Quenching of the singlet and triplet state of benzene by halogenated alkanes in the vapor phase

Abstract

The second order rate constants (kq) for the quenching of the singlet state of benzene by several halogenated alkanes (viz. CH2Br2, CCl4, CH3CCl3, CHCl3, CHF3, CH3Cl, CH3F, CH2F2, CH2Cl2, CFCl3, CF2Cl2, CF3Cl, CF4) in the vapor phase have been measured. For CCl4, CHCl3, CH2Cl2, and CH3Cl, which were previously studied by Das Gupta and Phillips, there is satisfactory agreement between the kq values obtained in the separate investigations. The linear dependence of the electron affinity of quencher with the corresponding quenching rate constants supports an exciplex mechanism in accordance with Klein's model. Charge transfer occurs from benzene to halogenated alkane and is greater for the chloro alkanes than for mixed fluoro–chloro alkanes. In the case of the triplet state of benzene, whose behaviour was monitored using the sensitized emission of biacetyl technique, quenching effects were at best one order of magnitude smaller and probably involve more than one bimolecular process.