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