Exploratory photochemistry of polyfluorinated aryl azides in neutral and acidic media

Laser flash photolysis (LFP) of perfluoro-2-naphthyl azide in various solvents produces only weak transient absorption. The transient species can be assigned as a triplet nitrene and an azirine/ketenimine mixture. No catalysis of intersystem crossing (ISC) by methanol was observed for singlet perfluoro-2-naphthylnitrene. The singlet nitrene is rather short-lived (ca. 0.2 ns) as supported by the fact that it requires neat pyridine to be intercepted in modest yield. The short lifetime of the singlet nitrene may be due to facile cyclization of the nitrene to form azirine intermediates on the basis of a molecular orbital effect. The azirine/ketenimine mixture can be easily trapped by dilute pyridine to give a nitrene-ketenimine ylide, which provides the basis for measuring the lifetime of the azirine/ketenimine mixture and the pyridine rate constant. The azirine/ketenimine lifetime hinges upon reaction of the transient with azide, or in the case of methanol with solvent. Triplet perfluoro-2-naphthyl nitrene can be detected upon photolysis of the azide at cryogenic temperature by matrix UV-Vis and EPR spectroscopy.;LFP of polyfluorinated phenyl azides in acetonitrile generates singlet nitrenes which have lifetimes of tens to hundreds of nanoseconds. The lifetimes are controlled by ring expansion and ISC to the lower energy triplet state. In the presence of sulfuric acid the singlet nitrene can be protonated to form a new species whose transient absorption is attributed to a nitrenium ion. This assignment is based in part on analogy to other nitrenium ion spectra and studies reported recently by Falvey, McClelland and Novak. Upon photolysis of the azides in EPA glass at 77 K containing sulfuric acid the persistent UV-Vis spectra of triplet nitrenes and nitrenium ions are observed. However only the EPR spectrum of a triplet nitrene is observed with the sample. Thus the polyfluorinated phenyl nitrenium ion likely has a singlet ground state, consistent with recent calculations of the parent structure.;The kinetics of singlet perfluoro-4-biphenylyl nitrene (with pyridine) and singlet pentafluorophenyl nitrene (with alkenes) has also been investigated, as well as the synthetic, biophysical and biochemical aspects of water-soluble, DNA-intercalating phenyl azide derivatives and azidophenothiazine derivatives.