1. Electronic Structure Calculations Of Inter-ring Torsional Potentials Of Regioregular Poly (3-methyl Thiophene) Oligomers n
Ram S. Bhatta and David S. Perry
Department of Chemistry
The University of Akron, OH

2. Motivation Objective P3MT
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 n
Objective
P3MT
Investigation of torsional potential and charge delocalization on P3MT oligomers
1Yi-Kang Lan et al., Polym Int 2010, 59, 16

3. 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

4. Molecular structure computation and convergence
P3MT dimer
Bithiophene
1Fedor, A. M. et al., Vib. Spectrosc., 49 (2009) 124

5. Electronic properties and extended conjugation
HOMO orbital
LUMO 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

6. 1-D torsional potentials
tetramer
Cis & trans planar geometries are stabilized for longer oligomers
hexamer
dimer

7. Computation of the 3-D torsional potential
Head
Tail α γ β
Computation of the 3-D torsional potential
Head torsion (α) / deg
Central torsion (β) / deg
Tail torsion (γ) / deg 90
180
-90
-180
Fit to Fourier series
nearest neighbor rings
2nd nearest neighbors

8. 2-D slices of the 3-D fit potential
Full potential
Central torsion (β) / deg
Head torsion (α) / deg
Relative energy / cm-1
1000
γ = 00
Head torsion (α) / deg
Cen. tor. (β) / deg
Relative energy / cm-1
Coupling terms only
(terms involving 2nd nearest neighbor rings)

9. 2-angle coupling terms involving 2nd nearest neighbor rings
Head vs. central
Tail vs. central

10. Nearest neighbor rings
3-D fit and scaling of the coupling terms
Head
Tail α  β
Nearest neighbor rings
2nd nearest neighbors
Residuals

11. Potential dependence on the relative signs of the torsion anglesα β γ
γ=00 -α

12. 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

13. Sylvestre Twagirayezu
Acknowledgements
Dr. David S. Perry
Dr. Harlan Wilk
Sylvestre Twagirayezu
Mahesh Dawadi
Jonathan Martens