Luminescent complexes of the zinc triad with N-substituted 8-amino-quinoline ligands: Synthesis and comparative study on the stability constants and related photophysical properties

The potentially fluorescent terdentate ligand bis-quinolin-8-yl-amine (BQAH) yields the bis-chelate complexes [M(BQAH)2](ClO4)2 (M = Zn, Cd, Hg) and the mono-chelate [M(BQAH)Cl2] (M = Zn, Cd). The aminic proton of the coordinated BQAH displays a remarkable acidity. Thus, in polar solvents (CH3CN and methanol) the formation of the deprotonated derivatives [M(BQA)2] and [M(BQA)Cl] is observed whose absorption and fluorescent spectra are identical with those of independently synthesized complexes (M = Zn). The affinity of the ligand BQAH with the metals of the zinc triad was studied in CH3CN; the stability constants related to the complex [M(BQAH)(CH3CN)]2+ (β1) and [M(BQAH)2]2+ (β2) were determined and compared with those calculated in the case of the ligand 8-[(2-pyridylmethyl)amino]-quinoline (NNN(Qui)) in the same solvent. Owing to the enhanced rigidity of the ligand BQAH, a marked selectivity in coordinating the Zn2+ cation with respect to the larger Cd2+ was apparent. In the case of mercury, the equilibrium constant value was also confirmed by means of 1H NMR technique. The low lying excited state of the BQAH and NNN(Qui) systems is ligand centered and fluo-solvato-chromism analysis reveals that in protic solvents an inter-molecular hydrogen bond between the aminic proton in the excited state and the solvent itself efficiently quenches the fluorescent signal. Coordination with metals induces a hypsochromic displacement of the absorbance maxima measured in CH2Cl2 with respect to those of the free ligands. On the contrary in CH3OH the complete deprotonation of the coordinated BQAH induces a bathochromic displacement of the absorption maxima at 480 nm. In CH3OH the fluorescent emissions of the mono- and bis-chelate deprotonated BQA− complexes at ≈600 nm display a very low quantum yield and a reduced Stokes shift as compared with that of the protonated species. Such an increase can be related to the enhanced rigidity of the deprotonated ligand inducing a tight coplanarity of the aromatic rings in the first excited state. Eventually the metal coordination, while reducing the energy of the fluorescent emission of both ligands in CH2Cl2, does not inhibit the non radiative relaxation pathways in the BQAH system.