Synthesis of Carbene-Stabilized PNPN Fragments and Their Carbene-Dependent Redox Properties.

Herein, we report the synthesis of carbene-stabilized 1,3-diaza-2,4-diphosphabutenes CAACMePNPNCAACMe 4CAAC (CAACMe = 1-[2,6-bis(isopropyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene) and IPrPNPNIPr 4NHC (IPr = 1,3-Bis(2,6-diisopropylphenyl)-imidazol-2-ylidene). The bonding in both systems is defined by a delocalized polar covalent π-system, with 4NHC exhibiting increased conjugation relative to 4CAAC. The nature of the stabilizing carbene also influences the redox properties of the compound, with 4CAAC undergoing potassium-mediated reduction to the closed-shell P-P bonded dimer K252, which upon treatment with Kryptofix-2,2,2 converts to the transient radical anion [Kcrypt][5], the formal one-electron reduction product of 4CAAC. In contrast, 4NHC undergoes reversible one-electron oxidation to the stable radical cation [6NHC][SbF6]. Computational and spectroscopic analyses of both radical species are suggestive of unevenly delocalized spin, with the bulk of the spin density residing on phosphorus in both cases.

Breaking of Lateral Symmetry in Two-Dimensional Crystallization-Driven Self-Assembly on a Surface.

Symmetry breaking is prevalent in nature and provides distinctive access to hierarchical structures for artificial materials. However, it is rarely explored in two-dimensional (2D) entities, especially for lateral asymmetry. Herein, we describe a unique symmetry breaking process in surface-initiated 2D living crystallization-driven self-assembly. The 2D epitaxial growth occurs only at one lateral side of the immobilized cylindrical micelle seeds, accessing unilateral platelets with the yield increasing with the seed length, the growth temperature, and poly(2-vinylpyridine) corona length (maximum = 92%). Generally, the tilted immobilization of seeds blocks one lateral side and triggers the lateral symmetry breaking, where the intensity and spatial arrangement of seed-surface interactions dictate the regulation. Segmented unilateral platelets with segmented corona regions are further fabricated with the addition of different blended unimers. Remarkably, discrete slope-like and dense blade-like platelet arrays grow off the surface when seeds are compactly aligned either with spherical micelles or themselves. This strategy provides nanoscale insights into the symmetry breaking in long-range self-assembly and would be promising for the design of innovative colloids and smart surfaces.

Functional Noncentrosymmetric Nanoparticle-Nanofiber Hybrids via Selective Fragmentation.

Noncentrosymmetric nanostructures are an attractive synthetic target as they can exhibit complex interparticle interactions useful for numerous applications. However, generating uniform, colloidally stable, noncentrosymmetric nanoparticles with low aspect ratios is a significant challenge using solution self-assembly approaches. Herein, we outline the synthesis of noncentrosymmetric multiblock co-nanofibers by subsequent living crystallization-driven self-assembly of block co-polymers, spatially confined attachment of nanoparticles, and localized nanofiber fragmentation. Using this strategy, we have fabricated uniform diblock and triblock noncentrosymmetric π-conjugated nanofiber-nanoparticle hybrid structures. Additionally, in contrast to Brownian motion typical of centrosymmetric nanoparticles, we demonstrated that these noncentrosymmetric nanofibers undergo ballistic motion in the presence of H2O2 and thus could be employed as nanomotors in various applications, including drug delivery and environmental remediation.

Mechanism of Action and Design of Potent Antibacterial Block Copolymer Nanoparticles.

Self-assembled polymer nanoparticles are promising antibacterials, with nonspherical morphologies of particular interest as recent work has demonstrated enhanced antibacterial activity relative to their spherical counterparts. However, the reasons for this enhancement are currently unclear. We have performed a multifaceted analysis of the antibacterial mechanism of action of 1D nanofibers relative to nanospheres by the use of flow cytometry, high-resolution microscopy, and evaluations of the antibacterial activity of pristine and tetracycline-loaded nanoparticles. Low-length dispersity, fluorescent diblock copolymer nanofibers with a crystalline poly(fluorenetrimethylenecarbonate) (PFTMC) core (length = 104 and 472 nm, height = 7 nm, width = 10-13 nm) and a partially protonated poly(dimethylaminoethyl methacrylate) (PDMAEMA) corona (length = 12 nm) were prepared via seeded growth living crystallization-driven self-assembly. Their behavior was compared to that of analogous nanospheres containing an amorphous PFTMC core (diameter of 12 nm). While all nanoparticles were uptaken into Escherichia coli W3110, crystalline-core nanofibers were observed to cause significant bacterial damage. Drug loading studies indicated that while all nanoparticle antibacterial activity was enhanced in combination with tetracycline, the enhancement was especially prominent when small nanoparticles (ca. 15-25 nm) were employed. Therefore, the identified differences in the mechanism of action and the demonstrated consequences for nanoparticle size and morphology control may be exploited for the future design of potent antibacterial agents for overcoming antibacterial resistance. This study also reinforces the requirement of morphological control over polymer nanoparticles for biomedical applications, as differences in activity are observed depending on their size, shape, and core-crystallinity.

High-resolution cryo-electron microscopy structure of block copolymer nanofibres with a crystalline core.

Seeded growth of crystallizable block copolymers and π-stacking molecular amphiphiles in solution using living crystallization-driven self-assembly is an emerging route to fabricate uniform one-dimensional and two-dimensional core-shell micellar nanoparticles of controlled size with a range of potential applications. Although experimental evidence indicates that the crystalline core of these nanomaterials is highly ordered, a direct observation of their crystal lattice has not been successful. Here we report the high-resolution cryo-transmission electron microscopy studies of vitrified solutions of nanofibres made from a crystalline core of poly(ferrocenyldimethylsilane) (PFS) and a corona of polysiloxane grafted with 4-vinylpyridine groups. These studies show that poly(ferrocenyldimethylsilane) chains pack in an 8-nm-diameter core lattice with two-dimensional pseudo-hexagonal symmetry that is coated by a 27 nm 4-vinylpyridine corona with a 3.5 nm distance between each 4-vinylpyridine strand. We combine this structural information with a molecular modelling analysis to propose a detailed molecular model for solvated poly(ferrocenyldimethylsilane)-b-4-vinylpyridine nanofibres.

Surface-Initiated Living Self-Assembly of Polythiophene-Based Conjugated Block Copolymer into Erect Micellar Brushes.

Nanostructured conjugated polymers are of widespread interest due to their broad applications in organic optoelectronic devices, biomedical sensors and other fields. However, the alignment of conjugated nanostructures perpendicular to a surface remains a critical challenge. Herein, we report a facile method to directly self-assemble a poly(3-(2-ethylhexyl)thiophene), P3EHT-based block copolymer into densely aligned micellar brushes through surface-initiated living crystallization-driven self-assembly. The presence of an ethyl pendant on the side group intrinsically moderates the crystallization rate of the polythiophene main chains, and hence favors the controlled living growth of long conjugated fibers and the subsequent fabrication of conjugated micellar brushes. The corona of the micellar brush can be further decorated with platinum nanoparticles, which enables the formation of erect nanoarrays with heights up to 2700 nm in the dried state. This also renders the micellar brush catalytically active toward hydrogen evolution reaction, which shows a low overpotential of 27 mV at 10 mA cm-2 . Notably, the P3EHT-based micellar brush can simultaneously grow with polyferrocenyldimethylsilane, PFS-based micellar brush on the same surface without any significant interference between the two systems. Thus, these two micellar brushes can be patterned through site-selective immobilization of two types of seeds followed by independent living self-assembly.

A Crystalline Monomeric Phosphaborene.

We report the synthesis of the monomeric phosphaborene Ar*P═B(TMP) (2) (Ar* = 2,6-bis(triisopropylphenyl)-3,5-diisopropylphenyl) containing 2-coordinate phosphorus and boron centers. Compound 2 has a PB bond length of 1.741(3) Å, the shortest reported to date. Computational examination of the bonding in 2 reveals, in addition to the σ bond, the presence of a single classical π bond and a large Wiberg bond index of 1.9707, consistent with double bond, and not triple bond, character. The chemistry of 2 is marked by its low reactivity, which is rationalized by examination of the frontier molecular orbitals and steric considerations.

Defect-Induced Secondary Crystals Drive Two-Dimensional to Three-Dimensional Morphological Evolution in the Co-Self-Assembly of Polyferrocenylsilane Block Copolymer and Homopolymer.

Bottom-up fabrication protocols for uniform 3D hierarchical structures in solution are rare. We report two different approaches to fabricate uniform 3D spherulites and their precursors using mixtures of poly(ferrocenyldimethylsilane) (PFS) block copolymer (BCP) and PFS homopolymer (HP). Both protocols are designed to promote defects in 2D assemblies that serve as intermediate structures. In a multistep seeded growth protocol, we add the BCP/HP mixture to (1D) rod-like PFS micelles in a selective solvent as first-generation seeds. This leads to 2D platelet structures. If this step is conducted at a high supersaturation, secondary crystals form on the basal surface of these platelets. Co-crystallization and rapid crystallization of BCP/HP promote the formation of defects that act as nucleation sites for secondary crystals, resulting in multilayer platelets. This is the key step. The multilayer platelets serve as second-generation seeds upon subsequent addition of BCP/HP blends and, with increasing supersaturation, lead to the sequential formation of uniform (3D) hedrites, sheaves, and spherulites. Similar structures can also be obtained by a simple one-pot direct self-assembly (heating-cooling-aging) protocol of PFS BCP/HP blends. In this case, for a carefully chosen but narrow temperature range, PFS HPs nucleate formation of uniform structures, and the annealing temperature regulates the supersaturation level. In both protocols, the competitive crystallization kinetics of HP/BCP affects the morphology. Both protocols exhibit broad generality. We believe the morphological transformation from 2D to 3D structures, regulated by defect formation, co-crystallization, and supersaturation levels, could apply to various semicrystalline polymers. Moreover, the 3D structures are sufficiently robust to serve as recoverable carriers for nanoparticle catalysts, exhibiting valuable catalytic activity and opening new possibilities for applications requiring exquisite 3D structures.

Synthesis of a Carbene-Stabilized (Diphospha)aminyl Radical and Its One Electron Oxidation and Reduction to Nonclassical Nitrenium and Amide Species.

Herein, we report the synthesis of an acyclic carbene-stabilized diphospha(aminyl) PNP radical CAACMePNPCAACMe 4 (CAACMe = 1-[2,6-bis(isopropyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene) by a facile one-pot, seven-electron reduction of hexachlorophosphazene chloride [Cl3PNPCl3][Cl]. The PNP radical 4 features a conjugated framework with spin density primarily localized on the central nitrogen atom as well as the flanking carbenes. Unlike other tripnictogen radicals, 4 undergoes facile one-electron oxidation and reduction to yield nonclassical nitrenium and amide species [5]+ and [6]-, respectively. The cation [5]+ exhibits conformational flexibility in the solution state between the expected W-shaped geometry [5b]+ and a previously unobserved linear heteroallene-type structure [5a]+, which was characterized in the solid state. The equilibrium was explored both computationally and experimentally, showing that [5a]+ is favored over [5b]+ both enthalpically (ΔH = -2.9 × 103 ± 80 J mol-1) and entropically (ΔS = 4.2 ± 0.25 J mol-1 K-1). The formal amide [6]- displays remarkable flexibility in its coordination chemistry due to the presence of multiple Lewis basic centers, as evidenced by the structure of its potassium complex K262, which exhibits µ, κ-P, κ-P, and η3-PNP coordination modes. Protonation of [6]- leads to the formation of an amine 7, which features a trigonal planar geometry around nitrogen.

Tailored Energy Funneling in Photocatalytic π-Conjugated Polymer Nanofibers for High-Performance Hydrogen Production.

The creation of artificial high-performance photosynthetic assemblies with a tailorable antenna system to deliver absorbed solar energy to a photosynthetic reaction center, thereby mimicking biological photosynthesis, remains a major challenge. We report the construction of recyclable, high-performance photosynthetic nanofibers with a crystalline π-conjugated polyfluorene core as an antenna system that funnels absorbed solar energy to spatially defined sensitized Co(II) porphyrin photocatalysts for the hydrogen evolution reaction. Highly effective energy funneling was achieved by tuning the dimensions of the nanofibers to exploit the very long exciton diffusion lengths (>200 nm) associated with the highly crystalline polyfluorene core formed using the living crystallization-driven self-assembly seeded growth method. This enabled efficient solar light-driven hydrogen production from water with a turnover number of over 450 for 8 h of irradiation, an H2 production rate of ca. 65 mmol h-1 g-1, and an overall quantum yield of 0.4% in the wavelength region (<405 nm) beyond the absorption of the molecular photocatalyst. The strategy of using a tailored antenna system based on π-conjugated polymers and maximizing exciton transport to a reaction center reported in this work opens up future opportunities for potential applications in other fields such as solar overall water splitting, CO2 reduction, and photocatalytic small molecule synthesis.

Polyferrocenylsilane Block Copolymer Spherulites in Dilute Solution.

Self-assembly of block copolymers (BCP) into uniform 3D structures in solution is an extremely rare phenomenon. Furthermore, the investigation of general prerequisites for fabricating a specific uniform 3D structure remains unknown and challenging. Here, through a simple one-pot direct self-assembly (heating and cooling) protocol, we show that uniform spherulite-like structures and their precursors can be prepared with various poly(ferrocenyldimethylsilane) (PFS) BCPs in a variety of polar and non-polar solvents. These structures all evolve from elongated lamellae into hedrites, sheaf-like micelles, and finally spherulites as the annealing temperature and supersaturation degree are increased. The key feature leading to this growth trajectory is the formation of secondary crystals by self-nucleation on the surface of early-elongated lamellae. We identified general prerequisites for fabricating PFS BCP spherulites in solution. These include corona/PFS core block ratios in the range of 1-5.5 that favor the formation of 2D structures as well as the development of secondary crystals on the basal faces of platelets at early stages of the self-assembly. The one-pot direct self-assembly provides a general protocol to form uniform spherulites and their precursors consisting of PFS BCPs that match these prerequisites. In addition, we show that manipulation of various steps in the direct self-assembly protocol can regulate the size and shape of the structures formed. These general concepts show promise for the fabrication and optimization of spherulites and their precursors from semicrystalline BCPs with interesting optical, electronic, or biomedical properties using the one-pot direct self-assembly protocol.

Tuning the Functionality of Self-Assembled 2D Platelets in the Third Dimension.

The decoration of 2D nanostructures using heteroepitaxial growth is of great importance to achieve functional assemblies employed in biomedical, electrical, and mechanical applications. Although the functionalization of polymers before self-assembly has been investigated, the exploration of direct surface modification in the third dimension from 2D nanostructures has, to date, been unexplored. Here, we used living crystallization-driven self-assembly to fabricate poly(ε-caprolactone)-based 2D platelets with controlled size. Importantly, surface modification of the platelets in the third dimension was achieved by using functional monomers and light-induced polymerization. This method allows us to selectively regulate the height and fluorescence properties of the nanostructures. Using this approach, we gained unprecedented spatial control over the surface functionality in the specific region of complex 2D platelets.

Transition-Metal-Free Dehydropolymerization of Phosphine-Boranes at Ambient Temperature.

Stoichiometric reaction of phosphine-borane adducts RR'PH⋅BH3 (R=Ph, R'=H, Ph, Et, and R=R'=t Bu) with the strong acid HNTf2 (Tf=SO2 CF3 ) leads to H2 elimination and the formation of the triflimido derivatives, RR'PH⋅BH2 (NTf2 ). Subsequent deprotonation by using bases, such as diisopropylethylamine or the carbene IPr (IPr=N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), led to the formation of P-mono- or -disubstituted polyphosphinoboranes [RR'P-BH2 ]n . Evidence for the intermediacy of transient phosphinoborane monomers, RR'PBH2 , was provided by trapping reactions.

Titanium-Catalyzed Polymerization of a Lewis Base-Stabilized Phosphinoborane.

The reaction of the Lewis base-stabilized phosphinoborane monomer tBuHPBH2 NMe3 (2 a) with catalytic amounts of bis(η5 :η1 -adamantylidenepentafulvene)titanium (1) provides a convenient new route to the polyphosphinoborane [tBuPH-BH2 ]n (3 a). This method offers access to high molar mass materials under mild conditions and with short reaction times (20 °C, 1 h in toluene). It represents an unprecedented example of a transition metal-mediated polymerization of a Lewis base-stabilized Group 13/15 compound. Preliminary studies of the substrate scope and a potential mechanism are reported.

Metal Organic Framework Cubosomes.

We demonstrate a general strategy for the synthesis of ordered bicontinuous-structured metal organic frameworks (MOFs) by using polymer cubosomes (PCs) with a double primitive structure (Im 3 ‾ ${\bar{3}}$ m symmetry) as the template. The filling of MOF precursors in the open channel of PCs, followed by their coordination and removal of the template, generates MOF cubosomes with a single primitive topology (Pm 3 ‾ ${\bar{3}}$ m) and average mesopore diameters of 60-65 nm. Mechanism study reveals that the formation of ZIF-8 cubosomes undergoes a new MOF growth process, which involves the formation of individual MOF seeds in the template, their growth and eventual fusion into the cubosomes. Their growth kinetics follows the Avrami equation with an Avrami exponent of n=3 and a growth rate of k=1.33×10-4 , indicating their fast 3D heterogeneous growth mode. Serving as a bioreactor, the ZIF-8 cubosomes show high loading of trypsin enzyme, leading to a high catalytic activity in the proteolysis of bovine serum albumin.

Customizable Supraparticles Constructed from Catechol-Terminated Molecular Building Blocks with Controllable Intermolecular Interactions.

Colloidal supraparticles integrated with multicomponent primary particles come with emerging or synergetic functionalities. However, achieving the functional customization of supraparticles remains a great challenge because of the limited options of building blocks with tailorability and functional extensibility. Herein, we developed a universal approach to construct customizable supraparticles with desired properties from molecular building blocks obtained by the covalent conjugation of catechol groups with a series of orthogonal functional groups. These catechol-terminated molecular building blocks can assemble into primary particles driven by various intermolecular interactions (i.e. metal-organic coordination, host-guest, and hydrophobic interactions), and then further assemble into supraparticles governed by catechol-mediated interfacial interactions. Our strategy enables the formation of supraparticles with diverse functionalities, such as dual-pH responsiveness, light-controllable permeability, and non-invasive fluorescence labeling of living cells. The ease with which these supraparticles can be fabricated, and the ability to tailor their chemical and physical properties through the choice of metals and orthogonal functional groups used, should enable a variety of applications.

Polyphosphinoborane Block Copolymer Synthesis Using Catalytic Reversible Chain-Transfer Dehydropolymerization.

An amphiphilic block copolymer of polyphosphinoborane has been prepared by a mechanism-led strategy of the sequential catalytic dehydropolymerization of precursor monomers, H3 B ⋅ PRH2 (R=Ph, n-hexyl), using the simple pre-catalyst [Rh(Ph2 PCH2 CH2 PPh2 )2 ]Cl. Speciation, mechanism and polymer chain growth studies support a step-growth process where reversible chain transfer occurs, i.e. H3 B ⋅ PRH2 /oligomer/polymer can all coordinate with, and be activated by, the catalyst. Block copolymer [H2 BPPhH]110 -b-[H2 BP(n-hexyl)H]11 can be synthesized and self-assembles in solution to form either rod-like micelles or vesicles depending on solvent polarity.

"Self-Adaptive" Coassembly of Colloidal "Saturn-like" Host-Guest Complexes Enabled by Toroidal Micellar Rubber Bands.

The creation of inclusion complexes with "Saturn-like" geometries has attracted increasing attention for supramolecular systems, but expansion of the concept to nanoscale colloidal systems remains a challenge. Here, we report a strategy to assemble toroidal polyisoprene-b-poly(2-vinylpyridine) (PI-b-P2VP) block copolymer micelles with a PI core and a P2VP corona and inorganic (e.g., silica) nanoparticles of variable shape and dimensions into "Saturn-like" constructs with high fidelity and yield. The precise nesting of the nanoparticles between the toroidal building units is realized by virtue of hydrogen bonding and self-adaptive expansion of the flexible toroidal units enabled by a flexible, low Tg PI core. Once the toroidal units are cross-linked, the self-adaptive feature is lost and coassembly yields instead out-of-cavity bound nanoparticles. "Saturn-like" assemblies can also be formed along silica nanosphere-decorated cylindrical micelles or, alternatively, at the hydroxyl-functionalized termini of cylindrical micelles to yield colloidal [3]rotaxanes.

Exploring the "Living" Growth of Block Copolymer Nanofibers from Surface-Confined Seeds by In Situ Solution-Phase Atomic Force Microscopy.

Living crystallization-driven self-assembly of polymeric and molecular amphiphiles is of growing interest as a seeded growth route to uniform 1D, 2D, and more complex micellar nanoparticles with controlled dimensions and a range of potential applications. Although most studies have been performed using colloidally stable seeds in bulk solution, growth of block copolymer (BCP) nanofibers from seeds confined to a surface is attracting increased attention. Herein, we have used atomic force microscopy (AFM) to undertake detailed studies of the growth of BCP nanofibers from immobilized seeds located on a Si surface. Through initial ex situ AFM studies and in situ AFM video analysis in solution, we determined that growth occurred in four stages, whereby an initial surface-bound growth regime transitions to surface-limited growth. As the nanofiber length increases, surface influence is diminished as the newly grown micelle segment is no longer bound to the Si substrate. Finally, a surface-independent regime occurs where nanofiber growth continues into bulk solution. In addition to the anticipated nanofiber elongation, our studies revealed occasional examples of AFM tip-induced core fragmentation. In these cases, the termini of the newly formed fragments were also active to further growth. Furthermore, unidirectional growth was detected in cases where the seed was oriented at a significant angle with respect to the surface, thereby restricting unimer access to one terminus.

Scalable and Uniform Length-Tunable Biodegradable Block Copolymer Nanofibers with a Polycarbonate Core via Living Polymerization-Induced Crystallization-Driven Self-assembly.

Uniform 1D block copolymer (BCP) nanofibers prepared by the seeded-growth approach termed living crystallization-driven self-assembly (CDSA) offer promising potential for various applications due to their anisotropy, length tunability, and variable core and coronal chemistries. However, this procedure consists of a multi-step process involving independent BCP synthesis and self-assembly steps, where the latter is performed at low solution concentrations (<1 wt %), hindering scale-up. Here, we demonstrate the use of a one-pot BCP synthesis and self-assembly process, polymerization-induced CDSA (PI-CDSA), to access length-disperse nanofibers with a biodegradable crystalline poly(fluorenetrimethylenecarbonate) (PFTMC) core and a hydrophilic poly(ethylene glycol) (PEG) corona derived from PEG-b-PFTMC at concentrations up to 20 wt %, 400 times higher than those previously reported. Furthermore, living PI-CDSA could be used to access scalable, low dispersity, and length-tunable 1D PEG-b-PFTMC nanofibers at concentrations of up to 10 wt %. This provides the first example of living PI-CDSA involving an all-organic and biodegradable BCP that utilizes a conveniently implemented BCP synthesis protocol and does not involve living anionic polymerization. Significantly, samples of low-dispersity nanofibers of controlled lengths from 100 to 660 nm (Lw/Ln = 1.08-1.20) were prepared, allowing for upscaled access to well-defined biodegradable nanofibers at useful length-scales for applications in nanomedicine. Interestingly, detailed studies revealed a key role for PFTMC homopolymer impurities in the BCP prepared in situ in the formation of nanofibers under the reaction conditions used.