Electronic Effects on Triplet and Singlet Excited-State Carbonyl Formation in the Thermolysis 3-Aryl-3-methyl-1,2-dioxetanes

Article abstract of DOI:10.1021/ja00379a021

A series of para- and meta-substituted 3-aryl-3-methyl-1,2-dioxetanes (1) was studied in order to evaluate the electronic effect of substituents on the efficiencies of excited-state carbonyl production.All substituents reduced the efficiency of triplet carbonyl production.Several correlations point to the formation of a triplet carbonyl exciplex, originating from a triplet biradical, in the thermolysis of 1.It also appears that substituent variation in the proacetophenone portion of 1 results in triplet efficiency changes primarily in formaldehyde, which can be rationalized in terms of a triplet exciplex.Substituent effects on singlet (S₁) efficiency are markedly different from those observed with triplet efficiencies.The possibility of heavy-atom effects in 1 was pursued with p-Br and m-Br substituents.No detectable heavy-atom effect was observed with singlet (S₁) efficiencies, but the p-Br substituent appeared to decrease the triplet efficiency.This suggests that a p-Br heavy-atom effect may operate from the triplet exciplex, providing the approximations used in the evaluations of the heavy-atom effect are valid.The effect of substituents on rate of thermolysis of 1 provides further evidence for a biradical mechanism.