The Bis-Salphen Zn(II) Unit: A Versatile Building Block for Self-Assembled Heteroleptic Coordination Cages

Abstract

Heteroleptic coordination cages enable not only a high control over the cavity properties but also multifunctionality. However, their synthesis is far from trivial and often relies on precious 4d and 5d transition metals such as Pd(II). The transfer to more cost-effective, earth abundant metal ions such as Zn(II) is highly desirable for increasing their application potential in real-life scenarios. In this work, we report on a self-assembly strategy employing the ditopic Zn(II) bis-salphen unit Zn₂L in combination with tritopic pyridyl-based ligands to construct novel prismatic cages. As a proof of concept, two distinct cages are presented, utilizing either a phenyl- or triazine-based tritopic ligand (cage 1 and cage 2, respectively). Both cages exhibit remarkable stability at low concentrations while displaying dynamic behaviour at high concentrations. Investigation of their photophysical properties reveals a striking "on-off" emission behaviour: the phenyl-based cage 1 features a significant orange emission (Φ = 6%), while cage 2 is non-emissive. This emission quenching can be attributed to the electron-withdrawing nature of the triazine ligand, which dominates the electronic relaxation pathway of the Zn₂L unit. Although both cages successfully bind aromatic guests in host-guest studies, their encapsulation results in partial cage decomposition, which prevents to further investigate whether cage 1 qualifies as an optical sensor material. However, the findings herein introduce a versatile route to emissive bis-salphen coordination cages, marking a significant step towards developing new heteroleptic zinc(II) cages for optical sensing.