Light-driven monodirectional molecular rotor

Abstract

Attempts to fabricate mechanical devices on the molecular level have yielded analogues of rotors, gears, switches, shuttles, turnstiles and ratchets. Molecular motors, however, have not yet been made, even though they are common in biological systems. Rotary motion as such has been induced in interlocked systems and directly visualized for single molecules, but the controlled conversion of energy into unidirectional rotary motion has remained difficult to achieve. Here we report repetitive, monodirectional rotation around a central carbon-carbon double bond in a chiral, helical alkene, with each 360 degrees rotation involving four discrete isomerization steps activated by ultraviolet light or a change in the temperature of the system. We find that axial chirality and the presence of two chiral centres are essential for the observed monodirectional behaviour of the molecular motor. Two light-induced cis-trans isomerizations are each associated with a 180 degrees rotation around the carbon-carbon double bond and are each followed by thermally controlled helicity inversions, which effectively block reverse rotation and thus ensure that the four individual steps add up to one full rotation in one direction only. As the energy barriers of the helicity inversion steps can be adjusted by structural modifications, chiral alkenes based on our system may find use as basic components for 'molecular machinery' driven by light.

An autonomous chemically fuelled small-molecule motor

“…22 Their designs culminated in a system that employed chemical reactions to bias a 120 degree rotation of a triptycene residue in one direction, 23 but attempts to extend this approach to repetitive 360 degree directional rotation proved unsuccessful. 24 Light-driven rotary molecular motors based on overcrowded alkenes 12,13 and imines 14,16 have been developed by the groups of Feringa and Lehn, while our group 25,26 and others [27][28][29] have made molecules in which the components can be rotated directionally step-wise by repetitively carrying out several chemical reactions in sequence. The latter systems all operate through Brownian ratchet mechanisms, differentiating the rates of random thermal motion of the components in each direction by the manipulation of kinetic (mainly steric) barriers.…”

“…12 The combined organic layers were washed with brine, dried over Na 2 SO 4 and concentrated under reduced pressure.…”

The tiniest Lego: a tale of nanoscale motors, rotors, switches and pumps

“…In 1999, after early experiments with shuttles and switches, the field took a big step forward with the creation of the first synthetic molecular motor 8 . Built by Feringa's team, it was a single molecule containing two identical 'paddle' units connected by a carbon-carbon double bond.…”

Light‐Gated Rotation in a Molecular Motor Functionalized with a Dithienylethene Switch

“…Light‐driven molecular motors based on overcrowded alkenes represent unique photoresponsive systems in the sense that they undergo unidirectional rotation around their central double bond (Scheme 1 a) 43, 44, 45. Promising applications have been demonstrated in nanotechnology,11, 46, 47 catalysis,48, 49 and anion binding,50, 51 amongst others.…”