Electronic Structure Calculations Of Inter-ring Torsional Potentials Of Regioregular Poly (3-

methyl Thiophene) Oligomers

Ram S. Bhatta and David S. PerryDepartment of Chemistry

The University of Akron, OH 44325-3601

n

Motivation Use of conductive organic polymers in several electronics and photovoltaic devices, e.g.

Solar cell rr-P3HT is promising polymer with ≈0.1 cm2V-1s-1 mobility1

Key steps for solar cell operation:

* Absorption of incident photon - surface properties

* Formation of exciton - band structure

* Separation of electron-hole pair - charge distribution

* Collection of charge at electrodes Efficiency depends on no. of independent charge carriers produced Important steps are influenced by conformations and charge delocalization

Objective

Investigation of torsional potential and charge delocalization on P3MT oligomers

1Yi-Kang Lan et al., Polym Int 2010, 59, 16

n

P3MT

Outline Computational procedure

Electronic properties and extended conjugation

Inter-ring torsional potentials* 1-D torsional potentials: dimer, tetramer, hexamer

* 3-D torsional potential: tetramer

Conclusions

Molecular structure computation and convergence

1Fedor, A. M. et al., Vib. Spectrosc., 49 (2009) 124

Bithiophene P3MT dimer

Electronic properties and extended conjugationLUMO orbitalHOMO orbital

Eg = |EH – EL|

HOMO-LUMO energy gap

Head torsion (α) = Tail torsion (γ) =00

Eg for orthogonal geometry is greater than pure cis and trans geometries

1Yi-Kang Lan et al., Polym Int 2010, 59, 16

Electronic coupling

Head torsion (α) = Tail torsion (γ) =00

Coupling for orthogonal geometry is less than pure cis and trans geometries

1

1-D torsional potentials

dimer tetramer

Cis & trans planar geometries are stabilized for longer oligomers

hexamer

Computation of the 3-D torsional potential

Head torsion (α) / deg

Cen

tral

tors

ion

(β)

/ deg

Tail torsion (γ) / deg 90

0

90

180

180

-90

180-180

090

0-90 -180

Head

Tail

α

γ

β

Fit to Fourier series

V (n1

6

n

k cosn

n

k cosn

n

k cosn)

k sin sin

k sin sin)

n,m

k cosn cosm

m1

3

n1

4

n,l

k cosn cos l

l1

3

n1

4

nearestneighbor rings

2nd nearest neighbors

2-D slices of the 3-D fit potential

Head torsion (α) / deg

Cen. tor. (β) / deg

Rel

ativ

e en

ergy

/ cm

-1

Coupling terms only(terms involving 2nd nearest neighbor rings)

Full potential

Central torsion (β) / degHead torsio

n (α) / deg

Rel

ativ

e en

ergy

/ cm

-1

1000

γ = 00

2-angle coupling terms involving 2nd nearest neighbor rings

Tail vs. centralHead vs. central

3-D fit and scaling of the coupling terms

Head

Tail

α

β

V (n1

6

n

k cosn

n

k cosn

n

k cosn)

k sin sin

k sin sin)

n,m

k cosn cosm

m1

3

n1

4

n,l

k cosn cos l

l1

3

n1

4

10

2

3

4

5

6789

100

2R

MS

var

iatio

n / c

m-1

321Nearest

neighbor rings

2nd nearest neighbors

Residuals

Potential dependence on the relative signs of the torsion angles

α β γ

γ=00 γ=00

β-α

βα

Conclusions Extended conjugation causes coupling between P3MT rings.

A mixture of cis and trans geometries can be expected in a disordered P3MT.

Barriers are low enough to allow cis-trans conversion at room temperature.

Scaling of inter-ring coupling is consistent with an exponential model:

1st nearest neighbor > 2nd nearest > 3rd nearest

Acknowledgements

Dr. David S. PerryDr. Harlan Wilk

Sylvestre TwagirayezuMahesh Dawadi

Jonathan Martens