Alan Kwun-Wa Chan

Multiaddressable molecular rectangles with reversible host-guest interactions: Modulation of pH-controlled guest release and capture

Significance There has been considerable interest in the construction of stimuli-responsive metallosupramolecular architectures because of their versatile structural and bonding properties, imparting them with the capability to exhibit specific functions. Despite a variety of metal–organic architectures reported, those involving noncovalent metal–metal interactions and luminescence changes that depend on the nature of the guests are rather underexplored. Herein, a series of alkynylplatinum(II) terpyridine molecular rectangles has been shown to exhibit reversible capture and release of anticancer therapeutic guests under different pH conditions with instant and drastic color and luminescence responses. The fundamental understanding of these host–guest interactions has led to the development of a unique proof-of-principle multiaddressable model system that illustrates the capability of reversible guest capture and release processes for therapeutic delivery. A series of multiaddressable platinum(II) molecular rectangles with different rigidities and cavity sizes has been synthesized by endcapping the U-shaped diplatinum(II) terpyridine moiety with various bis-alkynyl ligands. The studies of the host–guest association with various square planar platinum(II), palladium(II), and gold(III) complexes and the related low-dimensional gold(I) complexes, most of which are potential anticancer therapeutics, have been performed. Excellent guest confinement and selectivity of the rectangular architecture have been shown. Introduction of pH-responsive functionalities to the ligand backbone generates multifunctional molecular rectangles that exhibit reversible guest release and capture on the addition of acids and bases, indicating their potential in controlled therapeutics delivery on pH modulation. The reversible host–guest interactions are found to be strongly perturbed by metal–metal and π–π interactions and to a certain extent, electrostatic interactions, giving rise to various spectroscopic changes depending on the nature of the guest molecules. Their binding mode and thermodynamic parameters have been determined by 2D NMR and van’t Hoff analysis and supported by computational study.

Supramolecular Assembly of Isocyanorhodium(I) Complexes: An Interplay of Rhodium(I)···Rhodium(I) Interactions, Hydrophobic–Hydrophobic Interactions, and Host–Guest Chemistry

A series of tetrakis(isocyano)rhodium(I) complexes with different chain lengths of alkyl substituents has been found to exhibit a strong tendency toward solution state aggregation upon altering the concentration, temperature and solvent composition. Temperature- and solvent-dependent UV-vis absorption studies have been performed, and the data have been analyzed using the aggregation model to elucidate the growth mechanism. The aggregation is found to involve extensive Rh(I)···Rh(I) interactions that are synergistically assisted by hydrophobic-hydrophobic interactions to give a rainbow array of solution aggregate colors. It is noted that the presence of three long alkyl substituents is crucial for the observed cooperativity in the aggregation. Molecular assemblies in the form of nanoplates and nanovesicles have been observed in the hexane-dichloromethane solvent mixtures, arising from the different formation mechanisms based on the alkyl chain length of the complexes. Benzo-15-crown-5 moieties have been incorporated for selective potassium ion binding to induce dimer formation and drastic color changes, rendering the system as potential colorimetric and luminescent cation sensors and as building blocks for ion-controlled supramolecular assembly.

Supramolecular Self-Assembly and Dual-Switch Vapochromic, Vapoluminescent, and Resistive Memory Behaviors of Amphiphilic Platinum(II) Complexes

A series of amphiphilic platinum(II) complexes with tridentate N-donor ligands has been synthesized and characterized. Different supramolecular architectures are constructed using the amphiphilic molecules as the building blocks through the formation of Pt···Pt and π-π stacking interactions in aqueous media. The aggregation-deaggregation-aggregation self-assembly behavior together with obvious spectroscopic changes could be fine-tuned by the addition of THF in aqueous media. More interestingly, one of the complexes is found to show fast response and high selectivity toward alcohol and water vapors with good reversibility, leading to drastic color and luminescence changes, and hence unique dual switching behavior, with the water molecules readily displaced by the alcohol vapor. Rapid writing and erasure have been realized via the control of a jet or a stream of alcohol vapor flow. In addition, it has been employed as active materials in the fabrication of small-molecule solution-processable resistive memory devices, exhibiting stable and promising binary memory performance with threshold voltages of ca. 3.4 V, high ON/OFF ratios of up to 105 and long retention times of over 104 s. The vapochromic and vapoluminescent materials are demonstrated to have potential applications in chemosensing, logic gates, VOC monitoring, and memory functions.

Construction of Discrete Pentanuclear Platinum(II) Stacks with Extended Metal–Metal Interactions by Using Phosphorescent Platinum(II) Tweezers

Discrete pentanuclear PtII stacks were prepared by the host-guest adduct formation between multinuclear tweezer-type PtII complexes. The formation of the PtII stacks in solution was accompanied by color changes and the turning on of near-infrared emission resulting from Pt⋅⋅⋅Pt and π-π interactions. The X-ray crystal structure revealed the formation of a discrete 1:1 adduct, in which a linear stack of five PtII centers with extended Pt⋅⋅⋅Pt interactions was observed. Additional binding affinity and stability have been achieved through a multinuclear host-guest system. The binding behaviors can be fine-tuned by varying the spacer between the two PtII moieties in the guests. This work provides important insights for the construction of discrete higher-order supramolecular metal-ligand aggregates using a tweezer-directed approach.

Rhodium(I) Complexes of Tridentate N-Donor Ligands and Their Supramolecular Assembly Studies

New classes of tridentate N-donor rhodium(I) complexes have been synthesized and demonstrated to exhibit interesting induced self-assembly behavior by variation of external stimuli, as a result of extensive Rh(I)···Rh(I) interactions, with the assistance of π-π stacking and hydrophobic-hydrophobic interactions. An isodesmic aggregation mechanism has also been identified in the temperature-dependent process. Upon aggregation in acetone solution, the complex molecules form wire-like nanostructures with their shape dependent on the π-conjugation of the tridentate ligands. On the other hand, crystalline needles of rhodium(I) complexes obtained from recrystallization have also been shown to exhibit conductivity on the order of 10(-3) S cm(-1).

Versatile Control of Directed Supramolecular Assembly via Subtle Changes of the Rhodium(I) Pincer Building Blocks

Various rhodium(I) pincer complexes with different structural features have been prepared and found to display interesting self-assembly properties due to the extensive Rh(I)···Rh(I) interactions. The incorporation of electron-withdrawing -CF3 substituent has been found to improve the stability of the complexes and also facilitate the directed assembly of complex molecules, providing an opportunity for the systematic investigation of the various noncovalent interactions in their versatile self-assembly behaviors and insights into the structure-property relationship in governing the intermolecular interactions. An isodesmic growth mechanism is identified for the solvent-induced aggregation process. The complex molecules exhibit intense low-energy absorption bands corresponding to the absorptions of the dimers, trimers, and higher order oligomers upon aggregation, with energies related to the electronic properties of the tridentate N-donor ligand. Chiral auxiliaries have also been introduced into the rhodium(I) complexes to build up helical supramolecular assemblies and soft materials.

Adaptive Coordination-Driven Supramolecular Syntheses toward New Polymetallic Cu(I) Luminescent Assemblies

A thermally activated delayed fluorescence (TADF) tetrametallic Cu(I) metallacycle A behaves as a conformationally adaptive preorganized precursor to afford, through straightforward and rational coordination-driven supramolecular processes, a variety of room-temperature solid-state luminescent polymetallic assemblies. Reacting various cyano-based building blocks with A, a homometallic Cu(I) 1D-helical coordination polymer C and Cu8M discrete circular heterobimetallic assemblies DM (M = Ni, Pd, Pt) are obtained. Their luminescence behaviors are studied, revealing notably the crucial impact of the spin-orbit coupling offered by the central M metal center on the photophysical properties of the heterobimetallic DM derivatives.

Solvent-Induced and Temperature-Promoted Aggregation of Bipyridine Platinum(II) Triangular Metallacycles and Their Near-Infrared Emissive Behaviors

A series of bipyridine platinum(II) complexes with different sizes of triangular metallacycles and alkyl/oligoether chains has been synthesized and characterized. They are packed in a zig-zag fashion with the formation of dimeric structures according to their X-ray crystal structures. Different emission origins are observed due to the different sizes of the triangular ligands. Their morphologies could be tuned by the modification of the molecular structures with different metallacyclic alkynyl ligands and alkyl/oligoether chains and solvents. More interestingly, unusual electronic absorption changes and upfield shifts of the aromatic proton resonances are observed upon increasing the temperature, suggesting further aggregation of the architectures. Near-infrared (NIR) emission is also realized through the tuning of the π-π stacking, Pt⋅⋅⋅Pt interactions, and the packing of planar metallacycles.

Amphiphilic Carbazole-Containing Compounds with Lower Critical Solution Temperature Behavior for Supramolecular Self-Assembly and Solution-Processable Resistive Memories

The self-organization and resistive memory performances of a series of newly synthesized water-soluble amphiphilic carbazole derivatives have been explored. Temperature-dependent UV/Vis absorption spectroscopy has been conducted to study the isodesmic self-assembly mechanism of the carbazole-containing compounds. This class of compounds also exhibits interesting lower critical solution temperature properties, which are sensitive to concentration and ionic additives. One of the compounds has been solution-processed and utilized as an active material in the engineering of resistive memory devices, exhibiting a switching voltage of about 3.9 V, a constant ON/OFF current ratio of 106 , and a long retention time of 104  s. The present work demonstrates the versatile potential applications of water-soluble amphiphilic carbazole-containing compounds in supramolecular chemistry and resistive memory devices.