SEMIEMPIRICAL DETERMINATION OF TORSIONAL POTENTIALS AND ELECTRONIC-PROPERTIES OF BITHIOPHENE, TERTHIOPHENE AND 3',4'-DIHEXYL-2,2'-5',2''-TERTHIOPHENE IN THEIR GROUND AND FIRST EXCITED SINGLET AND TRIPLET ELECTRONIC STATES

We report a detailed study of the conformational behavior of bithiophe ne (BT), terthiophene (TT) and 3',4'-dihexyl-2,2':5' ,2 ''-terthiophen e (DHTT) in their ground (S-0) and first excited singlet (S-1) and tri plet (T-1) states. Ground state fully optimized torsional potentials a re calculated using the AM1, PM3 and ZINDO/S semiempirical calculation methods. ZINDO/S is also used to obtain the energy of the S-1 <-- S-0 electronic transitions as well as the S-1 and T-1 excited state energ ies. The calculated singlet-singlet transition energies are compared w ith UV spectra. It is found that the PM3 method does not reproduce acc urately the torsional potential of oligothiophenes. In contrast, AM1 c alculations reproduce minima of the BT potential curve at similar dihe dral angles to those measured in the vapor phase. AM1 calculations ind icate that BT and TT, in their ground state, have slightly twisted tra ns conformations (torsional angle between adjacent rings approximate t o 150 degrees) whereas DHTT is predicted to be much more twisted due t o steric hindrance caused by the presence of the hexyl chains, ZINDO/S performed on AM1 optimized conformers suggests that, after excitation , all the molecules relax to planar SI and Ti conformations such that the excited state energy barriers are predicted to be higher than thos e observed for the ground electronic states of these oligomers.