Photochemical generation of triplet-triplet nitrene pairs in aromatic diazide crystals

The molecular and crystal structures of 4-amino-2,6-diazido-3,5-dichloropyridine and 6-amino2,4-diazido-1,3,5-triazine, as well as the paramagnetic photolysis products of their crystals at 77 K, were studied using X-ray diffraction analysis and ESR spectroscopy. Triplet nitrenes generated during the photolysis of diazidopyridine form triplet-triplet nitrene pairs, whose ESR spectrum corresponds to the quintet spin state. The high-spin state (S = 2) results from the exchange interaction between two triplet molecules with the zero-field splitting parameters vertical bar D vertical bar = 1.0280 cm(-1) and vertical bar E vertical bar = 0.0038 cm(-1) and the theta angle between two C-N nitrene bonds equal to 133 degrees. This angle is close to an angle of 136.2 degrees between the C-N bonds of two adjacent molecules in the crystal structure. No formation of the triplet-triplet nitrene pairs is observed during the photolysis of crystalline diazidotriazine, whose molecules lie in the parallel planes.