Multicomponent Assembled Systems Based on Platinum(II

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

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Multicomponent Assembled Systems Based on Platinum(II) Terpyridine Complexes Published as part of the Accounts of Chemical Research special issue “Supramolecular Chemistry in Confined Space and Organized Assemblies”. Zhao Gao,† Yifei Han,† Zongchun Gao, and Feng Wang*

Acc. Chem. Res. Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 10/24/18. For personal use only.

CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China CONSPECTUS: Platinum(II) terpyridine complexes have received tremendous attention in recent years because of their square-planar geometry and fascinating photophysics. Bottomup self-assembly represents an intriguing approach to construct well-ordered supramolecular architectures with tunable optical and electronic properties. Until now, much effort has been devoted to the fabrication of monocomponent platinum(II) terpyridine-based assemblies. The next step is to develop multicomponent coassembled systems via the combination of platinum(II) terpyridine complexes with other π-organic and -organometallic molecules. The implementation of electron/energy transfer processes renders advanced functionality to the resulting coassemblies. For the fabrication of discrete multicomponent architectures, a feasible protocol is to construct preorganized molecular tweezers and macrocycles with the involvement of platinum(II) terpyridine complexes as the panel units. In view of their planar surface and positively charged character, such supramolecular receptors are capable of encapsulating electron-rich polyaromatic hydrocarbons and organometallic guests via donor−acceptor charge-transfer and/or metal−metal interactions. Intermolecular hydrogen bonds can be further incorporated between the molecular tweezers receptor and the polyaromatic hydrocarbon guests, giving rise to the strengthened binding affinity and sensitive stimuli-responsiveness. On this basis, multilayer donor− acceptor stacks have been obtained via the precise control over the number of pincers, which feature enhanced complexation strength and superior functionality. Moreover, platinum(II) terpyridine-based macrocycles are more suitable for guest accommodation than the corresponding molecular tweezers receptors in light of their definite size and constrained environment. Stimuli-responsive elements can be conveniently implemented into the rigid spacers of the molecular tweezers and macrocyclic receptors, facilitating the capture and release of the sandwiched guests in a highly controlled manner. On the other hand, long-range-ordered supramolecular polymers have been successfully fabricated with linear, hyperbranched, and cross-linked topologies by employing platinum(II) terpyridine-based molecular tweezers/guest recognition motifs as the non-covalent connecting unit. The degree of polymerization of the resulting donor−acceptor-type supramolecular polymers can be efficiently modulated by incorporating intermolecular hydrogen bonds between the molecular tweezers receptor and the complementary guest unit. An alternative approach toward extended multicomponent donor−acceptor assemblies is to mimic the structure of Magnus’ green salt. A delicate balance of non-covalent driving forces between homo- and heterocomplexation processes and a deeper understanding of thermodynamic and kinetic behaviors play the decisive roles in the final arrangement of the coassembled structures. Overall, multicomponent coassembly of platinum(II) terpyridine complexes into well-ordered nanostructures would open up a new avenue toward functional supramolecular materials that are especially promising for sensing, optoelectronics, and catalytic applications.

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INTRODUCTION

orbitals of the Pt centers, leading to the emergence of metalmetal-to-ligand charge transfer (MMLCT) transitions in the low-energy visible/NIR regions.2 The presence or absence of such MMLCT bands endows rich solid-state polymorphism to platinum(II) terpyridine-based crystalline materials.1 For the

Platinum(II) terpyridine complexes are regarded as an important class of transition metal coordination compounds with intriguing photophysical behaviors. Their square-planar geometry and coordinatively unsaturated character induce the formation of non-covalent Pt···Pt metal−metal and/or π−π interactions in the stacked form.1−5 As a consequence, orbital splitting occurs for both the occupied 5dz2 and unoccupied 6pz © XXXX American Chemical Society

Received: July 7, 2018

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DOI: 10.1021/acs.accounts.8b00340 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

stimuli-responsiveness of multicomponent architectures toward supramolecular functional materials.

further manipulation of the distance and strength of Pt···Pt metal−metal and/or π−π forces, the solution-based selfassembly technique represents a feasible approach. In recent years, Yam and co-workers have devoted much effort to monocomponent self-assembly of platinum(II) terpyridine complexes.2,6 As a next step, it is intriguing to combine platinum(II) terpyridine complexes with other π-conjugated molecules toward multicomponent supramolecular architectures. With the implementation of electron/energy transfer processes, the resulting coassemblies would show promising prospects for sensing, emitting, light harvesting, and photovoltaic applications.7−9 To attain this objective, a feasible protocol is to incorporate platinum(II) terpyridine units into preorganized molecular tweezers, macrocycles, and cages as supramolecular receptors. Although a variety of discrete organoplatinum(II)-based architectures have been reported,10,11 the metal units predominantly act as hinges for organic panels in these structures. It would be fascinating to incorporate platinum(II) terpyridine complexes as the key component of panel structures, which would render fascinating electronic, optical, and redox properties to the supramolecular receptors. In light of their planar π surface and positively charged character, electron-rich organic and organometallic guests can be encapsulated into the receptor’s cavity via charge transfer (CT) interactions. Briefly, acceptor−donor−acceptor (ADA)type discrete supramolecular architectures are prone to form when the centroid−centroid distance of platinum(II) terpyridine complexes is around 7 Å (twofold CT interactions).12 With delicate modulation of the spacer length, moderate-sized aromatic stacks with more than three π-aromatic units can be further achieved.13 Besides the discrete supramolecular systems, it is also intriguing to develop long-range-ordered multicomponent assemblies. In this regard, Magnus’ green salt, {[Pt(NH3)4][PtCl4]}n, provides a fascinating prototype.14,15 In its crystal structure, the [Pt(NH3)4]2+ and [PtCl4]2− building blocks are alternatively aggregated to form one-dimensional platinum(II) arrays. However, upon simple mixing of platinum(II) terpyridine with the complementary π-conjugated molecules, it is rather difficult to achieve the specific spatial arrangement in the final D−A coassemblies. The problem lies in the dynamic nature of the multicomponent system, leading to the mutual interplay between D−A heterocomplexation and A−A or D−D homocomplexation processes. As a result, a variety of orientation modes, such as segregated stacks, random mixing, diblock mixing, and alternate stacking, could exist as the most thermodynamic favorable state.16 To address this issue, a plausible approach is to develop a stepwise assembly methodology that directs the specific D−A organization in the nonequilibrium kinetically trapped state.17 An alternative strategy is to precisely modulate the homo- and heterocomplexation strengths at the molecular level.18 The aim of this Account is to highlight the most recent developments and contributions of discrete and infinite multicomponent assembled systems based on platinum(II) terpyridine complexes. To stimulate further research activity in this area, special attention is paid to the following three aspects: (1) molecular design principles to promote spatial control over the multicomponent organization, (2) spectroscopic properties of the resulting coassemblies influenced by non-covalent CT and metal−metal interactions, and (3)

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PREORGANIZED MOLECULAR TWEEZERS FOR ENCAPSULATION OF PAHS Yam and co-workers have reported molecular tweezers 1a, which possess two alkynylplatinum(II) terpyridine pincers connected by a rigid diphenylpyridine spacer (Figure 1a).19

Figure 1. (a) Chemical structures of platinum(II) terpyridine-based molecular tweezers 1a, 1b, and 2. (b) X-ray crystal structure of molecular tweezers 1a. (c) Plot of free energy changes for molecular tweezers/guest complexation versus the size of the π surface of the guest. Adapted with permission from ref 19. Copyright 2013 WileyVCH Verlag GmbH.

Because of the large steric hindrance imparted by the tert-butyl groups, intermolecular π−π stacking of the pincer units can be severely hampered. As a consequence, 1a shows excellent solubility in common organic solvents such as CH2Cl2, CHCl3, THF, CH3CN, and MeOH. In CH2Cl2, 1a displays low-energy absorptions at 380−520 nm, which are assigned to an admixture of metal-to-ligand and ligand-to-ligand charge transfer (MLCT/LLCT) transitions. Meanwhile, a triplet MLCT/LLCT emission centered at 612 nm is observed for 1a with a phosphorescence lifetime of 2.6 μs. Thanks to the presence of strong σ-donating alkynyl ligands, molecular tweezers 1a feature enhanced luminescence properties as compared with those of 2 (Figure 1a). For the latter compound, quenching of the emissive excited state occurs through nonradiative decay, ascribed to the thermally accessible low-lying d−d transition of chloroplatinum(II) terpyridyl pincers.20 As shown by X-ray crystallography, the two Pt(II) terpyridine units in 1a are arranged in a nearly face-to-face geometry, with interplanar distances of 6.781 to 7.143 Å (Figure 1b). In chlorinated solvents, 1a facilitates 1:1 encapsulation of electron-rich polyaromatic hydrocarbon (PAH) guests into its cavity. Divalent D−A CT interactions exist in the sandwiched structure, which are obviously stronger than the guest-outside binding mode with the monovalent CT force. The phosphorescence emission intensity is enhanced upon addition of naphthalene as the guest into 1a, while emission quenching takes place for anthracene, pyrene, or perylene as the guest via a reductive electron transfer pathway. B


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Figure 2. Graphical representation of the formation of D−A-type supramolecular polymers based on molecular tweezers/PAH guest complexation. Adapted with permission from ref 22. Copyright 2014 Wiley-VCH Verlag GmbH.

(DP) value (DP = 13 for 3 at 70 mM). As has been widely documented, the DP value can be theoretically calculated as DP = (Ka[C]0)1/2, where [C]0 is the monomer concentration, when 4Ka[C]0 ≫ 1.23 Hence, to achieve an appreciable DP value for D−A-type supramolecular polymers, it is necessary to improve the non-covalent molecular tweezers/guest complexation strength. Although enlarging the π surface of the PAH guest represents a possible approach, the tedious synthesis and limited solubility prevent its experimental feasibility. To address this issue, our research group has pursued an alternative protocol to enhance the molecular tweezers/guest binding affinity. Intriguingly, we have found that the pyridine unit located on the spacer of molecular tweezers 1c (Figure 3) can act as a hydrogen-bond acceptor site. When a hydrogenbond donor unit is attached on the complementary PAH guest, an intermolecular hydrogen bond tends to form and thereby contributes to binding affinity enhancement. For example, 2-

The binding constant (Ka) between 1a and coronene was determined to be 1.02 × 104 M−1, while no observable interaction was found for benzene as a guest. Accordingly, the π surface area exerts a crucial impact on the molecular tweezers/guest complexation strength (Figure 1c). In addition, non-covalent complexation between molecular tweezers and PAH guests is sensitive to solvent polarity changes. For example, the Ka value between the structurally similar tweezers 1b (Figure 1a) and coronene guest in 3:1 v/v chloroform/ acetonitrile was determined to be 640 M−1,21 which is much lower than that of 1a/coronene in pure chloroform (1.02 × 104 M−1). Notably, molecular tweezers/guest recognition systems feature high binding directionality, exact stoichiometry, and moderate complexation strength. By taking advantage of these properties, our research group has proposed the “tweezeringdirected self-assembly” strategy to fabricate D−A-type supramolecular polymers.22 In detail, the heteroditopic AB-type monomer 3 (Figure 2) was designed on the basis of the noncovalent 1a/pyrene complexation motif (Ka = 2.27 × 103 M−1 in CHCl3). Head-to-tail assembly of 3 is concentrationdependent, as evidenced by the abrupt slope changes for the double logarithmic plots of specific viscosity versus concentration (below 20 mM, slope = 1.29; above 20 mM, slope = 1.83). The stronger concentration dependence indicates the formation of linear supramolecular polymers at high monomer concentration via the ring−chain transition mechanism.23 On this basis, 9-methylanthracene is utilized as the chain stopper unit (Ka = 3.33 × 103 M−1 for 1a/9-methylanthracene in CHCl3), resulting in disassembly of the linear supramolecular polymers derived from 3. Upon the successive addition of bis(2-methoxyethyl) dicyanofumarate, it undergoes the quantitative Diels−Alder reaction with 9-methylanthracene, thus giving rise to the reversible formation of linear supramolecular polymers.

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HYDROGEN-BOND-ENHANCED MOLECULAR TWEEZERS/PAH GUEST COMPLEXATION Despite the aforementioned progress on the tweezeringdirected self-assembly strategy, supramolecular polymers derived from 3 suffer from a low degree of polymerization

Figure 3. Hydrogen-bond-enhanced molecular tweezers/PAH guest complexation. Data were taken from refs 24, 25, 26, and 29. C


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Figure 4. Graphical representation of the formation of D−A-type supramolecular polymer networks and their gelation capability. Adapted from ref 29. Copyright 2017 American Chemical Society.

naphthol displays a significant enhancement in the binding affinity toward 1c compared with the unsubstituted naphthalene guest (Ka = 1.2 × 104 M−1 for 1c/2-naphthol vs ∼101 M−1 for 1c/naphthalene).24 According to density functional theory calculations, the H···N distance was determined to be approximately 1.6 Å for the optimized 1c/2-naphthol geometry, with an O−H···N angle of 173°. More interestingly, the “hydrogen-bond-enhanced tweezers/guest complexation” strategy can be further expanded to azobenzene-, pyrene-, and carbazole-based PAH guests (Figure 3).25−29 Taking complex 1c/4 as an example, an intermolecular N−H···N hydrogen bond (N−H···N length = 2.4 Å; N−H···N angle = 174°) is formed between the amide moiety on 4 and the pyridine unit on 1c.25 Consequently, 1c/4 exhibits an 80-fold enhancement of the binding strength compared with 1c/pyrene (Ka = 1.80 × 105 M−1 for 1c/4 vs 2.27 × 103 M−1 for 1c/pyrene in CHCl3). The molecular tweezers/guest binding affinity can be varied to a large extent by modulating the strength of the intermolecular hydrogen bonds. For instance, 1,1,1,3,3,3hexafluoroisopropanol (HFIP) is capable of interfering with hydrogen bonds, leading to a 1000-fold decrease in the binding affinity between 1c and 4 [when 0%, 2%, and 8% v/v HFIP is added to complex 1c/4 in chloroform, Ka declines from 1.80 × 105 to 3.03 × 103 and 1.69 × 102 M−1, respectively].25 In addition to chemical stimuli, light can be also employed to regulate molecular tweezers/guest complexation via the “decaging” strategy.24,26 Specifically, when the hydroxyl group on 5 (Figure 3) is protected by an o-nitrobenzyl dimethyl ether caging unit, the resulting compound 6 (Figure 3) lacks the hydrogen-bond donor group and thus fails to complex with 1c. Removal of the photocleavable group is achieved via 365 nm UV light irradiation, leading to the revival of 1c/5 complexation.

On this basis, our research group sought to fabricate highmolecular-weight D−A-type supramolecular polymers by taking full advantage of the hydrogen-bond-enhanced molecular tweezers/guest complexation strategy. Accordingly, equimolar amounts of AA- and BB-type monomers based on the 1c/4 recognition motif were mixed together in chloroform solution.28 As the monomer concentration was increased from 3.00 to 30.0 mM, the diffusion coefficient values revealed a 178-fold decrease from 5.74 × 10−10 to 3.23 × 10−12 m2 s−1, suggesting a dramatic size expansion to form long-rangeordered supramolecular polymers at high monomer concentrations. The DP value was calculated to be 56 at a monomer concentration of 10 mM, which is significantly higher than that of the above system in the absence of intermolecular hydrogen bonds (DP = 13 for 3 at 70 mM). In addition to the formation of linear supramolecular polymers, the hydrogen-bond-enhanced molecular tweezers/ guest complexation strategy can also be employed for the fabrication of D−A-type supramolecular networks and gels.29 Conventionally, supramolecular polymer networks are constructed by grafting non-covalent recognition motifs onto the polymer side chains.30 Unlike these side-chain-type structures, we sought to develop main-chain-type supramolecular πconjugated polymer networks (SCPNs) in which π-conjugated polycarbazoles 8 (Figure 4) serve as the non-covalent linkages. The design principle is based on non-covalent complexation between molecular tweezers 1c and NH-type carbazole (Ka = 2.7 × 105 M−1 in CHCl3; Figure 3), which involves an intermolecular N−H···N hydrogen bond. Supramolecular gels form in chloroform (Figure 4) as a result of the multivalent recognition between polycarbazoles 8 and cross-linker 7. Remarkably, a trace amount of HFIP (5% v/v) destroys the supramolecular gels because of its ability to weaken hydrogenD


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3-fold enhancement of the binding affinity toward guest 9 compared with 1c (Ka = 1.10 × 105 M−1 for 14/9 in CHCl3). Remarkably, molecular tweezers/organometallic guest complexation can be maintained not only in chlorinated solvents but also in more polar media such as acetonitrile, DMSO, methanol, and even water. It is also tolerant to the presence of other non-covalent recognition motifs. For example, our research group introduced complex 1c/9 onto the rhombusshaped supramolecular coordination complex platform.35 The orthogonal recognition between Pt−N coordination and molecular tweezers/guest complexation leads to the formation of near-infrared-emissive supramolecular polymers (λmax = 795 and 831 nm for MMLCT bands in solution and the solid state, respectively). In another example, A2B3-type supramolecular hyperbranched polymers were hierarchically fabricated in CHCl3/CH3CN solution with the elaborate combination of 1c/11 and benzo-21-crown-7/secondary ammonium salt recognition motifs.36

bonding strength. Hence, it provides an efficient route to organize π-conjugated polymers into three-dimensional soft materials, which are promising for flexible optoelectronic applications.

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PREORGANIZED MOLECULAR TWEEZERS FOR ENCAPSULATING ORGANOMETALLIC GUESTS Besides PAH guests, planar organometallic guests can be also sandwiched into the cavity of alkynylplatinum(II) terpyridine molecular tweezers.12,20,31−36 For example, the solution color changes from yellowish-orange to deep red upon the addition of neutral organoplatinum guest 9 (Figure 5) to a CH2Cl2

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MOLECULAR TWEEZERS/GUEST COMPLEXATION WITH MULTILAYER STACKS In addition to the ADA-type molecular tweezers/guest complexation mode, the D−A stacking number can be further regulated. In this respect, Yam and co-workers reported the novel double-decker molecular tweezers 15 (Figure 6), which

Figure 5. Molecular tweezers/guest complexation with the presence of non-covalent metal−metal interactions.

solution of 1a (Ka = 5.5 × 104 M−1 for complex 1a/9). Simultaneously, a low-energy absorption band at ca. 500−600 nm emerges, accompanied by the appearance of an emission band centered at 772 nm.12 These newly formed bands are assigned to MMLCT transitions, ascribed to the proximity of the Pt atoms in complex 1a/9. Similar photophysical behaviors are observed upon mixing 1a with the negatively charged guest 10 (Figure 5). Apart from the homologous Pt(II)···Pt(II) interactions, Pt(II)···Au heterologous interactions could also exist. However, the poor energy match of Pt/Au frontier orbitals results in insignificant spectroscopic changes for complexes 1a/11 and 1a/12 (Figure 5).12 Moreover, the rigid spacer unit influences the fidelity and affinity of molecular tweezers/guest binding. For molecular tweezers 13 with the 2,2′-iminodibenzoyl backbone (Figure 5), the two alkynylplatinum(II) terpyridine pincers distort from each other to form a V-shaped structure (Pt···Pt distance = 18.5 Å), which is much more twisted than that of 1c (Pt···Pt distance = 7.56 Å).32 As a consequence, 13 shows weaker binding affinity than 1c toward the same guest (Ka = 3.43 × 104 M−1 for 1c/9 and 1.25 × 104 M−1 for 13/9 in CHCl3). In contrast, for molecular tweezers 14 (Figure 5) with the rigid dibenz[c,h]acridine backbone, carbon−carbon bond rotations for the spacer unit are restricted.33 Accordingly, 14 displays a

Figure 6. Chemical structures of double-decker molecular tweezers 15 together with the crystalline geometries of dimeric 152 and complex 15/16. Adapted with permission from refs 13 and 37. Copyright 2013 and 2017 Wiley-VCH Verlag GmbH.

consists of three cationic alkynylplatinum(II) terpyridine units connected by an oligophenylenepyridylene backbone.13 Notably, in the solid state 15 exists in a dimeric form in which two symmetrical complexes are mutually intercalated with each other (Figure 6). The double-decker molecular tweezers permit the accommodation of two organoplatinum guests into its cavities. The Hill coefficients (n) for complexes 15/92 (Ka = 3.16 × 1011 M−2) and 15/102 (Ka = 3.16 × 1013 M−2) are calculated to be 1.56 and 1.71, respectively. Such phenomena reveal the positive cooperativity for the multivalent recognition processes. In their extensive studies, the authors further designed tweezer-like guest 16 (Figure 6). Mixing 15 of 16 facilitated the formation of a discrete ADADA-type E


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Accounts of Chemical Research architecture with the presence of Pt(II)···Pt(II) interactions (Ka = 1.38 × 106 M−1 for complex 15/16 in CH2Cl2).37 Our research group has exploited the photocatalytic efficiency of multilayer tweezering architectures.38 The discrete DADA-type complex 14/17 (Ka = 2.89 × 106 M−1 in CHCl3; Figure 7) shows enhanced binding affinity compared with the

Figure 8. (a) Graphical representation of conformationally switchable molecular tweezers/guest complexation in response to pH and zinc cation stimuli. The red-colored pincer denotes a 4,4′,4″-tri-tert-butyl2,2′:6′,2″-terpyridineplatinum unit. (b) On-demand revival and loss of photocatalytic efficiency of complex 18b/9 for several repeated cycles. Adapted with permission from refs 39 and 40. Copyright 2018 Royal Society of Chemistry.

Figure 7. Graphical representation of the construction of DADA-type tweezering complex 14/17 for photocatalytic applications. POSS denotes polyhedral oligomeric silsesquioxane, which serves as the solubilizing group. Adapted from ref 38. Copyright 2017 American Chemical Society.

quently, 18a undergoes the mechanical transition from the Ushaped to the W-shaped conformation. This triggers the release of the guest from its cavity and thereby deactivates the Pt(II)···Pt(II) MMLCT bands. The established molecular tweezers/guest recognition motifs can be further incorporated into self-assembled polymers, leading to pH-mediated emergence/disappearance of Pt(II)···Pt(II) triplet MMLCT bands at the macroscopic scale. Furthermore, we designed another conformationally switchable molecular tweezers, 18b (Figure 8a), in which 2,2′:6′,2″terpyridine serves as the spacer unit.40 When 9 is sandwiched into the cavity of U-shaped supramolecular receptor 18b, the emergence of Pt(II)···Pt(II) interactions leads to excellent visible-light photosensitization behavior. It can be further utilized for photocatalyzed oxidative cyanation of N-phenyl1,2,3,4-tetrahydroisoquinoline (Figure 8b). Upon addition of Zn(OTf)2, 18b undergoes a U-shaped to W-shaped mechanical motion via Zn2+−terpyridine coordination. The reversible “on/off” switching of Pt(II)···Pt(II) spectroscopic bands (Figure 8b inset) provides extra control over their photocatalytic efficiency. With the successive addition of unsubstituted terpyridine to trap Zn2+ ion, the photocatalytic efficiency can be recovered “on-demand” for several repeated cycles (Figure 8b).

DAD- and ADA-type counterparts (Ka = 5.05 × 103 and 5.67 × 104 M−1, respectively). Since complex 14/17 features the maximized MMLCT absorbance at 564 nm, it can be photoirradiated via low-energy visible light. In detail, when a mixture of 14/17 and diamagnetic 2,2,6,6-tetramethylpiperidine (TEMP) is photoexcited with an OLED lamp (12 W, 590 nm), three lines with equal intensity are observed in the electron paramagnetic resonance spectrum. This phenomenon indicates the capture of singlet oxygen (1O2) generated in situ by TEMP, leading to the formation of the TEMPO radical. Because of the high reactivity of 1O2, oxidation of the secondary amine to the imine can be further achieved by employing complex 14/17 as a homogeneous photocatalyst (Figure 7). When 0.10 mol % 14/17 is loaded, a product yield of 98% (turnover number = 163.3 h−1) can be achieved upon photoirradiation for 6 h. Remarkably, the photocatalytic efficiency for DADA-type complex 14/17 is significantly higher than those of the DAD- and ADA-type counterparts. Therefore, both bathochromic-shifted MMLCT transition bands and high binding affinity are crucial for the excellent photocatalytic efficiency of complex 14/17.

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CONFORMATIONALLY SWITCHABLE MOLECULAR TWEEZERS/GUEST COMPLEXATION Our research group has designed and synthesized molecular tweezers 18a (Figure 8a) with the incorporation of the omethoxyphenyl−pyridine−o-methoxyphenyl triad (ortho-Py) spacer.39 As expected, 18a prefers a U-shaped conformation in the neutral state, which is prone to sandwich organometallic guest 9. Upon addition of trifluoroacetic acid (TFA), twofold N−H···O hydrogen bonds form between the methoxy units and the protonated pyridine moiety (Figure 8a). Conse-

MACROCYCLIC RECEPTORS FOR GUEST ENCAPSULATION In addition to molecular tweezers, macrocycles and cages are regarded as alternative types of supramolecular receptors for guest accommodation. In 2015, Yam and co-workers designed a series of macrocycles 19−22 with closed cavities (Figure 9) by attaching an auxiliary spacer on the aforementioned molecular tweezers unit.41 The spacer rigidity exerts crucial impacts on the macrocycle/guest complexation strength. In F


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Figure 9. Chemical structures of platinum(II) terpyridine-based macrocycles 19−22 and their reversible host−guest complexation properties. Adapted with permission from ref 41. Copyright 2015 National Academy of Sciences.

Figure 10. Self-assembled platinum(II) terpyridine-based macrocycles 23−24 and 27 together with their host−guest complexation behaviors. Adapted with permission from refs 42 and 44. Copyright 2005 and 2016 Wiley-VCH Verlag GmbH.

pyridine moieties were incorporated into the backbone of macrocycles 19 and 20, facilitating reversible modulation of the host−guest complexation (Figure 9). Addition of 2 equiv of HCl to 19 leads to protonation of the pyridine nitrogen atom. Complex 19/9 consequently dissociates, accompanied

detail, macrocycle 20 with two rigid spacers displays the highest binding affinity toward guest 9 (Ka = 2.04 × 106 M−1 in CH3CN), while 22 possessing two flexible oligo(ethylene glycol) chains exhibits the weakest binding strength (Ka = 3.09 × 104 M−1 for 22/9 in CH3CN). Furthermore, pH-sensitive G


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Figure 11. Graphical representation of the formation of two-component supramolecular fibers and gels. Adapted with permission from refs 18 and 46. Copyright 2018 Royal Society of Chemistry.

Figure 12. Graphical representation of the wire-to-wheel metamorphism process via a ligand substitution reaction. Adapted with permission from ref 47. Copyright 2008 Wiley-VCH Verlag GmbH.

= 107 M−1 in CD3CN at 253 K), which increases the solubility of pentacene in CH3CN up to the millimolar concentration level. In terms of naphthalene, anthracene, and tetracene guests, they prefer to adopt a 1:2 host−guest binding stoichiometry. Interestingly, Ka,2/Ka,1 > 4 was found for naphthalene and anthracene guests, suggesting the involvement of positive cooperative recognition events (Figure 10).

by the disappearance of the MMLCT absorption (λmax = 562 nm) and emission (λmax = 762 nm) bands. The sequential addition of NEt3 allows for the re-formation of complex 19/9. Notably, the organoplatinum and organogold guests have been shown to display anticancer therapeutic behavior. Hence, ondemand capture and release of such guest molecules from macrocyclic receptors provide a proof-of-principle model for designing pH-responsive drug delivery systems. Besides the covalent synthetic method, macrocycles can be also constructed via the metal−coordination approach.42−44 In this respect, Bosnich and co-workers developed macrocycles 23 and 24 (Figure 10) by associating chloroplatinum(II) terpyridyl molecular tweezers with dipyridine linkers.42 Intriguingly, the resulting macrocycles display 1:2 binding stoichiometry toward guest 26, with the involvement of positive allosteric complexation character. In comparison, the relatively larger guest 25 cannot be fully encapsulated into the macrocyclic receptors and thus shows the absence of binding allosteric behaviors. Additionally, Nabeshima and co-workers designed self-assembled macrocycle 27,44 with an interplanar distance of 7.2 Å for the cofacial bis(terpyridyl)butadiene moieties (Figure 10). Non-covalent complexation between 27 and pentacene as the guest in a 1:1 molar ratio takes place (Ka

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INFINITE SUPRAMOLECULAR MULTICOMPONENT ARCHITECTURES Inspired by the fascinating structure of Magnus’ green salt, we and others have sought to develop infinite supramolecular D− A copolymers with the involvement of non-covalent Pt(II)··· Pt(II) metal−metal forces.18,45,46 Specifically, we have designed positively charged platinum(II) terpyridine complex 28b as the acceptor monomer (Figure 11).18 It self-assembles in apolar 95:5 v/v MCH/DCE (MCH = methylcyclohexane, DCE = 1,2-dichloroethane) through intermolecular π−πstacking and van der Waals interactions. As a result, onedimensional nanofibers form via the nucleation−elongation cooperative mechanism.23 Notably, Pt(II)···Pt(II) metal− metal forces are ruled out for the neighboring alkynylplatinum(II) terpyridine units on 28b because of the presence of H



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sterically bulky tert-butyl units. Upon addition of monomer 29a (Figure 11) to 28b, intermolecular D−A CT interactions tend to form because of their similar π surfaces. Simultaneously, the reduced steric hindrance between 28b and 29a gives rise to the formation of Pt(II)···Pt(II) metal−metal interactions, which enhance the heterocomplexation strength over the homocomplexation one. As a consequence, the alternate arrangement for coassembly 28b/29a exists as the thermodynamic stable state, which is proven to follow the isodesmic coassembly mechanism.23 Intriguingly, the MMLCT absorption bands for coassembly 28b/29a are shifted to the lower energies (ranging from 550 to 700 nm). Such an “emergence upon coassembly” property is applied for red-lightirradiated oxidation of α-terpinene to ascaridole, which is unattainable for the individual species. In a more recent study, we have also investigated the coassembly behaviors of 28a and 29b (Figure 11). Monomer 28a is totally insoluble in apolar 95:5 decane/DCE, while 29b forms a yellow-colored solution. When they are mixed equivalently, two-component darkcolored metallogels are prone to form with a critical gelation value of 12 mM. The value can be decreased to 3 mM when 28a is replaced by a bis[alkynylplatinum(II) terpyridine]-based cross-linking agent. Che and co-workers have reported an interesting twocomponent supramolecular system that exhibits a morphological transition phenomenon. The design principle relies on the subtle interplay between Pt(II)···Pt(II) and electrostatic interactions. Specifically, the neutral monomer 30 and cationic monomer 31 (Figure 12) are prone to coassemble into nanowires in concentrated acetone solution.47 When the chloride unit on 30 is replaced by an acetonitrile ligand, the coassembly undergoes the wire-to-wheel metamorphism process. Higher reaction temperature and excess amount of NH4PF6 are beneficial for the evolution to the final nanowheel structure. Hence, it is feasible to construct submicrometersized nonlinear superstructures via a multicomponent coassembly strategy.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Feng Wang: 0000-0002-3826-5579 Author Contributions †

Z. Gao and Y. Han contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies Zhao Gao received his bachelor’s degree in chemistry from Chang’an University in 2014. He then joined in Prof. Feng Wang’s group at the University of Science and Technology of China to pursue his Ph.D. in chemistry. His current research interests are concentrated on responsive supramolecular organoplatinum materials. Yifei Han got his master’s degree in organic chemistry from the Central South University of China in 2015. After graduation, he joined Prof. Feng Wang’s group at the University of Science and Technology of China as a research assistant. His current interests are mainly focused on platinum-based molecular tweezers and their host− guest recognition behaviors. Zongchun Gao received his bachelor’s degree in polymer materials and engineering from Anhui University in 2015. He then joined Prof. Feng Wang’s group at the University of Science and Technology of China to pursue his Ph.D. in chemistry. His current research interests are focused on the construction and functionalization of chiral organometallic assemblies. Feng Wang received his Ph.D. in chemistry from Zhejiang University under the supervision of Prof. Feihe Huang in 2009. He joined Prof. E. W. Meijer’s group at the Eindhoven University of Technology as a postdoctoral fellow. He then moved to the University of Science and Technology of China as an associate professor in 2011 and was promoted to full professor in 2016. His current research interests are focused on supramolecular organometallic assemblies for biomedical, optoelectronic, and catalytic applications.

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CONCLUSIONS AND PERSPECTIVES Platinum(II) terpyridine complexes with square-planar geometry and fascinating photophysics represent ideal building blocks for multicomponent coassembled architectures. When they are incorporated into preorganized molecular tweezers and macrocycles, the resulting supramolecular receptors tend to encapsulate electron-rich guests, thereby providing convenient access to discrete D−A coassemblies with defined πaromatic units. Meanwhile, infinite multicomponent assemblies have been successfully obtained with long-range order by delicately balancing non-covalent driving forces between homo- and heterocomplexation processes. Stimuli-responsive elements can be further embedded into these supramolecular assemblies, facilitating “on-demand” switching of their properties and functionality at the macroscopic level. For future studies in this research field, more attention should be paid to the polymorphism (thermodynamic vs kinetically trapped states) of multicomponent coassembled systems.48 Moreover, the pursuit of potential applications for multicomponent assemblies in biomedical, optoelectronics, energy conversion, and catalytic fields, together with the establishment of structure−functionality relationships, should also be emphasized.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21674106 and 21871245), the CAS Youth Innovation Promotion Association (2015365), and the Fundamental Research Funds for the Central Universities (WK3450000004).

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Multicomponent Assembled Systems Based on Platinum(II

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