Harnessing light beyond equilibrium: from molecular photoswitches to out-of-equilibrium supramolecular assemblies

Abstract

Supramolecular chemistry enables dynamic, functional architectures to be constructed through reversible, non-covalent interactions. Metal-organic cages, in particular, offer a versatile platform for exploring light-responsive behaviour, owing to their defined geometry, internal cavities, and capacity for structural reconfiguration. This thesis explores how light can be harnessed to control and drive the behaviour of these assemblies, progressing from reversible molecular photoswitching to out-of-equilibrium transformations sustained by continuous irradiation. <br/>

Three interconnected projects examine how metal coordination, ligand geometry, and continuous light input influence the switching behaviour and structural dynamics of self-assembled cages. The first project explores the influence of metal coordination on the photochemical properties of azobenzene derivatives. A tridentate azobenzene-based ligand was designed to form a mononuclear Pd(II) complex, enabling the systematic investigation of the effects of coordination on isomerisation efficiency, thermal relaxation, and reversibility. These studies provide a foundation for understanding how proximity to metal centres modulates switching behaviour. <br/>

The second project investigates how variations in coordination vectors, resulting from ligand connectivity and <em>E</em>/<em>Z</em> isomerisation, influence the geometry and responsiveness of metal-organic cages. Comparative studies of <em>para</em>- and <em>meta</em>-substituted photoswitchable azobenzene ligands revealed that subtle structural changes can have a significant impact on the extent and reversibility of light-induced reconfiguration in Co(II)-based assemblies. The conditions under which concerted, reversible switching can be achieved were also examined in this work. <br/>

The final project applies these principles to out-of-equilibrium systems, demonstrating how continuous light input can drive a metallo-supramolecular assembly away from equilibrium via a molecular ratchet mechanism. Pd(II)-based structures incorporating azobispyrazole ligands were designed to undergo directional transformations between discrete assemblies under sustained irradiation. As both the <em>E</em> and <em>Z</em> states absorb visible light, the system remains responsive under constant white-light and sunlight exposure, operating autonomously and temporarily storing light energy as chemical energy. <br/>

Together, these studies advance our understanding of how molecular photoswitching translates into controllable supramolecular behaviour, and how light can be harnessed to not only modulate structure, but also drive directional, energy-fuelled transformations. By establishing design principles that preserve switching function, elucidating how ligand geometry governs responsiveness, and demonstrating autonomous operation under continuous irradiation, this work bridges the gap between molecular photoswitches and functional, light-powered supramolecular machines. These insights pave the way for future designs that couple structural adaptability with energy-driven functionality, providing a blueprint for artificial molecular devices capable of converting light into stored chemical energy.