Femtosecond hot-exciton emission in a ladder-type π-conjugated rigid-polymer nanowire

Abstract

A hot exciton is usually the initial elementary excitation product of the solid phase, particularly in low-dimensional photonic materials, which is a bottleneck to all subsequent processes. Measurement of hot-exciton emission (HExEm) is a great challenge due to fast EK relaxation and thus very weak transient emission. Here, we report the unambiguous observation of femtosecond HExEm from thin films of a model quasi-one-dimensional π-conjugated organic rigid-rod quantum nanowire, methyl-substituted ladder-type poly(para-phenylenes), using femtosecond time-resolved fluorescence spectroscopy. The results show clear HExEm from the cooling hot excitons, having a lifetime of ∼500 to ∼800 fs, and concomitant very weak density-dependent singlet-singlet annihilation (SSA) due to this ultrashort dwell time. The ultrafast dispersive migration of the relaxing excitons toward the bottom of the density of states occurs immediately after HExEm, which is simultaneous to the strong density-dependent SSA effect enhanced by the lengthening dwell time.