Synthesis, characterization, luminescence and NLO properties of binuclear copper(I) complexes containing carbon-rich alkynyl functionalized salicylaldimine ligand

The binuclear carbon-rich alkynyl functionalized copper(I) complexes of the type [Cu₂(L₁/L₂)(phen/bipy)₂] (1a-d) and [Cu₂(L₃/L₄)(phen/bipy)₂] (2a-d); (where, L₁= 4,4'-(1,4-phenylbis(ethyne-2,1-diyl))bis(2-(E−((4-nitrophenyl)imino)methyl)phenol); L₂= 4,4'-(1,4-phenylbis(ethyne-2,1-diyl))bis(2-(E((4-methoxyphenyl)imino)methyl)phenol) and L₃= 4,4'-([1,1′-biphenyl]-4,4′-diylbis(ethyne-2,1-diyl))bis(2-E−((4-nitrophenyl)imino)methyl)phenol); L₄= 4,4'-([1,1′-biphenyl]-4,4′-diylbis(ethyne-2,1-diyl))bis(2-E−((4-methoxypheny)imino)methyl)phenol) and phen= 1,10-phenanthroline and bipy = 2,2′-bipyridine) have been prepared and characterized on the basis of elemental analysis, FTIR, ¹H NMR and mass spectroscopic studies. These thermally stable complexes exhibit quasireversible redox behavior corresponding to Cu(I)/Cu(II) and Cu(II)/Cu(I) couples. Notably, the phenanthroline complexes appear at more positive potentials than their bipyridine counterparts, likely due to the π-acidic character of phenanthroline. At room temperature, all complexes show red-shifted emission, attributed to fluorescence resulting from intra-ligand charge transfer (ILCT) mixed with some metal-to-ligand charge transfer (MLCT). Further, the excited state of 1a-d and 2a-d are stabilized and red shifted in their emission wavelength with increasing polarity of the solvents. The second harmonic generation (SHG) efficiency of the complexes, measured using the Kurtz-powder technique, indicates that all complexes exhibit significant potential as nonlinear optical materials.