DNA-Inspired Strand-Exchange for Switchable PMMA-Based Supramolecular Morphologies.

Inspired by nanotechnologies based on DNA strand displacement, herein we demonstrate that synthetic helical strand exchange can be achieved through tuning of poly(methyl methacrylate) (PMMA) triple-helix stereocomplexes. To evaluate the utility and robustness of helical strand exchange, stereoregular PMMA/polyethylene glycol (PEG) block copolymers capable of undergoing crystallization driven self-assembly via stereocomplex formation were prepared. Micelles with spherical or wormlike morphologies were formed by varying the molecular weight composition of the assembling components. Significantly, PMMA strand exchange was demonstrated and utilized to reversibly switch the micelles between different morphologies. This concept of strand exchange with PMMA-based triple-helix stereocomplexes offers new opportunities to program dynamic behaviors of polymeric materials, leading to scalable synthesis of "smart" nanosystems.

Control of Amphiphile Self-Assembly via Bioinspired Metal Ion Coordination.

Inspired by marine siderophores that exhibit a morphological shift upon metal coordination, hybrid peptide-polymer conjugates that assemble into different morphologies based on the nature of the metal ion coordination have been designed. Coupling of a peptide chelator, hexahistidine, with hydrophobic oligostyrene allows a modular strategy to be established for the efficient synthesis and purification of these tunable amphiphiles (oSt(His)6). Remarkably, in the presence of different divalent transition metal ions (Mn, Co, Ni, Cu, Zn, and Cd) a variety of morphologies were observed. Zinc(II), cobalt(II), and copper(II) led to aggregated micelles. Nickel(II) and cadmium(II) produced micelles, and multilamellar vesicles were obtained in the presence of manganese(II). This work highlights the significant potential for transition metal ion coordination as a tool for directing the assembly of synthetic nanomaterials.

Synthesis, Optoelectronic Properties, Self-Association, and Base Pairing of Nucleobase-Functionalized Oligothiophenes.

Device-relevant π-conjugated oligothiophenes with the canonical nucleobases directly embedded into the π-framework have been designed, synthesized, and characterized. These oligomers offer the ability to tune optoelectronic properties via the intimate merging of the nucleobase molecular electronic structure with base-pairing fidelity. Analysis of their optical and electronic properties in a hydrogen-bond-disrupting solvent (DMF) indicates that the nucleobase identity influences the intrinsic electronic properties of the semiconductors. These differences are supported by DFT calculations which demonstrate that the HOMO/LUMO orbitals are distributed differently for each compound. The solubility and competition between self-association and base pairing in a hydrogen-bond-supporting solvent (chloroform) was studied to better understand the oligomer behavior under conditions relevant for downstream solution processing into thin-film devices. These solution studies reveal that in each case base-pairing is preferred to self-aggregation; the relatively weak heteroassociation of 1A-1U (35 ± 5 M-1) should be amenable to facile solution processing and successive hydrogen bond formation in the solid state, while the strong heteroassociation between 1G and 1C (>104 M-1) should enable assemblies to be preformed in solution. These results are expected to enable the synthesis of more complex π-conjugated architectures and facilitate their extension to optoelectronic devices.

Synthesis, optical properties, and electronic structures of nucleobase-containing π-conjugated oligomers.

The molecular recognition properties of the nucleobases instruct the formation of complex three-dimensional architectures in natural and synthetic systems; relatively unexplored is their use as building blocks for π-conjugated materials where they might mutually tune electronic and supramolecular structures. Toward this goal, an introductory set (1a-d and 2a-d) of six purine-terminated and two pyrimidine-terminated π-conjugated oligomers has been synthesized and used to develop experimental electronic and photophysical structure-property trends. Unlike 2,2':5',2″-terthiophene (TTT) derivatives 2a-d, intramolecular charge transfer dominates oligomers 1a-d bearing a 4,7-bisthienylbenzothiadiazole (TBT) spacer due to the strong electron-accepting ability of its benzothiadiazole (BTD) ring. The resulting donor-acceptor-donor systems feature lower HOMO-LUMO gaps than the terthiophene-linked nucleobases (ΔE(g) ∼ 1.8 eV vs 2.4 eV based on electrochemical measurements), and the lowest so far for π-conjugated molecules that include nucleobases within the π-framework. Experiments reveal a dependence of photophysical and electronic structure on the nature of the nucleobase and are in good agreement with theoretical calculations performed at the B3LYP/6-31+G** level. Overall, the results show how nucleobase heterocycles can be installed within π-systems to tune optical and electronic properties. Future work will evaluate the consequences of these information-rich components on supramolecular π-conjugated structure.

Triple Function Lubricant Additives Based on Organic-Inorganic Hybrid Star Polymers: Friction Reduction, Wear Protection, and Viscosity Modification.

Polymer-based lubricant additives for friction reduction, wear protection, or viscosity improvement have been widely studied. However, single additives achieving all three functions are rare. To address this need, we have explored the combination of polymer topology with organic-inorganic hybrid chemistry to simultaneously vary the temperature- and shear-dependent properties of polymer additives in solution and at solid surfaces. A topological library of lubricant additives, based on statistical copolymers of stearyl methacrylate and methyl methacrylate, ranging from linear to branched star architectures, was prepared using ruthenium-catalyzed controlled radical polymerization. Control over the polymerization yielded additives with low dispersity and comparable molecular weights, allowing evaluation of the influence of polymer architecture on friction reduction, wear protection, and bulk viscosity improvement in a commercial base oil (Yubase 4). Structure-performance relationships for these functions were assessed by a combination of a high-speed surface force apparatus (HS-SFA) experiments, wear track profilometry, quartz crystal microbalance analysis, and solution viscometry. The custom-built HS-SFA provides a unique experimental environment to measure the boundary lubrication performance under extreme shear rates (≈107 s-1) for prolonged times (24 h), mimicking the extreme conditions of automotive applications. These experiments revealed that the performance of the additives as boundary lubricants and wear protectants scales with the degree of branching. The branched architectures prohibit ordering of the additives in thin films under high-load conditions, leading to a thicker absorbed polymer brush boundary layer and therefore enhanced film fluidity and lubricity. Additionally, star polymers with increasing arm number lead to bulk viscosity modification, reflected by a significant increase in the viscosity index compared to the commercial base oil. Although outperformed by linear polymers for bulk viscosity improvement, the (hybrid) star polymers successfully combine the three distinct lubricant additive functions: friction reduction, wear protection, and bulk viscosity improvement-in a single polymeric structure. It should also be noted that, judging from HS-SFA experiments, hybrid stars carrying a silicate-based core outperform their fully organic analogues as boundary lubricants. The enhanced performance is most likely driven by attractive forces between the silicate cores and the employed metallic surfaces. Combining three function in one minimizes formulation complexity and thus opens a route to fundamentally understand and formulate key design parameters for the development of novel multifunction lubricant additives.

Novel Strategy for Photopatterning Emissive Polymer Brushes for Organic Light Emitting Diode Applications.

A light-mediated methodology to grow patterned, emissive polymer brushes with micron feature resolution is reported and applied to organic light emitting diode (OLED) displays. Light is used for both initiator functionalization of indium tin oxide and subsequent atom transfer radical polymerization of methacrylate-based fluorescent and phosphorescent iridium monomers. The iridium centers play key roles in photocatalyzing and mediating polymer growth while also emitting light in the final OLED structure. The scope of the presented procedure enables the synthesis of a library of polymers with emissive colors spanning the visible spectrum where the dopant incorporation, position of brush growth, and brush thickness are readily controlled. The chain-ends of the polymer brushes remain intact, affording subsequent chain extension and formation of well-defined diblock architectures. This high level of structure and function control allows for the facile preparation of random ternary copolymers and red-green-blue arrays to yield white emission.

Efficient syntheses of the unknown quinolino[2,3-c]cinnolines; synthesis of neocryptolepines.

A facile, efficient, three-step protocol for the synthesis of the unknown quinolino[2,3-c]cinnoline 5 is introduced. In addition, a new approach for the preparation of the biologically active neocryptolepines 8 in good overall yields is described.