Self-Assembly of Regioregular, Amphiphilic Polythiophenes into Highly Ordered π-Stacked Conjugated Polymer Thin Films and Nanocircuits

An asymmetric 2,3-fluoranthene imide building block for regioregular semiconductors with aggregation-induced emission properties

For organic semiconductors, the development of electron-deficient building blocks has lagged far behind that of the electron-rich ones. Moreover, it remains a significant challenge to design organic molecules with efficient charge transport and strong solid-state emission simultaneously. Herein, we describe a facile synthetic route toward a new π-acceptor imide building block, namely 2,3-fluoranthene imide, based on which four regioregular small molecules (F1–F4) are synthesized by tuning the imide orientations and the central linkage bridges. All molecules exhibit attractive aggregation-induced emission (AIE) characteristics with strong far-red emission in the powder state, and F3 shows the highest photoluminescence quantum yield of 5.9%. F1 and F3 with a thiophene bridge present an obvious p-type characteristic, while for F3 with an outward imide orientation, the maximum hole mobility from a solution-processed field-effect transistor (FET) device reaches 0.026 cm² V⁻¹ s⁻¹, being ∼10⁴ times higher than the value of F1 with an inward imide orientation. By using a fluorinated thiophene bridge, the resulting F2 and F4 can be turned into n-type semiconductors, showing an electron mobility of ∼1.43 × 10⁻⁴ and ∼3.34 × 10⁻⁵ cm² V⁻¹ s⁻¹, respectively. Our work not only demonstrates that asymmetric 2,3-fluoranthene imide is a promising building block for constructing organic materials with high carrier mobility and strong solid-state emission, but also highlights the importance of regioregular structures in the materials' properties.

A new electron-deficient 2,3-fluoranthene imide unit was easily synthesized through a one-pot reaction for constructing small molecule regioregular semiconductors with good carrier transport ability and strong solid-state emission.

Sandwich-Type Near-Infrared Conjugated Polymer Nanoparticles for Revealing the Fate of Transplanted Human Umbilical Cord Mesenchymal Stem Cells

Near-infrared conjugated polymer nanoparticles (NIR-CPNs) have been widely used in in vivo imaging fields. However, most of them face the aggregation-induced fluorescence quenching (ACQ) dilemma and serious dye leakage behavior, which impedes the long-term monitoring of transplanted cells in vivo. In the present work, a novel strategy of sandwich-type encapsulation of the conjugated polymer interlayer in the crystalline SiO2 core + shell (SSiO2@SPFTBT@CSiO2) is developed, which works well to avoid the ACQ problem by homogeneously dispersing poly((9,9-dioctylfluorene-2,7-diyl)-alt-(4,7-di(thiophene-2-yl)-2,1,3-benzothiadiazole)-5',5″-diyl) (PFTBT) and suppressing intermolecular π-π stacking. Furthermore, the unparalleled nanostructure efficiently stabilizes nanoparticles and successfully achieves long-term biocompatibility without interfering the biological characteristics of stem cells, indicating the potential of SSiO2@SPFTBT@CSiO2 in cell labeling. In addition, the fate of human umbilical cord mesenchymal stem cells (hucMSCs) in a mouse model with acute liver injury was disclosed. We found that the hucMSCs mainly migrated from the lungs to the injured liver and most transplanted hucMSCs were cleared up by the liver at 8 days post-injection. Revelation of the shuttle process and period will benefit in improving the clinical efficacy of hucMSCs, and the sandwich-type encapsulation strategy could also open a new avenue to obtain bright and robust NIR-CPNs for long-term fluorescence imaging.

Conjugated polymers as Langmuir and Langmuir-Blodgett films: Challenges and applications in nanostructured devices

Initially developed for classic systems composed of fatty acids and phospholipids, the Langmuir and Langmuir-Blodgett (LB) techniques allow the fabrication of nanometer-scale devices at self-assembly interfaces with high control over the thickness and molecular architecture. Their application in the research and production of new plastic materials has grown considerably over the past few decades due to the efficiency of conjugated polymers (CPs) for the production of light-emitting diodes, flexible displays, solar cells, and other photoelectronic devices. The structuring of polymers at different interfaces is not trivial as this class of macromolecules can undergo through different processes of folding/unfolding, which hinders the formation of stable Langmuir monolayers and, consequently, the production of Langmuir-Blodgett films. With these ideas in mind, the present article aims to review a series of elements related to the formation of stable Langmuir and Langmuir-Blodgett films of CPs, especially those based on poly(phenylene vinylene)s, polyfluorenes, and polythiophenes. This review is divided into two parts where we first discuss the formation of neat CP films, and then the strategies for the formation of stable CP films based on the co-immobilization with fatty acids, other polymers, and enzymes as mixed films.

Formal preparation of regioregular and alternating thiophene-thiophene copolymers bearing different substituents

Differently substituted thiophene–thiophene-alternating copolymers were formally synthesized employing a halo-bithiophene as a monomer. Nickel-catalyzed polymerization of bithiophene with substituents at the 3-position, including alkyl-, fluoroalkyl-, or oligosiloxane-containing groups, afforded the corresponding copolymers in good to excellent yield. The solubility test in organic solvents was performed to reveal that several copolymers showed a superior solubility. X-ray diffraction analysis of the thin film of the alternating copolymers composed of methyl and branched oligosiloxane substituents was also performed, and the results suggested the formation of a dual-layered film structure.

Recent developments in the synthesis of regioregular thiophene-based conjugated polymers for electronic and optoelectronic applications using nickel and palladium-based catalytic systems

Thiophene-based conjugated polymers are important conjugated polymers due to their exceptional optical and conductive properties, over the past few decades many researchers have designed novel strategies to reach more efficient materials for electronic applications.

Control of optical and electrical properties of nanosheets by the chemical structure of the turning point in a foldable polymer

Oligomers of tetra(ethylene glycol)-disubstituted phenyl-capped bithiophene (Ph2TPh) linked by catechol and resorcinol were prepared. Catechol and resorcinol link the monomers via the ortho- and meta-positions of the benzene ring, respectively, and function as turning points in the folding process of the polymer. It was confirmed that the ortho-linked 8mer (o-8mer) and meta-linked 8mer (m-8mer) could form nanosheets through the self-assembly of folded polymers in o-dichlorobenzene. We confirmed that the arrangement of thiophene units inside the nanosheets was controllable by changing the chemical structure of the turning point. The different arrangements of the Ph2TPh units led to changes in other physical properties such as UV-Vis absorption, nanosheet thickness and charge carrier transport. The absorption spectrum of the o-8mer nanosheets suggested that the Ph2TPh units are arranged vertical to the lateral direction of the nanosheets. On the other hand, the Ph2TPh units in the m-8mer nanosheets were considered to have a tilted orientation. The change in the Ph2TPh tilt angle inside the nanosheets was supported by the different thicknesses of the o-8mer and m-8mer nanosheets. The relationship between the absorption spectrum and Ph2TPh unit arrangement was discussed based on the DFT calculation. Intrinsic charge carrier transport properties were evaluated by a noncontact microwave-based method. The o-8mer nanosheets showed higher conductivity than the m-8mer and triazole-linked-8mer nanosheets. The lifetime of charge carriers in the nanosheet was longer than that in the lamellar structure of the drop-cast film.

Organic soluble and uniform film forming oligoethylene glycol substituted BODIPY small molecules with improved hole mobility

A propeller type donor imparts organic solubility to oligoethylene glycol substituted conjugated small molecules. The oligoethylene glycol facilitates better intermolecular contacts and improved organic field effect transistor efficiency.

Designing new symmetrical facial oligothiophene amphiphiles

In this study we designed a new class of symmetrical facial oligothiophene amphiphiles, which could be obtained in fewer steps than for previously reported analogues, but still possess the specific substituent sequence to control their backbone curvature. This novel design allows the late-stage introduction of hydrophilic groups, aiding both purification and ease of structure variation. Following the new synthetic scheme, symmetrical ter- and sexi-thiophenes were synthesized, analysed and their properties were compared to their non-symmetrical analogues. Surprisingly, the self-assembly behaviour in water, aggregate morphologies and photo-physical properties turned out to be significantly different despite the same ratio of hydrophilic and hydrophobic substituents. The new substitution pattern resulted in a drastic decrease of the critical aggregation concentration and an increase of the aggregate size. The symmetrical positioning of the substituents also heavily influenced the photo-physical properties. The changes were observed as large blue shifts in the absorption and emission spectra in water when compared to similar regio-regular oligothiophene amphiphiles.

Polythiophene-gold nanoparticle hybrid systems: Langmuir-Blodgett assembly of nanostructured films

In this work, we demonstrate a simple method of synthesizing nanoscale polythiophene-gold nanoparticle (AuNP) hybrid systems assembled by the Langmuir-Blodgett (LB) method. Regio-regular poly(3-(2-methoxyethoxy)ethoxymethyl)thiophene-2,5-diyl (PMEEMT) and poly(3-dodecylthiophene) (PDDT) were employed as the polymeric constituents. The presence of PDDT improved the amphiphilicity of PMEEMT by addressing the phase separation that occurred due to convective hydrodynamic instability on the substrate. 4 layer stacks of 90% and 99% PMEEMT films exhibited uniform film structure with a significant reduction in phase separation. A detailed mechanism for minimization of the surface effect has been proposed based on the interaction of polythiophenes with the substrate. For the first time, an ex situ approach has been adopted to incorporate AuNPs into LB films without affecting the film morphology and uniformity. The incorporation of AuNPs into the polythiophene matrix, aided by the affinity of sulphur for gold, was strongly dependent on the molecular arrangement of the matrix, which in turn depended on the composition of the matrix. The hybrid polythiophene films exhibited enhanced conductivity and can be applied in sensors, photovoltaics and memory devices.

Improving the performance of P3HT-fullerene solar cells with side-chain-functionalized poly(thiophene) additives: a new paradigm for polymer design

The motivation of this study is to determine if small amounts of designer additives placed at the polymer-fullerene interface in bulk heterojunction (BHJ) solar cells can influence their performance. A series of AB-alternating side-chain-functionalized poly(thiophene) analogues, P1-6, are designed to selectively localize at the interface between regioregular poly(3-hexylthiophene) (rr-P3HT) and PC(n)BM (n = 61, 71). The side chains of every other repeat unit in P1-6 contain various terminal aromatic moieties. BHJ solar cells containing ternary mixtures of rr-P3HT, PC(n)BM, and varying weight ratios of additives P1-6 are fabricated and studied. At low loadings, the presence of P1-6 consistently increases the short circuit current and decreases the series resistance of the corresponding devices, leading to an increase in power conversion efficiency (PCE) compared to reference P3HT/PC(61)BM cells. Higher additive loadings (>5 wt %) lead to detrimental nanoscale phase separation within the active layer blend and produce solar cells with high series resistances and low overall PCEs. Small-perturbation transient open circuit voltage decay measurements reveal that, at 0.25 wt % incorporation, additives P1-6 increase charge carrier lifetimes in P3HT/PC(61)BM solar cells. Pentafluorophenoxy-containing polymer P6 is the most effective side-chain-functionalized additive and yields a 28% increase in PCE when incorporated into a 75 nm thick rr-P3HT/PC(61)BM BHJ at a 0.25 wt % loading. Moreover, devices with 220 nm thick BHJs containing 0.25 wt % P6 display PCE values of up to 5.3% (30% PCE increase over a control device lacking P6). We propose that additives P1-6 selectively localize at the interface between rr-P3HT and PC(n)BM phases and that aromatic moieties at side-chain termini introduce a dipole at the polymer-fullerene interface, which decreases the rate of bimolecular recombination and, therefore, improves charge collection across the active layer.

Formation of net-like patterns of gold nanoparticles in liquid crystal matrix at the air-water interface

Controlled patterning and formation of nanostructures on surfaces based on self-assembly is a promising area in the field of "bottom-up" nanomaterial engineering. We report formation of net-like structures of gold nanoparticles (Au NPs) in a matrix of liquid crystalline amphiphile 4'-n-octyl-4-cyanobiphenyl at the air-water interface. After initial compression to at least 18 mN m(-1), decompression of a Langmuir film of a mixture containing both components results in formation of net-like structures. The average size of a unit cell of the net is easily adjustable by changing the surface pressure during the decompression of the film. The net-like patterns of different, desired average unit cell areas were transferred onto solid substrates (Langmuir-Blodgett method) and investigated with scanning electron microscopy and X-ray reflectivity (XRR). Uniform coverage over large areas was proved. XRR data revealed lifting of the Au NPs from the surface during the formation of the film. A molecular mechanism of formation of the net-like structures is discussed. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11051-012-0826-4) contains supplementary material, which is available to authorized users.

Effect of self-assembled monolayers on charge injection and transport in poly(3-hexylthiophene)-based field-effect transistors at different channel length scales

Charge injection and transport in bottom-contact regioregular-poly(3-hexylthiophene) (rr-P3HT) based field-effect transistors (FETs), wherein the Au source and drain contacts are modified by self-assembled monolayers (SAMs), is reported at different channel length scales. Ultraviolet photoelectron spectroscopy is used to measure the change in metal work function upon treatment with four SAMs consisting of thiol-adsorbates of different chemical composition. Treatment of FETs with electron-poor (electron-rich) SAMs resulted in an increase (decrease) in contact metal work function because of the electron-withdrawing (-donating) tendency of the polar molecules. The change in metal work function affects charge injection and is reflected in the form of the modulation of the contact resistance, R(C). For example, R(C) decreased to 0.18 MΩ in the case of the (electron-poor) 3,5-bis-trifluoromethylbenzenethiol treated contacts from the value of 0.61 MΩ measured in the case of clean Au-contacts, whereas it increased to 0.97 MΩ in the case of the (electron-rich) 3-thiomethylthiophene treated contacts. Field-effect mobility values are observed to be affected in short-channel devices (<20 μm) but not in long-channel devices. This channel-length-dependent behavior of mobility is attributed to grain-boundary limited charge transport at longer channel lengths in these devices.

Stimuli-responsive azulene-based conjugated oligomers with polyaniline-like properties

Novel azulene building blocks, prepared via the cycloaddition of thiophene-S,S-dioxides and fulvenes, allow for incorporation of the seven-membered ring of the azulene nucleus directly into the backbone of conjugated materials. This unique mode of incorporation gives remarkably stable, stimuli-responsive materials upon exposure to acid. This simple doping/dedoping strategy provides for effective optical band gap control and on/off fluorescence switching, reminiscent of polyaniline.

Scanning electrochemical microscopy studies of micropatterned copper sulfide (Cu(x)S) thin films fabricated by a wet chemistry method

Patterned copper sulfide (Cu(x)S) microstructures on Si (1 1 1) wafers were successfully fabricated by a relatively simple solution growth method using copper sulfate, ethylenediaminetetraacetate and sodium thiosulfate aqueous solutions as precursors. The Cu(x)S particles were selectively deposited on a patterned self-assembled monolayer of 3-aminopropyltriethoxysilane regions created by photolithography. To obtain high quality Cu(x)S films, preparative conditions such as concentration, proportion, pH and temperature of the precursor solutions were optimized. Various techniques such as optical microscopy, atomic force microscopy (AFM), X-ray diffraction, optical absorption and scanning electrochemical microscopy (SECM) were employed to examine the topography and properties of the micro-patterned Cu(x)S films. Optical microscopy and AFM results indicated that the Cu(x)S micro-pattern possessed high selectivity and clear edge resolution. From combined X-ray diffraction analysis and optical band gap calculations we conclude that Cu(9)S(5) (digenite) was the main phase within the resultant Cu(x)S film. Both SECM image and cyclic voltammograms confirmed that the Cu(x)S film had good electrical conductivity. Moreover, from SECM approach curve analysis, the apparent electron-transfer rate constant (k) in the micro-pattern of Cu(x)S dominated surface was estimated as 0.04 cm/s. The SECM current map showed high edge acuity of the micro-patterned Cu(x)S.

Molecular dynamics of ions in two forms of an electroactive polymer

Molecular dynamics (MD) are performed in all-atom simulations of two polymer species based on polythiophene. In one case the amphiphilic polymer forms a monolayer interface between a vacuum and an aqueous layer containing ions. The electroactive nature of the polymer is invoked by conferring a negative charge on it to compensate for charge imbalance in the Na(+) and Cl(-) concentrations of the aqueous layer. The effects of hydrostatic pressure and charge imbalance on the stability of the monolayer are investigated. In another simulation a polythiophene oligomer is wound into a helix where it serves as an ion channel between two aqueous regions on both sides of a phospholipid bilayer membrane.

Amphiphilic conjugated thiophenes for self-assembling antenna systems in water

Newly developed conjugated terthiophene surfactants are able to aggregate in water and to act as a host for hydrophobic chromophores, creating a multiple donor-acceptor energy transfer (ET) system by self-assembly.

The role of intrachain and interchain interactions of regioregular poly(3-octylthiophene) chains on the optical properties of a new amphiphilic conjugated random copolymer in solution

The synthesis of a random copolymer through free radical copolymerization of a properly vinyl monofunctionalized regioregular poly(3-octylthiophene) (rrP3OT) macromonomer and N,N'-dimethylacrylamide (DMAM) is presented. The optical properties of the copolymer in water and in several organic solvents of varying polarity, as well as in THF/water and THF/methanol mixtures, were explored using UV-vis and photoluminescence spectroscopy. It is demonstrated that the rrP3OT chains adopt a coil conformation in solvents such as tetrahydrofuran (THF) and chloroform with the appearance of the absorption and emission maxima at 439 and 565 nm, respectively. On the contrary, the rrP3OT chains are organized on a single chain packing form (intrachain interactions) in polar solvents such as ethanol and methanol, as it is verified with the observation of the characteristic three vibronic features of the absorption spectra of the copolymer with maxima at 513, 550, and 603 nm and emission maxima at 560 nm. However, when water is used as solvent, the rrP3OT chains self-assemble into a stacklike structure due to the increased interchain interactions, as confirmed by the different aggregation process of the rrP3OT chains in the THF/water mixture, the broader absorption spectrum in water compared to those recorded in ethanol and methanol, and the 80 nm red-shifted emission maximum, centered at 640 nm.

Polymer-templated self-assembly of a 2-dimensional gold nanoparticle network

We here report on the formation of well-ordered 2D gold nanostructures at the air/water interface. Spreading a mixture of alkanethiol-capped gold nanoparticles (AuNPs) and an amphiphilic poly(p-phenylene) on a water surface and compressing the mixture to a surface pressure of 40 mN/m lead to the formation of a network of well-ordered gold nanostructures. The structures are transferred horizontally (Langmuir-Schäfer) onto a solid substrate and investigated with TEM, AFM, and X-ray reflectivity, showing a pattern that is repeating over several micrometers. AFM and X-ray reflectivity data at different surface pressures reveal that the polymer is lifting the AuNPs 1.5-2 nm in the vertical direction, away from the polymer layer, when the pressure is increased from 20 to 40 mN/m.

Ab initio calculations on π-stacked thiophene dimer, trimer, and tetramer: Structure, interaction energy, cooperative effects, and intermolecular electronic parameters

Pi-stacked complexes formed by two, three, and four thiophene rings have been investigated using abinitio quantum mechanical calculations. The relative orientation between the rings was investigated for each complex by exploring the corresponding potential energy surface at the MP2/6-31+G(d,p) level, the inter-ring distance, and the degree of tilting being examined in each case. Interaction energies were calculated at the MP2, MP3, MP4, and CCSD, levels of theory. Negligible or even slightly positive n-body effects have been predicted for the stacked thiophene arrangements studied in this work. This is consequence of the cancellation of favorable induction contribution by the destabilizing dispersion component. On the other hand analysis of the optimized geometries obtained for the trimer and tetramer revealed that the orientation of the rings presents a preferred degree of periodicity. Finally, we found that the lowest transition energy decreases when the size of the complex increases, this feature being attributed to desestabilization of the HOMO and stabilization of the LUMO that occur simultaneously.

Effect of composition on the conductivity and morphology of poly(3-hexylthiophene)/gold nanoparticle composite Langmuir–Schaeffer films

Ultrathin Langmuir-Schaeffer (LS) films were fabricated from blends of regioregular poly(3-hexylthiophene) (P3HT) and highly monodispersed dodecanethiolate-capped gold nanoparticles (Au NPs) mixed in varying weight ratios. The morphology of the ultrathin films was investigated by UV-visible absorption spectroscopy, atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). The results of the structural investigations were correlated with the lateral conductivity of the films, with P3HT in its unintentionally doped state, probed by scanning electrochemical microscopy (SECM), which proved to be a very sensitive technique. Control over the P3HT/Au NP ratio led to remarkable changes in the morphology and lateral conductivity of the films. Inclusion of Au NPs into P3HT was found to influence the ordering of P3HT, which ultimately determined the macroscopic charge transport characteristics of the films. Composite films with ca. 33% by weight of Au NPs were found to be the most ordered and exhibited the highest conductivity, substantially higher than P3HT alone. To provide insight into the film formation process, LS composite films comprising equal quantities of P3HT and Au NPs (by weight) were transferred at several surface pressures and investigated by SECM, AFM and FE-SEM.

A water-soluble hexa-peri-hexabenzocoronene: synthesis, self-assembly and role as template for porous silica with aligned nanochannels

A water-soluble hexa-peri-hexabenzocoronene was prepared and shown to undergo ordered columnar self-assembly either in water solution or bulk and therefore served as template for the fabrication of porous silica with aligned nanochannels.