Π-conjugated Polyelectrolyte Research Articles

Water binding and hygroscopicity in π-conjugated polyelectrolytes

The presence of water strongly influences structure, dynamics and properties of ion-containing soft matter. Yet, the hydration of such matter is not well understood. Here, we show through a large study of monovalent π-conjugated polyelectrolytes that their reversible hydration, up to several water molecules per ion pair, occurs chiefly at the interface between the ion clusters and the hydrophobic matrix without disrupting ion packing. This establishes the appropriate model to be surface hydration, not the often-assumed internal hydration of the ion clusters. Through detailed analysis of desorption energies and O–H vibrational frequencies, together with OPLS4 and DFT calculations, we have elucidated key binding motifs of the sorbed water. Type-I water, which desorbs below 50 °C, corresponds to hydrogen-bonded water clusters constituting secondary hydration. Type-II water, which typically desorbs over 50–150 °C, corresponds to water bound to the anion under the influence of a proximal cation, or to a cation‒anion pair, at the cluster surface. This constitutes primary hydration. Type-III water, which irreversibly desorbs beyond 150 °C, corresponds to water kinetically trapped between ions. Its amount varies strongly with processing and heat treatment. As a consequence, hygroscopicity—which is the water sorption capacity per ion pair—depends not only on the ions, but also their cluster morphology.

Surface-Plasmonic-Coupled Photodetector Based on MAPbI3 Perovskites with Au Nanoparticles: Significantly Enhanced Photoluminescence and Photodetectivity

Organic–inorganic metal halide perovskites (OMHPs) are promising active materials suitable for highly efficient solar cells, photodetectors, light-emitting diodes, and sensors. In this study, methylammonium lead iodide (MAPbI3) thin sheets (TSs) were synthesized as OMHPs using both hybrid vapor-solution method for optical study and anti-solvent solution method for the photodetector. π-Conjugated polyelectrolyte (π-CPE), such as poly(9,9-bis(4′-sulfonatobutyl)fluorene-alt-1,4-phenylene) potassium (FPS-K), was spin-coated on the MAPbI3 TS, and functionalized gold nanoparticles (Au-NPs) were hybridized. The laser confocal microscope photoluminescence (PL) intensity of the MAPbI3 TS was significantly enhanced after hybridization with Au-NPs/FPS-K, owing to the passivating effect of the FPS-K and the generation of extra-photoexcited charges by local surface plasmon resonance (LSPR) coupling with Au-NPs. These results were supported by the variation in the exciton lifetime measured from the time-resolved PL decay curves. The photocurrent of the MAPbI3 photodetector increased up to 1.1 × 104 times, and the photoresponsivity (R) and photodetectivity (D*) increased by 70 and 13 times, respectively, with the hybridization of Au-NPs/FPS-K. The highest D* of the Au-NPs/FPS-K/MAPbI3 photodetector was measured to be 7.5 × 1010 Jones at 735 nm excitation. The power and wavelength dependencies of R and D* for the MAPbI3 photodetector were also significantly improved by the Au-NPs/FPS-K hybrid. These results support the development of high-performance perovskite photodetectors utilizing LSPR coupling with the π-CPE layer.

Optimizing ionic strength of interfacial electric double layer for ultrahigh external quantum efficiency of photomultiplication-type organic photodetectors

A comprehensive study on the effects of the interfacial electric double layers on the performances of photomultiplication-type organic photodetectors is proposed by employing π-conjugated polyelectrolytes with various ionic densities.

Triphenylamine-Based Conjugated Polyelectrolyte as a Hole Transport Layer for Efficient and Scalable Perovskite Solar Cells.

π-Conjugated polyelectrolytes (CPEs) have been studied as interlayers on top of a separate hole transport layer (HTL) to improve the wetting, interfacial defect passivation, and crystal growth of perovskites. However, very few CPE-based HTLs have been reported without rational molecular design as ideal HTLs for perovskite solar cells (PeSCs). In this study, the authors synthesize a triphenylamine-based anionic CPE (TPAFS-TMA) as an HTL for p-i-n-type PeSCs. TPAFS-TMA has appropriate frontier molecular orbital (FMO) levels similar to those of the commonly used poly(bis(4-phenyl)-2,4,6-trimethylphenylamine) (PTAA) HTL. The ionic and semiconducting TPAFS-TMA shows high compatibility, high transmittance, appropriate FMO energy levels for hole extraction and electron blocking, as well as defect passivating properties, which are confirmed using various optical and electrical analyses. Thus, the PeSC with the TPAFS-TMA HTL exhibits the best power conversion efficiency (PCE) of 20.86%, which is better than that of the PTAA-based device (PCE of 19.97%). In addition, it exhibits negligible device-to-device variations in its photovoltaic performance, contrary to the device with PTAA. Finally, a large-area PeSC (1 cm2 ) and mini-module (3 cm2 ), showing PCEs of 19.46% and 18.41%, respectively, are successfully fabricated. The newly synthesized TPAFS-TMA may suggest its great potential as an HTL for large-area PeSCs.

Charge-Transfer Effect and Enhanced Photoresponsivity of WS2- and MoSe2-Based Field Effect Transistors with π-Conjugated Polyelectrolyte

The characteristics of field effect transistors (FETs) fabricated using two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be modulated by surface treatment of the active layers. In this study, an ionic π-conjugated polyelectrolyte, poly(9,9-bis(4'-sulfonatobutyl)fluorene-alt-1,4-phenylene) potassium (FPS-K), was used for the surface treatment of MoSe2 and WS2 FETs. The photoluminescence (PL) intensities of monolayer (1L)-MoSe2 and 1L-WS2 clearly decreased, and the PL peaks were red-shifted after FPS-K treatment, suggesting a charge-transfer effect. In addition, the n-channel current of both the MoSe2 and WS2 FETs increased and the threshold voltage (Vth) shifted negatively after FPS-K treatment owing to the charge-transfer effect. The photoresponsivity of the MoSe2 FET under light irradiation (λex = 455 nm) increased considerably, from 5300 A W-1 to approximately 10 000 A W-1, after FPS-K treatment, and similar behavior was observed in the WS2 FET. The results can be explained in terms of the increase in electron concentration due to photogating. The external quantum efficiency and photodetectivity of both FETs were also enhanced by the charge-transfer effect resulting from surface treatment with FPS-K containing mobile cations (K+) and fixed anions (SO3-), as well as by the photogating effect. The variation in charge-carrier density due to the photogating and charge-transfer effects is estimated to be approximately 2 × 1012 cm-2. The results suggest that π-conjugated polyelectrolytes such as FPS-K can be a promising candidate for the passivation of TMDC-based FETs and obtaining enhanced photoresponsivity.

Associative Phase Separation of Aqueous π-Conjugated Polyelectrolytes Couples Photophysical and Mechanical Properties

The associative phase separation of water-soluble polyelectrolytes is important across many different fields including food science, biomedicine, materials science, and prebiotic organization. Spec...

New Hybrid Copper Nanoparticles/Conjugated Polyelectrolyte Composite with Antibacterial Activity.

In the search for new materials to fight against antibiotic-resistant bacteria, a hybrid composite from metallic copper nanoparticles (CuNPs) and a novel cationic π-conjugated polyelectrolyte (CPE) were designed, synthesized, and characterized. The CuNPs were prepared by chemical reduction in the presence of CPE, which acts as a stabilizing agent. Spectroscopic analysis and electron microscopy showed the distinctive band of the metallic CuNP surface plasmon and their random distribution on the CPE laminar surface, respectively. Theoretical calculations on CuNP/CPE deposits suggest that the interaction between both materials occurs through polyelectrolyte side chains, with a small contribution of its backbone electron density. The CuNP/CPE composite showed antibacterial activity against Gram-positive (Staphylococcus aureus and Enterococcus faecalis) and Gram-negative (Escherichia coli and Salmonella enteritidis) bacteria, mainly attributed to the CuNPs’ effect and, to a lesser extent, to the cationic CPE.

Cationic π-Conjugated Polyelectrolyte Shows Antimicrobial Activity by Causing Lipid Loss and Lowering Elastic Modulus of Bacteria.

Cationic, π-conjugated oligo-/polyelectrolytes (CCOEs/CCPEs) have shown great potential as antimicrobial materials to fight against antibiotic resistance. In this work, we treated wild-type and ampicillin-resistant (amp-resistant) Escherichia coli (E. coli) with a promising cationic, π-conjugated polyelectrolyte (P1) with a phenylene-based backbone and investigated the resulting morphological, mechanical, and compositional changes of the outer membrane of bacteria in great detail. The cationic quaternary amine groups of P1 led to electrostatic interactions with negatively charged moieties within the outer membrane of bacteria. Using atomic force microscopy (AFM), high-resolution transmission electron microscopy (TEM), we showed that due to this treatment, the bacterial outer membrane became rougher, decreased in stiffness/elastic modulus (AFM nanoindentation), formed blebs, and released vesicles near the cells. These evidences, in addition to increased staining of the P1-treated cell membrane by lipophilic dye Nile Red (confocal laser scanning microscopy (CLSM)), suggested loosening/disruption of packing of the outer cell envelope and release and exposure of lipid-based components. Lipidomics and fatty acid analysis confirmed a significant loss of phosphate-based outer membrane lipids and fatty acids, some of which are critically needed to maintain cell wall integrity and mechanical strength. Lipidomics and UV-vis analysis also confirmed that the extracellular vesicles released upon treatment (AFM) are composed of lipids and cationic P1. Such surface alterations (vesicle/bleb formation) and release of lipids/fatty acids upon treatment were effective enough to inhibit further growth of E. coli cells without completely disintegrating the cells and have been known as a defense mechanism of the cells against cationic antimicrobial agents.

Multivalent anions as universal latent electron donors.

Electrodes with low work functions are required to efficiently inject electrons into semiconductor devices. However, when the work function drops below about 4electronvolts, the electrode suffers oxidation in air, which prevents its fabrication in ambient conditions. Here we show that multivalent anions such as oxalate, carbonate and sulfite can act as powerful latent electron donors when dispersed as small ion clusters in a matrix, while retaining their ability to be processed in solution in ambient conditions. The anions in these clusters can even n-dope the semiconductor core of π-conjugated polyelectrolytes that have low electron affinities, through a ground-state doping mechanism that is further amplified by a hole-sensitized or photosensitized mechanism in the device. A theoretical analysis of donor levels of these anions reveals that they are favourably upshifted from ionic lattices by a decrease in the Coulomb stabilization of small ion clusters, and by irreversibility effects. We attain an ultralow effective work function of 2.4electronvolts with the polyfluorene core. We realize high-performance, solution-processed, white-light-emitting diodes and organic solar cells using polymer electron injection layerswith these universal anion donors, demonstrating a general approach to chemically designed and ambient-processed Ohmic electron contacts for semiconductor devices.

Intercalation of conjugated polyelectrolytes in layered titanate nanosheets for enhancement in photocatalytic activity

Cationic π-conjugated polyelectrolytes (CPEs) were successfully intercalated into layered titanate (TiO2), while exfoliated layered titanates were reassembled into lamellar structures. The resulting composites showed significantly enhanced visible light absorbance and enhanced photocatalytic activity with respect to layered titanate, which was proven by the photodegradation of methylene blue in aqueous media under irradiation by visible light. Notably, some amount of tetrabutylammonium (TBA) used for the delamination of a layered titanate could be co-intercalated with CPEs, and the residual TBA could be removed via continuously repeating the intercalation process, maintaining the spatial arrangement and content of CPEs in the confined area of layered titanates. However, with the presence of TBA, the hybrid demonstrated greater performance in the photodegradation of methylene blue at any given period of time and almost complete degradation of such dye within 8 min. The significant difference in photocatalytic efficiency with or without TBA suggested that the TBA effectively reduced the surface energy at the interface between the organic components and aqueous medium, which was evidenced by the dispersity assessment of hybrids with or without TBA.

Enhanced Electron Transfer Mediated by Conjugated Polyelectrolyte and Its Application to Washing-Free DNA Detection.

Direct electron transfer between a redox label and an electrode requires a short working distance (<1-2 nm), and in general an affinity biosensor based on direct electron transfer requires a finely smoothed Au electrode to support efficient target binding. Here we report that direct electron transfer over a longer working distance is possible between (i) an anionic π-conjugated polyelectrolyte (CPE) label having many redox-active sites and (ii) a readily prepared, thin polymeric monolayer-modified indium-tin oxide electrode. In addition, the long CPE label (∼18 nm for 10 kDa) can approach the electrode within the working distance after sandwich-type target-specific binding, and fast CPE-mediated oxidation of ammonia borane along the entire CPE backbone affords high signal amplification.

Self-assembled hybrid hydrogels based on an amphipathic low molecular weight peptide derivative and a water-soluble poly(para-phenylene vinylene)

The self-assembly of an amphipathic low molecular weight peptide and a water-soluble π-conjugated polyelectrolyte is studied in order to form hybrid hydrogel materials with synergistic properties.

Effect of π-Conjugated Polyelectrolyte on Performance of White Polymer Light-Emitting Diodes Based on Excitons and Exciplexes Having Long Intermolecular Distances

The authors studied white polymer light-emitting diodes (PLEDs) based on blue exciton emission by poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PFD) and orange-to-red emission from exciplexes formed at the interface between the PFD and poly(4-methyl-triphenylamine-co-acetoaldehyde) (TPA-AA). It is thought that the orange-to-red light-emitting exciplexes formed at the TPA-AA/PFD interface have intermolecular charge-transfer (CT) characteristics, namely, TPA-AA+·PFD–, in which TPA-AA and PFD act as a donor (D) and an acceptor (A), respectively, and long intermolecular distances. TPA-AA is a nonconjugated polymer with good solubility in chloroform and acts as a hole transport layer (HTL). A bilayer PLED, ITO (indium tin oxide)/TPA-AA/PFD/Al, was fabricated by spin-coating, and white electroluminescence (EL), corresponding to Commission Internationale d’Eclairage (CIE) chromaticity coordinates of (0.36, 0.28) and a brightness of 39.6 cd/m2, was achieved at 16 V. In order to decrease the turn-on voltage and enhanc...

π-Conjugated Polyelectrolytes Derived from 2-Ethynylpyridine: The Effect of Quaternization Agent and Reaction Conditions on the Polymer Structure and SERS Characterization of Nanocomposites with Ag-Nanoparticles

The reaction of 2-ethynylpyridine (2EP) with stoichiometric equivalent of an alkyl halide RX (R is ethyl, nonyl, or hexadecyl and X is Br or I) gives a poly(N-alkyl-2-ethynylpyridinium halide) type ionic polymer that belongs to the family of π-conjugated polyelectrolytes (CPEs). Reaction conditions significantly influence configuration of the polymer main chains: polymerization in acetonitrile solution gives polymers with high content of cis units while bulk polymerization provides irregular cis/trans polymers. Increased regularity of the high-cis polymers is documented by the NMR and IR as well as SERS (surface enhanced Raman scattering) spectra measured on Ag-nanoparticles/CPE systems. Both polymerization processes give polymers in which 2EP monomeric units are ionized only from ca one half, which exhibit good stability in air and good solubility in polar solvents such as MeOH, DMF, and DMSO and those with N-ethylpyridinium groups even in water.

Influence of polymeric electron injection layers on the electrical properties of solution-processed multilayered polymer light-emitting diodes

In this study, we fabricated multilayered polymer-based light-emitting diodes (pLEDs) with various solution-processed electron-injection layers (EILs), and investigated the influence of the EILs on the electrical properties of pLEDs in indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT:PSS)/poly[(9,9-dioctylfluorene-alt-(1,4-phenylene((4-sec-butylphenyl)amino)-1,4-phenylene))] (TFB) (HTL)/poly(9,9-dioctylfluorene-alt-1,4-benzothiadiazole) (F8BT) (EML)/EIL/Al structures. We have used the quaternized ammonium π-conjugated polyelectrolyte derivative (poly[(9,9-di(3,3′-N,N′-trimethylammonium)propylfluorenyl-2,7-diyl)-co-(1,4-phenylene)]diiodide salt) (PF–PDTA), a mixture of PF–PDTA and CS2CO3, and the aliphatic-amine-based polymer poly(ethylene imine) (PEI) as solution-processed EILs, and compared them with LiF as a solvent-free EIL. The EILs enhanced the electron injection and improve the pLED performance. High external quantum efficiencies of nearly 4% were obtained in the pLEDs with the combination of a multilayered structure fabricated by a transfer printing technique and EILs of a PF–PDTA:CS2CO3 mixture and PEI. On the other hand, the device with PF–PDTA exhibited lower efficiency, higher driving voltage, and larger leakage current at lower voltage. The migration of ionic charges was suggested from the abnormal dielectric behaviors, and serious damage on the electrode material occurred when both an acid hole-injection layer (PEDOT:PSS) and PF–PDTA were used. On the other hand, the pLEDs with ultrathin PEI showed high performance and stable device operation in terms of the influence of ionic charges.

Self-assembly and hybridization mechanisms of DNA with cationic polythiophene.

The combination of DNA and π-conjugated polyelectrolytes (CPEs) represents a promising approach to develop DNA hybridization biosensors, with potential applications for instance in the detection of DNA lesions and single-nucleotide polymorphisms. Here we exploit the remarkable optical properties of a cationic poly[3-(6'-(trimethylphosphonium)hexyl)thiophene-2,5-diyl] (CPT) to decipher the self-assembly of DNA and CPT. The ssDNA/CPT complexes have chiroptical signatures in the CPT absorption region that are strongly dependent on the DNA sequence, which we relate to differences in supramolecular interactions between the thiophene monomers and the various nucleobases. By studying DNA-DNA hybridization and melting processes on preformed ssDNA/CPT complexes, we observe sequence-dependent mechanisms that can yield DNA-condensed aggregates. Heating-cooling cycles show that non-equilibrium mixtures can form, noticeably depending on the working sequence of the hybridization experiment. These results are of high importance for the use of π-conjugated polyelectrolytes in DNA hybridization biosensors and in polyplexes.

Patterning a π-conjugated polyelectrolyte through sequential polymerization of a bifunctional ionic liquid monomer

An electronically conductive polyelectrolyte is prepared by the sequential polymerization of a bifunctional imidazolium-based ionic liquid (IL) monomer, composed of a thienyl and vinyl containing cation paired with a tetrafluoroborate anion. In the first step, potentiodynamic electropolymerization of the thienyl moiety forms a cationic polyalkylthiophene that is soluble in select organic solvents. Cyclic voltammetry (CV) was used to determine the polymer p-doping potential (0.31 V) and the bipolaronic state (1.49 V). The polymer exhibits electrochromism, converting from red in the neutral state (λmax = 443 nm) to dark blue in the polaronic state (λmax = 819 nm). The solution-processable polymer can be cast into a film, masked and patterned by UV-initiated free radical polymerization of the vinyl moiety. Small-angle X-ray scattering (SAXS) revealed that the insoluble crosslinked polyalkylthiophene–polyvinylimidazolium adopts a lamellar structure with a lattice spacing of 3.3 nm. Four-probe d.c. conductivity measurements determined the de-doped electrical conductivity was 1.0 × 10−2 S/cm. The results underscore the importance of the anion in controlling the polymerization of IL monomers.

Synthesis and characterization of PEDOT particles in aqueous dispersion of π-conjugated polyelectrolyte

The synthesis by in situ oxidative polymerization of poly(5-methoxyl-2-(3-sulfopropoxyl)-1,4-phenylenevinylene) (MPS-PPV)/poly(3,4-ethylenedioxythiophene) (PEDOT) composite particles with high surface area in aqueous dispersion of MPS-PPV was reported. The nanostructure and growth mechanism of MPS-PPV/PEDOT composite particles were investigated in detail by FE-SEM and TEM, as well as XRD. PEDOT appeared to have used the tangled-worm like structures of MPS-PPV assembled in the presence of FeCl3 as a scaffold for polymerization, wrapping itself around the tangled worms like bark on a tree. Cyclic voltammetry (CV) was used to qualitatively characterize the pseudo-capacitance properties of MPS-PPV/PEDOT composite particles. Their mass-specific capacitance and cycling stability were calculated by cyclic voltammetry cycles at different potential scan rates. The results showed that the maximum mass specific capacitance for MPS-PPV/PEDOT composite particles increases with the increase of the weight ratio of MPS-PPV, which can be achieved as high as 102Fg−1. Moreover, the materials exhibited high cycling stability and retained approximately 45% of their maximum total capacitance, which was higher than that of pure PEDOT (approximately 40%), as the scan rate approaches 25mVs−1. These results make the composite materials available in the field of electronic devices.

Chirality in DNA–π-conjugated polymer supramolecular structures: insights into the self-assembly

The self-assembly of DNA and π-conjugated polyelectrolytes leads to chiral structures, with specific right- or left-handed assemblies ruled by the DNA sequence and topology.

Aggregation-Induced Amplified Quenching in Conjugated Polyelectrolytes with Interrupted Conjugation

A pair of anionic conjugated polyelectrolytes that contain three-ring (phenylene ethynylene) units linked by a single -CH(2)- or -O- tether (P1 and P2, respectively) are studied. The linkers serve to interrupt the π conjugation along the polymer backbone. Fluorescence spectroscopy reveals that P2 forms a fluorescent aggregate in methanol and water; however, the fluorescence of P1 is much weaker in water, and P1 exhibits only weak aggregate fluorescence. Fluorescence quenching of the polymers was examined using methyl viologen (MV(2+)) as a cationic quencher. P1 shows only a weak amplified quenching effect, with a Stern-Volmer quenching constant of K(SV) ≈ 6 × 10(5) M(-1) in methanol. Interestingly, for P2 in methanol, the aggregate emission is strongly quenched with K(SV) ≈ 5 × 10(6) M(-1), which is comparable to the highest quenching efficiency observed for fully π-conjugated polyelectrolytes. By contrast, the monomer emission is quenched much less efficiently, with K(SV) ≈ 2 × 10(5) M(-1). The results are explained by a model in which -O- linked polymer P2 is able to fold into a helical conformation in solution, which facilitates the formation of extended π-stacked aggregates allowing long-distance exciton transport.