Substituent Effects upon Efficiency of Excited-State Acetophenones Produced on Thermolysis of 3,4-Diaryl-3,4-dimethyl-1,2-dioxetanes

Article abstract of DOI:10.1021/jo00390a033

The effects of meta and para substituents upon the triplet (α_T) and singlet (α_S) efficiencies in the thermolysis of 3,4-diaryl-3,4-dimethyl-1,2-dioxetanes 3, 3-aryl-3,4,4-trimethyl-1,2-dioxetanes 4, and 3-aryl-3-(bromomethyl)-4,4-dimethyl-1,2-dioxetanes 5 are reported.Triplet efficiencies for series 3 dioxetanes, where the two proketone moieties are identical, are sensitive to aryl substituent changes.Attempted correlation of log α_T with Hammett-type substituents constants failed, and the best correlation of log α_T was with the lowest triplet energy of the acetophenones T) = (0.77+/-0.19)E_T1(ArCOCH3)-55.05+/-14.00, r=0.834, S_y*x=+/-0.336>.This type of correlation was previously observed with 3-aryl-3-methyl- and 3-aryl-3-methyl-3,3-(2,2'-biphenyldiyl)-1,2-dioxetanes, where the slopes (S values) are 0.38+/-0.14 and 0.52+/-0.08, respectively.In these two dioxetane series, the triplet energies of the companion proketones (formaldehyde and fluorenone) to ArCOCH3 are approximately equal to and less than E_T1(ArCOCH3).For the 4 and 5 series dioxetanes, there was no change in triplet efficiency with various aryl substituents, where the average α_T values are 29.1+/-2.4percent for 4 and 28.1+/- 0.7percent for 5.For these two dioxetane series, the triplet energy of the companion proketone (acetone) is higher than E_T1(ArCOCH3).With regard to triplet efficiencies, all of these dioxetane series fall into two categories: (i) where the companion proketones possess triplet energies approximately equal to or less than E_T1(ArCOCH3) and where α_T is dependent upon substituent changes in the ArCOCH3 moiety; (ii) where the companion proketone possesses a triplet energy higher than E_T1(ArCOCH3) and α_T is independent of substitution changes in the ArCOCH3 moiety.An exciplex or encounter complex process is proposed to explain the apparent communication between the two proketone moieties in category i dioxetanes.Activation parameters for series 3-5 were typical of most other tetrasubstituted dioxetanes.The ρ values for ?⁺ correlations of series 3 and 4 plus 5 dioxetanes are -0.285 +/- 0.033 and -0.20 +/- 0.05, respectively, which is consistent with a 1,4-dioxy biradical decomposition process.