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.

A Thiophene Backbone Enables Two-Dimensional Poly(arylene vinylene)s with High Charge Carrier Mobility.

Linear conjugated polymers have attracted significant attention in organic electronics in recent decades. However, despite intrachain π-delocalization, interchain hopping is their transport bottleneck. In contrast, two-dimensional (2D) conjugated polymers, as represented by 2D π-conjugated covalent organic frameworks (2D c-COFs), can provide multiple conjugated strands to enhance the delocalization of charge carriers in space. Herein, we demonstrate the first example of thiophene-based 2D poly(arylene vinylene)s (PAVs, 2DPAV-BDT-BT and 2DPAV-BDT-BP, BDT=benzodithiophene, BT=bithiophene, BP=biphenyl) via Knoevenagel polycondensation. Compared with 2DPAV-BDT-BP, the fully thiophene-based 2DPAV-BDT-BT exhibits enhanced planarity and π-delocalization with a small band gap (1.62 eV) and large electronic band dispersion, as revealed by the optical absorption and density functional calculations. Remarkably, temperature-dependent terahertz spectroscopy discloses a unique band-like transport and outstanding room-temperature charge mobility for 2DPAV-BDT-BT (65 cm2  V-1 s-1 ), which far exceeds that of the linear PAVs, 2DPAV-BDT-BP, and the reported 2D c-COFs in the powder form. This work highlights the great potential of thiophene-based 2D PAVs as candidates for high-performance opto-electronics.

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.

Polymer Acceptors Containing B←N Units for Organic Photovoltaics.

ConspectusOrganic photovoltaics (OPVs), in which blend films of organic or polymer electron donor and electron acceptor are used as the active layer, are a promising photovoltaic technology with the great advantages of solution processing, low cost, and flexibility. The development of small molecular or polymer electron acceptors has boosted power conversion efficiency (PCE) of OPVs from 10% to 18%. Among them, polymer acceptors have the merits of superior morphology stability and excellent mechanical properties. However, owing to the key requirement of very low-lying LUMO/HOMO energy levels for polymer acceptors, very few conjugated polymers can work as polymer acceptors in OPVs. The majority of polymer electron acceptors are based on strong electron-withdrawing imide units or cyano substituents. Since 2015, conjugated polymers containing the boron-nitrogen coordination bond (B←N) have emerged as a new kind of polymer electron acceptor with excellent photovoltaic performance in various kinds of organic photovoltaic devices. In this Account, we summarize our research progress on polymer acceptors containing B←N units.At first, we introduce the principle of B←N to greatly down shift LUMO/HOMO energy levels, which enables B←N to be used to design polymer acceptors. Then we describe the two molecular design strategies for polymer acceptors containing B←N units. For high-efficiency OPVs, polymer acceptors should have wide absorption spectra, proper LUMO/HOMO energy levels, high electron mobility, and good donor/acceptor blend morphology. We discuss how to use molecular design to finely tune the absorption spectra, energy levels, and electron mobility of the B←N-containing polymer acceptors. We also discuss how to improve the phase separation morphology of the blends of these polymer acceptors with small molecular donors or polymer donors. These improvements lead to excellent performance of the polymer acceptors containing B←N units in three kinds of organic photovoltaic devices. The small molecular donor/polymer acceptor type organic solar cells show excellent thermal stability and PCE of 8.0%, which is the highest value reported so far. The all-polymer solar cells exhibit PCE of 10.1%. The all-polymer indoor photovoltaics show PCE as high as 27.4% under fluorescent lamp illumination at 2000 lx. This PCE is fairly comparable to those of the best organic or inorganic indoor photovoltaics. These results provide a solid foundation for future advances. Finally, we propose that great attention should be paid to further PCE enhancement of OPVs and indoor photovoltaic applications of this new emerging kind of polymer acceptor.

A New Electron-Rich Unit for Polymer Electron Acceptors: 4,4-Difluoro-4H-cyclopenta[2,1-b:3,4-b']dithiophene.

We report 4,4-difluoro-4H-cyclopenta[2,1-b:3,4-b']dithiophene (fCDT) as a new electron-rich unit to design polymer electron acceptors. Owing to the fluoro substitutes, fCDT unit exhibits downshifted LUMO energy level, diminished steric hindrance effect and strong intermolecular interaction. The resulting polymer electron acceptor exhibits low-lying LUMO energy level and high electron mobility, as well as good all-polymer solar cell device performance.

Polymer Acceptor Based on B←N Units with Enhanced Electron Mobility for Efficient All-Polymer Solar Cells.

We demonstrate that polymer electron acceptors with excellent all-polymer solar-cell (all-PSC) device performance can be developed from polymer electron donors by using B←N units. By alleviating the steric hindrance effect of the bulky pendant moieties on the conjugated polymers that contain B←N units, the π-π stacking distance of polymer backbones is decreased and the electron mobility is consequently enhanced by nearly two orders of magnitude. As a result, the power conversion efficiency of all-PSCs with the polymer acting as the electron acceptor is greatly improved from 0.12 % to 5.04 %. This PCE value is comparable to that of the best all-PSCs with state-of-the-art polymer acceptors.

Analgesic effects of pectoserratus plane block and intermediate cervical plexus block for transaxillary endoscopic thyroidectomy: a prospective randomized controlled trial.

BACKGROUND: Transaxillary endoscopic thyroidectomy (TAET) is favored for its favorable cosmetic outcomes and the preservation of anterior cervical function. Despite these benefits, postoperative analgesia has traditionally relied on pharmacological interventions, and regional anesthetic procedures may be an alternative method. This study aimed to evaluate the efficacy of an ultrasound-guided pectoserratus plane block (PSPB) combined with an intermediate cervical plexus block (ICPB) for TAET. METHODS: Forty patients undergoing TAET were randomized into two groups: the nerve block group (N.=20) received ultrasound guided PSPB with 20 ml of 0.375% ropivacaine and ICPB with 8 mL of 0.3% ropivacaine, while the control group (N.=20) received no block. The primary outcome was the Visual Analog Scale (VAS) scores for postoperative neck and axillary pain at different time points (1, 6, 12, 24 h) during rest and movement post-TAET. The secondary outcomes included intraoperative remifentanil consumption, incidence of postoperative nausea and vomiting (PONV), number of remedial analgesic requirements, and patient satisfaction postoperatively. RESULTS: Compared to the control group, patients in the nerve block had significantly lower VAS scores of the neck and axilla whether at rest or movement, and 1, 6, 12, and 24 h postoperatively (P<0.0125). The nerve block group showed higher patient satisfaction (P<0.001). No difference was observed in intraoperative remifentanil consumption, need for rescue analgesics, or other adverse effects 48 h postoperatively. CONCLUSIONS: Ultrasound-guided PSPB with ICPB significantly alleviated postoperative pain and improved patient satisfaction with TAET.

Large piezoelectric response in a Jahn-Teller distorted molecular metal halide.

Piezoelectric materials convert between mechanical and electrical energy and are a basis for self-powered electronics. Current piezoelectrics exhibit either large charge (d33) or voltage (g33) coefficients but not both simultaneously, and yet the maximum energy density for energy harvesting is determined by the transduction coefficient: d33*g33. In prior piezoelectrics, an increase in polarization usually accompanies a dramatic rise in the dielectric constant, resulting in trade off between d33 and g33. This recognition led us to a design concept: increase polarization through Jahn-Teller lattice distortion and reduce the dielectric constant using a highly confined 0D molecular architecture. With this in mind, we sought to insert a quasi-spherical cation into a Jahn-Teller distorted lattice, increasing the mechanical response for a large piezoelectric coefficient. We implemented this concept by developing EDABCO-CuCl4 (EDABCO = N-ethyl-1,4-diazoniabicyclo[2.2.2]octonium), a molecular piezoelectric with a d33 of 165 pm/V and g33 of ~2110 × 10-3 V m N-1, one that achieved thusly a combined transduction coefficient of 348 × 10-12 m3 J-1. This enables piezoelectric energy harvesting in EDABCO-CuCl4@PVDF (polyvinylidene fluoride) composite film with a peak power density of 43 µW/cm2 (at 50 kPa), the highest value reported for mechanical energy harvesters based on heavy-metal-free molecular piezoelectric.

Engineering hydrogen bonding to align molecular dipoles in organic solids for efficient second harmonic generation.

Considering nearly infinite design possibilities, organic second harmonic generation (SHG) molecules are believed to have long-term promise. However, because of the tendency to form dipole-antiparallel crystals that lead to zero macroscopic polarization, it is difficult to design a nonlinear optical (NLO) material based on organic molecules. In this manuscript, we report a new molecule motif that can form asymmetric organic solids by controlling the degree of hydrogen bonding through protonation. A conjugated polar organic molecule was prepared with a triple bond connecting an electron-withdrawing pyridine ring and an electron-donating thiophene ring. By controlling the degree of hydrogen bonding through protonation, two different crystal packing motifs are achieved. One crystallizes into the common dipole-antiparallel nonpolar P1̄ space group. The second crystallizes into the uncommon dipole-parallel polar P1 space group, in which the molecular dipoles are aligned along a single axis and thus exhibit a high macroscopic polarization in its solid-state form. Due to the P1 polar packing, the sample can generate second harmonic light efficiently, about three times the intensity of the benchmark potassium dihydrogen phosphate. Our findings show that crystal engineering by hydrogen bonding in a single molecular backbone can be used for controlling the macroscopic NLO properties.

Engineering Electro-Optic BaTiO3 Nanocrystals via Efficient Doping.

Electro-optic (EO) modulators provide electrical-to-optical signal conversion relevant to optical communications. Barium titanate (BaTiO3 ) is a promising material system for EO modulation in light of its optical ultrafast nonlinearity, low optical loss, and high refractive index. To enhance further its spontaneous polarization, BaTiO3 can be doped at the Ba and Ti sites; however, doping is often accompanied by ion migration, which diminishes EO performance. Here, donor-acceptor doping and its effect on EO efficiency are investigated, finding that La-doped BaTiO3 achieves an EO coefficient of 42 pm V-1 at 1 kHz, fully twice that of the pristine specimen; however, it is also observed that, with this single-element doping, the EO response falls off rapidly with frequency. From impedance spectroscopy, it is found that frequency-dependent EO is correlated with ion migration. Density functional theory calculations predict that the ion-migration barrier decreases with La3+ doping but can be recovered with further Mn2+ doping, a finding that prompts to prevent ion migration by incorporating Mn2+ into the Ti-site to compensate for the charge imbalance.

Simple and Efficient Green-Light-Emitting Diodes Based on Thin Organolead Bromide Perovskite Films via Tuning Precursor Ratios and Postannealing Temperature.

Organometal halide perovskites (OHPs) are becoming a hot topic in the field of display and lighting. Unlike the strategy used for solar cells, that is, using several hundred nanometers thick OHP film for fully absorbing solar light to convert electricity, thin-film OHPs (<50 nm) are advantageous to restrain its self-absorption drawback and thus beneficial for preparing efficient light-emitting diodes (LEDs). Here we manipulate the excess molar ratio of MABr/PbBr2 precursors and the post-annealing temperature to obtain uniform thin-film OHPs and suppress the nonradiative defects. Using this simple process, high efficient green perovskite light-emitting diode (PeLED) was obtained, with a maximum luminance of 6124 cd m-2, current efficiency of 15.26 cd A-1, and external quantum efficiency of 3.38%, which is nearly three-fold enhancement with respect to the previous reported best PeLED based on thin perovskite films (<50 nm).