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.     

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

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.     

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.     

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.     

Acceleration of a nanomotor: electronic control of the rotary speed of a light-driven molecular rotor

A new second-generation light-driven molecular motor was designed, in which the presence of a potential electronic push-pull system leads to a significant increase of the rate of rotation compared to previous motor systems, without disturbing its overall unidirectionality.     

Efficient light-driven carbon-free cobalt-based molecular catalyst for water oxidation

The abundant-metal-based polyoxometalate complex [Co(4)(H(2)O)(2)(PW(9)O(34))(2)](10-) is a hydrolytically and oxidatively stable, homogeneous, and efficient molecular catalyst for the visible-light-driven catalytic oxidation of water. Using a sacrificial electron acceptor and photosensitizer, it exhibits a high (30%) photon-to-O(2) yield and a large turnover number (>220, limited solely by depletion of the sacrificial electron acceptor) at pH 8. The photocatalytic performance of this catalyst is superior to that of the previously reported precious-metal-based polyoxometalate water oxidation catalyst [{Ru(4)O(4)(OH)(2)(H(2)O)(4)}(γ-SiW(10)O(36))(2)](10-).     

A redesign of light-driven rotary molecular motors

Structural modification of unidirectional light-driven rotary molecular motors in which the naphthalene moieties are exchanged for substituted phenyl moieties are reported. This redesign provides an additional tool to control the speed of the motors, and should enable the design and synthesis of more complex systems.     

Control of rotor function in light-driven molecular motors

A study is presented on the control of rotary motion of an appending rotor unit in a light-driven molecular motor. Two new light driven molecular motors were synthesized that contain aryl groups connected to the stereogenic centers. The aryl groups behave as bidirectional free rotors in three of the four isomers of the 360° rotation cycle, but rotation of the rotors is hindered in the fourth isomer. Kinetic studies of both motor and rotor functions of the two new compounds are given, using (1)H NMR, 2D-EXSY NMR, and UV-vis spectroscopy. In addition, we present the development of a new method for introducing a range of aryl substituents at the α-carbon of precursors for molecular motors. The present study shows how the molecular system can be photochemically switched between a state of free rotor rotation and a state of hindered rotation and reveals the dynamics of coupled rotary systems.     

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.     

In control of the speed of rotation in molecular motors. Unexpected retardation of rotary motion

Surprisingly, a new motor with a tetrahydronaphthalene upper part rotates slower than the original molecular motor with a tetrahydrophenanthrene upper part despite decreased steric hindrance.     

180 degree unidirectional bond rotation in a biaryl lactone artificial molecular motor prototype

A bifunctional biaryl lactone has been synthesized that should be capable of iterative unidirectional aryl-aryl bond rotation via: (1) a diastereoselective lactone ring opening, (S)-1 to (P,S)-2 or (M,S)-2; (2) a chemoselective lactonization, (P,S)-2 or (M,S)-2 to (S)-3; and (3) a chemoselective hydrolysis, (S)-3 to (S)-1. Preliminary results of a racemic sample have indicated unidirectional 180 degrees rotation with very high directional selectivity per individual artificial molecular motor molecule through the first two steps of this sequence. [reaction: see text]     

Next-generation quantum theory of atoms in molecules for the S1/S0 conical intersections in dynamics trajectories of a light-driven rotary molecular motor

Next-generation quantum theory of atoms in molecules was applied to analyze, along an entire bond path, intramolecular interactions known to influence the photoisomerization dynamics of a light-driven rotary molecular motor. The 3D bond-path framework set B_0,1 constructed from the least and most preferred directions of electronic motion, provided new insights into the bonding leading to different S₁ state lifetimes including the first quantification of covalent character of a closed-shell intramolecular bond path. We undertook the first use of the stress tensor trajectory T_σ(s) analysis on selected nonadiabatic molecular dynamics trajectories with the electron densities obtained using the ensemble density functional theory method. The stress tensor T_σ(s) analysis was found to be well suited to follow the dynamics trajectories that included the S₀ and S₁ electronic states through the conical intersection and also provided to a new measure to assess the degree of purity of the axial bond rotation for the design of rotary molecular motors.     

Controlling the speed of rotation in molecular motors. Dramatic acceleration of the rotary motion by structural modification

Substitution of a 6-membered by a 5-membered ring upper half in the light driven second generation molecular motors resulted in a dramatic increase of the speed of rotation.     

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.     

Traps for charge carriers in molecular materials formed by dipolar species: towards light-driven molecular switch

The influence of polar species on the transport and trapping of charge carriers is discussed. Calculations performed on a model molecular lattice demonstrate that polar dopants locally modify the polarization energy thus creating traps for charge carriers in the vicinity of the dipole. The presence of polar dopants in disordered solids gives rise to a broadening of the density-of-states function. A scheme of a molecular switch has been put forward, based on electrostatic interactions between photochromic moieties and charge carriers travelling on a molecular wire (conjugated polymer chain).     

Artificial photosynthesis: from molecular catalysts for light-driven water splitting to photoelectrochemical cells

Photosynthesis has been for many years a fascinating source of inspiration for the development of model systems able to achieve efficient light-to-chemical energetic transduction. This field of research, called "artificial photosynthesis," is currently the subject of intense interest, driven by the aim of converting solar energy into the carbon-free fuel hydrogen through the light-driven water splitting. In this review, we highlight the recent achievements on light-driven water oxidation and hydrogen production by molecular catalysts and we shed light on the perspectives in terms of implementation into water splitting technological devices.     

Collisional alignment of molecular rotation

The alignment of the axes of rotation of diatomic molecules produced in successive collisions with atoms is examined in classical trajectory calculations. For I₂ subjected to collisions with Ar under conditions for the generation of seeded molecular beams with high velocity slip, the ratio of I₂ molecules with axes of rotation parallel to the flow to those with axes perpendicular to the flow is estimated to be 1:3.     

Fine tuning of the rotary motion by structural modification in light-driven unidirectional molecular motors

The introduction of bulky substituents at the stereogenic center of light-driven second-generation molecular motors results in an acceleration of the speed of rotation. This is due to a more strained structure with elongated C=C bonds and a higher energy level of the ground state relative to the transition state for the rate-limiting thermal isomerization step. Understanding the profound influence that variation of the substituent at the stereogenic center holds over the rotational speed of the light-driven molecular motor has enabled the development of the fastest molecular motor reported thus far.     

Effect of molecular rotation on enantioseparation

Recently, several laser schemes have been proposed to separate racemic mixtures of enantiomers by splitting a molecular beam into subbeams consisting of molecules of definite chirality [Y. Li, C. Bruder, and C. P. Sun, Phys. Rev. Lett. 99, 130403 (2007); X. Li and M. Shapiro, J. Chem. Phys. 132, 194315 (2010)]. These ideas rely on laser-induced effective gauge potentials in an adiabatic basis which lead to a chirality dependent force on the center-of-mass. However, the effect of molecular rotation has been neglected in these studies. Accounting for the full molecular quantum state we find that the potentials from the adiabatic dressed state approach cannot be recovered once the molecular orientation dynamics is included, even in the rotational ground state. This affects substantially the ability to perform enantioseparation in the above mentioned setups.