Structural and Spectroscopic Study of 6,7-Dicyano-Substituted Lumazine with High Electron Affinity and Proton Acidity

The introduction of cyano groups
into lumazine (pteridine-2,4-(1H,3H)­dione) at the C6 and C7 positions
enhances its electron affinity, proton acidity, and solubility in
solvents. As a result, 6,7-dicyanolumazine (DCNLH2) forms
charge transfer (CT) complexes with donors such as tetrathiafulvalene,
2,3,5,6-tetramethyl-1,4-phenylenediamine, and 3,3′,5,5′-tetramethylbenzidine
and readily dissociates a proton from the N1 nitrogen to form a monoanionic
salt with tetrabutylammonium (TBA+). Crystal structures
of the CT complexes consist of mixed stacks in which DCNLH2 interacts with donors in face-to-face configurations, but they form
intermolecular hydrogen bonds differently depending on the donor type.
In the TBA+ salt, two deprotonated DCNLH– monoanions form a unique dianionic dimer connected by two centrosymmetric
hydrogen bonds, N3–H···O–C2, which is
electronically isolated by the presence of bulky TBA+ countercations
and the absence of a proton at the N1 hydrogen-bonding site. This
dimer fluoresces yellowish green (fluorescence quantum yield Φ
= 0.04). Because the DCNLH– anion only shows weak
blue fluorescence in aqueous solution (Φ < 0.01), we suggest
that the dimer formation is responsible for the fluorescence enhancement
with a large emission band shift to the low-energy side.