Phosphorescence and triplet state energies of oligothiophenes.

The phosphorescence spectra of a series of small oligothiophenes (nT, n = 1-3) incorporating a variety of substituents, end cappers, and functional groups have been recorded for the first time using gated detection in combination with nanosecond excitation in frozen solution at 80 K. The vibrationally resolved emission spectra provide accurate estimates of the T(1) and S(1) levels, and the singlet-triplet energy gap. Theoretical quantum chemical calculations performed at the DFT (B3LYP/6-31G*) level reproduce all experimental trends accurately and provide quantitative description of the S(0)-T(1) energy difference. The geometry relaxation in the excited state shows that the "natural" size of the triplet exciton is about 3-4 thiophene units.