Porphyrin Boxes - Accounts of Chemical Research (ACS Publications)

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Porphyrin Boxes Published as part of the Accounts of Chemical Research special issue “Supramolecular Chemistry in Confined Space and Organized Assemblies”. Rahul Dev Mukhopadhyay,†,§ Younghoon Kim,‡,§ Jaehyoung Koo,‡,§ and Kimoon Kim*,†,‡ †

Center for Self-Assembly and Complexity (CSC), Institute for Basic Science (IBS), Pohang 37673, Republic of Korea Department of Chemistry, Pohang University of Science and Technology, Pohang 37673, Republic of Korea

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‡

CONSPECTUS: In order to fabricate efficient molecular photonic devices, it has been a long-held aspiration for chemists to understand and mimic natural light-harvesting complexes where a rapid and efficient transfer of excitation energy between chlorophyll pigments is observed. Synthetic porphyrins are attractive building blocks in this regard because of their rigid and planar geometry, high thermal and electronic stability, high molar extinction, small and tunable band gap, and tweakable optical as well as redox behavior. Owing to these fascinating properties, various types of porphyrin-based architectures have been reported utilizing both covalent and noncovalent approaches. However, it still remains a challenge to construct chemically robust, well-defined three-dimensional porphyrin cages which can be easily synthesized and yet suitable for useful applications both in solution as well as in solid state. Working on this idea, we recently synthesized box-shaped organic cages, which we called porphyrin boxes, by making use of dynamic covalent chemistry of imine condensation reaction between 4-connecting, square-shaped, tetraformylporphyrin and 3connecting, triangular-shaped, triamine molecules. Various presynthetic, as well as postsynthetic modifications, can be carried out on porphyrin boxes including a variation of the alkyl chain length in their 3-connecting subunit, chemical functionalization, and metalation of the porphyrin core. This can remarkably tune their inherent properties, e.g., solubility, window size, volume, and polarity of the internal void. The porphyrin boxes can therefore be considered as a significant addition to the family of multiporphyrin-based architectures, and because of their chemical stability and shape persistency, the applications of porphyrin boxes expand beyond the photophysical properties of an artificial light-harvesting complex. Consequently, they have been exploited as porous organic cages, where their gas adsorption properties have been investigated. By incorporating them in a lipid bilayer membrane, an iodide selective synthetic ion channel has also been demonstrated. Further, we have explored electrocatalytic reduction of carbon dioxide using Fe(III) metalated porphyrin boxes. Additionally, the precise size and ease of metalation of porphyrin boxes allowed us to utilize them as premade building blocks for creating coordination-based hierarchical superstructures. Considering these developments, it may be worth combining the photophysical properties of porphyrin with the shape-persistent porous nature of porphyrin boxes to explore other novel applications. This Account summarizes our recent work on porphyrin boxes, starting with their design, structural features, and applications in different fields. We also try to provide scientific insight into the future opportunities that these amazing boxes have in store for exploring the still uncharted challenging domains in the field of supramolecular chemistry in a confined space.

1. INTRODUCTION

As a result, there exists a plethora of reports on porphyrinbased architectures covalently or noncovalently built with more than three porphyrin units including one-dimensional (1D) nanotapes,1 two-dimensional (2-D) nanorings2 and dendrimers.3 For instance, 2-D porphyrin nanorings reminiscent of the chromophoric arrangement in natural lightharvesting complexes have been extensively synthesized and studied by Kobuke, Osuka, Anderson, and others.2,4−6 Porphyrin-based 2-D/3-D infinite arrays, such as metal− organic frameworks7,8 or covalent-organic frameworks,9,10 have

In nature, light-harvesting complexes embedded in the chloroplast transfer the energy of incident photons to the reaction center with very high efficiency. A densely packed arrangement of cofacially oriented, slip-stacked chlorophyll molecules results in such easy and rapid inter-ring energy transfer. These complexes have inspired chemists to design various porphyrin-based architectures, making use of both covalent and noncovalent interactions.1−3 Apart from being used as artificial light-harvesting antennae, these structures possess the potential to find suitable applications in catalysis, photodynamic therapy, and molecular electronics. © XXXX American Chemical Society

Received: June 23, 2018

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DOI: 10.1021/acs.accounts.8b00302 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

exploring porphyrin chemistry in this box-shaped nanoplatform.

also been reported. Three-dimensional box or cagelike porphyrin assemblies have been reported by Osuka,6 Aida,11,12 and others13 utilizing reversible noncovalent interactions. In particular, a series of porphyrin-based metal− organic cages have been recently reported by Nitschke and coworkers.14−17 However, creating a 3-D space encompassed by multiple (more than three) porphyrin subunits using a covalent strategy remains rare18 and synthetically challenging, although such cages, if synthesized in high yield, can be exploited for many interesting applications. Moving away from a coordination chemistry approach (due to well-known, chemically labile metal−organic linkages) and aiming toward more persistent molecular structures, efforts toward designing robust, shape-persistent 3-D porphyrin-based molecular containers existed in our group as early as 2008. An ideal prologue to this story may start with our research interest in utilizing container-shaped molecules such as cucurbiturils for the encapsulation of guest molecules both in solution and in solid state, which dates back to the early 2000s.19,20 Almost at the same time, we were also working on the synthesis of other enclosed nanostructures such as polymer nanocapsules by cross-linking planar disc-shaped building blocks, possessing multiple in-plane reactive functionalities and ditopic linkers with irreversible covalent linkages.21 However, atomically precise structures were not obtained making use of this strategy. With the advent of pioneering research on shapepersistent 3-D porous organic cages (POCs) by Cooper and others,22,23 we were inspired to exploit the dynamic covalent chemistry, established by Lehn, Sanders, Stoddart, and others,24,25 to design robust, porous, and shape persistent 3D organic cages composed of planar as well as rigid building blocks like porphyrin in high yield. Although Mastalerz and coworkers used boronate ester chemistry to synthesize a shapepersistent, porous [8 + 12] cage, having the highest surface area reported so far (3758 m2 g−1),26 the reversible boronate ester linkages in it are known to be unstable in the presence of moisture. Therefore, in order to make a balance between the structural rigidity and moisture stability, we designed porphyrin-based [6 + 8] cages constructed from the imine condensation reaction between six porphyrin aldehydes and eight amine linkers. These box-shaped cages, named as porphyrin boxes (PBs), were moisture stable and also possessed a Brunauer−Emmett−Teller (BET) surface area up to 1370 m2 g−1.27 We have further demonstrated that PBs not only are potential materials for gas storage but also are useful for other applications. For example, the porous cavity of PBs can be used for encapsulation of guest molecules in solution. Besides, metalated PBs may form host−guest complexes with pyridylfunctionalized molecules assisted by coordination bonds. At the same time, it was interesting to see if the intrinsic pore and windows of PBs can act as channels for transport of ions and whether the transport properties can be tuned by proper functionalization of the pore windows or walls. One can even think of using the pore cavity for catalysis,28 considering that metalloporphyrins have been widely exploited as catalysts in the literature. Finally, PBs with atomically precise structures can also be used as nanosized premade building blocks to construct rationally designed hierarchical superstructures. This Account will describe the unique design and structural features of these PBs, their applications in the aforementioned fields, and the scope of future perspectives of research in

2. PORPHYRIN BOXES: SYNTHESIS Shape-persistent 3-D organic cages with large cavities can be rationally designed and synthesized by using the combination of two differently shaped building units as the components of Archimedean solids. For example, rhombicuboctahedronshaped POCs can be obtained by combining eight triangular (3-connecting) and six square shaped (4-connecting) building units (Figure 1a). Although a similar strategy was used to

Figure 1. (a) Design of a (6 + 8) POC based on an Archimedean solid. (b) Synthetic scheme and crystal structures of porphyrin boxes (PB-1 and PB-2). Hydrogen atoms and aliphatic chains are omitted for clarity. Adapted with permission from ref 27. Copyright 2015 Wiley-VCH.

synthesize an organic cage by Warmuth and co-workers,29 neither its X-ray crystal structure nor application has been studied. The use of a rigid building unit is important to obtain POCs which show high structural stability and retain their porosity even after removal of guest solvent molecules. Among many candidates, porphyrin having a large planar shape, interesting physical, chemical, and catalytic properties and ease-of-functionalization is an excellent choice for designing POCs with potential applications. Furthermore, porphyrinB


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Accounts of Chemical Research based POCs may pave a new research avenue in the field of purely organic 3-D porphyrin-based architectures. We therefore set out a project to build large shape-persistent 3-D organic cages based on porphyrins. In 2010, we successfully synthesized the first rationally designed PB, PB1, which consisted of triangular (3-connecting) and square (4connecting) building units.30 The one-pot imine condensation reaction between two different building blocks, meso-tetra(pformylphenyl)porphyrin and triangular-shaped (2,4,6-tributoxybenzene-1,3,5-triyl)trimethanamine produced PB-1, in nearly quantitative yield (Figure 1b). The extremely simple 1H NMR spectrum of PB-1 which consists of 444 hydrogen atoms gave us a hint that the formed product was highly symmetric. The [6 + 8] formation was further confirmed from matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass analysis (m/z = 6981.4, [M + H]+).30 Computational studies suggested PB-1 has a rhombicuboctahedral structure, but it took nearly 4 years before the structure was finally confirmed by single crystal X-ray diffraction (SC-XRD) studies (see below). The well-known reversibility of imine bond formation leads to structural defect rectification but at the same time can cause the decomposition of the product, leading to low chemical stability. However, PB-1 exhibited an unexpected chemical stability in aqueous media in the pH range 4.8 to 13, which may be attributed to the hydrophobic aliphatic chains of the triamine ligands, initially, introduced for enhancing its solubility. We further demonstrated that the use of a different 3connecting triangular unit (TREN, tris(2-aminoethyl)amine) can lead to the synthesis of new PB (PB-2) (Figure 1b), which indicates that an isoreticular synthetic approach can be utilized to produce other PBs, having larger accessible void volume and wide windows, allowing the encapsulation of larger guest molecules. Other synthetic modifications such as variation of the alkyl chain length can further affect the solubility of PBs.

Figure 2. (a) Packing structure of PB-1 along the b axis and (b) view of the 1-D channels of PB-1 along the a axis. (c) N2 sorption isotherm of PBs (solid symbols, adsorption; open symbols, desorption) at 77 K. Adapted with permission from ref 27. Copyright 2015 Wiley-VCH.

isotherm (195 K) further revealed microporosity in PB-2 crystals with a BET surface area (935 m2 g−1). Despite the difference in their structures and nitrogen adsorption behavior, both PBs exhibited permanent porosity. To further explore their gas adsorption properties, selective uptake of CO2 over N2 and CH4 was investigated. Although the BET surface area of PB-1 is larger than that of PB-2, a higher selectivity for CO2 was observed for PB-2 compared to PB-1 at 273 K. It is considered that the introduction of extra tertiary amine sites provided the higher CO2 selectivity for PB2. As shown in Figure 1b, the length of alkyl chains of PB-1 can be varied from butyloxy to hexyloxy or octyloxy. A decrease in the BET surface area by changing the length of the aliphatic chains from butyloxy (PB-1, 1370 m2/g)27 to hexyloxy (PB-1(6), 539 m2/g)32 was observed. Whereas the PB functionalized with octyloxy chains (PB-1(8))32,33 turned out to be nonporous after removal of occluded solvent molecules even using supercritical CO2 activation. Metalation of the PBs using zinc ions, however, caused an insignificant change in their porosity (BET surface area of Zn-PB-1(6): 522 m2/g).32 This work illustrates that PBs, the gas sorption behavior of which can be tuned by choosing appropriate building blocks, may be useful for gas storage and separation.

3. PORPHYRIN BOXES: STRUCTURES AND POROSITY SC-XRD analysis unambiguously proved that the structure of PB-1 is a rhombicuboctahedron which consists of six porphyrin units and eight triamine ligands, occupying the square and triangular faces of the polyhedron, respectively. PB1 possesses a large accessible cavity (1.9 nm in diameter) and 12 windows (6.6 × 8.5 Å) (Figure 1b). The close packing of the PB-1 molecules in the crystal results in the formation of a 1-D channel passing through the windows (Figure 2a,b). Nitrogen sorption analysis at 77 K, after supercritical (SC) CO2 activation of the PB-1 crystals, showed a microporous adsorption isotherm (type I) with small adsorption/desorption hysteresis, and one of the highest BET surface area (1370 m2 g−1) among POCs (Figure 2c). The cavity diameter (1.7 nm) obtained from a narrow pore-size distribution well-matched with that obtained from its X-ray crystal structure (1.9 nm). PB-1 retained its crystallinity even after SC CO2 activation followed by gas adsorption measurements. Compared to PB-1, PB-2 possessed a slightly distorted rhombicuboctahedral structure with a smaller cavity (1.1 nm in diameter) and windows (1.1 × 3.0 Å) (Figure 1b). PB-2 exhibited an unusual nitrogen adsorption isotherm with large adsorption/desorption hysteresis (Figure 2c). This behavior resembles a gate-opening phenomenon, related to the narrow windows with a relatively large cavity and flexibility of the structure, similar to soft-porous crystals.31 CO2 adsorption

4. ION CHANNEL Numerous attempts to mimic the natural ion transporting behavior using various synthetic molecules (such as macroC


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Accounts of Chemical Research cycles, barrels, etc.) have been made so far.34 We have previously reported the synthetic ion channel behavior of cucurbit[n]uril (CB[n], n = 5 and 6)35 and MOP-18 (metal− organic polyhedron)36 when integrated into a lipid bilayer membrane. These results encouraged us to investigate the properties of PBs as a synthetic ion channel. In 2017, we reported the ion transporting behavior of PB-1 when incorporated into lipid bilayer membrane as well as in living cells (Figure 3a).33 PBs are potential ion transporters

Figure 3. (a) Schematic illustration for the incorporation of PB-1(8) into lipid bilayer membranes. (b) Time-dependent decay in the normalized fluorescence intensity ratio of HPTS associated with the transport of different anions through PB-1(8). Inset: schematic illustration of HPTS assay. (c) Typical on−off current profile measured with PB-1(8) during voltage clamp experiments. Adapted with permission from ref 33. Copyright 2017 American Chemical Society.

aromatic or aliphatic CH···anion interactions may be the plausible driving force behind such a behavior. The anion transporting mechanism was further investigated using chloride-sensitive lucigenin assay. With increasing concentration of PB-1(8), a linear increase in anion transport was observed, indicating that anions pass through the PB-1(8) incorporated into the membrane, and not through membrane leakage. Additionally, transport activity was substantially decreased by changing the intravesicular anions to more hydrophilic sulfate, instead of nitrate, which may indicate a dominating antiport (Cl−/NO3−) over symport (Na+/Cl−) mechanism. Among the anions, iodide showed the most significant quenching of fluorescence, which can be correlated to the dehydration free energy of the anions. The sequence follows the Hofmeister series: weakly hydrated iodide anions exhibited the highest transport activity while the lowest transport activity was observed for the strongly hydrated sulfate. The dehydration of the anions is therefore crucial for their transport through PB-1(8). Voltage clamp experiments showed a single channel current profile which was long-lived and having distinct open and closed states. An unusual nonlinear relationship (i.e., nonohmic behavior) in current−voltage profile was, however, observed. Nevertheless, this result confirmed that transporting process of PB-1(8) follows a dynamic channel mechanism (Figure 3c). The high chemical stability of PB-1(8) encouraged us to incorporate it in a real system, e.g., into a living-cell membrane (Figure 4). The passage of iodide through the PB-1(8) channels led to the quenching of fluorescence of yellow fluorescent protein (YFP) expressed in HEK-293T cells. The fluorescence was, however, recovered by changing the extracellular anion from iodide to chloride, showing the reversible nature of the iodide transport process. PB-1(8) can, therefore, be considered as a promising synthetic ion channel to study iodide transporting process in living cells as well as in facilitating or triggering biological functions related to iodide. The advantage of an artificial ion channel is the possibility of controlling its transport properties by synthetic modification. The PB as an ion channel also offers many such options. For example, it is anticipated that, by changing the length of aliphatic chains of PB, the dynamics of ion transfer behavior may be affected.37 Also, the chemical environment inside the PB can be modulated by metalation or insertion of a pillar linker.38 At the same time, stimuli-responsive ion transport channels, with a switchable on−off system, can be designed by incorporating appropriate linkers (such as azobenzene) inside the cavity of PBs.

because of their inherent properties. First, they are shapepersistent and possess a large inner void with 12 accessible windows which provide a suitable pathway for ion transport. Second, PBs like PB-1 (outer diameter: 3.6 nm) can be incorporated into a lipid bilayer (thickness of lipid bilayer membrane: ∼4 nm) membrane assisted by its hydrophobic aliphatic chains, without being buried. PB-1 also exhibited high chemical stability in the pH range 4.8−13. To improve the solubility and lipophilicity, we used PB-1(8) with octyloxy chains for the ion channel experiments. HPTS (8-hydroxypyrene-1,3,6-trisulfonic acid) fluorescence assay proved that PB-1(8) primarily acts as an anion transporter (Figure 3b). On the other hand, a minimal response was observed with cations in a similar assay. The

5. CATALYSIS Metalloporphyrins are omnipresent in biological processes such as reduction of O2 (cytochrome c oxidase), substrate oxidation (cytochrome P450), and electron transfer (cytochrome c). Understanding and mimicking the active sites of these enzymatic systems and utilizing them for catalyzing various organic reactions has therefore been actively pursued by researchers for a long time.39 For instance, the synthesis of cofacial porphyrins is one way of mimicking the natural catalytic porphyrin moieties which involve in a four-electron reduction of dioxygen.40 The distance of two opposite porphyrin moieties in a PB is too far (>4−5 Å) to participate in cofacial 4 electron reduction for molecular oxygen; however, the enzyme mimicking interior voids (around 1.9 nm) and D


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matrix in which the catalyst is embedded. Fe-PB was then used as a heterogeneous catalyst in neutral water by depositing onto a glassy carbon electrode precoated with carbon nanotubes. After saturation with CO2, there is a dramatic current increase at the FeI/Fe0 couple with a positive shift of the onset potential, suggesting catalytic CO2 reduction. On the other hand, a lower current intensity was observed when Fe-TPP was used under the same conditions. Electroactive surface area measurement showed that the number of electrochemically active iron centers in Fe-PB is 16% more compared to aggregated Fe-TPP. Inefficient electron transfer or limited substrate accessibility leads to the poor electrocatalytic performance in Fe-TPP. Long-term bulk electrolysis revealed that the amount of CO produced by electrocatalytic reduction of CO2 using Fe-PB is double of that obtained by using Fe-TPP under similar conditions. Moreover, the turnover number per electroactive iron in case of Fe-PB is 69% higher than that of Fe-TPP (Figure 5c), consistent with the results of CO specific current density of Fe-PB (Figure 5d). The electrocatalytic performance can be further improved by designing isoreticular porphyrin boxes using appropriate building units, resulting in larger inner space and higher surface area. An alternate route to develop new structures with larger accessible space for substrates is to connect individual porphyrin box units into hierarchical superstructures to emerge new outer spaces. The self-assembly strategies, involving various noncovalent interactions, adopted in this direction will be discussed in the next section. Figure 4. Schematic illustration of PB-1(8) assisted iodide transport in YFP-HEK-293T cells and corresponding real-time cell images. Scale bar is 50 μm. Adapted with permission from ref 33. Copyright 2017 American Chemical Society.

6. HIERARCHICAL SUPERSTRUCTURES Self-assembled hierarchical superstructures made of predesigned nanoparticles have been widely explored as catalysts and sensors, as well as for delivery and storage.44,45 Spherical or polyhedral colloidal particles (usually polymers or silica-based) have been conventionally used to construct spontaneously selfassembled hierarchical superstructures, having a long-range ordered array of nanoparticles with narrow-sized distribution and uniform morphology.46,47 However, making functionalized building blocks of uniform size and shape and connecting them through a particular lattice preference still remains a challenge. In this context, POCs can be rationally used for constructing predictable hierarchical superstructures having a long-range order. The truncated cubic geometry and six porphyrin faces of PBs prompted us to utilize the metalated PBs as premade secondary building units (SBUs). They can be either connected by bridging the metalated porphyrin faces using coordination linkages or by utilizing π−stacking or charge transfer interactions with acceptor molecules such as C60.48 Exploiting the pentacoordinate geometry of zinc metalated porphyrin (Zn-PB), we utilized bipyridyl-terminated rigid ligands to construct porphyrin box-based hierarchical superstructures (PSSs) (Figure 6a).32 There is an optimal range of length of the bridging ligands (9.4−13.6 Å) in which Zn-PB units slightly expanded or shrunk themselves to adjust into the PSSs. In the case of a ligand in the longer extreme, 2,6di(pyridine-4-yl)naphthalene (13.6 Å), a 2-D layered structure was observed. Molecular dynamics (MD) simulations confirmed that the 3-D structure was stabilized by interpenetration and π−π interactions between adjacent Zn-PBs besides the coordination bonds. If the bridging ligands were too short or long, superstructures were not constructed. The PSS built with Zn-PB having hexyloxy chains (Zn-PB-1(6)) and 1,4-di(4-

large surface area of PB prompted us to explore its catalytic activities for other transformations that may benefit from such structural porosity. Advantages of electrochemical CO2 reduction over thermal and photochemical methods inspired Chang and Yaghi to utilize cobalt porphyrin-based covalent organic frameworks (COFs) for catalytic reduction of CO2 in neutral water, with high Faradaic efficiency and turnover numbers at low overpotential.41,42 Likewise, we envisaged that the PBs may possess higher reaction efficiency, catalytic activity, and selectivity due to their packing geometry, enlarged surface area, and isolated internal cavities. We and Chang, therefore, compared the electrocatalytic properties of a chloroiron(III) porphyrin box (Fe-PB) with iron tetraphenylporphyrin monomer (Fe-TPP), which is an established catalyst for the electrolytic reduction of CO2 to CO (Figure 5a).43 While FeTPP shows a close packed structure, Fe-PB (synthesized from PB-1(8)) with an intrinsic cavity possessed a BET surface area of 490 m2/g exhibiting 3.67 wt % CO2 uptake at 298 K. Due to the aforementioned structural advantages, the diffusion of CO2 to the catalytically active sites is more favored in Fe-PB and a higher electrochemical activity and more efficient mass transport than Fe-TPP was expected and that was the case (Figure 5b). Homogeneous cyclic voltammetry studies of Fe-PB and FeTPP suggest that both have similar electronic structures, indicating that our supramolecular approach does not interfere with intrinsic characteristics but rather can provide a new E


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Figure 5. (a) Molecular structures of Fe-TPP and Fe-PB. (b) Proposed mechanism by which the porosity of Fe-PB facilitates diffusion of substrate and electrolyte around the catalytic iron centers. (c) Comparative CO and H2 production during bulk electrolysis under CO2 and (d) CO specific current densities for Fe-PB and Fe-TPP. Adapted with permission from ref 43. Copyright 2018 Wiley-VCH.

occurs much faster with PSS than with Zn-PB, which establishes its potential for other photocatalytic transformations as well as for applications such as photodynamic therapy. Apart from using the coordination approach, one may also utilize other noncovalent forces, such as the donor−acceptor interactions as one between porphyrin and fullerenes49 to construct hierarchical superstructures. Such materials can have intriguing electronic properties and may find suitable

pyridyl)benzene showed much higher porosity (Langmuir surface area: 1060 m2/g) compared to the parent PB (PB1(6), Langmuir surface area: 554 m2/g) presumably due to the extrinsic porosity created upon formation of the hierarchical superstructures (Figure 6b). Due to its extrinsic porosity, PSS shows a better photocatalytic performance as compared to its precursor, Zn-PB.32 Singlet oxygen-mediated photooxidation of 1,5-dihydroxynaphthalene to juglone, a natural product, F


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combination with the photonic properties of a multiporphyrinbased architecture can further culminate into other interesting applications (Figure 7).

Figure 7. Overall view of the chemistry and applications ofPBs.

One obvious application of PBs yet to be explored is encapsulation of guest molecules such as fullerenes and oligopeptides inside its cavity as Nitschke and co-workers demonstrated with porphyrin-based 3-D metal−organic cages.14,16,52 Besides hydrophobic interactions, donor−acceptor interactions can be utilized to encapsulate guest molecules like C60 inside PBs. These host−guest complexes may be useful for studying energy or electron transfer processes and exploring some emergent photophysical behavior. One can also insert a specifically designed central pillar inside the void of metalated PBs utilizing coordination chemistry. Such pillar linker inserted PBs may also be used as a model to study molecular rotors or gyroscopes.53 An alternative strategy to amalgamate the properties of the porphyrin molecules with that of a POC is to use the photophysical properties of its triplet state which has been widely explored for generation of singlet oxygen and associated applications, e.g., catalysis, photodynamic therapy, and bioimaging.54 However, for such biological applications as mentioned, PBs need to be permeable and compatible with the cellular medium. Our experience with the incorporation of PBs as ion channels in real cell membranes inspires us to look forward in this direction. This nanoplatform can also be useful for applications such as bioimaging and photon upconversion.55 The rational design of PBs can, therefore, help us answer complex and intriguing challenges with the aid of a rational supramolecular approach. In fact, one should be able to program larger number of porphyrin monomers and its complementary vertex linkers to form predesigned larger multiporphyrin-based architectures similar to the viral icosahedral procapsids or the protein, ferritin. In recent years, Fujita’s group has produced a large number of such well-defined capsules using directional metal−ligand interactions.56 Similarly, synthesis of such multiporphyrin-based

Figure 6. (a) Schematic representation of construction of PSS s utilizing zinc-metalated porphyrin box (Zn-PB) and bridging ligands. (b) Crystal structure of a PSS built with Zn-PB and 1,4-di(4pyridyl)benzene, shown as a wire frame. Adapted with permission from ref 32. Copyright 2018 American Chemical Society.

applications as charge transport materials in solar cells and in field effect transistor devices.50,51

7. SUMMARY AND PERSPECTIVES PBs can be considered as a very recent and valuable addition to the family of multiporphyrin-based architectures which can be synthesized in high yields making use of the dynamic covalent chemistry approach. Their rational synthetic design and stability both in solution and in solid state makes them suitable for diverse applications. The rigid 3-D structure with a large cavity of PBs allowed us to explore them as porous molecular solids. Furthermore, when embedded into a lipid bilayer membrane, they behaved as synthetic ion channels that can selectively transport iodide ions across the membrane. By taking advantage of the porosity of PBs facilitating the transport of substrates, we successfully demonstrated that Fe(III) metalated PBs deposited on an electrode can catalyze the electrochemical conversion of CO2 to CO with high efficiency and selectivity. Finally, the atomically precise, truncated cubic molecules also prompted us to use coordination chemistry to rationally design and construct PB-based hierarchical superstructures, having larger surface area and better photocatalytic performance than their parent PB-based building units. These properties when explored in G


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gigantic hollow capsules using covalent bonds may be possible but it will surely be a grueling challenge in front of future organic chemists. The PB is an exciting start, but it is good time that we start thinking beyond and much outside this “box”.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kimoon Kim: 0000-0001-9418-3909 Author Contributions §

R.D.M., Y.K., and J.K. contributed equally.

Notes

The authors declare no competing financial interest. Biographies Rahul Dev Mukhopadhyay received both his B.Sc. (2008) and M.Sc. (2010) degrees in chemistry from the University of Calcutta. He then obtained his Ph.D. (2017) from the Academy of Scientific and Innovative Research (AcSIR), working with Prof. Ayyappanpillai Ajayaghosh. Currently, he is working as a postdoctoral research fellow at CSC, IBS. His research interests include supramolecular polymerization, stimuli-responsive materials, and metal−organic frameworks. Younghoon Kim received his M.S. in chemistry at Chungnam National University (CNU) in 2015 and is currently a Ph.D. student at POSTECH under the guidance of Prof. Kimoon Kim. His research focuses on the synthesis of porous organic cages and their applications. Jaehyoung Koo received his B.S. in chemistry at Sungkyunkwan University (SKKU) in 2015 and is currently a Ph.D. student at POSTECH under the guidance of Prof. Kimoon Kim. His current research focuses on applications of novel porous materials including metal−organic frameworks and porous organic cages. Kimoon Kim studied chemistry at Seoul National University (B.S., 1976), KAIST (M.S., 1978), and Stanford University (Ph.D., 1986). After postdoctoral work at Northwestern University, he joined POSTECH where he is now a Distinguished University Professor. He has also been director of CSC, IBS. His current research focuses on developing novel functional materials and systems based on supramolecular chemistry.

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ACKNOWLEDGMENTS We thank all the co-workers and collaborators who have contributed to this research as cited. We gratefully appreciate the financial support of the Institute for Basic Science (IBS) [IBS-R007-D1].

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REFERENCES

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