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 DCNLH ₂ 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.