New insights on near-infrared emitters based on Er-quinolinolate complexes: Synthesis, characterization, structural, and photophysical properties

Erbium quinolinolates, commonly assumed to be mononuclear species with octahedral co-ordination geometry, have been proposed as promising materials for photonic devices but difficulties in obtaining well defined products have so far limited their use. We report here the conditions to obtain in high yields three different kinds of pure neutral erbiurn quinolinolates by mixing an erbium salt with 8-quinolinol (HQ) and 5,7-dihalo-8-quinolinol (H5,7XQ: X = Cl and Br): i) the trinuclear complex Er(3)Q(9) (1) which is obtained with HQ deprotonated by NH3 in water or ethanol/water mixtures; ii) the already known dimeric complexes based on the unit [Er(5,7XQ)(3)(H2O)(2)] [X = Cl (2) and Br (3)]; iii) the mononuclear [Er(5,7XQ)(2)(H5,7XQ)(2)Cl] [X = Cl (4) and Br (5)] complexes, obtained in organic solvents without base addition, where the ion results coordinated to four ligands, two deprotonated chelating, and two as zwitterionic monodentate oxygen donors. These results represent a further progress with respect to a recent reinvestigation on this reaction, which has shown that obtaining pure and anhydrous octahedral ErQ(3), the expected reaction product, is virtually impossible, but failed in the isolation of I and of the neutral tetrakis species based on H5,7XQ ligands. Structural data provide a detailed description of the molecules and of their packing which involves short contacts between quinoxaline ligands, due to pi-pi interactions. Electronic and vibrational studies allow to select the fingerprints to distinguish the different products and to identify the presence of water. The structure/property relationship furnishes a satisfactory interpretation of the photo-physical properties. Experimental evidence confirms that the most important quenchers for the erbium emission are the coordinated water molecules and shows that the ligand emission is significantly affected by the pi-pi interactions.