Second Generation Light-Driven Molecular Motors. Unidirectional Rotation Controlled by a Single Stereogenic Center with Near-Perfect Photoequilibria and Acceleration of the Speed of Rotation by Structural Modification

Abstract

Nine new molecular motors, consisting of a 2,3-dihydro-2-methylnaphtho[2,1-b]thiopyran or 2,3-dihydro-3-methylphenanthrene upper part and a (thio)xanthene, 10,10-dimethylanthracene, or dibenzocycloheptene lower part, connected by a central double bond, were synthesized. A single stereogenic center, bearing a methyl substituent, is present in each of the motors. MOPAC93-AM1 calculations, NMR studies, and X-ray analysis revealed that these compounds have stable isomers with pseudoaxial orientation of the methyl substituent and less-stable isomers with pseudoequatorial orientation of the methyl substituent. The photochemical and thermal isomerization processes of the motors were studied by NMR and CD spectroscopy. The new molecular motors all show two cis-trans isomerizations upon irradiation, each followed by a thermal helix inversion, resulting in a 360 degrees rotation around the central double bond of the upper part with respect to the lower part. The direction of rotation is controlled by a single stereogenic center created by the methyl substituent at the upper part. The speed of rotation, governed by the two thermal steps, was adjusted to a great extent by structural modifications, with half-lives for the thermal isomerization steps ranging from t(1/2)(theta) 233-0.67 h. The photochemical conversions of two new motors proceeded with near-perfect photoequilibria of 1:99.

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.…”

In control of switching, motion, and organization

“…A more pronounced effect is seen when X is altered. By changing from X = S (6) to X = CH 2 (8), the half-life for the thermal isomerization step could be decreased from 215 h to 40 min at room temperature [29]. Much to our surprise, in the motor systems 9 and 10, shown in Fig.…”

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

“…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.…”