Photochemistry of 1,1-Diazenes. Direct and Sensitized Photolyses of N-(2,2,5,5-Tetramethylpyrrolidyl)nitrene, dl-N-(2,5-Diethyl-2,5-dimethylpyrrolidyl)nitrene, and N-(2,2,6,6-Tetramethylpiperidyl)nitrene

Article abstract of DOI:10.1021/ja00388a031

The photochemistry of the 1,1-diazenes N-(2,2,5,5-tetramethylpyrrolidyl)nitrene (1), dl-N-(2,5-diethyl-2,5-dimethylpyrrolidyl)nitrene (2), and N-(2,2,6,6-tetramethylpiperidyl)nitrene (3) is reported.The fluorescence spectrum of 1,1-diazene 1 has a O-O band at 607 nm, which is the maximum.The spacing between the peaks at 607 and 672 nm corresponds to the N=N stretch of S₀ consistent with the 1638 cm^-1 stretch obtained from the infrared spectrum of 1.The fluorescence quantum yields are Φ_F = 2 x 10^-3 (MTHF, -78 deg C), 7 x 10^-3 (MTHF, -196 deg C), and 1 x 10^-3 (EPA, -196 deg C).The fluorescence lifetime of 1 is τ_F = 2.3 x 10^-8 s (CFCl3, -196 deg C).Direct irradiation of 1 (466-610 nm, -78 deg C) affords four hydrocarbon products, 54percent 4, 44percent 5, 2percent 6 + 7 and tetrazene 8.Triplet-sensitized photodecomposition afforded 74percent 4, 24percent 5, 2percent 6 + 7 and tetrazene 8.An approximate quantum yield for decomposition on direct irradiation is Φ_D = 1.1 x 10^-2.From S₁, k_N2 is > 3.4 x 10⁵ s^-1, and reaction of S₀ with S₁, k_DIM, is > 4.2 x 10⁷ L mol^-1 s^-1 (at -78 deg C).The spectrum of 1,1-diazene 2 reveals a structured absorption with λ_max 507 nm and a O-O band at 568 nm ( ε = 20).The vibrational spacing is 1270 cm^-1.The fluorescence spectrum of 1,1-diazene 2 has a O-O band at 620 nm, which is the maximum.The spacing between the maxima at 620 and 690 nm corresponds to the N=N stretch of S₀ consistent with the 1630 cm^-1 stretch obtained for the infrared spectrum of 2.The fluorescence quantum yield Φ_F = 9 x 10^-3 (MTHF, -196 deg C).The direct and sensitized irradiation of 2 in the visible affords hydrocarbon products 14-19 and tetrazene 20 in different ratios.The retention of stereochemistry in the cyclobutane products in the direct and sensitized photodecomposition was 98 and 68percent, respectively, similar to the spin correlation effect seen in corresponding 1,2-diazene isomer.This indicates that for 2 (and by extension 1) k_isc << k_N2, consistent with a large S₁-T₁ gap in 1,1-diazenes.For 1,1-diazene 3 the fluorescence spectrum has a single maximum at 684 nm.The fluorescence quantum yield Φ_F = 4 x 10^-4 (MTHF, -196 deg C).The estimated fluorescence lifetime is τ = 4 x 10^-9 s.Direct irradiation of 3 in the visible at -78 deg C afforded three hydrocarbon products, 29percent 21, 68percent 23 and tetrazene 25.