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

Abstract

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 (³ 1N). The formation of ³ 1N was verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λ_max ~ 375 nm) in cryogenic 2-methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of 1 in methanol formed ³ 1N (λ_max ~ 370 nm, 2.85 × 10⁷ s^-1 ). On shorter timescales, a transient absorption (λ_max ~ 390 nm) that decayed with a similar rate constant (3.11 × 10⁷ s^-1 ) 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_A ) or stilbene moiety (T_Tw , twisted and T_Fl , flat). The transient was assigned to T_Tw based on its calculated spectrum. CASPT2 calculations gave a singlet-triplet energy gap of 16.6 kcal mol^-1 for 1 N; thus, intersystem crossing of ¹ 1N to ³ 1N is unlikely at ambient temperature, supporting the formation of ³ 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.