Optical isomerism photochemistry

Polarized light absorption orients both isomers of photisomerizahle chromo-phores, and quantified photo-orientation both reveals the symmetrical nature of the isomers photochemical transitions and shows how chromophores move upon isomerization. Photo-orientation theory has matured by merging optics and photochemistry, and it now provides analytical means for powerful characterization of photo-orientation by photoisomerization. In azobenzenes, it was found that the photochemical quantum yields and the rate of the cis—>trans thermal isomerization strongly influence photo-... 

Chromium, tris(hexafluoroacetylacetonato)-optical isomerism, 28 Chromium, tris(l,10-phenaiithroline)-ligand field photochemistry, 398 Chromium, tris(trifluoroacetylacetonato)-isomerism, 28 Chromium complexes geometric isomerism, 11 Chromium (0) complexes magnetic behavior, 273 Chromium(I) complexes magnetic behavior, 272 Chiomium(II) complexes magnetic behavior, 272 spectra, 252 thiocyanate... 

In this chapter, I discuss the phenomenon of molecular orientation induced by photoisomerization whereby experiment merges with theory to assess molecular movement during isomerization. The theory unifies photochemistry with optics, and it provides rigourous analytical tools for powerful quantification of coupled photoisomerization and photo-orientation. Experiments on spectrally distinguishable isomers detail the mechanisms of chromophore reorientation during photo- and thermal isomerization. In particular, I... 

Further studies on the photoisomerization of ci5-cyclohexene and cycloocta-1,3-diene have been reported. Again the work has focused on enantiodiffer-entiation. In this case a series of optically active chiral sensitisers (3) have been used under conditions where solvent and temperature have been varied. Some of the o-disubstituted and tetra substituted amide sensitisers afford mixtures with enantioisomeric excesses of 14%. The influence of pressure and temperature on the asymmetric photochemistry of cyclooctene has been reported. A variety of chiral sensitisers were used. Some of these are shown in (4). Other work has shown that aromatic phosphates, phosphinates and phosphines (e.g. 5-8) can also sensitise the isomerism of cyclooctene. Moderate stationary-state ratios were obtained. 

Linkage isomerizations have also been observed for other small molecules such as N2, N02, NCS, S02, and dimethylsulfoxide (DMSO).15 The isomers are likely participants as intermediates in biorelevant ligand interchange or addition processes, in the NO recombination after photolysis, or even in the route to NO release from the ES in aqueous, room-temperature photochemistry (Section 7.7). Indeed, the photo-switchable nitrosyl compounds are an attractive class of materials with favorable photochromic and photorefractive properties, potentially useful for optical and biomedical applications. 

The (Z)-stilbene (6) does not form a rotaxane with dibenzo-24-crown-8 but, when the stilbene is isomerized (sensitized by benzil) to the E form, a pseudo-rotaxane is formed.Lewis and Crompton have reported that the photoacidity of the stilbene derivatives (7) and (8) is dependent on the position of the hydroxy group on the stilbene, and as a consequence the meta derivative undergoes isomerism.The influence of the nitrogen atoms on the photochemistry of the 1,4-distyrylbenzene analogue (9) and related compounds has been assessed. The photophysics and conformational analysis of the stilbene dimer (10) have been studied. Modelling of the optical properties of the stilbenophane (11) has been reported. 

---