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 flu orescence decays for adamantyldiazirine in a variety of solvents, over a wide range of temperatures (77-320 K) and across the diazirine abso rption band (330-371 nm). The fluorescence Lifetime of the primary dec ay component is on the order of 240 ps at ambient temperature and incr eases at lower temperatures. Arrhenius treatment of the fluorescence l ifetime data indicates that the rate-limiting barrier for activated pr ocesses 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 fluorescen ce emission spectra of adamantyldiazirine in a variety of solvents, as well as the infrared spectrum (KBr). We interpret the spectra with th e help of ab initio (RHF/6-31G and CIS/6-31G*), density functional (B 3LYP/6-31G), and semiempirical (PM3) calculations. The fluorescence q uantum yield of adamantyldiazirine at ambient temperature was calculat ed to be similar to 0.0012. Analysis of our data in the light of previ ous research leads us to conclude that little or no intermolecular che mistry is attributable to photoexcited adamantyldiazirine in solution at ambient temperature. Rather, fluorescence competes with one or more intramolecular photochemical processes.