Tuning Molecular Motors: Molecular Dynamics of Novel Overcrowded Alkenes in Gas and Condensed Phase

Machines in biological cells have been honed through billions of years of evolution to perform specific tasks with exquisite efficiency. Artificial molecular machines aim to mimic cellular machines, but to fulfil new tasks harnessed to human health and technology. Molecular motors are essential components for the development of molecular machines. The family of photochemical molecular motors (PMMs), based on the overcrowded alkene motif, permit the conversion of light (free, readily available, and wireless) into the directional molecular rotational motion required by machines. Recently coupled pairs of PMMs have delivered concerted directional intramolecular rotation to support molecular translation. Here, we develop a strategy to optimise efficiency of PMMs and provide novel means for control of their function. We achieve this through the development and exploitation of new photochemical experiments in concert with expert collaborators in chemical synthesis.
PMMs operate through successive photoisomerization and thermal helix inversion (THI) steps to achieve unidirectional rotation. PMM efficiency is a product of the rate of inversion and the yield of isomerization. Tremendous synthetic efforts optimised the rate-determining THI, yielding PMMs capable of supporting MHz rotation rates. However, each successive improvement in rotation rate led to a reduction in isomerization yield. With the aim of optimising yield in the fastest PMMs, we characterised the detailed mechanism of isomerization using ultrafast spectroscopy. This characterisation revealed an important role for ‘sudden polarization’ along the reaction coordinate. We reasoned that substitution on the motor with electron donating and withdrawing groups would modulate this polarization and modify the yield. In one case we achieved a 99% yield in nonpolar media. Further, the reaction could be turned off by polar solvents, indicating medium control of motor performance. Here, we build on that preliminary observation in an extensive program aimed at controlling both yield enhancement and medium tunability. We will develop novel experiments for gas and condensed phase photochemical studies and apply them to a range of unsymmetric molecular motors. We will precisely determine excited state barriers and structural dynamics as a function of medium, achieved through fine synthetic control of PMM substituent position and character.
An important recent extension of PMMs was the synthesis of third-generation motors, comprising two unidirectional PMMs acting as ‘wheels’, supporting translational motion by successive isomerization/THI cycles. This is a major step forward and we made the first study of third-generation PMM photoisomerization dynamics. Key questions for this new family concern control over the translational motion, in particular over which bond isomerizes. Once one bond in one wheel has reacted, the symmetry is reduced. Can this change be exploited to ensure the second photoreaction is at the second bond? A tandem ion mobility experiment capable of photochemically or thermally preparing isomers, separating them on a millisecond timescale, and probing their isomerization dynamics will provide answers. A range of newly synthesised symmetrically and asymmetrically substituted third-generation PMMs will be studied. Gas phase measurements will be complemented by ultrafast spectroscopy in solution to resolve structural dynamics through the full photocycle, and to investigate the effect of medium polarity on bond switching order and yield.
In summary, bespoke experiments will probe photochemical reaction dynamics in a carefully designed series of novel PMMs. From these measurements we will learn how to optimise efficiency and control operation of PMMs, and thus enable technology that will drive the next generation of molecular machines.