Triplet state isomerizations biacetyl sensitized

At least four applications of this technique can be cited. Quantum yields for triplet formation in benzene108 and fluorobenzene109 have been estimated by comparing the phosphorescence yields of biacetyl produced by sensitization to that produced by direct irradiation. Intersystem crossing yields of a number of organic molecules in solution have been obtained by measuring the quantum yield with which they photosensitize the cis-trans isomerization of piperylene (1,3-pentadiene) and other olefins.110 As will be discussed later, the triplet states of... 

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... 

In order to increase the quantum yield of cis -> trans isomerization, it is proposed to use aromatic hydrocarbons such as anthracene as a carrier of the quantum chain process. (Z,E)-(cis-18) and (E,E)-l,4-di(3,5-di-tert-butylstyryl) benzene (trans-18) underwent mutual isomerization on biacetyl sensitization with a quantum chain process to afford a 4>c >t value of 1.2-1.3 in a cis isomer concentration of 1 X 10 M [96]. The quantum chain process proceeds through the energy transfer from the trans triplet (H ) to the ground state cis isomer ( c). On addition of anthracene (AN) as a quencher or a carrier of the excited state, the quantum... 

On biacetyl sensitization 22 undergoes adiabatic one-way ZZ ZE and ZZ - EE isomerizations in the triplet manifold (Scheme 8) [98]. The ZE isomer also isomerizes to EE. The quantum yield of ZE - EE isomerization (0ze->ee) nd the photostationary state isomer ratios, ([EE]/[ZE])p , increase with increasing diene concentration. The ze->ee value is about 14 at... 

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