Highly selective synthesis and near-infrared photothermal response of heterometallic double trefoil knot and D-type [2]catenanes

The selective construction of sophisticated supramolecular topologies has always attracted the remarkable attention of researchers. Thus, various molecular knots and catenanes have recently been reported and were obtained either accidentally or by following a previously planned synthetic strategy. Herein, we introduce a series of supramolecular topologies including double trefoil knot (1), trefoil knot (2) and [2]catenanes (4, 5), which were prepared via an elaborated self-assembly strategy by using half-sandwich units based on a flexible pyridyl linker L1. The formation of all the structures has been confirmed by single crystal X-ray diffraction analysis and NMR spectroscopy, and ESI-MS. Remarkably, the central potassium ion of 1 coordinates six oxygen atoms belonging to six amide groups from two trefoil segments. The double trefoil knot 1 and [2]catenanes 4, 5 are stabilized by parallel-displaced π⋯π stacking interactions (of interlayer distance 3.34–3.82 Å) between the pyridyl fraction and phenyl moieties of three ligands L1, and by edge-to-face-type CH⋯π interactions (2.64–2.83 Å) between BiBzIm conjugated planes and phenyl hydrocarbon bond. Interestingly, the addition/removal of potassium ions resulted in topological transformation between the double trefoil knot 1 and the trefoil knot 2. In addition, the addition of DMF-d₇ to a methanolic solution of the double trefoil knot 1 induced its structural conversion into the metallamacrocycle 3. NIR photothermal studies of 1, 4, and 5 showed different photothermal responses in solid and solution states. Thus, the photothermal conversion efficiencies of 4, and 5 were within the limit of 16.8%–45.1% based on different power areas, the number and the fashion of π⋯π stacking interactions can directly affect the photothermal conversion efficiency, and EPR experiments confirmed the recorded results.