Triplet exciton generation, transport and relaxation in isolated polydiacetylene chains: Subpicosecond pump-probe experiments Original Research Article

Triplet states of 3- and 4-BCMU isolated polydiacetylene chains dispersed in their single-crystal monomer matrix are studied by subpicosecond pump-probe experiments. Triplet population is monitored by an intense and narrow TT∗ photoinduced absorption band near 1.35 eV at 20 K. In our experimental conditions, we find that triplets are generated by two different processes: fission of singlets generated by the pump (1-quantum process); or fission of highly excited singlets produced by absorption of pump photons by the singlet exciton (2-quantum process). The lowest triplet lies slightly above half the singlet energy. Thus the final state T∗ of the induced absorption is ≤ 0.1 eV below the known electron-hole generation gap. No clear evidence of self-trapping of the triplet exciton is observed. Triplet population decay is governed by diffusion of triplets on the chain (hopping rate ≈ 1011 s−1) and efficient annihilation of triplet pairs at contact (rate ≈4 × 1011 s−). It is also shown that this TT∗ transition is very strong: one order of magnitude stronger than the ground state to singlet exciton transition.