Photoisomerization pathways and Raman activity of 1,1'-difluorostilbene.

The photoisomerization of 1,1'-difluorostilbene, following S→S optical excitation in solution, was studied with femtosecond broadband transient absorption and stimulated Raman spectroscopy, and by quantum-chemical calculations. In n-hexane, trans-to-cis (t→c) isomerization starts with Franck-Condon relaxation (τ = 0.07 ps) followed by nearly barrierless torsion around the ethylenic bond (τ ≈ 0.3 ps) to a perpendicular conformation P. About 50% of the excited molecules are trapped in P, while others reach the S(cis) conformation adiabatically. For the opposite cis-to-trans (c→t) path, the dynamics in n-hexane (τ = 0.04 ps, τ = 0.7 ps) suggest a 5 kJ/mol barrier between the relaxed S(cis) and P states. The subsequent P decay with τ = 0.4 ps is followed by much slower ground-state recovery (τ ≈ 3 ps), indicating an intermediate state X. The t→P and c→P torsion depend on solvent viscosity and polarity, whereas the P→X→S relaxation and residual torsion is viscosity-independent but still polarity-dependent. Photoisomerization yields are derived from the transient absorption data and compared to those from actinometric measurements. Low-frequency oscillations in the transient signal are assigned to nuclei motions. Transient and stationary stimulated Raman spectra are compared to calculations. Early Franck-Condon Raman spectra differ from those of the quasistationary trans or cis S state. The photoisomerization behavior of stilbene and vinyl-substituted derivatives is compared and the general features are discussed.