Photo‐responsive Helical Motion by Light‐Driven Molecular Motors in a Liquid‐Crystal Network

Abstract Controlling sophisticated motion by molecular motors is a major goal on the road to future actuators and soft robotics. Taking inspiration from biological motility and mechanical functions common to artificial machines, responsive small molecules have been used to achieve macroscopic effects, however, translating molecular movement along length scales to precisely defined linear, twisting and rotary motions remain particularly challenging. Here, we present the design, synthesis and functioning of liquid‐crystal network (LCN) materials with intrinsic rotary motors that allow the conversion of light energy into reversible helical motion. In this responsive system the photochemical‐driven molecular motor has a dual function operating both as chiral dopant and unidirectional rotor amplifying molecular motion into a controlled and reversible left‐ or right‐handed macroscopic twisting movement. By exploiting the dynamic chirality, directionality of motion and shape change of a single motor embedded in an LC‐network, complex mechanical motions including bending, walking and helical motion, in soft polymer materials are achieved which offers fascinating opportunities toward inherently photo‐responsive materials.


Introduction
Thep rospects of dynamic molecular systems, [1][2][3] using artificial molecular machines [4][5][6] to induce motion and perform complex mechanical tasks reminiscent of the omnipresent "machinery of life", [7,8] has greatly stimulated scientists to design molecular motors and machines that can power specific movements or allow several distinct mechanical operations. [9,10] Molecular muscles, [11][12][13] chemical synthesizers, [14,15] multitasking catalysts, [16,17] self-sorting machines, [18] transporters, [19,20] pumps [21][22][23] and responsive surfaces [24,25] are illustrative of machine-like dynamic functions demonstrated in recent years.C ontrolled mechanical movement has been achieved in solution, on surfaces and in materials including rotation, [26][27][28] translation [29][30][31] and bending [32,33] motions.D espite these advances the development of systems in which molecular movement is translated and amplified along multiple length scales to induce macroscopic motion as abasis for actuator materials has been limited. [34,35] Facing the challenge to achieve multiple distinctive autonomous motions in as oft material powered by light we envisioned that our photochemical rotary motors [36,37] offer unique opportunities to achieve more complex mechanical movements.Inparticular, these systems allow control of directionality of rotary movement and are driven non-invasively using light energy. [36,37] In addition, designing single molecular motor systems and organizing these in such aw ay that they can induce various types of motion, reminiscent of distinct mechanical actions by abiological muscle,represents amajor next step in the field of molecular machines.Photochromic switches have been used, for example,i nr esponsive crystals, [38][39][40][41] polymers, [42] LC materials [43][44][45][46][47][48][49][50] and gels, [51][52][53] while motors have shown photochemical rotary motion of micro-objects on LC-surfaces [54,55] and heat-driven anisotropic deformations of LC films. [56] Katsonis and co-workers [57,58] elegantly demonstrated macroscopic helical tendril-like motion using an intricate combination of light of distinct wavelengths,anLC-polymer network, chiral dopant and azobenzene photoswitch taking advantage of specific morphology and alignment in the LC material. In our approach all the key parameters,that is,dynamic chirality, dopant function, shape change and photo-responsiveness,are embedded in asingle rotary motor that acts as across-linker unit in ap olymer LC network. Furthermore,asingle wavelength of light enables fast and reversible amplification of motion resulting not only in bending and walking but also inducing left-or right-handed macroscopic twisting motions of the polymer.The precise control of the structure with light and the inherent and dynamic helical chirality (racemic,Por M) of the molecular motor govern the multiple complex movements at the macroscopic level in the material.

Design and Synthesis
Al iquid-crystal polymer network forms the basis for our material that reorganizes into macroscopic helices in af ully reversible manner upon illumination ( Figure 1). In our approach am olecular rotary motor with distinct upper and lower halves (denoted as as econd-generation motor) M1 is used as al ight-responsive chiral cross-linker in an aligned nematic liquid-crystal polymer network. Key to the functioning of these systems is the photochemical-induced change of mesoscopic order as the motor M1 simultaneously functions as chiral LC dopant, controlling the initial helicoidal molecular organization, and the light-responsive unit inducing major changes in shape and chirality of the polymer.L arge deformations are expected as there is as trong coupling between mechanical strain, chirality of the dopant and orientational order in the LC polymer network.
Them aterial used for the LC matrix is obtained by photopolymerization of amixture of acrylate-functionalized nematic liquid crystals (RM 82, RM 105, RM 23) (Figure 1), in ar atio that allows processing of the monomers at moderate temperatures and enough flexibility after polymerization to permit deformation of the polymer films.The designed motor cross-linker molecule M1 ( Figure 1) contains two essential parts:i )a no vercrowded-alkene-based second-generation light-driven rotary motor as acentral core and ii)two acrylate moieties for co-polymerization in the liquid-crystal network ( Figure 1). In the present study,w ee mployed as econdgeneration motor with ac yclopentene upper-a nd af luorenene-lower half (Figure 1) as the core structure because motors of related structures have high rotary speeds (t 1/2 = 1-3min at rt), [59] which are suitable for fast actuation [45,53] and these structures also allow for easy functionalization at both upper and lower halves.AC-6 carbon spacer was placed between the motor core and the acrylate moieties to act as abifunctional cross-linker while providing enough free space for the motor to rotate inside the polymer network ( Figure 1). Motor M2 with as ingle acrylate functional group was prepared to serve as ac ontrol compound (Scheme S1).
Thes ynthesis of bisacrylate functionalized molecular motor M1 started with tetralone 3 which was converted in three steps (deprotection, reprotection, thionation) in TBSprotected thioketone 6 (Scheme S1). Thelower half,ketone 7 was treated with hydrazine monohydrate and the resulting hydrazone 8 oxidized with MnO 2 at 0 8 8Ct og enerate the corresponding diazo compound 9.T he key step in the synthesis of the overcrowded alkene is aB arton-Kellogg coupling reaction which requires heating of the thioketone 6 and diazo compound 9 at 75 8 8Cf or 3hfollowed by desulfurization of the resulting episulfide with PPh 3 to provide the Amixture of the LC monomers and racemic motor (R,S)-M1 (3 wt %) is aligned from homeotropic to planar.The mixture is cured into ah omogeneous film and is cut along the rubbing direction. The obtained ribbon is able to bend upon UV light irradiation (365 nm) or walk over asurface. B) LC monomers with enantiomerically pure motors (R)-M1 and (S)-M1 (red dashed square). The LC monomers are mixed with (R)-M1 or (S)-M1 motor (1 wt %) and aligned into atwisted nematic structure. The mixture is cured into ahomogeneousfilm and is cut along the rubbing direction. The resulting ribbon with R-motor shows left-handed helical motion when irradiated with UV light, while the ribbon with S-motor shows right-handed helical motion. overcrowded alkene 10.A fter deprotection of the phenol groups,t he attachment of two linker chains using 6-bromohexan-1-ol was performed in the presence of abase and TBAI and the resulting motor 12 was treated with acryloyl chloride in the presence of base yielding the target bis-acrylate motor molecule M1.Mono-acrylate motor M2, not able to function as crosslinker,was obtained using asimilar route ( Figure 2A). Enantiomeric pure (R)-M1 and (S)-M1 were obtained by preparative chiral HPLC.T he separation and characterization of (R)-M1 and (S)-M1 are detailed in the Supporting Information.

Rotary Cycleo fM olecular Motor
As econd-generation molecular motor undergoes af ull 360-degree rotary cycle of the upper rotor part with respect to the lower stator part with the central olefinic double bond functioning as the rotational axle ( Figure 2A). The4 -step rotary cycle involves two photoinduced isomerization processes (step 1a nd 3) around the central double bond each followed by athermal helix inversion (THI) step (step 2a nd 4). Upon irradiation with UV-light (l max = 365 nm) of M1,the stable E-isomer [(R, M)-stable-E-M1]i sc onverted to an unstable isomer in which the methyl group at the stereogenic center is forced to adopt an energetically less-favored pseudoequatorial orientation with ac hange in helicity of the molecule,r esulting in (R, P)-unstable-Z-M1 isomer (Figure 2A). Asubsequent thermal helix inversion step results in release of the structural strain providing (R, M)-stable-Z-M1, with the methyl group at the stereocenter in amore favorable pseudoaxial orientation and an inversion of the helicity from P to M. Another set of photochemical and thermal isomerization processes completes the 3608 8 rotary cycle (Figure 2A). Figure 2B and Cs how the associated UV/Vis spectra of M1 and M2 in DCM solution at 263 K; both spectra displayed bathochromic shifts of the absorption upon irradiation at 365 nm with an increased band at 393 nm, which is characteristic of the formation of the unstable isomers (Figure 2B,C). [59] Thes amples were irradiated until no further change was observed and the photostationary states (PSS) were reached. Next the samples were kept in the dark at rt and the original spectra were regained as the result of the thermal helix inversion step.T he kinetic studies ( Figure S25 and S27) at different temperatures (T = 283, 273, 263, 253 K) provided the rate constants of the first-order thermal isomerization process,a nd the Gibbs energy of activation based on Eyring analysis ( Figure S26 and S28), was 79.33 kJ mol À1 for the THI from unstable-Z to stable-Z isomer and 78.87 kJ mol À1 for THF from the unstable-E to stable-E isomer.T he half-lives (t 1/2 )w ere calculated to be 15.5 sa nd 12.9 s, respectively,for the unstable-E and unstable-Z isomers at room temperature.T hese values are similar to secondgeneration motors with related core structures, [59] indicating that the introduction of the functional groups has no significant influence on the overall rate-determining thermal helix inversion step.T he results confirm that the motor M1 employed in the present study has ah igh rotary speed, considered crucial for the photo-responsive behavior.
In addition, circular dichroism was used to confirm the photochemical and thermal steps of motor M1.(R)-M1 shows an egative CD absorption at 380 nm ( Figure 2D,b lack line) and upon irradiation with UV-light anew positive CD band is observed at 420 nm, which is in accordance with the change of molecular helicity due to the formation of the unstable M1 isomer (Figure 2A,step 1). Independent study of enantiomer (S)-M1 showed similar but inverse CD effects ( Figure 2E). It should be noted that clear isosbestic points are maintained indicating unimolecular processes and keeping the samples in dark at rt (thermal helix inversion, Figure 2A,s tep 2) after photoisomerization resulted in the original spectra in accordance with selective isomerization. And another cycle of irradiation and heating shows similar spectroscopic changes (Figure 2A,s tep  3a nd 4) indicative of the complete 3608 8 rotary cycle and in accordance with our previous studies on second-generation motors. [37] Bending and Walking Motion of Photo-responsive LC Films Following the characterization of the rotary motion of the motor in solution, the functioning of the motor as crosslinker and as ap hoto-actuator in LCN thin films was studied. Am ixture of 3wt% racemic M1 and LC monomers was stirred at 80 8 8Cf or 3h prior to application. Before the curing process,t he resulting mixture was introduced in splayed cells by capillary suction at 80 8 8C.
We chose the splayed cell for the liquid-crystal network (LCN) formation as splayed alignment usually gave rise to larger deformations of free-standing films upon irradiation when compared to the samples with different alignments (uniaxial or parallel). [45] Thec ells were subsequently cooled down to 45 8 8Cand during this process,the doped liquid-crystal monomer material undergoes ac hange from the isotropic phase to anematic phase at around 60 8 8C( Figure S32) and the cell configuration enabled the LC mixture containing motor to align from homeotropic to parallel as shown in Figure 3A. Next the mixture was copolymerized using blue (455 nm) light irradiation with al ight intensity of 80 mw cm À2 .T he liquid-crystal films were annealed at 125 8 8Cf or 10 min and cooled down to rt. Thepolarized optical microscopic (POM) images of the prepared films show ad ark field when it was placed parallel to the cross-polarized light and abright field at 458 8 ( Figure S33), which indicates that the polymeric LC films have as played alignment with au nidirectional projection of the orientation vector parallel to the rubbing direction of the lower substrate. [45,58] Following isolation of the polymer LC films from the cells, ribbons with aw idth of 3mmw ere cut along the alignment direction at the planar orientation side of the sample. Irradiations were performed on free-standing ribbons with UV light (365 nm) at an intensity of 100 mW cm À2 (see Supporting Information, for experimental set-up). In the actuation study,the UV LED was placed at the homeotropic side of the film, the ribbons bend away from the light source ( Figure 3B). Theribbons could reach the saturated state in 2s during illumination and recovered to their initial position instantaneously after switching off of light (Supporting Movie S1). Theribbon was studied with UV/Vis spectroscopy during the irradiation cycles.
Upon UV light (365 nm) illumination, the absorption of the film shows ad ecrease at 380 nm with ac oncomitant increase at 430 nm ( Figure 3C), which is similar to the change of the motor in solution ( Figure 2B). It indicates the rotary motion of the motor in the LC ribbon during the irradiation. When the UV-light was switched off,the original spectra were regained. Thep hoto-actuation and recovery have been repeated several times and the system operates without any fatigue during the cycles ( Figure 3B,D,Supporting Movie S1). In order to explore the potential of the fast bending motion to induce translational motion, [45] the LC film prepared by the above method was cut into asmall piece with alength of 5mm and placed on af lat glass surface.U pon irradiation, the LC film was able to walk as hort distance in ad irection towards the light source ( Figure 3E,S upporting Movie S2). For comparison, 3wt% racemic M2,c ontaining as ingle acrylate group,w as used as dopant together with 18 wt %R M2 3, 31 wt %R M8 2, 46 wt %R M1 05 and 2wt% IRG 819. The control film was formed under identical conditions as those of M1 using as played cell with at hickness of 25 mm, and also studied under the same photochemical conditions.H owever, no shape change was observed after UV light irradiation using this control compound not able to function as across-linking unit in al iquid-crystal network ( Figure S30, Supporting Movie S3). It strongly supports the notion that the observed actuation of the LCN ribbon based on motor M1 is predominantly due to the rotation and change in shape of the motor, and its effect on the order parameter of the LCN and cannot be attributed to ah eating effect. [58] Ther otational motion of the motor,a nd its effect on the polymer main chains,r educe the order parameter of the mesogenic units which results in shrinkage along the molecular direction and expansion orthogonal to the ribbon. As ar esult of the applied splayed configuration the expansion difference at both sides of the film makes the film to bend.

Helical Motion of Liquid-Crystal Network
In order to study helical motion, we next used enantiomerically pure motors.W ef irst tested the helical twisting power (HTP) of (R)-M1 and (S)-M1.( R)-M1 and (S)-M1 were mixed with E7 respectively and the resulting mixtures were then filled into wedge cells by capillary force.The wedge cells were heated to 70 8 8Ca nd then cooled down to room temperature,w hereupon distinctive disclination lines were observed in the LC film through POM ( Figure S31). The helical pitch measured by the Grandjean-Cano method, was obtained using P = 2 Rtanq,w here R represents the distance between disclination lines and q is the wedge angle of the wedge cells (tanq = 0.0078). With 1wt%of (R)-M1 or (S)-M1 as ac hiral dopant in E7, the helical pitch was determined to be 1.90 AE 0.04 mma nd 1.92 AE 0.05 mm, respectively.T his represents the helical twist power for both (R)-M1 and (S)-M1 being AE 115 mm À1 ,w hich indicates M1 being as trong chiral dopant. [47,54,60] Next the chiral doped LC material was prepared from (R)-M1 with the LC monomer mixture (18 wt %R M2 3, 32 wt %R M8 2, 46 wt %R M1 05 and 2wt% IRG 819) at 80 8 8C. Theresulting LC mixture was filled in planar cells above the isotropic transition temperature and subsequently cooled to 40 8 8Ctofurther process the mixture in its chiral-nematic phase.
To our delight, after curing with blue light, astructure with helical organization was obtained in the LC films ( Figure 4A) which was confirmed by the reflective colors in the POM images,a lthough al ime structure is present that might point to some distortion of the helices from their pure planar organization ( Figure 4B). Furthermore,t he enantiomeric form of the motor (S)-M1, was also incorporated as ac hiral dopant in the LCN and the twisted nematic phase was formed similar to that of the R enantiomer ( Figure 4C). Ribbons with aw idth of 3mmw ere cut along the rubbing direction of the alignment layer. Next, the free-standing ribbons were irradiated with 365 nm UV light at an intensity of 100 mW cm À2 . Theprepared LC films showed fast helical motion during the actuation experiment. Only 1wt% (R)-M1 is needed to achieve left-handed helical motion after irradiation (Figure 4D,S upporting Movie S4) while the S-enantiomer (S)-M1 displayed right-handed motion ( Figure 4E,S upporting Movie S5). Ther ibbons were found to reach their saturated state in 2sduring illumination and recovered to their initial position as soon as the light was switched off ( Figure 4D,E; see also Supporting Movies S4, S5). Several cycles could be performed by subsequent irradiation and switching off of the light and the system did not show significant fatigue.Control experiments on two samples with opposite helices,due to the presence of enantiomeric chiral dopants as well as an achiral azobenzene photoswitch actuator,s howed indeed twist effects upon irradiation with opposite helicity ( Figure S34). This supports the molecular origin of the helicity change,t he correlated gradient in stress and the chiral amplification from the molecular via the mesoscopic to the macromolecular level. In the present study,t he fast helical motion is due to several unique features of these molecular rotary motors that are able to act as chiral dopant, responsive cross-linker and photochemical -actuator in the LCN film. Them otor is as trong chiral dopant when it was embedded in small amounts (1 wt %) in LC monomers and as ac onsequence induces the LC monomers to align in ah elix structure.T he approach presented here,t aking advantage of the various intrinsic properties of ar otary motor in combination with controlled alignment, provides af acile method to obtain responsive materials to amplify and control several characteristic motions.The approach is distinctly different from earlier ways to achieve actuation involving LCN material which requires additional chiral dopant besides photochromic molecules to make controlled helix alignment in an LC film with twisted cell. [57] Embedding all key features in as ingle motor, the change in helicity of the rotary motor, used as chiral dopant as well as cross-linker,e nables fast actuation and both left-and right-handed helical twisting motions can be readily induced.

Conclusion
Responsive soft polymeric materials which allow multiple distinct and well-defined motions triggered by light are of major importance to enable the development of complex mechanical actuating systems.H ere we show how al ightdriven molecular rotary motor incorporated in aliquid-crystal (LC) polymer network can induce bending, walking and both left and right-handed helical motion in the material.
As econd-generation molecular rotary motor functionalized with diacrylate moieties was incorporated as crosslinker in an LC-polymer network without compromising its photochemical driven rotary motion. Them otor unit has multiple functions that is,c ross linker for the LC network, intrinsic chiral dopant and photo-responsive units to allow autonomous motion upon irradiation with as ingle wavelength of light. Both racemic and homochiral motors were introduced in the LC network. Thee xperimental data show that, in contrast to the racemic motor which did not affect the orientation of monomers in different alignments,t he use of enantiomers resulted in the induction of different helical orientations of the LC monomers.T he polymer ribbons with splayed alignment obtained from racemic motors show fast bending motion and surface walking upon irradiation. In contrast, the samples prepared with R and S chiral motors shown fast right-handed or left-handed helical motion, respectively,d uring illumination with UV light. Control experiments indicate that the sense of helicity induced in the polymer ribbons is governed by the intrinsic chirality of the motor dopant and the twisting motions are fully reversible following the isomerization steps and associated dynamic helicity change of the motor unit during the rotary cycle.
Distinct from other approaches to achieve the conversion of light energy in reversible helical motion and amplification of mechanical effects along length scales from the molecular to macroscopic level, all key functions are embedded in as ingle molecular structure.U sing am olecular motor as crosslinker and chiral dopant in an LC-network, taking advantage of the shape change,dynamic chirality and control of directionality of movement, complex mechanical responses including bending,walking and twisting motions are achieved. Theability to govern complex autonomous motions based on the delicate interplay of intrinsic photo-responsive motor function, dynamic chirality and organization as shown here open new avenues for future smart materials.