Singlet and triplet energies of α-oligothiophenes:: A spectroscopic, theoretical, and photoacoustic study:: Extrapolation to polythiophene

The triplet state energies, E-T(0-0), for the alpha-oligothiophenes (alpha n's) with n = 1-7, were determined and vary from 18 000 cm(-1) (alpha 2) to 13 000 cm(-1) (alpha 7). The data were obtained from spectroscopic and calorimetric measurements. It was found that the triplet energies decrease with the increase ring number (n), leveling off beginning with the alpha-oligothiophene with n = 5. A linear relation was obtained for the triplet energy as a function of the reciprocal of n. The data has allowed the extrapolation to polythiophene (n = infinity) and the evaluation of its triplet energy (congruent to 10 000 cm(-1)). The energies (0-0) of the first singlet excited state of n = 1-7 were also evaluated and again a linear relation was found for the E[S-1(0-0)] as a function of 1/n. The S-1-T-1 energy splitting, Delta E(S-1-T-1), was determined and also found to decrease with the increase in the number ring. Semiempirical molecular orbital theoretical calculations reproduce this same trend of decreasing Delta E(S-1-T-1) with n. Extended FEMO theory was applied to the singlet and triplet energies as a function of the number of pi electrons of the system. The data obtained are discussed regarding the length of the chain needed for an effective conjugation similar to that of the polymer (n = infinity) and the geometries of the singlet and triplet states of the alpha-oligothiophenes. (C) 1999 American Institute of Physics. [S0021-9606(99)00236-6].