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

Abstract

In the photoisomerization path of stilbene, a perpendicular state P on the S potential energy surface is expected just before internal conversion through a conical intersection S/S. For decades the observation of P was thwarted by a short lifetime τ 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 (τ = 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, is attributed to stretching vibrations of the phenyl rings. Transient electronic states, resonance conditions, and corresponding Raman signals are discussed.

Effect of charge-transfer enhancement on the efficiency and rotary mechanism of an oxindole-based molecular motor

“…7 ) and a low S 1 to S 0 oscillator strength. Hence, we assign the SE decay and ESA spectral shifts – observed in the first 400 to 600 fs – to the formation of this perpendicular state, reminiscent of the phantom state described in the isomerisation of stilbene derivatives 80,81 and of the dark state in molecular motors. 60,82 However, despite its pronounced charge-transfer character the perpendicular state of 1 does not display the highly pyramidalised carbon expected for such states.…”

Ab Initio Excited-State Transient Raman Analysis

“…To demonstrate the capability of the transient Raman protocol presented here to realistically capture the temporal evolution of a vibrational mode, we have computed the transient nonresonant Raman signals following a vertical electronic excitation in the trans -stilbene (Figure ) molecule, which has been extensively studied from both the experimental and theoretical vantage. − …”

Designing light-driven rotary molecular motors

“…112,113 Due to this peculiarity, the region near the excited state minimum is usually referred as "dark", a feature shared with stilbenes and stiff-stilbenes. 111,[114][115][116] This state has a relatively long excited state lifetime, and relaxes back to the ground state in the order of 1 ps. 112 However, the exact origin and structure of this dark state remains unclear, especially in the presence of substituents with different electronic features installed on the motor, but it is proposed to possess a perpendicular arrangement of upper half and lower half.…”