Application of the transient grating method to the investigation of the photo-thermalization process of malachite green in room temperature ionic liquids

Transient grating spectroscopy has been applied to the investigation of the photo-thermalization process of malachite green (MG) in several imidazolium-based room temperature ionic liquids (RTILs). MG was simultaneously photo-excited to the S2 state by two sub-picosecond laser pulses (390 nm) with an angle of 30°, and the time profile of the transient grating due to the population and energy dynamics of MG was monitored by the diffraction of a probe pulse (780 nm). The population grating signal in RTILs showed a fast decay component (ca. 4 ps). After the decay of the population signal, there appeared an acoustic oscillation signal mainly due to the heat dissipation from the photo-excited MG molecule. In the highly viscous ILs such as 1-butyl-3-methylimidazolium hexafluorophosphate, the TG signal showed an additional slower rise component, which cannot be explained by the fast heat dissipation process. The signal was analyzed in terms of the structure relaxation of RTILs, and the sound velocities at the high frequency have been determined.