CONFORMATIONAL AND SPECTROSCOPIC ANALYSIS OF SELECTED 2,2'-BITHIOPHENE DERIVATIVES

A spectroscopic study has been performed on selected substituted 2,2'- bithiophenes. Absorption and fluorescence spectra of bithiophenes (BT) substituted with alkyl and alkoxy groups in positions 4,4' and 3,3' h ave been measured in solvents of various polarity and viscosity. AMI a nd ZINDO/S semiempirical calculations have also been performed on all molecules in order to evaluate the torsional potential energy surfaces and the singlet-singlet transition energies. It is found by a combine d theoretical and experimental technique in which 3,3'-derivatives are more twisted than their 4,4'-analogs in the ground state. After excit ation, molecules relax to much more planar SI excited states. The sole exceptions are 3,3'-alkoxybithiophene derivatives which seem to remai n in the same conformation in their respective relaxed SI excited stat e. For molecules in their Ti excited states, planar and perpendicular conformations are the most stable. It is also observed that fluorescen ce quantum yields of all bithiophene derivatives are weak but weaker f or 3,3'-substituted compounds. Results are interpreted in terms of a p ossible substitutional effect on the intersystem crossing process invo lved in these systems.