The excited state (S_1) lifetime of a triphenylmethane (TPM) dye, malachite green (MG), the nonradiative decay of which is couple to the diffusive rotation of the phenyl ring, was monitored by means of femtosecond pump-probe spectroscopy in water-alcohol binary solvents. The dynamics observed in the ultrafast time range of ≤3.0ps was not very sensitive to the bulk solvent viscosity, indicating that the phenyl rotation is only influenced by the ambient local solvation structure.
