Photoisomerization Dynamics of Stiff-Stilbene in Solution

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• 作者：M. Quick ; F. Berndt ; A. L. Dobryakov ; I. N. Ioffe ; A. A. Granovsky ; C. Knie ; R. Mahrwald ; D. Lenoir ; N. P. Ernsting ; S. A. Kovalenko
• 刊名：Journal of Physical Chemistry B
• 出版年：2014
• 出版时间：February 6, 2014
• 年：2014
• 卷：118
• 期：5
• 页码：1389-1402
• 全文大小：969K
• ISSN：1520-5207

文摘

Photoinduced isomerization of 1,1鈥?bis-indanyliden (stiff-stilbene) in solution was studied with broadband transient absorption and femtosecond Raman spectroscopies, and by quantum-chemical calculations. Trans-to-cis S₁ isomerization proceeds over a 600 and 400 cm^鈥? barrier in n-hexane and acetonitrile, respectively. The reaction develops on multiple time scales with fast (0.3鈥?.4 ps) viscosity-independent and slower (2鈥?6 ps) viscosity-dependent components. In the course of intramolecular torsion (which should be the main reaction coordinate) some excited molecules pass through the perpendicular conformation P and reach the cis geometry, to be temporarily trapped there. Subsequently they relax back to P and further to the ground state S₀. The cis-to-trans isomerization reveals ultrafast (0.06 ps) oscillatory relaxation followed by 13 ps decay in n-hexane and 2 ps decay in acetonitrile, corresponding to barriers of 800 and 400 cm^鈥?, respectively. Raman S₀ and S₁ spectra are reported and discussed. The perpendicular conformation P was not detected, possibly due to its low oscillator strength and short lifetime, or because of strong overlap with hot product spectra. XMCQDPT2 calculations locate a stationary S₁ point on the cis side and two perpendicular-pyramidalized stationary points, to be reached from the former over 300 and 680 cm^鈥? barrier. Implications for parent stilbene are discussed; in this case we also see evidence for the trans-to-cis adiabatic path, as in stiff-stilbene. Very similar viscosity dependence for the two compounds supports the common isomerization pathway: torsion about the central double bond.