Ground- and excited-state isomerization of triphenylmethane dyes in the femtosecond regime

We previously confirmed the existence of two ground-state isomers of crystal violet (CV) in alcohol (Maruyama, Y.; Ishikawa, M.; Satozono, H. J. Am. Chem. Soc. 1996, 118, 6257). These isomers are called `solvation isomers' because they are differentiated by the solvation of alcohol. Here, to investigate if solvation isomers are possible in triphenylmethane (TPM) dyes other than CV, we measured transient differential absorption (ΔOD) spectra of two classes of TPM dyes: symmetric (ethyl violet and parafuchsin) and asymmetric (malachite green and brilliant green) using a femtosecond spectral hole-burning technique. The symmetric TPM dyes that we studied (including CV) showed two spectral holes in the early part of photobleaching, whereas the asymmetric dyes showed three spectral holes. The ΔOD spectra of all of the TPM dyes depended on the pump wavelength. This dependence is the key experimental observation that confirms the existence of ground-state isomers, and thus the asymmetric TPM dyes have three ground-state isomers. Solvation isomers are possible in TPM dyes in alcohol. To obtain insight into the deactivation mechanism of TPM dyes, we examined a plateau region that appears in the early part of photobleaching recovery. The plateau is evidence for the existence of an intermediate state between the lowest singlet excited state (S1) and the ground state. The absence of the plateau only in parafuchsin, which is due to the difference in a positive charge distribution on a nitrogen atom, suggests that the intermediate state is a distorted excited state in which a positive charge is partially shifted to a nitrogen atom in an amino group.