Supramolecular architectures and luminescent properties of the salts containing flexible bis(pyridyl) cations with aliphatic diamine spacer: effects of inorganic anions, alkalinity and conformation of the bis(pyridyl) cations

Abstract

Three flexible bis(pyridyl) molecules with an aliphatic diamine spacer, N,N′-bis(pyridin-4-ylmethyl)ethane-1,2-diamine (M1), N,N′-bis(pyridin-3-ylmethyl)ethane-1,2-diamine (M2), and N,N′-bis(pyridin-2-ylmethyl)ethane-1,2-diamine (M3), react with different inorganic acids (HCl, HNO₃ and HClO₄) in different ratios (1 : 2 and 1 : 4), thus leading to the formation of seventeen salts, H₂M1²⁺·2Cl⁻ (1), H₂M2²⁺·2Cl⁻ (2), H₂M1²⁺·2NO₃⁻·H₂O (3), H₂M2²⁺·2NO₃⁻ (4), H₂M3²⁺·2NO₃⁻·H₂O (5), H₂M1²⁺·2ClO₄⁻·H₂O (6), H₂M2²⁺·2ClO₄⁻ (7), H₂M3²⁺·2ClO₄⁻ (8), H₄M1⁴⁺·4Cl⁻ (9), H₄M2⁴⁺·4Cl⁻ (10), H₄M3⁴⁺·4Cl⁻·2H₂O (11), H₄M1⁴⁺·4NO₃⁻ (12), H₄M2⁴⁺·4NO₃⁻ (13), H₄M3⁴⁺·4NO₃⁻ (14), H₄M1⁴⁺·4ClO₄⁻ (15), H₄M2⁴⁺·4ClO₄⁻ (16), and H₄M3⁴⁺·4ClO₄⁻ (17), which have been characterized by elemental analysis such as IR, TG, PL, powder and single-crystal X-ray diffraction. Structural analyses indicate that the nature of the three inorganic anions, alkalinity and conformation of the three flexible bis(pyridyl) cations can effectively influence the supramolecular patterns of the seventeen salts through diverse non-covalent bonding interactions. Leaving other interactions aside, these salts exhibit diverse patterns with the assistance of the traditional hydrogen bonds. Salts 1–4, 9, 10, and 12 exhibit classic ladder chain II, whereas 6 exhibits ladder chain IV. Ladder chain III is observed in salts 11 and 13. Salts 14 and 17 contain both ladder chain I and ladder chain III. Salts 7 and 16 present layer II, III while salt 15 exhibits 3-D hydrogen bonding network involving layer II. The thermal stability of the seventeen salts presents a regular decrease with the sequence of perchlorates, chlorates and nitrates. Luminescent analyses indicate that all these seventeen salts exhibit blue emission maxima in the range of 420–456 nm in the solid-state at room temperature.