A computational study of regioisomers formed from the σ-dimerisation of asymmetric terthiophenes

The structural and spectroscopic properties of the three possible regioisomers generated from the σ-dimerisation of 3′-styryl substituted terthiophene have been investigated theoretically using density functional theory calculations. The calculated geometries on the neutral species show that the thiophene chain is only slightly altered between isomers, however upon oxidation to the polaron or bipolaron species the bonding in the thiophene chain is sensitive to the regioisomerisation. The structural distortion caused by the polaron is confined between the substituents in the case of the HH isomer (where the substituents point towards the σ-link); this is not the case for the other two regioisomers. The calculated Raman spectra of the neutral species are similar for the three isomers, however, distinct spectral differences are predicted when the isomers are oxidised. For both the neutral and oxidised species the correlation of experimental and calculated spectra is superior for the HH isomer.