New Concepts for Hemithioindigo Photoswitches and Machines

Molecular switches and machines have reached almost every field of application, from medicine to materials science. Heat, pH changes, and chemical inputs are often required for their operation. However, there are also examples that operate under light irradiation, so-called photoswitches. Light as an energy source is of great interest, as its properties can be easily controlled by external electronic devices. Furthermore, light allows a high degree of spatiotemporal control without the generation of waste products. As a consequence, the field of molecular photoswitches has been vastly developed, encompassing different molecular frameworks. Among these, hemithioindigos (HTIs) represent a promising class since they possess many beneficial properties such as addressability with visible light and high thermal stability. To improve the performance of HTI photoswitches, the threefold substituted HTIs were further developed into fourfold substituted diaryl-HTIs and new design principles for high performance photoswitches were established in a systematic study. A combination of strong electron donor groups within the molecular setup and cross-conjugation were employed for this purpose. In addition, a further design element in the form of a pyridine ring was introduced to achieve late-stage modifications and metal-ion binding, which was finally exploited via a specific acid/base response of diaryl-HTIs for the application in logic operations. In a further study, HTI photoswitches were considered under the aspect of sustainable chemistry. Here, a new synthetic strategy based on dynamic covalent chemistry of amines with alkyl bromides was established to reversibly convert a symmetric, non photoswitchable precursor into a diaryl-HTI ammonium salt photoswitch. The resulting photoswitches showcase high performance and the symmetric precursor experienced a gain of function. Interestingly, the ammonium HTI species could be reverted back to the symmetric precursor in a dealkylation step and functionalization of the recycled precursor with another alkyl residue can be accomplished. Additionally, a new level of control was established to influence the recyclability in an on/off manner and to generate long-lived and thermally stable ammonium salts. This was achieved via a reversible exchange of the counterions. In summary, a new basis for the design of novel, recyclable photoswitches was developed, which holds great potential for future applications. Finally, a novel approach to introduce directionality in HTI-based molecular motors was tackled. Conventionally, the directionality of HTI motor motion originates from the stereoinformation of the sulfoxide. Within this concept, the transfer of stereochemical information from a chiral ligand in a metal complex to the HTI architecture is envisioned. For this purpose, HTI ligands for the complexation of metal ions were designed. The coordinating nature of sulfur together with nitrogen containing heterocycles incorporated in the HTI core or in the periphery was used to construct bidentate ligands. This approach allows the design of new HTI motors with beneficial properties such as strongly red-shifted absorptions and accelerated rotational motions.