Ligand isomerization, photo

Allosteric control of a ligand-gated ion channel75 and light-controlled expulsion of molecules from mesostructured silica nanoparticles,76 both based on the photo-induced isomerization of azobenzene, have also been reported. 

Transition metal catalysts can also be used in photochemical organic synthesis. For example, the photo-Bergman reaction (cycloaromatization see also Scheme 6.62) of (Z)-hex-3-en-l,5-diyne [(Z)-589], which is obtained by photostationary E Z isomerization (Section 6.1.1) from ( )-589, occurs via transition metal-catalysed cycloaromatization reaction followed by photochemical dissociation of the arene ligand from the complex 590 (Scheme 6.289).1530 The product, 1,2-di( -propyl)benzene (591), is obtained in 91% chemical yield. 

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. 

Mechanistic photochemistry of metal )8-diketonate complexes has received increasing attention. Near-ultraviolet irradiation of diketonate complexes of Mn ", Fe , Co ", No" and Cu" eventuates in one-electron reduction by the ligand of the metal center whereas Cr " complexes photo-isomerize. Irradiation of franj-Rh(CF3COCHCOMe)3 appears to lead to two photoactive excited states which may have significant radical character. Photoisomerization to the cis form occurs in inert solvents whereas decomposition takes place in the presence of potential hydrogen atom donors such as alcohols. ... 

Figure 13.25. Photo and thermal isomerization of azobenzene ligands in the photoresponsive aerogel, (a) As-prepared, (b) after UV irradiation of sample (a) for 40 min, (c) after room light exposure of sample (b) for 60 min, (d) after heating sample (b) to 100°C for 10 min.

---