HF/3-21G*ab initio calculations on methoxy-substituted bithiophenes

We report, for the first time, the lowest energy structures as well as a detailed conformational analysis of several bithiophenes substituted with methoxy and methyl groups in various positions. For each molecule, geometrical parameters and potential energy surfaces were obtained from HF/3-21G* ab initio calculations. The effect of the position of substituents on structural parameters and on torsional potentials is examined. These calculations give results, which are in good agreement with available experimental data. For oligomers having methoxy groups in positions 3,3' and 3,4', planar anti conformations are well stabilized with large rotational barriers at 90 degrees. This phenomenon was interpreted in terms of charge transfer in the thiophene units induced by the methoxy groups in the 3- and/or 3'-positions and delocalization throughout the molecular frame. For 4,4'-methoxy-2,2'-bithiophene, the alkoxy groups do not show any influence on the electronic structure of the molecule and do not create steric hindrance such that the most stable conformer is very similar to that of the unsubstituted bithiophene molecule. In contrast, it is found that the most stable conformer of 3,4'-dimethoxy-4,3'-dimethyl-2,2'-bithiophene is much twisted owing to the presence of the methyl group in the 3'-position. This work also reports on theoretical results performed at lower levels (AM1 and PM3) as these levels of calculation are often the only one practically available for large substituted oligothiophenes. It is shown that these calculations fail to predict accurate conformations of the molecules studied here mainly because of the methoxy group substituent. A ZINDO/S analysis performed on the geometries obtained by the various theoretical methods employed is also reported. It is observed that, for the same torsional angle, the ZINDO/S method is very sensitive to the starting geometry. (C) 1999 Elsevier Science B.V. All rights reserved.