Intrinsic excitonic luminescence in crystalline terthiophene

The intrinsic excitonic luminescence of crystalline terthiophene samples is reported and discussed in the context of their peculiar crystal structure, which is slightly different from that of all other unsubstituted oligothiophenes (OT's). Starting from the pinwheel aggregate model, it is demonstrated that free-exciton radiative recombination is in principle allowed. Nevertheless, it can not be experimentally observed. Its oscillator strength is indeed so low that even the occurrence of cooperative effects, which enhance the corresponding transition in other OT crystals, cannot be effective in this case.