Initial photooxidation mechanism leading to reactive radical formation of polythiophene derivatives

Light-induced Delivery of Charged Species using Ion-selective Core-Shell Nanoparticles

Controlled release systems have gained considerable attention owing to their potential to deliver molecules, including ions and drugs, in a customized manner. We present a light-induced ion-transfer platform consisting of a dispersion of nanoparticles (NPs, ~300 nm) with the conductive polymer poly(3-octylthiophene-2,5-diyl) (POT) in the core and a potassium (K+)-selective membrane in the shell. Owing to the photoactive nature of POT, POT NPs can be used for a dual purpose: as a host for positively charged species and as an actuator to trigger the subsequent release. POT0 and doped POT+ coexist in the core, allowing K+ encapsulation in the shell. As POT0 is photo-oxidized to POT+, K+ is released to the (aqueous) dispersion phase to preserve the neutrality of the NPs. This process is reversible and can be simultaneously assessed using the native fluorescence of POT0 and via potentiometric measurements. The NP structure and its mechanism of action were thoroughly studied with a series of control experiments and complementary techniques. Understanding the NP and its surrounding interactions will pave the way for other nanostructured systems, facilitating sophisticated applications. The delivery of ionic drugs and interference/pollutant catching for advanced sensing/restoration will be considered in future research.

Polythiophene as a Double-Electrostatic Template for Zinc Oxide and Gold: Multicomponent Nano-Objects for Enhanced Photocatalysis

Using electrostatic self-assembly and electrostatic nanotemplating, a quaternary nanostructured system consisting of zinc oxide nanoparticles, gold nanoparticles, poly[3-(potassium-4-butanoate)thiophene-2,5-diyl] (PT), and methyltrioctylammonium chloride (MTOA) (PT-MTOA-ZnO-Au) was designed for aqueous photocatalysis. The PT-MTOA hollow sphere aggregates served as an electrostatic template for both individual inorganic nanoparticles controlling their morphology, stabilizing the nanoparticles, and acting as a photosensitizer. The hybrid structures included spherical ZnO nanoparticles with a diameter of d = 2.6 nm and spherical Au nanoparticles with d = 6.0 nm embedded in PT-MTOA hollow spheres with a hydrodynamic radius of R_H = 100 nm. The ZnO nanoparticles acted as the main catalyst, while the Au nanoparticles acted as the cocatalyst. As a photocatalytic model reaction, the dye degradation of methylene blue in aqueous solution using the full spectral range from UV to visible light was tested. The photocatalytic activity was optimized by varying the Zn and Au loading ratios and was substantially enhanced regarding the components; for example, it was increased by about 61% using PT-MTOA-ZnO-Au compared to the composite without gold particles. A photocatalytic mechanism of the methylene blue degradation was proposed when catalyzed by these multicomponent nano-objects. Thus, a simple procedure of templating two different nanoparticle species within the same cocatalytically active template has been demonstrated, which can be extended to other inorganic particles, making a variety of task-specific catalysts accessible.

Group 16 conjugated polymers based on furan, thiophene, selenophene, and tellurophene

Five-membered aromatic rings containing Group 16 elements (O, S, Se, and Te), also referred as chalcogenophenes, are ubiquitous building blocks for π-conjugated polymers (CPs). Among these, polythiophenes have been established as a model system to study the interplay between molecular structure, solid-state organization, and electronic performance. The judicious substitution of alternative heteroatoms into polythiophenes is a promising strategy for tuning their properties and improving the performance of derived organic electronic devices, thus leading to the recent abundance of CPs containing furan, selenophene, and tellurophene. In this review, we first discuss the current status of Kumada, Negishi, Murahashi, Suzuki-Miyaura, and direct arylation polymerizations, representing the best routes to access well-defined chalcogenophene-containing homopolymers and copolymers. The self-assembly, optical, solid-state, and electronic properties of these polymers and their influence on device performance are then summarized. In addition, we highlight post-polymerization modifications as effective methods to transform polychalcogenophene backbones or side chains in ways that are unobtainable by direct polymerization. Finally, the major challenges and future outlook in this field are presented.

Thermally Induced Formation of HF4TCNQ- in F4TCNQ-Doped Regioregular P3HT

The prototypical system for understanding doping in solution-processed organic electronics has been poly(3-hexylthiophene) (P3HT) p-doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ). Multiple charge-transfer states, defined by the fraction of electron transfer to F₄TCNQ, are known to coexist and are dependent on polymer molecular weight, crystallinity, and processing. Less well-understood is the loss of conductivity after thermal annealing of these materials. Specifically, in thermoelectrics, F₄TCNQ-doped regioregular (rr) P3HT exhibits significant conductivity losses at temperatures lower than other thiophene-based polymers. Through detailed spectroscopic investigation of progressively heated P3HT films coprocessed with F₄TCNQ, we demonstrate that this diminished conductivity is due to formation of the nonchromophoric, weak dopant HF₄TCNQ^-. This species is likely formed through hydrogen abstraction from the α aliphatic carbon of the hexyl chain at the 3-position of thiophene rings of rr-P3HT. This reaction is eliminated for polymers with ethylene glycol-containing side chains, which retain conductivity at higher operating temperatures. In total, these results provide a critical materials design guideline for organic electronics.

Photochemistry of Organic Retinal Prostheses

Organic devices are attracting considerable attention as prostheses for the recovery of retinal light sensitivity lost to retinal degenerative disease. The biotic/abiotic interface created when light-sensitive polymers and living tissues are placed in contact allows excitation of a response in blind laboratory rats exposed to visual stimuli. Although polymer retinal prostheses have proved to be efficient, their working mechanism is far from being fully understood. In this review article, we discuss the results of the studies conducted on these kinds of polymer devices and compare them with the data found in the literature for inorganic retinal prostheses, where the working mechanisms are better comprehended. This comparison, which tries to set some reference values and figures of merit, is intended for use as a starting point to determine the direction for further investigation.

Structure-Function Relationship of Organic Semiconductors: Detailed Insights From Time-Resolved EPR Spectroscopy

Organic photovoltaics (OPV) is a promising technology to account for the increasing demand for energy in form of electricity. Whereas the last decades have seen tremendous progress in the field witnessed by the steady increase in efficiency of OPV devices, we still lack proper understanding of fundamental aspects of light-energy conversion, demanding for systematic investigation on a fundamental level. A detailed understanding of the electronic structure of semiconducting polymers and their building blocks is essential to develop efficient materials for organic electronics. Illuminating conjugated polymers not only leads to excited states, but sheds light on some of the most important aspects of device efficiency in organic electronics as well. The interplay between electronic structure, morphology, flexibility, and local ordering, while at the heart of structure-function relationship of organic electronic materials, is still barely understood. (Time-resolved) electron paramagnetic resonance (EPR) spectroscopy is particularly suited to address these questions, allowing one to directly detect paramagnetic states and to reveal their spin-multiplicity, besides its clearly superior spectral resolution compared to optical methods. This article aims at giving a non-specialist audience an overview of what EPR spectroscopy and particularly its time-resolved variant (TREPR) can contribute to unraveling aspects of structure-function relationship in organic semiconductors.

Controlling Molecular Doping in Organic Semiconductors

The field of organic electronics thrives on the hope of enabling low-cost, solution-processed electronic devices with mechanical, optoelectronic, and chemical properties not available from inorganic semiconductors. A key to the success of these aspirations is the ability to controllably dope organic semiconductors with high spatial resolution. Here, recent progress in molecular doping of organic semiconductors is summarized, with an emphasis on solution-processed p-type doped polymeric semiconductors. Highlighted topics include how solution-processing techniques can control the distribution, diffusion, and density of dopants within the organic semiconductor, and, in turn, affect the electronic properties of the material. Research in these areas has recently intensified, thanks to advances in chemical synthesis, improved understanding of charged states in organic materials, and a focus on relating fabrication techniques to morphology. Significant disorder in these systems, along with complex interactions between doping and film morphology, is often responsible for charge trapping and low doping efficiency. However, the strong coupling between doping, solubility, and morphology can be harnessed to control crystallinity, create doping gradients, and pattern polymers. These breakthroughs suggest a role for molecular doping not only in device function but also in fabrication-applications beyond those directly analogous to inorganic doping.

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.