Strategic synthesis and near-infrared photothermal conversion of unique single/double trefoil knots and [2]catenane assemblies

The rare molecular structures and application possibilities in a variety of fields has attracted the attention to molecular knots and catenanes in recent years. However, their selective synthesis remains a major challenge. Herein, a new diimidazole-based flexible ligand precursor L1 possessing two phenyl conjugated groups and two methylene moieties has been synthesized. In addition, five half-sandwich-based building units (B1-B5) featuring different sizes and functional groups were selected to combine with compound L1 for generating different molecular knots and catenanes, by following a classical coordination-driven self-assembly strategy. Thus, one double trefoil knot, two trefoil knots, one D-type [2]catenane and one metallamacrocycle have been isolated in remarkable yields. Interestingly, the D-shaped [2]catenane 2 could be converted into metallamacrocycle 1 upon light irradiation and introduction of Cl⁻ ions. The formation of all the indicated structures has been unequivocally confirmed by single-crystal X-ray diffraction analysis, NMR spectroscopy and ESI-TOF-MS. Photothermal conversion studies showed that the double trefoil knot 5 exhibits remarkable photothermal conversion efficiency in near-infrared (NIR) experiments (31.32% - 40.28%), which can be attributed to its unique topological structure and π-π stacking interactions. Electron paramagnetic resonance (EPR) experiments fully confirmed the recorded results.