Coassembly-Directed Fabrication of an Exfoliated Form of Alternating

Aug 6, 2015 - International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki...
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Coassembly-Directed Fabrication of an Exfoliated Form of Alternating Multilayers Composed of a Self-assembled Organoplatinum(II) Complex−Fullerene Dyad Satoru Sato,† Toshiaki Takei,† Yoshitaka Matsushita,‡ Takeshi Yasuda,‡ Tatsuhiro Kojima,§ Masaki Kawano,§ Masato Ohnuma,∥ and Kentaro Tashiro*,† †

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan § Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), Korea ∥ Laboratory of Quantam Beam System Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan S Supporting Information *

ABSTRACT: The self-assembly of covalently linked dyad 1a of cyclometalated organoplatinum(II) complex and fullerene afforded alternating multilayers of electron-rich and -deficient molecular components. On the other hand, the coassembly of 1a with organoplatinum(II) complex 2 having no fullerene moiety gave an exfoliated form of the multilayers, by inhibiting the interdigitation of organoplatinum(II) complex moieties of 1a. The coassembled 1a/2 transports both of the photogenerated holes and electrons, while the self-assembled 1a allows only the transportation of electrons under the same conditions. Figure 1. Molecular structures of (a) 1 and (b) 2, coupled with perspective views (50% thermal ellipsoids) of (a) 1b and (b) 2. Hydrogen atoms are omitted for clarity.

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he fabrication of an exfoliated form of a multilayered structure with a thickness of some nanometers keeps attracting much attention of researchers in diverse fields, particularly because the exfoliated form often exhibits different and even more interesting properties than the corresponding multilayered form.1−3 As a representative example, mechanical peel-off of graphene from graphite provided a 2D material with a remarkable charge-carrier mobility,1 while cation exchange of double-layered hydroxides allows the preparation of inorganic nanosheets that are fabricatable into ferroelectric materials having an ultrahigh κ value.2 In contrast to a large number of related works with these and other materials of robust in-plane networks,3 as well as many reports on the fabrication of 2D organic thin films,4 trials of the straightforward exfoliation on the layers of self-assembled molecules, which are generally associated with each other via less strong intralayer interactions, have hardly been successful because of the accompanied destruction of each fragile molecular layer in the course of exfoliation.5 One of the alternative approaches to addressing this issue could be changing the direction of the assembling process to selectively afford an equivalent structure of the exfoliated form by means of coassembly. In spite of the large potential of this approach, however, almost no exploration has been made so far. Here we report that a covalently linked dyad of the cyclometalated organoplatinum(II) complex and fullerene (1a; Figure 1a) self-assembled to afford alternating multilayers of its © XXXX American Chemical Society

structural components, while its coassembly with a reference organoplatinum(II) complex having no fullerene moiety (2;6 Figure 1b) allowed one to give an exfoliated the multilayered structure. The coassembled 1a and 2 exhibited the signs of hole and electron transportations under photoirradiation. In contrast, the self-assembled form of 1a showed only electron-transporting behaviors under the same conditions. Square-planar platinum(II) complexes are one of the representative classes of compounds that exhibit metal−metal interactions. In 2009, Chen and co-workers reported that a derivative of 2 bearing a CF3 substituent at the periphery of its pincer ligand crystallized into photoresponsive p-type semiconductor sheets, where the complex stacked each other to make 1D columns containing an infinite linear chain of interacting platinum(II) centers.6 With such intriguing information in mind, we newly designed dyads 1 (Figure 1a) by expecting that the intrinsically homoselective PtII−PtII interaction induces segregation of the organoplatinum(II) complex and fullerene moieties of 1 in their self-assemblies to give p/n molecular heterojunctions.7 Dyad 1a was synthesized by NaOH-mediated coupling of the corresponding organoplatinum(II) complex Received: May 29, 2015

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DOI: 10.1021/acs.inorgchem.5b01183 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry having a coordinated chlorine anion, with a fullerene8 bearing an alkyne unit, and unambiguously characterized by 1H NMR, IR, and mass spectrometry analyses.9 Similarly, dyad 1b was also obtained and further analyzed by X-ray crystallography. The perspective view of 1b proves that its organoplatinum(II) complex moiety has PtII−C(sp), C(sp)−C(sp), and C(sp)− C(sp3) bond lengths quite similar to those of 2 (Figure 1).9 The absorption spectrum of 1a or 1b in their good solvents such as CH2Cl2 is basically composed of the summation of the absorptions of the corresponding two starting materials (Figure S3), showing the absence of detectable intramolecular groundstate electronic interaction between the organoplatinum(II) complex and fullerene moieties. On the other hand, emissions typical for the organoplatinum(II) complexes were not observed (Figure S6), indicating their quenching due to the presence of fullerene in the structure. Slow diffusion of hexane or cyclohexane into a toluene solution of 1a for 3 days afforded brown precipitates (Figure S7b). Scanning electron microscopy (SEM) on those precipitates revealed that they are aggregates of 2D sheets with a lateral size of several micrometers (Figures 2a and S9). Sonication of a

structure of self-assembled 1a, where the bilayer of the fullerene moiety and the monolayer of the interdigitated organoplatinum(II) complex moiety alternately stack to form their multilayer with a repeating d spacing of 2.9 nm. On the other hand, 1b, as revealed by X-ray crystallography, self-assembles into a different packing structure, where fullerene−fullerene and pincer−pincer contacts are less efficient (Figure S14). The multilayered structure of self-assembled 1a inspired us to challenge the fabrication of its exfoliated form by inhibiting the interdigitation of its organoplatinum(II) complex moieties through coassembly with 2. Although diffusion of hexane into a toluene solution of 1a caused its precipitation, a stoichiometric mixture of 1a and 2 under the same conditions apparently kept its homogeneity even after standing for 3 days (Figure S8b). On the other hand, the absorption spectrum of the resultant solution exhibited a clear increase in absorbance at 450−800 nm, suggesting that the clustering of fullerenes took place (Figure S4). A Job’s plot of the change in absorbance at 600 nm allowed us to optimize the molar ratio of [2]/[1a] to 3:2 (Figure S5). TEM observations on the drop-cast solution demonstrated the presence of 2D sheets with a lateral size of submicrometers (Figure 3a), which, however, were not tolerant under the

Figure 2. (a) SEM, (b) AFM, (c) TEM, (d) SAED, (e) HR-TEM, and (f) SWAXS data of 1a self-assembled in toluene/cyclohexane (1:3, v/v). (g) Schematic representations of a 3D packing diagram for selfassembled 1a. Carbon atoms of its organoplatinum(II) complex and fullerene moieties are colored in blue and orange, respectively.

Figure 3. (a) TEM and (b) AFM images of 1a and 2 coassembled in toluene/hexane (1:3, v/v). (c) SWAXS pattern experimentally obtained from coassembled 1a and 2 (top) and that simulated from the singlecrystal structure of 2 (bottom). (d) Schematic representations of a 3D packing diagram for coassembled 1a and 2.

cyclohexane suspension of these aggregates gave individualized sheets (Figures S10 and S11), whose thicknesses were distributed mostly in the range of 10−200 nm, as evaluated by atomic force microscopy (AFM; Figure 2b). Transmission electron microscopy (TEM) on the individualized sample visualized a thin semitransparent sheet (Figure 2c), whose selected-area electron diffraction (SAED) pattern demonstrated that it is crystalline with periodicities of d = 1.00, 0.79, and 0.43 nm (Figure 2d). High-resolution TEM (HR-TEM) observation on the same sheet exhibited regularly oriented black dots that are assignable to two dimensionally packed fullerene moieties of 1a (Figure 2e). Small- and wide-angle X-ray scattering (SWAXS) measurement on the as-prepared precipitates of 1a displayed multiple diffraction peaks (Figure 2f), all of which were indexed to a 3D monoclinic lattice with parameters a, b, c, and β of 18.0 Å, 28.8 Å, 10.1 Å, and 99.7°, respectively, by using DICVOL04 (Table S1). Figure 2g shows a 3D packing diagram of 1a that satisfies all of the experimentally collected information on the

conditions for HR-TEM as well as SEM. The thickness of a single sheet, as evaluated to be 4.1 nm by AFM (Figures 3b and S13), is within the expected range for the exfoliated form and distinctively thinner than those of self-assembled 1a (Figure 2b). SWAXS measurement on the coassembled 1a and 2, as obtained by evaporation of the solvents followed by washing with hexane, displayed diffractions at d = 1.00, 0.89, 0.78 0.50, 0.47, and 0.43 nm (Figure 3c), which were successfully indexed to a 2D oblique lattice with parameters a, b, and β of 20.1 Å, 10.6 Å, and 96.8°, respectively (Table S2). No diffraction peak assignable to the single crystal of 2 was observed (Figure 3c), excluding the possible contamination of the coassembly with macroscopically phase-separated 2. Figure 3d shows a plausible 3D packing diagram of 1a and 2 in their coassembly, where a bilayer of the fullerene moiety of 1a is sandwiched by monolayers composed of 2 and the organoplatinum(II) complex moiety of 1a. CPK model B

DOI: 10.1021/acs.inorgchem.5b01183 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

complex−fullerene dyad 1a. Evaluation on the charge-transporting property of the exfoliated form (coassembled 1a and 2), together with those of the corresponding two self-assemblies (self-assembled 1a and 2), revealed the positive meaning of the coassembly for the transportation of photogenerated holes.

studies suggested that the observed 3:2 molar ratio for 2/1a is suitable to construct a layer of densely packed organoplatinum(II) complexes (Figure 3d, top view). The charge-transporting properties of self-assembled 1a and its coassembly with 2 were compared by means of a bottomcontact field-effect transistor (FET) configuration.9 A toluene/ hexane solution (1:3, v/v) containing coassembled 1a and 2 was drop-cast onto 50-μm-gap source and drain electrodes. At dark, the coassembled sample exhibited electron-transporting behaviors, as the source−drain current (Isd) increased with elevation of the gate voltage (Vg) at the positive source−drain voltage (Vsd) side in the output characteristics (Figure S16a).10 On the other hand, no sign of the hole-transporting behavior was observed at the negative Vsd region. Photoirradiation on the coassembled sample with halogen light (λ = 400−900 nm) at Vg = 0 V enhanced Isd with a maximum on/off ratio over 10 (Figure S18a). The output characteristics of the photoirradiated coassembly at the positive Vsd side with variable positive Vg proved that the electron-transporting nature of the sample is preserved under light (Figure 4a). Of interest, Isd at the negative Vsd side in the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01183. Synthesis of dyads 1 and characterization of their assemblies (PDF) Crystallographic details [CCDC 1041759 (1b) and 1048124 (2)] (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-29-8604879. Fax: +81-29-860-4706. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The synchrotron radiation experiments were performed at NW2A of PF-AR, KEK, with its approval (Proposal 2014G048). S.S. thanks JSPS for a Research Fellowship for Young Scientists.

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REFERENCES

(1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666. (2) Sasaki, T.; Watanabe, M.; Hashizume, H.; Yamada, H.; Nakazawa, H. J. Am. Chem. Soc. 1996, 118, 8329. (3) (a) Geng, F.; Ma, R.; Nakamura, A.; Akatsuka, K.; Ebina, Y.; Yamauchi, Y.; Miyamoto, N.; Tateyama, Y.; Sasaki, T. Nat. Commun. 2013, 4, 1632. (b) Xu, M.; Liang, T.; Shi, M.; Chen, H. Chem. Rev. 2013, 113, 3766. (c) Junggeburth, S. C.; Diehl, L.; Werner, S.; Duppel, V.; Sigle, W.; Lotsch, B. V. J. Am. Chem. Soc. 2013, 135, 6157. (4) Baek, K.; Hwang, I.; Roy, I.; Shetty, D.; Kim, K. Acc. Chem. Res. 10.1021/acs.accounts.5b00067. (5) Transformation of multilayered lamellar structures into bilayered vesicles by changing the ratio of two surfactants has been known. For example, see: Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (6) Chen, Y.; Li, K.; Lu, W.; Chui, S. S.-Y.; Ma, C.-W.; Che, C.-M. Angew. Chem., Int. Ed. 2009, 48, 9909. (7) (a) Würthner, F.; Chen, Z.; Hoeben, F. J. M.; Osswald, P.; You, C.C.; Jonkheijm, P.; van Herrikhuyzen, J.; Schenning, A. P. H. J.; van der Schoot, P. P. A. M.; Meijer, E. W.; Beckers, E. H. A.; Meskers, S. C. J.; Janssen, R. A. J. J. Am. Chem. Soc. 2004, 126, 10611. (b) Yamamoto, Y.; Fukushima, T.; Suna, Y.; Ishii, N.; Saeki, A.; Seki, S.; Tagawa, S.; Taniguchi, M.; Kawai, T.; Aida, T. Science 2006, 314, 1761. (c) Sisson, A. L.; Sakai, N.; Banerji, N.; Fürstenberg, A.; Vauthey, E.; Matile, S. Angew. Chem., Int. Ed. 2008, 47, 3727. (8) Hizume, Y.; Tashiro, K.; Charvet, R.; Yamamoto, Y.; Saeki, A.; Seki, S.; Aida, T. J. Am. Chem. Soc. 2010, 132, 6628. (9) See the Supporting Information. (10) From the transfer characteristics, the field-effect electron mobility was calculated to be 6.8 × 10−7 cm2 V−1 s−1 (Figure S17). (11) The output characteristics at the negative Vsd side under photoirradiation displayed hysteresis, indicating the temporal trapping of photogenerated holes in the sample.

Figure 4. Output characteristics of (a) 1a coassembled with 2 and (b) self-assembled 1a upon photoirradiation (λ = 400−900 nm).

same output characteristics was more largely amplified by the application of more negative Vg, demonstrating that the coassembled sample can also transport photogenerated holes.11 Namely, 1a coassembled with 2 displayed an ambipolar chargetransporting character when charge carriers were photogenerated. On the other hand, while the responses of selfassembled 1a to the applications of Vg at dark (Figure S16b) or photoirradiation at Vg = 0 V (Figure S18b) were qualitatively similar to those of the coassembled sample (Figures S16a and S18a, respectively), the former assembly behaved differently from the latter under negative Vsd and Vg upon exposure to light, where Isd through the self-assembly became less by the application of more negative Vg (Figure 4b). These observations allow one to conclude that self-assembled 1a, in contrast to its coassembly with 2, transports only electrons but not holes even under photoirradiation. It should also be noted that 2 alone, treated under the same conditions for the coassembly, did not exhibit any detectable FET responses irrespective of photoirradiation (Figure S19). X-ray crystallography on a single crystal of 2, as obtained from the same conditions, revealed that 2 packs into not a 1D columnar but a dimeric structure (Figure S15). Therefore, the coassembly of 1a and 2 is positive not only for 1a but also for 2 because it provides the opportunity for them to be packed in a proper way for hole transportation, which cannot be achieved by their individual self-assemblies. In conclusion, by means of a coassembly-directed approach, we successfully fabricated an exfoliated form of the alternating multilayers of electron-rich and -deficient molecular components, as obtained by the self-assembly of organoplatinum(II) C

DOI: 10.1021/acs.inorgchem.5b01183 Inorg. Chem. XXXX, XXX, XXX−XXX