Two-Dimensional Living Supramolecular Polymerization: Improvement in Edge Roughness of Supramolecular Nanosheets by Using a Dummy Monomer.

Supramolecular polymers are formed through nucleation (i. e., initiation) and polymerization processes, and kinetic control over the nucleation process has recently led to the realization of living supramolecular polymerization. Changing the viewpoint, herein we focus on controlling the polymerization process, which we expect to pave the way to further developments in controlled supramolecular polymerization. In our previous study, two-dimensional living supramolecular polymerization was used to produce supramolecular nanosheets with a controlled area; however, these had rough edges. In this study, the growth of the nanosheets was controlled by using a 'dummy' monomer to produce supramolecular nanosheets with smoothed edges.

Polymorphism in Squaraine Dye Aggregates by Self-Assembly Pathway Differentiation: Panchromatic Tubular Dye Nanorods versus J-Aggregate Nanosheets.

A bis(squaraine) dye equipped with alkyl and oligoethyleneglycol chains was synthesized by connecting two dicyanomethylene substituted squaraine dyes with a phenylene spacer unit. The aggregation behavior of this bis(squaraine) was investigated in non-polar toluene/tetrachloroethane (98:2) solvent mixture, which revealed competing cooperative self-assembly pathways into two supramolecular polymorphs with entirely different packing structures and UV/Vis/NIR absorption properties. The self-assembly pathway can be controlled by the cooling rate from a heated solution of the monomers. For both polymorphs, quasi-equilibrium conditions between monomers and the respective aggregates can be established to derive thermodynamic parameters and insights into the self-assembly mechanisms. AFM measurements revealed a nanosheet structure with a height of 2 nm for the thermodynamically more stable polymorph and a tubular nanorod structure with a helical pitch of 13 nm and a diameter of 5 nm for the kinetically favored polymorph. Together with wide angle X-ray scattering measurements, packing models were derived: the thermodynamic polymorph consists of brick-work type nanosheets that exhibit red-shifted absorption bands as typical for J-aggregates, while the nanorod polymorph consists of eight supramolecular polymer strands of the bis(squaraine) intertwined to form a chimney-type tubular structure. The absorption of this aggregate covers a large spectral range from 550 to 875 nm, which cannot be rationalized by the conventional exciton theory. By applying the Essential States Model and considering intermolecular charge transfer, the aggregate spectrum was adequately reproduced, revealing that the broad absorption spectrum is due to pronounced donor-acceptor overlap within the bis(squaraine) nanorods. The latter is also responsible for the pronounced bathochromic shift observed for the nanosheet structure as a result of the slip-stacked arranged squaraine chromophores.

Control over the Aspect Ratio of Supramolecular Nanosheets by Molecular Design.

Recent developments in kinetically controlled supramolecular polymerization permit control of the size (i.e., length and area) of self-assembled nanostructures. However, control of molecular self-assembly at a level comparable with organic synthetic chemistry and the achievement of structural complexity at a hierarchy larger than the molecular level remain challenging. This study focuses on controlling the aspect ratio of supramolecular nanosheets. A systematic understanding of the relationship between the monomer structure and the self-assembly energy landscape has derived a new monomer capable of forming supramolecular nanosheets. With this monomer in hand, the aspect ratio of a supramolecular nanosheet is demonstrated that it can be controlled by modulating intermolecular interactions in two dimensions.

A Block Supramolecular Polymer and Its Kinetically Enhanced Stability.

Biomolecular systems serve as an inspiration for the creation of multicomponent synthetic supramolecular systems that can be utilized to develop functional materials with complexity. However, supramolecular systems rapidly reach an equilibrium state through dynamic and reversible noncovalent bonds, resulting in a disorganized mixture of components rather than a system in which individual components function cooperatively and/or independently. Thus, efficient synthetic strategies and characterization methods for intricate multicomponent supramolecular assemblies need to be developed. Herein, we report the synthesis of porphyrin-based supramolecular polymers (SPs) in which two distinct block segments consisting of different metal porphyrins are connected: i.e., block supramolecular polymers (BSPs). BSPs with a controlled length and narrow polydispersity were achieved through seeded-growth by a solvent mixing protocol. Interestingly, the block structure permitted the SP as an inner block to coexist with a reagent that was otherwise incompatible with the SP alone. We infer that the inner SP block is compartmentalized in the block structure and endowed with the kinetic stability. Molecular simulations revealed that monomer exchange occurs from the termini of the SP, which corroborated the enhanced stability of the BSP. These results are expected to pave the way for the design of more complex multicomponent supramolecular systems.

Direct Observation and Manipulation of Supramolecular Polymerization by High-Speed Atomic Force Microscopy.

Despite recent advances in mechanistic understanding and controlled-synthesis methodologies regarding synthetic supramolecular assemblies, it has remained challenging to capture the molecular-level phenomena in real time, thus hindering further progress in this research field. In this study, we applied high-speed atomic-force microscopy (AFM), which has extraordinary spatiotemporal resolution (1 nm and sub-100 ms), to capture dynamic events occurring during synthetic molecular self-assembly. High-speed AFM permitted the visualization of unique dynamic behavior, such as seeded growth and self-repair in real time. Furthermore, scanning-probe AFM permitted the site-specific manipulation and functionalization of a molecular self-assembly. This powerful combination of bottom-up and top-down approaches at the molecular level should enable targeted syntheses of unprecedented functional nanoarchitectures.

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.

Mechanism of self-assembly process and seeded supramolecular polymerization of perylene bisimide organogelator.

The mechanism of supramolecular polymerization has been elucidated for an archetype organogelator molecule composed of a perylene bisimide aromatic scaffold and two amide substituents. This molecule self-assembles into elongated one-dimensional nanofibers through a cooperative nucleation-growth process. Thermodynamic and kinetic analyses have been applied to discover conditions (temperature, solvent, concentration) where the spontaneous nucleation can be retarded by trapping of the monomers in an inactive conformation, leading to lag times up to more than 1 h. The unique kinetics in the nucleation process was confirmed as a thermal hysteresis in a cycle of assembly and disassembly processes. Under appropriate conditions within the hysteresis loop, addition of preassembled nanofiber seeds leads to seeded polymerization from the termini of the seeds in a living supramolecular polymerization process. These results demonstrate that seeded polymerizations are not limited to special situations where off-pathway aggregates sequester the monomeric reactant species but may be applicable to a large number of known and to be developed molecules from the large family of molecules that self-assemble into one-dimensional nanofibrous structures. Generalizing from the mechanistic insight into our seeded polymerization, we assert that a cooperative nucleation-growth supramolecular polymerization accompanied by thermal hysteresis can be controlled in a living manner.

Single molecular resistive switch obtained via sliding multiple anchoring points and varying effective wire length.

A single molecular resistive (conductance) switch via control of anchoring positions was examined by using a molecule consisting of more than two same anchors. For this purpose, we adopted the covered quaterthiophene (QT)-based molecular wire junction. The QT-based wire consisted of two thiophene ring anchors on each side; thus, shift of anchors was potentially possible without a change in the binding modes and distortion of the intramolecular structure. We observed three distinct conductance states by using scanning tunneling microscope-based break junction technique. A detailed analysis of the experimental data and first-principles calculations revealed that the mechanism of the resistive switch could be explained by standard length dependence (exponential decay) of conductance. Here, the length is the distance between the anchoring points, i.e., length of the bridged π-conjugated backbone. Most importantly, this effective tunneling length was variable via only controlling the anchoring positions in the same molecule. Furthermore, we experimentally showed the possibility of a dynamic switch of anchoring positions by mechanical control. The results suggested a distinct strategy to design functional devices via contact engineering.

Conductive poly(2,5-substituted aniline)s highly soluble both in water and organic solvents.

Highly soluble conductive polyanilines were synthesized from newly designed aniline derivatives: 2,5-bis(2-methoxyethoxy)aniline (2) and 2,5-bis[2-(2-methoxyethoxy)ethoxy]aniline (3). The corresponding polyanilines, P2 and P3, were characterized by means of fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), and UV-VIS-NIR spectroscopies. The electrical conductivities at room temperature of emeraldine salt forms of P2 (P2-ES) and P3 (P3-ES) were evaluated to be sigma(rt) = 2.4 x 10(-3) and 1.7 x 10(-3) S/cm, respectively. The length of ethylene-1,2-dioxy chains on the polyaniline scarcely affected the electronic conductivity. A simple modification at 2,5-positions of aniline by introducing 1,2-ethlenedioxy groups dramatically altered the solubility of polyanilines in common organic solvents and water (400 g/L for P3-ES).

Kinetic control over pathway complexity in supramolecular polymerization through modulating the energy landscape by rational molecular design.

Far-from-equilibrium thermodynamic systems that are established as a consequence of coupled equilibria are the origin of the complex behavior of biological systems. Therefore, research in supramolecular chemistry has recently been shifting emphasis from a thermodynamic standpoint to a kinetic one; however, control over the complex kinetic processes is still in its infancy. Herein, we report our attempt to control the time evolution of supramolecular assembly in a process in which the supramolecular assembly transforms from a J-aggregate to an H-aggregate over time. The transformation proceeds through a delicate interplay of these two aggregation pathways. We have succeeded in modulating the energy landscape of the respective aggregates by a rational molecular design. On the basis of this understanding of the energy landscape, programming of the time evolution was achieved through adjusting the balance between the coupled equilibria.

Picket-fence polythiophene and its diblock copolymers that afford microphase separations comprising a stacked and an isolated polythiophene ensemble.

All-polythiophene diblock copolymers, comprising one unsheathed block and one fenced block, were synthesized through catalyst-transfer polycondensation. The unsheathed block self-assembles through π-π stacking, thereby inducing microphase separation. Consequently, we have succeeded in creating a microphase separation comprising an ensemble of stacked and isolated polythiophenes. This achievement could be extended to various unexplored applications as a result of the integration of the contrasting functions of the two blocks.

Synthesis of polyaniline with low polydispersity by using a supramolecular ionic assembly as the reaction medium.

A supramolecular ionic assembly comprised of an anionic oligo(phenylene ethynylene) and anilinium cations provides a unique reaction medium in which anilinum cations are concentrated and aligned. The oxidative polymerization (see figure) of aniline using the supramolecular ionic assembly (gray) yielded polyaniline (green/blue) with a number-average molar mass of 20,500 and polydispersity of 1.3.

Individually separated supramolecular polymer chains toward solution-processable supramolecular polymeric materials.

Herein, we present a simple design concept for a monomer that affords individually separated supramolecular polymer chains. Random introduction of alkyl chains with different lengths onto a monomer prevented its supramolecular polymers from bundling, permitting the preparation of concentrated solutions of the supramolecular polymer without gelation, precipitation, or crystallization. With such a solution in hand, we succeeded in fabricating self-standing films and threads consisting of supramolecular polymers.

Multistep, site-selective noncovalent synthesis of two-dimensional block supramolecular polymers.

Although the principles of noncovalent bonding are well understood and form the basis for the syntheses of many intricate supramolecular structures, supramolecular noncovalent synthesis cannot yet achieve the levels of precision and complexity that are attainable in organic and/or macromolecular covalent synthesis. Here we show the stepwise synthesis of block supramolecular polymers from metal-porphyrin derivatives (in which the metal centre is Zn, Cu or Ni) functionalized with fluorinated alkyl chains. These monomers first undergo a one-dimensional supramolecular polymerization and cyclization process to form a toroidal structure. Subsequently, successive secondary nucleation, elongation and cyclization steps result in two-dimensional assemblies with concentric toroidal morphologies. The site selectivity endowed by the fluorinated chains, reminiscent of regioselectivity in covalent synthesis, enables the precise control of the compositions and sequences of the supramolecular structures, as demonstrated by the synthesis of several triblock supramolecular terpolymers.

Efficient linking of two epoxides using potassium thioacetate in water and its use in polymerization.

Here, we show that two epoxides can be efficiently linked using potassium thioacetate (AcSK) in water even at their imbalanced stoichiometric ratios. We found that the first reaction between epoxide and AcSK gave rise to an intermediate that underwent the second reaction with another epoxide with a reactivity much higher than that of AcSK. Time-dependent 1H NMR measurements revealed that the rate constant of the second reaction was 31 times larger than that of the first reaction. Using this reaction, we succeeded in nonstoichiometric polymerization of a bifunctional epoxide. Furthermore, in the presence of a multifunctional epoxide, we obtained hydrogels and self-standing films. We expect that this straightforward and efficient reaction of versatile reagents, epoxide and AcSK, in water would lead to new applications of epoxides.

A self-threading polythiophene: defect-free insulated molecular wires endowed with long effective conjugation length.

Herein, we report on a self-threading polythiophene whose conjugated molecular wire is sheathed within its own cyclic side chains. The defect-free insulating layer prevents electronic cross-communication between the adjacent polythiophene backbone even in the solid film. Notably, the covalently linked cyclic side chains extend the effective conjugation length of the interior polythiophene backbone, which results in an excellent intrawire hole mobility of 0.9 cm(2) V(-1) s(-1).

Supramolecular double-stranded Archimedean spirals and concentric toroids.

Connecting molecular-level phenomena to larger scales and, ultimately, to sophisticated molecular systems that resemble living systems remains a considerable challenge in supramolecular chemistry. To this end, molecular self-assembly at higher hierarchical levels has to be understood and controlled. Here, we report unusual self-assembled structures formed from a simple porphyrin derivative. Unexpectedly, this formed a one-dimensional (1D) supramolecular polymer that coiled to give an Archimedean spiral. Our analysis of the supramolecular polymerization by using mass-balance models suggested that the Archimedean spiral is formed at high concentrations of the monomer, whereas other aggregation types might form at low concentrations. Gratifyingly, we discovered that our porphyrin-based monomer formed supramolecular concentric toroids at low concentrations. Moreover, a mechanistic insight into the self-assembly process permitted a controlled synthesis of these concentric toroids. This study both illustrates the richness of self-assembled structures at higher levels of hierarchy and demonstrates a topological effect in noncovalent synthesis.

Conducting polymer networks cross-linked by "isolated" functional dyes: design, synthesis, and electrochemical polymerization of doubly strapped light-harvesting porphyrin/oligothiophene monomers.

Dye-functionalized conducting polymers: A new concept in the design of monomers gives bithiophene-based monomers as "isolated" functional dyes, thus yielding electroactive films through anodic electrochemical polymerization (see picture).We have synthesized doubly strapped porphyrin derivatives with four bithiophene segments that diverge from the porphyrin core, namely, Por(BT)(4) and PorZn(BT)(4). These molecules are designed as electrochemically polymerizable monomers that will yield the highly cross-linked conducting polymeric networks poly[Por(BT)(4)] and poly[PorZn(BT)(4)], respectively, through an oxidative coupling reaction among the bithiophene moieties. Selective synthesis of the distal doubly strapped porphyrin derivatives was successful from the bis(formylphenyl) and bis(dipyrrolylmethylphenyl) straps under Lindsey conditions. Slow kinetics observed for the zinc insertion reaction toward the doubly strapped porphyrin derivative revealed that both faces of the porphyrin plane are overlaid with alkyl chain straps. To probe this "isolation" effect, photophysical properties of the monomers and their model components were investigated by UV/Vis and fluorescence spectroscopic analysis. An absorption spectral comparison between the diluted solution and the thin solid film of Por(BT)(4) demonstrated that the porphyrin molecule is shielded by the double strap and self-aggregation is prevented. It is noteworthy that the fluorescence of the spin-coated film of the doubly strapped porphyrin monomer was twice as strong as that of an unstrapped porphyrin, thus indicating that the double strap can suppress undesired deactivation processes from the photoexcited state. In addition, fluorescence spectral measurements revealed that quantitative energy transfer from the oligothiophene segments to the porphyrin molecule takes place, which demonstrates an effective electronic interaction between these two chromophores. Electrochemical polymerization of the monomers Por(BT)(4) and PorZn(BT)(4) gave robust films that showed stable electrochemistry. Absorption spectral measurements and electrochemical characterization of the obtained films showed that the doubly strapped porphyrins are incorporated into the conducting polymer networks without any decomposition and protonation. Given all these observations above, our new monomer design based on functional dyes shielded by the double strap will lead to new organic optoelectronic materials in which functional molecules are spatially incorporated and isolated and yet show an effective electronic interactions with the conducting polymer backbones.

Living supramolecular polymerization based on reversible deactivation of a monomer by using a 'dummy' monomer.

Although living supramolecular polymerization (LSP) has recently been realized, the scope of the monomer structures applicable to the existing methods is still limited. For instance, a monomer that spontaneously nucleates itself cannot be processed in a manner consistent with LSP. Herein, we report a new method for such a "reactive" monomer. We use a 'dummy' monomer which has a similar structure to the reactive monomer but is incapable of one-dimensional supramolecular polymerization. We show that in the presence of the dummy monomer, the reactive monomer is kinetically trapped in the dormant state. In this way, spontaneous nucleation of the reactive monomer is retarded; yet, addition of seeds of a supramolecular polymer can initiate the supramolecular polymerization in a chain growth manner. As a result, we obtain the supramolecular polymer of the reactive monomer with a controlled length, which is otherwise thermodynamically inaccessible. We believe that this concept will expand the scope of LSP for the synthesis of other functional supramolecular polymers, and thus lead to a variety of applications.

Rod-like transition first or chain aggregation first? ordered aggregation of rod-like poly(p-phenyleneethynylene) chains in solution.

The rod-like configuration of conjugated polymer chains with its low energetic disorder is the key to utilizing the backbone as a highly electrically-conductive wire. An energetic disorder that is higher than 0.1 eV, coupled with vibronic modes of the chains, leads to the localization of charges. Herein, we have tracked precisely the rod-like transition of poly(p-phenyleneethynylene) (PPE) chains as a function of temperature in diluted solutions, and shown a steep increase in persistence length at 230 K. The resulting rod-like configuration of the PPE chains with its extended electronic conjugation exhibited an extremely small energetic disorder of ∼70 meV, and was stabilized by subsequent polymer aggregate formation.