Effect of temperature on oxygen quenching of excited states polycyclic aromatic compounds in the vapor phase

Abstract

The temperature dependences of oxygen quenching rate constants of singlet (\( k_S^{O_2 } \)) and triplet (\( k_T^{O_2 } \)) states, as well as of the fractions of singlet (\( q_S^{O_2 } \)) and triplet (\( q_T^{O_2 } \)) states quenched by oxygen in vapor pahse of polycyclic aromatic hydrocarbons (PAHs), namely, anthracene, 2-aminoanthracene, 9-methylanthracene, 9,10-dibromoanthracene, chrysene, phenanthrene, fluoranthene, and carbazole, are studied. It is found that in the temperature range 433–673 K, the oxygen quenching rate constants of polycyclic aromatic hydrocarbons either do not change or slightly decrease with increasing temperature. The quenching activation energies Ea for these cases are determined. The effect of the stored vibrational energy and the free energy of electron transfer from a PAH to oxygen on the temperature dependences of \( k_S^{O_2 } \) and \( k_T^{O_2 } \) is considered. The role played by charge-transfer complexes in the observed temperature dependences \( k_S^{O_2 } \) of and \( k_T^{O_2 } \), as well as the applicability of a model of electron transfer concerted with reorganization of reactants for description of quenching in heated vapors is discussed.