Relaxation dynamics of excitons in polydiacetylene crystals

The lifetimes of self-trapped excitons (STE) in poly-[8-(4-bromophenylthio)-1-(3-chlorophenylcarbamoyloxy)-5,7-octadiyne] (PDA-TPC) with excitonic transition energy Eex=1.75 eV and poly-[1,6-dicarbazolyl-2,4-hexadiyne] (PDA-DCHD) with Eex=1.90 eV were measured to be 0.6±0.1 and 1.3±0.5 ps, respectively, at room temperature. The value of Eex in PDA-TPC is the smallest among all polydiacetylenes (PDA) studied so far because of the electron donor nature of the side substituent group. From the data, together with those of seven other PDAs, the shortest lifetime in PDA-TPC is explained in terms of its low potential barrier between the minima of the STE and the ground state. A comparison of the lifetime and Eex among many PDAs shows that a previous model of STE needs slight modification.