Conjugated Biradical Intermediates: Spectroscopic, Kinetic, and Trapping Studies of 2,2‐Dimethyl‐1,3‐perinaphthadiyl

The biradical 2,2‐dimethyl‐1,3‐perinaphthadiyl (a) was generated from two different precursors, the naphthocyclopropane 1 and the azo compound 2, and from each by three different pathways (pyrolysis, direct photolysis, and triplet sensitization, Scheme 1). The combined evidence from flash photolysis, low‐temperature spectroscopy, and product analyses provides a detailed mechanistic picture of the formation and decay of this reactive intermediate which is persistent at 77 K in the triplet ground state (3a) and rather long‐lived (400 μs) at room temperature. When formed in its lowest singlet state (1a), the biradical is too short‐lived to undergo intersystem crossing to 3a or bimolecular reactions. Thus, 3a is formed exclusively from the excited triplet state of the precursor compounds, 31* or 33*. The monomolecular decay of 3a is retarded by the spin barrier; 3a initiates the polymerization of acrylonitrile and is trapped by 3O2.