Photo-excitation dynamics of Phenol Blue

We measured transient absorption spectra after the photo-excitation of Phenol Blue in various liquids using a sub-picosecond dye laser. The ground state bleach signal shows a recovery mainly with two different time constants: the faster one is about 0.5 ps, and the slower one more than 10 ps. Both time constants depend on the solvents. We assigned the faster time constant to the lifetime of the excited state. The slower time constant can be understood by the ground state dynamics due to solvation, vibrational cooling and/or structural relaxation. The relationship between the back electron transfer rate and the reaction free energy estimated by analysis of the absorption spectrum is presented.