High‐Performance pH‐Switchable Supramolecular Thermosets via Cation–π Interactions

Supramolecular chemistry has provided versatile and affordable solutions for the design of intelligent soft materials, but it cannot be applied in stiff materials. This paper describes a new concept for the design of high‐performance supramolecular thermosets by using the noncovalent cation–π interaction as cross‐linking. These supramolecular thermosets are a class of infusible and insoluble stiff polymers having excellent mechanical properties even at temperatures exceeding 300 °C. The cation–π interaction can be locally and reversibly installed and removed by aqueous treatments at high or low pH, respectively. Local manipulation of cross‐linking confers these thermosets with multiple stimuli‐responsive functions, such as recyclability, healability, adhesion, and nondestructive detection of cross‐linking and mechanical properties.     

Creating Graphitic Carbon Nitride Based Donor‐π–Acceptor‐π–Donor Structured Catalysts for Highly Photocatalytic Hydrogen Evolution

Conjugated polymers with tailored donor–acceptor units have recently attracted considerable attention in organic photovoltaic devices due to the controlled optical bandgap and retained favorable separation of charge carriers. Inspired by these advantages, an effective strategy is presented to solve the main obstructions of graphitic carbon nitride (g‐C₃N₄) photocatalyst for solar energy conversion, that is, inefficient visible light response and insufficient separation of photogenerated electrons and holes. Donor‐π–acceptor‐π–donor polymers are prepared by incorporating 4,4′‐(benzoc 1,2,5 thiadiazole‐4,7‐diyl) dianiline (BD) into the g‐C₃N₄ framework (UCN‐BD). Benefiting from the visible light band tail caused by the extended π conjugation, UCN‐BD possesses expanded visible light absorption range. More importantly, the BD monomer also acts as an electron acceptor, which endows UCN‐BD with a high degree of intramolecular charge transfer. With this unique molecular structure, the optimized UCN‐BD sample exhibits a superior performance for photocatalytic hydrogen evolution upon visible light illumination (3428 µmol h^?1 g^?1), which is nearly six times of that of the pristine g‐C₃N₄. In addition, the photocatalytic property remains stable for six cycles in 3 d. This work provides an insight into the synthesis of g‐C₃N₄‐based D‐π–A‐π–D systems with highly visible light response and long lifetime of intramolecular charge carriers for solar fuel production.     

Solution Adsorption Formation of a π‐Conjugated Polymer/Graphene Composite for High‐Performance Field‐Effect Transistors

Semiconducting polymers with π‐conjugated electronic structures have potential application in the large‐scale printable fabrication of high‐performance electronic and optoelectronic devices. However, owing to their poor environmental stability and high‐cost synthesis, polymer semiconductors possess limited device implementation. Here, an approach for constructing a π‐conjugated polymer/graphene composite material to circumvent these limitations is provided, and then this material is patterned into 1D arrays. Driven by the π–π interaction, several‐layer polymers can be adsorbed onto the graphene planes. The low consumption of the high‐cost semiconductor polymers and the mass production of graphene contribute to the low‐cost fabrication of the π‐conjugated polymer/graphene composite materials. Based on the π‐conjugated system, a reduced π–π stacking distance between graphene and the polymer can be achieved, yielding enhanced charge‐transport properties. Owing to the incorporation of graphene, the composite material shows improved thermal stability. More generally, it is believed that the construction of the π‐conjugated composite shows clear possibility of integrating organic molecules and 2D materials into microstructure arrays for property‐by‐design fabrication of functional devices with large area, low cost, and high efficiency.     

Chiral Nanostructures from Helical Copolymer–Metal Complexes: Tunable Cation–π Interactions and Sergeants and Soldiers Effect

Poly(phenylacetylene) (PPA) copolymers containing (R)‐ or (S)‐MPA as minor chiral pendant can be forced to selectively adopt the right‐ o left‐handed helix, in the presence of small amounts of Na⁺ or Ag⁺ (“Sergeants and Soldiers Effect”) by addition of a donor cosolvent. The helical sense depends exclusively on the chiral monomer/donor cosolvent ratio, and this allows a perfect on/off tuning of the helicity of the copolymer. When the amount of the donor cosolvent is low, the metal ion complex is stabilized by a cation–π interaction, which is selectively cleaved when the amount of cosolvent is higher. Macroscopically chiral nanospheres and nanotubes composed by helical copolymers with P or M helical sense are also described. Our results demonstrate that it is possible to obtain the two enantiomeric helical structures (P and M helicities) and the corresponding nanospheres and nanotubes from a single helical copolymer, by controlled activation/deactivation of the Sergeant and Soldiers Effect with a donor cosolvent.     

Strong,Conductive, Foldable Graphene Sheets by Sequential Ionic and π Bridging

The goal of this work is to develop an inexpensive low‐temperature process that provides polymer‐free, high‐strength, high‐toughness, electrically conducting sheets of reduced graphene oxide (rGO). To develop this process, we have evaluated the mechanical and electrical properties resulting from the application of an ionic bonding agent (Cr³⁺), a π–π bonding agent comprising pyrene end groups, and their combinations for enhancing the performance of rGO sheets. When only one bonding agent was used, the π–π bonding agent is much more effective than the ionic bonding agent for improving both the mechanical and electrical properties of rGO sheets. However, the successive application of ionic bonding and π–π bonding agents maximizes tensile strength, toughness, long‐term electrical stability in various corrosive solutions, and resistance to mechanical abuse and ultrasonic dissolution. Using a combination of ionic bonding and π–π bonding agents, high tensile strength (821 MPa), high toughness (20 MJ m^?3), and electrical conductivity (416 S cm^?1) were obtained, as well as remarkable retention of mechanical and electrical properties during ultrasonication and mechanical cycling by both sheet stretch and sheet folding, suggesting high potential for applications in aerospace and flexible electronics.     

Interchain Interactions in Organic π‐Conjugated Materials: Impact on Electronic Structure,Optical Response,and Charge Transport

The pioneering work of Heeger, MacDiarmid, and Shirakawa, rewarded by the 2000 Nobel Prize in Chemistry, has paved the way for the development of the fields of plastic electronics and photonics. Functional organic molecular materials and conjugated oligomers or polymers now allow the low‐cost fabrication of thin films for insertion into new generations of electronic and optoelectronic devices. The performance of these devices relies on the understanding and optimization of several complementary processes (see sketch). Our goal is to discuss, from a theoretical standpoint, the electronic structure characteristics and interfacial properties that are of importance in all these areas. The concept of interface should be taken here in the microscopic sense, i.e., molecular interactions among two or several chains/molecules (of the same or of a different nature). Specifically, we will address the impact of interchain interactions within an organic layer on the transport and optical properties. These issues will therefore be more directly related to transistor and light‐emitting diode applications; however, in all instances, the aspects related to interfacial charge or energy transfer processes will dictate the ultimate performance of a material in a given device.     

Ion‐Migration Inhibition by the Cation–π Interaction in Perovskite Materials for Efficient and Stable Perovskite Solar Cells

Migration of ions can lead to photoinduced phase separation, degradation, and current–voltage hysteresis in perovskite solar cells (PSCs), and has become a serious drawback for the organic–inorganic hybrid perovskite materials (OIPs). Here, the inhibition of ion migration is realized by the supramolecular cation–π interaction between aromatic rubrene and organic cations in OIPs. The energy of the cation–π interaction between rubrene and perovskite is found to be as strong as 1.5 eV, which is enough to immobilize the organic cations in OIPs; this will thus will lead to the obvious reduction of defects in perovskite films and outstanding stability in devices. By employing the cation‐immobilized OIPs to fabricate perovskite solar cells (PSCs), a champion efficiency of 20.86% and certified efficiency of 20.80% with negligible hysteresis are acquired. In addition, the long‐term stability of cation‐immobilized PSCs is improved definitely (98% of the initial efficiency after 720 h operation), which is assigned to the inhibition of ionic diffusions in cation‐immobilized OIPs. This cation–π interaction between cations and the supramolecular π system enhances the stability and the performance of PSCs efficiently and would be a potential universal approach to get the more stable perovskite devices.     

Patterning π‐Conjugated Polymers

π‐Conjugated polymers show promise as active materials in application areas such as microelectronics, electro‐optics, opto‐electronics, and photonics. A critical feature in this emerging technology is device fabrication and the reproducible deposition of active material. This review focuses on current trends in the spatial deposition of conjugated polymers.     

All‐Carbon Electrode Molecular Electronic Devices Based on Langmuir–Blodgett Monolayers

Nascent molecular electronic devices, based on monolayer Langmuir–Blodgett films sandwiched between two carbonaceous electrodes, have been prepared. Tightly packed monolayers of 4‐((4‐((4‐ethynylphenyl)ethynyl)phenyl)ethynyl)benzoic acid are deposited onto a highly oriented pyrolytic graphite electrode. An amorphous carbon top contact electrode is formed on top of the monolayer from a naphthalene precursor using the focused electron beam induced deposition technique. This allows the deposition of a carbon top‐contact electrode with well‐defined shape, thickness, and precise positioning on the film with nm resolution. These results represent a substantial step toward the realization of integrated molecular electronic devices based on monolayers and carbon electrodes.