Exciton-exciton annihilation in copper-phthalocyanine single-crystal nanowires

Femtosecond one-color pump-probe spectroscopy was applied to study exciton dynamics in single-crystal α-phase copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a biexponential decay function with lifetimes of 22 and 204 ps, but the corresponding relative amplitudes depend on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton-exciton annihilation process. Detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, the exciton-exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusion-limited, which possibly involves intrachain exciton diffusion along 1D molecular stacks.