Singlet and triplet energies of alpha-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 cal orimetric measurements. It was found that the triplet energies decrease wit h the increase ring number (n), leveling off beginning with the alpha-oligo thiophene 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 extrapolati on 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 exci ted state of n = 1-7 were also evaluated and again a linear relation was fo und 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 incre ase in the number ring. Semiempirical molecular orbital theoretical calcula tions reproduce this same trend of decreasing Delta E(S-1-T-1) with n. Exte nded FEMO theory was applied to the singlet and triplet energies as a funct ion of the number of pi electrons of the system. The data obtained are disc ussed regarding the length of the chain needed for an effective conjugation similar to that of the polymer (n = infinity) and the geometries of the si nglet and triplet states of the alpha-oligothiophenes. (C) 1999 American In stitute of Physics. [S0021-9606(99)00236-6].