The Quest for Molecular Grippers: Photo-Electric Control of Molecular Gripping Machinery

The quest for nanoscale molecular machines has inspired the search for their close relatives, molecular grippers. This path was paved by the development of resorcin[4]arene cavitands and their quinone-based redox-active congeners. In this Concept article, the efforts to design and establish the control of quinone-functionalized resorcin[4]arenes by electronic and electromagnetic stimuli is described. This was achieved by relying on paramagnetic semiquinone radical anions formed electrochemically or by photoredox catalysis. The gripper-like motion of such species could not be studied by conventional NMR spectroscopy. Instead, an entirely different approach had to be developed that included various electroanalytical and spectroelectrochemical methods, including UV/Vis/NIR spectroelectrochemistry, pulsed EPR and Davies H-1 ENDOR spectroscopy, transient absorption spectroscopy, and time-resolved luminescence measurements, besides density functional theory calculations and X-ray crystallography. The conceptual breakthroughs are reviewed as well as the current state and future perspectives of photoredox-switchable molecular grippers.