Effects of transition metal cations and temperature on the luminescence of a 3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrole-2-olate anion

Abstract

Two new mononuclear complexes based on the anion 3-cyano-4-dicyanomethylene-5-oxo-4,5-dihydro-1H-pyrrol-2-olate (C₈HN₄O₂⁻ = HA⁻) and Mn²⁺ or Zn²⁺ cations, namely [Mn(HA)₂(H₂O)₄]·2H₂O (1) and [Zn(HA)₂(H₂O)₄] (2), were synthesized and characterized by single-crystal X-ray diffraction and photoluminescence spectroscopy. Complexes 1 and 2 crystallize in the triclinic space group P. Units [M(HA)₂(H₂O)₄] (M = Mn, Zn) are the basic building blocks in both 1 and 2 crystal structures. The metal cation, located on an inversion center, is bound to two nitrogen atoms of two HA⁻ and four water oxygen atoms. Units [M(HA)₂(H₂O)₄] are connected by π–π stacking interactions between anions HA⁻ and by hydrogen bonds to form a 3D crystal structure. Complexes 1 and 2 are isostructural with previously reported complexes [Cd(HA)₂(H₂O)₄]·2H₂O and [Cu(HA)₂(H₂O)₄], respectively. A comparison between the luminescence properties of isostructural complexes confirms that the incompletely filled 3d subshell (3d⁵, 3d⁹) of a divalent metal cation is a key factor leading to the total luminescence quenching of anion HA⁻ upon complexation with metal cations. Salts M(HA)₂ (M = Mn, Cd, Cu, or Zn) adsorbed on cellulose paper display thermo-sensitive luminescence. The luminescence is due to HA⁻ anions. The intensity of the luminescence peak reversibly decreases by 6–7 times upon heating from 27 up to 123 °C.