Femtosecond fluorescence study of the reaction pathways and nature of the reactive S1 state of cis-stilbene

We report a femtosecond time-resolved fluorescence study of cis-stilbene, a prototypical molecule showing ultrafast olefinic photoisomerization and photocyclization. The time-resolved fluorescence signals were measured in a nonpolar solvent over a wide ultraviolet-visible region with excitation at 270 nm. The time-resolved fluorescence traces exhibit non-single exponential decays which are well fit with bi-exponential functions with time constants of τA = 0.23 ps and τB = 1.2 ps, and they are associated with the fluorescence emitted from different regions of the S1 potential energy surface (PES) in the course of the structural change. Quantitative analysis revealed that the two fluorescent components exhibit similar intrinsic time-resolved spectra extending from 320 nm to 700 nm with the (fluorescence) oscillator strength of fA = 0.32 and fB = 0.21, respectively. It was concluded that the first component is assignable to the fluorescence from the untwisted S1 PES region where the molecule reaches immediately after the initial elongation of the central CC bond, while the second component is the fluorescence from the substantially twisted region around a shallow S1 potential minimum. The quantitative analysis of the femtosecond fluorescence data clearly showed that the whole isomerization process proceeds in the one-photon allowed S1 state, thereby resolving a recent controversy in quantum chemical calculations about the reactive S1 state. In addition, the evaluated oscillator strengths suggest that the population branching into the isomerization/cyclization pathways occurs in a very early stage when the S1 molecule still retains a planar Ph–CC–Ph skeletal structure. On the basis of the results obtained, we discuss the dynamics and mechanism of the isomerization/cyclization reactions of cis-stilbene, as well as the electronic structure of the reaction precursor.