Length control of supramolecular polymeric nanofibers based on stacked planar platinum(II) complexes by seeded-growth

Seeded Polymerization of an Amide-Functionalized Diketopyrrolopyrrole Dye in Aqueous Media

The self-assembly of an amide-functionalized dithienyldiketopyrrolopyrrole (DPP) dye in aqueous media was achieved through seed-initiated supramolecular polymerization. Temperature- and time-dependent studies showed that the spontaneous polymerization of the DPP derivative was temporally delayed upon cooling the monomer solution in a methanol/water mixture. Theoretical calculations revealed that an amide-functionalized DPP derivative adopts an energetically favorable folded conformation in the presence of water molecules due to hydration. This conformational change is most likely responsible for the trapping of monomers in the initial stage of the cooperative supramolecular polymerization in aqueous media. However, the monomeric species can selectively interact with externally added fragmented aggregates as seeds through concerted π-stacking and hydrogen-bonding interactions. Consequently, the time course of the supramolecular polymerization and the morphology of the aggregated state can be controlled, and one-dimensional fibers that exhibit a J-aggregate-like bathochromically shifted absorption band can be obtained.

Controlling the supramolecular polymerization and metallogel formation of Pt(ii) complexes via delicate tuning of non-covalent interactions

Direct influence of noncovalent ionic and hydrogen bonding interactions on supramolecular polymerization mechanisms and their impact on gel formation of luminescent platinum complexes have been comprehensively investigated.

Solution self-assembly of ABC triblock terpolymers with a central crystallizable poly(ferrocenyldimethylsilane) core-forming segment

The synthesis and solution self-assembly behavior of a range of linear ABC triblock terpolymers with a central crystallizable poly(ferrocenyldimethylsilane) core-forming segment have been explored.

Cationic magnesium hydride [MgH]+ stabilized by an NNNN-type macrocycle

A magnesium hydride cation [(L)MgH]⁺ supported by a macrocyclic ligand (L = Me₄TACD; 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) has been shown to react with Lewis acids as well as with unsaturated substrates including pyridine.

Scalable Fiber-like Micelles and Block Co-micelles by Polymerization-Induced Crystallization-Driven Self-Assembly

Self-assembled 1D block copolymer nanoparticles (micelles) are of interest for a range of applications. However, morphologically pure samples are often challenging to access, and precise dimensional control is not possible. Moreover, the development of synthetic protocols that operate on a commercially viable scale has been a major challenge. Herein, we describe the preparation 1D fiber-like micelles with crystalline cores at high concentrations by a one-pot process termed polymerization-induced crystallization-driven self-assembly (PI-CDSA). We also demonstrate the formation of uniform fibers by living PI-CDSA, a process in which block copolymer synthesis, self-assembly, and seeded growth are combined. We have demonstrated that the method is successful for block copolymers that possess the same composition as that of the seed (homoepitaxial growth) and also where the coronal chemistries differ to give segmented 1D fibers known as block co-micelles. We have also shown that heteroepitaxial growth allows the formation of scaled-up block co-micelles where the composition of both the core and corona was varied. These proof-of-concept experiments indicate that PI-CDSA is a promising, scalable route to a variety of polydisperse or uniform 1D nanoparticles based on block copolymers with different crystalline core chemistries and, therefore, functions.

Extending the Scope of "Living" Crystallization-Driven Self-Assembly: Well-Defined 1D Micelles and Block Comicelles from Crystallizable Polycarbonate Block Copolymers

Fiber-like block copolymer (BCP) micelles offer considerable potential for a variety of applications; however, uniform samples of controlled length and with spatially tailored chemistry have not been accessible. Recently, a seeded growth method, termed "living" crystallization-driven self-assembly (CDSA), has been developed to allow the formation of 1D micelles and block comicelles of precisely controlled dimensions from BCPs with a crystallizable segment. An expansion of the range of core-forming blocks that participate in living CDSA is necessary for this technique to be compatible with a broad range of applications. Few examples currently exist of well-defined, water-dispersible BCP micelles prepared using this approach, especially from biocompatible and biodegradable polymers. Herein, we demonstrate that BCPs containing a crystallizable polycarbonate, poly(spiro[fluorene-9,5'-[1,3]-dioxan]-2'-one) (PFTMC), can readily undergo living CDSA processes. PFTMC- b-poly(ethylene glycol) (PEG) BCPs with PFTMC:PEG block ratios of 1:11 and 1:25 were shown to undergo living CDSA to form near monodisperse fiber-like micelles of precisely controlled lengths of up to ∼1.6 μm. Detailed structural characterization of these micelles by TEM, AFM, SAXS, and WAXS revealed that they comprise a crystalline, chain-folded PFTMC core with a rectangular cross-section that is surrounded by a solvent swollen PEG corona. PFTMC- b-PEG fiber-like micelles were shown to be dispersible in water to give colloidally stable solutions. This allowed an assessment of the toxicity of these structures toward WI-38 and HeLa cells. From these experiments, we observed no discernible cytotoxicity from a sample of 119 nm fiber-like micelles to either healthy (WI-38) or cancerous (HeLa) cell types. The living CDSA process was extended to PFTMC- b-poly(2-vinylpyridine) (P2VP), and addition of this BCP to PFTMC- b-PEG seed micelles led to the formation of well-defined segmented fibers with spatially localized coronal chemistries.

Multicomponent Assembled Systems Based on Platinum(II) Terpyridine Complexes

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

Regression Analysis for Nucleation-Elongation Model of Supramolecular Assembly: How To Determine Nucleus Size

Nucleation-elongation is known to give satisfactory descriptions of many supramolecular polymerization systems in thermal equilibrium. Its key feature is the necessity to form a "nucleus" consisting of a certain number of monomer units before being able to grow into a longer polymer chain. The size of the nucleus has significant implications for the understanding of the supramolecular polymerization mechanism. Here we investigate how experiments can give information on the nucleus size by regression analysis of various types of measurements. The measurements of free monomer concentrations, diffusion coefficients, and calorimetric response as functions of concentration or temperature are considered. The nucleation-elongation model with a general value for the nucleus size is used to provide mathematical expressions for these experimental observables. Numerical experiments are performed where experimental errors are simulated by computer-generated random numbers, and it is investigated whether least-squares fitting analyses can give the correct values of the nucleus size in the presence of experimental errors. It is recommended that the calorimetric measurements such as differential scanning calorimetry (DSC) or isothermal titration calorimetry (ITC) be performed under various conditions to correctly determine the nucleus size experimentally.

Probing the Growth Kinetics for the Formation of Uniform 1D Block Copolymer Nanoparticles by Living Crystallization-Driven Self-Assembly

Living crystallization-driven self-assembly (CDSA) is a seeded growth method for crystallizable block copolymers (BCPs) and related amphiphiles in solution and has recently emerged as a highly promising and versatile route to uniform core-shell nanoparticles (micelles) with control of dimensions and architecture. However, the factors that influence the rate of nanoparticle growth have not been systematically studied. Using transmission electron microscopy, small- and wide-angle X-ray scattering, and super-resolution fluorescence microscopy techniques, we have investigated the kinetics of the seeded growth of poly(ferrocenyldimethylsilane)- b-(polydimethylsiloxane) (PFS- b-PDMS), as a model living CDSA system for those employing, for example, crystallizable emissive and biocompatible polymers. By altering various self-assembly parameters including concentration, temperature, solvent, and BCP composition our results have established that the time taken to prepare fiber-like micelles via the living CDSA method can be reduced by decreasing temperature, by employing solvents that are poorer for the crystallizable PFS core-forming block, and by increasing the length of the PFS core-forming block. These results are of general importance for the future optimization of a wide variety of living CDSA systems. Our studies also demonstrate that the growth kinetics for living CDSA do not exhibit the first-order dependence of growth rate on unimer concentration anticipated by analogy with living covalent polymerizations of molecular monomers. This difference may be caused by the combined influence of chain conformational effects of the BCP on addition to the seed termini and chain length dispersity.

Biomimetic temporal self-assembly via fuel-driven controlled supramolecular polymerization

Temporal control of supramolecular assemblies to modulate the structural and transient characteristics of synthetic nanostructures is an active field of research within supramolecular chemistry. Molecular designs to attain temporal control have often taken inspiration from biological assemblies. One such assembly in Nature which has been studied extensively, for its well-defined structure and programmable self-assembly, is the ATP-driven seeded self-assembly of actin. Here we show, in a synthetic manifestation of actin self-assembly, an ATP-selective and ATP-fuelled, controlled supramolecular polymerization of a phosphate receptor functionalised monomer. It undergoes fuel-driven nucleation and seeded growth that provide length control and narrow dispersity of the resultant assemblies. Furthermore, coupling via ATP-hydrolysing enzymes yielded its transient characteristics. These results will usher investigations into synthetic analogues of important biological self-assembly motifs and will prove to be a significant advancement toward biomimetic temporally programmed materials.

Controllable supramolecular polymerization via a chain-growth mechanism

A naphthalene-diimide appended carboxylic acid either spontaneously self-assembles (P) by an open-chain H-bonding or can be arrested in an intra-molecularly H-bonded monomeric state (M) depending on the sample preparation method. Sonication of P produces a seed (S) that triggers controllable supramolecular polymerization (SP) of M via a chain growth route with first order kinetics, a tunable mesoscopic structure and a living nature as demonstrated by the chain-extension up to three cycles. SP of an achiral M, initiated by a chiral S, illustrates chiral amplification by the "sergeant and soldiers" effect.

Templated Formation of Luminescent Virus-like Particles by Tailor-Made Pt(II) Amphiphiles

Virus-like particles (VLPs) have been created from luminescent Pt(II) complex amphiphiles, able to form supramolecular structures in water solutions, that can be encapsulated or act as templates of cowpea chlorotic mottle virus capsid proteins. By virtue of a bottom-up molecular design, icosahedral and nonicosahedral (rod-like) VLPs have been constructed through diverse pathways, and a relationship between the molecular structure of the complexes and the shape and size of the VLPs has been observed. A deep insight into the mechanism for the templated formation of the differently shaped VLPs was achieved, by electron microscopy measurements (TEM and STEM) and bulk analysis (FPLC, DLS, photophysical investigations). Interestingly, the obtained VLPs can be visualized by their intense emission at room temperature, generated by the self-assembly of the Pt(II) complexes. The encapsulation of the luminescent species is further verified by their higher emission quantum yields inside the VLPs, which is due to the confinement effect of the protein cage. These hybrid materials demonstrate the potential of tailor-made supramolecular systems able to control the assembly of biological building blocks.

Solvent dependent pathway complexity and seeded supramolecular polymerization

cNDI-1 exhibits an off-pathway aggregate in cyclic hydrocarbon (MCH) but produces a helical supramolecular polymer in linear alkane (decane) by well-defined J-aggregation.

Tunable Energy Landscapes to Control Pathway Complexity in Self-Assembled N-Heterotriangulenes: Living and Seeded Supramolecular Polymerization

Herein, the synthesis and self-assembling features of N-heterotriangulenes 1-3 decorated in their periphery with 3,4,5-trialkoxy-N-(alkoxy)benzamide moieties that enable kinetic control of the supramolecular polymerization process are described. The selection of an appropriate solvent results in a tunable energy landscape in which the relative energy of the different monomeric or aggregated species can be regulated. Thus, in a methylcyclohexane/toluene (MCH/Tol) mixture, intramolecular hydrogen-bonding interactions in the peripheral side units favor the formation of metastable inactivated monomers that evolve with time at precise conditions of concentration and temperature. A pathway complexity in the supramolecular polymerization of 1-3 cannot be determined in MCH/Tol mixtures but, importantly, this situation changes by using CCl₄ . In this solvent, the off-pathway product is a face-to-face H-type aggregate and the on-pathway product is the slipped face-to-face J-type aggregate. The autocatalytic transformation of the metastable monomeric units, as well as the two competing off- and on-pathway aggregates allow the realization of seeded and living supramolecular polymerizations. Interestingly, the presence of chiral, branched side chains in chiral (S)-2 noticeably retards the kinetics of the investigated transformations. This work brings to light the relevance of controlling the pathway complexity in self-assembling units and opens new avenues for the investigation of complex and functional supramolecular structures.

Living Supramolecular Polymerization of a Perylene Bisimide Dye into Fluorescent J-Aggregates

The self-assembly of a new perylene bisimide (PBI) organogelator with 1,7-dimethoxy substituents in the bay position affords non-fluorescent H-aggregates at high cooling rates and fluorescent J-aggregates at low cooling rates. Under properly adjusted conditions, the kinetically trapped "off-pathway" H-aggregates are transformed into the thermodynamically favored J-aggregates, a process that can be accelerated by the addition of J-aggregate seeds. Spectroscopic studies revealed a subtle interplay of π-π interactions and intra- and intermolecular hydrogen bonding for monomeric, H-, and J-aggregated PBIs. Multiple polymerization cycles initiated from the seed termini demonstrate the living character of this chain-growth supramolecular polymerization process.

Dimensional Control and Morphological Transformations of Supramolecular Polymeric Nanofibers Based on Cofacially-Stacked Planar Amphiphilic Platinum(II) Complexes

Square-planar platinum(II) complexes often stack cofacially to yield supramolecular fiber-like structures with interesting photophysical properties. However, control over fiber dimensions and the resulting colloidal stability is limited. We report the self-assembly of amphiphilic Pt(II) complexes with solubilizing ancillary ligands based on polyethylene glycol [PEG_n, where n = 16, 12, 7]. The complex with the longest solubilizing PEG ligand, Pt-PEG₁₆, self-assembled to form polydisperse one-dimensional (1D) nanofibers (diameters <5 nm). Sonication led to short seeds which, on addition of further molecularly dissolved Pt-PEG₁₆ complex, underwent elongation in a "living supramolecular polymerization" process to yield relatively uniform fibers of length up to ca. 400 nm. The fiber lengths were dependent on the Pt-PEG₁₆ complex to seed mass ratio in a manner analogous to a living covalent polymerization of molecular monomers. Moreover, the fiber lengths were unchanged in solution after 1 week and were therefore "static" with respect to interfiber exchange processes on this time scale. In contrast, similarly formed near-uniform fibers of Pt-PEG₁₂ exhibited dynamic behavior that led to broadening of the length distribution within 48 h. After aging for 4 weeks in solution, Pt-PEG₁₂ fibers partially evolved into 2D platelets. Furthermore, self-assembly of Pt-PEG₇ yielded only transient fibers which rapidly evolved into 2D platelets. On addition of further fiber-forming Pt complex (Pt-PEG₁₆), the platelets formed assemblies via the growth of fibers selectively from their short edges. Our studies demonstrate that when interfiber dynamic exchange is suppressed, dimensional control and hierarchical structure formation are possible for supramolecular polymers through the use of kinetically controlled seeded growth methods.

Complex and Hierarchical 2D Assemblies via Crystallization-Driven Self-Assembly of Poly(l-lactide) Homopolymers with Charged Termini

Poly(l-lactide) (PLLA)-based nanoparticles have attracted much attention with respect to applications in drug delivery and nanomedicine as a result of their biocompatibility and biodegradability. Nevertheless, the ability to prepare PLLA assemblies with well-defined shape and dimensions is limited and represents a key challenge. Herein we report access to a series of monodisperse complex and hierarchical colloidally stable 2D structures based on PLLA cores using the seeded growth, "living-crystallization-driven self-assembly" method. Specifically, we describe the formation of diamond-shaped platelet micelles and concentric "patchy" block co-micelles by using seeds of the charge-terminated homopolymer PLLA₂₄[PPh₂Me]I to initiate the sequential growth of either additional PLLA₂₄[PPh₂Me]I or a crystallizable blend of the latter with the block copolymer PLLA₄₂-b-P2VP₂₄₀, respectively. The epitaxial nature of the growth processes used for the creation of the 2D block co-micelles was confirmed by selected area electron diffraction analysis. Cross-linking of the P2VP corona of the peripheral block in the 2D block co-micelles using Pt nanoparticles followed by dissolution of the interior region in good solvent for PLLA led to the formation of novel, hollow diamond-shaped assemblies. We also demonstrate that, in contrast to the aforementioned results, seeded growth of the unsymmetrical PLLA BCPs PLLA₄₂-b-P2VP₂₄₀ or PLLA₂₀-b-PAGE₈₀ alone from 2D platelets leads to the formation of diamond-fiber hybrid structures.

Programmable Assembly of Peptide Amphiphile via Noncovalent-to-Covalent Bond Conversion

Controlling the number of monomers in a supramolecular polymer has been a great challenge in programmable self-assembly of organic molecules. One approach has been to make use of frustrated growth of the supramolecular assembly by tuning the balance of attractive and repulsive intermolecular forces. We report here on the use of covalent bond formation among monomers, compensating for intermolecular electrostatic repulsion, as a mechanism to control the length of a supramolecular nanofiber formed by self-assembly of peptide amphiphiles. Circular dichroism spectroscopy in combination with dynamic light scattering, size-exclusion chromatography, and transmittance electron microscope analyses revealed that hydrogen bonds between peptides were reinforced by covalent bond formation, enabling the fiber elongation. To examine these materials for their potential biomedical applications, cytotoxicity of nanofibers against C2C12 premyoblast cells was tested. We demonstrated that cell viability increased with an increase in fiber length, presumably because of the suppressed disruption of cell membranes by the fiber end-caps.

Autocatalytic Time-Dependent Evolution of Metastable Two-Component Supramolecular Assemblies to Self-Sorted or Coassembled State

Despite substantial effort devoted in the history of supramolecular chemistry, synthetic supramolecular systems still lag behind biomolecular systems in terms of complexity and functionality. This is because biomolecular systems function in a multicomponent molecular network under out-of-equilibrium conditions. Here we report two-component supramolecular assemblies that are metastable and thus show time-dependent evolution. We found that the systems undergo either self-sorting or coassembly in time depending on the combination of components. Interestingly, this outcome, which had been previously achievable only under specific conditions, emerged from the two-component systems as a result of synergistic or reciprocal interplay between the coupled equilibria. We believe that this study sheds light on the similarity between synthetic and biomolecular systems and promotes better understanding of their intricate kinetic behaviors.

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.

Facet-Selective Growth of Organic Heterostructured Architectures via Sequential Crystallization of Structurally Complementary π-Conjugated Molecules

In contrast to those for their polymeric counterparts, the controlled construction of organic heterostructured architectures derived from π-conjugated organic molecules has been rare and remains a great challenge. Herein, we develop a simple single-step solution strategy for the realization of organic heterostructures comprising coronene and perylene. Under a sequential crystallization process, an efficient doping step for coronene and perylene domains enables their perfect lattice matching, which facilitates facet-selective epitaxial growth of perylene domains on both the tips and the side surfaces of the preformed seed microwires by manipulating the growth pathways of the two pairs of materials. The present synthetic route provides a promising platform to investigate the detailed formation mechanism of complex organic heterostructures with specific topological configurations, further directing the construction of more functional heterostructured materials.

Phosphorescent Pt(ii) complexes bearing a monoanionic C^N^N luminophore and tunable ancillary ligands

A new monoanionic pincer luminophore is presented, yielding phosphorescent Pt(ii) complexes bearing a neutral 1,2,3-triazole ring introduced via click chemistry. The overall charge, intermolecular interactions and excited state properties can be manipulated and controlled.

Control over differentiation of a metastable supramolecular assembly in one and two dimensions

Molecular self-assembly under kinetic control is expected to yield nanostructures that are inaccessible through the spontaneous thermodynamic process. Moreover, time-dependent evolution, which is reminiscent of biomolecular systems, may occur under such out-of-equilibrium conditions, allowing the synthesis of supramolecular assemblies with enhanced complexities. Here we report on the capacity of a metastable porphyrin supramolecular assembly to differentiate into nanofibre and nanosheet structures. Mechanistic studies of the relationship between the molecular design and pathway complexity in the self-assembly unveiled the energy landscape that governs the unique kinetic behaviour. Based on this understanding, we could control the differentiation phenomena and achieve both one- and two-dimensional living supramolecular polymerization using an identical monomer. Furthermore, we found that the obtained nanostructures are electronically distinct, which illustrates the pathway-dependent material properties.

Photoregulated Living Supramolecular Polymerization Established by Combining Energy Landscapes of Photoisomerization and Nucleation-Elongation Processes

The significant contribution of conventional living polymerization to polymer science assures that living supramolecular polymerization will also lead to a variety of novel phenomena and applications. However, the monomer scope still remains limited in terms of the self-assembly energy landscape; a kinetic trap that retards spontaneous nucleation has to be coupled with a supramolecular polymerization pathway, which is challenging to achieve by molecular design. Herein, we report a rational approach to addressing this issue. We combined the supramolecular polymerization and photoisomerization processes to build the energy landscape, wherein the monomer can be activated/deactivated by light irradiation. In this way, the supramolecular polymerization and kinetic trap can be independently designed in the energy landscape. When the "dormant" monomer was activated by light in the presence of the seed of the supramolecular polymer, the "activated" free monomer was polymerized at the termini of the seed in a chain-growth manner. As a result, we achieved supramolecular polymers with controlled lengths and a narrow polydispersity. Although photoisomerization has been extensively employed in supramolecular polymer chemistry, most studies have focused on the stimuli responsiveness. In this respect, the present study would provoke supramolecular chemists to revisit stimuli-responsive supramolecular polymer systems as potential candidates for devising living supramolecular polymerization.