Fine Tuning of the Rotary Motion by Structural Modification in Light-Driven Unidirectional Molecular Motors

Abstract

The introduction of bulky substituents at the stereogenic center of light-driven second-generation molecular motors results in an acceleration of the speed of rotation. This is due to a more strained structure with elongated C=C bonds and a higher energy level of the ground state relative to the transition state for the rate-limiting thermal isomerization step. Understanding the profound influence that variation of the substituent at the stereogenic center holds over the rotational speed of the light-driven molecular motor has enabled the development of the fastest molecular motor reported thus far.

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.…”

“…24 Thus, if one of the thermal steps is accelerated by modifying the steric size of the substituents, then the other thermal step may be decelerated on the same grounds. 24 In second-generation rotary molecular motors (e.g., IIa, IIb and IIc in Scheme 1), which contain a single stereocenter on the rotator, 18,23,25,26,28 this limitation is circumvented by employing a nearly symmetric stator, whereby the two thermal steps are similarly affected by the stereogenic substituent. 24 The type IIa motors typically employ a xanthene (X = O), thioxanthene (X = S) or anthracene-based (X = CH 2 ) stator combined with a naphthopyran (Y = O), naphthothiopyran (Y = S) or phenanthrylidenebased (Y = CH 2 ) rotator.…”

Stimuli‐Directed Dynamic Reconfiguration in Self‐Organized Helical Superstructures Enabled by Chemical Kinetics of Chiral Molecular Motors

“…In contrast to the chiral overcrowded alkenes reported before,48, 57, 61 the new chiral motor M1 in the present study was designed with structural modification on the rotor region (Figure 1a). As reported previously,49 the length of the rigid substituents at the 6‐position of naphthalene ring play an crucial role on the entropic barrier for rotation, but insignificantly affect the enthalpy of activation.…”

“…Due to the dynamic controllability of unidirectional rotation, the stereoselectively chiroptical inversion between the stable ( P ) form and the unstable ( M ) form is reversible with a variable half‐life by structural modification from 8 ns to 1.4 × 10 3 years 47, 48, 49, 50. The resulting tunability of geometrical charity has been significantly investigated to create photoactiving chiral catalysts in solvents51, 52 and polar switchovers on solid surfaces 53, 54, 55.…”