Abstract
In recent years, much progress has been made in the design, synthesis and operation of light-driven rotary molecular motors based on chiral overcrowded alkenes. Through consecutive cis–trans photoisomerization and thermal helix inversion steps, where the latter dictate the overall rate of rotation, these motors achieve a full 360° unidirectional rotation around the carbon–carbon double bond connecting the two (rotator and stator) alkene halves. In this work, we report quantum chemical calculations indicating that a particularly fast-rotating overcrowded alkene-based motor capable of reaching the MHz regime, can be made to rotate even faster by the substitution of a rotator methyl group with a methoxy group. Specifically, using density functional theory methods that reproduce the rate-limiting ∼35 kJ mol−1 thermal free-energy barriers shown by the methyl-bearing motor with errors of ∼5 kJ mol−1 only, it is predicted that this substitution reduces these barriers by a significant 15–20 kJ mol−1. This prediction is preceded by a series of benchmark calculations for assessing how well density functional theory methods account for available experimental data (crystallographic, UV-vis absorption, thermodynamic) on the rotary cycles of overcrowded alkenes, and a detailed examination of the thermal and photochemical reaction mechanisms of the original motor of this type.
Highlights
The construction and operation of motors of molecular dimensions that can execute useful functions is a formidable challenge and cornerstone activity in nanotechnology.[1,2,3] Molecular motors are molecules that can perform work by absorbing external energy and converting the energy to directed mechanical motion such as rotation or translation.[4,5,6,7,8,9,10] Motors that produce unidirectional rotary motion are referred to as rotary molecular motors
Using density functional theory methods that reproduce the rate-limiting $35 kJ molÀ1 thermal free-energy barriers shown by the methyl-bearing motor with errors of $5 kJ molÀ1 only, it is predicted that this substitution reduces these barriers by a significant 15–20 kJ molÀ1
This prediction is preceded by a series of benchmark calculations for assessing how well density functional theory methods account for available experimental data on the rotary cycles of overcrowded alkenes, and a detailed examination of the thermal and photochemical reaction mechanisms of the original motor of this type
Summary
The construction and operation of motors of molecular dimensions that can execute useful functions is a formidable challenge and cornerstone activity in nanotechnology.[1,2,3] Molecular motors are molecules that can perform work by absorbing external energy and converting the energy to directed (i.e., non-Brownian) mechanical motion such as rotation or translation.[4,5,6,7,8,9,10] Motors that produce unidirectional rotary motion are referred to as rotary molecular motors (or molecular rotors). Besides being able to control the direction (clockwise or counterclockwise) of rotation, such systems are characterized by their ability to rotate a full 360 and to repeat the rotation for a large number of cycles through consumption of energy. While Nature's biological machinery contains a number of complex protein assemblies that convert the energy stored in chemical bonds into directed rotary motion,[11] such as ATP synthase,[12] the rst synthetic unidirectional rotary molecular motors were developed by Kelly[13] and Feringa[14,15] and their coworkers in the late nineties. Upon uptake of chemical and light energy, respectively, these systems produce motion consisting of rotation around a covalent bond: the former motor around a carbon–carbon single bond in a triptycene derivative[13] and the latter around a carbon–carbon double bond in a sterically overcrowded alkene.[15] For both motors, chirality is an essential feature for the unidirectional rotary motion
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