No 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 temperature dependence (10-160 K) of the phosphorescence lifetimes and intensities of 2,4,5-trimethylbenzaldehyde (TMB) in crystalline durene have been investigated. Two phosphorescence bands (407"5 and 411'2 nm) are observed to grow in with increasing temperature, reach a maximum intensity, and decrease in intensity at higher temperatures. Exponential phosphorescence decays are observed in the 10 to 95 K and 110 to 160 K ranges, while non-exponential decays are observed in the 95 to 110 K region. Ultra-violet photolysis, particularly at higher temperatures, is seen to decrease the intensity of the infra-red carbonyl stretching band at 1703 cm -x, but not the one at 1684 cm -1. A model in which two inequivalent sets of TMB molecules in the durene lattice undergo their own unique photochemical and photophysical processes is suggested to explain the results. It is proposed that within each site the lower, predominantly Z~rrr* state of the anti TMB conformer is in thermal equilibrium with the higher-lying 3nrr* state of the syn conformer which is formed by out-of-plane rotation of the aldehyde group in the excited state. In one site, intramolecular photoenol formation occurs via thermal activation from the 3nr* of the syn conformer, while in the other site intermolecular hydrogen abstraction occurs, leading to the simultaneous production of a photoenol and a duryl radical. This model is shown to be consistent with previous knowledge of the primary photochemical processes of TMB in durene.

No abstract