Conjugated Polymers That Respond to Oxidation with Increased Emission

Multicolor Conjugated Polymer Thin Films with Tunable Responsivity to Oxidative and Reductive Environments

Dynamic colors that respond to environmental changes are of great interest for diverse areas of science and technology ranging from chemical and biological sensors to smart information display. Here, we demonstrate a multitude of responsive colors from a conjugated polymer film arising from a thin-film interference. This mechanism provides an excellent control over the thin-film color by varying the film thickness, type of substrate, and degree of polaron population and is generally applicable to various conjugated polymers for further color variation. Furthermore, multiple sets of responsive colors are achieved from a single polymer layer by patterning the underlying substrate to spatially modify the interference conditions. Using this system, we demonstrate the reversible color changes induced by an oxidative or reductive environment with color responsivity controllable with the nature of the polaron state.

Sulfur-bridged chromophores for photofunctional materials: using sulfur oxidation state to tune electronic and structural properties

The use of a heteroatom, such as sulfur, as a linker or bridge, in π-conjugated materials has advantages over purely carbon-based ones due to the accessibility of higher oxidation states as a result of hypervalence. Materials containing a sulfide bridge (S) can be systemically oxidized into sulfoxides (SO) and sulfones (SO2), each of which can then influence how a material interacts with light, playing a large role in dictating the photophysical and sometimes photochemical properties. In this perspective, we summarize the progress that our group and others have made, showing how oxidation of a sulfur bridge in symmetric bichromophoric dimers and in diimine ligands can influence the excited state behavior in organic π-conjugated materials and metal complexes.

The variability of soils and vegetation of hydrothermal fields in the Valley of Geysers at Kamchatka Peninsula

The picturesque and high conservation value thermal landscapes of the Valley of Geysers feature endothermal (heated by endogenous fluids) soils which support endangered and unique species. However, such soils have not been distinguished as a separate taxon within most classification systems. In this study, we described the soil morphology at macro-, meso- and micro-scales, chemistry, mineralogy and vegetation of these landscapes as they are affected by the steam-heated acid-sulfate waters. The studied catenary sequence from exothermal (non-heated) to endothermal soils was characterized by decreasing contents of soil organic carbon, sand fraction, essential nutrients (Ca, K, Mg, Mn and Si), increasing soil acidity, amounts of fine particle-size fractions and contents of trace elements (Al, As, Co, Cr, Cu, Fe, Pb, Ti and V) as well as the development of sodium-sulfate salinity, kaolinization and ferrugination. In phytocenoses supported by endothermal soils, species of order Rosales and Asparagales were overrepresented among obligate and facultative thermophytes respectively, and species of order Poales were underrepresented among facultative thermophytes in relation to the flora of the Valley of Geysers. Phytocenoses on the non-heated Andosols were enriched in Polypodiopsida species. The results of our comparative analysis of the thermally-induced variability in the soils and vegetation contribute to the general understanding of mineralogical, bio-abiotic and biological systems affected by steam-heated acid-sulfate waters. We hope that our findings will provide a basis for future transdisciplinary studies of the influence of steam-heated waters of a hot spring on the thermal landscapes.

Trace Detection of Hydrogen Peroxide via Dynamic Double Emulsions

Hydrogen peroxide is a dynamic signaling molecule in biological systems. We report herein a versatile double emulsion sensor that can detect femtomolar quantities of aqueous hydrogen peroxide. The mechanism responsible for this sensitivity is a peroxide induced change in double emulsion structure, which results in a modified directional emission from dyes dissolved in the high index organic phase. The morphology (structure) of the double emulsion is controlled via interfacial tensions and a methyltrioxorhenium catalyzed sulfide oxidation results in an enhancement of the surfactant effectiveness. The incipient polar sulfoxide induced decrease of the interfacial tension at the organic-water (O-W) interface results in an increased interfacial area between the organic phase and water and a diminished emission perpendicular to the supporting substrate. The modularity of our sensory system is demonstrated through cascade catalysis between methyltrioxorhenium and oxidase enzymes, with the latter producing hydrogen peroxide as a byproduct to enable for the selective and sensitive detection of molecular and ionic enzymatic substrates.

The primary dipole of flipper probes

Despite their growing popularity in biology to image membrane tension, central design principles of flipper probes have never been validated. Here we report that upon deletion of their primary dipole, from electron-poor and electron-rich dithienothiophenes, absorptions blue-shift, lifetimes shorten dramatically, and mechanosensitivity in cells vanishes not partially, but completely.

Improved, Odorless Access to Benzo[1,2-d;4,5-d']- bis[1,3]dithioles and Tert-butyl Arylsulfides via C-S Cross Coupling

Benzo[1,2-d;4,5-d′]bis[1,3]dithioles are important building blocks within a range of functional materials such as fluorescent dyes, conjugated polymers, and stable trityl radicals. Access to these is usually gained via tert-butyl aryl sulfides, the synthesis of which requires the use of highly malodorous tert-butyl thiol and relies on S_NAr-chemistry requiring harsh reaction conditions, while giving low yields. In the present work, S-tert-butyl isothiouronium bromide is successfully applied as an odorless surrogate for tert-butyl thiol. The C-S bond formation is carried out under palladium catalysis with the thiolate formed in situ resulting in high yields of tert-butyl aryl sulfides. The subsequent formation of benzo[1,2-d;4,5-d′]bis[1,3]dithioles is here achieved with scandium(III)triflate, a less harmful reagent than the usually used Lewis acids, e.g., boron trifluoride or tetrafluoroboric acid. This enables a convenient and environmentally more compliant access to high yields of benzo[1,2-d;4,5-d′]bis[1,3]dithioles.

Cellular imaging using emission-tuneable conjugated polymer nanoparticles

New materials that exhibit tuneable optical properties, notable emission across the visible spectrum, are of immense interest to biologists as they present a broad palette of colours from a single imaging agent that can be utilised in biological detection. Such a flexible system, when combined with the advantages of using conjugated polymer nanoparticles in cell imaging results in a widely useful medical diagnostic system. Here, we describe tuneable emission observed through oxidation of a conjugated polymer followed by the formation of nanoparticles and their subsequent use in cell imaging.

Controlled Synthesis of CuInS2/ZnS Nanocubes and Their Sensitive Photoluminescence Response toward Hydrogen Peroxide

We synthesized uniform CuInS₂/ZnS nanocubes by adjusting reaction parameters at the ZnS growth stage. Higher temperature and zinc concentration were shown to drive resultant crystals to have cubic morphology, which could be ascribed to the facet-dependent ligand dynamics on the crystal surface and concomitantly preferred directions of crystal growth. It was found that these nanocubes exhibit sensitive responses, as of photoluminescence quenching, toward hydrogen peroxide, compared to pyramid-shaped nanocrystals. The origin of quenching was further analyzed to be the oxidation of thiolate ligands that leaves the quenching center on the surface. It was noted that the quenched photoluminescence could be fully recovered by introducing additional ligand molecules into the system. Being adopted in the shape-controlled crystal growth, the ligand-to-crystal interaction was shown to still govern the interfacial reaction, the oxidation by hydrogen peroxide, of faceted crystals in our system. It turns out that the reactivity at the crystal surface depends on the exposed facets, especially induced by shape control, and the weak ligand-binding nature of the nanocube renders it vulnerable to the surface reaction.

A pH and redox dual responsive homopolypeptide: synthesis, characterization, and application in “smart” single-walled carbon nanotube dispersion

We present the synthesis and characterization of a pH and redox dual responsive homopolypeptide which can be used as a dual responsive stabilizer to control the aggregation of carbon nanotubes in water.

On the Origins of Nonradiative Excited State Relaxation in Aryl Sulfoxides Relevant to Fluorescent Chemosensing

We provide herein a mechanistic analysis of aryl sulfoxide excited state processes, inspired by our recent report of aryl sulfoxide based fluorescent chemosensors. The use of aryl sulfoxides as reporting elements in chemosensor development is a significant deviation from previous approaches, and thus warrants closer examination. We demonstrate that metal ion binding suppresses nonradiative excited state decay by blocking formation of a previously unrecognized charge transfer excited state, leading to fluorescence enhancement. This charge transfer state derives from the initially formed locally excited state followed by intramolecular charge transfer to form a sulfoxide radical cation/aryl radical anion pair. With the aid of computational studies, we map out ground and excited state potential energy surface details for aryl sulfoxides, and conclude that fluorescence enhancement is almost entirely the result of excited state effects. This work expands previous proposals that excited state pyramidal inversion is the major nonradiative decay pathway for aryl sulfoxides. We show that pyramidal inversion is indeed relevant, but that an additional and dominant nonradiative pathway must also exist. These conclusions have implications for the design of next generation sulfoxide based fluorescent chemosensors.

Ratiometric Array of Conjugated Polymers-Fluorescent Protein Provides a Robust Mammalian Cell Sensor

Supramolecular complexes of a family of positively charged conjugated polymers (CPs) and green fluorescent protein (GFP) create a fluorescence resonance energy transfer (FRET)-based ratiometric biosensor array. Selective multivalent interactions of the CPs with mammalian cell surfaces caused differential change in FRET signals, providing a fingerprint signature for each cell type. The resulting fluorescence signatures allowed the identification of 16 different cell types and discrimination between healthy, cancerous, and metastatic cells, with the same genetic background. While the CP-GFP sensor array completely differentiated between the cell types, only partial classification was achieved for the CPs alone, validating the effectiveness of the ratiometric sensor. The utility of the biosensor was further demonstrated in the detection of blinded unknown samples, where 121 of 128 samples were correctly identified. Notably, this selectivity-based sensor stratified diverse cell types in minutes, using only 2000 cells, without requiring specific biomarkers or cell labeling.

Zinc-mediated diastereoselective assembly of a trinuclear circular helicate

We have designed a triptycene-based ditopic ligand H₂L, which is able to adopt an adequate topology required for optimal binding to naked Zn²⁺ leading to a neutral trinuclear circular helicate Zn₃L₃.

Optical properties and solvatofluorochromism of fluorene derivatives bearing S,S-dioxidized thiophene

Fluorene derivatives having phenylthiophene or benzothiophene showed strong solvatofluorochromism by the oxidation of the thiophene ring.

Synthesis and Preliminary Characterization of a PPE-Type Polymer Containing Substituted Fullerenes and Transition Metal Ligation Sites

A substituted fullerene was incorporated into a PPE-conjugated polymer repeat unit. This subunit was then polymerized via Sonogashira coupling with other repeat units to create polymeric systems approaching 50 repeat units (based on GPC characterization). Bipyridine ligands were incorporated into some of these repeat units to provide sites for transition metal coordination. Photophysical characterization of the absorption and emission properties of these systems shows excited states located on both the fullerene and aromatic backbone of the polymers that exist in a thermally controlled equilibrium. Future work will explore other substituted polyaromatic systems using similar methodologies.

Direct synthesis of poly(p-phenyleneethynylene)s from calcium carbide

An efficient method for the preparation of poly(p-phenyleneethynylene)s (PPEs) from direct coupling reactions between aryl diiodides and inexpensive chemical feedstock calcium carbide is developed.

Oxidation-induced photoluminescence of conjugated polymers

Here, we report an unusual oxidation-induced photoluminescence (PL) turn-on response of a poly(3-alkoxythiophene), poly(3-{2-[2-(2-ethoxyethoxy)ethoxy]ethoxy}thiophene) (PEEEET). PEEEET shows a significantly red-shifted absorption spectrum compared to polyalkylthiophenes and is almost nonfluorescent (quantum yield ≪ 1%) in its pristine state. The introduction of sulfonyl defects along the polymer backbone by the oxidation of PEEEET with meta-chloroperbenzoic acid (m-CPBA) increased the emission quantum yield with the intensity increasing with the degree of oxidation. Molecular modeling data indicated that the oxidation-induced PL increase cannot be explained by the nature of monomer units and radiative rate changes. We attributed the enhanced fluorescence to the reduced nonradiative rate caused by the increased band gap, according to the energy gap law, which is consistent with the observed blue shifts in absorption and PL spectra accompanied by the PL increase.

Putting anion-π interactions into perspective

Supramolecular chemistry is a field of scientific exploration that probes the relationship between molecular structure and function. It is the chemistry of the noncovalent bond, which forms the basis of highly specific recognition, transport, and regulation events that actuate biological processes. The classic design principles of supramolecular chemistry include strong, directional interactions like hydrogen bonding, halogen bonding, and cation-π complexation, as well as less directional forces like ion pairing, π-π, solvophobic, and van der Waals potentials. In recent years, the anion-π interaction (an attractive force between an electron-deficient aromatic π system and an anion) has been recognized as a hitherto unexplored noncovalent bond, the nature of which has been interpreted through both experimental and theoretical investigations. The design of selective anion receptors and channels based on this interaction represent important advances in the field of supramolecular chemistry. The objectives of this Review are 1) to discuss current thinking on the nature of this interaction, 2) to survey key experimental work in which anion-π bonding is demonstrated, and 3) to provide insights into the directional nature of anion-π contact in X-ray crystal structures.

Properties and transport behavior of perfluorotripentylamine (FC-70)-doped amorphous teflon AF 2400 films

Teflon AF 2400 films are known to imbibe solvents, making films in the presence of solvents less fluorous than they might otherwise be. Herein, we demonstrate that doping films with perfluorotripentylamine (Fluorinert FC-70) maintains the fluorous nature of Teflon AF 2400 and improves transport selectivity for fluorine-containing organic compounds. Density measurements on the FC-70-doped films reveal that free volume decreases dramatically as the dopant concentration increases (0-12 wt %) and then increases to approach that of pure FC-70. Remarkably, films from 0 to 12 wt % FC-70 have the same w/v concentration of Teflon AF 2400, indicating that FC-70 fills the free volume of Teflon AF 2400. This is consistent with the observed increased storage modulus and significant decrease (compared to undoped films) of solute diffusion coefficients in the same range of FC-70 concentrations. In contrast, FC-70 at concentrations greater than 12 wt % dilutes Teflon AF 2400, leading to a decrease of storage modulus and dramatic increase in solute diffusion coefficients. Sorption of chloroform decreases from 11.8 g of chloroform/100 g of film (pure Teflon film) to 3.8 g of chloroform/100 g of film (27 wt % FC-70-doped Teflon film), less than the solubility of chloroform in pure FC-70 (4.06 g of chloroform/100 g of FC-70). Solute partition coefficients from chloroform to FC-70-doped films generally decrease with increased dopant concentration. However, within a series of toluenes and nitrobenzenes, selectivity for F-containing solutes over analogous H-containing solutes increases as dopant concentration increases if the substitution is on the aromatic ring but not if it is on the methyl group (toluene). Transport (partitioning × diffusion) rates, as they involve both thermodynamic and kinetic factors, are not simply related to composition.