CONFORMATIONAL-ANALYSIS (AB-INITIO HF 3-21G-ASTERISK) AND OPTICAL-PROPERTIES OF SYMMETRICALLY DISUBSTITUTED TERTHIOPHENES/

We report a conformational analysis of several substituted terthiophen es using ab initio calculations performed at the HF/3-21G level. Geom etries of terthiophenes having methoxy substituents in 3,3'' positions (DMOTT), methyl,groups in the same positions (DMTT), and ethyl substi tuents in 3',4' positions (DETT) are compared with that of the unsubst ituted molecule (TT). For all these symmetrical molecules, it is obser ved that the two dihedral angles are independent of each other. The mo st stable conformation of TT is found for dihedral angles theta = phi = 147.2 degrees, whereas three maxima are located at 0 degrees, 90 deg rees: and 180 degrees. The insertion of methoxy groups in 3,3'' positi ons favors a more planar conformation with a higher rotational barrier at 90 degrees. This behavior is explained by the electron donor prope rties of the methoxy groups. By contrast, the addition of two methyl g roups at the same positions induces a twisting in the molecule which i s caused by the steric hindrance between the methyl substituents and t he sulfur atom. The presence of two ethyl groups in 3',4' positions cr eates an even stronger steric effect, giving rise to a more twisted co nformation for DETT compared to that of DMTT. Absorption and fluoresce nce spectra of each terthiophene derivative are also reported and are correlated with their respective potential energy surfaces. The more p lanar molecule (DMOTT) shows a red-shifted absorption band with a high er vibrational resolution and a smaller bandwidth. For more twisted mo lecules, the blue shift and the bandwidth of the absorption bands incr ease with twisting while the absorption coefficient decreases. The flu orescence bands, in all molecules, show a better vibrational resolutio n with a smaller bandwidth compared to their absorption counterparts, while their maximum wavelengths are practically the same, showing that in the first excited singlet state., all molecules relax to a more pl anar conformation.