Temperature dependence of the phosphorescence lifetimes of benzene-chloroform complexes

Abstract

The temperature dependence (10-90 K) of the phosphorescence lifetime of benzene complexed with chloroform and deuterated chloroform in 3-methylpentane has been studied. Nonexponential decays are observed at all temperatures and attributed to emission from 1:1 and 2:1 chloroform:benzene complexes. The temperature dependence of the two deconvoluted lifetimes have been fit to two Arrhenius-type rate expressions and the Arrhenius parameters interpreted in terms of the thermal production of a chloroformor solvent-substituted hexatriene from the benzene triplet state. It is proposed that the precursor to the substituted hexatriene is an exterplex, an excited state complex formed between chloroform, triplet benzene, and the solvent. The comprehensive experiments of Simons and co-workers are shown to be consistent with this suggestion. It is further proposed that the known solvent and temperature dependence of (uncomplexed) benzene phosphorescence in hydrocarbon matrices arises from solvent-substituted hexatriene formation via a precursor benzene-solvent exciplex. Closed-and open-shell INDO molecular orbital calculations on the triplet and ground state benzene-H2 model system are supportive of benzene-solvent exciplex formation. The possible existence of benzene exciplexes in other phases is discussed. In liquid alkanes, where benzene is known to be a radiation shield under 7 irradiation, protection of the solvent from CH bond rupture via energy transfer in a benzene-solvent exciplex is suggested.

The Photolysis of Chlorocarbonylbis(triphenylphosphine)iridium(I) in Chloroform

“…The thermodynamics of the chloroform‐benzene association were determined in cyclohexane by Creswell and Allred, who found an equilibrium constant of 1.1 and an association enthalpy of −8 kJ mol −1 (39). Phosphorescence spectra of the benzene–chloroform complex, distinct from the phosphorescence of benzene itself, have been reported (40,41). It is reasonable to presume that C–Cl bond fission occurs through excitation of the benzene in a hydrogen‐bonded complex.…”