Singlet quenching of polycyclic aromatic hydrocarbons by polychlorobenzenes in acetonitrile

Abstract

The quenching of the excited singlet states of naphthalene, phenanthrene and pyrene by polychlorobenzenes has been studied in acetonitrile by static and time-resolved techniques. The bimolecular rate constants correlate well with the Gibbs energy change for a charge-transfer process in which the hydrocarbon acts as an electron donor, when the overall Gibbs energy change for the reaction is negative. The dechlorination reaction was not observed even after several hours of continuous irradiation at the longest absorption band of the hydrocarbon. In agreement with this, laser flash photolysis experiments do not show evidence for the formation of the radical ions of the hydrocarbons. Instead the only transient species observed were the triplet states of the hydrocarbons. In the endergonic region of the charge-transfer process, the experimental rate constants are higher than those expected from extrapolation of the results in the exergonic region. In some cases, the rate constants depart from the correlation line by more than two orders of magnitude. However, these experimental rate constants, corrected for the number and type of substituents in the quencher, do correlate satisfactorily with the singlet–triplet energy difference between the polycyclic aromatic (donor) and the polychlorobenzene (acceptor).