Quenchers photoisomerization

The direct photoisomerizations are singlet-state reactions, as shown by their independence of oxygen and the addition of triplet quenchers. Ilge et a/.31 103 demonstrated that no potential barriers in the photoisomerizations, EE - EZ, and EZ — ZZ, were found. In contrast, there was a small barrier in the photoisomerization of ZZ - EZ or ZE a veiy weak fluorescence was detected at 77 K, and the fluorescence of the ZZ isomer disappeared at temperatures above 130 K. 

The mechanism of the enaminonitrile - imidazole conversion has been the subject of extensive study, although only a few mechanistic features are known with certainty. The nitrile and the amine must have a cis relationship in the starting material. In the case of diaminomaleonitrile, the initial step would therefore be a photoisomerization to diaminofumaronitrile, via the triplet excited state of the enaminonitrile (equation 22)60. The rearrangement to imidazole, however, is presumed to occur via the singlet excited state, since triplet sensitizers do not promote the reaction and triplet quenchers do not inhibit it56b 59b. 

Coordination compounds are known for their photochemical properties, which may result in photodissociation, photosubstitution, photoisomerization, photoreduction, or photooxidation, for example (3). In addition to inner-sphere rearrangements, the complexes in excited states are susceptible to transfer their energy or charges into other species, such as solvent molecules, ion-pair partners, and other nonbonded quenchers. 

Effects of Quenchers. The cis trans photoisomerization of 1,2-diarylethylenes can be influenced by properties of the medium, nature of the sensitizer (Section III), and also several additives acting as quenchers [192-202], The effect of quenchers on the quantum yields and the photo-stationary trans/cis ratio has been used as a mechanistic probe for exploration of isomerization pathways. Since quenching can occur with singlet as well as with triplet states, measurements on various systems have been performed under direct and sensitized excitation conditions. 

Three methods were applied to investigate the effect of these quenchers on direct photoisomerization ... 

In view of the well-documented ability of Oj to function generally as an acceptor of triplet energy, it had been expected that its presence should also increase ([t]/[c])3 ratios for the triplet sensitized photoisomerization of stilbene-like olefins. However, as was first noted for nitrostilbenes, the quenching of the olefin triplets by O2 does not alter ([t]/[c])s (42,43). For the parent stilbene it was shown that when azulene is used as a quencher of stilbene triplets the slope of the ([t]/[c])s-vs. -[Az] plot is strongly attenuated by O2 see Figure 5 (41,44). Since the intercept of the azulene plot was not influenced by O2, it was reasoned that O2 deactivates p without changing the decay firaction 8 as predicted by Schemes 1 and 2 (44). To account for stilbene triplet deactivation by O2 without change in 8 two possibilities were considered ... 

The potential energy surface of the isomerization is discussed in terms of adiabatic and diabatic processes [1-4,12,18,71]. Two-way isomerization in the triplet manifold without a quencher takes place as a diabatic process by deactivation at p. However, as mentioned above, photochemical cis- trans one-way isomerization in the triplet state proceeds by an adiabatic process where the excited state of a starting material, c, undergoes adiabatic conversion to the excited state of the product, t, followed by either unimolecular deactivation to the, ground state of the product, t, or energy transfer to c to give t and c. The isomerization of 5b also proceeds partly by way of an adiabatic process. Deactivation from t occurs as an adiabatic process, but that from p proceeds as a diabatic process [25]. Therefore, two-way photoisomerization usually takes place as a diabatic process, whereas one-way photoisomerization and isomerization... 

A fiuorescent-photochrome method of quantifying the orientation of solid-phase antibodies immobilized on a silica plate was proposed [9]. The method is based on the measurement of fiuorescence quenching by a quencher in solution, rates of trans-cis photoisomerization and photodestruction of a stilbene-labeled hapten in an antibody binding site. These experimental parameters enable a quantitative description of the order of binding sites of antibodies immobilized on a surface and can be used to characterize the micro viscosity and steric hindrance in the vicinity of the binding site. Furthermore, a theoretical method for the determination of the depth of immersion of the fiuorescent label in a two-phase system was developed [9]. 

Similar photoisomerization of c-2a and f-2a occurs in other solvents such as acetonitrile and ethyl acetate to afford the same PSS mixture as that in benzene. Direct irradiation of p-substituted vinylidenecyclopropanes 2b-d also afforded 1 1 PSS mixtures. The results are summarized in Table 31.1. These photoi-somerizations were not quenched by typical triplet quenchers such as molecular dioxygen, 2-methyl-1,3-butadiene, and cyclohexa-1,3-diene. Therefore, this photoreaction proceeds through a singlet mechanism. 

Cyclooct-2-enone and cyclohept-2-enone undergo Z —> isomerization on irradiation. While E-q cloocten-2-one can be characterized spectroscopically at room temperature, E-cyclohep-2-enone is stable only at temperatures around -ISO C, but can be trapped with dienes in a dark reaction at -78°C. The photoisomerization of these medium-ring cycloalkenones appears to occur much faster than inter-molecular processes, e.g., cycloaddition to alkenes or energy transfer to triplet quenchers. Another seven-membered ring enone, 2,3-benzocyclohepta-2,6-dienone (55) was found to afford trans-fased [4+2]... 

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