Photoisomerization dynamics of azobenzene in solution with S1 excitation: A femtosecond fluorescence anisotropy study

Chih Wei Chang, Ying Chih Lu, Tsai Te Wang, Eric Wei Guang Diau

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Abstract

Measurements of anisotropy of femtosecond fluorescence after direct excitation of the S1(n,π*) state of azobenzene in hexane and ethylene glycol solutions have been carried out to address the controversy about inversion and rotation in the mechanism of photoisomerization. The observed anisotropies in hexane decay to a nonzero asymptotic level with a relaxation period the same as that for slow decay of the corresponding biexponential transient; this effect is attributed to involvement of the out-of-plane CNNC-torsional motion on approach to a twisted conical intersection along the "rotation channel" that depolarizes the original in-plane transition moment. In contrast, when the rotational channel becomes substantially hindered in ethylene glycol, the anisotropies show no discernible decay feature, but the corresponding transients show prominent decays attributed to involvement of in-plane symmetric motions; the latter approach a planar-sloped conical intersection along a "concerted inversion channel" for efficient internal conversion through vibronic coupling. The proposed mechanism is consistent with theoretical calculations and rationalizes both results on quantum yields and ultrafast observations.

Original languageEnglish
Pages (from-to)10109-10118
Number of pages10
JournalJournal of the American Chemical Society
Volume126
Issue number32
DOIs
Publication statusPublished - 2004 Aug 18

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All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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