Independent Control of the Width and Length of Semiconducting 2D Nanorectangles via Accelerated Living Crystallization-Driven Self-Assembly.

Self-assembly of conjugated polymers offers a powerful method to prepare semiconducting two-dimensional (2D) nanosheets for optoelectronic applications. However, due to the typical biaxial growth behavior of the polymer self-assembly, independent control of the width and length of 2D sheets has been challenging. Herein, we present a greatly accelerated crystallization-driven self-assembly (CDSA) system of polyacetylene-based conjugated polymer to produce 2D semiconducting nanorectangles with precisely controllable dimensions. In detail, rectangular 2D seeds with tunable widths of 0.2-1.3 µm were produced by changing the cosolvent% and grown in the length direction by uniaxial living CDSA up to 11.8 µm. The growth rate was effectively enhanced by tuning the cosolvent%, seed concentration, and temperature, achieving up to 27-fold increase. Additionally, systematic kinetic investigation yielded empirical rate equations, elucidating the relationship between growth rate constant, cosolvent%, seed concentration, and seed width. Finally, the living CDSA allowed us to prepare penta-block comicelles with tunable width, length, and height.

Size-Tunable Semiconducting 2D Nanorectangles from Conjugated Polyenyne Homopolymer Synthesized via Cascade Metathesis and Metallotropy Polymerization.

Size-tunable semiconducting two-dimensional (2D) nanosheets from conjugated homopolymers are promising materials for easy access to optoelectronic applications, but it has been challenging due to the low solubility of conjugated homopolymers. Herein, we report size-tunable and uniform semiconducting 2D nanorectangles via living crystallization-driven self-assembly (CDSA) of a fully conjugated polyenyne homopolymer prepared by cascade metathesis and metallotropy (M&M) polymerization. The resulting polyenyne with enhanced solubility successfully underwent living CDSA via biaxial growth mechanism, thereby producing 2D nanorectangles with sizes precisely tuned from 0.1 to 3.0 µm2 with narrow dispersity mostly less than 1.1 and low aspect ratios less than 3.1. Furthermore, living CDSA produced complex 2D block comicelles with different heights from various degrees of polymerization (DPs) of unimers. Based on diffraction analyses and DFT calculations, we proposed an interdigitating packing model with an orthorhombic crystal lattice of semiconducting 2D nanorectangles.

Dynamics and Equilibration Mechanisms in Block Copolymer Particles.

Self-assembly of block copolymers into interesting and useful nanostructures, in both solution and bulk, is a vibrant research arena. While much attention has been paid to characterization and prediction of equilibrium phases, the associated dynamic processes are far from fully understood. Here, we explore what is known and not known about the equilibration of particle phases in the bulk, and spherical micelles in solution. The presumed primary equilibration mechanisms are chain exchange, fusion, and fragmentation. These processes have been extensively studied in surfactants and lipids, where they occur on subsecond time scales. In contrast, increased chain lengths in block copolymers create much larger barriers, and time scales can become prohibitively slow. In practice, equilibration of block copolymers is achievable only in proximity to the critical micelle temperature (in solution) or the order-disorder transition (in the bulk). Detailed theories for these processes in block copolymers are few. In the bulk, the rate of chain exchange can be quantified by tracer diffusion measurements. Often the rate of equilibration, in terms of number density and aggregation number of particles, is much slower than chain exchange, and consequently observed particle phases are often metastable. This is particularly true in regions of the phase diagram where Frank-Kasper phases occur. Chain exchange in solution has been explored quantitatively by time-resolved SANS, but the results are not well captured by theory. Computer simulations, particularly via dissipative particle dynamics, are beginning to shed light on the chain escape mechanism at the molecular level. The rate of fragmentation has been quantified in a few experimental systems, and TEM images support a mechanism akin to the anaphase stage of mitosis in cells, via a thin neck that pinches off to produce two smaller micelles. Direct measurements of micelle fusion are quite rare. Suggestions for future theoretical, computational, and experimental efforts are offered.

Conformation Dynamics of Single Polymer Strands in Solution.

Conformational changes in macromolecules significantly affect their functions and assembly into high-level structures. Despite advances in theoretical and experimental studies, investigations into the intrinsic conformational variations and dynamic motions of single macromolecules remain challenging. Here, liquid-phase transmission electron microscopy enables the real-time tracking of single-chain polymers. Imaging linear polymers, synthetically dendronized with conjugated aromatic groups, in organic solvent confined within graphene liquid cells, directly exhibits chain-resolved conformational dynamics of individual semiflexible polymers. These experimental and theoretical analyses reveal that the dynamic conformational transitions of the single-chain polymer originate from the degree of intrachain interactions. In situ observations also show that such dynamics of the single-chain polymer are significantly affected by environmental factors, including surfaces and interfaces.

Synchronous Preparation of Length-Controllable 1D Nanoparticles via Crystallization-Driven In Situ Nanoparticlization of Conjugated Polymers.

Precise size control of semiconducting nanomaterials from polymers is crucial for optoelectronic applications, but the low solubility of conjugated polymers makes this challenging. Herein, we prepared length-controlled semiconducting one-dimensional (1D) nanoparticles by synchronous self-assembly during polymerization. First, we succeeded in unprecedented living polymerization of highly soluble conjugated poly(3,4-dihexylthiophene). Then, block copolymerization of poly(3,4-dihexylthiophene)-block-polythiophene spontaneously produced narrow-dispersed 1D nanoparticles with lengths from 15 to 282 nm according to the size of a crystalline polythiophene core. The key factors for high efficiency and length control are a highly solubilizing shell and slow polymerization of the core, thereby favoring nucleation elongation over isodesmic growth. Combining kinetics and high-resolution imaging analyses, we propose a unique mechanism called crystallization-driven in situ nanoparticlization of conjugated polymers (CD-INCP) where spontaneous nucleation creates seeds, followed by seeded growth in units of micelles. Also, we achieved "living" CD-INCP through a chain-extension experiment. We further simplified CD-INCP by adding both monomers together in one-shot copolymerization but still producing length-controlled nanoparticles.

Semi-conducting 2D rectangles with tunable length via uniaxial living crystallization-driven self-assembly of homopolymer.

Semi-conducting two-dimensional (2D) nanoobjects, prepared by self-assembly of conjugated polymers, are promising materials for optoelectronic applications. However, no examples of self-assembled semi-conducting 2D nanosheets whose lengths and aspect ratios are controlled at the same time have been reported. Herein, we successfully prepared uniform semi-conducting 2D sheets using a conjugated poly(cyclopentenylene vinylene) homopolymer and its block copolymer by blending and heating. Using these as 2D seeds, living crystallization-driven self-assembly (CDSA) was achieved by adding the homopolymer as a unimer. Interestingly, unlike typical 2D CDSA examples showing radial growth, this homopolymer assembled only in one direction. Owing to this uniaxial growth, the lengths of the 2D nanosheets could be precisely tuned from 1.5 to 8.8 µm with narrow dispersity according to the unimer-to-seed ratio. We also studied the growth kinetics of the living 2D CDSA and confirmed first-order kinetics. Subsequently, we prepared several 2D block comicelles (BCMs), including penta-BCMs in a one-shot method.

Rapid formation and real-time observation of micron-sized conjugated nanofibers with tunable lengths and widths in 20 minutes by living crystallization-driven self-assembly.

Preparing well-defined semiconducting nanostructures from conjugated polymers is of paramount interest for organic optoelectronic devices. Several studies have demonstrated excellent structural and size control from block copolymers (BCPs) containing non-conjugated blocks via crystallization-driven self-assembly (CDSA); however, the precise control of their size and shape remains a challenge due to their poor solubility, causing rapid and uncontrolled aggregation. This study presents a new type of fully conjugated BCP comprising two polyacetylene derivatives termed poly(cyclopentenylene-vinylene) to prepare semiconducting 1D nanofibers. Interestingly, the widths of nanofibers were tuned from 12 to 32 nm based on the contour lengths of their crystalline core blocks. Their lengths could also be controlled from 48 nm to 4.7 µm using the living CDSA. Monitoring of the growth kinetics of the living CDSA revealed the formation of micron-sized 1D nanofibers in less than 20 min. The rapid CDSA enabled us to watch real-time growth using confocal fluorescence microscopy.

Morphologically Tunable Square and Rectangular Nanosheets of a Simple Conjugated Homopolymer by Changing Solvents.

Two-dimensional (2D) polymer nanosheets have been attracting immense attention owing to their potential applications in optical devices, membranes, and catalysis. However, creating uniform monolayered 2D nanosheets through polymer self-assembly is very challenging, especially when using homopolymers. In this work, we designed a new crystalline polyacetylene that contains fluorenes and triisopropylsilyl side chains, which could self-assemble into sharp-edged 5-nm-thick square nanosheets with a narrow length dispersity of 1.01, by simple heating and aging in dichloromethane (DCM). Interestingly, the addition of tetrahydrofuran (THF) or chloroform to the heated polymer solution in DCM changed the morphology from square to rectangle. The aspect ratios increased linearly, from 1.0 to 10.6, according to the amount of THF or chloroform added, while maintaining narrow length dispersities less than 1.06. These unique fluorescent semiconducting nanosheets with tunable shapes exhibit high potential for optoelectronic applications.

Conformation of Tunable Nanocylinders: Up to Sixth-Generation Dendronized Polymers via Graft-Through Approach by ROMP.

Well-defined dendronized polymers (denpols) bearing high-generation dendron are attractive nano-objects as high persistency provides distinct properties, contrast to the random coiled linear polymers However, their syntheses via graft-through approach have been very challenging due to their structural complexity and steric hindrance retarding polymerization. Here, we report the first example of the synthesis of poly(norbornene) (PNB) containing ester dendrons up to the sixth generation (G6) by ring-opening metathesis polymerization. This is the highest generation ever polymerized among dendronized polymers prepared by graft-through approach, producing denpols with molecular weight up to 1960 kg/mol. Combination of size-exclusion chromatography, light scattering, and neutron scattering allowed a thorough structural study of these large denpols in dilute solution. A semiflexible cylinder model was successfully applied to represent both the static and dynamic experimental quantities yielding persistent length (l p), cross-sectional radius (R cs), and contour length (L). The denpol persistency seemed to increase with generation, with l p reaching 27 nm (Kuhn length 54 nm) for PNB-G6, demonstrating a rod-like conformation. Poly(endo-tricycle[4.2.2.0]deca-3,9-diene) (PTD) denpols exhibited larger persistency than the PNB analogues of the same generation presumably due to the higher grafting density of the PTD denpols. As the dendritic side chains introduce shape anisotropy into the denpol backbone, future work will entail a study of these systems in the concentrated solutions and melts.

Synthesis of Functional Polyacetylenes via Cyclopolymerization of Diyne Monomers with Grubbs-type Catalysts.

Metathesis cyclopolymerization (CP) of α,ω-diynes is a powerful method to prepare functional polyacetylenes (PAs). PAs have long been studied due to their interesting electrical, optical, photonic, and magnetic properties which make them candidates for use in various advanced applications. Grubbs catalysts are widely used throughout synthetic chemistry, largely due to their accessibility, high reactivity, and tolerance to air, moisture, and many functional groups. Prior to our entrance into this field, only a few examples of CP using modified Grubbs catalysts existed. Inspired by these works, we saw an opportunity to expand the accessibility and utility of Grubbs-catalyzed CPs. We began by exploring CP with popular and commercially available Grubbs catalysts. We found Grubbs third-generation catalyst (G3) to be an excellent catalyst when we used strategies to stabilize the propagating Ru carbene, such as decreasing the polymerization temperature or using weakly coordinating solvent or ligands. Controlled living polymerizations were demonstrated using various 1,6-heptadiyne monomers and yielded polymers with exclusively 5-membered rings (via α-addition) in the polymer backbone. The strategy of stabilizing the Ru carbene was also critical to successful CP with Hoveyda-Grubbs second-generation (HG2) and Grubbs first-generation (G1) catalysts. We found that decomposed Ru species were catalyzing side reactions which could be completely shut down by decreasing the reaction temperature or using weakly coordinating ligands. While HG2 generally led to uncontrolled polymerizations, we found it to be an effective catalyst for monomers with very large side chains. G1 displayed broader functional group tolerance and thus broader monomer scope than G3. We next looked at our ability to change the regioselectivity of the polymerization by using Z-selective catalysts which favor ß-addition and the formation of 6-membered rings in the polymer backbone. While modest ß-selectivity could be obtained using Grubbs Z-selective catalyst at low temperatures, we found that by using one of Hoveyda and co-workers' catalysts with decreased carbene electrophilicity, we could achieve exclusive formation of 6-membered rings. We also pursued alternative routes to achieve 6+-membered rings in the polymer backbone by using diyne monomers with increased distance between alkynes. We found that optimizing the monomer structure for CP was an effective strategy to achieve controlled polymerizations. By using bulky substituents (maximizing the Thorpe-Ingold effect) and/or using heteroatoms (shorter bonds) to bring the alkynes closer together, controlled living CP could be achieved with various 1,7-octadiyne and 1,8-nonadiyne monomers. Finally, we took advantage of several inherent properties of controlled CP techniques to prepare polymers with advanced architectures and nanostructures. For instance, the living nature of the polymerization enabled production of block copolymers, the tolerance of very large substituents enabled production of dendronized and brush polymers, and the insolubility or crystallinity of some monomers was utilized for the spontaneous self-assembly of polymers into various one- and two-dimensional nanostructures. Overall, the strategies of stabilizing the propagating Ru carbene, modulating the selectivity and reactivity of the Ru carbene, and enhancing the inherent reactivity of monomers were key to improving the utility and performance of CP with Grubbs-type catalysts. The insight provided by these studies will be important for future developments of CP and other metathesis polymerizations utilizing ring-closing steps.

Preparing Semiconducting Nanoribbons with Tunable Length and Width via Crystallization-Driven Self-Assembly of a Simple Conjugated Homopolymer.

Precise control of width and length of one-dimensional (1D) semiconducting nanostructures has attracted much attention owing to its potential for optoelectronic applications. However, regulating both their length and width using conjugated polymers or even block copolymers is a huge challenge. To solve this problem, we synthesized a unique conjugated polyacetylene homopolymer by living cyclopolymerization, which spontaneously formed 1D nanoribbons via in situ nanoparticlization. Interestingly, their widths could be controlled from 8 to 41 nm, which were directly proportional to their degree of polymerization. Furthermore, a self-seeding technique via crystallization-driven self-assembly (CDSA) was used to control the length of the nanoribbons up to 5.2 µm with narrow distributions less than 1.1. Interestingly, adding a block copolymer unimer to these nanoribbons produced triblock comicelles by the living CDSA mechanism. Finally, these nanoribbons were visualized directly by super-resolution optical fluorescence microscopy. Now, one can modulate both length and width of 1D nanoribbons simultaneously.

Direct Formation of Large-Area 2D Nanosheets from Fluorescent Semiconducting Homopolymer with Orthorhombic Crystalline Orientation.

Semiconducting polymers have been widely investigated due to their intriguing optoelectronic properties and their high crystallinity that provides a strong driving force for self-assembly. Although there are various reports of successful self-assembly of nanostructures using semiconducting polymers, direct in situ self-assembly of these polymers into two-dimensional (2D) nanostructures has proven difficult, despite their importance for optoelectronics applications. Here, we report the synthesis of a simple conjugated homopolymer by living cyclopolymerization of a 1,6-heptadiyne (having a fluorene moiety) and its efficient in situ formation of large-area 2D fluorescent semiconducting nanostructures. Using high-resolution imaging tools such as atomic force microscopy and transmission electron microscopy, we observed the solvent-dependent self-assembly behaviors of this homopolymer; the identical starting polymer formed 2D nanosheets with different shapes, such as rectangle, raft, and leaf, when dissolved in different solvents. Furthermore, super-resolution optical microscopy enabled the real-time imaging of the fluorescent 2D nanosheets, revealing their stable and uniform shapes, fluorescence, and solution dynamics. Notably, we propose an orthorhombic crystalline packing model to explain the direct formation of 2D nanostructures based on various diffraction patterns, providing important insight for their shape modulation during the self-assembly.

A one-pot synthesis of polysulfane-bearing block copolymer nanoparticles with tunable size and refractive index.

A one-pot synthesis of sulfur-rich polymer nanoparticles through a ring-opening metathesis polymerization is reported. The nanoparticles are formed in situ from diblock copolymers containing a polynorbornene derivative bearing cyclic polysulfanes. The refractive indices of the resulting nanoparticles are readily controlled in the range from 1.54 to nearly 1.65.

Construction of protein chip to detect binding of Mitf protein (microphthalmia transcription factor) and E-box DNA.

A protein chip was constructed to detect the binding of microphthalmia-associated transcription factor (Mitf) and E-box DNA. Mitf, a key regulatory transcriptional factor of pigmentation-related genes such as tyrosinase, binds to specific sequence (CATGTG) in E-box DNA within the promoter of tyrosinase in the melanocytes. We produced Mitf as a maltose-binding protein (MBP) fusion protein in Escherichia coli, purified it using an affinity column, and immobilized it on beta-cyclodextrin-coated glass plate. Binding of Mitf to its target DNA, E-box oligomer, was monitored by surface plasmon resonance (SPR), SPR imaging (SPRi), and fluorescence-based system. Among these detection methods, fluorescence method was the most reliable. In this method, fluorescent intensity was proportional to the DNA concentration (up to 20 microM) and Mitf (up to 500 microg/ml). Kinetics of DNA binding with Mitf showed Langmuir isotherm, and its kinetic constants were determined. It is expected that Mitf-E-box DNA chip can be used as a screening tool for depigmenting agents in the cosmetic industry.