Visible Light Mediated Photopolymerization in Single Crystals

Communication Cite This: Chem. Mater. 2018, 30, 577−581

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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*,† †

Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur Campus, Mohanpur 741246, India ‡ Department of Physical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur Campus, Mohanpur 741246, India S Supporting Information *

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, e.g., 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 (e.g., 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 stress 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 single-crystal-to-single-crystal (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, until 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, biocompatibility, abundancy, etc.12 Not only in materials © 2018 American Chemical Society

Figure 1. (a) Photopolymerization of orange BIT-dodeca2 (left side) monomer crystals yield yellow PBIT-dodeca2 crystals (right side) after 2 h of exposure to sunlight. Reverse depolymerization reaction takes place under thermal conditions (at 195 °C) to produce orange color crystals of monomers. (b) Images of a PBIT-dodeca2 crystal decomposing with time to monomer at 195 °C (from t = 0 s to 5 min).

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 color single crystals of BIT-dodeca2, in 4−5 days, which grew along crystallographic baxis, with lengths ranging from 0.5−3 cm. The freshly grown orange crystals when exposed to sunlight for about 3 h 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 Received: November 12, 2017 Revised: January 12, 2018 Published: January 12, 2018 577

DOI: 10.1021/acs.chemmater.7b04756 Chem. Mater. 2018, 30, 577−581

Communication

Chemistry of Materials were heated at ∼195 °C, the crystals converted to orange color again (thermochromism). To investigate the reversible color changes, at the 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

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 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 PBITdodeca2 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 20.4087 to 20.916 Å) whereas b- and c-axes contracted (b = 4.9438 to 4.8563 Å; c = 19.5578 to 19.218 Å) after irradiation. 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 toward 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 photomechanical bending with an angle of nearly 73° by about 52 s of exposure time (Figure 4a). The bending was toward 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

Table 1. Selected Parameters from Single Crystal X-ray Diffraction Data of Monomer (BIT-dodeca2) and Polymer (PBIT-dodeca2) Crystal parameters Space group Cell lengths (Å)

Cell angles (deg) Cell volume (Å3) dc−c (Å) dπ−π (Å)

BIT-dodeca2 (before irradiation)

PBIT-dodeca2 (after irradiation)

P21/c a = 20.4087 (15) b = 4.9438 (3) c = 19.5578 (16) β = 113.954 (9)

P21/c a = 20.916 (4) b = 4.8563 (7) c = 19.218 (4) β = 116.10 (2)

V = 1800.3 (Z = 2)

V = 1753.5 (Z = 2)

3.228 3.478

1.612 3.099

analysis of orange crystals, BIT-dodeca2 revealed that it 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), whereas 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 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 the dark (at 195 °C) did not initiate polymerization. The polymerization mechanism is postulated 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

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. 578


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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 photoinduced bending.

of the bent crystal by heating uniformly in an oven at 195 oC for 5 min. 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 nonpolymerization) chemical reactions or isomerization in photomechanical effects shown in literature. Exploration of these aspects in these fast 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 shows 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

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

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 toward light source (Figures 4a). Repetition of the experiment with a well-defined laser source of 405 nm (beam diameter: 3 mm; 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. Because the topochemical polymerization reaction is reversible under thermal conditions, we attempted unbending 579


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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

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04756. NMR, TGA, DSC data and additional results (PDF) Movie of photomechanical effect (MPG) Movie of photomechanical effect (MPG) Crystallographic data of monomer (CIF) Crystallographic data of polymer (CIF)

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REFERENCES

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Figure 5. 3D AFM height images of (a) BIT-dodeca2 and (b) PBITdodeca2. The graphs (c) and (d)show different degrees of roughness for (BIT-dodeca2) monomer and (PBIT-dodeca2) polymer, respectively.

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AUTHOR INFORMATION

Corresponding Author

*C. Malla Reddy. E-mail: [email protected]. ORCID

C. Malla Reddy: 0000-0002-1247-7880 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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. 580


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Visible Light Mediated Photopolymerization in Single Crystals

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