Increased Speed of Rotation for the Smallest Light-Driven Molecular Motor

Abstract

In this paper we present the smallest artificial light-driven molecular motor consisting of only 28 carbon and 24 hydrogen atoms. The concept of controlling directionality of rotary movement at the molecular level by introduction of a stereogenic center next to the central olefinic bond of a sterically overcrowded alkene does not only hold for molecular motors with six-membered rings, but is also applicable to achieve the unidirectional movement for molecular motors having five-membered rings. Although X-ray analyses show that the five-membered rings in the cis- and trans-isomer of the new molecular motor are nearly flat, the energy differences between the (pseudo-)diaxial and (pseudo-)diequatorial conformations of the methyl substituents in both isomers are still large enough to direct the rotation of one-half of the molecule with respect to the other half in a clockwise fashion. The full rotary cycle comprises four consecutive steps: two photochemical isomerizations each followed by a thermal helix inversion. Both photochemical cis-trans isomerizations proceed with a preference for the unstable diequatorial isomers over the stable diaxial isomers. The thermal barriers for helix inversion of this motor molecule have decreased dramatically compared to its six-membered ring analogue, the half-life of the fastest step being only 18 s at room temperature.

Computational design of faster rotating second-generation light-driven molecular motors by control of steric effects

“…24,44 A major experimental effort has therefore been invested in accelerating the rate-limiting thermal steps of overcrowded alkene-based rotary molecular motors, 18,19,21,[23][24][25]27,28,33 exploring the influence of conformational, steric and electronic factors. From such studies, it has been found that one way of lowering the thermal freeenergy barriers of type IIa motors is to contract the sizes of the rotator and stator by employing a cyclopenta[a]naphthalenylidene rotator and a fluorene stator, as done in type IIb motors.…”

“…[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] Several examples of such systems known as either first-generation or second-generation rotary molecular motors are shown in Scheme 1.…”

A Reversible Synthetic Rotary Molecular Motor

“…Inspired by such biological motors and by Feynman_s celebrated discussion (4) of the miniature ratchet and pawl first introduced (5) by Smoluchowski, efforts have been made to design molecules that exhibit directional control over submolecular rotary motion (6)(7)(8)(9)(10). Unidirectional rotation about single (11,12), double (13)(14)(15)(16), and mechanical (17) bonds has been achieved, but unlike F 1 F 0 -ATPase, these artificial motor molecules are only able to rotate in one direction but not the other. We now report on a molecular structure in which a fragment can be circumrotated in either direction, and we probe features of the underlying mechanism.…”

Computational Design of Molecular Motors and Excited-State Studies of Organic Chromophores

“…By variationally optimizing the electronic energy for this single-determinantal form of the wavefunction under the constraint of orthonormalization of the one-electron MOs φ i , we obtain the HF equations 18) to obtain matrix eigenvalue equations known as the Roothaan-Hall equations, 43,44 FC = SCε, (2.19) where F is the Fock matrix, F kl = <µ k |F|µ l >; S is the overlap matrix, S kl = <µ k |µ l >; C is the matrix of MO coefficients,{c ki }; and ε is the diagonal matrix of MO energies, {ε i }.…”

“…A major experimental effort has therefore been invested to improve the rates and efficiencies of overcrowded alkene-based molecular motors. 9,10,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] In this regard, computational methods are particularly useful as they can provide detailed insights into the mechanisms and dynamics of the chemical reactions that produce the rotary motion. Such insights may not be easily obtained by experimental techniques, because of the difficulties associated with detecting transient species such as reactive intermediates and transition structures (TSs) or measuring the timescales of ultrafast processes.…”