Peculiarities of crystal structures and photophysical properties of GaIII/LnIII metallacrowns with a non-planar [12-MC-4] core

Abstract

A new series of gallium(III)/lanthanide(III) metallacrown (MC) complexes (Ln-1) was synthesized by the direct reaction of salicylhydroxamic acid (H₃shi) with Ga^III and Ln^III nitrates in a CH₃OH/pyridine mixture. X-ray single crystal analysis revealed two types of structures depending on whether the nitrate counterion coordinate or not to the Ln^III: [LnGa₄(shi)₄(H₂shi)₂(py)₄(NO₃)](py)₂ (Ln = Gd^III, Tb^III, Dy^III, Ho^III) and [LnGa₄(shi)₄(H₂shi)₂(py)₅](NO₃)(py) (Ln = Er^III, Tm^III, Yb^III). The representative Tb-1 and Yb-1 MCs consist of a Tb/YbGa₄ core with four [Ga^III–N–O] repeating units forming a non-planar ring that coordinates the central Ln^III through the oxygen atoms of the four shi³⁻ groups. Two H₂shi⁻ groups bridge the Ln^III to the Ga^III ring ions. The Yb^III in Yb-1 is eight-coordinated while the ligation of the nine-coordinated Tb^III in Tb-1 is completed by one chelating nitrate ion. Ln-1 complexes in the solid state showed characteristic sharp f–f transitions in the visible (Tb, Dy) and near-infrared (Dy, Ho, Er, Yb) spectral ranges upon excitation into the ligand-centered electronic levels at 350 nm. Observed luminescence lifetimes and absolute quantum yields were collected and discussed. For Yb-1, luminescence data were also acquired in CH₃OH and CD₃OD solutions and a more extensive analysis of photophysical properties was performed. This work demonstrates that while obtaining highly luminescent lanthanide(III) MCs via a direct synthesis is feasible, many factors such as molar absorptivities, triplet state energies, non-radiative deactivations through vibronic coupling with overtones of O–H, N–H, and C–H oscillators and crystal packing will strongly contribute to the luminescent properties and should be carefully considered.