On the possibility to accelerate the thermal isomerizations of overcrowded alkene-based rotary molecular motors with electron-donating or electron-withdrawing substituents.

Baswanth Oruganti,Bo Durbeej

doi:10.1007/s00894-016-3085-y

Baswanth Oruganti, Bo Durbeej

Open Access

https://doi.org/10.1007/s00894-016-3085-y

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Journal: Journal of molecular modeling	Publication Date: Aug 24, 2016
Citations: 61	License type: CC BY 4.0

Affiliation: Linköping University

Abstract

We employ computational methods to investigate the possibility of using electron-donating or electron-withdrawing substituents to reduce the free-energy barriers of the thermal isomerizations that limit the rotational frequencies achievable by synthetic overcrowded alkene-based molecular motors. Choosing as reference systems one of the fastest motors known to date and two variants thereof, we consider six new motors obtained by introducing electron-donating methoxy and dimethylamino or electron-withdrawing nitro and cyano substituents in conjugation with the central olefinic bond connecting the two (stator and rotator) motor halves. Performing density functional theory calculations, we then show that electron-donating (but not electron-withdrawing) groups at the stator are able to reduce the already small barriers of the reference motors by up to 18 kJ mol−1. This result outlines a possible strategy for improving the rotational frequencies of motors of this kind. Furthermore, exploring the origin of the catalytic effect, it is found that electron-donating groups exert a favorable steric influence on the thermal isomerizations, which is not manifested by electron-withdrawing groups. This finding suggests a new mechanism for controlling the critical steric interactions of these motors.Graphical The introduction of electron-donating groups in one of the fastest rotary molecular motors known to date is found to reduce the free-energy barriers of the thermal steps that limit the rotational frequencies by up to 18 kJ mol−1.Electronic supplementary materialThe online version of this article (doi:10.1007/s00894-016-3085-y) contains supplementary material, which is available to authorized users.

Highlights

Many of nature’s complex biological tasks are carried out using molecular-sized machines oftentimes referred to as molecular motors
Molecular motors that exhibit unidirectional rotary motion are commonly known as rotary molecular motors
The Blower^ half is known as the Bstator^, as it is immobilized on a surface in the functionalized form of the motor [35, 38,39,40,41], and the Bupper^ half is known as the Brotator^ that rotates around the central carbon-carbon double-bond (Baxle^) connecting the two halves

Summary

Introduction

Many of nature’s complex biological tasks are carried out using molecular-sized machines oftentimes referred to as molecular motors. In a recent computational study, the relative stabilities of the four different conformations and their potential roles in the rotary cycle of a slightly modified second-generation type II motor combining a thioxanthene stator with a cyclopenta[a]napthalenylidene rotator were assessed using density functional theory (DFT) methods [42] This motor, hereafter referred to as motor 1a, is shown in Scheme 2, together with the rotary cycle predicted by these calculations [42]. This finding suggests an approach for improving the rotational frequencies of overcrowded alkene-based motors that is complementary to the approach based on optimization of the steric bulkiness of the rotator substituent [42]

Methods

Results and discussion

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