Supracluster Rotaxanes with Adaptive Al8 Macrocycle for Tunable Optical Limiting

Abstract

Abstract Mechanically interlocked molecules like rotaxane have attracted considerable attention in supramolecular chemistry and have been shown useful for the construction of molecular machines. Metal coordination chemistry plays an important role to expand the rotaxanes and immobilize the ring components to the axial molecules. Although many types of macrocycles have been extensively used to make rotaxanes, those based on metal-organic macrocycles are relatively undeveloped. Therefore, there is a large potential for exploring their supracluster assemblies for target properties. Different from the currently limited study on "axle-H···ring” noncovalent binding mode in host-guest inorganic-organic supramolecular chemistry, we herein developed an opposite supracluster assembly pattern naming "ring-H···axles". The key to the success of donor and acceptor role reversal is the use of a flexible Al8 macrocycle: its capsule-shaped cavity with sufficient hydrophobic aromatic inner wall and hydrophilic -OH sites provide an excellent environment to form and adapt to the axles. Such "ring-H···axles" noncovalent binding mode pattern altered the fixed binding mode of ring and axle in reported hybrid inorganic-organic rotaxanes and versatility of the axles involving molecules, metal complexes and infinite metal coordination “strings”. The capsule-shaped Al8 macrocycle acts as a confined reactor demonstrating the directional, stepwise, and controlled assembly process from molecules, metal complexes to metal coordination “strings”, resulting in the isolation from [2]-pseudorotaxane, [2]-rotaxane, to higher-ordered polyrotaxanes. Different from the traditional solution-phase method to make rotaxane, this work provides an atomically precise synthesis route for constructing supracluster rotaxane. Such controlled synthesis in the atomic-level confined cavity provides an excellent platform for property study. A rigid Al8 macrocycle can trap metal centers through electrostatic interaction, while flexible organic ligands can interact with aromatic guests through π···π stacking. Therefore, the heavy metal effect and conjugation effect can be introduced simultaneously to remarkably improve photo-related properties such as optical limiting and third-order nonlinear responses.