Photolysis of 3-azido-1-indenone (1) with a light-emitting diode (LED, lambda = 405 nm) or mercury arc lamp (Pyrex) resulted in the formation of heterodimer 3 in excellent yield, through dimerization of triplet vinylnitrene 32. At ambient temperature, vinylnitrene (3)2 (lambda(max) at 340 and 480 nm) was detected directly with laser flash photolysis of vinyl azide 1. The vinylnitrene intermediate was also characterized directly with IR and ESR spectroscopy in cryogenic matrices. The ESR spectrum of vinylnitrene (3)2 yielded a zero-field splitting parameter |D/hc| of 0.460 cm(-1) and |E/hc| of 0.015 cm(-1), which reveals that vinylnitrene (3)2 has significant 1,3-biradical character. The proposed mechanism for the formation and reactivity of triplet vinylnitrene (3)2 was supported with density functional theory (DFT) calculations, which highlight that the steric demand of the five-membered ring in vinylnitrene (3)2 prevents intersystem crossing to the corresponding azirine (10). CASSCF and CASPT2 calculations showed that the energy gap between the singlet and triplet configurations of vinylnitrene 2 is only 10 kcal/mol. In spite of this small energy gap, vinylnitrene (3)2 does not decay by intersystem crossing, but rather by dimerization. Thus, triplet vinylnitrenes can be selectively formed with visible light and used to form new C-N bonds in synthetic applications.