Free-Standing Nanocrystalline Materials Assembled from Small

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Free-Standing Nanocrystalline Materials Assembled from Small Molecules Tamar Wolf, Angelica Niazov-Elkan, Xiaomeng Sui, Haim Weissman, Ilya Bronshtein, Mark Raphael, H. Daniel Wagner, and Boris Rybtchinski J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13638 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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Journal of the American Chemical Society

Free-Standing Nanocrystalline Materials Assembled from Small Molecules Tamar Wolf,†§ Angelica Niazov-Elkan,†§ XiaoMeng Sui,‡ Haim Weissman,† Ilya Bronshtein,† Mark Raphael,† H. Daniel Wagner,‡ and Boris Rybtchinski*† †

Department of Organic Chemistry, and ‡Department of Materials and Interfaces, the Weizmann Institute of Science, Rehovot 76100, Israel

Supporting Information Placeholder Adding an acetone solution of 1 to water (resulting in a 4·10-5 M solution of 1 in acetone:water=1:4, v/v), followed by aging for 8-10 days, results in high-aspect ratio flexible nanocrystalline fibers, as revealed by cryo-TEM and SEM imaging (Figures 1c and S5). The nanofibers were typically 20-130 nm in width and several micrometers in length (Figures 1c and S6). They were stable in solution, with no appreciable precipitation for at least one month. Film fabrication was carried out by filtering the aged acetone/water mixture of 1 over a 0.45 μm polyether sulphone (PES) support in a controlled pressure setup (Figure S7) to afford ONC film (F1a) deposited on top of the PES support. The film was dried, after which it could be detached manually. Typically, freestanding macroscopic films with a diameter of 10 mm (Figure 1d, inset) and a thickness of ~20 m (Figure S8) were obtained following deposition of 40 ml of the aged acetone/water solution of 1. SEM images of F1a revealed a porous network of intertwined nanocrystalline fibers (Figure 1d) resembling electrospun polymer nanofiber films.18 X-ray diffraction (XRD) confirmed that the film was crystalline (Figure 2e). SEM images of F1a revealed extensive ONC contacts and good 3D connectivity. Remarkably, a very small amount of compound 1 (~0.4 mg) was sufficient for successful film delamination following the deposition of aged solutions. The film failed to delaminate when it was prepared from short ONCs formed using a nonaged solution of 1 (Figure S6a). Similarly, depositing nanocrystals of non-uniform dimensions formed in THF:water=1:9 (v/v) solutions (Figure S9) of 1 did not result in the formation of freestanding films. The lack of cohesiveness in these systems is due to poor 3D connectivity. To expand our molecular toolbox and expedite fabrication, we developed a complementary method for cases where long and/or nonuniform ONCs form rapidly and tend to precipitate. Upon preparation of the aqueous crystallization solutions of 1-3 in THF: water=3:7 (v/v), they immediately formed crystals, and we applied sonication in order to induce formation of uniform crystalline fibers19 (Figures S10 and S11). In the case of 1 and 2, the sonication step was followed by the addition of a molecular solution of the PDI derivative in organic solvent (THF) in order to elongate the ONCs, which act as seeds20 (Figures S10a-d and S11). In the case of 3, the sonication resulted in formation of elongated uniform crystals (Figure S10e,f) without monomer

ABSTRACT: We demonstrate a solution-based fabrication of centimeter-size free-standing films assembled from organic nanocrystals based on common organic dyes (perylene diimides, PDIs). These nanostructured films exhibit good mechanical stability, and thermal robustness superior to most plastics, retaining the crystalline microstructure and macroscopic shape even upon heating up to 250-300°C. The films also show nonlinear optical response and can be used as ultrafiltration membranes. The macroscopic functional materials based on small molecules can be alternative or complementary to materials based on macromolecules.

There is growing interest in creating analogs of polymeric materials that are crystalline and based entirely on small molecules.1,2 Such “crystalline molecular plastics” may be prone to facile fabrication and recycling, and can exhibit robustness3,4 as well as advantageous optoelectronic properties.5 However, unlike large polymeric molecules that entangle, creating stable 3D networks in the bulk, molecular crystals normally do not form cohesive materials, owing to the lack of uniformity, and morphologies incompatible with 3D connectivity. To address this challenge, we envisaged that organic nanocrystals (ONCs),6,7 with “polymerlike” uniform morphologies can create macroscopic 3D networks with extensive van der Waals contacts. Herein, we report on freestanding materials based on fibrous or nanosheet ONCs that exhibit exceptional thermal robustness, good mechanical properties, and can be used as ultrafiltration membranes and nonlinear optical materials. Perylene diimides (PDIs) are widely employed as industrial colorants and organic semiconductors,8-11 as well as building blocks for diverse self-assembled nanoarrays,2,12,13 including soluble ONCs,14 and ONC-based hybrid materials.15 Crystalline materials based on PDIs exhibit useful photonic and electronic properties3,8,11,12,16,17 and possess high thermal robustness.8,16,17 To create bulk materials constructed entirely from small molecules, we chose compounds 1-4 (Figures 1b, 4a) since the conditions for their self-assembly can be easily adjusted to form soluble elongated ONCs (1-3), or ONC nanosheets (2D plates, 4). The elongated ONCs were expected to interact as depicted in Figure 1a. The 2D ONCs were anticipated to interact via their extended surfaces.

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addition, owing to high propensity of the crystals to fuse. We employed sonication methodology to fabricate free-standing crystalline films F1b, F2, and F3 (from 1-3, respectively, Figures 1eg). The resulting films are built of ONCs having distinct morphologies (diameters and lengths) for each molecular crystal, giving rise to the diverse morphologies of the films. The sonication method significantly expands the scope of fabrication; we hypothesize that free-standing films can be generated by this method from a wide variety of aromatic ONCs. We analyzed the thermal behavior of the crystalline films by differential scanning calorimetry (DSC), revealing the high thermal robustness of the films. The melting temperatures of F1a and F1b are in the range of 330-333°C, and F2 begins to decompose at ~320°C without melting (Figures S12a-c). In the case of F1a, narrowing of the XRD peaks was observed upon heating up to 250° or 300°C (Figure 2e), indicating increase of the crystal size,

consistent with increased thickness of the fibers (150-1000 nm vs 20-130 nm for the nonheated F1a) as observed by SEM (Figures 2a and S13a). Overall, SEM and XRD studies indicated that upon heating to 250°C, the films retained their crystalline microstructure and macroscopic shapes (Figures 2a-c,e). Films F1b and F2 underwent only slight changes upon heating to 300°C, as revealed by preserving the morphology (Figure S13) and XRD patterns (Figures 2e) upon heating. Such thermal stability is superior to that of most polymeric materials,21 it can be attributed to strong interactions between the aromatic building blocks,4,16 large interaction areas between the ONCs, and extensive 3D connectivity. F3 was the least stable, featuring more substantial morphological and structural changes upon heating (Figures 2d-e, S13d). Thus, XRD diffractograms indicate that F3 undergoes a phase transition to give larger crystals with known structure,22 resulting in alteration of the film morphology (Figure 2d).

Figure 1. (a) Illustration describing film fabrication from fibrous ONCs. (b) Molecular structure of compounds 1-3. (c) Cryo-TEM image of nanocrystals of 1 formed in acetone/water solution (1:4, v/v; 4·10-5 M), after aging at 20-21°C for one month. Inset: Fast Fourier Transform (FFT)-filtered magnification of a marked region showing crystalline fringes. (d) SEM images of a representative dry film, F1a, prepared from a 4·10-5 M acetone/water solution (1:4, v/v) of 1 aged for 10 days. Insets: left – magnified image of 1a; right – free-standing film F1a with a diameter of 10 mm collected with forceps. (e)-(g) SEM images of films prepared using sonication: (e) F1b, (f) F2 , (g) F3. Insets: self-standing films (diameters of 10 mm) collected with forceps. F4 built from stacks of nanosheets (Figure 3b, c). The XRD diffractogram indicates that their crystal structure fits the one reported for 4 (edge-to-face motif).23 In the case of 4, the sonication promotes the development of the 2D crystalline nanosheets from amorphous aggregates and small crystals (Figure S14). DSC and SEM revealed that F4 was thermally stable upon heating up to 230°C (Figure S15). However, upon heating above 250°C the platelets underwent morphological changes and partially fused (Figure S15c, d). XRD analysis indicated that the crystal structure of F4 is preserved upon heating up to 280°C (Figure 3d).

The scope of the ONC free-standing materials was further expanded utilizing 2D crystalline building blocks. Compound 4 (Figure 3a) forms 2D nanosheets with micrometric surface area in THF:water=4:6 (v/v, promoted by sonication, see Supporting Information for details). Formation of 2D crystals in this case is apparently due to a distinct crystalline structure of 4 that features edge-to-face arrangement of molecules23 (rather than face-to-face stacking typical of PDI) because of steric bulk of dimethyl penthyl groups. Deposition of the 2D crystals of 4 on a PVDF support and subsequent delamination yielded free-standing film


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Figure 3. (a). Molecular structure of 4, (b) SEM image, top view of F4; inset: a photograph of F4, (c) SEM image of the crosssection of F4, (d) XRD diffractograms of F4 at room temperature (black trace), after heating to 150°C (red trace), 220°C (blue trace), 280°C (pink trace), and subsequent cooling.

Figure 2. (a-d) SEM images of films 1-3 heated to 250°C under inert conditions and cooled. (a) F1a, (b) F1b, (c) F2, and (d) F3. Insets show the photographs of the free-standing films after heating to 250°C followed by cooling. Except for F3, the SEM images show that the microstructure is maintained up to 250°C. (e) XRD diffractograms of F1-3 at room temperature (black trace), after heating to 250°C (red trace), after heating to 300°C (blue trace), and subsequent cooling. The XRD patterns remained similar, except when F3 was heated to 300°C.

for electrospun nonwoven polymer fiber films.24,25 The variations in mechanical properties of the films appear to result from their morphology. Thus, F1a and F1b exhibit somewhat higher elongation and lower Young’s moduli (Table 1), consistent with the apparently more flexible nature of the constituent ONCs (Figure 1c, d, e). Higher modulus and lower elongation of F3 can be a consequence of larger ONCs that are at times fused, resulting in stiffer network (Figure 1g). We note that yield behavior of F4 differs from that of F2-3, as its stress/strain curves exhibit a short plastic region after a linear elastic one (Figure S18e), which (albeit less pronounced) is also observed for F1a and F1b. In order to rationalize the observed differences, we performed SEM imaging of the films following yield upon tensile test. It revealed that F3 that is constructed from partially fused crystals (Figure 1g) shows sharp rupture boundary, indicative of breaking intermolecular bonds within crystals (Figure S19a, b). F1a and F1b that are constructed from entangled fibrous crystals (Figure 19c-f), showed significant fiber disentanglement upon yield, consistent with their lower Young’s moduli and slightly higher resilience. This was also observed in the case of F2, although to a lesser extent (Figure S19g, h). Yield of F4 apparently involves detachment (sliding) of nanosheets (Figure S19i, j), which may explain the plastic region upon yield. The films can be bent (Figure S20c). Studies aimed at further understanding how the nature of the ONCs, defects, and various additives influence the mechanical properties are currently in progress.

The DSC of polycrystalline materials based on 1-4 (precipitated from solutions) reveal thermal behavior similar to that of films F1-4 (Figure S16). Regarding molecular thermal stability, thermal gravimetric analysis (TGA) revealed that 1 and 3 are stable up to 400°C, 2 is stable up to 320°C, and 4 is stable up to 420°C, in agreement with the known robustness of PDI molecules8 (Figure S17). The mechanical properties of films F1-4 were evaluated by tensile test performed on the bulk films (Table 1, Table S1, and Figure S18). F3 had the highest Young’s modulus (300 MPa) and the lowest elongation (0.6%), whereas films F1a, F1b, and F2 exhibited moduli between 80 and 140 MPa. The Young’s moduli and tensile strengths of the films are comparable to values observed


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ACKNOWLEDGMENT Sample

Young’s Modulus (MPa)

Tensile Strength (MPa)

Elongation (%)

Toughness (MPa)

F1a

80±30

1.2±0.2

2.2±0.5

1.6±0.3

F1b

90±20

2.0±1.0

2.3±0.6

3.0±2.0

F2

140±50

1.3±0.5

0.9±0.1

0.7±0.4

F3

300±90

1.8±0.5

0.6±0.05

0.5±0.2

F4

128±18

1.1±0.1

1.2±0.2

0.9±0.3

This work was supported by grants from the Israel Science Foundation, the Minerva Foundation, and the Kimmel Center for Molecular Design. It was also made possible by the generosity of the Harold Perlman family. The EM studies were conducted at the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging (Weizmann Institute). We thank V. Kalchenko for assisting with SHG microscopy experiments, and I. Kaplan-Ashiri for low voltage SEM imaging. A.N.E. is supported by the Adams Fellowship Program, and H.D.W. is the recipient of the Livio Norzi Professorial Chair in Materials Science.

REFERENCES

Table 1. Tensile test results.

(1) Aida, T.; Meijer, E. W.; Stupp, S. I. Science 2012, 335, 813-8177. (2) Sun, H. J.; Zhang, S. D.; Percec, V. Chem. Soc. Rev. 2015, 44, 3900-3923. (3) Watson, M. D.; Fechtenkotter, A.; Müllen, K. Chem. Rev. 2001, 101, 1267-1300. (4) Grimme, S. Angew. Chem. Int. Ed. 2008, 47, 3430-3434. (5) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals and Polymers; Oxford University Press: Oxford, 1999. (6) Fery-Forgues, S. Nanoscale 2013, 5, 8428-8442. (7) Baba, K.; Kasai, H.; Nishida, K.; Nakanishi, H. In Nanocrystal; Masuda, Y., Ed.; InTech: Rijeka, Croatia, 2011. (8) (a) Würthner, F. Chem. Commun. 2004, 1564-1579. (b) Herrmann, A.; Müllen, K., Chem. Lett. 2006, 35, 978-985. (9) Zhan, X. W.; Facchetti, A.; Barlow, S.; Marks, T. J.; Ratner, M. A.; Wasielewski, M. R.; Marder, S. R. Adv Mater 2011, 23, 268-284. (10) Weil, T.; Vosch, T.; Hofkens, J.; Peneva, K.; Müllen, K. Angew. Chem. Int. Ed. 2010, 49, 9068-9093. (11) Yasukuni, R.; Sliwa, M.; Hofkens, J.; De Schryver, F., C; Herrmann, A.; Müllen, K.; Asahi, T. Jpn. J. Appl. Phys. 2009, 48, 065002. (12) Würthner, F.; Saha-Moller, C. R.; Fimmel, B.; Ogi, S.; Leowanawat, P.; Schmidt, D. Chem. Rev. 2016, 116, 962-1052. (13) Chen, S.; Slattum, P.; Wang, C. Y.; Zang, L. Chem. Rev .2015, 115, 11967-11998. (14) Rosenne, S.; Grinvald, E.; Shirman, E.; Neeman, L.; Dutta, S.; BarElli, O.; Ben-Zvi, R.; Oksenberg, E.; Milko, P.; Kalchenko, V.; Weissman, H.; Oron, D.; Rybtchinski, B. Nano Lett. 2015, 15, 7232-7237. (15) Niazov-Elkan, A.; Weissman, H.; Dutta, S.; Cohen, S. R.; Iron, M. A.; Pinkas, I.; Bendikov, T.; Rybtchinski, B. Adv. Mater. 2018, 30, 1705027. (16) Huang, C.; Barlow, S.; Marder, S. R. J. Org. Chem. 2011, 76, 23862407. (17) Chen, S.; Slattum, P.; Wang, C.; Zang, L. Chem. Rev. 2015, 115, 11967-11998. (18) Frenot, A.; Chronakis, I. S. Curr. Opin. Colloid. In. 2003, 8, 64-75. (19) Guo, Z.; Zhang, M.; Li, H.; Wang, J.; Kougoulos, E. J. Cryst. Growth 2005, 273, 555-563. (20) Carnall, J. M. A.; Waudby, C. A.; Belenguer, A. M.; Stuart, M. C. A.; Peyralans, J. J.-P.; Otto, S. Science 2010, 327, 1502-1506. (21) Hiemenz, P. C.; Lodge, T.P. Polymer Chemistry; 2nd ed.; CRC Press: Boka Raton, Florida, 2007. (22) Klebe, G.; Graser, F.; Hadicke, E.; Berndt, J. Acta Crystallogr. B 1989, 45, 69-77. (23) Langhals, H.; Krotz, O.; Polborn, K.; Mayer, P. Angew. Chem. Int. Ed. 2005, 44, 2427-2428. (24) Carrizales, C.; Pelfrey, S.; Rincon, R.; Eubanks, T. M.; Kuang, A. X.; McClure, M. J.; Bowlin, G. L.; Macossay, J. Polym. Advan. Technol. 2008, 19, 124-130. (25) Veleirinho, B.; Rei, M. F.; Lopes-da-Silva, J. A. J. Polym. Sci. Pol. Phys. 2008, 46, 460-471. (26) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. Rev. 1994, 94, 31-75. (27) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537-2574.

The ONC films also exhibited advantageous photonic properties. Thus, F2 showed a strong nonlinear optical (NLO) response (second harmonic generation, SHG; Figure S21d) owing to the NLO activity of ONCs based on 2.14 Long-term thermal stability of macroscopic organic NLO materials presents a challenge.26 Since F2 is thermally stable, it displays unchanged NLO response over time and after heating (Figure S21), advancing a concept of thermally robust ONC-based bulk NLO materials. Films F1-4 are emissive (Figure S22), which may be useful for sensing based on fluorescence quenching of the films by various analytes.27 ONCs impose nanoporous film structure (F1-3) that can be utilized for ultrafiltration. Thus, F2 exhibited a uniform porosity, enabling its application as a membrane for size-selective filtration of gold and silica nanoparticles in aqueous medium, demonstrating 60-nm cutoff (Figures S23 and S24). F2 can be easily fabricated and, when necessary, disassembled (dissolved with organic solvents), cleaned, and reassembled, which can be used to manage membrane fouling and recycling. In summary, we have presented a solution-based method of fabricating free-standing crystalline films constructed from ONCs. The films are assembled entirely from small molecular building blocks (readily available organic dyes) and exhibit high thermal stability, advantageous optical properties, and sizeselective filtration ability. They represent a new class of macroscopic functional nanomaterials, whose molecular nature, porosity, and crystallinity may enable a wide range of applications.

ASSOCIATED CONTENT Supporting Information Synthesis and characterization of compounds 1-4, electron microscopy images, DSC and TGA thermograms, tensile test results, SHG and Fluorescence microscopy images. This material is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *[email protected]

ORCID Boris Rybtchinski: 0000-0002-2071-8429 Author Contributions §

These authors contributed equally Notes The authors declare no competing financial interests.


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