Rational Design and Construction of Hierarchical Superstructures

This work may also provide a new insight into the construction of metal–organic .... The unusual sorption behavior of PSS-1' may be due to the long ...
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Rational Design and Construction of Hierarchical Superstructures Using Shape-persistent Organic Cages: Porphyrin Box-based Metallosupramolecular Assemblies Younghoon Kim, Jaehyoung Koo, In-Chul Hwang, Rahul Dev Mukhopadhyay, Soonsang Hong, Jejoong Yoo, Ajaz Ahmad Dar, Ikjin Kim, Dohyun Moon, Tae Joo Shin, Young Ho Ko, and Kimoon Kim J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 1, 2018

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Journal of the American Chemical Society

Rational Design and Construction of Hierarchical Superstructures Using Shape-persistent Organic Cages: Porphyrin Box-based Metallosupramolecular Assemblies Younghoon Kim,†‡ Jaehyoung Koo,†‡ In-Chul Hwang,† Rahul Dev Mukhopadhyay,† Soonsang Hong,† Jejoong Yoo,† Ajaz Ahmad Dar,† Ikjin Kim,†§ Dohyun Moon,¶ Tae Joo Shin,# Young Ho Ko,† and Kimoon Kim*†‡§ †

Center for Self-assembly and Complexity, Institute for Basic Science, Pohang 37673, Republic of Korea Department of Chemistry and §Division of Advanced Materials Science, Pohang University of Science and Technology, Pohang 37673, Republic of Korea ¶ Supramolecule Crystallography, Pohang Light Source II, Pohang 37673, Republic of Korea # UNIST Central Research Facilities & School of Natural Science, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea ‡

Supporting Information Placeholder ABSTRACT: We report a new approach to building hierarchical superstructures using a shape-persistent porous organic cage, which acts as a pre-made secondary building unit, and coordination chemistry. To illustrate the principle, zinc-metallated porphyrin box (Zn-PB), a corner-truncated cubic porous cage, was connected by suitable dipyridyl terminated bridging ligands to construct PB-based hierarchical superstructures (PSSs). The PSSs were stabilized not only by the coordination bonds between Zn ions and bipyridyl-terminated ligands but also by π-π interactions between the corners of the Zn-PB units. By varying the length of the linker, we identified an optimum range of the linker length for construction of PSSs. The PSSs have large void volumes and extrinsic surface area compared to the parent PBs, which can be exploited for the selective encapsulation and interior functionalization of the PSSs for various applications, including catalysis. We observed that singlet oxygen induced synthesis of the natural product, juglone, is more efficiently catalyzed by PSS1 than its constituent component Zn-PB.

Spontaneously self-assembled hierarchical superstructures generated from precisely designed subunits have been extensively investigated not only to understand the self-assembly of complex architectures found in natural system (such as virus capsids),1 but also to utilize them as useful materials for many applications such as storage, catalysis, sensor, delivery, and photonics.2 Current hierarchical superstructures have mostly been constructed using spherical building units or polyhedral inorganic nanoparticles.1,2 The key requirements for the desired hierarchical superstructures are narrow-sized distribution and shape homogeneity of such building blocks. Moreover, the non-covalent interactions between those building blocks should be directional so that the superstructures can be constructed with highly ordered arrangements.3 However, it is still challenging to precisely control the size and shape of such building blocks and functionalize the surface of materials for specific binding. In order to tailor-make superstructures with desired dimensions and properties, a rational design strategy and a synthetically tractable secondary building unit (SBU) with a well-defined size are required.

Shape-persistent porous organic cages, which can act as a premade SBU, have attracted relatively little attention in the field of hierarchical superstructures despite many advantages such as atomically-precise nanometer-sized structure, accessible internal void, shape persistence, good processability, and ease-offunctionalization.4 Furthermore, by virtue of their polyhedral shape, they can also act as organic polyhedral building blocks that impart directional information to dictate the self-assembly of highly ordered hierarchical superstructures. However, there are only a few examples of porous organic cages being used to generate hierarchical superstructures. For instance, Cooper and co-workers reported a three-dimensional (3D) metal–organic network composed of an in situ generated zinc metal cluster acting as SBU at a node and tetrahedral porous organic cage as a bridging ligand.5 More recently, Pan and co-workers utilized a triangular prism-shaped porous organic cage and metal ion (Na+) to construct a cage network.6 Despite the success, the construction of self-assembled hierarchical superstructures using atomically precise, hollow organic polyhedral building blocks is still at an early stage and new, rational strategies are required to obtain hierarchical self-assemblies in a predictable fashion.

Scheme 1. Construction of a hierarchical superstructure utilizing porous organic cage, porphyrin box (PB) and coordination chemistry. Recently, we reported rationally designed shape-persistent porous organic cages, the porphyrin boxes (PBs), which consist of six square-shaped porphyrin units and eight triangular units.7,8 PBs have a truncated cube structure (rhombicuboctahedron) with a large outer size of 3.65 nm in diameter. The truncated cubic

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geometry and six porphyrin faces of PBs prompted us to explore metallated PBs as pre-made SBUs to build hierarchical superstructures connected by suitable bridging ligands.

Figure 1. (a) The construction of PB-based superstructure PSS-1 utilizing Zn-PB (Zn: cyan, C: grey, N: blue, alkoxy group: red), (b) X-ray crystal structure of PSS-1, shown as a wireframe, with 2-fold interpenetration (red and blue networks). The severely disordered octyloxy chains and hydrogen atoms are omitted for clarity. Herein, we present a rational approach to the design and construction of hierarchical superstructures made of a porous organic cage and suitable organic bridging ligands (Scheme 1). Namely, we utilized the aforementioned metallated PB, as a premade, shape persistent SBU, and rigid bipyridyl-terminated ligands to construct the porphyrin box-based hierarchical superstructures PSSs. The PSSs were formed through coordination bonds between Zn ions and bipyridyl-terminated ligands as well as π-π interactions between Zn-PB units. The resulting PSS can be modulated by adjusting the length of the bridging ligands or by adding functional groups to the ligands. The PSSs have protein-like large unit cells (≈40 Å), spacious interiors (void volume up to 62%), and low density (as low as 0.516 g/cm3 (calculated density for solvent-free structures)). This work may also provide a new avenue into the construction of metal–organic frameworks with hierarchical structures.9 Simple mixing of Zn-PB with octyloxy chains10 in chloroform and a ditopic bridging ligand (1,4-di(4-pyridyl)benzene, L4; 11.5 Å, 3 equiv.) in tetrahydrofuran under ambient conditions (Figure

1a), followed by letting the solution sit on bench for 1 day produced violet, uniform sized (14.4 ± 2.1 µm, Figure S1) rhombic dodecahedron-shaped crystals of PSS-1.11 PSS-1 has been fully characterized by single-crystal X-ray diffraction analysis (SC-XRD), elemental analysis (EA), thermogravimetric analysis (TGA) (see details in SI). The X-ray crystal structure of PSS-1 which crystallizes in the space group Pn-3n (a = 39.258(5) Å), revealed a simple cubic net consisting of Zn-PB and L4 with a molecular formula [(Zn6C552H624N48O24)(C16H12N2)3]. More specifically, the nodes of the 3D net are occupied by Zn-PB as an SBU while the edges are occupied by L4 as a bridging ligand. The pyridyl moiety (Py) of L4 is coordinated to the Zn metal center of Zn-PB (Zn-N = 2.081(5) Å). There are two identical, mutually interpenetrating networks (Figure 1b) and the whole framework is stabilized not only by the coordination bonds between Zn and Py but also by π-π interactions between the benzene rings at the corner of each Zn-PB with eight neighboring Zn-PBs. Despite 2-fold interpenetration of the network, PSS-1 possesses an extensive void volume. There are two different types of void: the intrinsic void of the Zn-PB unit and extrinsic void from the network. Both voids are occupied by highly disordered solvent molecules. The intrinsic void volume is almost the same as that of free Zn-PB. A close inspection of the extrinsic void revealed interconnected 1D channels (10 Å x 10 Å) along the all a, b, and c axes. Because of the hollow nature, PSS-1 has extremely low density (calculated density for solvent-free structures = 0.516 g/cm3) compared to the parent organic cage PB-1 with butyloxy chains (0.733 g/cm3).7a Having successfully constructed the hierarchical superstructure (PSS-1), we tested the generality of the strategy by using different ditopic ligands. We first utilized a similar bridging ligand with dimethoxy group, (4,4'-(2,5-dimethoxy-1,4-phenylene)dipyridine, L5; 11.5 Å). As expected, the functionalized bridging ligand (L5) produced isomorphic crystalline material PSS-2, as characterized by SC-XRD analysis: space group Pn-3n (a = 39.074(5) Å). PSS2 has essentially the same superstructure with disordered dimethoxy groups on L5. The successful construction of PSS-2 with L5 suggests that we can readily introduce functional moieties into the extrinsic void of the PSS, which may be useful for various applications. Next, we explored how the length of the bridging ligands affected the superstructure. We prepared a series of ditopic bridging ligands with different lengths ranging from 2.7 Å to 18.2 Å. Among them, 1,2-di(4-pyridyl)ethylene (L3; 9.3 Å) produced PSS-3, which is also isomorphic to PSS-1 with a slightly smaller unit cell (a = 38.945(5) Å) (Figure 2a). Interestingly, despite switching the bridging ligand from L4 (11.5 Å) to shorter L3 (9.4 Å), the node to node distance of PSS-3 decreases only by ~0.3 Å (from 39.258(5) Å to 38.945(5) Å). It appears that the Zn-PB expands itself to compensate the short bridging ligand L3. Hence, the distance between the opposite faces of the Zn-PB unit increased (from 23.026(8) Å to 24.661(9) Å) in each direction, illustrating the structural flexibility of Zn-PB (Figure S3a). We anticipated that the longer ligand 2,6-di(pyridine-4yl)naphthalene (L6; 13.6 Å) would also produce a similar 3D net with Zn-PB. Surprisingly, however, instead of a 3D cubic net, the resulting PSS, (PSS-4) has a two-dimensional layer structure (AB stacked) as shown in Figure 2b. The Zn-PB units in PSS-4 were deformed into cuboid-shaped building units (Figure S3b). In the case of other bridging ligands (L1, L2, L7, and L8), no formation of a PSS was observed, probably because the length of these ligands are too short or too long for such 3D superstructures with Zn-PB. To obtain molecular insights into the stability of the superstructures, we performed molecular dynamics (MD) simulations (see Methods for simulation details). Firstly, the simulations of PSS-1 with and without interpenetration clearly

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Journal of the American Chemical Society showed that the interpenetration is essential for the structural stability (compare Supporting Movies 1 and 2). Secondly, the π-π stacking interactions between neighboring porphyrin box units play a significant role in stabilizing the superstructure (Figure S4d and e). Finally, the simulations also revealed that PBs in PSS are elastically deformable over a range of a few angstroms, consistent with the experimental observations (Figure S5).

Figure 2. Crystal structures of (a) 3D PSS-3, and (b) 2D PSS-4, shown as a wireframe, obtained using L3, and L6, respectively. The severely disordered octyloxy chains and hydrogen atoms are omitted for clarity. (c) Dipyridyl organic bridging ligands used in the study. The bridging ligands are arranged by length; the optimal range for construction of PSSs is marked in red. These results illustrate that there is an optimal range of the length of the bridging ligands to achieve network structures (Figure 2c). This range corresponds to that where π-π interactions between neighboring Zn-PB units supplement the coordination bonds between Zn and Py to stabilize the structure. We next sought to measure the porosity of the formed superstructures. Attempts to measure the porosity of PSSs using N2 sorption isotherms at 77 K were hampered by structural collapse upon removal of solvent molecules. Interestingly, however, the analogous frameworks PSS's constructed with ZnPB' containing hexyloxy chains10 were more robust in the activation process, which allowed us to study N2 sorption isotherms at 77 K.12 Note that the overall framework structures of PSS's were the same as those of PSSs as confirmed by PXRD (Figure S6). Most importantly, PSS's showed much higher porosity (Langmuir surface area: 1060 m2/g) than the parent PB' (Langmuir surface area: 554 m2/g) (Figure S7) presumably due to the extrinsic porosity created upon formation of the hierarchical superstructures.13,14 The increased surface areas suggested that PSSs may possess enhanced catalytic performance compared to PBs. The unique photochemistry associated with the triplet excited state of porphyrins has been widely exploited for applications ranging from light-harvesting,15a photon upconversion,15b singlet oxygen generation,15c,d etc. Specifically, the singlet oxygen produced by porphyrin based supramolecular assemblies can be utilized for catalyzing different reactions and a better catalytic performance is often achieved by increasing the substrate accessible surface area.

The evolution of 1O2 mediated by PSS-1 was detected using 1,3-diphenylisobenzofuran (DPBF), where a decrease in the DPBF absorbance at 411 nm was observed only upon irradiation with visible-light (λ > 420 nm) (Figure 3a, S9a). Further the 1O2 produced was utilized for the photooxidation of 1,5dihydroxynaphthalene (DHN). The formation of the oxidized natural product, juglone, can be confirmed from the increase of its absorbance at 420 nm and a decrease of the structured absorbance (λ < 350 nm) of the starting material DHN (Figure 3b). The catalytic conversion was much faster with PSS-1 as compared to Zn-PB (Figure 3c). Note that catalytic conversion of DHN to juglone was observed only upon excitation with a visible light source (Figure 3d). PSS-1 can also be recycled for at least 5 cycles without significant loss of catalytic performance (Figure S11).

Figure 3. Time-dependant UV/Vis absorption spectra of (a) DPBF and (b) DHN in the presence of PSS-1, oxygen and visiblelight irradiation (λ > 420 nm). (c) Comparative catalytic performance of PSS-1 (black) and Zn-PB (red) for 1O2 induced oxidation of DHN and (d) photo switchable (on-off) catalytic cycles utilizing PSS-1. In summary, we report a rational strategy to construct hierarchical superstructures using shape-persistent porous organic cages and coordination chemistry. Zn-PB, which serves a premade SBU, is linked by ditopic bridging ligands to construct PSSs. The network structure of PSSs can be modulated by the length of the bridging ligands. The stability of PSSs is affected not only by the coordination bonds, but also by π-π interactions between the benzene rings on neighboring SBUs. This work may also provide a new insight into the construction of metal–organic frameworks with hierarchical structures. The PSSs have many benefits including large void volume and ease-offunctionalization. For example, the intrinsic and extrinsic voids offer great potential for selective guest encapsulation and interior functionalization, which may be useful for various applications including catalysis. As a proof-of-concept, we have demonstrated better 1O2 induced catalytic oxidation of DHN to juglone by PSS1 as compared to Zn-PB, which validates the rationale behind designing a suitable superstructure from PBs. This work may pave a new direction in construction of hierarchical superstructures and kindle new ideas in supramolecular catalysis, light-havesting donor-acceptor arrays, photodynamic therapy, etc.

ASSOCIATED CONTENT Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthetic and experimental details, SC-XRD, PXRD studies, MD simulation, gas sorption, and photocatalysis results (PDF) Crystallographic data (CIF) Supplementary movies on PSS-1 structure with noninterpenetration and 2-fold interpenetration (AVI)

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank Drs. M. R. Rohman and J. Murray for helpful discussions. This work was supported by the Institute for Basic Science (IBS) [IBS-R007-D1] and [IBS-R007-Y1]. X-ray crystallography experiments with synchrotron radiation were performed at the Pohang Accelerator Laboratory (PLS-II BL2D SMC, BL6D C&S UNIST-PAL beamlines). Experiments on the PLS-II 6D beamline were supported in part by MSIT, POSTECH, and UCRF.

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Schröder, R. R.; Mastalerz, M. Chem.—Eur. J. 2018, 24, 1816. (n) Zhang, G.; Presly, O.; White, F.; Oppel, I. M.; Mastalerz, M. Angew. Chem., Int. Ed. 2014, 53, 5126. (5) Swamy, S. I.; Bacsa, J.; Jones, J. T. A.; Stylianou, K. C.; Steiner, A.; Ritchie, L. K.; Hasell, T.; Gould, J. A.; Laybourn, A.; Khimyak, Y. Z.; Adams, D. J.; Rosseinsky, M. J.; Cooper, A. I. J. Am. Chem. Soc. 2010, 132, 12773. (6) Zhang, L.; Xiang, L.; Hang, C.; Liu, W.; Huang, W.; Pan, Y. Angew. Chem., Int. Ed. 2017, 56, 7787. (7) (a) Hong, S.; Rohman, M. R.; Jia, J.; Kim, Y.; Moon, D.; Kim, Y.; Ko, Y. H.; Lee, E.; Kim, K. Angew. Chem., Int. Ed. 2015, 54, 13241. (b) Benke, B. P.; Aich, P.; Kim, Y.; Kim, K. L.; Rohman, M. R.; Hong, S.; Hwang, I.-C.; Lee, E. H.; Roh, J. H.; Kim, K. J. Am. Chem. Soc. 2017, 139. 7432. (c) Smith, P. T.; Benke, B. P.; Cao, Z.; Kim, Y.; Nichols, E. M.; Kim, K.; Chang, C. J. Angew. Chem., Int. Ed. 2018, 57, 9684. (8) In recent years, Nitschke group reported porphyrin-based cubic metal–organic cages. (a) Meng, W.; Breiner, B.; Rissanen, K.; Thoburn, J. D.; Clegg, J. K.; Nitschke, J. R. Angew. Chem., Int. Ed. 2011, 50, 3479. (b) Brenner, W.; Ronson, T. K.; Nitschke, J. R. J. Am. Chem. Soc. 2017, 139, 75. (9) Related to this work, metal–organic polyhedra have been used as premade SBUs to construct metal–organic frameworks. (a) Li, J.-R.; Timmons, D. J.; Zhou, H.-C. J. Am. Chem. Soc. 2009, 131, 6368. (b) Sudik, A. C.; Côté, A. P.; Wong-Foy, A. G.; O’Keeffe, M.; Yaghi, O. M. Angew. Chem., Int. Ed. 2006, 45, 2528. (c) Alkordi, M. H.; Brant, J. A.; Wojtas, L.; Kravtsov, V. Ch.; Cairns, A. J.; Eddaoudi, M. J. Am. Chem. Soc. 2009, 131, 17753. (d) Mossine, A. V.; Mayhan, C. M.; Fowler, D. A.; Teat, S. J.; Deakyne, C. A.; Atwood, J. L. Chem. Sci. 2014, 5, 2297. (e) Wang, H.-N.; Meng, X.; Yang, G.-S.; Wang, X.-L.; Shao, K.-Z.; Su, Z.M.; Wang, C.-G. Chem. Commun. 2011, 47, 7128. (10) The original free-base porphyrin box, PB-1, reported in ref. 7a, contained butyloxy chains. To improve the solubility for this and other studies,7b,c we later synthesized porphyrin boxes with different alkyl chains such as hexyloxy and octyloxy. Unless otherwise specified, the porphyrin box used here is the one functionalized with octyloxy chains (PB) and the hierarchical superstructures obtained from PB are referred to as PSSs. Porphyrin box functionalized with hexyloxy chains is referred to as PB'. PB'-derived superstructures (PSS's) were used to study gas sorption properties. A list of abbreviations of the different porphyrin box derivatives is provided along with their structures in the Supporting Information (Scheme S1). (11) Stock, N.; Biswas, S. Chem. Rev. 2012, 112, 933. (12) We observed that the length of alkyl chains drastically affected the surface area of the porphyrin boxes. For example, PB-1 with butyloxy chains and PB' with hexyloxy chains showed a BET surface area of 1370 m2/g (ref 7a) and 539 m2/g, respectively. Whereas, PB with octyloxy chains turned out to be nonporous after removal of solvent molecules even by supercritical CO2 activation. Metallation of the PB' using zinc ions, however, caused an insignificant change in their porosity (BET surface area of Zn-PB': 522 m2/g). (13) While a typical type I isotherm was observed for PSS-3' and PSS4', the N2 sorption isotherm of PSS-1' showed a steep initial uptake before P/P0 = 0.1 followed by a slowly and linearly increasing adsorption isotherm. The unusual sorption behavior of PSS-1' may be due to the long alkyl chains on the PB' windows, which may kinetically hinder the entry of gas molecules as already documented by others.14 (14) Hu, X.-Y.; Zhang, W.-S.; Rominger, F.; Wacker, I.; Schröder, R. R.; Mastalerz, M. Chem. Commun. 2017, 53, 8616. (15) (a) Lee, J. Y.; Farha, O. K.; Roberts, J.; Scheidt K. A.; Nguyen, S. T.; Hupp, J. T. Chem. Soc. Rev. 2009, 38, 1450. (b) Yanai, N.; Kimizuka, N. Acc. Chem. Res. 2017, 50, 2487. (c) Ding, Y.; Chen, Y.-P.; Zhang, X.; Chen, L.; Dong, Z.; Jiang, H.-L.; Xu, H.; Zhou, H.-C. J. Am. Chem. Soc. 2017, 139, 9136. (d) Park, J.; Feng, D.; Yuan, S.; Zhou, H.-C. Angew. Chem., Int. Ed. 2015, 54, 430.

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Table of contents Title: Rational Design and Construction of Hierarchical Superstructures Using Shape-persistent Organic Cages: Porphyrin Box-based Metallosupramolecular Assemblies

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