Abstract
Synthetic molecular motors driven by E/Z photoisomerization reactions are able to produce unidirectional rotary motion because of a structural asymmetry that makes one direction of rotation more probable than the other. In most such motors, this asymmetry is realized through the incorporation of a chemically asymmetric carbon atom. Here, we present molecular dynamics simulations based on multiconfigurational quantum chemistry to investigate whether the merits of this approach can be equaled by an alternative approach that instead exploits isotopic chirality. By first considering an N-methylpyrrolidine–cyclopentadiene motor design, it is shown that isotopically chiral variants of this design undergo faster photoisomerizations than a chemically chiral counterpart, while maintaining rotary photoisomerization quantum yields of similarly high magnitude. However, by subsequently considering a pyrrolinium–cyclopentene design, it is also found that the introduction of isotopic chirality does not provide any control of the directionality of the photoinduced rotations within this framework. Taken together, the results highlight both the potential usefulness of isotopic rather than chemical chirality for the design of light-driven molecular motors, and the need for further studies to establish the exact structural circumstances under which this asymmetry is best exploited.
Highlights
Molecular motors are molecules that use energy from an external source to produce mechanical motion and have the ability to control the direction of the resulting motion.[1−3] Among the different types of synthetic molecular motors available today, those that achieve 360° unidirectional rotary motion through UV- or visible-light-powered photoisomerization reactions around a double bond are commonly referred to as light-driven rotary molecular motors, with examples including overcrowded-alkene,[4,5] hemithioindigo,[6,7] dibenzofulvene,[8] and N-alkyl-imine[9,10] motors
nonadiabatic molecular dynamics (NAMD) trajectories, these results suggest that at least within certain molecular frameworks, the potency of isotopic chirality in facilitating high-quantum yields (QYs) rotary motion in light-driven molecular motors is comparable to that of chemical chirality, despite that isotopic chirality affords less steric hindrance to control the directionality of the photoisomerizations
We have performed extensive NAMD simulations to investigate potential merits of using isotopic chirality instead of chemical chirality to induce unidirectional rotary motion in molecular motors operated by E/Z photoisomerization reactions around an olefinic bond
Summary
Molecular motors are molecules that use energy from an external source to produce mechanical motion and have the ability to control the direction of the resulting motion.[1−3] Among the different types of synthetic molecular motors available today, those that achieve 360° unidirectional rotary motion through UV- or visible-light-powered photoisomerization reactions around a double bond are commonly referred to as light-driven rotary molecular motors, with examples including overcrowded-alkene,[4,5] hemithioindigo,[6,7] dibenzofulvene,[8] and N-alkyl-imine[9,10] motors. The required control of the photoinduced rotation of such systems is attained by the introduction of chirality into the motors and the asymmetry between clockwise (CW) and counterclockwise (CCW) photoisomerization directions that chirality imparts on their excited-state dynamics. While most of these motors contain a chiral center, it is not absolutely essential that they do. Complementing these structure-based approaches to introduce the desirable asymmetry, it is possible to steer the rotary motion of light-driven molecular motors by means of the chirality of the radiation supplied to them.[13]
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