Geometrical manipulation of complex supramolecular tessellations by hierarchical assembly of amphiphilic platinum(II) complexes.

Here we report complex supramolecular tessellations achieved by the directed self-assembly of amphiphilic platinum(II) complexes. Despite the twofold symmetry, these geometrically simple molecules exhibit complicated structural hierarchy in a columnar manner. A possible key to such an order increase is the topological transition into circular trimers, which are noncovalently interlocked by metal···metal and π-π interactions, thereby allowing for cofacial stacking in a prismatic assembly. Another key to success is to use the immiscibility of the tailored hydrophobic and hydrophilic sidechains. Their phase separation leads to the formation of columnar crystalline nanostructures homogeneously oriented on the substrate, featuring an unusual geometry analogous to a rhombitrihexagonal Archimedean tiling. Furthermore, symmetry lowering of regular motifs by design results in an orthorhombic lattice obtained by the coassembly of two different platinum(II) amphiphiles. These findings illustrate the potentials of supramolecular engineering in creating complex self-assembled architectures of soft materials.

Supramolecular assembly of bent dinuclear amphiphilic alkynylplatinum(ii) terpyridine complexes: diverse nanostructures through subtle tuning of the mode of molecular stacking.

A new class of bent amphiphilic alkynylplatinum(ii) terpyridine complexes was found to adopt different modes of molecular stacking to give diverse nanostructures. In chlorinated solvents, the complexes aggregate in the presence of water droplets and assist in the formation of porous networks, while in DMSO solutions, they self-assemble to give fibrous nanostructures. The complexes would adopt a head-to-tail tetragonal stacking arrangement, as revealed by X-ray crystallographic studies, computational studies and powder X-ray diffraction (PXRD) studies. Their self-assembly follows a cooperative growth mechanism in DMSO and an isodesmic growth mechanism in DMSO-H2O mixture. The balance between hydrophobic and hydrophilic components of the complex system, in conjunction with the nuclearity and the positioning of the substituents, are found to govern the mode of molecular stacking and the fabrication of precise functional nanostructures. This class of complexes serve as versatile building blocks to construct orderly packed molecular materials and functional materials in a well-controlled manner.

Platinum(II) Probes for Sensing Polyelectrolyte Lengths and Architectures.

Platinum(II) polypyridine complexes of a square-planar geometry have been used as spectroscopic reporters for quantification of various charged species through non-covalent metal-metal interactions. The characterization of molecular weights and architectures of polyelectrolytes represents a challenging task in polymer science. Here, we report the utilization of platinum(II) complex probes and non-covalent metal-metal interactions for sensing polyelectrolyte lengths and architectures. It is found that the platinum(II) probes can bind to linear polyelectrolytes via electrostatic attractions and give rise to significant spectroscopic changes associated with the formation of metal-metal interactions, and the extent of the spectroscopic changes is found to increase with the lengths of the linear polyelectrolytes. Besides, the platinum(II) probes have been found to co-assemble with the linear polyelectrolytes to form well-defined nanofibers, and the lengths of the linear polyelectrolytes can be directly estimated from the diameter of the nanofibers under transmission electron microscopy observation. Interestingly, upon mixing with the platinum(II) probes, polyelectrolytes with bottlebrush architectures have been found to exhibit larger spectroscopic changes than linear polyelectrolytes with the same chemical composition. Combined with the reported theoretical studies on counterion condensation of polyelectrolytes, the platinum(II) complexes are found to function as spectroscopic probes for sensing the charge densities of the polyelectrolytes with different lengths and diverse architectures. Moreover, platinum(II) probes pre-organized in nanostructured aggregates have been found to intercalate into double-stranded DNA, which are naturally occurring biological polyelectrolytes with helical architectures and intercalation sites, to give significant enhancement of spectroscopic changes when compared to the intercalation of monomeric platinum(II) probes into double-stranded DNA.

Multiresponsive Luminescent Cationic Cyclometalated Gold(III) Amphiphiles and Their Supramolecular Assembly.

A new class of amphiphilic tridentate cyclometalated gold(III) complexes has been designed and synthesized as luminescent supramolecular building blocks. Positively charged trimethylammonium (-CH2NMe3+) containing alkynyl ligands have been incorporated to introduce the electrostatic interactions. The X-ray crystal structures of two of the complexes have been determined, and the existence of π-π interactions between molecules has been observed. Steady-state and time-resolved absorption and emission studies have been carried out to investigate the nature of the excited states. The complexes are found to exhibit self-assembly properties with the assistance of π-π stacking and hydrophobic interactions and possibly weak Au···Au interaction, resulting in notable emergence of low-energy absorption bands and luminescence changes. The presence of a large hydrophobic moiety is found to be crucial for the formation of aggregates, especially in polar media where hydrophobic interactions play an important role. The nature of the counterion has been shown to have a significant effect on the extent of self-assembly in different media. Upon aggregation, nanofibers are formed in polar media, while nanorods are observed in nonpolar media in one of the representative complexes. Interestingly, a small modification on the alkynyl ligand resulted in the formation of nanoribbons instead. Intriguing luminescence mechanochromic properties have also been observed. This orthogonal and rational molecular design strategy has been shown to be effective in the construction of gold(III)-based smart and multiresponsive materials.

Rational Design of Multi-Stimuli-Responsive Scaffolds: Synthesis of Luminescent Oligo(ethynylpyridine)-Containing Alkynylplatinum(II) Polypyridine Foldamers Stabilized by Pt···Pt Interactions.

A series of oligo(ethynylpyridine)-containing alkynylplatinum(II) terpyridine/bzimpy (bzimpy = 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine) metallofoldamers has been designed and synthesized to investigate the potential applications of metallofoldamers imparted by the rich spectroscopic responses of Pt···Pt interactions. Apart from the control of the folding/unfolding processes by solvent compositions and temperatures, this class of metallofoldamers has also been found to exhibit reversible folding/unfolding behaviors mediated by the addition of acids/bases due to the incorporation of the acid-sensitive oligo(ethynylpyridine) derivatives. More importantly, the intramolecular Pt···Pt interaction has been found to play a crucial role in governing the folded state conformation. The conformation of this class of metallofoldamers has been investigated by 2D ROESY NMR, electronic absorption, and emission spectroscopy, which provide further insights into the rational molecular design and multidimensional control of metallofoldamers upon the application of various external stimuli, leading to the preparation of multi-stimuli-responsive materials for potential applications in material sciences.

Directional Self-Assembly and Photoinduced Polymerization of Diacetylene-Containing Platinum(II) Terpyridine Complexes.

A series of newly designed and synthesized diacetylene-containing platinum(II) terpyridine complexes exhibited intriguing self-assembly properties. Facilitated by Pt⋅⋅⋅Pt, π-π stacking, hydrogen-bonding and hydrophobic-hydrophobic interactions, these complexes are preorganized to readily undergo topochemical polymerization reactions upon photoirradiation. The in situ polymerization of the diacetylene units to form polydiacetylene, indicated by the UV/Vis spectral changes, gel permeation chromatography and dynamic light scattering, was found to alter their assembly behaviours, as revealed by TEM images.

Energy Landscape in Supramolecular Coassembly of Platinum(II) Complexes and Polymers: Morphological Diversity, Transformation, and Dilution Stability of Nanostructures.

Establishment of energy landscape has emerged as an efficient pathway for improved understanding and manipulation of both thermodynamic and kinetic behaviors of complicated supramolecular systems. Herein, we report the establishment of energy landscapes of supramolecular coassembly of platinum(II) complexes and polymers, as well as the fabrication of nanostructures with enhanced complexity and intriguing properties from the coassembly systems. In the energy landscape, coassembly at room temperature has been found to only allow the longitudinal growth of platinum(II) complexes and block copolymers into core-shell nanofibers that are the kinetically trapped products. Thermal annealing can switch on the transverse growth of platinum(II) complexes and block copolymers to produce core-shell nanobelts that are the thermodynamically stable nanostructures. The extents of the transverse growth are found to increase with thermal annealing temperatures, leading to nanobelts with larger widths. Besides, rapid quenching of a hot coassembly mixture to room temperature can capture intermediate nanobelt- block-nanofiber nanostructures that are metastable and capable of converting to nanobelts upon further incubation at room temperature. Moreover, sonication treatment has been found to couple with the energy landscape of the coassembly system and open a unique energy-driven pathway to activate the kinetically forbidden nanofiber-to-nanobelt morphological transformation. Furthermore, based on the established energy landscapes, nanosphere- block-nanobelt nanostructures with distinct segmented architectures have been fabricated by thermal annealing of the ternary mixture of platinum(II) complexes, block copolymers, and polymer brushes in a one-pot and single-step procedure. Finally, the nanobelts and nanosphere- block-nanobelt nanostructures are found to possess intriguing morphological stability against acid and dilution, exhibiting characteristics that are important for promising biomedical applications.

Controlling Self-Assembly Mechanisms through Rational Molecular Design in Oligo( p-phenyleneethynylene)-Containing Alkynylplatinum(II) 2,6-Bis( N-alkylbenzimidazol-2'-yl)pyridine Amphiphiles.

The systematic control over association mechanisms of self-assembled materials has been demonstrated through the rational design and synthesis of a series of amphiphilic dinuclear alkynylplatinum(II) bzimpy (bzimpy = 2,6-bis( N-alkylbenzimidazol-2'-yl)pyridine) complexes containing the shape-persistent oligo( p-phenyleneethynylene)s. Multistage morphological transformations from plates to fibers and to spherical nanostructures under different solvent compositions have been demonstrated. The subtle balances between multiple noncovalent interactions including Pt···Pt, hydrophobic, hydrophilic, and π-π stacking interactions are found to have profound impact on the supramolecular assembly of the system, in which a change in the association mechanism from isodesmic to cooperative and back to isodesmic growth has been observed upon increasing hydrophilicity of the complexes.

Supramolecular assemblies of dinuclear alkynylplatinum(ii) terpyridine complexes with double-decker silsesquioxane nano-cores: the role of isomerism in constructing nano-structures.

Isomeric double-decker silsesquioxane-functionalized dinuclear alkynylplatinum(ii) terpyridine complexes demonstrate self-association behaviours via the stabilisation of hydrophobic, PtPt and/or π-π stacking interactions. These supramolecular architectures and molecular packings are found to be closely related to the isomeric configurations of the complexes and have been investigated using various spectroscopic studies.

Formation of 1D Infinite Chains Directed by Metal-Metal and/or π-π Stacking Interactions of Water-Soluble Platinum(II) 2,6-Bis(benzimidazol-2'-yl)pyridine Double Complex Salts.

A new class of water-soluble double complex salts (DCSs), [Pt{bzimpy(TEG)2}Cl][Pt{bzimpy(PrSO3)2}Cl] and its alkylplatinum(II) bzimpy derivatives (bzimpy = 2,6-bis(benzimidazol-2'-yl)pyridine, has been demonstrated to exhibit strong aggregation in water through Pt···Pt and π-π stacking interactions to give a variety of distinctive nanostructures based on the formation of one-dimensional (1D) infinite chains. The self-association process can be systemically controlled by varying the solvent composition and temperature and has been studied by 1H NMR, 2D NOESY NMR, mass spectrometry, electron and confocal fluorescence microscopy, UV-vis absorption, and emission spectroscopy.

Supramolecular assembly of a phosphole-based moiety into nanostructures dictated by alkynylplatinum(ii) terpyridine complexes through non-covalent Pt···Pt and π-π stacking interactions: synthesis, characterization, photophysics and self-assembly behaviors.

A new class of platinum(ii) terpyridine complexes with a phosphole-derived bridging alkynyl ligand have been prepared. The X-ray crystal structure of complex 2 has been determined, and reveals a polymeric zig-zag chain structure with the existence of π-π stacking interactions. The photophysical properties have also been studied, with 3MLCT/3LLCT phosphorescence exhibited in degassed CH2Cl2; the energy of which is varied by the π-conjugation of the terpyridine ligands. The solvent-induced assembly of complex 1 has been studied. The incorporation of hydrophobic hydrocarbon chains has been shown to play an important role in assisting the formation of self-assembled nanostructures via Pt···Pt, π-π stacking and hydrophobic-hydrophobic interactions. It has been established that an isodesmic growth mechanism operates in polar media to give nanospheres, while fibrous networks originate from the self-assembly of the complexes in non-polar media, predominantly driven by π-π stacking interactions.

A rational molecular design of triazine-containing alkynylplatinum(ii) terpyridine complexes and the formation of helical ribbons via PtPt, π-π stacking and hydrophobic-hydrophobic interactions.

The self-assembly of triazine-containing alkynylplatinum(ii) terpyridine complexes has revealed the formation of sophisticated helical ribbons. Fine control of the morphological transition from plates to helical structures has been demonstrated through an interplay of the bent molecular structure and PtPt, π-π stacking and hydrophobic-hydrophobic interactions.

Living supramolecular polymerization achieved by collaborative assembly of platinum(II) complexes and block copolymers.

An important feature of biological systems to achieve complexity and precision is the involvement of multiple components where each component plays its own role and collaborates with other components. Mimicking this, we report living supramolecular polymerization achieved by collaborative assembly of two structurally dissimilar components, that is, platinum(II) complexes and poly(ethylene glycol)-b-poly(acrylic acid) (PEG-b-PAA). The PAA blocks neutralize the charges of the platinum(II) complexes, with the noncovalent metal-metal and π-π interactions directing the longitudinal growth of the platinum(II) complexes into 1D crystalline nanostructures, and the PEG blocks inhibiting the transverse growth of the platinum(II) complexes and providing the whole system with excellent solubility. The ends of the 1D crystalline nanostructures have been found to be active during the assembly and remain active after the assembly. One-dimensional segmented nanostructures with heterojunctions have been produced by sequential growth of two types of platinum(II) complexes. The PAA blocks act as adapters at the heterojunctions for lattice matching between chemically and crystallographically different platinum(II) complexes, achieving heterojunctions with a lattice mismatch as large as 21%.

Synthesis of Luminescent Platinum(II) 2,6-Bis(N-dodecylbenzimidazol-2'-yl)pyridine Foldamers and Their Supramolecular Assembly and Metallogel Formation.

Dinuclear alkynylplatinum(II) metallofoldamers with an oligomeric m-phenyleneethynylene backbone have been designed with the incorporation of a sterically undemanding, π-conjugated, and hydrophobic 2,6-bis(N-dodecylbenzimidazol-2'-yl)pyridine pincer ligand. The complex with the optimal chain length has been found to exhibit gelation behavior via stabilization by noncovalent Pt···Pt and π-π stacking interactions in the hierarchical architecture constructed from the single-turn helix. The chain lengths of the complexes have been found to be a critical determinant for their gelation behavior, conformations, and morphologies. Such a gelation process has been found to undergo a cooperative assembly mechanism according to the nucleation-elongation model. Their self-assembly via the Pt···Pt and π-π stacking interactions has been studied by 1H NMR, 2D ROESY NMR (ROESY = rotating-frame Overhauser spectroscopy), electronic absorption, and emission spectroscopy, and density functional theory calculations have provided further insights into the folded state geometry of this class of metallofoldamers.

Supramolecular assembly of platinum-containing polyhedral oligomeric silsesquioxanes: an interplay of intermolecular interactions and a correlation between structural modifications and morphological transformations.

A series of alkynylplatinum(ii) terpyridine complexes functionalized with polyhedral oligomeric silsesquioxane (POSS) moieties has been demonstrated to exhibit drastic color changes and give various distinctive nanostructures with interesting multi-stage morphological transformations from spheres to nanoplates in response to solvent conditions through the interplay of various intermolecular interactions, including hydrophilic-hydrophilic, hydrophobic-hydrophobic, Pt···Pt and π-π stacking interactions. These supramolecular architectures can be systematically modified and controlled through the molecular design and the variation of solvent compositions. In particular, drastic changes in color in response to solvent polarity were observed through the incorporation of the charged moieties, representing a new class of potential candidates for functional materials with sensing or imaging capabilities. This class of complexes has been studied by 1H NMR spectroscopy, electron microscopy, UV-vis absorption and emission spectroscopy.

Self-Assembled Architectures of Alkynylplatinum(II) Amphiphiles and Their Structural Optimization: A Balance of the Interplay Among Pt···Pt, π-π Stacking, and Hydrophobic-Hydrophobic Interactions.

A series of alkynylplatinum(II) terpyridine complexes with triethylene glycol units was synthesized, and their self-assembly properties were investigated in solution by UV-vis absorption, emission, and 1H NMR spectroscopy. The aggregation behaviors of several water-soluble complexes were investigated in aqueous media. Some of them were found to give rise to uniform fibers, suggesting the important role that triethylene glycol units has in regulating their self-assembly properties. Further modifications of these structures through the incorporation of alkyl chains and changes in counter-anions have rendered the complexes more amphiphilic in nature, and the effect of their alkyl chain lengths was studied and optimized. The distinguishable color and spectral changes upon variations in solvent compositions might have potential applications in developing colorimetric and luminescent probes for the detection of microenvironment change. Furthermore, an optimum chain length, i.e., n-butyl chain, is required for the formation of stable and ordered nanostructures. This represents a delicate balance among Pt···Pt, π-π stacking, and hydrophobic-hydrophobic interactions and provides guiding principles into the construction of supramolecular materials with practical applications.

Synthesis, Electrochemistry, and Photophysical Studies of Ruthenium(II) Polypyridine Complexes with D-π-A-π-D Type Ligands and Their Application Studies as Organic Memories.

A new class of ruthenium(II) polypyridine complexes with a series of D-π-A-π-D type (D=donor, A=acceptor) ligands was synthesized and characterized by 1 H NMR spectroscopy, mass spectrometry, and elemental analysis. The photophysical and electrochemical properties of the complexes were also investigated. The newly synthesized ruthenium(II) polypyridine complexes were found to exhibit two intense absorption bands at both high-energy (λ=333-369 nm) and low-energy (λ=520-535 nm) regions. They are assigned as intraligand (IL) π→π* transitions of the bipyridine (bpy) and π-conjugated bpy ligands, and IL charge-transfer (CT) transitions from the donor to the acceptor moiety with mixing of dπ(RuII )→π*(bpy) and dπ(RuII )→π*(L) MLCT characters, respectively. In addition, all complexes were demonstrated to exhibit intense red emissions at approximately λ=727-744 nm in degassed dichloromethane at 298 K or in n-butyronitrile glass at 77 K. Nanosecond transient absorption (TA) spectroscopy has also been carried out, establishing the presence of the charge-separated state. In order to understand the electrochemical properties of the complexes, cyclic voltammetry has also been performed. Two quasi-reversible oxidation couples and three quasi-reversible reduction couples were observed. One of the ruthenium(II) complexes has been utilized in the fabrication of memory devices, in which an ON/OFF current ratio of over 104 was obtained.

Tuning of Supramolecular Architectures of l-Valine-Containing Dicyanoplatinum(II) 2,2'-Bipyridine Complexes by Metal-Metal, π-π Stacking, and Hydrogen-Bonding Interactions.

A series of newly synthesized dicyanoplatinum(II) 2,2'-bipyridine complexes exhibits self-assembly properties in solution after the incorporation of the l-valine amino units appended with various hydrophobic motifs. These l-valine-derived substituents were found to have critical control over the aggregation behaviors of the complexes in the solution state. On one hand, one of the complexes was found to exhibit interesting circularly polarized luminescence (CPL) signals at low temperature due to the formation of chiral spherical aggregates in the temperature-dependent studies. On the other hand, systematic transformation from less uniform aggregates to well-defined fibrous and rod-like structures via Pt⋅⋅⋅Pt and π-π stacking interactions has also been observed in the mixed-solvent studies. These changes were monitored by UV/Vis absorption, emission, circular dichroism (CD), and CPL spectroscopies, and morphologies were studied by electron microscopy.

Self-assembly of alkynylplatinum(II) terpyridine amphiphiles into nanostructures via steric control and metal-metal interactions.

A series of mono- and dinuclear alkynylplatinum(II) terpyridine complexes containing the hydrophilic oligo(para-phenylene ethynylene) with two 3,6,9-trioxadec-1-yloxy chains was designed and synthesized. The mononuclear alkynylplatinum(II) terpyridine complex was found to display a very strong tendency toward the formation of supramolecular structures. Interestingly, additional end-capping with another platinum(II) terpyridine moiety of various steric bulk at the terminal alkyne would lead to the formation of nanotubes or helical ribbons. These desirable nanostructures were found to be governed by the steric bulk on the platinum(II) terpyridine moieties, which modulates the directional metal-metal interactions and controls the formation of nanotubes or helical ribbons. Detailed analysis of temperature-dependent UV-visible absorption spectra of the nanostructured tubular aggregates also provided insights into the assembly mechanism and showed the role of metal-metal interactions in the cooperative supramolecular polymerization of the amphiphilic platinum(II) complexes.

Metal-Metal and π-π Interactions Directed End-to-End Assembly of Gold Nanorods.

The end-to-end aggregation of gold nanorods (GNRs) has been demonstrated to be directed by a thioacetate-containing alkynylplatinum(II) terpyridine complex. The in situ deprotected complex is preferentially attached at the ends of the gold nanorods (GNRs) and induce the aggregation of GNRs in an "end-to-end" manner by Pt···Pt and π-π interactions, which have been characterized by electron microscopy, energy dispersed X-ray (EDX) analysis, and UV-vis absorption spectroscopy. The assembly of the nanorods into chain-like nanostructures can be controlled by the concentration of the Pt(II) complexes.