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

Dalton Transactions Pub Date: 2010-08-24 DOI: 10.1039/C0DT00225A

Abstract

Two building blocks, 2-(1H-tetrazol-5-yl)quinoline-8-ol (8-hydroxyquinoline-2-tetrazole) (H2hqt) and 8-hydroxyquinoline-2-carboxylic acid (H2hqa) 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]K3 and [Ln(hqt)3]K3 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(H1/2hqa)3]2(Et3NH)3.Et3NHOTf have been crystallised from methanol solution. The X-ray crystal structure ([Nd(hqa)3]K3.7MeOH∞·MeOH, [Er(hqa)3]K3.4MeOH∞·3MeOH and [Nd(hqt)3]K3.5MeOH∞·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]K3 complexes in solution while the major solution geometry of [Ln(hqt)3]K3 complexes is “syn”. Sizable near-IR emission quantum yields were measured for the [Ln(hqa)3]K3 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.

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