Conformational analysis of oligothiophenes in the external electric field

Fully relaxed single-bond torsional potentials of oligothiophenes (nTs, n = 2, 4, 6) under the interaction of the parallel external electric field (EF) constructed by point charges have been evaluated with semi-empirical AM1 and PM3 calculations. Consistent evolutions of the torsional potential surfaces have been observed for three nTs as the EF increases. In particular, the equilibrium molecular geometries are deformed toward planar conformations, and the torsional barriers around the central C-alpha-C-alpha' bond are elevated. These features are sensitive to the conjugation length as expected. For the longer conjugation, the equilibrium geometries become more planar and the torsional barrier increases more rapidly. In addition, the electronic structures of nTs can be also modulated by the application of an external EF. The increase of EF leads to reduction of the energy gap between the LUMO and HOMO. (C) 2004 Elsevier B.V. All rights reserved.