Precise Modulation of Molecular Building Blocks from Tweezers to

Phosphorescent molecular tweezers have been constructed from the alkynylplatinum(II) terpyridine moiety to demonstrate host–guest interactions with ...
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Precise Modulation of Molecular Building Blocks from Tweezers to Rectangles for Recognition and Stimuli-Responsive Processes Published as part of the Accounts of Chemical Research special issue “Supramolecular Chemistry in Confined Space and Organized Assemblies”. Alan Kwun-Wa Chan and Vivian Wing-Wah Yam*

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Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China CONSPECTUS: Alkynylplatinum(II) terpyridine complexes have been increasingly explored since the previous decades, mainly arising from their intriguing photophysical properties and aggregation affinities associated with their extensive Pt(II)···Pt(II) and π−π stacking interactions. Through molecular engineering, one can modulate their fundamental properties and assembly behavior by introduction of various functional groups and structural features. They can therefore serve as ideal candidates to construct metal complex-based molecular architectures to provide an alternative to organic compounds. The metal-based framework can be simultaneously built from predetermined building blocks, giving rise to their well-defined, unique, and discrete natures for molecular recognition. The individual constituents can contribute to molecular architectures with their integrated properties, allowing the manipulation of the various noncovalent intermolecular forces and interactions for selective guest capture. In this Account, our recent progress in the development of these metallomolecular frameworks based on the alkynylplatinum(II) terpyridine system and their recognition properties toward different guest molecules will be presented. Phosphorescent molecular tweezers have been constructed from the alkynylplatinum(II) terpyridine moiety to demonstrate host−guest interactions with cationic, charge-neutral and anionic platinum(II), palladium(II), gold(I), and gold(III) complexes and their binding affinities were found to be perturbed by different metal···metal, π−π and electrostatic interactions. The host− guest assembly process has also resulted in dramatic color changes, together with the turning on of near-IR (NIR) emissions as a result of extensive Pt(II)···Pt(II) interactions. Further work has also been performed to demonstrate that the tweezers can selectively recognize π-surfaces of different planar π-conjugated organic guests. The framework of molecular tweezers has been extended to a double-decker tweezers structure, or a triple-decker structure, which can bind two equivalents of square-planar platinum(II) guests cooperatively to induce a significant color change in solution, representing rare examples of discrete Magnus’ green-like salts. By the approaches of structural modifications, we have further modulated the host architecture from molecular tweezers to molecular rectangles. The rectangles have been found to show selective encapsulation of different transition metal complex guests based on the size and steric environment of the host cavity. The molecular rectangles also exhibit reversible host−guest association, in which guest capture and ejection processes can be manipulated by the pH environment, illustrating a potential approach for precise and smart delivery of therapeutic reagents to the slightly more acidic cancer cells.

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terpyridine alkynyl complexes,9 which were found to have improved solubility and room-temperature solution-state luminescence compared to the chloro- counterpart.9,10 This system with attractive features has been further utilized to respond to various external stimuli, such as pH11 and metal ion-binding12 based on photoinduced electron transfer (PeT) luminescence-ON sensing,12 as well as variation in solvent

INTRODUCTION

Among the various transition metal complex systems, squareplanar d8 platinum(II) complexes have been considered as an important candidate with extensive exploration due to their strong tendency to exhibit metal···metal interactions and rich photophysical properties.1−10 Thanks to the square-planar coordination geometry, platinum(II) polypyridine complexes can exhibit rich solid state polymorphism,7,8 with facile access to Pt(II)···Pt(II) interactions and π−π interactions. In 2001, we reported the first preparation and isolation of platinum(II) © XXXX American Chemical Society

Received: July 7, 2018

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Accounts of Chemical Research Scheme 1. Strategic Molecular Design of Platinum(II) Hosts for Various Functional Properties and Applications, Demonstrating the Key Concept of “Function-by-Design”

composition,13 addition of ionic12 and polymeric species, and14 changes in pH15,16 and temperature.17,18 They can display drastic spectral changes induced by the formation of Pt(II)··· Pt(II) and π−π interactions. The aggregation behavior of the alkynylplatinum(II) terpyridine complexes, associated with the latter sensing mechanism, has been further applied to uniquely identify versatile biomolecules or biologically relevant molecules, such as single-strand DNA,19 heparin,20 thrombin,21 lysozyme,21 human serum albumin,22 kinase,23 phosphatase,23 ATP,23 G-quadruplex,24 trypsin,25 and sugars.26 These unique spectral and responsive features render the alkynylplatinum(II) terpyridine complexes as molecular receptors and spectroscopic reporters to detect a variety of compounds.27 The versatility offered by the platinum(II) system and the diverse nature of ligands has resulted in 2D and 3D building blocks for molecular recognition processes. The precise regulation over their dimensions and shapes, together with a large variety of available coordinating ligands, can control the size of voids for molecular recognition from angstroms to nanometers. The polarity and charge state of the desired molecular building blocks can also be readily perturbed by changing the corresponding functional groups. Generally, molecular recognition can be dominated by the structure of the host as well as the intermolecular interactions that exist in the host−guest adduct.28 Therefore, the understanding and manipulation of such forces for selective guest capture constitute a major challenge for supramolecular host−guest chemistry. There are various kinds of supramolecular host− guest associations, including electrostatic,29 ion-dipole,30 dipole−dipole,31 hydrogen bonding,32 cation−π,33 CH−π,34 π−π stacking,34 metal···metal,27 van der Waals,35 and donor− acceptor interactions36 on top of metal−ligand coordination.36 The receptor cavity of the host and the dimensions of the guest are also critical for these intermolecular interactions to operate. By the integration of unique optical properties and aggregation affinities of the alkynylplatinum(II) terpyridine complex, together with precise control of the shape, rigidity as well as the functional group at the ligand backbone, one can design platinum(II)-containing molecular architectures for selective guest capture that can be reported by distinctive color changes and luminescence enhancement.37 In this Account, we illustrate the rational molecular design that evolves from phosphorescent molecular tweezers to

double-decker tweezers and eventually to molecular rectangles of different topologies utilizing the alkynylplatinum(II) terpyridine motif as the receptor site for molecular recognition (Scheme 1). To allow an effective binding toward versatile square-planar guests, an optimal separation between the two platinum(II) binding sites of about 7 Å is required.38 In order to rationalize the contribution from the various intermolecular host−guest interactions, square-planar cationic, charge-neutral, and anionic platinum(II), palladium(II), gold(I), and gold(III) complexes as well as aromatic molecules of different π-surfaces have been utilized as guest molecules for systematic comparison. The pH-sensitive functionalities on the molecular backbone can be further modulated to achieve the reversible host−guest interaction, illustrating the multiaddressable features for the platinum(II)-containing molecular architectures. These works also describe our efforts for the “proof-ofprinciple” demonstration of organized molecular assemblies in confined space recognition, illustrating their potential for the construction of smart and multifunctional systems with unique optical properties.

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ALKYNYLPLATINUM(II) TERPYRIDINE MOLECULAR TWEEZERS Molecular tweezers are synthetic molecular receptors that contain an open cavity decorated with two interaction sites for guest binding and bridged by a spacer.39 By controlling the flexibility of the tweezers, complementary advantages can be provided toward molecular recognition. The molecular tweezers with flexible linkers can provide versatility in the guest capture process, offering multiple conformations that can be adopted by the tweezers.39 A signal can be detected upon conformational guest binding interactions for sensing purposes. Unfortunately, complete rotational freedom of the spacer has limited the binding affinity because of the entropic loss associated with reduction of conformational states upon guest binding. The entropic cost can be partially compensated by conformational restrictions, which can be provided by flexible spacers connected to rigid aromatic scaffolds of the binding site.40 To achieve more selective guest capture, rigid molecular clefts constructed by square-planar chloropalladium and chloroplatinum(II) terpyridine moieties have also been B

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Accounts of Chemical Research reported previously.38 An optimal separation between the two binding sites of about 7 Å has been determined for the effective binding toward charge-neutral square-planar metal complex guests. However, the anionic metal complex guests were found to give intractable precipitates upon the association with the host molecules. Such solubility problems precluded host−guest studies for the anionic complex systems. To overcome this problem, we have utilized the alkynylplatinum(II) terpyridine moiety as the binding site in the U-shape molecular tweezers (Figure 1).37 In addition to its better solubility, the motif

in nature with interplanar distances of 6.781−7.143 Å, making them spatially capable of serving as hosts for the insertion of planar molecules. The alkynylplatinum(II) terpyridine molecular tweezers have also been illustrated as the first phosphorescent molecular tweezers, serving as a versatile host to detect different molecules (Figure 1). Thanks to the improved solubility, it allows the systematic study of their host−guest interactions with cationic, charge-neutral and also anionic platinum(II), palladium(II), gold(I), and gold(III) complexes via drastic color and luminescence changes. From the binding affinity and the extent of energy shift, a rationalization of the influences from the metal center, πconjugation, charge, and molecular structure of the guest molecules can be achieved. The addition of a charge-neutral [Pt(C∧N∧C)(CN− C6H4−OMe-p)] guest into the molecular tweezers has been found to cause a tremendous color shift from yellowish-orange to deep red, showing a growth of a low-energy absorption band at ca. 570−620 nm in the UV−vis absorption spectra (Figure 2).37 The newly formed low-energy absorptions have been attributed to a metal−metal-to-ligand charge-transfer (MMLCT) transition due to the formation of Pt(II)···Pt(II) and π−π interactions associated with the host−guest interaction (Figure 2). On the other hand, the guest addition also leads to emission spectral changes with a decrease in the 3 MLCT emission intensity at 612 nm with a concomitant enhancement of emission band at 772 nm, which is ascribed to the triplet MMLCT excited state associated with the Pt(II)··· Pt(II) and π−π interactions.37 Similar spectroscopic responses have been found in the host−guest interactions with the anionic [Pt(C∧N∧C)(CC−C6H4−OMe-p)]NBu4 guest while other structural analogues, such as the charge-neutral [Pt(C∧N∧C)(DMSO)] with a bulky DMSO ligand, anionic [Pt(O∧N∧O)Cl](NBu4) with a less π-conjugated ligand, cationic [Pt(N∧N∧N)Cl](PF6), cationic [Pd(N∧N∧N)Cl]-

Figure 1. Molecular structures of phosphorescent alkynylplatinum(II) terpyridine tweezers and the platinum guest complexes. Reproduced with permission from ref 37. Copyright 2012 The Royal Society of Chemistry.

possesses enhanced spectroscopic features, including lowenergy absorptions and room temperature phosphorescence in solution states relative to the nonemissive chloro- counterparts, and the capability of turning on low-energy absorption and emission bands upon host−guest interaction. As illustrated in the X-ray crystal structure, two Pt(II) terpyridine binding units are aligned in a face-to-face geometry and are almost coplanar

Figure 2. Host−guest association of the alkynylplatinum(II) terpyridine molecular tweezers with [Pt(C∧N∧C)(CN−C6H4−OMe-p)] with the drastic color and luminescence changes. Reproduced with permission from ref 37. Copyright 2012 The Royal Society of Chemistry. C

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Accounts of Chemical Research (PF6) and charge-neutral [Au(C∧N∧C)(CC−C6H4−OMep)], have been alternatively found to show a drop in the absorbance of the MLCT/LLCT band with no significant changes in the low-energy absorption tail in the UV−vis studies. Concomitant with these absorption changes is the reduction in the 3MLCT emission intensity upon host−guest association.37 The results illustrate the capability of the alkynylplatinum(II) terpyridine molecular tweezers in differentiating the structure and dimension of guest molecules. A 1:1 binding stoichiometry between the alkynylplatinum(II) terpyridine molecular tweezers and the various metal complex guests has been determined by the Job’s method of continuous variation, ESI-mass spectroscopy and 2D NMR studies. Relatively larger binding constants (KS) of over 105 M−1 have been observed for the host−guest interactions of the molecular tweezers with the charge-neutral and anionic platinum(II) guest complexes, suggesting their relatively strong binding affinity. Upon careful and systematic comparison of the binding constants between the molecular tweezers with the cationic, charge-neutral, and anionic guests with different metal centers and π-conjugation, it is found that there are cumulative contributions from Pt(II)···Pt(II), π−π stacking, and electrostatic interactions (Figure 3). The strong host−guest

other metal centers and the cationic platinum(II) host, it also demonstrates a rare example of discrete platinum(II) double salts in solution state and the possible formation of intermolecular Pt(II)−M heterometallic interactions.

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ALKYNYLPLATINUM(II) TERPYRIDINE TWEEZERS WITH POLYAROMATIC HYDROCARBONS As the functions of the molecular host can be influenced by the guest panel units, it is also vital to explore the effect of intercalation of various planar guest molecules, which may perturb both the redox and optical properties upon host−guest adduct formation. Besides the square-planar metal complex guests, further works have been elaborated to study the host− guest interactions between the alkynylplatinum(II) terpyridine tweezers and different planar π-conjugated organic guests including polyaromatic hydrocarbons (PAHs) (Figure 4).42 This has allowed the rationalization and quantification of the effect of π-surface and substituent groups on the host−guest interactions. More importantly, such studies have provided insights into the control and fine-tuning of their excited features induced by guest binding. Host−guest binding studies of the platinum(II) molecular tweezers with a variety of planar aromatic guests such as benzene, naphthalene, anthracene, pyrene, acridine, triphenylene, perylene, coronene, and 9-methylcarbazole have been performed. A 1:1 binding stoichiometry between the molecular tweezers and the planar aromatic guests has been determined by the NMR Job’s method and ESI mass spectrometry.42 The 2D NOESY NMR studies also suggest the complete insertion of the planar aromatic guests into the cavity of the molecular tweezers to maximize the π−π interactions (Figure 4).42 The binding constants and Gibbs free energies (ΔG) have been found to gradually increase in the order of benzene < naphthalene < anthracene < pyrene < triphenylene < perylene < coronene, according to the UV−vis and emission titration studies (Figure 5).42 This indicates that stronger host−guest interactions result from an increase in the extent of π-surface area of the guest molecules, suggesting the significant contribution from π−π interactions in the association process. A plot of changes of free energy of host−guest association versus the number of double bonds in the resonance structures of aromatic guests, which is a reflection of the π-surface area, shows an increase in −ΔG value with increasing π-surface area from anthracene to coronene, which eventually reaches a plateau. This can be attributed to the limited π-surface area of the alkynylplatinum(II) terpyridine moieties for effective π−π interactions with the aromatic guest. Substituent effects of aromatic guests toward the host−guest interactions have also been explored for a series of 9substituted anthracenes with the −OMe, −NH2, −NO2, −CN, −Br, and −CHO substituent groups. A Hammett plot has been constructed to correlate the ΔG for host−guest association with the Hammett’s parameters of the substituent groups based on their electron-donating properties. For the −NH2, −OMe, −Br, and −NO2 substituted as well as the unsubstituted derivatives, a reasonably linear Hammett plot has been obtained with a ρ value of 1.27 (Figure 5).42 However, the −CHO and −CN substituted derivatives have resulted in more negative ΔG values than expected while the −OMe and −COMe group derivatives give less negative ΔG values from the linear plot. For the −CHO and −CN substituted derivatives, the findings have been explained by their extended π-conjugation that would participate in the

Figure 3. Binding constants for host−guest association of the alkynylplatinum(II) terpyridine molecular tweezers with various square-planar metal complex guests. Reproduced with permission from ref 37. Copyright 2012 The Royal Society of Chemistry.

interaction of the current system has also been assisted by the tweezers effect of its preorganized rigid structure, which enhances the intermolecular interaction. More interestingly, the addition of a negatively charged linear [Au(CCPh)2](PPN) guest to the host solution leads to a drop in the 3MLCT emission together with turning-on of a lower-energy emission from 600 to 800 nm. Such a phenomenon cannot be observed in the host−guest interactions between the molecular tweezers and the charge-neutral square-planar Au(III) guest. Even though the heterometallic Pt(II)−Au(I) interaction is expected to be weak because of the poor energy match of the frontier orbitals of the Pt(II) and Au(I) center,41 the growth of a low-energy emission band has been ascribed to the contribution of both heterometallic mixed-metal interactions and ligand π−π excimeric state. The very specific phosphorescent alkynylplatinum(II) molecular tweezers have illustrated their capability in associating with various square-planar d8 and linear d10 metal complex guests that results in unique colorimetric and luminescence responses, perturbed by metal···metal, π−π stacking and electrostatic interactions. With anionic platinum(II) guests or guests of D

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Figure 4. Host−guest association of the alkynylplatinum(II) terpyridine molecular tweezers with planar polyaromatic hydrocarbon (PAH) guests showing the association mode with coronene in 1:1 binding stoichiometry from NMR studies. Reproduced with permission from ref 42. Copyright 2013 Wiley-VCH.

Figure 5. Plot of changes of free energy of host−guest interaction versus the number of double bonds in the resonance structures of aromatic guests (left); and plot of changes of free energy of host−guest interaction at 298 K versus Hammett parameter (σp) (right). Reproduced with permission from ref 42. Copyright 2013 Wiley-VCH.

Figure 6. Molecular structures of triple-decker alkynylplatinum(II) terpyridine complexes and the X-ray crystal structure of the mutually intercalated complex cations. Reproduced with permission from ref 43. Copyright 2013 Wiley-VCH.

host−guest π−π interaction to give the more negative ΔG values at 298 K. As for the case of −OMe and −COMe

derivatives, the less negative ΔG values have been ascribed to the nonplanarity of the substituent group that leads to steric E

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Figure 7. Host−guest interactions of triple-decker hosts with two equivalents of neutral platinum guest to form the deep green pentanuclear Pt5 adduct; and the corresponding UV−vis and emission titrations studies. Reproduced with permission from ref 43. Copyright 2013 Wiley-VCH.

longer oligophenylenepyridylene backbone to connect to three alkynylplatinum(II) terpyridine interacting units (Figure 6).43 The triple-decker structure possesses two binding sites for the accommodation of square-planar metal complex guests, representing rare examples of models of discrete stacked arrays similar to that of the Magnus’ green-like salts. As revealed by the X-ray crystal structure of the triple-decker host,43 the complex cations have been found to pack in a dimeric form with the mutual intercalation of two symmetrical complexes, resulting in the relatively short Pt(II)···Pt(II) distances of 3.432 Å and interplanar separations of 3.436 Å (Figure 6).43 This indicates that the triple-decker host can favorably undergo self-association or dimerization in the solid state as driven by the extensive Pt(II)···Pt(II) and π−π interactions. It is also interesting to note that the addition of neutral [Pt(C ∧ N ∧ C)(CN−C 6 H 4 −OMe-p)] and anionic [Pt(C∧N∧C)(CC−C6H4−OMe-p)]NBu4 as the guest into the triple-decker host exhibits more dramatic color and luminescence response when compared to the previous molecular tweezers with single binding site (Figure 7).43 New lowerenergy absorptions at around 500−750 nm have been observed and this can be ascribed to the MMLCT transition due to the formation of Pt(II)···Pt(II) and π−π interactions upon host− guest association. There is also emergence of 3MMLCT emission band at about 770−790 nm, which is also much lower in energy compared to the previous molecular tweezers host− guest adducts (Figure 7).43 By the Job’s plot of continuous variation, a 1:2 stoichiometric binding ratio has been found, indicating the simultaneous capture of two platinum guests at the binding site. The relatively low energy of the MMLCT absorption and the 3MMLCT emission associated with the host−guest interactions, as compared to the previously reported tweezers with trinuclear array of platinum(II) host− guest adducts, suggests that significant Pt(II)···Pt(II) inter-

repulsions. These suggest that the alkynylplatinum(II) terpyridine tweezers are not only sensitive to the electronic properties but also to the π-surface area and steric factors, leading to more selective guest recognition.42 It is also interesting to observe the quenching of the 3MLCT emission of the platinum(II) molecular tweezers upon the addition of the PAH guests. In order to find out the origin of such phenomenon, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) and the triplet energies of the tweezers and the aromatic guests have been determined to work out the quenching mechanism.40 It has been found that the quenching processes in some cases can occur via reductive electron-transfer reaction with negative ΔG values. On the other hand, the triplet state energies of perylene (1.56 eV), anthracene (1.85 eV), pyrene (2.09 eV), and coronene (2.36 eV) guests have been determined to be relatively lower than that of the platinum(II) molecular tweezers (2.38 eV). Therefore, quenching through the energy transfer mechanism from the platinum(II) tweezers to these aromatic guests in the adduct has also been suggested.42

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DOUBLE-DECKER MOLECULAR TWEEZERS In view of the strong binding affinities and selectivity of the alkynylplatinum(II) terpyridine tweezers toward planar guest molecules with drastic color and luminescence responses, attempts have also been made to modify the molecular tweezers motif such that it can accommodate two or more guest molecules in the pockets in a double-sandwich fashion for the construction of high-order oligomers in a controlled and well-defined manner. To reach this goal, the framework of the molecular tweezers has been extended to a double-decker tweezers structure, or a triple-decker structure by using a F

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Figure 8. Molecular structures of alkynylplatinum(II) terpyridine rectangles with different cavity size and rigidity. Reproduced with permission from ref 46. Copyright 2015 Proceedings of the National Academy of Sciences of the United States of America.

Figure 9. UV−vis absorption and emission spectral changes upon addition of neutral platinum guest into the molecular rectangles. Insets show the color/emission changes upon addition of guest. Reproduced with permission from ref 46. Copyright 2015 Proceedings of the National Academy of Sciences of the United States of America.

molecular arrays for future partial oxidation to give chargetransporting properties along the direction of the mixedvalence metal centers, opening up a new research dimension in conducting materials and metal−organic field effect transistors (MOFETs) in a well-defined and controlled fashion. Alternatively, light-induced charge transport upon MLCT or MMLCT excitation may lead to photoconducting properties through the generation of a hole in the metal chain and an electron along the π-stack of the polypyridine ligand.

actions extending across the multiple platinum centers exist in the current pentanuclear platinum(II) adduct, representing rare examples of models similar to discrete Magnus’ green-like salts. In addition, the binding constants and the Hill coefficient (n) have been determined by the Hill’s plot based on the 1:2 host−guest associations in the triple-decker structure.43 The binding constants have been found to give relatively large values of over 1013 M−2 with positive cooperative effects (n = 1.4−1.7). The positive cooperative effect has been attributed to structural features of the triple-decker framework in that the binding of first guest would cause the rigidification and thus provide preorganized structure for the binding of a second guest. These special features of the triple-decker hosts also represent a rare example of an allosteric effect directed by metal−metal interactions to organize the platinum(II) complexes of different charge natures. The cooperativity and preorganized framework of the triple-decker host, which is unique compared to other molecular hosts and receptors,29−37 can therefore be utilized to prepare discrete multinuclear platinum stacks based on the host−guest interactions with sophisticated platinum guests. These well-defined discrete multiunit platinum-polypyridine molecular stacks may serve as

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MULTIADDRESSABLE MOLECULAR RECTANGLES The recognition characteristics of supramolecular architecture arise from the “information” inherited from their individual building blocks and the structural intrinsic properties that are dictated by the diversity of shapes and dimensions.26−28 The open structures of the molecular tweezers have restricted their functionality for application purposes.28−31 It is believed that by rigidifying the molecular structure and restricting the cavity of the host from tweezers to rectangles, only certain guest molecules that can fit into the cavity would be better accommodated, which in turn could result in more selective guest capture under a definite size and steric environment.44,45 In addition, incorporation of various responsive functionalities G

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Figure 10. Association mode of the host−guest adduct formed between the molecular rectangle and the charge-neutral platinum complex guest determined by NMR studies and DFT calculations. Reproduced with permission from ref 46. Copyright 2015 Proceedings of the National Academy of Sciences of the United States of America.

Figure 11. Schematic diagram representing the reversible host−guest association with the addition of two equivalents of hydrochloric acid and then triethylamine into the host−guest adduct; and the graphical representation of the reversible host−guest association with lock and key mechanism. Reproduced with permission from ref 46. Copyright 2015 Proceedings of the National Academy of Sciences of the United States of America

The molecular rectangles have been shown to encapsulate square-planar platinum(II), palladium(II), gold(III), and the related gold(I) complexes and to exhibit guest-specific UV−vis absorption and emission changes. The addition of [Pt(C∧N∧C)(CN−C6H4−OMe-p)] into the molecular rectangle in an acetonitrile solution has led to a color change from yellow to orange-brown, with the growth of a lower-energy MMLCT absorption band at about 520−650 nm and a NIR 3 MMLCT emission at 760 nm that arises from Pt(II)···Pt(II) and π−π interactions in the 1:1 host−guest adduct (Figure 9).46 By the systematic studies of other metal complex guests in the host−guest interactions, their binding affinities have been determined to be perturbed by both metal···metal, π−π, and electrostatic interactions as well as the cavity size of the rectangle, leading to selectivity-tunable guest capture by simply perturbing and manipulating these intermolecular interactions and geometrical factors. Density functional theory calculations have also been performed to provide the optimized geometry

into the rectangle could lead to multiaddressable host−guest interactions and functions that could not have been easily achieved by the rather open tweezers structure.45 By engineering the molecular framework of the doubledecker or triple-decker host through the end-capping process, the work has been extended to a series of alkynylplatinum(II) terpyridine molecular rectangles with different geometries, topologies and electronic properties dedicated to responsive guest encapsulation (Figure 8).46 Attempts have been made to utilize pH-sensitive pyridine moieties in the molecular backbone in order to manipulate the reversible host−guest interaction upon protonation/deprotonation of the pyridine nitrogen atom for multiaddressable functions. Different platinum and gold guest complexes, which have been demonstrated to display anticancer therapeutic properties,47 have been employed as guest molecules, leading to a smart recognition system that could control the guest uptake and release processes under specific pH conditions as a “proof-ofprinciple” model for controlled drug delivery. H

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discrete stacked arrays, which are stabilized by metal···metal interactions and other intermolecular forces, aligned in a controllable and orderly fashion to provide important implications for achieving desirable materials properties and functions for charge transport, photoconductivity and photoinduced charge-separation for energy, catalysis, and lightharvesting applications. The research into the integrated features of luminescence and the supramolecular assembly of these platinum(II) molecular functional architectures would represent an innovative and exciting area of chemistry, and it is envisaged that, through continuing efforts in the rational design and study of the complexes, novel classes of metal-based functional materials will emerge in near future.

for the binding mode, which is in a good agreement with the results from NMR studies (Figure 10), indicative of the encapsulation of the guest within the cavity of the molecular rectangle.46 More interestingly, the proton resonances have been downfield-shifted and resolved into the individual host and guest signals respectively upon addition of hydrochloric acid toward the 1:1 host−guest adduct of the molecular rectangle with the [Pt(C∧N∧C)(CN−C6H4−OMe)-p] guest. The acid addition has also led to the vanishing of the low-energy MMLCT absorption band at 562 nm (from orange-brown to yellow in color) and the 3MMLCT emission band at 762 nm. These suggest the ejection of free guest from the host−guest adduct (Figure 11). The addition of triethylamine reveals the regeneration of the original resonance signals of the host− guest adduct as well as the recovery of the low-energy absorption and emission bands, indicating the recapture of the guest.46 The guest release process has been found to be triggered by the geometric deformation between the rectangle backbone and the terpyridine unit upon protonation. Given the charge-neutral nature of the guest, electrostatic interactions do not seem to play a significant role in the guest release process. The processes have also been examined to be reproducible upon at least four repeated cycles, indicating a robust and reversible system.46 By taking advantage of the pH-modulated reversible guest capture and release properties of the molecular rectangle, the design of “proof-of-principle” multiaddressable system by “lock and key” mechanism for reversible release of anticancer therapeutic platinum and gold metal complex guests can be envisioned in the targeted controlled drug delivery to cancer cells, which are usually more acidic than normal cells. This allows naked-eye monitoring of the guest binding and release processes.46 However, for the flexible molecular rectangles, the process has been found to be irreversible upon addition of triethylamine, which can be ascribed to their lack of rigidity to recapture and reconfine the guest molecules within the cavity. This also suggests that a rigid molecular structure would be vital in designing reversible systems for the responsive capture and release of therapeutic reagents.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Vivian Wing-Wah Yam: 0000-0001-8349-4429 Notes

The authors declare no competing financial interest. Biographies Alan Kwun-Wa Chan received his B.Sc. (Hons) in 2011 and his Ph.D. in 2015 from The University of Hong Kong. He is now serving as a Postdoctoral Fellow at the same university. His research interests include the design and synthesis of luminescent functional transition metal complexes and their supramolecular assembly properties, responsive behavior, as well as structure−property correlation. Vivian Wing-Wah Yam obtained both her B.Sc. (Hons) and Ph.D. degrees from The University of Hong Kong and is currently the Philip Wong Wilson Wong Professor in Chemistry and Energy and Chair Professor of Chemistry there. Her research interests include photophysics and photochemistry of transition metal complexes, supramolecular chemistry, and metal-based molecular functional materials for luminescence sensing, optoelectronics, optical and resistive memories, and solar energy research.

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ACKNOWLEDGMENTS V.W.-W.Y. acknowledges support from The University of Hong Kong under the URC Strategically Oriented Research Theme (SORT) on Functional Materials for Molecular Electronics. This work has been supported by the University Grants Committee Areas of Excellence Scheme (AoE/P-03/ 08) and a General Research Fund (GRF) grant from the Research Grants Council of Hong Kong Special Adminstrative Region, P.R. China (HKU 17334216).

CONCLUDING REMARKS AND FUTURE OUTLOOK To summarize, unique classes of molecular architectures from tweezers, double-decker tweezers to rectangles have been constructed based on the luminescent alkynylplatinum(II) terpyridine moieties. They are found to exhibit host−guest interactions with various square-planar complexes as well as planar aromatic compounds via color and luminescence changes determined by host−guest interactions. By modification of the molecular backbone and the fine interplay of Pt(II)···Pt(II), π−π, and electrostatic interactions, cooperative and selective guest uptake and release have been achieved. They can also serve as multiaddressable model systems to illustrate the capability of proof-of-principle reversible guest capture and release processes. Therefore, the nature of these materials can be fine-tuned by modulation of molecular structures to give rise to various applications. The studies not only have provided fundamental understanding on structure− property relationships that govern versatile luminescence behavior of these metal complexes, but also provide the guiding principles for the formation of noncovalent metal··· metal interactions and self-assembly driven by external stimuli. More importantly, they may lead to the construction of

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