Crystalline acylhydrazone photoswitches with multiple mechanical

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Crystalline acylhydrazone photoswitches with multiple mechanical responses Poonam Gupta, Tamas Panda, Suryanarayana Allu, Silpisikha Borah, Anamika Baishya, anilkumar gunnam, Ashwini Nangia, Naba K. Nath, and Pance Naumov Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01860 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Crystal Growth & Design

Crystalline acylhydrazone photoswitches with multiple mechanical responses Poonam Gupta,† Tamas Panda,‡ Suryanarayana Allu,┴ Silpisikha Borah,† Anamika Baishya,† Anilkumar Gunnam,┴ Ashwini Nangia, ┴,§ Panče Naumov,*‡ and Naba K. Nath*† †

Department of Chemistry, National Institute of Technology, Meghalaya (India) ‡ ┴

§

New York University Abu Dhabi, Abu Dhabi (United Arab Emirates)

School of Chemistry, University of Hyderabad, Hyderabad 500046 (India)

CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008 (India)

ABSTRACT. Advancement of our understanding of stimuli-responsive molecular crystals has led to the realization that they hold great, yet unexplored potential as adaptive materials. Although molecular crystals that exhibit single mechanical response to single input stimulus are now abundant, crystals that are capable of response to multiple stimuli are rather scarce. Here we report two photoswitchable acylhydrazone derivatives, Ac-1 and Ac-2, which respond to light as well as to mechanical force. Upon application of localized mechanical stress, the anhydrous (Ac1a) and monohydrate (Ac-1h) crystals of Ac-1 undergo plastic shearing deformation and bending, whereas monohydrate crystals of Ac-2 undergo elastic deformation. If they are exposed to UV light, crystals of Ac-1h and Ac-2 undergo photoinduced bending, on the other hand, crystals of Ac-1a and thicker crystals of Ac-2 exhibit photosalient effect (light-induced leaping). It is demonstrated that synergistic action of multiple stimuli (UV light and force) elicits enhance

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mechanical response, and this strategy could be employed in future to boost the efficacy of single crystal actuators.

Stimuli-responsive molecular crystals are gaining the attention of materials scientists due to possible applications of these smart materials as dynamic components of miniature machines. This research direction has witnessed rapid growth in view of the discovery, characterization and design of such materials, as illustrated by reports on molecular crystals which can bend,1‒7 twist,7‒11 jump (i.e., photosalient12‒16 and thermosalient effects17‒21), crawl,22 walk,23 and roll23 in the presence of light or heat. Moreover, crystals that can bend reversibly (elastically)11,24‒28 or irreversibly (plastically)24,29‒34 have been also discovered and analyzed. As new examples of stimuli-responsive crystals are being reported,35‒38 deeper understanding of such phenomena at a molecular level becomes essential. The presence of slip planes bridged by weak intermolecular dispersive interactions is believed to be central to plastic bending, whereas isotropic intermolecular interactions with interlocked molecular packing are considered to be relevant to elastic bending.11 Heat-induced mechanical effects in molecular crystals require a sudden crystalto-crystal phase transition, in addition to other requirements. Currently, prediction of such effects poses significant challenges. Although molecular crystals that respond to single stimulus have been reported,1‒10,12‒22,25‒34 crystalline materials exhibiting multiple mechanical responses are not common; indeed, only three such cases have been reported thus far. Desiraju et al. discovered molecular crystals of halogenated aldimine which exhibit thermosalient effect and force-induced elastic bending.39 Similarly, crystals of a chiral azobenzene compound that undergo photoinduced bending, curling40 and heat-induced rolling and walking motion were reported by Koshima et al.23 Recently, we have reported a molecular cocrystal that shows heat-induced twisting, light-induced bending, elastic deformation and self-healing.11 The scarcity of molecular


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crystalline materials that are responsive to multiple external stimuli could be attributed to difficulties with predicting the response to external influence of the crystal lattice, and thus, of the intermolecular interactions and molecular packing. With novel derivatives of an acylhydrazone photoswitch, here we report a rare case of molecular crystals that display photomechanical bending, photosalient effect and force-induced plastic and elastic bending. The acylhydrazones are organic photochromic molecules that can undergo light-induced E → Z isomerization and thermal Z → E isomerization.41‒44 Unlike other molecular photoswitches such as azobenzenes, anthracenes, and diarylethenes, the photomechanical response of this class of photoswitches in the solid state has not been explored yet. The two acylhydrazones Ac-1 and Ac-2 (Figure 1a) were selected from a set of twelve synthesized acylhydrazone derivatives (Scheme S1). The two compounds were prepared using a reported procedure41 and crystallized (Figure S1 and Figure S2). Upon slow evaporation from acetonitrile and acetone solutions Ac-1 crystallized as two distinct habits, acicular and hemimorphic, respectively (Figure S3). Single crystal X-ray diffraction (Figure S4), differential scanning calorimetry (DSC) (Figure S5) and thermogravimetric analysis (TGA) (Figure S6) revealed that the acicular crystals are a monohydrate (Ac-1h) whereas the hemimorphic crystals are the anhydrous form (Ac-1a). Ac-2 crystallized from acetonitrile as elongated plates. X-ray diffraction, DSC and TGA confirmed that these crystals are monohydrate (Figures S4, S5 and S6). Anhydrous crystals of Ac-2 could not be isolated, even with repeated trials by crystallization from different solvents.


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Figure 1. (a) Molecular structures and isomerization of the acylhydrazone derivatives Ac-1 and Ac-2. (b) Absorption spectra of Ac-1 and Ac-2 showing their photoswitching behaviour in acetonitrile. Colour codes: black (solid line), before UV irradiation; blue (solid line), after UV irradiation; red (broken line), after UV irradiation, at 45 °C. The numbers in panel b correspond to the λmax values. The UV-visible spectra were recorded to investigate the photoswitching of Ac-1 and Ac-2 in acetonitrile (concentrations: 1.574 × 10‒5 M and 1.439 × 10‒5 M, respectively). The absorption spectra of Ac-1 exhibited two maxima, at 226 nm and 316 nm. Upon UV light irradiation (365 nm, power density 361 mW cm‒2) for 9 min, the absorbance of the 226 nm peak increased while that of 316 nm peak decreased due to E → Z conversion. The thermal back-isomerization, Z → E, was achieved by warming the solution at 45 °C for 270 min (Figure 1b). In case of Ac-2, the E → Z isomerization was achieved by irradiating the solution with UV light (365 nm, power density 361 mW cm‒2) for 5 min, whereupon the two absorption bands at 223 nm and 310 nm changed their positon and intensity, with a shift of the 223 nm band to 237 nm and disappearance


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of the 310 nm band. The reverse (thermal) Z → E isomerization was accomplished by heating the solution at 45 °C for 380 min (Figure 1b). Time dependent density functional theory (TDDFT) was employed to calculate the UV-vis spectra of Ac-1 and Ac-2 which is detailed in Supporting Information (Figure S7, Table S1). The obtained crystal forms were then tested for mechanical response by applying various external stimuli such as mechanical force, UV light and heat. Upon application of mechanical force on the (001) face with a metal needle, the Ac-1h crystals were found to bend irreversibly (Figure 2a, Figure S8, Video S1) whereas crystals of Ac-1a were found to be brittle. However when crystals of Ac-1a were mechanically stressed on (010) face (Figure S8), they undergo shear deformation33 and slices of the crystals were separated when they were stressed further (Figure 2b). The crystal structure of Ac-1h was solved and refined in the monoclinic space group Pc with four molecules each of Ac-1 and water in the crystal lattice (Figure S4, Table S2). The molecules in the crystal are stacked along the a axis and connected to the neighbouring stacks along the b axis via N—H···O hydrogen bonds with the water molecules (Figure S9). Along the c axis the stacks are interconnected via weak Cl···O interactions45 thereby forming slip planes with weak interactions parallel to the (001) plane (Figure 2c). On the other hand crystals of Ac1a was solved and refined in P1̅ space group with Z´ = 1. C=O group involve in bifurcated hydrogen bonding with N—H (dH···A, dD···A, and Ѳ are 2.36(2) Å, 3.144(2) Å and 160(2)⁰ ) and C—H (dH···A, dD···A, and Ѳ are 2.47 Å, 3.277(2) Å and 144⁰ ) to form infinite chains along a axis which in turn are stacked via C—H···π interactions along b axis (Figure S9). These 2D stacks then closely pack resulting in a layered structure with the molecular layers parallel to the (001) plane and wider face of the crystal (Figure 2d). As these layers are weekly interconnected


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therefore they can slide over one another irreversibly when the crystals are stressed parallel to the molecular layers causing shear deformation.

Figure 2. (a) Plastic bending of the Ac-1h crystals accomplished by application of pressure on their (001) face. (b) Shearing deformation and slice separation of Ac-1a crystals achieved by pressing the crystals on (010) face. (c) Molecular packing in Ac-1h crystals viewed through (100) plane forming slip plane in the (001) plane. Red arrows shows direction of stress applied for plastic bending. (d) Layers of close packed 2D stacks of Ac-1 molecules in the crystal structure of Ac-1a viewed through (100). Red arrows show molecular layers and direction of stress for shear deformation of the crystals. When held together with a pair of forceps and pressed with a metal needle on the wider face, (001), elongated platy crystals of Ac-2 bend elastically (Figure 3 a,b; Figure S8, Video S2). The crystals can be bent almost to 90°. However, when exposed to further pressure occasionally break into two pieces that are elastic and fly off the stage. To understand the nature of the elastic bending, the structure was determined using single crystal X-ray diffraction. The compound


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crystallizes in the monoclinic space group P21/c with one molecule of Ac-2 and one molecule of water in the crystal lattice (Figure S4). In the crystal structure, the Ac-2 molecules are connected along the a axis by O—H···O hydrogen bonds between the water O—H groups and the carbonyl C=O groups, forming infinite stacks (Figure 3c; dH···A, dD···A, and Ѳ are 1.99(4) Å, 2.835(3) Å, 160(3)° and 1.98(3) Å, 2.826(3) Å, 173(2)°). The adjacent molecular stacks are connected through N—H···O hydrogen bonding along the b axis that extends between the acylhydrazone N—H groups and water (dH···A, dD···A, Ѳ are 1.93(3) Å, 2.891(3) Å, 163(2) Å). The 3D packing is completed by weak C—H···N hydrogen bonding between the CN and C—H groups. Unlike most reported elastically bendable crystals, the packing of the molecules in Ac-2 is not interlocked, and the interactions are highly anisotropic, with short hydrogen bonds along two directions and long hydrogen bonds along the third direction. This observation is similar to the elastic bending of copper(II) acetylacetonate crystals reported recently.5

Figure 3. (a), (b) Elastic bending of Ac-2 monohydrate crystal. (c) Molecular stacks connected by water via O—H···O and O—H···N hydrogen bond. The crystals bend when they are pressed


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on their (001) face. (d) Weak C—H···N hydrogen bonding along the b axis. (e) Cartoon of the prominent faces of a typical crystal of Ac-2. The UV-visible spectra recorded by dispersing Ac-1 and Ac-2 in KBr matrix showed that the reversible E ↔ Z isomerization can be induced in the solid state by irradiating the sample with UV light (365 nm, power density 361 mW cm‒2), followed by heating (Figure S10). To check for any photomechanical response, single crystals of Ac-1h, Ac-1a and Ac-2 were irradiated with UV light from a mercury-xenon lamp (365 nm, power density 361 mW cm‒2, with 3 cm distance between the light source and the crystal). 10 crystals each of Ac-1h and Ac-2 were picked and their photomechanical response was recorded by exposing their wider face, (001) (Figure S8), to UV light. Crystals of Ac-1h (sizes: 7.94—8.56 × 0.03—0.15 × 0.01—0.04 mm) bent with tip deflection in the range 1.12 mm to 8.01 mm, corresponding to crystal sizes 8.12 × 0.09 × 0.04 mm and 8.56 × 0.03 × 0.01 mm respectively (Figure 4a,b,c; Figure S11; Video S3). Similarly, crystals of Ac-2 (sizes: 1.25—3.51 × 0.08—0.19 × 0.01—0.04 mm) bent with maximum tip deflection in the range 0.80 to 3.82 mm corresponding to crystal sizes 1.25 × 0.08 × 0.03 mm and 3.42 × 0.11 × 0.04 mm, respectively (Figure 4d,e,f; Figure S11; Video S4). A plot of the maximum crystal tip deflection against the crystal length shows dependence of the maximum deflection on the crystal sizes (Figure 4c,f), in line with previous reports and the existing models.3,36,46


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Figure 4. Stills of Ac-1h (a, d)) and Ac-2 crystals (b, e) before UV irradiation and after UV irradiation. White cursors show the direction of UV irradiation. (c) and (f) show the plot of the maximum tip deflection vs crystal length. The width and thickness of the crystals corresponding to each point are also given and color-coded. Longer and thinner crystals are expected to bend to greater extent than shorter and thicker crystals, as the latter are much stiffer than the former. On the other hand, wider crystals are more bendable due to greater extent of E → Z isomerization on the irradiated crystals surface which is due to absorption of greater number of photons. All these factors collectively determine the extent of crystal bending, and hence a complex trend was observed for both Ac-1h and Ac-2. After the crystals of Ac-1h and Ac-2 are bent by UV light, they remain bent, however they regain their straight shape by heating to 40 °C with loss of crystallinity (Figure S12). The retention of original straight shape by the crystals may be ascertained to thermal back isomerization (Z → E) and volume change due to loss of water molecules from crystal lattice.


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Figure 5. (a) Photosalient effect displayed by crystals of Ac-1a. UV induced bending followed by photosalient effect in Ac-1a (b) and Ac-2 (c) crystals. (d) A crystal of Ac-1h deformed by application of mechanical force is subsequently bent by UV light irradiation. (e) A crystal of Ac2 bent by UV light is bent further by applying mechanical force. Apart from the above mentioned effects, crystals of Ac-1a and Ac-2 also display the photosalient effect. Hemimorphic crystals of Ac-1a (sizes: 0.19—0.44 × 0.08—0.3 × 0.03—0.06 mm), when exposed to UV light (365nm, power density 361 mW cm‒2) on their (001) face (Figure S8), jump rapidly or break with separation of debris (Figure 5a, Video S5). When a crystal was glued at one end to a metal needle and its (001) face was exposed to UV light, it was observed that it bends prior to disintegration or jumping (Figure S5b, Video S6). Photosalient effect was also observed with Ac-2 crystals which are comparatively thicker (sizes: 1.30‒3.42 × 0.09‒0.21 × 0.03‒0.06) than the typical crystals of this compound. When these crystals of Ac-2 were attached at one end to a metal needle and their wider faces were exposed to UV light, the


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crystals initially bent and disintegrated afterwards (Figure 5c; Video S7). Due to the poor diffraction quality of the UV-irradiated single crystals, the attempts to determine the crystal structure of the product upon photoexcitation by ex situ X-ray diffraction were not successful, and therefore detailed mechanistic insights of the underlying processes could not be provided. Finally, combined impact of mechanical force and light on the crystals of Ac-1h and Ac-2 was also investigated. It was found that when deformed by UV light Ac-1h crystals lose their ability to bend plastically. However if the crystals are initially deformed by application of localised mechanical force, they can be bent further by exposure to UV light (Figure 5d). After deforming Ac-2 crystals by exposing them to UV light into a bent shape, they were mechanically bent by pressing them with a needle on their wider face. The crystals did not lose their ability to bend elastically (Figure 5e). In both the abovementioned cases the crystals of Ac-1h and Ac-2 could be bent to a greater extent due to application of two different stimuli (UV light and mechanical force), and this result points to the possibility of using combined stimulus to enhance molecular performance. In summary, multiple mechanical responses were observed in the crystal forms of two novel derivatives of the acylhydrazone molecule photoswitch. Mechanically stressed UV bendable molecular crystals of Ac-1h and Ac-2 undergo plastic and elastic bending, respectively. Thicker crystals of Ac-2 and Ac-1a show photosalient effect. It was shown for the first time that the different stimuli can exert synergistic effect to enhance the mechanical response from the same crystal. ASSOCIATED CONTENT


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Supporting Information. The following files are available free of charge. Synthesis, characterization, mechanical effects, supporting figures, crystallographic parameters (PDF) Crystallographic data for Ac-1a,Ac-1h and Ac-2 (cif) Supporting videos (avi)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT PG thanks NIT Meghalaya for a fellowship. SA thanks UGC-BSR-SRF and AG thanks CSIRSRF for fellowships. NKN thanks DST-SERB-ECR grant (ECR/2016/000331) and NIT Meghalaya CPDA for funding. We thank Dr. Tejender Thakur (CSIR-CDRI Lucknow) for single crystal X-ray diffraction, refinement, generating cif file for publication and face indexing of Ac-


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1a crystals. This research was partially carried out using Core Technology Platform (CTP) resources at New York University Abu Dhabi.

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Table of Contents Graphics


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Crystal forms of acylhydrazone derivatives exhibit mechanical response to both UV light and force by photomechanical bending, photosalient effect and elastic/plastic deformation, in a rare case of multiresponsive crystalline material.


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Crystalline acylhydrazone photoswitches with multiple mechanical

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