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We report the first confirmed fluorescence lifetime measurement for a diazirine. We have obtained time-correlated single-photon counting fluorescence decays for adamantyldiazirine in a variety of solvents, over a wide range of temperatures (77−320 K) and across the diazirine absorption band (330−371 nm). The fluorescence lifetime of the primary decay component is on the order of 240 ps at ambient temperature and increases at lower temperatures. Arrhenius treatment of the fluorescence lifetime data indicates that the rate-limiting barrier for activated processes in the diazirine excited state is between 2.7 and 2.9 kcal/mol. Adamantyldiazirine's fluorescence lifetime appears to be unaffected by deuteration of the solvent, solvent polarity, or excitation energy. We also report and discuss the steady-state absorption and fluorescence emission spectra of adamantyldiazirine in a variety of solvents, as well as the infrared spectrum (KBr). We interpret the spectra with the help of ab initio (RHF/6-31G* and CIS/6-31G*), density functional (B3LYP/6-31G*), and semiempirical (PM3) calculations. The fluorescence quantum yield of adamantyldiazirine at ambient temperature was calculated to be ∼0.0012. Analysis of our data in the light of previous research leads us to conclude that little or no intermolecular chemistry is attributable to photoexcited adamantyldiazirine in solution at ambient temperature. Rather, fluorescence competes with one or more intramolecular photochemical processes.

A new diffusion model is developed for radiative transfer in particulate media. It includes the effects of higher Legendre moments while avoiding the mathematical complexities of solving multiple coupled differential moment equations and satisfying higher-order boundary conditions. The method accurately extrapolates the conventional Eddington approximation to problems involving large absorption. Although the simplifying assumptions limit the model to nonbeam incidence and spatial homogeneity of the scattering material, they are nonrestrictive with regard to the size and shape of the medium, the character of background reflections, and the type of phase function.

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