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 44325-3601

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

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 P3MT dimer Bithiophene 1Fedor, A. M. et al., Vib. Spectrosc., 49 (2009) 124

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

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

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

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)

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

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

Potential dependence on the relative signs of the torsion angles α β γ γ=00 -α

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

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