Electronic states of symmetrically disubstituted s-tetrazines

Six symmetrically disubstituted derivatives of s-tetrazine have been studied by linear dichroism spectroscopy in the UV-visible region, magnetic circular dichroism spectroscopy, and theoretical calculations of structure and spectra. The electronic absorption spectra, recorded up to 48 000 cm−1, show three major, characteristic transitions: An out-of-plane polarized nπ∗ transition at lowest energy, an in-plane, short axis polarized ππ∗ transition at higher energies (in some compounds overlapping the first transition), and a third, long axis polarized transition at higher energy (for two compounds above 48 000 cm−1 and not observed in the spectra). The calculated and observed transition moments and energies for these transitions are in good agreement. In addition, several weaker transitions, both of nπ∗- and ππ∗-type are predicted for all compounds and several of these are observed and tentatively assigned. The linear dichroism spectra, recorded on solutions in stretched poly(ethylene) (PE) or, when solubility in PE was low, in poly(vinyl alcohol), demonstrate distinct differences in the degree of alignment of similarly shaped molecules. This is remarkable, since studies of well over 100 aromatic hydrocarbons have shown a fairly simple relation between molecular shape and alignment.