Excited-state torsional dynamics of c i s-stilbene from resonance Raman intensities

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Abstract
Resonance Raman spectra of cis‐stilbene have been obtained using excitation at 356, 299, 282, and 266 nm. Intense Raman lines at 403 and 560 cm−1 are observed in the frequency region characteristic of double bond torsions. Resonance spectra of trans‐stilbene show no Raman lines of comparable intensity which can be assigned to double bond torsions. These results suggest that electronically excited cis‐stilbene twists about its central double bond much more rapidly than does the trans isomer. To quantitate these observations, the Raman lines of cis‐stilbene at 165, 403, 560, and 963 cm−1 are assigned to specific out‐of‐plane normal modes by comparison with QCFF/PI calculations. We then model the excited state surface as a linear dissociative potential along the 560 cm−1 double bond torsion and a harmonic potential along all other modes. The experimental Raman intensities are used to determine the slope of the excited state torsional potential and the excited state origin displacements in the harmonic modes. Wave packet propagation on this excited state surface gives a 25° torsion about the central double bond of cis‐stilbene in only 20 fs. The differences in excited state torsional dynamics between cis‐ and trans‐stilbene are attributed to ground state conformational distortions due to the severe steric hindrance in the cis isomer.