Polyene isomerization, excited state

In addition, the results indicated that the efficiency of cis —> trans increased as the initial cis double bond configuration is shifted from the center of the polyenic chain, consistent with the 7j, triplet excited state potential curve that has a very shallow minimum at the 15-cis position compared to the deep minima at the all-trans position. The results strongly suggest that isomerization takes place via the 7j state of the carotenoid even in the case of direct photoexcitation, with their photosensitized process because of the very low intersystem crossing quantum yield, isc ([Pg.246]

Pulse radiolysis is used also for preparation of excited states of dienes and polyenes. This is done by irradiation of the diene/polyene in toluene solution. The radiolysis of toluene yield high concentration of molecules in the triplet excited state of the solute. Wilbrandt and coworkers61 pulse-radiolysed 1 mM solution of al I -lrans-1,3,5-heptatriene in toluene solution and observed the absorption spectra of the triplet state of the heptatriene with a maximum at 315 nm. The same group62 produced and measured the absorption spectra of several isomeric retinals in their lowest excited triplet state by pulse irradiation of their dilute solution in Ar-saturated benzene containing 10 2 M naphthalene. Nakabayashi and coworkers63 prepared the lowest triplet states of 1,3-cyclohexadiene,... 

The process entails shifting of double bonds along the polyene chain, with the formation of a "retro-retinal" structure. Peters et al. (301) interpreted their observations by identifying PBAT with an excited state of rhodopsin, where single proton transfer toward the Schiff base nitrogen leads to the formation of bathorhodopsin. This approach has been supported by the theoretical interpretation of the spectrum of rhodopsin in terms of a nonprotonated Schiff base (214-216). A mechanism involving deprotonation of the Schiff base has also been suggested (310). All these models do not require cis-trans isomerization as a primary event in the chromo-phore. 

The photoisomerization of several indole derivatives has been studied while isomerization around the C=N double bonds of N-methoxy-l-(l-pyrene)meth-imine has been described. In certain cases, the reversibility of the isomerization step depends on the spin multiplicity of the reacting state." Several reports have been concerned with the photophysical properties of cyanine dyes promoted into upper excited states. Indeed, for a series of cyanine dyes having different polyene backbones it was possible to establish that isomerization takes place... 

Z-E isomerization via simple geometric inversion (one-bond flip, OBF, Fig. 2.3A) involves the torsional relaxation of the perpendicular excited state via an adiabatic mechanism which implies a non-volume-conserving process. This is not compatible with the ultrafast CTI in polyenes, in particular retinyl chromophores, and two other possible ways of photo-CTI have been proposed over the past 15 years [11]. 

Polyenes (15-30) shown in Exhibit 4 exhibits an interesting aspect of photoisomerization. They undergo solvent polarity dependent trans-cis isomerization [76-87], In a nonpolar solvent like hexane, photoisomerization is found to be restricted to trisubstituted double bond (formation of 13-cis, 9-cis) isomers in vitamin A series, whereas in a polar solvent like MeOH or EtOH or acetonitrile, both trisubstituted and disubstituted double bonds (formation of 7-cis, 11-cis, 9-cis, and 13-cis) undergo isomerization. It is clear from these studies that solvent polarity plays a role in interacting with the singlet excited state of these polyenes... 

The idea that the various more or less planar conformers of a conjugated trlene—or of a polyene in general—upon n - n excitation do not interconvert during their short (singlet) excited-state lifetime arose in the early days of the study of the photoisomerization reactions in the vitamin D field (14, 63). Its introduction served the purpose of rationalizing the values of the quantum yields of isomerization of prevltamln D and tachysterol into their primary photoproducts. 

B. Valence isomerization. The conjugated polyenes in their excited biradical state can lead to intramolecular cyclization in a number of ways giving rise to a number of products. In such cyclization, valence bonds are reorganized without migration of atoms or groups but by migration of c or it electrons only. For example, on excitation of pentadiene, the following products are observed ... 

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