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 para -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- N, N -diethylamino- N ′-methylstilbazolium tosylate (DEST) derivatives with the ortho - and para -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 para - and ortho -isomers from the time-resolved electronic spectroscopy can be clearly differentiated by the time-resolved vibrational spectroscopy.