Triplet pathways in the cis⇆trans photoisomerization of 4-acetyl-and 4-benzoyl-stilbene

Abstract

The photochemical properties of 4-acetylstilbene (AS) and 4-benzoylstilbene (BS) have been studied in various solvents using steady-state irradiation and ns laser flash photolysis. The quantum yields of trans ⇄ cis photoisomerization (Φ_{t → c}, Φ_{c → t}) are substantial (0.3) over a wide temperature range, while the quantum yield of fluorescence is small even at –196 °C (e.g.Φ_f⩽ 0.04). Similar transient absorption spectra, showing maxima at ca. 380 and ca. 415 nm in fluid and frozen media, respectively, were recorded for trans-AS and also for cis-AS. The transient is assigned to the triplet state in either case; its lifetime between 25 and –196 °C varies by a factor of ca. 10⁵. These two virtually identical T–T absorption spectra are suggested to originate from the trans(³t*) and the cis(³c*) triplet state, respectively, since at –196 °C intersystem crossing occurs without significant configurational changes. Apart from small spectral differences BS behaves similarly. Quenching measurements by ferrocene of Φ_{t → c}, in comparison with those of the triplet decay, support that the triplet state is intermediate in the trans → cis photoisomerization pathway. The spectral and kinetic features of the triplet state are interpreted on the basis of the intermediacy of the twisting step from ³c* to the perpendicular conformation (³p*) in the cis → trans photoisomerization pathway, the ³t*⇄³p* equilibrium and decay to the ground states via the ³p*→¹p intersystem-crossing step.