title: On the possibility to accelerate the thermal isomerizations of overcrowded alkene-based rotary molecular motors with electron-donating or electron-withdrawing substituents.
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 Abstract 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).
authors.affiliation: Division of Theoretical Chemistry, IFM, Linköping University, SE-581 83, Linköping, Sweden.
authors.forename: Baswanth
authors.initials: B
authors.lastname: Oruganti
authors.affiliation: Division of Theoretical Chemistry, IFM, Linköping University, SE-581 83, Linköping, Sweden. bodur@ifm.liu.se.
authors.forename: Bo
authors.identifier: http://orcid.org/0000-0001-5847-1196
authors.initials: B
authors.lastname: Durbeej
keywords: Electronic effects; Molecular motors; Quantum chemistry; Rotary rates; Steric effects