Phosphorescence photoisomerization

Biacetyl sensitized photoisomerization of l,2-di-9-anthrylethane 7a does not lead to the 4n + 4n cyclomer 8a but yields exclusively the An + 2n cycloadduct 26 with a quantum yield of 0.1 [72]. Since the phosphorescence of biacetyl is quenched by dianthrylethane 7a at nearly diffusion controlled rate, the photochemical Diels-Alder reaction is explicable by triplet energy transfer from biacetyl to 7a. The photochemical isomerization of 10-benzoyl-l,2-di-9-anthrylethane 27 also proceeds exclusively by An + 2n cycloaddition and gives cycloadduct 28 with a quantum yield of 0.005 [73], The low fluorescence quantum yield of 27 (excited triplet state. Biacetyl sensitization of 27 leads to 28... 

Interestingly, the photoisomerizing styrylpyridine and l,2-di(4-pyridyl)-ethylene (dpe) rhenium complexes behave as luminescence switches [123-125], This phenomenon originates in the different nature of the lowest excited state of the trans and cis isomers, TI.(F) and MLCT, respectively. Near-UV irradiation converts the nonemissive trans isomer to the cis form, which shows strong phosphorescence from its 3MLCT state. Emission can be switched off by UV (254 rnn) light that induces a reverse cis—>trans isomerization. 

The Ti state (but not the Si state) of toluene sensitizes the photoisomerization of both ( )- and (Z)-2-heptene. Direct irradiation of either isomer of 1-phenyl-2-butene leads to photoisomerization. The fluorescence spectrum of l-phenyl-2-butene is simitar in shape to that of toluene but is slightly reduced in intensity. However, phosphorescence from 1-pheny 1-2-butene was not detected under conditions in which phosphoresence from toluene could readily be observed. Propose an explanation for the photoisomerization of l-phenyl-2-butene upon direct irradiation that is consistent with these observations. 

The role of the triplet state in the cis-trans isomerization of stilbenes effected by photosensitizers, such as acetophenone, benzophenone, or anthraquinone, which have large So Ti excitation energies, was first revealed in Ref. [65]. Theoretical considerations and experimental data on intermolecular triplet-triplet energy transfer leading to the sensitized stilbene photoisomerization are described in Section 4.2.2. It was shown that data on positional dependence of the heavy-atom effect on the cis-trans photoisomerization of bromostilbenes were consistent with the fact that, in contrast to the para position, the meta position is near a node in the highest occupied and the lowest unoccupied MO of stilbene [66]. According to [67], internal and external heavy-atom effects induce phosphorescence in frans-stilbene... 

Experimental results gave the evidence of an efficient nitroxide radical inhibitor effect upon the sensitized cis-trans photoisomerization of DMAAS by quenching the sensitizer triplet state. The phosphorescence triplet lifetime of erythrosin B in PPDC membranes was measured to be 3 x 10 s. The experimental quenching rate constant of the cascade reaction kq and the rate constant of the triplet-triplet energy transfer kx evaluated in 2D terms were obtained as kq = (1.05 0.08) X 10 cm /(mols) and kx = (1.26 0.21) x 10 cm /(mols). Taking into consideration the efficiency of the triplet-triplet energy transfer y = 3.5 %, the rate constant of encounters between the sensitizer and the photochrome was found as ke = 3.4x 10 cm /(mols). 

In the frame of CSTPM, the following dynamics parameters of the cascade system components can be experimentally measured the spin label rotation correlation time and spin relaxation parameters, the fluorescence and phosphorescence polarization correlation times, the singlet and triplet state quenching rate constants, the rate constant of photoisomerization, and the rate constant of the triplet-triplet energy transfer. This set of parameters is a cumulative characteristic of the dynamic state of biomembranes in the wide range of the probes amplitude and characteristic time. 

Studies of electronic structures in photoisomerization and photodimerization of cinnamic acid showed that phosphorescence of cinnamic groups occurs at about 20,000 cm Also, it was demonstrated when photosensitizers are present, the critical distance between donor, sensitizer, and acceptor molecules (cinnamic acid) is about 10 angstroms.Although all the details of incipient photocrosslinking of poly(vinyl cinnamate) have still not been fully worked out, most accept that all three mechanisms take place. These are dimerizations to truxillic and truxinic acid type structures and polymerizations through the double bonds. The excited states of the molecules can be produced by direct irradiation and also through intersystem crossing from an appropriate photosensitizer. ... 

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