SINGLET QUENCHING OF POLYCYCLIC AROMATIC-HYDROCARBONS BY POLYCHLOROBENZENES IN ACETONITRILE

The quenching of the excited singlet states of naphthalene, phenanthre ne and pyrene by polychlorobenzenes has been studied in acetonitrile b y static and time-resolved techniques. The bimolecular rate constants correlate well with the Gibbs energy change for a charge-transfer proc ess in which the hydrocarbon acts as an electron donor, when the overa ll Gibbs energy change for the reaction is negative. The dechlorinatio n reaction was not observed even after several hours of continuous irr adiation at the longest absorption band of the hydrocarbon. In agreeme nt 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 hydroc arbons. In the endergonic region of the charge-transfer process, the e xperimental rate constants are higher than those expected from extrapo lation 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 sati sfactorily with the singlet-triplet energy difference between the poly cyclic aromatic (donor) and the polychlorobenzene (acceptor).