Influence of Curvature on the Physical Properties and Reactivity of Triplet Corannulene Nitrene

Although nitrene chemistry is promising for the light-induced modification of organic compounds, the reactivity of large polycyclic aromatic compounds and the effects of their curvature remain unexplored. Irradiation of azidocorannulene (1) in methanol/acetonitrile followed by HCl addition produced diastereomers 5 and 5'. Azirine 2 is apparently trapped by methanol to form diastereomeric acetal derivatives that are hydrolyzed with HCl to yield 5 and 5'. ESR spectroscopy in a glassy matrix at 77 K showed that irradiation of 1 yields corannulene nitrene ³1N, which has significant 1,3-biradical character. Irradiation of 1 in a glassy matrix resulted in a new absorption band in the region of 360-440 nm, with λ_max at 360 and 410 nm, attributed to ³1N, as supported by time-dependent density function theory calculations, which placed the major electronic transitions of ³1N at 367 nm (f = 0.0407) and 440 nm (f = 0.0353). Laser flash photolysis of 1 revealed a similar absorption spectrum. Nitrene ³1N had a lifetime of only a few hundred nanoseconds and was efficiently quenched by oxygen, because of its 1,3-biradical character. CASPT2(12,11)/6-311G** calculations revealed small energy gap (7.2 kcal/mol) between singlet and triplet configurations, suggesting that ³1N is formed by intersystem crossing of ¹1N to ³1N. Spin-density, nucleus-independent chemical shift, and anisotropy of the induced current density calculations verified that ³1N is a triplet vinylnitrene with unpaired electrons localized on the C═C-N moiety; decaying by intersystem crossing to 2, which is more stable owing to its aromaticity, as supported by calculations (SA-CASSCF/QD-NEVPT2/CBS).