Net directed 180° aryl-aryl bond rotation in a prototypical achiral biaryl lactone synthetic molecular motor

Net directed 180° bond rotation was achieved through diastereoselective ring-opening reactions in an achiral biaryl lactone using a chiral nucleophile followed by re-lactonization. The efficiency of the directed bond rotation has been determined by HPLC-MS to be 50% and 20% with two different chiral nucleophiles. These results demonstrate the potential for a prototype of a chemically driven synthetic molecular motor which has the advantages of both simplicity and flexibility in operation and is the first example of the use of a chiral auxiliary to induce transient axial chirality resulting in net directed bond rotation.     

Synthesis and characterization of a functionalized chiral biaryl capable of exhibiting unidirectional bond rotation

A class of chiral biaryl molecules that have been designed to undergo unidirectional rotation about the aryl–aryl bond may show promise for future application in the area of synthetic molecular motors. These asymmetric molecules should be capable of channeling chemical energy into repeated 360° rotation in one direction. Herein we report the synthesis and characterization of one such biaryl molecule (1).     

A donor-acceptor substituted molecular motor: unidirectional rotation driven by visible light

A newly designed donor-acceptor substituted molecular motor 1 allows unidirectional rotation driven by visible light and shows some unique photophysical properties.     

MHz unidirectional rotation of molecular rotary motors

A combination of cryogenic UV-vis and CD spectroscopy and transient absorption spectroscopy at ambient temperature is used to study a new class of unidirectional rotary molecular motors. Stabilization of unstable intermediates is achieved below 95 K in propane solution for the structure with the fastest rotation rate, and below this temperature measurements on the rate limiting step in the rotation cycle can be performed to obtain activation parameters. The results are compared to measurements at ambient temperature using transient absorption spectroscopy, which show that behavior of these motors is similar over the full temperature range investigated, thereby allowing a maximum rotation rate of 3 MHz at room temperature under suitable irradiation conditions.     

Light-driven unidirectional rotation in a molecule: ROKS simulation.

We present a first-principles molecular dynamics study of the excited-state motion in a molecule that has recently been proven to exhibit light-driven unidirectional rotation. The simulations show that the directed motion is due to the complex excited-state dynamics on ultrashort timescales in the chiral system.     

Conformational isomers from rotation of diacetylenic bond in an ethynylpyrene-substituted molecular hinge

The first example of isolation and X-ray crystallographic structural characterization of two conformers arising from rotation along a diacetylenic bond is reported. In both the conformers extensive pi-pi interactions are observed in the solid state. VT-NMR and fluorescence spectroscopic studies in solution suggest that the closed and open conformers are in equilibrium and that the closed conformer is the predominant species at room temperature.     

A minimal 2D model of the free energy surface for a unidirectional natural molecular motor

A schematic model of a natural molecular motor is proposed. It uses the change of the free energy surface to an effective surface as long as the enzyme is active. This effective surface acts as a trapdoor and explains the power stroke in biomotors, as well as its unidirectional movement. Then a thermal relaxation can do the energy transduction of the motor. The model uses Newton trajectories to explain the movement of stationary points on the effective surface.     

Increased speed of rotation for the smallest light-driven molecular motor

In this paper we present the smallest artificial light-driven molecular motor consisting of only 28 carbon and 24 hydrogen atoms. The concept of controlling directionality of rotary movement at the molecular level by introduction of a stereogenic center next to the central olefinic bond of a sterically overcrowded alkene does not only hold for molecular motors with six-membered rings, but is also applicable to achieve the unidirectional movement for molecular motors having five-membered rings. Although X-ray analyses show that the five-membered rings in the cis- and trans-isomer of the new molecular motor are nearly flat, the energy differences between the (pseudo-)diaxial and (pseudo-)diequatorial conformations of the methyl substituents in both isomers are still large enough to direct the rotation of one-half of the molecule with respect to the other half in a clockwise fashion. The full rotary cycle comprises four consecutive steps: two photochemical isomerizations each followed by a thermal helix inversion. Both photochemical cis-trans isomerizations proceed with a preference for the unstable diequatorial isomers over the stable diaxial isomers. The thermal barriers for helix inversion of this motor molecule have decreased dramatically compared to its six-membered ring analogue, the half-life of the fastest step being only 18 s at room temperature.     

Photochemical and thermal behavior of light-driven unidirectional molecular motor with long alkyl chains

In order to utilize molecular motors in a molecular device or smart materials, their dynamic behavior when other groups are introduced at key positions has to be examined. A C₁₆ hydrocarbon chain has been introduced at the stereogenic centers of the first generation light-driven molecular motor based on sterically overcrowded biphenanthrylidene. It was found that this derivatization does not affect the unidirectional rotary capability of the motor and does not cause a reduction of its speed, opening new possibility for future functionalizations and applications.     

Light-driven molecular motors: stepwise thermal helix inversion during unidirectional rotation of sterically overcrowded biphenanthrylidenes

To investigate the unidirectional rotation of chiral overcrowded biphenanthrylidenes in more detail, the size of the substituent next to the double bond responsible for the unidirectionality of rotation was varied. The thermal and photochemical isomerization of three sterically overcrowded alkenes is described. The behavior of the biphenanthrylidenes with methyl and ethyl substituents is rather similar, and these compounds undergo a unidirectional 360 degrees rotation around the central double bond in a four-step sequence involving two photochemical cis-trans isomerizations and two thermal helix inversions. The only difference between these two true molecular motors was a small entropic effect, which causes the ethyl substituted molecular motor to rotate slightly faster. The behavior of the i-propyl substituted compound differs significantly from that of the other two. Although not all different isomers of the i-propyl substituted molecular motor were detected spectroscopically, experimental data led to the conclusion that this compound can also be considered as a molecular motor and is capable of performing a 360 degrees unidirectional rotation. (1)H NMR and X-ray analysis show a meso-like form as an intermediate in the unidirectional rotation, which proves that the thermal helix inversion is a stepwise process.     

A molecular orbital study of the rotation about the C-C bond in 1,3-butadiene

The geometry and energy of 1,3-butadiene have been calculated using the 6-311G** basis set as a function of the CCCC dihedral angle-0 ° (trans), 30 °, 60 °, 75 °, 90 °, 120 °, 135 °, 150 °, 165 ° and 180 ° (cis)-assuming that the vinyl groups remain planar. Potential minima are located at 0 ° and 141.4 °, with the trans structure more stable than the gauche by 13.2 kJ mol^–1. Potential maxima are located at 76.7 °, giving a barrier height of 25.4 kJ mol^–1 relative to the trans structure, and at 180 ° giving a barrier height of 3.0 kJ mol^–1 relative to the 141.4 °-gauche structure. Using the 6-31G* basis set the inclusion of electron correlation, accounting for about 52% of the correlation energy, was found to produce no significant change in the shape of the potential energy curve. The magnitude of the expectation energy differences is such that both barriers with respect to the 14l.4 °-gauche maximum structure can be categorized unequivocally as attractive-dominant, whereas the values for the energy barrier with respect to the trans structure, although characteristic of a repulsive-dominant barrier at the 6–311G** level, are sufficiently small that higher level calculations might give the opposite result. Analysis of V _nn for the conversion reactions cis 150 °-gauche, trans 60 °-gauche, and trans 90 °-gauche in terms of the individual contributions from the various internuclear interactions shows that nonbonded interactions are important, not only in initiating the destabilization of the crowded cis structure, but also through-out the entire range of CCCC dihedral angles, 0 ° to 180 °.     

Hindered rotation around C-N bond in nitroenamines

Conclusions The relation between the value of the rotation barrier around the C-N bond of nitroenamines and the type of solvent is determined mainly by the solvation of the nitroenamine molecule in the ground state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2392–2395, October, 1978.     

Bidirectional and unidirectional PCET in a molecular model of a cobalt-based oxygen-evolving catalyst

The oxidation of water to molecular oxygen is a kinetically demanding reaction that requires efficient coupling of proton and electron transfer. The key proton-coupled electron transfer (PCET) event in water oxidation mediated by a cobalt-phosphate-based heterogeneous catalyst is the one-electron, one-proton conversion of Co(III)-OH to Co(IV)-O. We now isolate the kinetics of this PCET step in a molecular Co(4)O(4) cubane model compound. Detailed electrochemical, stopped-flow, and NMR studies of the Co(III)-OH to Co(IV)-O reaction reveal distinct mechanisms for the unidirectional PCET self-exchange reaction and the corresponding bidirectional PCET. A stepwise mechanism, with rate-limiting electron transfer is observed for the bidirectional PCET at an electrode surface and in solution, whereas a concerted proton-electron transfer displaying a moderate KIE (4.3 ± 0.2), is observed for the unidirectional self-exchange reaction.     

Stereodynamics of bond rotation in tertiary aromatic amides

The degree to which the rotations about the C-N and Ar-CO bonds of aromatic amides occur in a concerted manner was investigated by a variety of NMR and kinetic techniques. Otherwise complex kinetic analyses were simplified by exploiting symmetry and asymmetry in the N-substituents of amides. In 2-unsubstituted 1-naphthamides bearing branched N-substituents, most conformational changes about the amide group were by correlated rotation, though uncorrelated Ar-CO rotation also occurred to some extent. In 2-substituted 1-naphthamides, correlated rotation accounted for all of the Ar-CO rotations, though a significant amount of uncorrelated C-N rotation also occurred. Naphthamides bearing branched N-substituents thus turn out to be efficient molecular gears: Compound 12 showed almost no gear slippage.     

Restricted rotation about the boron---nitrogen bond in aminoboranes

The application of variable temperature ¹³C NMR to the study of a series of chlorodialkylaminophenylboranes has enabled G values for the rotational barrier, about the boron---nitrogen bond, to be determined.     

Exploring the boundaries of a light-driven molecular motor design: new sterically overcrowded alkenes with preferred direction of rotation

Insight in the steric and electronic parameters governing isomerization processes in artificial molecular motors is essential in order to design more advanced motor systems. A subtle balance of steric parameters and the combination of helical and central chirality are key features of light-driven unidirectional rotary molecular motors constructed so far. In an approach to decrease the steric hindrance around the central olefinic bond (rotary axis) and thereby lowering the energy barrier for helix inversion resulting in an increased rotation rate, the boundaries of our molecular motor design are explored. In a new design of a light-driven molecular motor based on a sterically overcrowded alkene the methyl substituent adjacent to the stereogenic center, which is responsible for the control of the direction of rotation, is shifted one position away from the fjord region of the molecule compared to the second-generation motor systems. In contrast to previously developed light-driven molecular motors, there is a preference for the methyl substituent to adopt a pseudo-equatorial orientation. Nevertheless, this new type of motor is capable of functioning as a rotary molecular motor, albeit not with full unidirectionality. Under the combined influence of light and heat, there is a preferred clockwise rotation of one half of the molecule. Surprisingly, the effect of shifting the methyl substituent on the energy barrier for helix inversion is small and even a slight increase in the barrier is observed.     

Computational study on the working mechanism of a stilbene light-driven molecular rotary motor: sloped minimal energy path and unidirectional nonadiabatic photoisomerization

The working mechanism of a geometrically overcrowded, chiral stilbene light-driven molecular rotary motor [(2R,2R)-2,2',7,7'-tetramethyl-1,1'-bis(indanylidene), 3] has been investigated by a potential energy surface (PES) study. The reaction paths of the two photoinitiated cis-trans (or E/Z) isomerization processes, namely, (P,P)-stable-cis→(M,M)-unstable-trans-3 and (P,P)-stable-trans→(M,M)-unstable-cis-3, have been explored at the CASPT2//CASSCF level of theory. The minimal energy reaction paths (MEPs) of these two processes are nearly parallel on the PESs, separated by a ridge of high inversion barrier. The MEPs have a remarkably steep slope, which drives C═C bond rotation unidirectionally. The asymmetric bias on the excited-state MEPs is caused by the substituents on the "fjord" region as well as by the phenyl moieties. The overall photoisomerization reaction can be described as a three-state (1B→2A→1A) multimode mechanism: The molecule excited to the 1B state first crosses one of the sloped 1B/2A seams, and then follows two cooperative torsional reaction modes to cross preferentially one of the two 2A/1A conical intersections to reach the isomerized ground-state product.     

Decarboxylative biaryl synthesis in a continuous flow reactor

A practical protocol was developed that allows performing decarboxylative cross-coupling reactions in continuous flow reactors. Various biaryls were thus synthesized from aromatic carboxylic acids and aryl triflates using a Cu/Pd-catalyst system.     

Control of rotor motion in a light-driven molecular motor: towards a molecular gearbox

Controlled intramolecular movement and coupling of motor and rotor functions is exerted by this new molecular device. The rate of rotation of the rotor part of the molecule can be adjusted by alteration of the conformation of the motor part of the molecule. For all states of the motor part, different rates of rotation were measured for the rotor part. Conversion between the four propeller orientations was achieved by irradiation and heating.