Femtosecond isomerization of crystal violet in alcohols

The temporal rise of the spectral bleaching of crystal violet (CV) in alcohol solutions was studied using femtosecond pump-probe spectroscopy with tunable pump pulses. From the pump-wavelength dependence of transient differential absorption (Delta OD) spectra, we concluded that CV has two ground states. The isomerization rate, not dependent on solvent viscosity, was similar to 500 fs at 295 K. This shows that the structural difference between the isomers is not as large as a phenyl ring rotation, which has been regarded as the origin of the ground-state isomers. We proposed a ground-state model of CV, in which the molecular structure of CV has D-3 or C-3 symmetry, based on molecular orbital calculations. Our model explains not only our findings but also almost all of the previous findings that seemed to conflict with each other over the last 50 years.