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 tau(A) = 0.23 ps and tau(B) = 1.2 ps, and they are associated with the fluorescence emitted from different regions of the S-1 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 f(A) = 0.32 and f(B) = 0.21, respectively. It was concluded that the first component is assignable to the fluorescence from the untwisted S-1 PES region where the molecule reaches immediately after the initial elongation of the central C=C bond, while the second component is the fluorescence from the substantially twisted region around a shallow S-1 potential minimum. The quantitative analysis of the femtosecond fluorescence data clearly showed that the whole isomerization process proceeds in the one-photon allowed S-1 state, thereby resolving a recent controversy in quantum chemical calculations about the reactive S-1 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 S-1 molecule still retains a planar Ph-C-C-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.