Self-assembling molecular wires of halogen-bridged platinum complexes in organic media. Mesoscopic supramolecular assemblies consisting of a mixed valent Pt(II)/Pt(IV) complex and anionic amphiphiles

Structural Conversion of Supramolecular Assembly in Solution by Thermally Induced Intramolecular Electron Transfer of [Co2 Fe2 ] Complex

Combining metal complexes with amphiphilic molecules leads to a wide variety of functional self-assembled nanostructures. Metal complexes exhibiting spin transitions can be good candidates as the trigger to cause structural conversion of such assembly because they respond to various external stimuli. In this work, we studied a structural conversion of a supramolecular assembly containing a [Co2 Fe2 ] complex through a thermally induced electron transfer-coupled spin transition (ETCST). With an amphiphilic anion, the [Co2 Fe2 ] complex formed reverse vesicles in solution and showed thermal ETCST. In contrast, thermal ETCST in the presence of a bridging hydrogen-bond donor caused structural conversion from the reverse vesicle structure to entangled one-dimensional chains through hydrogen bond formation.

Helical-Ribbon and Tape Formation of Lipid Packaged [Ru(bpy)3]2+ Complexes in Organic Media

Anionic lipid amphiphiles with [RuII(bpy)3]2+ complex have been prepared. The metal complexes have been found to form ribbon and tape structures depending on chemical structures of lipid amphiphiles. Especially, the composites showed hypochromic effect and induced circular dichroism in organic media, and flexibly and weakly supramolecular control of morphological and optical properties have been demonstrated.

Luminescence of Amphiphilic PtII Complexes Controlled by Confinement

The formation of hybrid silica-based systems to study the effect of the confinement on the emission properties of self-assembled platinum(II) complexes is reported. The complexes behave as surfactants since they possess a hydrophobic moiety and, on the ancillary ligand, a relatively long hydrophilic chain terminated with a positively charged group. The compounds, soluble in water, self-assemble, even at very low concentration, in supramolecular structures which display an orange luminescence. The properties of the assemblies have been studied in detail and in order to stabilize these supramolecular architectures and to enhance their emission properties hybrid silica porous nanoparticles have been prepared. In particular the Pt^II complexes have been employed as co-surfactant for the template formation of mesoporous silica nanoparticles (MSNs) using a sol gel synthesis. Interestingly, upon encapsulation in the silica pores, the platinum aggregates exhibit an emission profile similar in energy to the complexes assembled in solution, but the photoluminescence quantum yields of the hybrid systems are significantly higher (up to 45 %), and the excited state lifetimes much longer than those recorded in solution. Such enhancement of the photophysical properties together with the possibility to process the hybrid silica nanomaterials can pave the way to new type of emitters.

Multi-molecular emission of a cationic Pt(ii) complex through hydrogen bonding interactions

The cationic Pt complexes with amide groups have been found to show dimer emission through hydrogen bonding interactions with counter anions even at low concentration. In order to investigate further details of dimer emission, we prepared three Pt complexes, Pt·B(C₆F₅)₄, Pt·Cl, and Pt·PF₆, whose counter anions possess different strengths of a hydrogen bonding acceptor. Hydrogen bonding interactions in the ground state and excited-state dynamics of the Pt complexes were evaluated by NMR analysis, temperature dependence, and kinetics of dimer emission. These studies revealed that the hydrogen bonding interaction in the ground state is essential for dimer emission, but too strong hydrogen bonding prevents dimer emission due to the inhibition of a stacked dimer formation in the excited state. Owing to this trade-off, the Pt complex with a moderate hydrogen bonding acceptor, PF₆^-, most effectively shows dimer emission. In general, a strong supramolecular interaction efficiently provides a desired assembled structure showing multi-molecular emission. We revealed a unique phenomenon that a moderate interaction is beneficial to effective multi-molecular emission.

Hierarchical Self-Assembly of Luminescent Tartrate-Bridged Chiral Binuclear Tb(III) Complexes in Ethanol

A new family of supramolecular metalloamphiphiles carrying two metal centers is developed. They are formed by bridging two coordinatively unsaturated lipophilic Tb³⁺ complexes (TbL⁺) with chiral dicarboxylate anions. The formation of bridging coordination bonds is confirmed using UV spectroscopy, induced circular dichroism (ICD), increased luminescence intensity of TbL⁺, and electrospray ionization mass spectrometry (ESIMS) analysis. These supramolecular metalloamphiphiles hierarchically self-assemble in ethanol to give luminescent nanospheres, as observed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The two hydroxyl groups introduced in the bridging ligands of [TbL]₂(d-/l-tartrate) significantly promote self-assembly by increasing coherent forces via intermolecular hydrogen bonding. The observed self-assembly in ethanol also merits mention because such polar alcoholic media have been unfavorable for conventional molecular self-assemblies. The present approach offers a new molecular design strategy for composable metalloamphiphiles.

Coupling of platinated triguanides with platinum-activated nitriles as a novel strategy for generation of dimetallic systems

One of two Pt(IV)-activated propanenitriles in trans-[PtCl4(EtCN)2] is involved in platinum(IV)-mediated nitrile-imine coupling with the platinum(II)-based metallacycles [PtCl2{NH=C(NR2)N(Ph)C(=NH)N(Ph)C(NR2)=NH}] [R2 = Me2 (1a), C5H10 (1b)] yielding diplatinum products, whose structures depend on molar ratios between the reactants. At a 1 : 1 ratio, the mixed-valence platinum(II)/platinum(IV) species [PtCl4{NH=C(NR2)N(Ph)C{=[(N(Et)C=NH)PtCl2(EtCN)]}N(Ph)C(NR2)=NH}] [R2 = Me2 (2a), (CH2)5 (2b)] were generated, whereas at a 1 : 2 ratio the dinuclear platinum(II)/platinum(II) complexes [PtCl2{NH=C(NR2)N(Ph)C{=[(N(Et)C=NH)PtCl2(EtCN)]}N(Ph)C(NR2)=NH}] [R2 = Me2 (3a), (CH2)5 (3b)] were obtained. In contrast to the nitrile-imine coupling observed for the platinum(IV) dinitrile complex, the reaction between the platinum(II) congener trans-[PtCl2(EtCN)2] and any one of 1a,b gives exclusively the substituted dimetallic platinum(II)/platinum(II) products [PtCl2{NH=C(NR2)N(Ph)C{=[(NH)PtCl2(EtCN)]}N(Ph)C(NR2)=NH}] [R2 = Me2 (6a), (CH2)5 (6b)] featuring platinum-containing guanidine 1 as one of the ligands. Complexes 2a,b, 3a,b, and 6a,b were characterized by elemental analyses (C, H, N), HRESI-MS, IR, (1)H NMR spectroscopy, and DTA/TG. The molecular and crystal structure of 2a·2CDCl3 was additionally studied by single-crystal X-ray diffraction. Complexes 2a,b undergo further redox transformation in solutions, and single crystals of [PtCl2{NH=C(NMe2)N(Ph)C{=[(N(Et)C=NH)PtCl2(MeCN)]}N(Ph)C(NMe2)=NH}]·2CH2Cl2 (3'a·2CH2Cl2) were obtained from 2a in a CH2Cl2-MeCN-C2H4Cl2 mixture and studied by X-ray crystallography. The driving forces for the generation of diplatinum products 2 and 3 were elucidated based on a quantum-chemical study.

Charge-bistable Pd(iii)/Pd(ii,iv) coordination polymers: phase transitions and their applications to optical properties

This paper reviews the recent progress in charge-bistable Pd(iii)/Pd(ii,iv) coordination polymers (MX chains). The temperature-, pressure- and photo-induced phase transitions have been demonstrated, proposing MX chains as a material for optical switching devices.

Bromide-bridged palladium(iii) chain complexes showing charge bistability near room temperature

We synthesized and characterized bromide-bridged Pd(iii) chain complexes, [Pd(en)₂Br](MalC_n–Y)₂·H₂O (en = ethylenediamine; MalC_n–Y = dialkyl sulfomalonate; n: the number of carbon atoms) (n = 7 and 12), which were in averaged valence states at room temperature.

EFFECT OF LIPID-PACKAGING ON THE CHARGE TRANSFER CHARACTERISTICS OF ONE-DIMENSIONAL, MIXED-VALENCE PLATINUM COMPLEXES

Lipid-packaged platinum complexes of [ Pt ( en )2][ Pt X2( en )2] (X= Cl , Br , I ) are newly synthesized and their morphology and charge transfer (CT) absorption characteristics are compared in organic media. They show remarkably red-shifted CT absorption characteristics, which are ascribed to the significantly promoted exciton delocalization along the chains.

Controlled polymerization and self-assembly of halogen-bridged diruthenium complexes in organic media and their dielectrophoretic alignment

Lipophilic paddlewheel biruthenium complexes [Ru(2)(μ-O(2)CR)(3)X](n) (O(2)CR = 3,4,5-tridodecyloxybenzoate, X = Cl, I) self-assemble in organic media to form halogen-bridged coordination polymers. The polymerization is accompanied by spectral changes in π(RuO,Ru(2)) → π*(Ru(2)) and π(axial ligand) → π*(Ru(2)) absorption bands. These polymeric complexes form lyotropic liquid crystals in n-decane at concentrations above ~100 unit mM. The bridging halogen axial ligands (X = Cl or I) exert significant influences on their electronic structures and self-assembling characteristics: the chloride-bridged polymers give hexagonally aligned ordered columnar structure (columnar hexagonal phase, Col(h)), whereas the iodide-bridged polymers form less ordered columnar nematic (Col(n)) phase, as revealed by small-angle X-ray diffraction measurements. Chloro-bridged coordination polymers dispersed in n-decane are thermally intact even at the elevated temperature of 70 °C. In contrast, iodo-bridged polymers show reversible dissociation and reassembly phenomena depending on temperature. These halogen-bridged coordination polymers show unidirectional alignment upon applying alternating current (ac) electric field as investigated by crossed polarizing optical microscopy and scanning electron microscopy. The unidirectionally oriented columns of chloro-bridged polymers are accumulated upon repetitive application of the ac voltage, whereas iodo-bridged coordination polymers show faster and reversible alignment changes in response to turning on-and-off the electric field. The controlled self-assembly of electronically conjugated linear complexes provide a potential platform to design electric field-responsive nanomaterials.

Self-assembly of a triangle-shaped, hexaplatinum-incorporated, supramolecular amphiphile in solution and at interfaces

The self-assembly and characterization of a novel supramolecular amphiphile built from a new 60 degree amphiphilic precursor that incorporates hydrophilic platinum(II) metals and hydrophobic dioctadecyloxy chains is reported. The amphiphilic macrocycle and its precursor compound have been characterized by multinuclear NMR spectroscopy, ESI-MS, and other standard techniques. The coacervate morphology of the amphiphile at the liquid-liquid interface has been studied by using confocal optical microscopy and in situ Raman spectroscopy. The self-assembly of the amphiphilic macrocycle at the air-water interface has been investigated through Langmuir-trough techniques. The study indicates the possible formation of surface micelle-like aggregates. The disparity between the experimental molecular areas and those derived from molecular models support the idea of aggregation. AFM images of the surface aggregates show the formation of a flat topology with arbitrary ridgelike patterns. Reasonable molecular-packing arrangements are proposed to explain the molecular organization within the observed structures.

Giant micelles of organoplatinum(II) gemini amphiphiles

Organoplatinum(II) gemini amphiphiles with two different chain lengths are synthesized and characterized. Self-assembly at the air-water interface is investigated as a function of chain length and reduction in surface area by using Langmuir-trough techniques. The Langmuir-trough experiments lead to a conjecture that surface aggregates may be the adsorbing units. Atomic force microscopy on the transferred Langmuir-Schaefer films reveals spontaneous formation of wormlike micellar aggregates. A shear-induced transition and alignment are proposed for the observed effects.

Metallo-supramolecular modules as a paradigm for materials science

Metal ion coordination in discrete or extended metallo-supramolecular assemblies offers ample opportunity to fabricate and study devices and materials that are equally important for fundamental research and new technologies. Metal ions embedded in a specific ligand field offer diverse thermodynamic, kinetic, chemical, physical and structural properties that make these systems promising candidates for active components in functional materials. A key challenge is to improve and develop methodologies for placing these active modules in suitable device architectures, such as thin films or mesophases. This review highlights recent developments in extended, polymeric metallo-supramolecular systems and discrete polyoxometalates with an emphasis on materials science.

Challenges and breakthroughs in recent research on self-assembly

The controlled fabrication of nanometer-scale objects is without doubt one of the central issues in current science and technology. However, existing fabrication techniques suffer from several disadvantages including size-restrictions and a general paucity of applicable materials. Because of this, the development of alternative approaches based on supramolecular self-assembly processes is anticipated as a breakthrough methodology. This review article aims to comprehensively summarize the salient aspects of self-assembly through the introduction of the recent challenges and breakthroughs in three categories: (i) types of self-assembly in bulk media; (ii) types of components for self-assembly in bulk media; and (iii) self-assembly at interfaces.

Investigation on solvent casting films of surfactant-encapsulated clusters

The surfactant-encapsulated cluster (SEC) composed of a hydrophobic dimethyl dioctadecyl ammonium (DODA) shell and an encapsulated hydrophilic polyoxoanion core can form casting films. The structure of the casting film is influenced by evaporation rates of organic solvent. When the casting films are prepared by slow evaporation of chloroform, the alkyl chains are considered to possess a partial interdigitation, and the interdigitated length is 1.6 nm. The casting film structure is characterized by scanning force microscopy (SFM), Fourier transformation infrared (FT-IR), wide-angle X-ray diffraction, and differential scanning calorimetry (DSC).

Investigation on γ-cyclodextrin nanotube induced by -diphenylbenzidine molecule

Dynamic light scattering (DLS) measurement provides an effective way to investigate the formation of nanotube of gamma-cyclodextrin (gamma-CD) induced by N,N'-diphenylbenzidine (DPB) in water. With the combination of steady-state fluorescence and fluorescence anisotropy experiments, it was found that for alpha- and beta-CD, only 1:2 (guest:host) inclusion complexes were formed and for gamma-CD, cyclodextrin nanotube was formed involving 16 gamma-CD units at maximum. The pH effect studies with both DLS and fluorescence anisotropy measurements indicated that the hydrogen bonding between neighboring CDs was necessary to the formation of cyclodextrin nanotube. In the temperature experiment, we found that the nanotube of DPB-gamma-CD could exist stably at relatively high temperatures and the transition point for structural collapse was estimated to be around 54 degrees C. The aggregation states of both gamma-CD itself and DPB-gamma-CD nanotube were observed with TEM.

Metal-complexed nanofiber formation in water from dicarboxylic valylvaline bolaamphiphiles

Nanofiber formation of dipeptide-based bolaamphiphiles, bis (N-alpha-amide--valyl--valine) 1,n-alkane dicarboxylate (n=6, 8, 10, and 12) in water was analyzed by TEM, SEM, IR, and XRD. The bolaamphiphiles proved to be coordinated to divalent transition-metal cations, such as Co2+, Ni2+, Cu2+, and Zn2+, giving precipitates, colloidal dispersions (loose hydrogels), and hydrogels upon self-assembly at 23 or 70 degrees C. Longer oligomethylene chains and strong interaction between the metal cations and the carboxylate anions are responsible for the hydrogel formation. Energy-filtering transmission electron microscopy (EF-TEM) and field-emission scanning electron microscopy (EF-SEM) images revealed that the colloidal dispersions and the hydrogels consist of a large number of nanofibers with widths of 15-20 nm and lengths of several micrometers. FT-IR and powder XRD measurement supported the existence of a beta-sheet structure-based nanofibers complexing with metal cations.

Heat-Set Gel-like Networks of Lipophilic Co(II) Triazole Complexes in Organic Media and Their Thermochromic Structural Transitions

A novel class of thermally responsive supramolecular assemblies is formed from the lipophilic cobalt(II) complexes of 4-alkylated 1,2,4-triazoles. When an ether linkage is introduced in the alkylchain moiety, a blue gel-like phase is formed in chloroform, even at very low concentration (ca. 0.01 wt %, at room temperature). The blue color is accompanied by a structured absorption around 580-730 nm, which is characteristic of cobalt (II) in the tetrahedral (T(d)) coordination. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) of the gel-like phase confirms the formation of networks of fibrous nanoassemblies with widths of 5-30 nm. The observed widths are larger than a molecular length of the triazole ligand (ca. 2.2 nm) and they are consisted of aggregates of T(d) coordination polymers. Very interestingly, the blue gel-like phase turned into a solution by cooling below 25 degrees C. A pale pink solution is obtained at 0 degrees C, indicating the formation of octahedral (O(h)) complexes. The observed thermochromic transition is totally reversible. The formation of gel-like networks by heating is contrary to the conventional organogels, which dissolve upon heating. Temperature dependence of the storage and loss moduli (G' and G") shows minima around at 27 degrees C, at which temperature they gave comparable values. On the other hand, G' exceeds G" both in the gel-like phase (temperature above 27 degrees C) and in the solution phase (temperature below 25 degrees C). These observations indicate that T(d) complexes are present as low-molecular weight species around at 25-27 degrees C. They are self-assembled to polymeric T(d) complexes by heating and form gel-like networks. Upon cooling the solution below 25 degrees C, T(d) complexes are converted to O(h) complexes and they also self-assemble into oligomeric or polymeric species at lower temperatures. The observed unique thermochromic transition (pink solution --> blue gel-like phase) is accompanied by an exothermic peak in differential scanning calorimetry (DSC), and is shown to be an enthalpy-driven process. The lipophilic modification of one-dimensional coordination systems provides unique solution properties and it would be widely applicable to the design of thermoresponsive, self-assembling molecular wires.

Pillared honeycomb nanoarchitectures formed on solid surfaces by the self-assembly of lipid-packaged one-dimensional Pt complexes

A highly lipophilic polyion complex [Pt(en)(2)][PtCl(2)(en)(2)](1)(4) (en, 1,2-diaminoethane) is prepared from one-dimensional mixed valence Pt(II)/Pt(IV) complex and newly designed chiral amphiphile 1. The powdery sample showed purple color, which is a result of the mixed valence absorption of the linear chlorobridged complex (Pt(II)-Cl-Pt(IV)-Cl-)(n). When the lipid complex is dispersed in dichloromethane, purple-colored dispersion is obtained at 0 degrees C, whereas the color disappears after heating the solution to 21 degrees C. The observed thermochromism is reversible with respect to the temperature changes and is ascribed to the reversible dissociation and reassembly of the self-assembling inorganic wires. Casting of the 0 degrees C-purple dispersion on solid substrates affords honeycomb nanostructures in addition to the nanowires with the width of about 20 nm. The honeycomb patterns seem to be templated by the condensed water droplets that are formed and aligned on the rapidly evaporating dichloromethane solution. On the other hand, more regular honeycomb structures are exclusively obtained by casting the 21 degrees C-colorless solution. These observations indicate that the ordered honeycomb structures are obtainable on solid surfaces by the self-assembly of molecularly dispersed components [Pt(en)(2)](1)(2) and trans-[PtCl(2)(en)(2)](1)(2.) Very interestingly, formation of double-layered honeycomb nanostructure is observed by scanning electron microscopy. The unit hexagons and pillars of the honeycombs are made of nanowires that are hierarchically assembled from the lipid-packaged Pt(II)/Pt(IV) complexes. The surface self-organization of lipophilic inorganic complexes has a potential to fabricate novel nanoarchitectures with conjugated electronic structures.

Hierarchical self-assembly of amphiphilic metallohosts to give discrete nanostructures

The synthesis and aggregation behavior of a new class of rigid metallohosts is described. The molecules consist of a ruthenium-bipyridine complex functionalized with a glycoluril-based receptor cavity. By specific molecular recognition processes in water, the metallohosts self-assemble to form large arrays of molecules. Depending on the size of the cavity side walls of the host, these arrays can grow in a hierarchical process into discrete rectangular and cigar-like aggregates of nanometer size, which can be visualized by electron microscopy.