Stimuli-Responsive Metal–Organic Cages

Abstract

Biological systems are capable of responding to diverse stimuli, then processing the signals and transferring them into potential changes in structures and/or properties. Studying the stimuli-responsiveness in metal–organic cages provides a platform to mimic biological systems and gain insights into the fundamental principles underlying specific functions. This will also offer valuable guidance for the design of novel responsive materials and devices, such as sensors. This thesis is focused on the design and synthesis of metal–organic cages that can respond to physical stimuli, investigating how these stimuli modulate host–guest chemistry within the cages. Two systems are presented with response to two different stimuli respectively: 1) a thermally induced spin-crossover system where spin states of the metal centers can be altered by the change of temperature; 2) a redox-active azopyridine-based system that can reversibly accept and donate electrons. In the first system, the strategy for constructing FeII spin-crossover cages has been specifically explored. The tetrahedral spin-crossover cage can encapsulate various guests, which stabilize different cage spin states depending on guest size. Conversely, the spin-crossover tetrahedron exhibits different affinities for guests in different spin states. Examination of spin-crossover thermodynamics across a series of host–guest complexes enabled sensitive probing of guest fit to the host cavity, providing information complementary to binding-constant determination. The construction of a series of novel metal–organic cages which contain redox-active azo groups coordinated to FeII centers is demonstrated in the second system. Upon reduction of the cages, their guests are released and may then be re-encapsulated when the cages are regenerated by oxidation. Since the redox centers are on the ligand arms, they are modular and can be attached to a variety of ligand cores to afford varying and predictable architectures. This method thus shows promise as a generalized approach for designing redox-controlled guest release and uptake systems.