Chemiluminescence of 1,1′-biisoquinolinium and 2,2′-biquinolinium salts. Reactions of electron-rich olefins with molecular oxygen

Abstract

2,2′-Dialkyl-1,1′-biisoquinolylidenes (BIQ), having been prepared by two-electron reduction of the corresponding diquaternary salts BIQ²⁺(X^–)₂, react with triplet dioxygen to produce chemiluminescence (CL). In aprotic solvents, the kinetics of the reaction are first-order with respect to the concentrations of the substrate and of oxygen. The second-order rate constants at 25 °C increase in the order DMF < MeCN DMSO, which correlate with the free energy change for an electron-transfer (ET) from BIQ to O₂, although this process is endothermic in all three solvents. In non-polar solvents such as benzene, the reaction proceeds much more slowly. From these results, a reaction mechanism is proposed as follows. An ET or partial charge-transfer gives a radical ion pair {BIQ˙⁺ · · · O₂˙^–} or a CT complex, in which intersystem crossing takes place from the triplet to the singlet state, and then radical coupling at the 1-position followed by cyclisation yields a 1,2-dioxetane, the postulated intermediate of the CL reaction. An electron-rich olefin having a closely related structure, 1,1′-dimethyl-2,2′-bisquinolylidene (BQ), shows a similar redox behaviour to that of BIQ. Reaction of BQ with O₂ produces CL, and the emitter has been identified as the singlet excited state of 1-methyl-2(1H)-quinolinone 8a. The effects of substituents in the quinoline ring on the autoxidation rate have been investigated in MeCN and DMSO. An olefin having the more negative oxidation potential shows the higher reactivity to O₂, suggesting that the autoxidation of BQ should take place via a similar reaction pathway as that of BIQ.