Fluorescence lifetime measurements and spectral analysis of adamantyldiazirine

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 similar to 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.