Design Concepts of Organic

Semiconductors for Plastic Electronics

Peter Skabara

EuroDisplay 2013

• Understand the concept of conjugation, aromaticity…

• Identify the main features of a conjugated polymer

• Identify the features that influence HOMO/LUMO energy

levels and hence the band gap

• Appreciate the effects of heteroatoms on electronic

properties

The aims for today’s workshop

Common conjugated polymers (CPs)

In summary:

Eg = Optical bandgap

Ef = Fermi energy level

Metal

Semi-metal

Insulator

Semiconductor

Band theory

• bond length alternation (BLA)

• the degree of planarity within the chain

• the aromatic resonance energy of the ring system

• the electronic push/pull effect of substituents

• contribution of intermolecular or interchain contacts

in the solid state

Criteria for band gap variation:

Consider the carbon atom in methane:

Configuration of carbon = 1s2 2s2 2p2.

The valence electrons available for bonding are underlined.

Some basic chemistry……

[1] Hybridisation

The new orbital will have 25% s character

and 75% p character. This is derived from

one s electron and three p electrons.

Therefore, the new hybrid orbital is known

as an sp3 orbital which denotes the

ratio of s and p characters in the orbital.

[1] Hybridisation

Ethene contains a double bond - the double bond possesses a bond and a bond

(methane has four bonds). The bond is composed of two p orbital electrons only.

This leaves one s orbital electron and two p orbital electrons for bonding to three

other atoms. On hybridisation, therefore, the new type of bonding orbital has sp2

character.

Some basic chemistry……

[1] Hybridisation

For alkynes (triple bonds) two p orbital electrons are used from each carbon atom to

form two bonds. Therefore, there is only one s electron and one p electron available

for bonding to two other atoms. The new hybrid orbital is therefore an sp orbital –

50% s character and 50% p character.

Some basic chemistry……

[1] Hybridisation

Delocalisation of

electrons

Some basic chemistry……

[2] Conjugation

Relates conjugation and substituents (auxochromes) to electronic

transitions and can be used to calculate max:

[1] Conjugation length

Greater length in conjugation leads to an increase in absorption maximum.

For example: adding an extra conjugated double bond to a diene shifts the

absorption maximum bathochromically by 30 nm.

[2] Substituent effects

Substituents on the conjugated chain can shift max bathochromically.

For example: –OR, 6 nm; -NR2, 60 nm; -SR, 30 nm; -Cl or Br, 5 nm; alkyl,

5 nm.

Some basic chemistry……

[3] Fieser-Woodward Rules

Example:

Theoretical max = 214 (standard value for a linear conjugated diene) + 30

(extended olefin) + 10 (alkyl groups at 1 and 4) + 5 (Cl at 1) + 60 (dialkylamino

Group at 6) = 319 nm

Some basic chemistry……

[3] Fieser-Woodward Rules

Electron donating substituents Electron withdrawing substituents

Strong: Strong:

-NH2, -NHR, NR2 , -NO2, -CF3, -N+R3, -P

+R3

-OH, -OR -CO2H, -CO2R, -C(O)R, -SO3H, -CN

Weak: Weak:

Alkyl, phenyl groups -F, -Cl, -Br, -I

Substituent effects

The stability of an aromatic compound recognised by considering the

Stabilisation Energy (SE) derived from specific heats of hydrogenation

Valence Bond (VB)/Bond Order (BO)

Molecular Orbital (MO) Approach

Hückel’s Rule

Planarity

[4] Aromaticity

Some basic chemistry……

Values found experimentally:

[4] Aromaticity

All bonds are 1.397 Å (i.e. equivalent), therefore, the bonds

must be delocalised throughout the C6 ring

[4] Aromaticity

SE = 36.0 kcal mole-1 (or 150.6 kJ mole-1)

(1 cal = 4.184 J)

db = double bond character

Benzene [4] Aromaticity

(i) Bond Order = 1.67

Bond Length = 1.37 Å

(ii) Bond Order = 1.33

Bond Length = 1.42 Å

Naphthalene

SE = 255 kJ mole-1 (found experimentally)

The value should be (2 x 150) = 300 kJ mole-1

Therefore, there is a loss of 45 kJ mole-1 due to the due to the

decrease in delocalisation.

[4] Aromaticity

(i) Bond Order = 1.75

(ii) Bond Order = 1.25

Anthracene

SE = 349 kJ mole-1

(experimentally found)

should be 450 kJ mole-1

[3 x 150 (benzene)]

Therefore, loss of SE

= 101 kJ mole-1

[4] Aromaticity

Decrease in stabilisation energy due to a decrease in

delocalisation of the electrons - nitrogen is more

electronegative than C, so there will be a slightly larger

concentration of negative charge located at the N atom.

Maximum delocalisation is reached in benzene where the

electrons are equally shared through all the 6 carbon atoms.

Aromatic molecules require planarity for the efficient

delocalisation of electrons throughout the system.

[4] Aromaticity – planarity is key

Monocyclic

Conjugated

Planar

(4n + 2) species (where n = 1, 2, 3, 4 etc.)

i.e. 6 , 10 , 14 , 18 , 22 etc. (Hückel’s rule)

…………………….are aromatic!!!

In general:

[4] Aromaticity

The conjugated chain:

n

dsdsdsdsBLA nn.....332211

BLA = Bond Length Alternation

Structural Features

Optical band gap of PA = 1.8 – 3.8 eV

Average bond length = 1.396 Å

BLA (calculated) = 0.051 Å

BLA (experimental) = 0.08 Å

Optical band gap of Si = 1.1 eV

DEGENERATE!!!

So why is there a band gap?

Structural Features

In theory, delocalisation in PA should be absolute:

However, a one-dimensional metal is liable to structural

distortion: single electrons become paired and localised.

Hence, a HOMO-LUMO gap develops.

This is known as Peierls theory.

Structural Features

Peierls Distortion

Structural Features

Non-degenerate ground state polymers

Poly(isothianaphthalene) (PITN)

Band gap = 1.1 eV

Structural Features

Non-degenerate ground state polymers

Torsion angle 46o Torsion angle 17o

S

SS

S

S

Torsion angle 4o Torsion angle 12o

Planarity revisited

Low level calculations for lowest energy conformers, but a good comparison

Conjugation pathways

What is the blue benzene ring conjugated to in each case?

Red rings are within conjugation pathways; black rings are not conjugated.

Focus on polythiophene (PT) for substituent effects

Substituents at the 3-position eliminate coupling –

maximises conjugation length and maintains a low band gap

Methyl groups also lower the

IP of the monomer by an

Inductive effect (0.2 eV)

But – side groups can also cause steric

hindrance and disrupt planarity

Eg = 0.95 eV

Eg = 1.65 eV

Eg = 0.36 eV

Electron rich side groups

Electrons pushed into chain

HOMO is raised

Electron poor side groups

Electrons pulled away from chain

LUMO is lowered

Mixed donor-acceptor polymers

PT

Conductivity = vast range from

insulator to 1000s S/cm

Band gap = 2.0-2.1 eV

Insoluble

Poly(3,4-dimethoxythiophene)

Conductivity = 90 S/cm

Band gap = 1.8-1.9 eV

Very poor solubility

PEDOT

Conductivity = 600 S/cm

Band gap = 1.6-1.7 eV

Soluble as oxidised material

with polystyrene sulfonate (PSS)

See: Star-shaped -Conjugated Oligomers and Their Applications in Organic Electronics and

Photonics, A. L. Kanibolotsky, I. F. Perepichka and P. J. Skabara, Chem. Soc. Rev., 2010, 39,

2695-2728.

Scherf, Müllen

Ladders polymers – planar, but at the expense of bulky substituents to allow solubility

J. M. Tour, J. Org. Chem., 2007, 72, 7477.

J. Mater. Chem., 2010, 20, 1112

Absorption maxima – PEDOT = 575 nm (Eg = 1.4 eV), PEDTT = 440 nm (Eg = 2.2 eV)

S

SS

S

SS

J. Mater. Chem., 2005, 15, 4783

X-Ray B3LYP/6-31G(d) level

EDTT-EDTT

Compound E1ox

(V)

E1red

(V)

HOMO

(eV)a

LUMO

(eV)a

Eg (eV) λmax

(nm)

PEDOT +0.40 -1.89 -4.0 -2.7 1.35,b

1.63c

578

PEDTT +1.18 -2.34 -4.9 -2.75 2.19,b

2.15c

441

POSO +0.45 -1.94 -4.3 -2.8 1.47,b

1.64c

553

PSOS +0.72 -1.95 -4.6 -2.7 1.89,b

2.14c

446

Polymer HOMO/eV LUMO/eV Eg/eV max/nm

PEDOT -4.0 -2.7 1.35,a

1.63b

578

PEDTT -4.9 -2.75 2.19,a

2.15b

441

PEDST -4.8 -3.3 1.55,a

1.79b

459

65%

95%

78%

B3LYP/6-31G(d) level

E1ox/ V E2ox/ V E1red/ V HOMO /

eV

LUMO / eV a Eg eV

Poly(1) -0.33 +0.31 -2.02 -4.24 -2.71 1.53

Poly(2) +0.64/0.52 +0.74 -2.05 -5.39 -2.9 2.49

E1ox / V E2ox / V E1red / V HOMO /

eV

LUMO /

eV

Electrochemicala

HOMO-LUMO gap /

eV

max /

nm

1 +0.46 +0.95 -1.98 -5.18 -2.97 2.21 347

2 +0.71/0.51 +1.00 -1.97 -5.42 -2.97 2.45 310

Chem. Mater., 2010, 22, 3000-3008

• bond length alternation (BLA)

• the degree of planarity within the chain

• the aromatic resonance energy of the ring system

• the electronic push/pull effect of substituents

• contribution of intermolecular or interchain contacts in

the solid state

Criteria for band gap variation