Singlet exciton relaxation in isolated polydiacetylene chains studied by subpicosecond pump-probe experiments

Singlet ${}^1{B}_u$ exciton relaxation in polydiacetylene chains isolated in their crystalline monomer matrix is studied by subpicosecond pump-probe experiments. Results on photoinduced absorption (PA) and on ${}^1{B}_u$ exciton absorption bleaching [photobleaching (PB)] are presented. It is shown that three excited states lie in the optical gap, apart from the triplet ${}^3{B}_u$ exciton. Two short-lived states (respective lifetimes τ≈450 fs and 2 ps) of very similar PA cross sections and PB efficiencies, belong to the same relaxation pathway. A third longer-lived state (τ⩾30 ps) is responsible for a slow PB component and exists independently of the triplet exciton, which has its own signature [B. Kraabel et al., Chem. Phys. 227, 83 (1998)]. There is a branching in the ${}^1{B}_u$ exciton relaxation, excluding a single cascade of these three states. The proposed nonradiative relaxation scheme involves two $A_g$ states. Self-trapping is also discussed and it is concluded that, if present, it cannot be instantaneous. Our data suggest that self-trapping is at best a minor component of the $B_u$ singlet exciton relaxation in polydiacetylene isolated chains.