Morphology Control of Selenophene–Thiophene Block Copolymers through Side Chain Engineering

We report the synthesis of three fully π-conjugated diblock copolymers containing selenophene- and thiophene-based repeating units. All of these diblock copolymers undergo phase separation, and by systematically changing the compatibility of the two blocks through side chain modification, we are able to access different thin film morphologies. Introducing a bulky 2-ethylhexyl side chain increases solubility while retaining crystallinity of the selenophene block. While poly­(3-hexylselenophene)-<i>b</i>-poly­(3-hexylthiophene) and poly­(3-(2-ethylhexyl)­selenophene-<i>b</i>-poly­(3-(2-ethylhexyl)­thiophene) form more disordered fibrillar structures, poly­(3-hexylthiophene)-<i>b</i>-poly­(3-(2-ethylhexyl)­selenophene) forms long (1–2 μm) solid state fibrillar structures that are reminiscent of the lamellae that are formed by nonconjugated block copolymers. We further use electron energy loss spectroscopy to visualize thiophene- and selenophene-rich domains at the nanometer scale in each of these examples. By studying new polymer compositions and relating them to solid state structure, we further our understanding of heterocycle induced phase separation and phase separation in general

Selenophene–Thiophene Block Copolymer Solar Cells with Thermostable Nanostructures

The nanostructure morphology and electron donor performance of a poly(3-hexylselenophene)-<i>block</i>-poly(3-hexylthiophene) (P3HS-<i>b</i>-P3HT) copolymer was studied in a photovoltaic device with a [6,6]-phenyl C61 butyric acid methyl ester (PCBM) acceptor. P3HS-<i>b</i>-P3HT forms fiberlike nanostructures spontaneously, which leads to an initial optimal device performance. Furthermore the nanostructure morphology is not greatly affected by annealing, which leads to a device stability that outperforms P3HT, P3HS, or a P3HS/P3HT mixture under identical conditions. External quantum efficiency, hole mobility, and current–voltage measurements show that the block copolymer also outperforms a ternary blend that consists of a physical mixture of P3HS, P3HT, and PCBM with the same overall composition. Overall, the observation of optimal device performance and morphology without annealing as well as enhanced thermal stability demonstrates the advantage of fully conjugated diblock copolymers in nanostructured devices

Solvent/Electrolyte Control of the Wall Thickness of Template-Synthesized Nanostructures

We study the morphology of polythiophene nanostructures synthesized by templated electrochemical oxidation of thiophene from different solvent/electrolyte systems. We find that the wall thickness of such nanostructures is greatly influenced by the choice of solvent/electrolyte. The role of solvent/electrolyte is related to the rate of polymerization. Solvent/electrolyte systems that increase the rate of thiophene polymerization yield solid nanowires, whereas systems that decrease the rate of polymerization yield thin-wall nanotubes. A solvent/electrolyte system that leads to an intermediate polymerization rate yields intermediate structures, namely thick wall nanotubes. These observations are explained by the thiophene electrochemical polymerization mechanism, which requires the loss of a proton, and thus the acidity or basicity of the solvent/electrolyte can be used to control the reaction rate

A Highly Electron-Deficient Analogue of Aniline, Soluble Oligomers, and Their Redox Properties

The synthesis and electrochemical oxidative coupling of a highly electron-deficient analogue of aniline results in the formation of soluble electron-deficient oligomers. Oligomers undergo related oxidation and reduction processes that are separated by a wide potential range. The mechanism behind this behavior is examined by cyclic voltammetry, optical absorption spectroscopy, 1H NMR spectroscopy, and density functional theory calculations. Mesomeric isomerization of the oxidized oligomers leads to a very stable oxidized state that requires a large (2.8 V) overpotential to return to the neutral form

Compositional Influence on the Regioregularity and Device Parameters of a Conjugated Statistical Copolymer

We describe a conjugated statistical copolymer, poly­(3-hexylthiophene)-<i>stat</i>-(3-thiohexylthiophene) (P3HT-<i>s</i>-P3THT), with 3-hexylthiophene:3-thiohexylthiophene monomer ratios ranging from 50:50 to 99:1. The copolymer is head-to-tail regioregular in both its hexylthiophene–hexylthiophene and hexylthiophene–thiohexylthiophene linkages, which is not observed in poly­(3-thioalkylthiophene) homopolymers. The polymer sequence is random, and the ¹H NMR spectra have eight distinct aromatic signals that correspond to the eight possible HT–HT regioisomer triads and differ from the spectra expected for the corresponding block or homopolymer systems. When testing the copolymers in bulk heterojunction devices with a fullerene-derivative (PC₇₁BM) acceptor, the copolymers have an 11–18% increase in the open-circuit voltage (<i>V</i>_oc) relative to the P3HT:PC₇₁BM device due to the deeper HOMO level of the 3-thiohexylthiophene unit. This increase is independent of copolymer composition over the 50:50 to 85:15 range and is still observed when there is just one 3-thiohexylthiophene unit in the polymer chain. This shows that statistical copolymers containing as low as 1% of a deep HOMO unit can be used to increase the <i>V</i>_oc of the device relative to the parent polymer. All device parameters change in a nonlinear manner as a function of composition, which highlights the distinct properties that can be achieved with conjugated statistical copolymers

A Highly Electron-Deficient Analogue of Aniline, Soluble Oligomers, and Their Redox Properties

The synthesis and electrochemical oxidative coupling of a highly electron-deficient analogue of aniline results in the formation of soluble electron-deficient oligomers. Oligomers undergo related oxidation and reduction processes that are separated by a wide potential range. The mechanism behind this behavior is examined by cyclic voltammetry, optical absorption spectroscopy, ¹H NMR spectroscopy, and density functional theory calculations. Mesomeric isomerization of the oxidized oligomers leads to a very stable oxidized state that requires a large (2.8 V) overpotential to return to the neutral form

Gold Nanotubes as Sensitive, Solution-Suspendable Refractive Index Reporters

Gold Nanotubes as Sensitive, Solution-Suspendable Refractive Index Reporter

Stable, Dual Redox Unit Organic Electrodes

The development of organic materials for electrochemical energy storage has attracted great attention because of their high natural abundance and relatively low toxicity. The bulk of these studies focus on small molecules, polymers, or porous/framework-type materials that employ one type of redox moiety. Here, we report the synthesis and testing of organic materials that incorporate two distinct types of redox units: triptycene-based quinones and perylene diimides. We examine this “dual redox” concept through the synthesis of both frameworks and small molecule model compounds with the redox units positioned at the vertices and connection points. Such a design increases the theoretical capacity of the material. It also imparts high stability because both examples are relatively rigid and highly insoluble in the electrolyte. Lithium-ion batteries consisting of the framework and the small molecule have an excellent cycling retention of 75 and 77%, respectively, over 500 cycles at 1 C. Our work emphasizes the advantages of using multiple redox units in the design of the cathodic materials and redox-active triptycene linkages to achieve high cycling stability

Homogenous Synthesis of Monodisperse High Oligomers of 3‑Hexylthiophene by Temperature Cycling

Whereas monodisperse polymers are ubiquitous in Nature, they remain elusive to synthetic chemists. Absolute control over polymer length and structure is essential to imparting chemical functionality, reproducible properties, and specific solid-state behavior. Precise polymer length has proven to be extremely difficult to control. The most successful examples are generally similar to solid-phase oligo nucleotide or peptide synthesis, wherein the polymer is built up one unit at a time with each sequential monomer addition requiring purification and deprotection (or other functional group activation) step. We have discovered a stepwise homogeneous catalyst-transfer polymerization to prepare monodisperse oligo­(3-hexylthiophene) using temperature to limit additions to one unit per chain per cycle. This is the first reported example of a one-pot synthesis of monodisperse oligomers that requires no additional purification or intermediate steps. It is our hope that the strategy of temperature cycling to “freeze” intermediates will be generalizable to other living polymerization techniques, such as other catalyst-transfer polymerization systems, and those where a resting state involves an association between the catalyst and growing chain