Photomechanical Bending and Thermomechanical Unbending

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Visible Light Mediated Photopolymerization in Single Crystals: Photomechanical Bending and Thermomechanical Unbending Ranita Samanta, Subhrokoli Ghosh, Ramesh Devarapalli, and C. Malla Reddy Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04756 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Chemistry of Materials

Visible Light Mediated Photopolymerization in Single Crystals: Photomechanical Bending and Thermomechanical Unbending Ranita Samanta,[a] Subhrokoli Ghosh,[b]Ramesh Devarapalli,[a] C. Malla Reddy*[a] [a] Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata, Mohanpur Campus, Mohanpur, Nadia-741246, West Bengal, India. [b] Department of Physical Sciences, Indian Institute of Science Education and Research (IISER), Kolkata, Mohanpur Campus, Mohanpur, Nadia-741246, West Bengal, India. ABSTRACT: Recently several molecular crystals have been reported to show photomechanical bending mediated by chemical dimerization or rearrangement reactions or supramolecular changes in single crystals. Here we report a photomechanical bending involving polymerization reaction in single crystals for the first time. Bending is observed under visible light in a single-crystal-tosingle-crystal (SCSC) fashion and a rare thermomechanical unbending (via decomposition reaction) upon heating in needle shaped crystals of 1,1'-dioxo-1H-2,2'-biindene-3,3'-diyl didodecanoate. The single crystalline polymeric material formed here via topochemical polymerization reaction may provide an opportunity to combine the attractive qualities of both the crystalline small molecules and durable polymers for realizing certain unique advantages, for instance for construction of durable high performance mechanical actuators.

C

onversion of light or heat into mechanical work by stimuli responsive molecular crystals has attracted much attention in recent years due to their potential advantages in various devices, eg. for biomedical, optoelectronic, sensor or actuation applications.1-3 In photomechanical bending of small molecule crystals, spatially resolved anisotropic local molecular changes caused by photochemical (eg. intramolecular electrocyclic, keto-enol or cis-trans photoisomerizations or intermolecular photodimerization) reactions lead to macroscale movement of the crystal.4-12 In all such cases the stress generally dissipates via reorganization of intermolecular interactions among molecules at reactive sites in structure. Compared to traditional polymer based actuators, the examples of highly ordered, small molecular crystal actuators are relatively new and have been projected lately for achieving higher efficiencies due to their prospective superior elastic properties, faster response times and greater conversion efficiencies.1-3 However, the small molecule based materials generally lag far behind the polymers on stability and durability aspects, the most important and bare-minimum qualifying aspects, for wide practical usage.13-14 Hence, a question arises as to how one would achieve high-performance actuating materials. Is it possible to combine the superior stabilities of polymers and higher efficiencies of crystalline materials? Photomechanical bending in such combination crystals would depend on whether they withstand the long range anisotropic stresses caused over a large number of monomer molecules upon polymerization. Here, we provide a proof-of-concept for photomechanical bending via polymerization reaction, using a novel diindene compound, namely, 1,1'-dioxo-1H-2,2'-biindene-3,3'-diyl

didodecanoate (BIT-dodeca2), which undergoes topochemical photopolymerization reaction under visible light in monomer crystals in SCSC fashion and thermal depolymerization in a reversible fashion (Figure 1).15-19 The SCSC polymerization reaction observed in single crystals of our compound is similar to the recent report by Dou et al.,20 but to the best of our knowledge no reports exist on photomechanical bending involving polymerization reaction in single crystals, either by UV or visible light, till date. In addition, we also demonstrate the thermomechanical unbending of the crystals mediated by depolymerization reaction, which is the third report in literature.21-22 Compared to UV light, visible light is more attractive owing to greater safety, biocompatability, abundancy etc.12 Not only in materials science but also in life the ability to trigger the properties of molecular systems by visible light holds great importance. The desired monomer BIT-dodeca2, was prepared (see S2 in the Supporting Information) and recrystallized from (1:1) dichloromethane: ethanol solution by slow evaporation method. We obtained long needle-like orange colour single crystals of BIT-dodeca2, in 4-5 days, which grew along crystallographic b-axis, with lengths ranging from 0.5 - 3 cm. The freshly grown orange crystals when exposed to sunlight for about 3 hrs converted to light yellow crystals (PBIT-dodeca2), with no visible loss of crystallinity, confirming their photochromic nature (Figure 1a). The same result was observed when we carried out the experiment with solar simulator using UV (400 nm) cutoff filter. Interestingly, when these light yellow crytals were heated at ~ 195 oC, the crystals converted to orange colour again (thermochromism).

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To investigate the reversible colour changes, at molecular level, single crystal X-ray diffraction (SCXRD) analysis of both the orange and light yellow crystals was carreid out. The crystallographic data are given in Table 1. The structural analysis of orange crystals, BIT-dodeca2 revealed that it

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longest dimension of the needle type crystals. Hence, the molecular polymeric chains are parallel to the crystal length. Structural analysis of organe crystals that are obtained upon heating the light yellow PBIT-dodeca2 crystals revealed the conversion of polymers to monomers (via thermal depolymerization reaction). This cycle could be repeated by alternate exposure to light and heat, respectively, confirming the reversible nature of the polymerization-depolymerization phenomenon. Although the overall crystal packing of the monomer and polymer crystals is very similar, the comparison of their SCXRD data (Table 1) revealed that the polymerization leads to significant changes in unit cell dimensions. The unit cell length along a-axis is elongated (from 20.4473 Å to 20.909 Å) while b- and c-axes contract (b = 4.9455 Å to 4.8571 Å; c = 19.5958 Å to 19.210 Å) after irradiation. Table 1. Selected parameters from single crystal X-ray diffraction data of monomer (BIT-dodeca2) and polymer (PBITdodeca2).

Figure 1. a) Photopolymerization of orange BIT-dodeca2 (left side) monomer crystals yields yellow PBIT-dodeca2 crystals (right side) after two hours of exposure to sunlight. Reverse depolymerization reaction takes place under thermal conditions (at 195 oC) to produce orange colour crystals of monomers. b) Images of a PBIT-dodeca2crystal decomposing with time tomonomer at 195 °C (from t = 0 s to 5 min).

BIT-dodeca2

PBIT-dodeca2

(before irradiation)

(after irradiation)

P21/c

P21/c

a = 20.4087 (15)

a = 20.916 (4)

b = 4.9438 (3)

b = 4.8563 (7)

c = 19.5578 (16)

c = 19.218 (4)

Cell angles (o)

β = 113.954 (9)

β = 116.10(2)

Cell volume (Å3)

V = 1800.3 (Z = 2)

V = 1753.5 (Z = 2)

dc-c (Å)

3.228

1.612

dπ-π (Å)

3.478

3.099

Crystal Parameters

Space group

Cell lengths (Å) crystallizes in monoclinic space group P21/c with a half molecule in the asymmetric unit. In the crystal structure, the aromatic diindine moieties stack along the length of the needle (i.e. b-axis), while long chains from adjacent molecules interdigitate as shown in Figure 2. The distance between two active unsaturated carbon atoms of the adjacent stacked monomer molecules was found to be 3.228 Å which is less than the 4.2 Å, a typical distance required for the topochemical polymerization reactions.23 The orange crystals of BIT-dodeca2 absorbed light in the 250 nm to 500 nm region (see Figure S3 in the Supporting Information). As light propagated, crystal became transparent, so the yield of single crystal polymerization was quantitative (~ 96 %; see S3, in the supporting information). We were unable to determine the molecular weight of the polymer due to its insoluble nature. It was also found that heating monomer crystals in dark (at 195 °C) did not initiate polymerization. The polymerization mechanism is believed to be similar to that reported earlier.20 The structural analysis of the light yellow crystals of PBITdodeca2 (monoclinic space group P21/c with a half molecule in the asymmetric unit) further confirmed the solid-state topochemical polymerization of monomer molecules via formation of covalent single bonds (1.612 Å) between the carbon atoms at 3, 3' positions of adjacent molecules, where hybridization changed from sp2 to sp3 (Figure 2). The overall crystal packing of the polymer, PBIT-dodeca2 was comparable to that of the monomer structure, BIT-dodeca2 (Figure 3). As a result, linear polymer chains of the molecules are formed along the stacking direction, i.e. parallel to the b-axis or

With this in mind, we aimed to irradiate the needle like single crystals of BIT-dodeca2, with a focused white light source (HBO 100), from one face so as to initiate the polymerization reaction preferentially from one surface. Because, the light is absorbed faster by the molecules on the front surface, compared to those placed closer to the rear surface (due to the gradient screening effect by absorbing molecules located closer to the front surface). If the crystal withstands the anisotropic stress generated at different locations, this shall lead to crystal bending assuming front surface contracts much faster than the rear surface under the experimental conditions. To observe such deformity, we mounted a needle shaped single crystal of about 1 cm in length on a wooden tip in such a way that (001) face of the crystal was pointing towards the white light source (HBO 100 with a beam spot size of ~ 3 mm at the focus). Consequently, when the crystal was exposed to the white light, we observed a gradual and smooth


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Chemistry of Materials

Figure 2. Crystal packing of a) monomer BIT-dodeca2 and b) polymer PBIT-dodeca2, viewed down along c-axis, to depict the interdigitation of long chains and stacking of aromatic groups.

Figure 3. Single crystal packing of a) BIT-dodeca2 and b) PBIT-dodeca2 viewed along different axes. c) Schematic depiction of the approximated crystal packing expected after photo-induced bending.



photomechanical bending with an angle of nearly 73° by about 52 s of exposure time (Figure 4a). The bending was towards the white light source and perpendicular to the longest dimension, i.e. b-axis, of the crystal (see Movie S1 in the Supporting Information). This suggests that the initial polymerization reaction started from the front surface. The polymerization led to accumulation and amplification of the contraction force through π-stacked columns parallel to (001) plane. The stress that was generated due to the difference in the degree of contraction between different layers led to the bending of the crystal towards light source (Figures 4a). Repetition of the experiment with a well defined laser source of 405 nm (beam diameter: 3mm; power : 50 mW) also resulted the similar bending (see Movie S2 in the Supporting Information and Figure 4b). Notably, the observed photomechanical bending effect by a combination of SCSC polymerization reaction and white/visible light is a rare phenomenon. Since the topochemical polymerization reaction is reversible under thermal conditions, we attempted unbending of the bent crystal by heating uniformly in an oven at 195 o C for five minutes. As anticipated, the bent crystal regained its linear shape due to unbending (Figure 4b), which we call thermomechanical unbending. Bending-unbending could be repeated (we tried up to 3 cycles) by irradiation with light and thermal heating in cycles (see Figure S7 in the Supporting Information). Although the bending and unbending process in single crystals was shown earlier with alternating UV and visible light exposure, thermal energy was used only in two earlier studies for the purpose.21-22 There are also several examples of thermosalient effects (crystal jumping) due to thermal phase transformations,24-27 but the thermomechanical bending remains a rarity. Moreover, the highly ordered polymers formed in single crystals here may provide an opportunity to achieve superior mechanical properties28-29 and stability compared to the simple (or non-polymerization) chemical reactions or isomerization in photomechanical effects shown in literature. Exploration of these aspects in these fast

Figure 5. 3D AFM height images of a) BIT-dodeca2 and b) PBIT-dodeca2. The graphs c) and d) show different degrees of roughness for (BIT-dodeca2) monomer and (PBIT-dodeca2) polymer, respectively.

emerging new class of single crystalline polymeric compounds, using crystal engeering approach30 is currently underway in our laboratory. In addition to the photo/ thermomechanical bending effects in crystals, we also studied average roughness of monomer and polymer single crystals by Atomic Force Microscopy (AFM). Before irradiation, the (001) face of the monomer showed an average roughness of 2.93±0.4 nm (standard deviation is from calculation of average roughness from three different positions of a measurement) and it has been increased to 32.14±0.5 nm after irradiation (Figure 5). The increase in roughness after irradiation could be due to molecular movements and associated surface defects during chemical reaction in the crystal. In conclusion, we have successfully demonstrated the visible light mediated photomechanical bending in a new class of compound, diindene derivative, involving topochemical polymerization reaction in single-crystal-to-single-crystal fashion. In addition, we also showed the reversal by a rare thermomechanical bending in a single crystal using thermal energy. Our study demonstrates the possibility of utilizing highly ordered single crystalline polymeric materials (achievable via topochemical polymerization reactions) for mechanical actuation, which may find some unique applications.

ASSOCIATED CONTENT Supporting information Crystallographic data of monomer and polymer, movies of photomechanical effect, NMR, TGA, DSC data and additional results. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 4. a) Bending process of BIT-dodeca2 during illumination. b) Photomechanical bending by light and thermomechanical reversal by heat.

AUTHOR INFORMATION Corresponding Author


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Chemistry of Materials *C. Malla Reddy. E-mail:[email protected]

ACKNOWLEDGMENT R.S. thanks UGC for senior research fellowship. C.M.R is grateful to DST (SJF/CSA-02/2014-15) for Swarnajayanti fellowship. We thank Dr. Ayan Banerjee (IISER Kolkata) for his support and help in performing bending experiments.

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Photomechanical Bending and Thermomechanical Unbending

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