The photophysics and photochemistry of complexes of s-tetrazine and acetylene

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Abstract
The photochemistry and photophysics of complexes of s‐tetrazine (T) with acetylene have been studied in a supersonic free jet using laser induced fluorescence spectroscopy. Two complexes of the form T–C2H2 and a complex of the form T–(C2H2)2 were observed. One of the monoacetylene complexes has a stacked structure and one has a planar structure. In all cases low frequency vibrational structure was observed in the emission spectrum, and this allowed measurement of the frequencies of intermolecular vibrations. At some levels of excitation, all three complexes had broad features in their emission spectra. It was shown that in these cases, the density of states was sufficiently high that the molecular complexes were in the statistical limit, and therefore the broad structure was attributed to intramolecular vibrational relaxation (IVR). At higher levels of excitation, dissociation of the complexes was observed, and in some instances both IVR and dissociation were observed. Rate constants were measured by comparing the relative intensity of emission produced by these two decay channels with the intensity of resonance fluorescence. These rate constants were found to vary depending on the complex and upon the initially excited state of a given complex. When dissociation was observed, not all energetically allowed vibrational states were observed. The diacetylene complex was observed to dissociate to produce both monoacetylene complexes, but never the monomer.