Substituent Effects on Exited-State Efficiencies: Thermolysis of 3,3-(2,2'-Biphenyldiyl)-4-methyl-4-aryl-1,2-dioxetanes

Article abstract of DOI:10.1021/jo00211a003

Triplet and singlet ketone efficiencies was determined for the thermolysis of a series of nine 3,3-(2,2'-biphenyldiyl)-4-methyl-4-aryl-1,2-dioxetanes, where substituent changes were made in the para and meta positions of the aryl group.Triplet efficiencies (α_T), which were determined by trans-stilbene isomerization, ranged from 2.0percent (Ar=p-BrC6H4) to 19.0percent (Ar=C6H5) and the best linear free energy correlation was with E_T1(ArCOCH3):log percent α_T=(0.518 +/- 0.079)E_T1(ArCOCH3)-36.62 +/- 5.73, S_y*x = +/- 0.144, which corresponds to +/- 1.4percent in percent α_T.Since no light enhancement resulted upon addition of 9,10-dibromoanthracene, it was concluded that the intercepted triplet ketone product was fluorenone.The results were considered in terms of a triplet ketone exciplex (Scheme I) and equilibrating solvent caged encounter complexes (Scheme II).The singlet efficiencies for all members in the series were essentially constant (percentα_S1 = 0.17percent).The chemiluminescent (CL) emission spectra of dioxetanes, where Ar = C6H5 and m-BrC6H4, match the fluorescence spectrum of fluorenone.In addition, the CL emission is not enhanced upon nitrogen purging.From a consideration of activation parameters and ρ reaction constants, a 1,4-dioxy biradical decomposition route for these dioxetanes appears most likely.