bonds constitute the skeleton of the molecules ü usually when σ bonds break, molecule gets degradation

C C

H H

H C C H

H H Organic functional materials: basic ingredients

ü the double bond consists of a part which arises from the formation of a σ bonds…

C C

H H C C H H Organic functional materials: basic ingredients

ü the double bond consists of a part which arises from the formation of a σ bonds… …. and a part due to the overlapping of the pz (py) atomic orbitals

C C

H H C C H H Alkenes: continuous chain of carbon atoms that contains the double bond.

üGeneral formula: CnH2n (homologous of cycloalkanes) üThe name given to the chain is obtained from the name of the corresponding alkane by changing the ending from -ane to -ene. H H

H2C CH2 H2C C H2C C CH3 CH3 C ethene propene H H

1-butene

H2 H2C C

C CH2 H2

cyclobutane üThe location of the double bond along an alkene chain is indicated by a prefix number that designates the number of the carbon atom that is part of the double bond and is nearest an end of the chain. The chain is always numbered from the end that brings us to the double bond sooner and hence gives the smallest number prefix. üIn propene the only possible location for the double bond is between the first and second carbons; thus, a prefix indicating its location is unnecessary. Organic functional materials: basic ingredients

ü although it is weaker than Bond Bond energyσ bond, the Calculated Bond length From π bond is strong enough to allows the Kcal/mol from Å formation of stable C-C trans80and cis isomersC H 1.53 C H ü the relatively low bond energy makes 2 6 2 6 π electrons less bound and more C=C 141 C2H4 1.34 C2H4

polarizableC≡C 195 C2H2 1.20 C2H2

Olefin Bond energy E Kcal/mol H3C CH3 π bond −27.665 Kcal/mol

ΔE = 1Kcal/mol σ bondH H 108

H3C H −28.6 Kcal/mol

H CH3 ü The cis-trans isomerisation of olefin involves 180° rotation about a C,C double bond. ü Except in strained cyclic olefins, the reaction is usually highly activated as a thermal process in the absence of catalysts. üOn the contrary, it occurs more easily if the molecule is at the first singlet or triplet excited state.

1t* ktp kcp 1c* 1P* k hν d hν αs (1−αs)

1t 1c H3C H H3C CH3

H CH3 H H One double bond is not enough… …the simplest case of 1,3-butadiene

It cannot be represented by a single Lewis formula There are some possible canonical forms (#3)

+ - - + H C C C CH CH C C CH CH C C CH 2 H H 2 2 H H 2 2 H H 2

The double bonds are not localized between the C1-C2 and C3-C4 carbon atoms, but π molecular orbitals arise from the linear combination of the four pz orbital, one belonging to each carbon atom.

It follows that: the double bond (C1-C2) and (C3-C4) is longer than a typical double bond the single bond is longer than a typical single bond (C2-C3)

Nomeclature: conjugated olefines, oligoenes, polyenes One double bond is not enough… …the simplest case of 1,3-butadiene

Energy

Ψ4

Ψ3

Ψ2

Ψ1 Conjugated systems

Conjugated molecules undergo not only typical reactions of olefins, possibly modified by the extended p overlapping, but also peculiar cycloaddition thermally or photochemically driven.

+ Giving some other π electrons more… …the oligoenes

ethylene butadiene octatetraene

CB π∗LUMO π∗LUMO π∗LUMO energy Eg

πHOMO πHOMO π VB HOMO Giving some other π electrons more… …the oligoenes

ethylene butadiene octatetraene polyacetylene

* 2 n *

CB π∗LUMO π∗LUMO π∗LUMO π∗LUMO energy Eg

πHOMO πHOMO πHOMO π VB HOMO Give me some other π electrons more… …the oligoenes

ethylene butadiene octatetraene polyacetylene

* 2 n *

CB π∗LUMO π∗LUMO π∗LUMO π∗LUMO energy Eg πHOMO πHOMO πHOMO VB πHOMO

üThe HOMO increases in energy with increasing conjugation length üThe LUMO decreases in energy with increasing conjugation length üThe band gap (Eg) is decreases with increasing conjugation length Properties: üIonization potential gets lower ü material gets more susceptible to electrophiles (more reactive, in general) ü spontaneous oxidations Peierls distortion

polyacetylene

* n *

π∗LUMO

πHOMO Electronic levels and optical properties: the UV-vis electronic spectra

antibonding MO

E = hν ΔE electronic excitation

bonding MO

Ground state Excited state

Electronic transitions

σ* Antibonding (single bonds)

π* Antibonding (double bonds) E n Non bonding

π Bonding (double bonds)

σ Bonding (single bonds) Structure Compound λmax (nm) ε

Ethene 171 15500

1,4- 178 --- pentadiene

1,3-butadiene 217 21000

2-methyl-1,3- 222,5 --- butadiene

trans-1,3,5- 268 36300 hexatriene

trans,trans- 330 ---- 1,3,5,7- octatetraene The UV-vis electronic spectra of oligoenes Conjugation: ethene acetylene propylene 1,3-butadiene not only linear double bonds… (alkene) (alkine) (allyle)

BENZENE

POLYCYCLIC BENZENOID HYDROCARBONS

CYCLIC POLYENES

HETEROCYCLES N N

5-MEMBERED HETEROCYCLES N O S

H furan tiophene pyrrole Conjugation: not only linear double bonds…

1.09 A BENZENE 1.39 A 120° H H

120° POLYCYCLIC BENZENOID HYDROCARBONS

Resonance Energy= 29.6 kcal mol-1

Benzene: cyclic vs homologous linear compound

benzene 1,3,5-hexatriene

E Conjugation: not only linear double bonds…

üIsoelectronic as a benzene (aromatic HETEROCYCLES compounds) N N ü thiophene: relative high chemical stability 5-MEMBERED HETEROCYCLES üthiophene: highly versatile in organic N O S H furan tiophene chemistry pyrrole Electronic levels and optical properties: the UV-vis electronic spectra of oligo-/poly-thiophene Electronic levels and optical properties: the UV-vis electronic spectra of oligothiophene

S S S S S S S S S S

T 2T 3T 4T

S S S S S S S S S S S

5T 6T

T2 T3 T4 T5 T6 absorbance

300 400 500 wavelength (nm) Electronic levels and optical properties: the UV-vis electronic spectra of oligothiophene

S S S S S S S S S S

T 2T 3T 4T

S S S S S S S S S S S

5T 6T

T2 T3 T4 S T5 S λmax = 305 nm T6

2T absorbance λmax = 245 nm

Thiophene: smaller resonance energy 300 400 500 wavelength (nm) smaller steric hindrance H2-H2’ Steric Hindrance

Courtesy of J. M. Rujas Electronic spectra (UV-vis)

t-Bu H C t-Bu 3 H3C t-Bu O O S t-Bu O S S S CH3 t-Bu S t-Bu O t-Bu CH3 O t-Bu t-Bu

t-Bu

S S

t-Bu t-Bu O t-Bu S t-Bu O S t-Bu O

t-Bu

absorbance t-Bu H3C CH O t-Bu 3 S O t-Bu S t-Bu O t-Bu CH t-Bu 3

S S

t-Bu

t-Bu O CH 3 400 600 800 wavelength (nm) Ladder polymers polyacenes

n n n

Egap = 0 eV (Bredas, Pomerantz) nmax= 7 Egap = 0.5 eV (Yamabe) Egap = 0.2 eV (Kao and Lilly) polyphenantrene

n n

pentacene (OFET) The electronic spectra (UV-vis): Ladder polymers

O O O O

R N N R R N R

n n O O O O Branched oligoenes Branched oligoenes Dendrimers

The attractive properties of multi-chromophores dendrimers suggested a new approach towards blue light-emitting materials with the following characters: (i) blue light emission is brought about by the presence of electronically decoupled polycyclic aromatic hydrocarbon (PAH) units; (ii) the units are incorporated into a rigid polyphenylene dendritic structure and thus adopt sterically defined positions and disallow intra-dendrimer chromophore-chromophore interactions; (iii) amorphous films are obtained due to the lack of intermolecular interactions; (iv) the amount of “useless” substituent and coupling units is kept at a minimum Electronic spectra: electrocyclization

Stilbene:

hν H O2 hν, Δ H

electrocyclisation

Analogous dithienylethenes

1.4

1.2

1.0

0.8

Abs 0.6

0.4

0.2

0.0 300 400 500 600 700 800 nm The side groups

Side groups are usually introduced onto the main molecular skeleton : ü solubility ü conjugation ü HOMO/LUMO levels and bandgap ü intermolecular interactions: solid state packing/self-assembly

#1: alkyl chains its main role is to increase solubility cooperate to the intermolecular packing through Van der Waals

interactions methyl CH3

hexyl

decyl

dodecyl

#2: functional groups selective/specific interactions post-functionalization electronic effect Steric effect of side groups: distortion of the skeleton

CH3

θ = 45°

λmax = 245 nm (εmax = 19000) λmax = 236 nm (εmax = 10000) Electronic effect of side groups: inductive effect

Inductive effect: polarization of a bond caused by the polarization of an adjacent bond

δδ+ δ+ δ-

H3C CH2 Cl

ü Strongly related to electronegativity ü the effect is greatest for adjacent bonds but may be felt far away ü The only effect in saturated hydrocarbons

- I Electron-withdrawing

+ I Electron-donating

The analogous effect which does not operate through bonds, but directly through space (or solvent molecules) is named field effect. It is often very difficult to separate these two effects. Electroactive substituents onto aromatic rings

Inductive effect donor acceptor CF CH3 3

Ring relatively rich of electrons Ring relatively poor of electrons (more reactive) (less reactive)

Resonance effect (mesomeric effect) Decrease in electron density in one position (and corresponding increase elsewhere) due to the presence of unshared electrons pair

+ + +NH :NH2 NH2 NH2 2

- - + M

- Electroactive substituents onto aromatic rings

COOH - I Inductive effect

O O OH O OH O OH OH

-M + +

+ examples -M: COOH CN NO2 CHO Electroactive substituents onto aromatic rings

: NH2 - I Inductive effect

Mesomeric effect + + +NH :NH2 NH2 NH2 2

- - + M

examples +M:

OH OMe NH2 Alogeni CH Electronic effect of side groups: 3 + I + mesomeric and inductive effects + I +

- I -

SiMe 3 + I +

NH 2 - I + M +

NO2 - I - M -

OH - I + M +

OMe - I + M +

COOH - I - M -

CHO - I - M -

CN - I - M -

F - I + M -

Cl - I + M -

Br - I + M - Effect of electo-active substituents on the electronic levels of the skeleton

LUMO

HOMO

D A

Introduction of electro-active groups: ü Affects the ionization potential and electron affinity, that is HOMO/LUMO

ü Affects EGAP üSelectively enhances electron vs hole transport ü Tunes the barrier org/Mt Effect of electo-active substituents on the electronic levels of the skeleton

H3C Good hole transporter N N

CH3

C12H25 CN S CN High electron affinity * S Good electron transporter

n *

C12H25 Substituted benzene: Electronic spectra

R Band at 200 nm Band at 256 nm λ (nm) ε λ (nm) ε --- 203 7400 256 220 CH3 206 7000 261 225 OH 211 6200 270 1450 SH 236 8000 271 630

NH2 230 8600 280 1430

CH2=CH 244 12000 282 750 donor-acceptor systems

D A Donor-acceptor hν ≈ ID-EA

+ O H N NO H N N 2 2 2 - O Charge-transfer

H2N COOH λmax = 289 nm (εmax = 18600)

λmax = 245 nm (εmax = 19000) donor-acceptorsystems: effect on the electronic levels

Egap (polymer) = 1.2 eV M.C. Gallazzi et al., Macromol. Chem. Phys. 2001, 202, 2074