Experimental Evidence Steady-State Photolysis

It has been established that the PFR occurs from the lowest singlet excited state, on the basis of the convCTsions and product distributions obtained in photosensitization and quenching experiments [18,19] however, a minor contribution of uppo- triplet excited states has been suggested in some cases [20-22]. 

As regards the proposed caged radical pair model, it is consistent with the results obtained upon photolysis of phenyl acetate in the vapor phase, which gives almost exclusively the escape product (phenol) [10, 11]. Conversely, in rigid matrixes (where the cage effect is enhanced), only radical recombination products are obtained [23]. 

SCHEME 31,2 Inteimolecular PFR between pinacolone andp-trifluoromethylphenol. 

Unambiguous chanical evidence for the involvement of cyclohexadienones as PFR intermediates has been provided by the isolation of acylnaphthalenones in the irradiation of partially blocked naphthyl estas (Schane 31.3) [25,26]. 

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