Exciton polaron formation and hot-carrier relaxation in rigid Dion-Jacobson-type two-dimensional perovskites.

The efficiency of two-dimensional Dion-Jacobson-type materials relies on the complex interplay between electronic and lattice dynamics; however, questions remain about the functional role of exciton-phonon interactions. Here we establish the robust polaronic nature of the excitons in these materials at room temperature by combining ultrafast spectroscopy and electronic structure calculations. We show that polaronic distortion is associated with low-frequency (30-60 cm-1) lead iodide octahedral lattice motions. More importantly, we discover how targeted ligand modification of this two-dimensional perovskite structure manipulates exciton-phonon coupling, exciton polaron population and carrier cooling. At high excitation density, stronger exciton-phonon coupling increases the hot-carrier lifetime, forming a hot-phonon bottleneck. Our study provides detailed insight into the exciton-phonon coupling and its role in carrier cooling in two-dimensional perovskites relevant for developing emerging hybrid semiconductor materials with tailored properties.

Sequence-Defined Conjugated Oligomers in Donor-Acceptor Dyads.

Conjugated macromolecules have a rich history in chemistry, owing to their chemical arrangements that intertwine physical and electronic properties. The continuing study and application of these systems, however, necessitates the development of atomically precise models that bridge the gap between molecules, polymers, and/or their blends. One class of conjugated polymers that have facilitated the advancement of structure-property relationships is discrete, precision oligomers that have remained an outstanding synthetic challenge with only a handful of reported examples. Here we show the first synthesis of molecular dyads featuring sequence-defined oligothiophene donors covalently linked a to small-molecule acceptor. These dyads serve as a platform for probing complex photophysical interactions involving sequence-defined oligomers. This assessment is facilitated through the unprecedented control of oligothiophene length- and sequence-dependent arrangement relative to the acceptor unit, made possible by the incorporation of hydroxyl-containing side chains at precise positions along the backbone through sequence-defined oligomerizations. We show that both the oligothiophene sequence and length play complementary roles in determining the transfer efficiency of photoexcited states. Overall, the work highlights the importance of the spatial arrangement of donor-acceptor systems that are commonly studied for a range of uses, including light harvesting and photocatalysis.

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.

Dopant-Stabilized Assembly of Poly(3-hexylthiophene).

Polymer self-assembly is a powerful approach for forming nanostructures for solution-phase applications. However, polymer semiconductor assembly has primarily been driven by solvent interactions. Here, we report poly(3-hexythiophene) homopolymer assembly driven and stabilized by oxidative doping with iron (III) p-toluenesulfonate in benzonitrile. By this improved method, dopant mol % and addition temperature determine the size and morphology of oxidized polymer nanostructures. The dopant counterion provides colloidal stability in a process of dopant-stabilized assembly (DSA). Each variable governing polymer assembly is systematically varied, revealing general principles of oxidized nanostructure assembly and allowing the polymer planarity, optical absorption, and doping level to be modulated. Oxidized nanostructure heights, lengths, and widths are shown to depend on these properties, which we hypothesize is due to competing nanostructure formation and oxidation mechanisms that are governed by the polymer conformation upon doping. Finally, we demonstrate that the nanoparticle oxidative doping level can be tuned post-formation through sequential dopant addition. By revealing the fundamental processes underlying DSA, this work provides a powerful toolkit to control the assembly and optoelectronic properties of oxidatively doped nanostructures in solution.

Precision Synthesis of Conjugated Polymers Using the Kumada Methodology.

Since the discovery of conductive poly(acetylene), the study of conjugated polymers has remained an active and interdisciplinary frontier between polymer chemistry, polymer physics, computation, and device engineering. One of the ultimate goals of polymer science is to reliably synthesize structures, similar to small molecule synthesis. Kumada catalyst-transfer polymerization (KCTP) is a powerful tool for synthesizing conjugated polymers with predictable molecular weights, narrow dispersities, specific end groups, and complex backbone architectures. However, expanding the monomer scope beyond the well-studied 3-alkylthiophenes to include electron-deficient and complex heterocycles has been difficult. Revisiting the successful applications of KCTP can help us gain new insight into the CTP mechanisms and thus inspire breakthroughs in the controlled polymerization of challenging π-conjugated monomers.In this Account, we highlight our efforts over the past decade to achieve controlled synthesis of homopolymers (p-type and n-type), copolymers (diblock and statistical), and monodisperse high oligomers. We first give a brief introduction of the mechanism and state-of-the-art of KCTP. Since the extent of polymerization control is determined by steric and electronic effects of both the catalyst and monomer, the polymerization can be optimized by modifying monomer and catalyst structures, as well as finding a well-matched monomer-catalyst system. We discuss the effects of side-chain steric hindrance and halogens in the context of heavy atom substituted monomers. By moving the side-chain branch point one carbon atom away from the heterocycle to alleviate steric crowding and stabilize the catalyst resting state, we were able to successfully control the polymerization of new tellurophene monomers. Inspired by innocent role of the sterically encumbered 2-transmetalated 3-alkylthiophene monomer, we introduce the treatment of hygroscopic monomers with a bulky Grignard compound as a water-scavenger for the improved synthesis of water-soluble conjugated polymers. For challenging electron-deficient monomers, we discuss the design of new Ni(II)diimine catalysts with electron-donating character which enhance the stability of the association complex between the catalyst and the growing polymer chain, resulting in the quasi-living synthesis of n-type polymers. Beyond n-type homopolymers, the Ni(II)diimine catalysts are also capable of producing electron-rich and electron-deficient diblock and statistical copolymers. We discuss how density functional theory (DFT) calculations elucidate the role of catalyst steric and electronic effects in controlling the synthesis of π-conjugated polymers. Moreover, we demonstrate the synthesis of monodisperse high oligomers by temperature cycling, which takes full advantage of the unique character of KCTP in that it proceeds through distinct intermediates that are not reactive. The insight we gained thus far leads to the first example of isolated living conjugated polymer chains prepared by a standard KCTP procedure, with general applicability to different monomers and catalytic systems. In summarizing a decade of innovation in KCTP, we hope this Account will inspire future development in the field to overcome key challenges including the controlled synthesis of electron-deficient heterocycles, complex and high-performance systems, and degradable and recyclable materials as well as cutting-edge catalyst design.

Homogeneous Synthesis of Monodisperse Sequence-Defined Conjugated Oligomers by Temperature Cycling.

Sequence-defined synthetic oligomers and polymers provide unprecedented opportunities for polymer chemists to finely control properties such as chain folding, self-assembly, and optoelectronic performance of materials. However, absolute control over both chain-length and monomer sequence has been a long-standing "grand challenge" for decades. Herein, we report a novel strategy to synthesize monodisperse sequence-defined conjugated oligomers in a homogeneous manner by temperature cycling, thereby achieving single-monomer precision in conjugated polyheterocycles. A series of sequence-defined oligomers with up to twelve repeating units, four different monomers, and various sequences were successfully synthesized. Monomer sequence was also proved to affect optical properties. We believe this strategy not only exhibits general applicability to the synthesis of group 16 conjugated oligomers and polymers, but also has far-reaching potential for other polymer systems.

Rigid Conjugated Diamine Templates for Stable Dion-Jacobson-Type Two-Dimensional Perovskites.

Hybrid organic-inorganic perovskites (HOIPs) have garnered widespread interest, yet stability remains a critical issue that limits their further application. Compared to their three-dimensional (3D) counterparts, two-dimensional (2D)-HOIPs exhibit improved stability. 2D-HOIPs are also appealing because their structural and optical properties can be tuned according to the choice of organic ligand, with monovalent or divalent ligands forming Ruddlesden-Popper (RP) or Dion-Jacobson (DJ)-type 2D perovskites, respectively. Unlike RP-type 2D perovskites, DJ-type 2D perovskites do not contain a van der Waals gap between the 2D layers, leading to improved stability. However, bifunctional organic ligands currently used to develop DJ-type 2D perovskites are limited to commercially available aliphatic and single-ring aromatic ammonium cations. Large conjugated organic ligands are in demand for their semiconducting properties and their potential to improve materials stability further. In this manuscript, we report the design and synthesis of a new set of larger conjugated diamine ligands and their incorporation into DJ-type 2D perovskites. Compared with analogous RP-type 2D perovskites, DJ 2D perovskites reported here show blue-shifted, narrower emissions and significantly improved stability. By changing the structure of rings (benzene vs thiophene) and substituents, we develop structure-property relationships, finding that fluorine substitution enhances crystallinity. Single-crystal structure analysis and density functional theory calculations indicate that these changes are due to strong electrostatic interactions between the organic templates and inorganic layers as well as the rigid backbone and strong π-π interaction between the organic ligands themselves. These results illustrate that targeted engineering of the diamine ligands can enhance the stability of DJ-type 2D perovskites.

Trends in Conjugated Chalcogenophenes: A Theoretical Study.

Heavy atom substitution in chalcogenophenes is a versatile strategy for tailoring and ultimately improving conjugated polymer properties. While thiophene monomers are commonly implemented in polymer designs, relatively little is known regarding the molecular properties of the heavier chalcogenophenes. Herein, we use density functional theory (DFT) calculations to examine how group 16 heteroatoms, including the radioactive polonium, affect polychalcogenophene properties including bond length, chain twisting, aromaticity, and optical properties. Heavier chalcogenophenes are more quinoidal in character and consequently have reduced band gaps and larger degrees of planarity. We consider both the neutral and radical cationic species. Upon p-type doping, bond length rearrangement is indicative of a more delocalized electronic structure, which combined with optical calculations is consistent with the polaron-model of charge storage on conjugated polymer chains. A better understanding of the properties of these materials at their molecular levels will inevitably be useful in material design as the polymer community continues to explore more main group containing polymers to tackle issues in electronic devices.

Thiophene- and selenophene-based conjugated polymeric mixed ionic/electronic conductors.

Mixed ionic/electronic conductors (MIECs) are desirable materials for next-generation electronic devices and energy storage applications. Polymeric MIECs are attractive from the standpoint that their structure can be controlled and anticipated to have mechanically robust properties. Here, we prepare and investigate conjugated copolymers containing thiophene and selenophene repeat units and their homopolymer counterparts. Specifically, thiophene bearing a triethylene glycol (EG3) side chain was polymerized and copolymerized with dodecyl thiophene/selenophene monomers. The synthesis leads to a class of copolymers that contain either S or Se and are blocky in nature. The Li-ion conductivity of ionically doped copolymers, P3DDT-s-P3(EG3)T and P3DDS-s-P3(EG3)T (9.7 × 10-6 and 8.2 × 10-6 S/cm, respectively), was 3-4 fold higher than that of the ionically doped constituent homopolymer, P3(EG3)T (2.2 × 10-6 S/cm), at ambient conditions. The electronic conductivity of the oxidatively doped copolymers was significantly higher than that of the constituent homopolymer P3(EG3)T, and most notably, P3DDS-s-P3(EG3)T reached ∼7 S/cm, which is the same order of magnitude as poly(3-dodecylthiophene) and poly(3-dodecylselenophene), which are the highest oxidatively doped conductors based on control experiments. Our findings provide implications for designing new MIECs based on copolymerization and the incorporation of heavy atom heterocycles.

Isolation of Living Conjugated Polymer Chains.

Living polymerizations currently play a central role in polymer chemistry. However, one feature of these polymerizations is often overlooked, namely, the isolation of living polymer chains. Herein we report the isolation of living π-conjugated polymer chains, synthesized by catalyst-transfer polycondensation. Successful preservation of the nickel complex at polymer chain ends is evidenced by nuclear magnetic resonance spectroscopy, end group analysis, and chain extension experiments. When characterizing living chains by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, we discovered a unique photoionization-photodissociation fragmentation process for polymers containing a nickel phosphine end group. Living chains are isolated for several types of conjugated polymers as well as discrete living oligomers. Additionally, we are able to recycle the catalysts from the isolated polymer chains. Catalyst recycling after π-conjugated polymerization has previously been impossible without chain isolation. This strategy not only exhibits general applicability to different monomers but also has far-reaching potential for other catalytic systems.

Boramidine: A Versatile Structural Motif for the Design of Fluorescent Heterocycles.

Sodium cyanoborohydride-derived N-alkylnitriliumboranes were found to be versatile precursors for the synthesis of novel boron-containing heterocycles. The reaction between N-alkylnitriliumboranes and 2-aminopyridines, imidazoles, oxazoles, or isoxazoles leads to the incorporation of the [B-C] motif into a five-membered boramidine, which exists as a mixture of Z and E isomers. The resulting heterocycles are blue fluorescent in both the solid state and in solution with ca. 2700-8400 cm-1 Stokes shifts and quantum yields in the 65-74% range in water and in the 42-84% range in organic solvents. The combination of photophysical properties, structural tunability, stability, and solubility in various media is expected to find application in a range of disciplines.

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.

Highly Stable and Inert Complexation of Indium(III) by Reinforced Cyclam Dipicolinate and a Bifunctional Derivative for Bead Encoding in Mass Cytometry.

H2 cb-te2pa, a cross-bridged cyclam functionalized by two picolinate arms, was used for the formation of an incredible inert InIII chelate. The inertness of the complex was evaluated by UV/Vis experiments in several competitive media and was highlighted by the comparison with [In(dota)]- and [In(dtpa)]2- (H4 dota = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, H5 dtpa = diethylenetriamine pentaacetic acid), which are currently used in biological applications. For the first time, a bifunctional analogue of H2 cb-te2pa was prepared by C-functionalization to keep its coordination properties intact. However, this strategy leads to the formation of two diastereoisomers as evidenced and studied by NMR experiments and DFT calculations. Kinetic studies proved nevertheless that both isomers of the complex are equally inert. They were therefore used without distinction for their covalent grafting on polystyrene beads. The so-called metal-encoded beads were tested for imaging mass cytometry. The detection of 115 In allows the generation of images with high quality, proving the great potential of the bifunctional [In(cb-te2pa)]+ derivatives for single-cell analysis by mass cytometry.

Helicoidal Patterning of Gold Nanorods by Phase Separation in Mixed Polymer Brushes.

The spatial distribution of polymer ligands on the surface of nanoparticles (NPs) is of great importance because it determines their interactions with each other and with the surrounding environment. Phase separation in mixtures of polymer brushes has been studied for spherical NPs; however, the role of local surface curvature of nonspherical NPs in the surface phase separation of end-grafted polymer ligands remains an open question. Here, we examined phase separation in mixed monolayers of incompatible polystyrene and poly(ethylene glycol) brushes end-capping the surface of gold nanorods in a good solvent. By varying the molar ratio between these polymers, we generated a range of surface patterns, including uniform and nonuniform polystyrene shells, randomly distributed polystyrene surface patches, and, most interestingly, a helicoidal pattern of polystyrene patches wrapping around the nanorods. The helicoidally patterned nanorods exhibited long-term colloidal stability in a good solvent. The helicoidal wrapping of the nanorods was achieved for the mixtures of polymers with different molecular weights and preserved when the quality of the solvent for the polymers was reduced. The helicoidal organization of polymer patches on the surface of nanorods can be used for templating the synthesis or self-assembly of helicoidal multicomponent nanomaterials.

Applying Heteroatom Substitution in Organic Photovoltaics.

Poly(3-alkylthiophene) (P3AT) has been a central focus of research on organic photovoltaics (OPVs) for well over a decade. Due to their controlled synthesis P3ATs have proven to be a vital model system for developing an understanding of the effects of polymer structure on optoelectronic properties and blend morphology in bulk heterojunction OPVs. Similar to their thiophene counterparts, selenophene and tellurophene can be polymerized in a controlled manner. As single atom substitution results in significant differences in absorption, charge transport and self-assembly these model systems provide a unique opportunity to probe fundamental structure-property relationships. In this account, we provide an overview of our work on copolymers of thiophene and selenophene and examine how the optoelectronic and morphological behavior of these materials can be strategically adjusted through polymer design. We also highlight recent developments on poly(3-alkyltellurophene) and comment on its future in fundamental and applied studies.

Self-Organization and Charge Transport Properties of Selenium and Tellurium Analogues of Polythiophene.

A series of conjugated polymers comprising polythiophene, polyselenophene, and polytellurophene with branched 3,7-dimethyloctyl side chains, well-matched molecular weight, dispersity, and regioregularity is synthesized. The ionization potential is found to vary from 5.14 to 5.32 eV, with polytellurophene having the lowest potential. Field-effect transistors based on these materials exhibit distinct hole transport mobility that varies by nearly three orders of magnitude, with polytellurophene having the highest mobility (2.5 × 10-2 cm² V-1 s-1 ). The large difference in mobility demonstrates the significant impact of heteroatom substitution. Although the series of polymers are very similar in structure, their solid-state properties are different. While the thin film microstructure of polythiophene and polyselenophene is identical, polytellurophene reveals globular features in the film topography. Polytellurophenes also appear to be the least crystalline, even though their charge transport properties are superior to other samples. The torsional barrier and degree of planarity between repeat units increase as one moves down group-16 elements. These studies show how a single atom in a polymer chain can have a substantial influence on the bulk properties of a material, and that heavy group-16 atoms have a positive influence on charge transport properties when all other variables are kept unchanged.

Microwave Synthesis of Thionated Naphthalene Diimide-Based Small Molecules and Polymers.

Thionated naphthalene diimides (NDIs) are promising materials for n-type organic semiconductors; despite their potential, synthetic routes to thionated NDIs are generally lengthy, nonselective, and low yielding and their polymeric analogues have yet to be reported in the literature. Here, we describe the rapid and selective thionation of thiophene- and selenophene-flanked NDIs using microwave irradiation and excess Lawesson's reagent. Remarkably, >99% conversion to the trans-dithionated product is observed by NMR within 45 min. Steric effects imparted by NDI core substituents prevent excess thionation, simplifying purification procedures. We apply this methodology to the postpolymerization thionation of NDI-based polymers to afford a series of polymers with varying degrees of thionation. Thionated NDIs exhibit bathochromic shifts of up to ∼100 nm in localized absorption maxima and increased electron affinities.

The rise of organic electrode materials for energy storage.

Organic electrode materials are very attractive for electrochemical energy storage devices because they can be flexible, lightweight, low cost, benign to the environment, and used in a variety of device architectures. They are not mere alternatives to more traditional energy storage materials, rather, they have the potential to lead to disruptive technologies. Although organic electrode materials for energy storage have progressed in recent years, there are still significant challenges to overcome before reaching large-scale commercialization. This review provides an overview of energy storage systems as a whole, the metrics that are used to quantify the performance of electrodes, recent strategies that have been investigated to overcome the challenges associated with organic electrode materials, and the use of computational chemistry to design and study new materials and their properties. Design strategies are examined to overcome issues with capacity/capacitance, device voltage, rate capability, and cycling stability in order to guide future work in the area. The use of low cost materials is highlighted as a direction towards commercial realization.

Correction: The rise of organic electrode materials for energy storage.

Correction for 'The rise of organic electrode materials for energy storage' by Tyler B. Schon et al., Chem. Soc. Rev., 2016, DOI: .

Patchy Nanofibers from the Thin Film Self-Assembly of a Conjugated Diblock Copolymer.

An unexpected morphology comprising patchy nanofibers can be accessed from the self-assembly of an all-conjugated, polyselenophene-block-polythiophene copolymer. This morphology consists of very small (<10 nm), polythiophene- and polyselenophene-rich domains and is unprecedented for both conjugated polymers and diblock copolymers in general. We propose that the patchy morphology occurs from the enhanced miscibility of the blocks arising from the longer alkyl chains in comparison to similar block copolymers with shorter alkyl chains, which fully phase separate, as well as the difference in rigidity between the polythiophene and polyselenophene blocks. This work demonstrates a facile way to tune the self-assembly behavior of conjugated block copolymers by modification of the side chain substituents.