
Journal of the Chemical Society - Faraday Transactions p. 4027 - 4034 (1993)
Update date:2022-08-11
Topics:
Goerner, Helmut
The photochemical properties of 4-acetylstilbene (AS) and 4-benzoylstilbene (BS) have been studied in various solvents using steady-state irradiation and ns laser flash photolysis.The quantum yields of trans -->/<-- cis photoisomerization (Φt->c, Φc->t) are substantial (>/=0.3) over a wide temperature range, while the quantum yield of fluorescence is small even at -196 deg C (e.g. Φf=0.04).Similar transient absorption spectra, showing maxima at ca. 380 and ca. 415 nm in fluid and frozen media, respectively, were recorded for trans-AS and also for cis-AS.The transient is assigned to the triplet state in either case; its lifetime between 25 and -196 deg C varies by a factor of ca. 1E5.These two virtually identical T-T absorption spectra are suggested to originate from the trans (3t*) and the cis (3c*) triplet state, respectively, since at -196 deg C intersystem crossing occurs without significant configurational changes.Apart from small spectral differences BS behaves similarly.Quenching measurements by ferrocene of Φt->c, in comparison with those of the triplet decay, support that the triplet state is intermediate in the trans --> cis photoisomerization pathway.The spectral and kinetic features of the triplet state are interpreted on the basis of the intermediacy of the twisting step from 3c* to the perpendicular conformation (3p*) in the cis --> trans photoisomerization pathway, the 3t* -->/<-- 3p* equilibrium and decay to the ground states via the 3p* --> 1p intersystem-crossing step.
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