Modulation of Fluorescence Emissions of Copper(II) 2,2′-biquinoline-4,4′-dicarboxylates

Abstract

Interplay of weak interactions of di-aqua-bis-ethylenediammine copper(II) 2,2′-biquinoline-4,4′-dicarboxylate hydrate (1) were used to modulate solid state fluorescence emissions. Complex 1 forms porous self-assembly in solid state by hydrogen bonds between cations, anions and lattice water molecules. Fluorescence spectroscopy, thermogravimetry, powder XRD, IR-spectroscopy and surface analysis were used to establish the breakdown and regeneration of porous hydrogen bonded sub-assemblies by dehydration and rehydration. Complex 1 is non-fluorescent in solid state but anhydrous form of the complex 1 is highly fluorescent. In the solid state self-assembly of the complex 1, π-stacking among biquinoline rings contributes to quenching. Dehydration of solid sample of complex 1 generates a fluorescent state, and subsequent absorption of moisture by the anhydrous complex quenches fluorescence. In solution, competition between chelation by ethylenediamine and coordination by 2,2′-biquinoline-4,4′-dicarboxylic acid with copper(II) ions is reflected in the ON and OFF states generated by judicious manipulation of chelation effects. Complex 1 is one among a series of bis-chelated ethylendiamine copper(II) aromatic carboxylate complexes which have layered structures. It is also a precursor of nano-dimensional copper oxide; 25–50 nm size copper oxide particles are formed by thermal decomposition of complex 1 at 500 °C.