Reversible photoisomerisations

Reversible photoisomerisation [23] numbers among the readiest reactions in photochemistry. The presence of a photoswitchable moiety is a prerequisite for this reaction. Since photoisomerisation commands comparable interest in physics and chemistry - as well as biology - it will be considered here at the conclusion of the photophysical chapter and before the beginning of the following chapter on chemical reactions. 

A new procedure has been described which enables the quantum yields of a reversible photoisomerisation to be determined. ... 

Besides the reversible photoisomerisation with a consecutive thermal step according to Section 2.1,4.4 and summarised in Table 2.1 a large number of other reactions exist. Just a few mechanisms are discussed in the following. The differential equations for the changes of the concentrations with time can be given in all cases. 

In case of a reversible photoisomerisation without a thermal back-reaction one can define A to be the trans and B the cis form... 

Fig. 18 (a) Chemical structures of trans- and c -azobenzene derivatives which undergo reversible photoisomerisation, (b) Putative self-assemblies only nanofibres generated from the trans-isomer aligned in a sound-induced fluid flow. Reproduced with permission from [90]. Copyright 2014 The Royal Society of Chemistry... 

Willner et al. also investigated the reversible photoisomerisation of trans-quinone (6-hydroxynaphthacene-5,12-dione) to the ana-quinone isomer state in a mixed monolayer [213]. The resulting densely packed monolayer includes the trans-quinone in a rigidified configuration (Fig. 40). When irradiated with light between 320 and 380 nm the frans-quinone isomerises to ana-quinone, which is electrochemically inactive but which can be removed by reaction with primary amines. 

The photoisomerisable nitrospiropyran flavoenzyme glucose oxidase, SP-GOD, was assembled as a monolayer onto a gold surface. The mono-layer revealed reversible photoisomerisable properties and illumination of... 

The light-induced reversible trans-cis photoisomerisation of franv-stilbene molecule in condensed media includes at least four macroscopic stages excitation of the stilbene chromophore, radiative deactivation of the excited state with the rate constant kr, medium relaxation around the excited stilbene molecule with the rate constant km (to provide space for torsional distortion during the photoisomerisation process) and eventually, twisting transition with the rate constant kt c. 

An intramolecular charge transfer excited state may be involved, and the inverse relationship between fluorescence and photoisomerisation suggests a singlet state mechanism. Irradiation of (Z)-urocanic acid, 3-(lH-imidazoI-4-yl)prop-2-enoic acid, in the presence of nitro blue tetrazolium and sodium azide promotes its photoisomerisation in a process which involves reversible addition of the azidyl radical to the double bond. ... 

In Section 5.6.1.1 quantitative results for the partial photochemical quantum yields of the photoreaction of stilbene-1 were given. A photoisomerisation was assumed to be the mechanism as a first step. This proposal is supported by a reaction chromatogram of the photodegradation reaction of this laser dye. The two parts of the diagram (see Fig. 5.58) prove a photo-reversible isomerisation as a first step [190]. This information was used for the evaluation mentioned. 

Makushenko AM, Neporent BS, Stolbova OV. 1971. Reversible orientational photodi chroism and photoisomerisation of aromatic azo compounds. I. The model of the system and II. Azobenzene and substituted azobenzene derivatives. Opt Spectr 31 295 299 and 741 748. 

The photochromic complex [Ru(bpy)2(pySO)] (pySO = 2-(isopropyl sulfinylmethyl)pyridine) undergoes photo reversible S- O linkage isomerisations blue light irradiation promotes S->0 isomerisation, which can be reversed by illumination with green light. Time constants of the process are solvent-dependent. The photoisomerisation of two... 

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