Excited state electronic structure and dynamics in sym -tetrazine vapor

Abstract

The excited state dynamics of sym ‐tetrazine are investigated following dye laser excitation into single vibronic levels of the ¹B_3u and ³B_3u states. The lifetime of the ¹B_3u state, τ_f ≤ 100 nsec, is probably controlled by unimolecular internal conversion into the ¹A_g ground state. Intersystem crossing is not observed at low pressure and is not induced by hard sphere collisions. The ³B_3u → ¹A_g phosphorescence, τ_p = 72 ± 2 μsec, is reported for the first time. Theory suggests that this lifetime is not controlled by radiationless intersystem crossing, and it may be that the ³B_3u lifetime is controlled by a radiative transition to a possible lower lying ³B_1g or ³B_2g state. Energy transfer experiments from triplet p ‐benzoquinone to ³B_3u sym ‐tetrazine confirm the lifetime and spin‐forbidden character of the phosphorescence emission.