Comprehensive evaluation of the absorption, photophysical, energy transfer, structural, and theoretical properties of alpha-oligothiophenes with one to seven rings

by Becker, R. S.; deMelo, J. S.; Macanita, A. L.; Elisei, F.

A large basis set of alpha-oligothiophenes with two to seven rings (alpha 2-alpha 7), also including thiophene, alpha 1, have been investigated in five solvents regarding absorption, fluorescence and phosphorescence, quantum yields of fluorescence (phi(P)) and tripler formation (phi(T)), lifetimes of fluorescence and the triplet state, quantum yields of singlet oxygen production (phi(Delta)), all rate constants k(F), k(IC), k(ISC), and several of the foregoing as a function of temperature. Ten different theoretical calculations across several levels including three levels of ab initio have been carried out regarding which conformer is lowest in energy and the Delta H's among all conformers of alpha 2, alpha 3 and alpha 5, as well as calculations of transitions energies of the alpha-oligothiophenes. We have shown that the (1) (1)Bu state is the lowest singlet state for all alpha 2-alpha 7 in any solvent, in contradiction to previous predictions for the higher members. Based on absorption and fluorescence data and calculations of atomic charges in S-0 and S-1, the ground state is twisted while the excited state is planar (quinoidal-like). Significant charge transfer occurs between S-0 and S-1 but not S-0 and T-1. For all alpha 2-alpha 7, phi(IC) is small, k(F)(0) is approximately constant while k(f)(0) decreases significantly from alpha 2 to alpha 7. The decrease is k(ISC) is believed to arise from a decrease in matrix elements of the type ([(1) Psi(CT)\H'\(3) Psi(1)]. The essential lack of phosphorescence is assigned as originating from inter-ring twisting mode coupling between T-1 and S-0. Tripler energy transfer to O-3(2) to produce O-1(2) is highly efficient fur alpha 2-alpha 5. Based on all data, the first alpha n representative of alpha-polythiophene is alpha 5.

Journal of Physical Chemistry
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