Perpendicular State of an Electronically Excited Stilbene: Observation by Femtosecond-Stimulated Raman Spectroscopy

In the photoisomerization path of stilbene, a perpendicular state P on the S1 potential energy surface is expected just before internal conversion through a conical intersection S1/S0. For decades the observation of P was thwarted by a short lifetime τP in combination with slow population flow over a barrier. But these limitations can be overcome by ethylenic substitution. Following optical excitation of trans-1,1′-dicyanostilbene, P is populated significantly (τP = 27 ps in n-hexane) and monitored by an exited-state absorption band at 370 nm. Here we report stimulated Raman lines of P. The strongest, at 1558 cm–1, is attributed to stretching vibrations of the phenyl rings. Transient electronic states, resonance conditions, and corresponding Raman signals are discussed.