Brenton A. G. Hammer

Hierarchical Helical Assembly of Conjugated Poly(3-hexylthiophene)-block poly(3-triethylene glycol thiophene) Diblock Copolymers

We report on the solution-state assembly of all-conjugated polythiophene diblock copolymers containing nonpolar (hexyl) and polar (triethylene glycol) side chains. The polar substituents provide a large contrast in solubility, enabling formation of stably suspended crystalline fibrils even under very poor solvent conditions for the poly(3-hexylthiophene) block. For appropriate block ratios, complexation of the triethylene glycol side chains with added potassium ions drives the formation of helical nanowires that further bundle into superhelical structures.

Dimensional Evolution of Polyphenylenes: Expanding in All Directions

Polyphenylenes (PPs) represent a class of conjugated polymers that have been used in applications ranging from organic electronic devices, sensors, polymer film additives to manipulate their mechanical properties, and even fluorescent tags or nanocarriers in biological media.1-3 The versatility of PPs stem from innovative synthetic strategies that have evolved throughout the years to provide avenues that precisely tune their architecture and function for specific purposes. This Review will cover the state of the art research on various PPs related to the relationship between their structure and resulting properties.

Morphology-dependent electronic properties in cross-linked (P3HT-b-P3MT) block copolymer nanostructures.

Combined Kelvin probe force microscopy and wavelength-resolved photoluminescence measurements on individual pre- and post-cross-linked poly(3-hexylthiophene)-b-poly(3-methyl alcohol thiophene) (P3HT-b-P3MT) nanofibers have revealed striking differences in their optical and electronic properties driven by structural perturbation of the crystalline aggregate nanofiber structures after cross-linking. Chemical cross-linking from diblock copolymer P3HT-b-P3MT using a hexamethylene diisocyanate cross-linker produces a variety of morphologies including very small nanowires, nanofiber bundles, nanoribbons, and sheets, whose relative abundance can be controlled by reaction time and cross-linker concentration. While the different cross-linked morphologies have almost identical photophysical characteristics, KPFM measurements show that the surface potential contrast, related to the work function of the sample, depends sensitively on nanostructure morphology related to chain-packing disorder.

Reversible, Self Cross-linking Nanowires from Thiol-Functionalized Polythiophene Diblock Copolymers

Poly(3-hexylthiophene)-block-poly(3-(3-thioacetylpropyl) oxymethylthiophene) (P3HT)-b-(P3TT) diblock copolymers were synthesized and manipulated by solvent-induced crystallization to afford reversibly cross-linked semiconductor nanowires. To cross-link the nanowires, we deprotected the thioacetate groups to thiols and they subsequently oxidized to disulfides. Cross-linked nanowires maintained their structural integrity in solvents that normally dissolve the polymers. These robust nanowires could be reduced to the fully solvated polymer, representing a novel, reversible cross-linking procedure for functional P3HT-based nanowire fibrils. Field-effect transistor measurements were carried out to determine the charge transport properties of these nanostructures.

The polar side of polyphenylene dendrimers

Polyphenylene dendrimers (PPDs) represent a unique class of dendrimers based on their rigid, shape persistent chemical structure. These macromolecules are typically looked at as nonpolar precursors for conjugated systems. Yet over the years there have been synthetic achievements that have produced PPDs with a range of polarities that break the hydrophobic stereotype, and provide dendrimers that can be synthetically tuned to be used in applications such as stable transition metal catalysts, nanocarriers for biological drug delivery, and sensors for volatile organic compounds (VOCs), among many others. This is based on strategies that allow for the modification of PPDs at the core, scaffold, and surface to introduce numerous different groups, such as electrolytes, ions, or other polar species. This review is aimed to demonstrate the versatility of PPDs through their site-specific chemical functionalization to produce robust materials with various polarities.

Covalent attachment and release of small molecules from functional polyphenylene dendrimers

Herein, we report the synthesis of 2nd generation PPDs functionalized with free thiol moieties within the scaffold, which were used as anchor points for the covalent attachment of guest species (p-nitrophenol derivatives) through the oxidative formation of disulfide linkages. The disulfide bonds were then cleaved under reductive conditions using dithiothreitol to discharge the molecules.

Robust polythiophene nanowires cross-linked with functional fullerenes

Poly(3-hexyl thiophene) (P3HT)-block-poly(3-(3-aminopropyl)oxymethyl thiophene) (P3AmT) diblock copolymers were synthesized and assembled into nanowires by solvent-induced crystallization. Bis(4-[1,6-hexyldiisocyanate]benzylpyrrolidine)-C60 was synthesized and used to covalently cross-link the structures, affording robust p-type/n-type nanowires. These cross-linked nanowires proved stable to solvents and temperatures that would disrupt conventional P3HT-nanowires, as characterized by transmission electron microscopy (TEM) and ultraviolet-visible (UV-Vis) spectroscopy. Photoluminescence spectroscopy showed quenching of the PL signal of the fullerene-crosslinked material, suggesting electronic communication between the polymer and fullerene in these novel donor/acceptor assemblies. Grazing incidence X-ray diffraction (GIXD) showed a similar crystal structure for nanowires before and after cross-linking, while field effect transistor transfer measurements of the cross-linked nanowires showed hole and electron mobilities of 3.5 × 10−5 cm2 V−1 s−1 and 4.6 × 10−5 cm2 V−1 s−1, respectively.

Controlling Cellular Uptake and Toxicity of Polyphenylene Dendrimers by Chemical Functionalization

Polyphenylene dendrimers (PPDs) represent a unique class of macromolecules based on their monodisperse and shape‐persistent nature. These characteristics have enabled the synthesis of a new genre of “patched” surface dendrimers, where their exterior can be functionalized with a variety of polar and nonpolar substituents to yield lipophilic binding sites in a site‐specific way. Although such materials are capable of complexing biologically relevant molecules, show high cellular uptake in various cell lines, and low to no toxicity, there is minimal understanding of the driving forces to these characteristics. We investigated whether it is the specific chemical functionalities, relative quantities of each moiety, or the “patched” surface patterning on the dendrimers that more significantly influences their behavior in biological media.

Expanding the limits of synthetic macromolecular chemistry through Polyphenylene Dendrimers

Polyphenylene dendrimers (PPDs) are a unique class of macromolecules because their backbone is made from twisted benzene repeat units that result in a rigid, shape-persistent architecture as reported by Hammer et al. (Chem Soc Rev 44:4072–4090, 2015) and Hammer and Müllen (Chem Rev 116:2103–210, 2016) These dendrimers can be synthetically tailored at their core, scaffold, and surface to introduce a wide range of chemical functionalities that influence their applications. It is the balance between the macromolecular properties of polyphenylene dendrimers with grandiose synthetic ingenuity that presents a template for the next generation of synthetic dendrimers to achieve complex structures other chemistry fields cannot. This perspective will look at how advances in synthetic chemistry have led to an explosion in the properties of polyphenylene dendrimers from their initial stage, as PPDs that were used as precursors for nanographenes, to next-generation dendrimers for organic electronic devices, sensors for volatile organic compounds (VOCs), nanocarriers for small molecules, and even as complexes with therapeutic drugs and viruses, among others. Ideally, this perspective will illustrate how the evolution of synthetic chemistry has influenced the possible structures and properties of PPDs and how these chemical modifications have opened the door to unprecedented applications.