Title:	Making molecular motors more efficient
DNr:	SNIC 2015/1-419
Project Type:	SNIC Medium Compute
Principal Investigator:	Bo Durbeej <bo.durbeej@liu.se>
Affiliation:	Linköpings universitet
Duration:	2015-12-01 – 2016-12-01
Classification:	10407 10405 10603
Homepage:	http://www.liu.se/forskning/foass/bo-durbeej
Keywords:

Abstract

This project, which is supported by Linköping University (career contract) and the Swedish Research Council (grant for young researchers), involves computational chemistry studies for making molecular motors more efficient. Molecular motors are molecules that can perform work by absorbing energy and converting the energy into directed mechanical motion such as rotation around a chemical bond. Because of their ability to execute a number of useful functions, such as rotating objects thousand-fold heavier than themselves and acting as wheels in perfectly manoeuvrable nanocars, one class of synthetic light-driven molecular motors that have attracted a considerable interest in recent years are those developed from sterically overcrowded alkenes. However, to fully realize the nanotechnological potential of these motors, which accomplish full (i.e., 360 degrees) rotary motion through sequential photochemical and thermal steps, it is imperative to design and synthesize systems capable of achieving as high rotational frequencies as possible, beyond the current record of ~3 MHz. In the last two years, and thanks to previous SNAC Medium allocations on Triolith, we have contributed to the development of faster molecular motors of this type by identifying several ways to accelerate the thermal steps of the rotary cycles. Specifically, by exploring the ground-state potential energy surfaces of the motors and locating the relevant stationary points by means of quantum chemical calculations, we have discovered a variety of steric, electronic and conformational approaches for lowering the free-energy barriers of the thermal steps (see, for example, RSC Adv. 2014, 4, 10240; Phys. Chem. Chem. Phys. 2015, 17, 21740). Using such approaches, we have shown that it is indeed possible to bring the rotational frequencies of overcrowded alkene-based molecular motors into new territory, beyond the MHz regime. However, in order to improve the rotational frequencies of the motors even further, it is critical to also gain an understanding of how the photochemical steps of the rotary cycles can be accelerated. This is the goal of the present project. From a computational point of view, this goal is more difficult to attain, because it is not sufficient to explore the associated excited-state potential energy surfaces through static quantum chemical calculations alone. Rather, since the excited-state surfaces are much shallower than the ground-state surfaces and since the motors need to promptly decay to the ground state upon completion of the photochemical steps, such understanding can only be gained by performing non-adiabatic excited-state molecular dynamics simulations using so-called surface hopping techniques. Through simulations of this kind, it is possible to predict how long the photochemical steps take, to estimate the corresponding quantum yields, and to investigate whether the decay to the ground state does indeed occur promptly. All in all, by carrying out simulations for a variety of different motors, this project will provide a broad picture of ways to accelerate the rotary motion of light-driven overcrowded alkene-based molecular motors.