Ultrafast Relaxation Dynamics from the S_2 State of Malachite Green Studied with Femtosecond Upconversion Spectroscopy

摘要

We have studied the relaxation dynamics in a triphenyl methane (TPM) dye, malachite green (MG), following S_2-state excitation using fluorescence up-conversion measurements in ethanol and ethylene glycol solutions. Information on the mechanism and dynamics of radiationless transitions from higher excited states in TPM dyes is interesting from the torsional dynamic aspects of the phenyl rings and for the applications of TPM dyes involving knowledge of the excited-state relaxation channels. Kinetic measurement at different wavelengths suggested a cascade population relaxation along the S_2 state with a time constant of ～130 fs that is almost independent of solvent viscosity in the two solvents used, contrary to the relaxation dynamics of the S_1 state. Wavelength-dependent time-resolved anisotropy measurements along the S_2 and S_1 emission bands displayed successive reductions in the anisotropy values, indicating a continuous evolution to the S_1 surface. A detailed analysis led us to propose a relaxation pathway along a surface contact of S_2 and S_1 potential surfaces, generally known as conical intersection, which leads to characteristics of mixed vibrational levels of S_2 and S_1. We propose that the conical intersection is promoted by a torsional coordinate of the unsubstituted phenyl ring of the dye, where the S_2 transition energy is localized. The proposal of a conical intersection between the S_2 and S_1 potential surfaces provides first-hand information on such a model in TPM dyes, which is substantially supported by time-resolved anisotropy measurements.