Photophysical properties of Kuratowski-type coordination compounds [MIIZn4Cl4(Me2bta)6] (MII = Zn or Ru) featuring long-lived excited electronic states

Abstract

The syntheses of Kuratowski-type pentanuclear clusters featuring {MZn₄Cl₄} cores (M^II = Ru or Zn) that incorporate triazolate ligands are described. The coordination compounds are characterized by single-crystal X-ray diffraction, X-ray powder diffraction (XRD), FTIR- and UV-vis spectroscopy. [Ru^IIZn₄Cl₄(Me₂bta)₆]·2DMF (Me₂bta⁻ = 5,6-dimethyl-1,2,3-benzotriazolate) (1) crystallizes in the cubic system, while [Zn₅Cl₄(ta)₆] (ta⁻ = 1,2,3-triazolate) (3) crystallizes in the tetragonal system. Both compounds feature structurally similar cluster topologies in which the central octahedrally coordinated metal ion is coordinated to six triazolate ligands. Each triazolate ligand is coordinated with two zinc ions (μ₃-bridging mode), leading altogether to a pentanuclear cluster of T_d point group symmetry. Photophysical investigations reveal that compound [Zn₅Cl₄(Me₂bta)₆]·2DMF (2) shows a short-lived excited electronic state, which can be populated with high quantum yield. The isostructural compound [Ru^IIZn₄Cl₄(Me₂bta)₆]·2DMF (1), on the other hand, shows a long-lived photoexcited state, owing to an internal singlet to triplet conversion of the electronic states, as revealed by time-resolved fluorescence spectroscopy. Insights gained from these studies open up novel design strategies towards photocatalytically active metal–organic frameworks incorporating photoactive Kuratowski-type secondary building units such as MFU-4 (Metal–Organic Framework Ulm University-4).