Structural and photophysical properties of trianionic nine-coordinated near-IR emitting 8-hydroxyquinoline-based complexes

Two building blocks, 2-(1H-tetrazol-5-yl)quinoline-8-ol (8-hydroxyquinoline-2-tetrazole) (H(2)hqt) and 8-hydroxyquinoline-2-carboxylic acid (H(2)hqa) provide convenient dianionic tridentate O,N,N- and O,N,O-chelating units containing the 8-hydroxyquinoline chromophore. These ligands form tris-chelate tris-anionic complexes [Ln(hqa)(3)]K(3) and [Ln(hqt)(3)]K(3) in water at pH = 12. At lower pH partially protonated species are formed. The Nd(III), Yb(III) and Er(III) complexes of hqa, and hqt and the partially protonated complex [Nd(H(1/2)hqa)(3)](2)(Et(3)NH)(3).Et(3)NHOTf have been crystallised from methanol solution. The X-ray crystal structure ([Nd(hqa)(3)]K(3).7MeOH(∞)·MeOH, [Er(hqa)(3)]K(3).4MeOH(∞)·3MeOH and [Nd(hqt)(3)]K(3).5MeOH(∞)·MeOH) show a helical arrangement of both ligands with an "anti" geometry for hqa and a "syn" geometry for hqt. Only tris-chelate complexes are formed in water and 0.04 M methanol solutions for both ligands. A "syn" geometry is found for the partially protonated dimeric complexes which is preserved in methanol solution. A statistical distribution of "anti" and "syn" species is found for [Ln(hqa)(3)]K(3) complexes in solution while the major solution geometry of [Ln(hqt)(3)]K(3) complexes is "syn". Sizable near-IR emission quantum yields were measured for the [Ln(hqa)(3)]K(3) complexes in solid state (0.06%, 0.18%, 0.0051% for Nd(III), Yb(III) and Er(III) respectively) and in methanol (0.063%, 0.28%, 0.0019% for Nd(III), Yb(III) and Er(III) respectively). All the values of quantum yields obtained for the hqt complexes are 5-17% higher than those measured for the hqa complexes. The trianionic complexes present a high solubility in organic and aqueous solvents and a good resistance to ligand dissociation compared to neutral tris 8-hydroxyquinoline complexes.