Role of charge-transfer complexes in oxygen quenching of excited states of anthracene derivatives in the gas phase

Abstract

Oxygen quenching of excited triplet and singlet states of gas-phase anthracene and its derivatives that have similar energies of the lower triplet levels but widely different oxidation potentials (0.44 < Eox < 1.89 V) was studied. Quenching rate constants for singlet (kSO2) and triplet (kTO2) states in addition to the fraction of oxygen-quenched singlet and triplet states qS1(T1O2 were determined from the decay rates, fluorescence intensities, and delayed fluorescence as functions of oxygen pressure. It was found that kSO2 values vary from 2·104 (9,10-dicyanoanthracene) to 1.2·107 sec−1·torr−1 (anthracene, 9-methylanthracene, 2-aminoanthracene) and kSO2 values from 5·102 to 1·105 sec−1·torr−1. The kSO2 values for anthracene, 9-methylanthracene, and 2-aminoanthracene, which have fast rates of interconversion from S1 to T1, are close to the rate constants for gas-kinetic collisions and are independent of the oxidation potentials (Eox). The quenching rate constants kSO2 for the other anthracene derivatives and kTO2 for all studied compounds decrease with increasing free energy of electron transfer ΔGET, which indicates the important role of charge-transfer interactions in the oxygen quenching of singlet S1- and triplet T1 states.