Knowledge Bank

The laser excitation and dispersed fluorescence spectra of the dimethyl-tetrazine dimer is observed in a supersonic free jet. Van der Waals is forces produce ground state dimers (preformed excimers) stable at the very low translational temperature in the jet, enabling one to Observe bound-bound transitions in absorption and emission. Dimer transitions are identified by a strong intensity dependence on the partial pressure of DMT. The shift at the dimer origin from that of the monomer indicates that the excimer ($S1$) is $548cm-1$ more tightly bound than the dimer ($S0$). This shift is much less than that of the typical hydrocarbon excimer ($6000cm-1$), but much greater than the typical van der Waals shift ($1-30cm-1$). It is suggested that the charge transfer interaction responsible for the large excimer shift in the $\pi ^{*} \leftarrow \pi $ transitions of hydrocarbons is less effective for the $\pi \ast \leftarrow n$ transition in DMT. Mild excimer formation is also observed for monomethyl-tetrazine dimer and s-tetrazine-DMT dimer. Unsubstituted s-tetrazine forms three distinct van der Waals complexes shifted +145, -27, and $-44$ $cm-1$, none of which is a mild excimer. Long progressions in the low-frequency dimer stretching vibrations appear in the mild excimer spectra.""