Controllable luminescent behaviors with pyrazine and benzimidazole groups: Syntheses, crystal structures and properties

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A set of new salts containing benzimidazole and pyrazine groups, namely, [HPZBI]+ X– nH2O (X = Cl (n = 1) (1), NO3 (n = 1) (2), ClO4 (n = 0) (3) and H2PO4 (n = 0) (4)) [PZBI = 2–(pyrazin-2-yl)–1H–benzimidazole], were obtained by the reaction of PZBI and a set of inorganic acid, respectively. The emission maxima can be found at 444, 472, 471, 432 and 443 nm for PZBI and its compounds 1–4 in the solid state at room temperature, respectively. Compared with the free PZBI, the emission maxima of compounds 1–4 are obviously blue/red-shifted for 1–4, indicating that the emission maxima are relative to the intermolecular packing distances. Their photoluminescent lifetime and quantum yield are 1.10 ns and 24.5% for 1, 0.99 ns and 26.4% for 2, 0.94 ns and 21.65% for 3, 2.86 ns and 20.57% for 4, respectively. Compounds 1–4 were structurally characterized by X–ray single crystal diffraction, FT-IR, and UV–Vis spectra. Single crystal X-ray diffraction reveals that π⋯π packing interactions and a set of hydrogen bonds can be observed. The shortest distances of π⋯π stacking interactions are 3.613, 3.534, 3.731 and 3.492 Å in compounds 1–4, respectively. Additionally, the thermal stabilities of compounds 1–4 were investigated in detail.