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 τ(A) = 0.23 ps and τ(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[double bond, length as m-dash]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[double bond, length as m-dash]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.