Synthesis, Characterization, and Langmuir-Blodgett Films of Fluorinated Polythiophenes

Influence of solvents on the morphology of Langmuir and Langmuir-Schaefer films of PCBM and PCBM-based oligomers and polymers

Fullerene-based polymers and oligomers combined with non-fullerene acceptors show extremely high efficiencies in organic photovoltaic devices. Furthermore, fullerene-based materials are of interest for use in anti-cancer and anti-viral treatments, where their presence can enhance the efficacy of medication considerably. Therefore, it remains important to understand their morphology and electronic properties to improve devices and technological applications. The main goal of this study is to prepare and characterize Langmuir and Langmuir-Schaefer films of PCBM-based materials to investigate the influence of different solvents such as chloroform, toluene, and xylene, and co-components on their morphology. PCBM-based materials were thus studied either alone or in mixtures with a polythiophene derivative (poly(3-hexythiophene), P3HT) commonly used in organic photovoltaic devices. The formation of Langmuir films was studied using surface pressure isotherms and Brewster's angle microscopy (BAM), where the homogeneity, phase behavior, and morphology of the films were investigated. In addition, Langmuir-Schaefer films were characterized by UV-visible absorption spectroscopy, atomic force microscopy (AFM), and Raman spectroscopy, providing information on the morphology of the solid films. This study has shown that it is possible to successfully fabricate Langmuir and Langmuir-Schaefer films of PCBM and PCBM-based oligomers and polymers, both pure and in mixtures with P3HT, to compare their organization, roughness, and optical properties. With the Langmuir films, it was possible to estimate the area of the molecules and visualize their aggregation through BAM images, establishing a relationship between the area occupied by these materials and the solvent used. All characterization techniques corroborate that the use of chloroform significantly reduced the roughness of the LS films mixed with P3HT and also presented a higher ordering compared to films prepared with xylene solutions.

Thiophene-Based Trimers and Their Bioapplications: An Overview

Certainly, the success of polythiophenes is due in the first place to their outstanding electronic properties and superior processability. Nevertheless, there are additional reasons that contribute to arouse the scientific interest around these materials. Among these, the large variety of chemical modifications that is possible to perform on the thiophene ring is a precious aspect. In particular, a turning point was marked by the diffusion of synthetic strategies for the preparation of terthiophenes: the vast richness of approaches today available for the easy customization of these structures allows the finetuning of their chemical, physical, and optical properties. Therefore, terthiophene derivatives have become an extremely versatile class of compounds both for direct application or for the preparation of electronic functional polymers. Moreover, their biocompatibility and ease of functionalization make them appealing for biology and medical research, as it testifies to the blossoming of studies in these fields in which they are involved. It is thus with the willingness to guide the reader through all the possibilities offered by these structures that this review elucidates the synthetic methods and describes the full chemical variety of terthiophenes and their derivatives. In the final part, an in-depth presentation of their numerous bioapplications intends to provide a complete picture of the state of the art.

Air-Liquid Interfacial Self-Assembly of Non-Amphiphilic Poly(3-hexylthiophene) Homopolymers

Here, we demonstrate that the self-assembly of poly(3-hexylthiophene) (P3HT) at the air-water interface can lead to free-standing films of densely packed P3HT nanowires. Interfacial self-assembly on various liquid subphases, such as water, diethylene glycol, and glycerol, indicates that the viscosity of the subphase is an important factor for the formation of well-ordered nanostructures. The thin-film morphology is also sensitive to the concentration of P3HT, its molecular weight (MW), and the presence of oxidative defects. The densely packed nanowire films can be easily transferred to solid substrates for device applications. The ultrathin films of P3HT prepared by the interfacial assembly showed significantly higher hole mobility (∼3.6 × 10^-2 cm²/V s) in a field-effect transistor than comparably thin spin-cast films. This work demonstrates that the air-liquid interfacial assembly is not limited to amphiphilic polymers and can, under optimized conditions, be applied to fabricate ultrathin films of widely used conjugated polymers with controlled morphologies.

Diketopyrrolopyrrole-based polymer with a semi-fluorinated side chain for high-performance organic thin-film transistors

A semi-fluorinated DPP based polymer showed hole mobility about 3 times higher than did its non-fluorinated analogue.

A novel thermally reversible soluble-insoluble conjugated polymer with semi-fluorinated alkyl chains: enhanced transistor performance by fluorophobic self-organization and orthogonal hydrophobic patterning

SFA-PQT exhibits self-assembly via a fluorophobic effect in a non-fluorous solvent, which leads to an enhanced electrical performance. Ambipolar transistors and inverters with p- and n- type bilayers are enabled by the unique thermally reversible soluble-insoluble properties of SFA-PQT. More importantly, the hydrophobicity of SFA-PQT facilitates orthogonal hydrophobic patterning and a patterned inverter exhibits low voltage dissipation, a narrow transition zone, a high gain value, and negligible hysteresis.

2008 Macromolecular Science and Engineering Division Award Lecture — Conjugated polymers: From micro-electronics to genomics

Conjugated polymers have received a lot of attention, since they combine the best features of metals or semiconductors with those of synthetic polymers. For instance, solar cells based on poly(2,7-carbazole) derivatives have revealed power-conversion efficiencies up to 6%. This class of materials could lead to printable and flexible photovoltaic devices. Moreover, water-soluble luminescent polythiophenes have allowed the specific, rapid, and ultra-sensitive detection of unlabelled DNA, RNA, or proteins. This new optical detection mechanism is based on electrostatic interactions between a cationic polythiophene derivative and negatively charged oligonucleotides. This method makes now possible the rapid assessment of the identity of single nucleotide polymorphisms (SNPs), genes, and pathogens without the need for nucleic acid amplification.

Synthesis and properties of perfluorinated conjugated polymers based on polyethylenedioxythiophene, polypyrrole, and polyfluorene. Toward surfaces with special wettabilities

The electrochemical deposition of organic materials is a convenient and straightforward method that affords rough films in mild conditions. The presence of fluorinated chains covalently attached on the polymer backbone allows the control of the second criterion which plays a role on the wetting properties of the surface, that is, the chemical composition. By modification of the nature of the polymer, films with different surface energies were obtained. Thus, original semifluorinated polypyrrole (PPy- RF n ), polyfluorene (PFl- RF n ), and polyethylenedioxythiophene (PEDOT- RF n ) have been chemically and electrochemically synthesized and characterized. On one hand, the chemical polymerization affords highly fluorinated soluble polymers. Soluble PFl- RF n exhibits blue fluorescence in solution while soluble PEDOT- RF n presents optical properties similar to those of PEDOT. Consequently, they represent interesting candidates for optical devices (OLEDs for PFl- RF n , electrochromic materials for PEDOT- RF n ). On the other hand, surface properties have been investigated on the electroformed polymers by goniometry and microscopy. Fluorinated surfaces of electrodeposited polypyrrole, like polythiophene, give birth to high hydrophobic and oleophobic surfaces, while the use of polyethylenedioxythiophene as the polymer increases sufficiently the surface energy to get combined superhydrophobicity and superoleophilicity. The influence of the chemical composition is discussed through the comparison of the wetting properties of polyethylenedioxythiophene and semifluorinated polythiophene and polyethylenedioxythiophene.

Fluorinated organic materials for electronic and optoelectronic applications: the role of the fluorine atom

In this article we highlight, by means of selected examples drawn from work performed in our or other laboratories, the features of some classes of fluorinated conjugated materials and their use in electronic devices such as electroluminescent diodes or field effect transistors. A variety of fluorinated conjugated systems, either molecular or polymeric, such as poly(phenylenevinylene)s, poly(phenyleneethynylene)s, polythiophenes, polyphenylenes, are dealt with. Attention is also focused on a different class of electroluminescent compounds, represented by the cyclometalated iridium complexes with various forms (mer and fac). In particular, fluorine atoms lower both the HOMO and LUMO energy levels. Consequently, the electron injection is made easier, the materials display a greater resistance against the degradative oxidation processes and organic n-type or ambipolar semiconducting materials may result. Moreover, the C-H...F interactions play an important role in the solid state supramolecular organization, originating a typical pi-stack arrangement which enhances the charge carrier mobility.

Electronegative Oligothiophenes Based on a Hexafluorocyclopentene-Annelated Thiophene Unit

[Structure: see text] The synthesis of hexafluorocyclopenta[c]thiophene and its based oligothiophenes is described. The effectiveness of a hexafluorocyclopentene unit to lower the LUMO level without disturbing the effective conjugation could be unambiguously clarified by spectroscopic measurements and X-ray analysis.

Optical sensors based on hybrid DNA/conjugated polymer complexes

Single-stranded DNA (ss-DNA) can specifically bind to various targets, including a complementary ss-DNA, ions, proteins, drugs, and so forth. When binding takes place, the oligonucleotide probe often undergoes a conformational transition. This conformational change of the negatively charged ss-DNA can be detected by using a water-soluble, cationic polythiophene derivative, which transduces the complex formation into an optical (colorimetric or fluorometric) signal without any labeling of the probe or the target. This simple and rapid methodology has enabled the specific and sensitive detection of nucleic acids and human thrombin. This new biophotonic tool can easily be applied to the detection of various other biomolecules and is also useful in the high-throughput screening of new drugs.

Stable superhydrophobic and lipophobic conjugated polymers films

With a view to developing repellent materials combining both low surface energy and rough structure, original semi-fluorinated polythiophenes have been chemically and electrochemically synthesized and characterized by cyclic voltammetry, GPC, and UV-visible measurements. Polymer films have been deposited onto different substrates by drop casting a soluble polymer fraction on glass plate or by electrodeposition on ITO plate. Surface properties and particularly water and oil repellencies have been investigated by goniometry and correlated with the surface morphology and structure observed by SEM. The incorporation of fluorocarbon chains in the rigid polythiophene backbone yields very low surface free energy materials. Moreover, the way of deposition has a huge influence on the quality and performance of the film surface properties. Electroformed polymers, due to rough surfaces, exhibit great super-hydrophobic and lipophobic properties together with exceptional time stability.

Highly Emissive Conjugated Polymer Excimers

Conjugated polymers often display a decrease of fluorescence efficiency upon aggregation due in large part to enhanced interpolymer interactions that produce weakly emissive species generally described as having excimer-like character. We have found that poly(phenylene ethynylene)s with fused pendant [2.2.2] ring structures having alkene bridges substituted with two ester groups function to give highly emissive, broad, and red-shifted emission spectra in the solid state. To best understand the origin of this new solid-state emissive species, we have performed photophysical studies of a series of different materials in solution, spin-coated thin films, solid solutions, and Langmuir films. We conclude that the new, red-shifted, emissive species originate from excimers produced by interchain interactions being mediated by the particular [2.2.2] ring system employed. The ability to design structures that can reliably produce highly emissive conjugated polymer excimers offers new opportunities in the emission tailoring of electroluminescence and sensory devices.

Building Blocks for N-Type Molecular and Polymeric Electronics. Perfluoroalkyl- versus Alkyl-Functionalized Oligothiophenes (nTs; n = 2−6). Systematic Synthesis, Spectroscopy, Electrochemistry, and Solid-State Organization

The synthesis, comparative physicochemical properties, and solid-state structures of five oligothiophene (nT) series differing in substituent nature and attachment, regiochemistry, and oligothiophene core length (n) are described. These five series include the following 25 compounds: (i) alpha,omega-diperfluorohexyl-nTs 1 (DFH-nTs, n = 2-6), (ii) beta,beta'-diperfluorohexyl-nTs 2 (isoDFH-nTs, n = 2-6), (iii) alpha,omega-dihexyl-nTs 3 (DH-nTs, n = 2-6), (iv) beta,beta'-dihexyl-nTs 4 (isoDH-nTs, n = 2-6), and (v) unsubstituted oligothiophenes 5 (alphanTs, n = 2-6). All new compounds were characterized by elemental analysis, mass spectrometry, and multinuclear NMR spectroscopy. To probe and address quantitatively how the chemistry and regiochemistry of conjugated core substitution affects molecular and solid-state properties, the entire 1-5 series was investigated by differential scanning calorimetry, thermogravimetric analysis, and optical absorption and emission spectroscopies. Single-crystal X-ray diffraction data for several fluorocarbon-substituted oligomers are also presented and compared. The combined analysis of these data indicates that fluorocarbon-substituted nT molecules strongly interact in the condensed state, with unit cell level phase separation between the aromatic core and fluorocarbon chains. Surprisingly, despite these strong intermolecular interactions, high solid-state fluorescence efficiencies are exhibited by the fluorinated derivatives. Insight into the solution molecular geometries and conformational behavior are obtained from analysis of optical and variable-temperature NMR spectra. Finally, cyclic voltammetry data offer a reliable picture of frontier MO energies, which, in combination with DFT computations, provide key information on relationships between oligothiophene substituent effects and electronic response properties.