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

Abstract

The photochemistry of the l, 1-diazenes N-(2,2,5,5-tetramethylpyrrolidyl)nitrene (1), d/-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 0-0 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 °C), 7 x 10^(-3) (MTHF, -196 °C), and l X 10^(-3) (EPA, -196 °C). The fluorescence lifetime of 1 is T_F = 2.3 x 10^(-8) s (CFCl_3, -196 °C). Direct irradiation of 1 (466-610 nm, -78 °C) affords four hydrocarbon products, 54% 4, 44% 5, 2% 6 + 7 and tetrazene 8. Triplet-sensitized photodecomposition afforded 74% 4, 24% 5, 2% 6 + 7 and tetrazene 8. An approximate quantum yield for decomposition on direct irradiation is Φ_D = 1.1 x 10^(-2). From S_1, kN_2 is >3.4 X 10^5 s^(-1), and reaction of S_0 with S_1. k_(DIM) is >4.2 x 10^7 L mo1^(-1) s^(-1) (at -78 °C). The spectrum of 1,1-diazene 2 reveals a structured absorption with λmax 507 nm and a 0-0 band at 568 nm (ε = 20). The vibrational spacing is 1270 cm^(-1). The fluorescence spectrum of 1,1-diazene 2 has a 0-0 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 S_0 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 °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 68%, 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 S_1-T_1 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 °C). The estimated fluorescence lifetime is T_F = 4 X 10^(-9)s. Direct irradiation of 3 in the visible at -78 °C afforded three hydrocarbon products, 29% 21, 3% 22, 68% 23 and tetrazene 25.