Strategies for Red-Light Photoswitching

Responsive, adaptive and even intelligent molecular systems have been identified as the key to next-generation pharmaceuticals and functional materials. Photoswitches, compounds that isomerise reversibly between two distinct ground-state species upon excitation with light and consequently give rise to a macroscopic effect, are an integral part of this future. Their potential application areas range from photopharmacology to optoelectronics and soft robotics. However, most conventional photoswitch structures such as azobenzenes absorb ultraviolet light, high-energy photons that are detrimental to many artificial materials and especially to living systems. To harness their full potential, photoswitches should function efficiently with visible light that is benign to the environment. Red or near-infrared light would be the ideal stimulus for switches utilised in biological context, as these wavelengths are least absorbed by living tissue. The same applies to light-driven molecular motors, compounds that exhibit unidirectional rotation upon photoexcitation. In addition to absorption in the red part of the visible spectrum, both switches and motors should exhibit efficient and fast photoisomerisation, favourable thermal isomerisation kinetics and tolerance towards different environments in order to be useful in real-life applications. In this light, it is crucial to understand the underlying fundamental mechanisms that govern these attributes.

In this thesis, we explore three different approaches to realise photoswitching with red light: (i) synthetic modifications of azobenzenes, (ii) utilisation of new photoswitch cores that inherently absorb low-energy photons, and (iii) indirect isomerisation with red-light photocatalysts. We study each strategy from a theoretical viewpoint and demonstrate that they all provide means to induce isomerisation with red light, each with unique advantages and challenges in terms of promoting efficient, fast and robust switching. As a result, a single optimal photoswitch system cannot be designed; instead, the challenge lies in identifying the best design for each application. The same principles can also be applied to molecular motors, giving rise to visible-light-powered unidirectional rotary motion on a molecular level. We show that drawing inspiration from red-light-absorbing photoswitches has repercussions not only on the visible-light absorption but also on enhanced rotation dynamics.