Intra- and intermolecular quenching of carbazole photoluminescence by imidazolidine radicals

Abstract

Mechanisms of carbazole photoluminescence quenching by the free and chemically bound nitroxyl radicals in the model bound system “carbazole (CBZ)—imidazolidine nitroxyl radical R•” were investigated and the photophysical properties of the system were studied and compared with those of free CBZ and R• in solution. The quantum yield and lifetime of fluorescence from the local singlet excited state of the carbazole moiety in the bound CBZ—R• system is three orders of magnitude lower than in free CBZ. The lifetime of the local triplet excited state of the carbazole moiety in the bound system is shorter than 50 ns. The rate constants for intermolecular quenching of the singlet and triplet excited states of free CBZ by R• in acetonitrile were found to be (1.4±0.1)·1010 and (1.5±0.2)·109 L mol−1 s−1, respectively. The most plausible mechanisms of both free and covalently bound carbazole luminescence quenching by nitroxyl radicals are exchange energy transfer and acceleration of internal conversion due to electron exchange.