The Photochemistry of Various Para-Substituted Tetrafluorophenyl Azides in Acidic Media and the Formation of Nitrenium Ions

Abstract

Laser flash photolysis of polyfluorinated aryl azides in acetonitrile at ambient temperature produces singlet nitrenes which have lifetimes of tens to hundreds of nanoseconds. The lifetimes are controlled by ring expansion and intersystem crossing 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 (J. Am. Chem. Soc. 1993, 115, 7254) and McClelland and Novak (J. Am. Chem. Soc. 1994, 116, 4513) and to the protonation of p-biphenylnitrene reported by Davidse et al. (J. Am. Chem. Soc. 1994, 116, 4513). The polyfluorinated nitrenium ions likely have singlet ground states, consistent with recent calculations of the parent structure.