A Reversibly Photoswitchable Zinc Metallocycle based on Azobenzene

Photoswitchable metallocycles and cages are of special interest for a photocontrolled uptake and/or release of guest molecules. While most compounds undergo a disassembly-re-assembly mechanism among photoisomerization, examples for reversibly switchable compounds without decomposition are extremely rare. Furthermore, most metallocycles and cages are based on 4d and 5d metal ions that form kinetically inert bonds. We here present a dinuclear Zn(II) metallocycle that despite the usually high ligand exchange rate of Zn(II) reversibly undergoes photoisomerization without disassembly. Therefore, we prepared the novel ligand 3,3’-Azobenz(BPA)₂ (1) that is based on an azobenzene scaffold equipped with two benzylpicolylamine (BPA) moieties for metal complexation. With this ligand, the Zn(II) complexes [{ZnCl₂}₂(1)] (2) and the cyclic 2 : 2 complex [Zn₂(1)₂](BF₄)₄ (3) were prepared and characterized. The photophysical switching properties of all compounds have been assessed using NMR and UV/Vis spectroscopy as well as quantum chemical calculations by density functional theory (DFT) at the CAM−B3LYP/D4/def2-TZVPP level. These studies show the highly quantitative and reversible photoinduced cyclic E/Z isomerization of 1–3. To the best of our knowledge, 3 thus represents the only second metallocycle based on azobenzene and first one using Zn(II), which reversibly undergoes photoisomerization.