Fluorescence quenching of vaporous polycyclic aromatic hydrocarbons by oxygen

摘要

The fluorescence quenching by oxygen of vapors of nine polycyclic aromatic hydrocarbons with different oxidation potentials 0.44 eV < E-ox < 1.61 cV (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 P-O2, the quenching rate constants k(S)(O2) for the excited singlet states S-1 and the fraction f S-O(2S) of the S-1 states quenched by oxygen are estimated. At P-O2=5 Torr, the k(S)(O2) constants vary from 1.2x10(7) to 3.0x10(5) 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 k(S)(O2) 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' k(S)(O2) are limited by the rate constants of gas-kinetic collisions k(gk) and do not depend on the electron-donor characteristics of fluorophores, while the fraction of quenched states f(S)(O2) changes with the oxidation potential. For compounds with k(S)(O2)< k(gk), both the rate constants k(S)(O2) and the fraction of quenched states f(S)(O2) depend on the E-ox of sensitizers, which demonstrates an important role played by the charge-transfer interactions in quenching of the S-1 states. The dependence of the rate constants k(S)(O2) on the free energy of electron transfer Delta G(et) is considered.