Quantum photoisomerisation

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]. 

These processes have poor quantum yields. As in photoisomerisation processes, the reactions are generally driven only by short-wavelength radiation, and cannot efficiently convert AMI solar radiation. 

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

The number of such reactions is large. The differential equations for the changes of the concentrations with time can be given in general. In the following example a consecutive reaction which contains two photoisomerisation steps (forward, backward) is followed by a consecutive thermal step. This reaction can be chosen to demonstrate the consequence of the definition of partial photochemical quantum yields (see Section 2.1.2.2), the linear dependencies, and the influence of the thermal reaction ... 

Mechanisms and photochemical quantum yields of the simple photoisomerisation in dependence on the photophysical pathway... 

As in the case of photoisomerisations, the reaction can proceed either via the singlet or the triplet intermediate. The component B is part of the reaction in contrast to the sensitisation mechanism discussed above. In addition both pathways via the singlet or triplet state can be quenched. The photophysical steps are summarised in Table 3.2 and the reaction scheme is derived in Appendix 6.6.1.1. In the following examples this information is used to derive the time laws and the quantum yields for 4 types of photoaddition reactions according to either the normal addition reaction... 

The quantum yields of the two linear dependent steps for the photoisomerisation are given in eq. (2.21). 

In the physically sensitised photoisomerisation treated in Section 3.1.1.4 the quantum yield is given according to eq. (3.6) by... 

The photoisomerisation of azobenzene in methanolic solution is a preferable photoreaction to be used in actinometry, since the mechanism has been well examined. The trans-cis photoreaction is photoreversible. The thermal reaction cis trans will not disturb photokinetics at normal conditions because of a half-life of approximately 1 week. Therefore a concentrated solution of trans-azobenzene in methanol at 6.4 x 1(H mol h, which totally absorbs radiation between 345 and 240 nm, can be used taking eq. (5.107) and the approximation given by eq. (5.109). In a first approximation the change in absorbance with time at a wavelength of observation is proportional to the intensity of radiation. This proportionality includes the photochemical quantum yields of the trans cis isomerisation step, the factor 1000 and the absorption coefficients at the observation wavelengths of the trans and cis isomers. 

Formal integration was also applied to the determination of the dependence of the quantum yields of the photoisomerisation of previtamin D3 on the wavelength of irradiation [173]. A more simple and approximate approach is given for i otochromic glasses [174]. 

It can be proved that the refractive index of the mixtures depends linearly on the percentage of ethylene glycol. In the S-1 photoreaction both the photoisomerisation steps strongly depend on the absolute viscosity, whereas 3 is very small and stays nearly constant during the photoreaction. Since the determined values of quantum yield fit even for different mixtures of water/ethylene glycol with viscosity adjusted to the same value by variation of temperature (see Fig. 5.53), polarity effects of the solvent will influence the photoreaction less than viscosity effects [178]. 

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. 

This review summarises and discusses the advances of computational photochemistry in 2012 and 2013 in both methodology and applications fields. The methodological developments of models and tools used to study and simulate non-adiabatic processes are highlighted. These developments can be summarised as assessment studies, new methods to locate conical intersections, tools for representation, interpretation and visualisation, new computational approaches and studies introducing simpler models to rationalise the quantum dynamics near and in the conical intersection. The applied works on the topics of photodissociation, photostability, photoisomerisations, proton/charge transfer, chemiluminescence and bioluminescence are summarised, and some illustrative examples of studies are analysed in more detail, particularly with reference to photostability and chemi/ bioluminescence. In addition, theoretical studies analysing solvent effects are also considered. We finish this review with conclusions and an outlook on the future. 

---