title: Structural and spectroscopic study of 6,7-dicyano-substituted lumazine with high electron affinity and proton acidity.
abstract: 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.
authors.affiliation: Department of Bio- and Material Photonics, Chitose Institute of Science and Technology, Chitose 066-8655, Japan. k-sakai@photon.chitose.ac.jp
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Sakai
authors.forename: Kenta
authors.initials: K
authors.lastname: Nagahara
authors.forename: Yuuya
authors.initials: Y
authors.lastname: Yoshii
authors.forename: Norihisa
authors.initials: N
authors.lastname: Hoshino
authors.forename: Tomoyuki
authors.initials: T
authors.lastname: Akutagawa
mesh_terms: D018360:Crystallography, X-Ray; D004583:Electrons; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D015394:Molecular Structure; D009570:Nitriles / Q000737:chemistry*; D011522:Protons; D011621:Pteridines / Q000737:chemistry*; D013050:Spectrometry, Fluorescence; D013056:Spectrophotometry, Ultraviolet
chemical_list: D009570:Nitriles; D011522:Protons; D011621:Pteridines; C008297:lumazine