On the dependence of optical properties on conformational changes in oligothiophenes I.: Electron absorption spectra

The geometries of 2,2'-bithiophene, 2,2':5',2 " -terthiophene and 2,2:5,2 " :5 " ,2''''-quaterthiophene molecules were optimized by using semiempirical AMI method for fixed equidistant dihedral angles between neighboring thiophene rings. Corresponding electron spectra are calculated by semempirical AM1 and ZINDO/S methods. The vibronic interaction between the low lying monoexcited electronic states is symmetry forbidden for any torsion coordinate in all planar oligothiophenes and similar compounds because these electron states are constructed from Tr molecular orbitals. Mutual orientation of torsion angles is not important for electronic structure of oligothiophenes. The dependence of maximal wavelength lambda (max) on the torsion angle is of similar shape as its squared sine function. Using this dependence the torsional angles for some alkylated oligothiophenes in solution are estimated. (C) 2001 Elsevier Science B.V. All rights reserved.