Excited State Geometry of and Radiationless Processes in the Lowest B3u(nπ*) Singlet State of s-Tetrazine

High resolution absorption and laser-induced emission spectra of the lowest B3u(nπ*) singlet state of s-tetrazine-h2 and -d2 in a benzene crystal at 1.8 K are presented and discussed. The absorption spectrum with origin at 17231 cm-1 (h2) is dominated by a progression in ν6a and a Herzberg-Teller origin which has been assigned as ν1. The absence of ν1 in the emission spectrum is explained as being due to a destructive vibronic interference effect. The Franck-Condon envelope of the unique ν6a progression in emission is used for a determination of the excited state structure and the limitations of this procedure are examined. Direct lifetime measurements using a dye laser and single photon counting techniques show the fluorescence lifetime of s-tetrazine-h2 and -d2 to be shorter than 1.5 ns. From a deconvolution of the emission pulse of dimethyl s-tetrazine its fluorescence lifetime in the gas phase is found to be 6.0 ± 0.3 ns. Through a comparison of the fluorescence quantum yield of s-tetrazine-h2 and dimethyl s-tetrazine we calculate for s-tetrazine-h2 a fluorescence lifetime of 1.5 ± 0.2 ns and a fluorescence quantum yield of 1.8 × 10-3. The ratio of the emissive lifetimes of s-tetrazine-d2 and -h2 was measured from relative fluorescence yields and found to be 1.18 ± 0.05. Photodissociation quantum yield studies on s-tetrazine-h2, -d2 and dimethyl for excitation into the origin of the 1B3u(nπ*) state show this yield to be in the range of 1.3 ± 0.3, and this could explain the low fluorescence yields of the s-tetrazines. The fluorescence quantum yields in the gas phase are found to vary among the vibronic levels of the 1B3u state. This finding is in agreement with earlier measurements, but the report that such quantum yield variations also occurred in the rovibronic structure is not confirmed