Formation of Stilbene Azo-Dimer by Direct Irradiation of p-Azidostilbene † .

Triplet arylnitrenes may provide direct access to aryl azo-dimers, which have broad commercial applicability. Herein, the photolysis of p-azidostilbene (1) in argon-saturated methanol yielded stilbene azo-dimer (2) through the dimerization of triplet p-nitrenostilbene ( 1N). The formation of 1N was verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λ  ~ 375 nm) in cryogenic 2-methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of 1 in methanol formed 1N (λ  ~ 370 nm, 2.85 × 10  s ). On shorter timescales, a transient absorption (λ  ~ 390 nm) that decayed with a similar rate constant (3.11 × 10  s ) was assigned to a triplet excited state (T) of 1. Density functional theory calculations yielded three configurations for T of 1, with the unpaired electrons on the azido (T ) or stilbene moiety (T , twisted and T , flat). The transient was assigned to T based on its calculated spectrum. CASPT2 calculations gave a singlet-triplet energy gap of 16.6 kcal mol for 1 N; thus, intersystem crossing of 1N to 1N is unlikely at ambient temperature, supporting the formation of 1N from T of 1. Thus, sustainable synthetic methods for aryl azo-dimers can be developed using the visible-light irradiation of aryl azides to form triplet arylnitrenes.