The spectroscopy, photophysics, and photochemistry of the dimer of dimethyl tetrazine

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Abstract
The dimer of dimethyl tetrazine (DMT)2 has been prepared in a supersonic free jet and its structure and dynamics have been studied by laser induced fluorescence spectroscopy. The binding energy of the excited π*←n electronic state of the dimer was significantly greater than the binding energy of the ground electronic state, and we have called the DMT dimer a mild excimer. The geometry of the DMT dimer was determined from the analysis of the rotational structure in the spectrum of the perdeutero isotopic species and was found to have the two DMT rings stacked and parallel with one ring slipped with respect to the other. At our lowest temperature the perdeutero species was found to have nonrotating methyl groups while the h12 isotopic species had freely rotating methyl groups at all temperatures. The structure of the DMT dimer was consistent with the observation of two exposed aromatic faces as determined by the observation of two van der Waals molecules formed by binding one or two helium atoms to the dimer. When excited vibrational levels of the excited electronic state were pumped, the emission spectrum was dominated by emission from levels reached by intramolecular vibrational relaxation of the initially excited level. The rate of intramolecular vibrational relaxation out of the 6a1 level was found to be 17±6×109 s−1 as determined from the line broadening observed in the fluorescence excitation spectrum. Emission from electronically excited monomers produced by photodissociation of the DMT dimer was observed, but relaxation was always the dominant process and appeared to be much faster than photodissociation.