Developing a new class of molecular machine: light-fuelled single- bond rotors Oliver M. Bayley, Beatrice S. L. Collins University of Bristol, UK Due to the central role of molecular machines in biological systems, chemists have long sought to create synthetic equivalents, for the development of future nanoscale technologies. [1] For molecular machines to play a diverse role in these technologies, a variety of machines which exhibit different types of motion or use different types of fuels are needed. This research looks at the development of a new class of molecular machine that uses light to drive rotation around an inherently non-light-responsive single C–C bond.
In our design, light controlled formation of one of two competing N–B dative bonds allows switchable control over two 180-degree rotary states of a borylated azo-biaryl scaffold ( 1 , Scheme 1). In the lowest energy state ( 1 -N azo –B), the azo nitrogen forms a dative bond with the boronic ester while the pyridine nitrogen forms an intramolecular hydrogen bond. Upon UV irradiation, the light responsive azobenzene moiety can undergo rapid E à Z photoisomerisation, breaking the N azo –B bond. [2] Breaking this N azo –B bond leads to the liberation of the boron’s p orbital, in turn causing the pyridine ring to undergo a 180° rotation around the central C–C bond to form a new N pyr –B bond ( 1 -N pyr –B). Upon thermal relaxation of the azobenzene to the original E isomer, the azo nitrogen becomes positioned to compete with the pyridine nitrogen for boronic ester binding. This in turn, leads to the reformation of the lower energy initial state ( 1 ‑N azo –B). If chirality transfer from the boronic ester to the biaryl axis is realised, it opens the possibility of controlling not just interconversion between the two states, but also the trajectory of that interconversion, thus providing the foundations for controlling directional rotation around the biaryl axis. To better understand the central N azo –B and N pyr –B bonds, a series of azobenzene and biaryl-pyridine model systems were synthesised, with their study providing novel insights into the nature of these N–B bonds and their involvement in photochemical processes. These studies have also proven that chirality associated with the ligands on the boron centre will influence the chirality of the biaryl axis (Scheme 2). The multistep synthesis of the target azo-biaryl was successful and used to make a variety of differently substituted azo-biaryl scaffolds. Using these different scaffolds, we have probed how different substitution patterns are able to fine-tune the competitive behaviour between the two possible N–B bonds. References 1. S. Kassem, T. van Leeuwen, A. S. Lubbe, M. R. Wilson, B. L. Feringa; D. A. Leigh, Chem. Soc. Rev. 2017, 46 , 2592-2621N. 2. Kano, J. Yoshino, T. Kawashima, OrganicLetters. 2005, 7 , 3909-3911.
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