Pyrazine π-Hole Interaction Preference in Crystal Structures and Spectroscopic Properties of Crystalline C8/C9 Alkyl-1H-5,6,10b-triazaacephenanthrylene-2-carbonitrile

The cooperative electrostatic and dispersive interactions of the pyrazine π-hole in fused triaazaacephenanthrylene (TAAP) systems influence the crystal structures and spectroscopic properties of solid TAAP and TAAP derivatives (8/9-tert-Bu, 8,9-diMe, and N5-4-Cl/OEtC₆H₄). In crystalline phases, the pyrazine π-hole (Cg3) induces face-to-face oriented centrosymmetric dimer formation by selecting either (1) a donor partner from the conjugated electron pair of the trivalent nitrogen atom N10b(sp²) in TAAP, (2) π-electrons of the imine C1═N11(sp²)–H11 moiety (in some TAAP derivatives), or (3) lone pair (lp) interactions of the solvate dioxane oxygen (O) atoms (sp³) in a centrosymmetric tetrameric cluster [comprising four (9-tert-BuTAAP) molecules and a dioxane molecule]. The calculated energy of the electrostatic noncovalent interaction between the dioxane lp and pyrazine π-hole was −4.64 kcal/mol, and the dispersion energy was −8.11 kcal/mol (with a Cg3···O separation of 3.170 Å). The electrostatic energy of one 9-tert-BuTAAP molecule in a centrosymmetric dimer was −6.23 kcal/mol, and the dispersion energy was −16.6 kcal/mol, with a Cg3···N11 separation of 3.080 Å. After changing the donor partner to an amide carbonyl group that competes with imine, the two methyl groups at positions 8 and 9 of TAAP caused the mutual orientation of the two dimer molecules to change from a centrosymmetric face-to-face relationship to parallel stacked molecules. In the stack, the strong dispersion (−16.9 kcal/mol) and electrostatic energies (including dipole–dipole interactions, −8.15 kcal/mol) influenced the carbonyl O atom interactions with Cg3 (separation of 3.414 Å) and the pyrazine C5a═N6 bond (3.196 Å).

The pyrazine π-hole interaction with π-type donors, i.e., imine nitrogen, carbonyl, conjugated trivalent nitrogen, and oxygen lone pair (lp), stabilizes the formation of centrosymmetric dimers with a head-to-tail arrangement, dimer stacks, translational stacks, or centrosymmetric tetrameric clusters. Fluorescence property of crystalline phases is modulated by pyrazine π-hole interaction preference. A relevant effect was observed for the π-hole interaction with the electron lp from the dioxane oxygen atom.