The field of synthetic molecular machines has quickly evolved in recent years, growing from a fundamental curiosity to a highly active field of chemistry. Many different applications are being explored in areas such as catalysis, self-assembled and nanostructured materials, and molecular electronics. Rotary molecular motors hold great promise for achieving dynamic control of molecular functions as well as for powering nanoscale devices. However, for these motors to reach their full potential, many challenges still need to be addressed. In this paper we focus on the design principles of rotary motors featuring a double-bond axle and discuss the major challenges that are ahead of us. Although great progress has been made, further design improvements, for example in terms of efficiency, energy input, and environmental adaptability, will be crucial to fully exploit the opportunities that these rotary motors offer.
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Symmetric molecular motors based
on two overcrowded alkenes with
a notable absence of a stereogenic center show potential to function
as novel mechanical systems in the development of more advanced nanomachines
offering controlled motion over surfaces. Elucidation of the key parameters
and limitations of these third-generation motors is essential for
the design of optimized molecular machines based on light-driven rotary
motion. Herein we demonstrate the thermal and photochemical rotational
behavior of a series of third-generation light-driven molecular motors.
The steric hindrance of the core unit exerted upon the rotors proved
pivotal in controlling the speed of rotation, where a smaller size
results in lower barriers. The presence of a pseudo-asymmetric carbon
center provides the motor with unidirectionality. Tuning of the steric
effects of the substituents at the bridgehead allows for the precise
control of the direction of disrotary motion, illustrated by the design
of two motors which show opposite rotation with respect to a methyl
substituent. A third-generation molecular motor with the potential
to be the fastest based on overcrowded alkenes to date was used to
visualize the equal rate of rotation of both its rotor units. The
autonomous rotational behavior perfectly followed the predicted model,
setting the stage for more advanced motors for functional dynamic
systems.
The visible-light-driven rotation
of an overcrowded alkene-based
molecular motor strut in a dual-function metal–organic framework
(MOF) is reported. Two types of functional linkers, a palladium–porphyrin
photosensitizer and a bispyridine-derived molecular motor, were used
to construct the framework capable of harvesting low-energy green
light to power the rotary motion. The molecular motor was introduced
in the framework using the postsynthetic solvent-assisted linker exchange
(SALE) method, and the structure of the material was confirmed by
powder (PXRD) and single-crystal X-ray (SC-XRD) diffraction. The large
decrease in the phosphorescence lifetime and intensity of the porphyrin
in the MOFs upon introduction of the molecular motor pillars confirms
efficient triplet-to-triplet energy transfer between the porphyrin
linkers and the molecular motor. Near-infrared Raman spectroscopy
revealed that the visible light-driven rotation of the molecular motor
proceeds in the solid state at rates similar to those observed in
solution.
Molecular rotary
motors based on oxindole which can be driven by
visible light are presented. This novel class of motors can be easily
synthesized via a Knoevenagel condensation, and the choice of different
upper halves allows for the facile tuning of their rotational speed.
The four-step rotational cycle was explored using DFT calculations,
and the expected photochemical and thermal isomerization behavior
was confirmed by NMR, UV/vis, and CD spectroscopy. These oxindole
motors offer attractive prospects for functional materials responsive
to light.
Exploring
routes to visible-light-driven rotary motors, the possibility
of red-shifting the excitation wavelength of molecular motors by extension
of the aromatic core is studied. Introducing a dibenzofluorenyl moiety
in a standard molecular motor resulted in red-shifting of the absorption
spectrum. UV/vis and 1H NMR spectroscopy showed that these
motors could be isomerized with light of wavelengths up to 490 nm
and that the structural modification did not impair the anticipated
rotary behavior. Extension of the aromatic core is therefore a suitable
strategy to apply in pursuit of visible-light-driven molecular motors.
We report on the active template synthesis of a [2]rotaxane through a Goldberg copper-catalyzed C-N bond forming reaction. A C2-symmetric cyclohexyldiamine macrocycle directs the assembly of the rotaxane, which can subsequently serve as a ligand for enantioselective nickel-catalyzed conjugate addition reactions. Rotaxanes are a previously unexplored ligand architecture for asymmetric catalysis. We find that the rotaxane gives improved enantioselectivity compared to a noninterlocked ligand, at the expense of longer reaction times.
A multiphotochromic hybrid system is presented in which a light‐driven overcrowded alkene‐based molecular rotary motor is connected to a dithienylethene photoswitch. Ring closing of the dithienylethene moiety, using an irradiation wavelength different from the wavelength applied to operate the molecular motor, results in inhibition of the rotary motion as is demonstrated by detailed 1H‐NMR and UV/Vis experiments. For the first time, a light‐gated molecular motor is thus obtained. Furthermore, the excitation wavelength of the molecular motor is red‐shifted from the UV into the visible‐light region upon attachment of the dithienylethene switch.
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