Observation of Near-Infrared Photothermal and Photoacoustic Effects in a Metallosupramolecular Trefoil Knot

Coordination-driven self-assembly is a facile and powerful strategy for the construction of metallosupramolecular mechanically interlocked molecules (MIMs). However, the limited stability of most MIMs constitutes a challenge with respect to the evaluation of their practical applications. Herein, we demonstrate a rational strategy to control the self-assembly outcome for selected metallosupramolecular structures. The reaction of a biphenyl-bridged bisimidazole ligand with dinuclear building blocks of varying lengths leads, depending on the size of the dinuclear building block, to a simple [1+1] metallacycle, a twisted [2+2] three-dimensional trapezoidal structure, or an intricate [3+3] metallosupramolecular trefoil knot. The trefoil knot exhibits exceptional structural stability, resisting decomposition upon dilution in polar solvent. Its knotted topology enforces strong intramolecular π⋅⋅⋅π stacking interactions, which red-shift the absorption band into the near-infrared (NIR) region. Benefiting from its stability and NIR absorption, the trefoil knot displays a high photothermal conversion efficiency (η = 73.5%) and a potent photoacoustic (PA) effect in water, even at low concentrations.