Mechanisms of Photoisomerization

Recent data on stUbene photoisomerization dynamics have been reviewed in Refs [12-20]. 

In the first step of the direct stilbene photoisomerization, the stUbene molecule is excited to the singlet or triplet state when it absorbs a quantum of energy. After that, solvent molecules undergo reorganization, thus reducing the energy of the system. Next step is the exdted-state decay by two competitive processes radiative (fluorescence or phosphorescene emission) or nonradiative (photochemical) deactivation. 

The foDowtng possible photochemical deactivation channels have been discussed  

Through double-bond twisting leading to trans-cis that consists of an initial emissive t state and subsequently a populated nonemissive p state (which is twisted by 90° about the C=C bond and undergoes internal conversion to give a mixture of trans- and ds-stUbene. 

Linear mechanism that suggests formation of a linear conjugated structure between amino substituent and phenyl group keeping the double bond free for 

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In addition, a 532 (visible) or 355 (UV region) nm laser-induced photoisomerization of allylic alcohols to aldehydes catalyzed by [Fe3(CO)i2] or [Fe(CO)4PPh3] was developed by Fan [176]. In this reaction, key intermediates such as the 7i-allyl hydride species [FeH(CO)3(q -C3H3ROH)] (R = H, Me) were detected by pulsed laser FTIR absorption spectroscopy. These results strongly support the 7i-allyl mechanism of photoisomerization of allyl alcohols. 

Liu, R.S.H., Yang, l.-Y. and Liu, J. (2007) Mechanisms of photoisomerization of polyenes in confined media from organic glasses to protein binding cavities. Photochem. Photobiol., 83, 2—10. 

Triplet state and Z/E isomerization of p-styrylstilbene induced by various sensitizers and the T, potential energy curves and one way photoisomerization (c -> t only) of styryl aromatics " are the subject of two papers on the behaviour of triplets of this class of compound. Time resolved resonance Raman spectra of the triplet state and radical cation of 5-dibenzosuberenol has been used to study and examine the mechanism of photoisomerization of this compound. ... 

Unlike alkenes, the unsaturated azo group in N=N or C=N derivatives possess an inplane lone electron pair in an n-orbital. As a result, both the n,Jt and Tt.Tt excited states can be involved in two limiting mechanisms of photoisomerization.1062 1080 1081 The first mechanism is a 180° rotation (twist) about the former double bond in conjunction with a reduced bond order, similar to that observed in alkenes (Scheme 6.157). In the second case, an in-plctne inversion proceeds due to rehybridization of one nitrogen without... 

The proposed mechanism of photoisomerization involves initial CO bond homolysis of 91 to form a biradical, which can subsequently transform to the 2-(2 -hydroxyphenyl)benzyl alcohol (93), via a spiroketone intermediate. " It was proposed that secondary photolysis of 93 leads to 92 via an initial dehydration reaction to form biphenyl quinone methide 94 ... 

Schuster, D. 1. and Brizzolara, D. F., The mechanism of photoisomerization of cyclohexenones. 10-hydroxymethyl-A > -2-octalone the question of hydrogen abstraction from benzene by ketone triplets, /. Am. Chem. Soc., 92, 4357,1970. 

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