Laser flash photolysis (LFP) of perfluorophenyl azide and perfluoro-4-biphenyl azide produces the corresponding singlet nitrenes which were detected by their transient absorptions at 330 and 350 nm, respectively. The absolute rate constants of the fundamental processes that consume the singlet nitrenes (intersystem crossing, k isc, rearrangement, k R; reaction with pyridine, k pyr) were determined by monitoring the decay of the singlet nitrene and by the growth of its reaction products (ketenimine, triplet nitrene, or pyridine ylide). In the case of singlet 4-perfluorobiphenylnitrene in CH2Cl2 k isc = (2.2 ± 0.1) × 106 s-1, k R = 1013.2±0.2 exp[−(9400 ± 400)/RT] s-1, and k pyr = 109.06±0.15 exp[−(2400 ± 200)/RT] M-1 s-1. In the case of singlet perfluorophenylnitrene in CH2Cl2 k isc = (1.05 ± 0.05) × 107 s-1, k R = 1013.8±0.3 exp[−(8800 ± 400)/RT] s-1, and k pyr = 109.00±0.13 exp[−(1600 ± 160)/RT] M-1 s-1.

Fluorescence and fluorescence excitation spectra of the photolysis products of perfluorophenyl azide and perfluoro-2-naphthyl azide in n-hexane and n-heptane at 5 K were recorded. The spectra with (0,0) transitions at 18 800 and 18 300 cm-1 were assigned to perfluorophenyl nitrene and perfluoro-2-naphthyl nitrene, respectively. A narrow spectrum with a (0,0) transition at 17 300 cm-1 was attributed to the aryl aminyl radical derived from perfluoro-2-naphthyl nitrene formed by abstraction of a hydrogen atom from the surrounding solvent matrix. The experimental results are consistent with calculations using semiempirical, B3LYP, and CASSCF/CASPT2 methods.

The photochemistry of perfluoro-2-naphthylazide was examined. At low temperature (77 K), photolysis of the azide produces a persistent triplet nitrene which can be characterized by UV−vis and EPR spectroscopy. Photolysis at ambient temperature releases a short-lived (τ < 1 ns) singlet nitrene which can be captured in neat pyridine to form an isolable ylide. Thus the lifetime of singlet perfluoro-2-naphthylnitrene is more similar to that of phenylnitrene where τ = 0.01−0.1 ns than to perfluorophenylnitrene where τ has been deduced to be ≈20 ns. In the absence of concentrated pyridine, singlet perfluoro-2-naphthylnitrene forms a closed-shell azirine or ketenimine intermediate which forms intractable polymeric material.