Developing a new class of molecular machine

: light‐fuelled single‐bond rotors

Abstract

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. The research herein details new work towards the development of a novel class of molecular machine which uses light to drive rotation around an inherently non-light-responsive single C–C bond. While many classes of molecular machines have been developed, as discussed in chapter 1, the class of light-fuelled single-bond rotor is yet to be realized.

In our design, light controlled formation of one of two competing N–B bonds allows switchable control over two 180° rotary states of a borylated azo-biaryl scaffold (states A and B). In the lowest energy state (state A), the azo nitrogen should form a dative bond with the boronic ester while the pyridine nitrogen should form an intramolecular hydrogen bond. Upon UV irradiation, the light responsive azobenzene moiety can undergo E to Z photoisomerization, breaking the N_azo–B bond. This should in turn cause the pyridine ring to undergo a 180° rotation around the central C–C bond to form a new N_pyr–B bond (state B). Upon thermal relaxation back to the E isomer, the azobenzene nitrogen outcompetes the pyridine nitrogen for boronic ester binding, causing the pyridine ring to undergo a second 180° rotation, reforming the lower energy initial state (State A). If the chirality of the biaryl axis can be controlled using a chiral boronic ester, it opens the possibility of controlling the trajectory of these pyridyl ring rotations and would provide the foundations for a light-fuelled single-bond rotor.

To better understand the central N_azo–B and N_pyr–B bonds, azobenzene and biaryl-pyridine model systems were synthesized, with their study described in chapters 2 and 3. These studies provided novel insights into the nature of these N–B bonds, their involvement in photochemical processes and demonstrated that chiral boron ligands had the ability to influence the chirality of the biaryl axis.
As discussed in chapter 4, the multistep synthesis of the target azo-biaryl proved challenging but ultimately successful and was used to make a variety of differently substituted azo-biaryl scaffolds. In chapter 5 a selection of these scaffolds were studied by ¹¹B and ¹⁵N NMR, compared against other benchmarking standards, and the NMR data used to understand how the balance between intramolecular bonds can be controlled by the pyridyl substitution. These findings are then used to identify a suitable motor candidate in which the N_azo–B bond formation is favoured in state A. Having established methods of favouring the N_azo–B bond formation in state A, the photoswitching between these N–B bonds was investigated and shown to be a viable process. Due to rapid rates of thermal relaxation in the hydroxy azobenzenes, the research shifted towards the development of new UV Vis and NMR equipment which allowed the azobenzene systems to be monitored under constant irradiation. Finally, the new avenues of research stemming from this work, the ongoing efforts and the future of the project are discussed in chapter 6.

Date of Award	5 Dec 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Beatrice Collins (Supervisor) & M C Galan (Supervisor)