Photoisomerisation

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. 

Photoisomerisations of azobenzene units have been found to proceed particularly clearly and to be well suited for experimental studies. The thermodynamically more stable E-isomer can be transformed photophysically into the Z-form, which can return to the E-form on photoexcitation or thermal treatment (Fig. 5.20). 

The isomers exhibit different absorption spectra The E-form displays an intense 7i7i band in the near UV range and a broad, symmetry-forbidden n7i band in the visible region. On isomerisation to the Z-form, the nn band is shifted to shorter wavelengths, with a concomitant increase of the nn band. The E- Z isomerisation engenders a pronounced structural change of the molecule, which is manifested in an increase of dipole moment (see also Section 8.8.1). 

The efficiency of the E/Z photoreaction - and especially the speed of the thermal Z/E reverse reaction - is strongly dependent upon the electronic and steric influence of the substituents on the arene units. 7r-Donors in para position accelerate it 7r-acceplors inhibit it push-pull substituents are particularly effective in lowering the barrier. 

If the periphery of a POPAM dendrimer bears azobenzene units then dye molecules can be included as guests in the dendrimer scaffold (see also host/ guest chemistry in Section 6.2.3) [23]. Here the E- and Z-isomers (or their enriched versions) differ in their capacity for accommodating guest molecules. In principle, guest molecule inclusion can be controlled (switched) in this way (Fig. 5.21). 

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Schroeder J 1997 Picosecond kinetics of trans-cis photoisomerisations from ]et-cooled molecules to compressed solutions Ber. Bunsenges Phys. Chem. 101 643... 

Mechanistic aspects of the photosubstitution and photoisomerisation reactions of d6-metal complexes. P. C. Ford, D. Wink and J. Dibenedetto, Prog. Inorg. Chem., 1983.30, 213-271 (182). 

We have also attempted to study the reactions of the tetrafluoro-o-phenylene di-radical (32) with benzene, by carrying out the photolysis of 1,2-di-iodotetrafluorobenzene in the presence of benzene. It is known that 1,2-di-iodoarenes give arynes on photolysis 70-72>. The only product derived immediately from o-dehydrotetrafluorobenzene was the 1,4-cyclo-adduct (24) which was, as expected, partially photoisomerised to (27) and a trace of (26) 73>. The photolysis resulted in the appearance of (24) and (27) before (26) was detected and hence we conclude that (26) was not a primary product. 

Frederick JH, Fujiwara Y, Penn JH, Yoshihara K, Petek H (1991) Models for stilbene photoisomerisation experimental and theoretical studies of the excited-states dynamics of 1,2-diphenylcycloalkenes. J Phys Chem 95 2845... 

After absorbing a photon, the 11-cis-retinal undergoes photoisomerisation into its geometric isomer all-trans-retinal (2) (Figure 12.3). 

Because of this photoisomerisation, structural changes occur within the confines of the binding cavity, which in turn produce changes in the opsin and the attached cell-membrane protein. This results in functional changes to the cell membrane, culminating in generation of a signal impulse which is sent to the brain. 

One alternative approach is to use photoisomerisable chiral compounds where the E and Z isomers have different helical twisting powers, e.g. menthone derivatives. By incorporating co-polymers, prepared from menthone containing monomers and cyano esters (5.5), as dopants into nematic LC mixtures materials, e.g. a mixture of cyanobiphenyls and cyanoterphenyls (E7 available from Merck), colour change can be effected by irradiating with UV light (365 nm). The colour obtained is dependent... 

Fig. 8. Schematic representation of the potential surfaces leading to photoisomerisation of (BQA)PtMe2I from mer to fac isomer via a sloped conical intersection at / -like geometries. Shown to the right are the branching space vectors the gradient difference (gd=x1), and the derivative coupling (dc=x2). The primary orbitals involved in the electronic transition are shown to the left [Adapted from Ref. (110) with permission].

The temperature dependence of the photoisomerisation can be rationalised in terms of the scheme shown in Figure 3. 

At very low temperatures, photoisomerisation becomes negligible, and the fluorescence intensity is proportional to... 

The stilbene carbon unit has also been peripherally bound to POPAM cores. Although it does not strictly belong to the hydrocarbon dendrimers, the formula of a G2 dendrimer of this type (Fig. 4.21) is depicted here as an example. It was obtained by alkylation of the corresponding eightfold mono-sulphonamide with 4-(bromomethyl)stilbene. Its fluorescence, E/Z isomerisation, photoisomerisation (see Section 5.2.2), and excimer formation were compared with those of non-dendritic stilbenes. The quantum yields of photoisomerisation (0.30) and fluorescence of the E isomer (0.014) of the dendrimer proved to be substantially lower [38]. 

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