Solvatomechanical Bending of Organic Charge Transfer Cocrystal

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Solvatomechanical Bending of Organic Charge Transfer Cocrystal Yanqiu Sun, Yilong Lei, Huanli Dong, Yonggang Zhen, and Wenping Hu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b00772 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

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Journal of the American Chemical Society

Solvatomechanical Bending of Organic Charge Transfer Cocrystal Yanqiu Sun,1,4,§ Yilong Lei,3,§ Huanli Dong,1 Yonggang Zhen1 and Wenping Hu1,2,* 1

Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China 3 Department of Chemistry, Hong Kong Baptist University, Hong Kong, China 4 University of Chinese Academy of Sciences, Beijing 100049, China Supporting Information Placeholder 2

ABSTRACT: Here, we report here a new ternary solvated (perylene-TCNB)•2THF cocrystal, which can transform into binary perylene-TCNB cocrystal reversibly by successive desorption or absorption of THF solvent. As a consequence, macroscopic mechanical bending would be realized when repeated stimulation with THF solvent. The present results clearly demonstrated that solvent induced mechanical bending is driven by structural change at the molecular scale. Such solvatomechanical bending behavior is clearly revealed for the first time. 1

Stimuli-responsive molecular crystals, which can respond 2 3 to appropriate external stimuli (e.g. heating, light radiation, 4 and external force ), commonly represent dramatic shape 5 6 changes or even mechanical motion as a result of the structural changes of constituent molecules at the molecular scale. A representative example is the case of photomechanical motion of BIT-dodeca2 crystals induced by polymerization reac7 8 tion. Besides photomechanical effect, it is still scarce to achieve crystal bending by different ways, such as solvent treatment. From another perspective, sometimes cocrystals exhibit novel and/or improved optoelectronic properties when com9 pared to their individual components. Similarly, the related works on stimuli-responsive molecular crystals may also be extended to organic cocrystals, such as remarkable dimensional changes of CuQ2-TCNQ cocrystal upon mechanical 4b stimulation and photomechanical motion of a photoisomeri1c zation diarylethene cocrystal . Inspired by the successes of 4b,10 the mechano-responsive TCNQ-based cocrystals, we wonder whether luminescent 1,2,4,5-tetracyanobezene 9c,11 (TCNB)-based charge-transfer (CT) cocrystals could also generate such changes under appropriate external stimuli. Herein, we report here a red-emitting solvated (peryleneTCNB)•2THF cocrystal (PT2TC). Due to the escape of THF molecules, the initial ternary cocrystal can switch to binary perylene-TCNB crystal (PTC), which would be accompanied by color changes. Importantly, macroscopic mechanical bending of the binary PTC was also achieved when exposed to high concentration of THF solvent/vapor. The striking responsive behavior of the PTC is caused by the inclusion and removal of THF molecules, which provides a desired platform to investigate the mechanism of mechanical motion of cocrystals induced by guest molecules.

Figure 1. (a) Reversible structural and color changes between ternary PT2TC and binary PTC. (b) Schematic diagram of mechanical bending of a cocrystal rod (“+” represents the inclusion of THF, “-” displays the removal of THF). We selected perylene and TCNB to form PTC considering that TCNB-based CT cocrystals commonly exhibit strong visible luminescence. Surprisingly, as confirmed by its single crystal X-ray diffraction (XRD) data, ternary PT2TC rather than binary PTC was achieved by slowly evaporating THF solution of perylene and TCNB (1:1). It was found that the as-prepared ternary cocrystal belongs to a monoclinic system with the following unit cell parameters: a = 13.039(2) Å, b = 14.398(2) Å, ° c = 7.8109(10) Å, and β = 97.60(2) and its chemical formula is C38H30N4O2. Similar to a large-sized PT2TC, micrometersized PT2TCs formed by rapid crystallization also show red color. Upon exposed to air within two minutes, the micrometer-sized PT2TCs would gradually become greyish-green, suggesting the removal of THF molecules or the oxidation of ° perylene. Actually, perylene is stable in air even above 100 C. Hence, the color change of PT2TC should result from the escape of THF molecules (Figure 1a), which was caused by weak intermolecular interaction between THF and TCNB, simi12 lar to solvated Alq3 crystals. As a result, the ternary PT2TC would transform into binary PTC. Unexpectedly, the above-mentioned PTC microrods display dramatic mechanical bending with color change when exposed to a sealed container containing THF solvent vapor (Figures 1b and 2). Fluorescence microscopy equipped with blue light was used for real-time observation. At the initial stage (t ~ 0.5s), the outer layer of a greyish-green binary PTC microrod would partially become red (Figure 2a-c). After then, the straight microrod would totally transform into red, which was accompanied by striking crystal bending (Figure 2d and 2e). Upon exposed to air, the curved rod with red emission would return back to greyish-green rapidly (Figure 2f and 2g). When compared to the case in the THF (Figure 2e), the crystal bending would slightly recover in reverse (Figure 2f and

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2g). While THF solvent was introduced again, the mechanical bending would proceed and be accompanied by the recovery of the red color (Figure 2h, 2j, 2l, 2n).

Figure 2. (a-o) Remarkable color and shape changes of a typical PTC microrod when exposed to THF vapor (2b-e, 2h, 2j, 2l, 2n) and air (2a, 2f, 2g, 2i, 2k, 2m, 2o), respectively. The scale bar is 5 μm. Upon repeated stimulation with THF solvent many times, the cocrystal microrod shows a continuous bending in shape and an obvious anisotropic expansion in thickness. As a result, it can even form a closed ring with a rough surface (Figure S1 and Video S3), which may be due to severe structural relaxation. However, the cocrystal microrod with/without THF still kept macroscopic integrity. Real-time videos of single rod-like PTC under exposure in THF solvent vapor and air were also recorded to obtain more deformation information. Specifically, continuous and rapid crystal bending for micrometer-sized rods can be achieved when exposed to low-doses of THF solvent vapor (Videos S1 and S2). While a drop of THF solvent was required to induce such a rapid change for centimeter-sized rods (Video S4), it can be hereby demonstrated that the change in molecular structures of the CT crystals before and after deformation drives the responsive processes. Next, we measured time-resolved FTIR spectra of PT2TC to investigate the composition change during structural transformation. The IR bands originated from perylene donor or TCNB acceptor almost keep unchanged when compared to the constituent materials (Figure S2). Thus, we can clearly confirm the removal of THF component from the ternary PT2TC due to the disappearance of characteristic IR peaks at around 1065, -1 1461, 2860, and 2974 cm when exposed to air for 10 min (Figure 3a). The present results again verify that ternary PT2TC would change into binary PTC, which was accompanied by a remarkable color change. To further determine the structural changes before and after deformation, powder XRD characterization of PT2TC and PTC were also carried out. After desorption of THF molecules from initial PT2TC, its XRD peaks (І in Figure 3c) would become weaker and display obvious shifts when compared to those of the former (І in Figure 3b). We hereby conclude that the solvent-free PTC has a weaker crystallinity considering that the removal of THF would leave some holes or defects in the binary crystal. After absorption of THF again, XRD patterns of the resultant crystals (ІІ in Figure 3b) are almost identical to the initial ternary rods (І in Figure 3b) except some additional weak diffraction peaks. Upon continuous removal and inclusion of THF molecules, the corresponding cocrystals exhibit

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similar XRD peaks to previous cases (І in Figure 3c, І and ІІ in Figure 3b). However, their crystallinity would become worse gradually, possibly due to multiple structural deformations.

Figure 3. (a) Time-resolved FTIR spectra of PT2TC when exposed to air for 10 min (from bottom to top). (b, c) PXRD patterns of PT2TC upon repeated (b) absorption and (c) desorption of THF solvent. (d) Time-dependent absorption and (e) PL spectra of PT2TC upon exposed to air. (f) Timeresolved fluorescence decay of PT2TC. Moreover, time-dependent absorption and photoluminescence (PL) spectra of PT2TCs were also performed to study its photophysical properties. As shown in Figure 3e, PT2TCs show an emission band at around 616 nm (red line, Figure 3e), which is due to CT interaction between perylene and TCNB, 9c,11 Upon prolongsimilar to previous TCNB-based cocrystals. ing the exposure time in air, the PL intensity would decrease gradually. Meanwhile, the PT2TCs display an absorption peak at 580 nm (red line, Figure 3d) and red-shifted to 610 nm (green line, Figure 3d) after 15 min due to the removal of THF. We thereby inferred that the CT interaction between perylene and TCNB in the binary PTC would become stronger, when compared to that of the ternary PT2TC. Theoretical analysis was also conducted to determine the CT degree of PT2TC and PTC. Both the CT amount and molecular orbital (MO) of 13 the two cocrystals were calculated (Figures S4 and S5). Compared to PT2TC, a larger CT amount (0.059e) and a narrowed HOMO-LUMO gap (1 eV) indicate stronger CT interaction in PTC, which is consistent well with the experiment results. Moreover, PT2TCs exhibit a mono-exponential fluorescence decay lifetime of 9.68 ns and a PL quantum yield of 5.04%, revealing its excellent crystallinity (Figure 3f). To clearly understand why THF solvent leads to such giant changes, it is necessary to reveal the structural change before and after deformation at the molecular scale. Obviously, the molecular packing of the PT2TC shown in Figure 4a exhibits that planar perylene donor and TCNB acceptor are stacked together in a DADAsequence along the c-axis. Similar to 14 the cases of solvated C60 crystals, the PTC could also accommodate solvent molecules with suitable sizes, such as THF, to occupy its voids. The intercalation of THF molecules would force TCNB to stack towards the edge of perylene. Specifically, the π-π overlap between one TCNB molecule and its adjacent perylene molecules is almost 50% of the perylene plane (Figure 4a). The rod-like appearance of the ternary cocrystal fits well with its predicted crystal morphology


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simulated by Material Studio (Figure 4b, Table S1). Typically, the simulation result reveals that PT2TC with a hexagonal cross section are grown along [001] direction, as confirmed by the in-situ environmental SEM results presented in Figure 4c and 4d.

After extensive experiments, we found that analogues of THF, such as 1,4-dioxane and pyridine, can also be used to achieve color change of PTC. While it is proven to be infeasible to realize this aim for most of organic solvents including DCM, cyclohexane, and methylated THF due to their incompatible molecule sizes and structures (Figure S8). As shown in Videos S5 and S6, the binary cocrystal rods would also become red when exposed to 1,4-dioxane solvent, which are accompanied by significant mechanical bending. The corresponding XRD patterns, PL spectrum, fluorescence lifetime, and quantum yield were also examined (Figures S9 and S10), further reveals that 1,4-dioxane plays a similar role to THF in the mechanical bending of PTC. In summary, we explore a luminescent solvated ternary PT2TC, which can transform to solvent-free binary PTC reversibly. Besides the color change, dramatic shape change was also observed upon stimulation with suitable solvents. The present cocrystal strategy provides a promising platform to investigate the mechanical motion of a CT cocrystal. By judicious selecting donors and acceptors, more types of vapor- or solvent-responsive CT cocrystals may be designed, further extending the applications of CT materials.

ASSOCIATED CONTENT Supporting Information Crystallographic data of PT2TC, movies, theoretical calculations and additional results. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Figure 4. (a) Molecular packing of PT2TC along the c-axis. (b) The corresponding predicted crystal morphology based on structural simulation. (c, d) SEM images of PT2TC microrods with a hexagonal cross section. (e) Schematic illustration of mechanical bending mechanism of PT2TC at the molecular scale. The PXRD pattern of PTC transformed from PT2TC is dif15 ferent from that PTC grown from CH2Cl2 reported before (Figure S6). We inferred that after removal of THF molecules, the arrangement of perylene and TCNB in PTC is still similar to their initial arrangement in PT2HC, but utterly different from perylene-TCNB grown from CH2Cl2 (Figure S7). Therefore we optimized the most stable molecular arrangement of PTC by 16 molecular mechanical (MM) method based on PT2TC molecular packing and obtained the geometry binary complex of PTC from our simulated result. Compared to the geometry of ternary complex of PTC (Figures 4a and S4a), the intermolecular distance between TCNB and perylene in PTC is shortened, thus increasing CT interaction and lattice stability. By means of the above structural analysis, we concluded its possible molecular packing to reveal the crystal bending mechanism (Figure 4e). Upon removal of THF molecules, the PTC would retain a straight rod-like appearance due to its rigid crystalline structure. As a result, some voids would appear in PTC and the strong CT interaction would drive TCNB to slightly shift towards the centre of the perylene plane with a torsion angle, clearly revealing a color change from red to greyishgreen. When exposed to THF solvent, the voids in the loose crystal would be again occupied by new THF molecules. The inclusion of THF molecules would force TCNB to recover to its original position, thus driving the interlayer sliding of TCNB towards the [010] direction. Afterward, the rigidity of PT2TH at the molecular scale decreased, further favoring the bending.

Corresponding Author *E-mail: [email protected];

Author Contributions §These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Thank Zhenjie Ni and Yang Li. This work was supported by the Ministry of Science and Technology of China (2016YFB04001100, 2013CB933403, 2013CB933504), the National Natural Science Foundation of China (91222203, 91233205, 91433115, 51303185, 21473222), Chinese Academy of Sciences (XDB12030300).

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Solvatomechanical Bending of Organic Charge Transfer Cocrystal

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