Optical Nonlinearities and Molecular Kinetics of Hypocrellin A

Abstract

Optical nonlinearities (including nonlinear absorption and nonlinear refraction) and the temporal properties of the nonlinear refraction of hypocrellin A (HA) in its resonant region have been investigated by using single-beam Z-scan and time-resolved two-color Z-scan techniques with picosecond pulses. The reverse saturable absorption effect can be attributed to the efficient absorption of excited states of both the normal and tautomeric forms, S1 and S1‘, with the contribution of S1 as the dominant mechanism. The change of the refractive index of HA increases with increasing irradiance I0 at low irradiances (<5 GW/cm2), while it hardly changes with I0 at high irradiances. The explanation for such an experimental phenomenon is that the averaged population densities of both S1 and S1‘ increase with I0 at low irradiance, while those of these two states become saturated at high irradiance. The refractive nonlinearities exhibit a very slowly decaying tail, which is attributed to the contributions of S1, S1‘, and the triplet state of the normal form, T1, and is dependent on the rise times and lifetimes of these intermediate states. The kinetic model for HA allows for the determination of the absorption cross sections of the ground state of the normal form of HA S0, the ground state of its tautomer S0‘, the excited state of its tautomer S1‘, and the refractive nonlinearities of S1, S1‘, and T1. The close match between the theory and experiment not only shows the completeness of the five-level kinetic model but also demonstrates the close relationship between the optical nonlinearities of HA and the dynamics involved in excited-state intramolecular proton transfer.