Fluorescence quenching of vaporous polycyclic aromatic hydrocarbons by oxygen

The fluorescence quenching by oxygen of vapors of nine polycyclic aromatic hydrocarbons with strongly different oxidation potentials 0.44 eV < Eox < 1.61 eV (anthracene, 9-methylanthracene, 2-aminoanthracene, 9,10-dibromanthracene, pyrene, chrysene, phenanthrene, fluoranthene, and carbazole) is studied. From the dependences of the fluorescence decay rates and intensities on the oxygen pressure PO2, the quenching rate constants kSO2 for the excited singlet states S1 and the fraction fSO2 of the S1 states quenched by oxygen are estimated. At PO2 = 5 Torr, the kSO2 constants vary from 1.2 × 107 to 3.0 × 105 s−1 Torr−1, while the fraction of the quenched excited singlet states changes from 0.1 (fluoranthene) to 0.7 (chrysene) and 0.8 (pyrene). The dependences of kSO2 on the photophysical and electron-donor characteristics of the fluorescing compounds are analyzed. It is shown that, in the gas phase of anthracene and its derivatives, the magnitudes of kSO2 are limited by the rate constants of gas-kinetic collisions kgk and do not depend on the electron-donor characteristics of fluorophores, while the fraction of quenched states fSO2 changes with the oxidation potential. For compounds with kSO2 < kgk, both the rate constants kSO2 and the fraction of quenched states fSO2 depend on the Eox of sensitizers, which demonstrates an important role played by the charge-transfer interactions in quenching of the S1 states. The dependence of the rate constants kSO2 on the free energy of electron transfer ΔGet is considered.