Twisted Intramolecular Charge Transfer of Stilbazolium Derivatives Studied by Time-Resolved Vibrational Spectroscopy

Ultrafast intramolecular charge transfer (ICT) often accompanies structural changes in chromophores, such as the rotation of the electron donor or acceptor group. A recent femtosecond stimulated Raman spectroscopy (FSRS) study of a diethylamino stilbazolium derivative featuring a <i>para</i>-methylpyridinium group revealed pronounced structural changes in the styrylpyridinium backbone during ICT. In this work, the detailed structural changes and ICT dynamics of two 4-<i>N</i>,<i>N</i>-diethylamino-<i>N</i>'-methylstilbazolium tosylate (DEST) derivatives with the <i>ortho</i>- and <i>para</i>-methylpyridinium groups were investigated by comparing the experimental results of time-resolved electronic and vibrational spectroscopy. Twisted ICTs of DEST isomers were probed by the skeletal vibrational modes of the styrylpyridinium backbone in the 1500-1650 cm^-1 range from FSRS measurements, where the seemingly identical ICT dynamics of the <i>para</i>- and <i>ortho</i>-isomers from the time-resolved electronic spectroscopy can be clearly differentiated by the time-resolved vibrational spectroscopy.