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.     

Design of a molecular memory element with an alternating circular array of dipolar rotors and rotation suppressors

As a new element for electric-field driven molecular memory, we developed a hexaarylbenzene derivative in which three difluorophenyl groups and three aryl groups as a dipolar rotor and a rotation suppressor, respectively, are alternately positioned on the central benzene core. This molecule has two rotational isomeric forms, both of which preserve their conformational states at room temperature but exhibit interconversion at high temperatures. Amorphous thin films fabricated from the hexaarylbenzene show a reversible change in surface potential by application of electric fields.

A hexaarylbenzene derivative with an alternating circular array of dipolar rotors and rotation suppressors holds promise as a new element for electric-field driven molecular memory.     

Giant Crystalline Molecular Rotors that Operate in the Solid State

Molecular motion in the solid state is typically precluded by the highly dense environment, and only molecules with a limited range of sizes show such dynamics. Here, we demonstrate the solid-state rotational motion of two giant molecules, i.e., triptycene and pentiptycene, by encapsulating a bulky N-heterocyclic carbene (NHC) Au(I) complex in the crystalline media. To date, triptycene is the largest molecule (surface area: 245 Ų; volume: 219 ų) for which rotation has been reported in the solid state, with the largest rotational diameter among reported solid-state molecular rotors (9.5 Å). However, the pentiptycene rotator that is the subject of this study (surface area: 392 Ų; volume: 361 ų; rotational diameter: 13.0 Å) surpasses this record. Single-crystal X-ray diffraction analyses of both the developed rotors revealed that these possess sufficient free volume around the rotator. The molecular motion in the solid state was confirmed using variable-temperature solid-state ²H spin-echo NMR studies. The triptycene rotor exhibited three-fold rotation, while temperature-dependent changes of the rotational angle were observed for the pentiptycene rotor.     

Theoretical design of a light-driven molecular rotary motor with low energy helical inversion: 9-(5-methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene

A light-driven molecular rotary motor of 9-(5-methyl-2-phenyl-2-cyclopenten-1-ylidene)-9H-fluorene (MPCPF) has been designed by means of ab initio complete active space self-consistent field and its second order multireference M?ller-Plesset perturbation methods. In the present model molecule of MPCPF, 9H-fluorene (as a stator) and 5-methyl-2-phenyl-2-cyclopenten-1-ylidene (as a rotor) are directly linked with each other by a C═C double bond. Even by a substitution of phenyl group, MPCPF comes to have a stable P-helical MPCPF and a metastable M-helical MPCPF, and exhibits unidirectionality around the C═C double bond. In addition, interchange of the helicity can proceed with a low energy barrier through a floppy phenyl torsional motion. This is in contrast to previous light-driven molecular rotary motors where the unidirectionality is ensured by rigid and sterically overcrowded rotors. In the full rotary process of MPCPF, therefore, constancy of the rotation speed is expected to be much more improved as well as unidirectionality.     

Guest-accelerated molecular rotor

A molecular rotor was designed in which the rate of rotation is accelerated by guest complexation. The binding of an acetate guest to the urea groups lowers the barrier of the adjacent C(aryl)-N(imide) bond by 2 to 4 kcal/mol. This behavior is in contrast to most molecular rotors in which guest complexation slows rotation.     

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.     

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.     

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.     

Auxiliary Methoxy Aided Triphenylamine and Dicyanoisophorone Based Flurophores with Viscosity and Polarity Sensitive Intramolecular Charge Transfer

Novel, multibranched “triphenylamine based donor with added auxiliary methoxy donor and dicyanovinyl acceptor” based fluorescent molecules are developed. The dicyanoisophorone moiety is used as a configurationally locked polyene system for π-conjugation linking between donor and acceptor, to control the unnecessary intramolecular rotations in the molecule, which can to act as a rotor. The synthesized dyes show good fluorescent molecular rotor properties and strong emission solvatochromism. Auxiliary methoxy donors shift both the absorption and emission maxima towards longer wavelengths compared to known analogues, along with increased Stokes shifts. Fluorescent molecular rotor properties of the dyes are influenced by a local excited state to twisted intramolecular charge transfer state transition, which is discussed in terms of emission solvatochromism and Lippert–Mataga, Weller and Rettig polarity plots. Three different viscous solvent systems i.e., paraffin oil–dichloromethane, polyethylene glycol-400–dichloromethane and polyethylene glycol-400–N,N-dimethylformamide are used to investigate the sensitivity of rotors towards the viscosity of the environment. A maximum 16-fold enhancement in emission intensity and 0.616 × value is achieved for rotor Dye-3. The polarity effect of a binary viscous solvent system, by the virtue of intramolecular charge transfer, on the viscosity sensing properties of rotors is explained by constructing the Weller and Rettig’s plots for different viscous systems.     

The use of McMurry coupling for the synthesis of indolophanes and cis-stilbenophanes

Treatment of 2 equiv of indole-3-aldehyde with o, m, p-xylyl, 2,5-dimethoxy-p-xylyl dibromides and 4,4′-bis(bromomethyl)-1,1′-biphenyl gave the bisalkylated products, which underwent McMurry coupling with low valent titanium to give indolophanes. Various cis-stilbenophanes with m-terphenyl building blocks were also synthesized by application of the McMurry coupling technique.     

Enantiopure Functional Molecular Motors Obtained by a Switchable Chiral‐Resolution Process

Molecular switches, rotors, and motors play an important role in the development of nano‐machines and devices, as well as responsive and adaptive functional materials. For unidirectional rotors based on chiral overcrowded alkenes, their stereochemical homogeneity is of crucial importance. Herein, a method to obtain new and functionalizable overcrowded alkenes in enantiopure form is presented. The procedure involves a short synthesis of three steps and a solvent‐switchable chiral resolution by using a readily available resolving agent. X‐ray crystallography revealed the mode of binding of the motor with the resolving agent, as well as the absolute configuration of the motor. ¹H NMR and UV/Vis spectroscopy techniques were used to determine the dynamic behavior of this molecular motor. This method provides rapid access to ample amounts of enantiopure molecular motors, which will greatly facilitate the further development of responsive molecular systems based on chiral overcrowded alkenes.     

OES and GC/MS Study of RF Plasma of Xylenes

The radio-frequency discharge of xylene isomers was monitored with optical emission spectroscopy (OES). It was found that the meta isomer showed relatively stronger excimer to monomer intensity ratio than the other two isomers. OES also indicates the formation of xylyl (methylbenzyl) radicals. The reaction products of low pressure xylene plasmas were analyzed by gas-chromatography mass spectrometry (GC/MS). It showed that the main composition of the reaction products was 1,2-di-p-tolylethane (DPTE), regardless the types of xylene isomers used. It is known that o- and m-xylyl radicals can undergo rearrangement and convert to p-xylyl radicals. Similar to the cases in benzene and toluene plasmas, the recombination reaction between two p-xylyl radicals is believed to be responsible for the formation of DPTE. Density functional theory calculations suggest that the direct conversion of xylene excimers to DPTE is unlikely.     

Conformational analysis,barriers to internal rotation and vibrational assignment for dimethylethylamine

The IR (50–3500 cm^?1) and Raman (20–3500 cm^?1) spectra have been recorded for gaseous and solid dimethylethylamine. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. Due to the fact that three distinct Raman lines disappear on going from the fluid phases to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers in the fluid phases with the gauche conformer being more stable and the only one present in the spectra of the unannealed solid. From the temperature study of the Raman spectrum of the liquid a rough estimate of 3.9 kcal mol^?1 has been obtained for ΔH. Relying mainly on group frequencies and relative intensities of the IR and Raman lines, a complete vibrational assignment is proposed for the gauche conformer. The potential functions for the three methyl rotors have been obtained, and the barriers to internal rotation for the two CH₃ rotors attached to the nitrogen atom have been calculated to be 3.51 and 3.43 kcal mol^?1, whereas the barrier for the CH₃ rotor of the ethyl group has been calculated to be 3.71 kcal mol^?1. The asymmetric torsional mode for the gauche conformer has been observed in both the IR and Raman spectra of the gas at 105 cm^?1 with at least one hot band at a lower frequency. Since the corresponding mode has not been observed for the trans conformer, it is not possible to obtain the potential function for the asymmetric rotation although estimates on the magnitudes of some of the terms have been made. Significant changes occur in the low-frequency IR and Raman spectra of the solid with repeated annealing; several possible reasons for these changes are discussed and one possible explanation is that a conformational change is taking place in the solid where the trans form is stabilized by crystal packing forces. These results are compared to the corresponding quantities for some similar amines.     

Induction of molecular chirality by circularly polarized light in cyclic azobenzene with a photoswitchable benzene rotor

New phototriggered molecular machines based on cyclic azobenzene were synthesized in which a 2,5‐dimethoxy, 2,5‐dimethyl, 2,5‐difluorine or unsubstituted‐1,4‐dioxybenzene rotating unit and a photoisomerizable 3,3′‐dioxyazobenzene moiety are bridged together by fixed bismethylene spacers. Depending upon substitution on the benzene moiety and on the E/Z conformation of the azobenzene unit, these molecules suffer various degrees of restriction on the free rotation of the benzene rotor. The rotation of the substituted benzene rotor within the cyclic azobenzene cavity imparts planar chirality to the molecules. Cyclic azobenzene 1 , with methoxy groups at both the 2‐ and 5‐positions of the benzene rotor, was so conformationally restricted that free rotation of the rotor was prevented in both the E and Z isomers and the respective planar chiral enantiomers were resolved. In contrast, compound 2 , with 2,5‐dimethylbenzene as the rotor, demonstrated the property of a light‐controlled molecular brake, whereby rotation of the 2,5‐dimethylbenzene moiety is completely stopped in the E isomer (brake ON, rotation OFF), while the rotation is allowed in the Z isomer (brake OFF, rotation ON). The cyclic azobenzene 3 , with fluorine substitution on the benzene rotor, was in the brake OFF state regardless of E/Z photoisomerization of the azobenzene moiety. More interestingly, for the first time, we demonstrated the induction of molecular chirality in a simple monocyclic azobenzene by circular‐polarized light. The key characteristics of cyclic azobenzene 2 , that is, stability of the chiral structure in the E isomer, fast racemization in the Z isomer, and the circular dichroism of enantiomers of both E and Z isomers, resulted in a simple reversible enantio‐differentiating photoisomerization directly between the E enantiomers. Upon exposure to r‐ or l‐circularly polarized light at 488 nm, partial enrichment of the (S)‐ or (R)‐enantiomers of 2 was observed.     

A study of the interaction energy between a glycoluril molecular clip and dihydroxybenzenes or diaminobenzenes: a justification for easy separation by affinity chromatography

Theoretical calculations were performed to determine the interaction energy between a glycoluril (GL) molecular clip and hydroxybenzenes (HBs) and aminobenzenes (ABs). The theoretical calculations on the GL and its interactions were carried out using the hybrid functional closed-shell RB3LYP and the 6-31G* basis set, employing gaussian 03. The stability in energy of the guest inside the GL, ΔE_T(1), was in the following order: m-DHB-GL > o-DHB-GL > m-THB-GL > m-DAB-GL > o-THB-GL. The geometric parameters, in particular the bond lengths are discussed for the host molecule GL and guest molecules DHB and DAB and their parameters are compared with the host-guest molecules DHB-GL and DAB-GL, respectively.     

The 1-D hindered rotor approximation

We offer an overview of the popular one- dimensional (1-D) hindered rotor model that is often used for quantum mechanical treatment of internal rotation. This model is put in context with other methods used for treating anharmonic motions. The 1-D hindered rotor scheme is general for tops of any symmetry and has been used to provide accurate treatment of hindered rotors in a wide range of systems. One obstacle preventing wider use of the model is its lack of incorporation into common electronic structure codes. We have developed an algorithm for consistently treating all tops in a molecule, and we present simple codes which interface with electronic structure codes to provide thermochemical properties (S, C _p , H) of individual species and reactions that have been corrected for internal rotations. Finally, we use this approach to give sensible advice about how the model can be used best. We show that dramatic changes in the reduced moment of inertia do not necessarily cause comparable changes in the properties of individual hindered rotors. We demonstrate that the rotational hindrance potential can be accurately determined using relatively coarse step sizes. Finally, we show that internal rotation in transition states can be treated using a “frozen transition state” approximation at a significant computational savings. We also discuss the relationship between calculated properties of hindered rotors and the choice of method and basis set used. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Enhancing the Viscosity-Sensitive Range of a BODIPY Molecular Rotor by Two Orders of Magnitude

Molecular rotors are a class of fluorophores that enable convenient imaging of viscosity inside microscopic samples such as lipid vesicles or live cells. Currently, rotor compounds containing a boron-dipyrromethene (BODIPY) group are among the most promising viscosity probes. In this work, it is reported that by adding heavy-electron-withdrawing −NO₂ groups, the viscosity-sensitive range of a BODIPY probe is drastically expanded from 5–1500 cP to 0.5–50 000 cP. The improved range makes it, to our knowledge, the first hydrophobic molecular rotor applicable not only at moderate viscosities but also for viscosity measurements in highly viscous samples. Furthermore, the photophysical mechanism of the BODIPY molecular rotors under study has been determined by performing quantum chemical calculations and transient absorption experiments. This mechanism demonstrates how BODIPY molecular rotors work in general, why the −NO₂ group causes such an improvement, and why BODIPY molecular rotors suffer from undesirable sensitivity to temperature. Overall, besides reporting a viscosity probe with remarkable properties, the results obtained expand the general understanding of molecular rotors and show a way to use the knowledge of their molecular action mechanism for augmenting their viscosity-sensing properties.     

Change of the molecular surface and volume during internal rotation and its effect on conformational equilibrium in solution

Conformational dependence of the molecular surface S and molecular volume V for hexane and 1,1,2-trichloroethane during rotation around a central bond in the molecules have been calculated. A model of overlapping spheres is used, the size of the spheres being determined by Van der Waals radii of individual atoms. Plots of S and V against torsional angle φ are compared with the potential of internal rotation of both molecules E(φ). The calculated molecular surfaces and volumes of the two molecules for the most stable conformers mutually differ by several percent as experimental results also indicate. We also show that the differences in S and V between individual conformers always affect conformational equilibrium in solution even if solvent-solute interaction energies are not explicitly considered. As a consequence of the mentioned volume changes during internal rotation in a molecule, the conformational energies for hydrocarbons in the condensed and gaseous phases can differ by as much as several kJ mol^?1.     

Electronic spectra of o-, m- and p-tolunitrile—substituent effect on internal rotation of the methyl group

The S₁ ← S₀ fluorescence excitation spectra and the S₁→S₀ dispersed fluorescence spectra of o-, m-and p-tolunitrile were measured in supersonic jets. Low-frequency bands due to internal rotation of the methyl group were observed in m- and p-tolunitrile. Observed band positions and relative intensities of the internal rotational bands were reproduced by a calculation using a free rotor basis set. From the analysis, the potential curve of the internal rotation was determined in both S₁ and S₀. It was found that the barrier height increases in going from S₀ to S₁ in m-tolunitrile, while it decreases in p-tolunitrile. In contrast, no low-frequency band was found in o-tolunitrile. It is concluded that the potential curve in o-tolunitrile does not change in going from S₀ to S₁. The change of the barrier height by electronic excitation in tolunitriles differs greatly from that observed in other toluene derivatives. It is suggested that the electronic properties of a substituent are important for the methyl rotation in the excited state.     

Molecular rotor of Cs2([18]crown-6)3 in the solid state coupled with the magnetism of [Ni(dmit)2

Nanoscale molecular rotors that can be driven in the solid state have been realized in Cs2([18]crown-6)3[Ni(dmit)2]2 crystals. To provide interactions between the molecular motion of the rotor and the electronic system, [Ni(dmit)2]- ions, which bear one S=1/2 spin on each molecule, were introduced into the crystal. Rotation of the [18]crown-6 molecules within a Cs2([18]crown-6)3 supramolecule above 220 K was confirmed using X-ray diffraction, NMR, and specific heat measurements. Strong correlations were observed between the magnetic behavior of the [Ni(dmit)2]- ions and molecular rotation. Furthermore, braking of the molecular rotation within the crystal was achieved by the application of hydrostatic pressure.