On the effect of donor and acceptor substituents on the behaviour of light-driven rotary molecular motors

Abstract

Light-driven rotary molecular motors based on overcrowded alkenes can be substituted with electron-donating and electron-withdrawing substituents (R = OMe, Cl and CN) in direct conjugation with the central double bond (the axis of rotation) without having a significant influence on the rate-limiting, thermal isomerisation step of their rotary cycle. This indicates that in this system, it is predominantly steric factors that determine the barrier to the thermal helix inversion. In contrast, the quantum yield and photoequilibria in the photochemical step were found to be quite sensitive to the combination of substituent and solvent employed.

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.…”

“…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. 23,28 Furthermore, for type IIb motors, using a bulkier stereogenic substituent than methyl has also been found to lower the thermal barriers. 23 Complementing these experimental efforts are a number of computational studies that have offered valuable mechanistic insight into the photochemical steps of overcrowded alkene-based rotary molecular motors, [45][46][47][48][49][50] and also identified other molecules as possible light-driven rotary 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.…”

Driving Unidirectional Molecular Rotary Motors with Visible Light by Intra- And Intermolecular Energy Transfer from Palladium Porphyrin

“…Substitution on this position of the motor has no effect on the thermal helix inversion, although the photostationary state is affected by electron donating or withdrawing substituents. 44 In a different molecular motor, the effect of substitution with large alkyl groups was also reported to be minimal, 45 so we do not expect a significant difference in behavior between the motor unit in 12 and compound 1.…”