Carlos

Luminescent mechanochromic materials are particularly appealing for the development of stimuli-responsive materials. Establishing the mechanism responsible for the mechanochromism is always an issue owing to the difficulty in characterizing the ground phase. Herein, the study of real crystalline polymorphs of a mechanochromic and thermochromic luminescent copper iodide cluster permits us to clearly establish the mechanism involved. The local disruption of the crystal packing induces changes in the cluster geometry and in particular the modification of the cuprophilic interactions, which consequently modify the emissive states. This study constitutes a step further toward the understanding of the mechanism involved in the mechanochromic luminescent properties of multimetallic coordination complexes.

A glowing report: The unusual mechanochromic luminescence properties of the studied copper iodide cluster result from local disruption of the crystal packing, which induces changes in the cluster geometry, thereby modifying the emissive properties (see figure).

Mesoporous carbon (m-C) has potential applications as porous electrodes for electrochemical energy storage, but its applications have been severely limited by the inherent fragility and low electrical conductivity. A rational strategy is presented to construct m-C into hierarchical porous structures with high flexibility by using a carbon nanotube (CNT) sponge as a three-dimensional template, and grafting Pt nanoparticles at the m-C surface. This method involves several controllable steps including solution deposition of a mesoporous silica (m-SiO₂) layer onto CNTs, chemical vapor deposition of acetylene, and etching of m-SiO₂, resulting in a CNT@m-C core–shell or a CNT@m-C@Pt core–shell hybrid structure after Pt adsorption. The underlying CNT network provides a robust yet flexible support and a high electrical conductivity, whereas the m-C provides large surface area, and the Pt nanoparticles improves interfacial electron and ion diffusion. Consequently, specific capacitances of 203 and 311 F g⁻¹ have been achieved in these CNT@m-C and CNT@m-C@Pt sponges as supercapacitor electrodes, respectively, which can retain 96 % of original capacitance under large degree compression.

Hybrid sponge: A rational strategy was developed by using a carbon nanotube (CNT) sponge as a 3D network template to fabricate hierarchical porous CNT@m-C@Pt core–shell hybrid structures (see picture: blue CNTs, green m-C (mesoporous carbon), silver Pt nanoparticles). These hybrid sponges show promising applications as high-performance flexible supercapacitor electrodes.

The incorporation of a phenanthrene moiety into a porphyrin framework results in the formation of a hybrid macrocycle—phenanthriporphyrin—merging the structural features of polycyclic aromatic hydrocarbons and porphyrins. An antiaromatic aceneporphyrinoid, adopting the trianionic {CCNN} core, is suitable for the incorporation of a phosphorus(V) center to form a hypervalent organophosphorus(V) derivative.

PAH–porphyrins: Phenanthriporphyrin, an antiaromatic hybrid macrocycle, merges structural facets of polycyclic aromatic hydrocarbons and porphyrins. The {CCNN} core of this aceneporphyrinoid involves phenanthrene carbon atoms which coordinate a hypervalent organophosphorus(V) center. Atom colors: C=gray; N=blue; O=red; H=white; P=purple.

A novel and efficient CP bond formation reaction of diarylphosphine oxides with aryl iodides was achieved by combining nickel catalysis and visible-light-induced photoredox catalysis. This dual-catalytic reaction showed a broad substrate scope, excellent functional group tolerance, and afforded the corresponding products in good to excellent yields. Compared with the previously reported use of photoredox/nickel dual catalysis in the construction of CC bonds, the methodology described herein was observed to be the first to allow for C-heteroatom bond formation.

Dual catalysis: A novel and efficient CP bond formation reaction of diarylphosphine oxides with aryl iodides was achieved by combining nickel catalysis and visible-light-induced photoredox catalysis (see scheme). This dual-catalytic reaction showed a broad substrate scope, excellent functional-group tolerance, and afforded the corresponding products in good to excellent yields. Compared with previously reported photoredox/nickel dual catalytic systems, this methodology is the first to allow for Cheteroatom bond formation.

This article assembles pertinent insights behind the concept of planarizable push–pull probes. As a response to the planarization of their polarized ground state, a red shift of their excitation maximum is expected to report on either the disorder, the tension, or the potential of biomembranes. The combination of chromophore planarization and polarization contributes to various, usually more complex processes in nature. Examples include the color change of crabs or lobsters during cooking or the chemistry of vision, particularly color vision. The summary of lessons from nature is followed by an overview of mechanosensitive organic materials. Although often twisted and sometimes also polarized, their change of color under pressure usually originates from changes in their crystal packing. Intriguing exceptions include the planarization of several elegantly twisted phenylethynyl oligomers and polymers. Also mechanosensitive probes in plastics usually respond to stretching by disassembly. True ground-state planarization in response to molecular recognition is best exemplified with the binding of thoughtfully twisted cationic polythiophenes to single- and double-stranded oligonucleotides. Molecular rotors, en vogue as viscosity sensors in cells, operate by deplanarization of the first excited state. Pertinent recent examples are described, focusing on λ-ratiometry and intracellular targeting. Complementary to planarization of the ground state with twisted push–pull probes, molecular rotors report on environmental changes with quenching or shifts in emission rather than absorption. The labeling of mechanosensitive channels is discussed as a bioengineering approach to bypass the challenge to create molecular mechanosensitivity and use biological systems instead to sense membrane tension. With planarizable push–pull probes, this challenge is met not with twistome screening, but with “fluorescent flippers,” a new concept to insert large and bright monomers into oligomeric probes to really feel the environment and also shine when twisted out of conjugation.

Lobsters, flippers, disorder, tension, and potential: The origins of the concept of mechanosensitive membrane probes—from fishmarket and scuba diving to the chemistry of vision, mechanochromic organic materials, and mechanosensitive channels—are briefly reviewed (see scheme).

Self-assembly of AB₂ and AB₃ type low molecular weight poly(aryl ether) dendrons that contain hydrazide units were used to investigate mechanistic aspects of helical structure formation during self-assembly. The results suggest that there are three important aspects that control helical structure formation in such systems with acyl hydrazide/hydrazone linkage: i) J-type aggregation, ii) the hydrogen-bond donor/acceptor ability of the solvent, and iii) the dielectric constant of the solvent. The monomer units self-assemble to form dimer structures through hydrogen-bonding and further assembly of the hydrogen-bonded dimers leads to macroscopic chirality in the present case. Dimer formation was confirmed by NMR spectroscopy and by mass spectrometry. The self-assembly in the system was driven by hydrogen-bonding and π–π stacking interactions. The morphology of the aggregates formed was examined by scanning electron microscopy, and the analysis suggests that aprotic solvent systems facilitate helical fibre formation, whereas introduction of protic solvents results in the formation of flat ribbons. This detailed mechanistic study suggests that the self-assembly follows a nucleation–elongation model to form helical structures, rather than the isodesmic model.

How will it gel? The J-type aggregation propensity, hydrogen-bond donor/acceptor ability of solvents, as well as the polarity of the medium have been identified as the main factors that control the self-assembly of helical structures by poly(aryl ether) based dendron derivatives with acyl hydrazone/hydrazide units.

The key parameters of conjugated polymers are lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Few approaches can simultaneously lower LUMO and HOMO energy levels of conjugated polymers to a large extent (>0.5 eV). Disclosed herein is a novel strategy to decrease both LUMO and HOMO energy levels of conjugated polymers by about 0.6 eV through replacement of a CC unit by a BN unit. The replacement makes the resulting polymer transform from an electron donor into an electron acceptor, and is proven by fluorescence quenching experiments and the photovoltaic response. This work not only provides an effective approach to tune the LUMO/HOMO energy levels of conjugated polymers, but also uses organic boron chemistry as a new toolbox to develop conjugated polymers with high electron affinity for polymer optoelectronic devices.

Transforming units: A novel strategy decreases both LUMO and HOMO energy levels of conjugated polymers, by about 0.6 eV, through replacement of a CC unit by a BN unit. The replacement transforms the resulting polymer from an electron donor into an electron acceptor.

Dinitriles bearing aggregation-induced emission (AIE)-active moieties [tetraphenylethylene (TPE) or diphenylphenanthrene (DPP)] were prepared. Compounds 4 (TPE-linked) and 8 (DPP-linked) showed considerably redshifted emission resulting from their large Stokes shifts and also strong fluorescence in the aggregated and solid states. Pure E and Z stereoisomers of both dinitriles were easily separated, and their isomerization equilibria and fluorescence properties were investigated. In addition to their pronounced AIEE behavior, 4 and 8 also showed various reversible chromic responses to external stimuli, namely, solvato-, piezo-, vapo-, and thermochromism, which make them potential candidates for smart materials.

What it takes to shine: Dinitriles bearing tetraphenylethylene (TPE) or diphenylphenanthrene (DPP) units showed distinct aggregation-induced emission (AIE) and various reversible chromic responses to external stimuli (solvato-, piezo-, vapo-, and thermochromism), depending on the isomer conformation and number of rotatable phenyl rings in the molecule.

Design and synthesis of porous solids employing both reversible coordination chemistry and reversible covalent bond formation is described. The combination of two different linkage modes in a single material presents a link between two distinct classes of porous materials as exemplified by metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). This strategy, in addition to being a compelling material-discovery method, also offers a platform for developing a fundamental understanding of the factors influencing the competing modes of assembly. We also demonstrate that even temporary formation of reversible connections between components may be leveraged to make new phases thus offering design routes to polymorphic frameworks. Moreover, this approach has the striking potential of providing a rich landscape of structurally complex materials from commercially available or readily accessible feedstocks.

Teamwork saves the day when coordination chemistry and covalent bond formation can both occur in a single material. A balance between the incubation time of the organic components and solvent decomposition/base formation governs the competition between the two processes and determines the phase outcome. Even the temporary formation of reversible connections between components can be leveraged to make new phases.

Highly efficient red–green–blue (RGB) tricolor luminescence switching was demonstrated in a bicomponent solid film consisting of (2Z,2′Z)-2,2′-(1,4-phenylene)bis(3-(4-butoxyphenyl)acrylonitrile) (DBDCS) and (2Z,2′Z)-3,3′-(2,5-bis(6-(9H-carbazol-9-yl)hexyloxy)-1,4-phenylene)bis(2-(3,5-bis(trifluoromethyl)phenyl)acrylonitrile) (m-BHCDCS). Reversible RGB luminescence switching with a high ratiometric color contrast (λ_em=594, 527, 458 nm for red, green, and blue, respectively) was realized by different external stimuli such as heat, solvent vapor exposure, and mechanical force. It was shown that Förster resonance energy transfer in the bicomponent mixture could be efficiently switched on and off through supramolecular control.

Reversible red–green–blue luminescence switching with a high ratiometric color contrast (λ_em=594, 527, 458 nm for red, green, and blue, respectively) was realized by different external stimuli such as heat, solvent-vapor exposure, and mechanical force. It was shown that Förster resonance energy transfer in a bicomponent mixture could be efficiently switched on and off through supramolecular control.

Multiple intramolecular motions consume the excited-state energy of luminogenic molecules upon photoexcitation and lower the emission efficiency. The low frequency rotational motion of aromatic rings can be facilely restricted by steric constraint in the condensed phase, but the high frequency bond stretching motion can hardly be suppressed by aggregation. In this work, three phosphorus-containing heterocycles, 1,2,3,4,5-pentaphenylphosphole-1-oxide (PPPO), 1,2,3-triphenylphosphindole-1-oxide (TPPIO), and 1,2,3-triphenylphosphindole (TPPI), were synthesized and characterized. Their optical properties, crystal-packing manners, electronic features, and fluorescence dynamics were systematically investigated, and theoretical calculations were performed to decipher structure–property relationships. The results reveal that these luminogens are weak emitters in solutions but show strong emission in aggregates, exhibiting obvious aggregation-induced emission (AIE) features. The aggregation-insensitive stretching motion, which is dominant in PPPO, is lowered in TPPIO, enabling TPPIO to fluoresce much more efficiently than PPPO in aggregates. The stretching motion is even more lowered in TPPI, but its relatively planar conformation suffers emission quenching due to strong π–π stacking interactions in aggregates. Therefore, a twisted molecular conformation consisting of a rigid stator and a rotatable periphery is demonstrated to be a rational design for more efficient AIE luminogens.

Keep the balance: 1,2,3-Triphenylphosphindole-1-oxide (see scheme, right) with a rigid central stator and flexible peripheral phenyl rotors fluoresces more efficiently than 1,2,3,4,5-pentaphenylphosphole-1-oxide (left) in the solid state, due to the greatly lowered stretching motion by a ring-fused strucutre.