Luminescence study on exciton relaxation in single crystals of symmetric diacetylenes

We have measured photoluminescence of the single crystals of symmetric diacetylenes with alkyl end groups. We found that the luminescence bands peaking around 2.6 eV are induced irrespective of the type of the end group. The 2.6-eV luminescence are characterized by large Stokes shifts and long decay time constants of 110 ms at 6 K. These characteristics are analogous to the 2.85-eV luminescence in 5,7-dodecadiyne-1,12-diole bis[phenyl carbamate] (DA-4Ph), in which it has been attributed to the recombination of the triplet self-trapped excitons. The results suggest that the excitons relax into the self-trapped exciton dominantly localized on the triple bonds. The 4.2-eV luminescence, which was tentatively assigned to the recombination luminescence of singlet self-trapped excitons in DA-4Ph, was not observed in the crystals used in the present study. It is inferred that the 4.2-eV luminescence is arising from the relaxed excited state localized at a phenyl ring.