Photoisomerization pathways and Raman activity of 1,1'-difluorostilbene.
The photoisomerization of 1,1'-difluorostilbene, following S0→S1optical 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 (τ1t= 0.07 ps) followed by nearly barrierless torsion around the ethylenic bond (τ2t≈ 0.3 ps) to a perpendicular conformation P. About 50% of the excited molecules are trapped in P, while others reach the S1(cis) conformation adiabatically. For the opposite cis-to-trans (c→t) path, the dynamics in n-hexane (τ1c= 0.04 ps, τ2c= 0.7 ps) suggest a 5 kJ/mol barrier between the relaxed S1(cis) and P states. The subsequent P decay with τ3= 0.4 ps is followed by much slower ground-state recovery (τ4≈ 3 ps), indicating an intermediate state X. The t→P and c→P torsion depend on solvent viscosity and polarity, whereas the P→X→S0relaxation 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 S1state. The photoisomerization behavior of stilbene and vinyl-substituted derivatives is compared and the general features are discussed.
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany.
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Dobryakov
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany.
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Quick
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany.
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Richter
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany.
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Knie
Department of Chemistry, Lomonosov Moscow State University, Moscow, Russia.
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Ioffe
Firefly Project, Moscow 117593, Russia.
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Granovsky
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany.
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Mahrwald
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany.
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Ernsting
Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, D-12489 Berlin, Germany.
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Kovalenko