The Motion of an Azobenzene Light-Controlled Switch: A Joint Theoretical and Experimental Approach

To gain further insight into the internal motion of molecular objects, we have synthesized a molecular turnstile AzoT composed of a rotor based on flexible tetraethyleneglycol (TEG) chains grafted on aromatic moieties and a stator containing a photoswitchable azobenzene (Azo) fragment. The control of the reversible light-induced E-AzoT⇆Z-AzoT isomerization is supported by both NMR spectroscopy and photophysical investigation, which show that the system exhibits a fatigueless isomerization switching process. Furthermore, 2D NMR spectroscopy points to the fact that the free internal motion is triggered by the E-AzoT⇆Z-AzoT isomerization. Using molecular dynamics simulations and DFT calculations we have investigated the nature of the internal motions. An internal rotation characterized by an energy barrier of 23 kJ/mol is found for the Z-AzoT isomer. In contrast, this barrier reaches 151 kJ/mol for the E-AzoT isomer, excluding any “classical” rotation at room temperature. This rotational movement could in principle occur via tunneling. A simple model calculation, however, excludes tunnelling to take place before 20 ms.