Benzo[1,2‐b:4,5‐b′]dithiophene‐alt‐terthiophene Copolymers Containing Styryl‐Triphenylamine Side Chains: Synthesis and Photovoltaic Performance Optimization with Fullerene Acceptors

A new two‐dimensional‐conjugated polymer (PBDTT3‐TPA) containing benzodithiophene (BDT) and a side chain isolation comonomer is designed and synthesized. Interestingly, PBDTT3‐TPA is compatible with higher lowest unoccupied molecular level (LUMO) acceptors of indene‐C₆₀ bisadduct (ICBA), and polymer solar cells based on PBDTT3‐TPA/ ICBA show an open‐circuit voltage (V_OC) of ca. 0.80 V and a power conversion efficiency of 2.48% under AM1.5G illumination of at 100 mW cm^?2. Furthermore, the energy loss in the corresponding fullerene acceptor devices is discussed, and the increase in the observed V_OC is explained quantitatively by the up‐shifted LUMO energy of ICBA (0.17 eV) and the reduced saturation current (J_SO) in the blends.

     

A Self-Healing Hierarchical Fiber Hydrogel That Mimics ECM Structure

Although there have been many studies on using hydrogels as substitutes for natural extracellular matrices (ECMs), hydrogels that mimic the structure and properties of ECM remain a contentious topic in current research. Herein, a hierarchical biomimetic fiber hydrogel was prepared using a simple strategy, with a structure highly similar to that of the ECM. Cell viability experiments showed that the hydrogel not only has good biocompatibility but also promotes cell proliferation and growth. It was also observed that cells adhere to the fibers in the hydrogel, mimicking the state of cells in the ECM. Lastly, through a rat skin wound repair experiment, we demonstrated that this hydrogel has a good effect on promoting rat skin healing. Its high structural similarity to the ECM and good biocompatibility make this hydrogel a good candidate for prospective applications in the field of tissue engineering.     

Biomaterial-based microstructures fabricated by two-photon polymerization microfabrication technology

Two-photon polymerization (TPP) microfabrication technology can freely prepare micro/nano structures with different morphologies and high accuracy for micro/nanophotonics, micro-electromechanical systems, microfluidics, tissue engineering and drug delivery. With the broad application of 3D microstructures in the biomedical field, people have paid more attention to the physicochemical properties of the corresponding materials such as biocompatibility, biodegradability, stimuli responsiveness and immunogenicity. Therefore, microstructures composed of biocompatible synthetic polymers, polysaccharides, proteins and their complexes have been widely studied. In this review, we briefly summarize the TPP mechanism, the photoinitiators for TPP microfabrication, photoresist based on biomaterials, their corresponding microstructures and subsequently their biomedical applications. We will point out the issues in previous research and provide a useful perspective on the future development of TPP microfabrication technology.

Two-photon polymerization (TPP) microfabrication technology can freely prepare micro/nano structures with different morphologies and high accuracy for micro/nanophotonics, micro-electromechanical systems, microfluidics, tissue engineering and drug delivery.     

The role of spatiotemporal and spectral cues in segregating short sound events: evidence from auditory Ternus display

Previous studies using auditory sequences with rapid repetition of tones revealed that spatiotemporal cues and spectral cues are important cues used to fuse or segregate sound streams. However, the perceptual grouping was partially driven by the cognitive processing of the periodicity cues of the long sequence. Here, we investigate whether perceptual groupings (spatiotemporal grouping vs. frequency grouping) could also be applicable to short auditory sequences, where auditory perceptual organization is mainly subserved by lower levels of perceptual processing. To find the answer to that question, we conducted two experiments using an auditory Ternus display. The display was composed of three speakers (A, B and C), with each speaker consecutively emitting one sound consisting of two frames (AB and BC). Experiment 1 manipulated both spatial and temporal factors. We implemented three ‘within-frame intervals’ (WFIs, or intervals between A and B, and between B and C), seven ‘inter-frame intervals’ (IFIs, or intervals between AB and BC) and two different speaker layouts (inter-distance of speakers: near or far). Experiment 2 manipulated the differentiations of frequencies between two auditory frames, in addition to the spatiotemporal cues as in Experiment 1. Listeners were required to make two alternative forced choices (2AFC) to report the perception of a given Ternus display: element motion (auditory apparent motion from sound A to B to C) or group motion (auditory apparent motion from sound ‘AB’ to ‘BC’). The results indicate that the perceptual grouping of short auditory sequences (materialized by the perceptual decisions of the auditory Ternus display) was modulated by temporal and spectral cues, with the latter contributing more to segregating auditory events. Spatial layout plays a less role in perceptual organization. These results could be accounted for by the ‘peripheral channeling’ theory.     

Optimization of the thermoelectric performance of layer-by-layer structured copper-phthalocyanine (CuPc) thin films doped with hexacyano-trimethylene-cyclopropane (CN6-CP)

Copper-phthalocyanine (CuPc), as a classical small molecular organic semiconductor, has been applied in many fields. However, the low intrinsic conductivity limits its application in thermoelectricity. Here, hexacyano-trimethylene-cyclopropane (CN6-CP), a strong electron acceptor, is synthesized as dopant for CuPc thin films to improve their conductivities. Multilayer thin films constructed from alternate thermally evaporated CuPc and CN6-CP thin layers are investigated. Under the optimized condition, the doped CuPc film with a conductivity of 0.76 S cm⁻¹ and a Seebeck coefficient of 130 μV K⁻¹, shows a high power factor of 1.3 μW m⁻¹ K⁻² and the carrier concentration is estimated to be 2.8 × 10²⁰ cm⁻³. Considering the relatively superior performance, the CN6-CP doped CuPc film is a promising small molecular organic thermoelectric (OTE) material. In addition, for those highly crystalline materials with poor solubility, the layer-by-layer structure offers a general strategy for investigation and optimization of their TE performance.

The alternately deposited multilayer structure of a small molecular semiconductor and dopant molecules offers a general strategy for investigating their TE performance.     

One-step solution synthesis of a two-dimensional semiconducting covalent organometallic nanosheet via the condensation of boronic acid

In this work, a 2D covalent organometallic nanosheet (COMS) was designed and successfully synthesized through the one-step conjunction of a terpyridine–metal–terpyridine (TMT) sandwich coordinate motif with borate ester covalent heterocyclic (B₃O₃) linkage via the condensation of boronic acid. The obtained 2D COMS with a cobalt coordination center (2D COMS-Co) showed promising p-type semiconducting properties.

A 2D covalent organometallic nanosheet (COMS-Co) was synthesized through the one-step conjunction of sandwich coordinate motif with covalent heterocyclic linkage via the condensation of boronic acid, and it showed promising semiconducting properties.

Two-dimensional (2D) metal–organic frameworks (MOFs) obtained by using the strong bonding effects between metal ions and organic molecule species have aroused great interest;^1–8 these include coordination nanosheets, which are a class of 2D materials featuring metal complex motifs.^9–12 Covalent bonds are considered to be an effective way to fabricate covalent organic frameworks (COFs) such as COF-1, which is mainly constructed by a phenyl ring and borate ester to form covalent heterocyclic (B₃O₃) linkage.^13–15 Based on the synergistic effects of metal coordination and covalent interactions, Jiang and co-workers demonstrated the important role of central metals in controlling π-electronic functions through Mpc-COF.¹⁶ Nevertheless, the coordination bond forms spontaneously within the ring of the porphyrin heterocycle and not as the linkage between organic components. In addition, the connections made between metal ions and ligand motifs cannot facilitate the proper tuning of the physicochemical properties of the structure.¹⁷ Combining the covalent and coordinate features may provide an opportunity to design new 2D materials and thus, the synergetic enhancement in many unique properties can be achieved. As far as we know, the fabrication of 2D semiconducting materials that combine the covalent heterocyclic linkage and coordinate linkage has not been reported. Therefore, it is a pressing need to construct a functional 2D organic framework through a one-step synthesis strategy, which would be a more attractive approach.Herein, we first proposed an efficient one-step strategy to synthesize a 2D material by the synergy of covalent and organometallic interactions; it was named as a covalent organometallic nanosheet (2D COMS-M, M = cobalt (Co)). The as-proposed 2D COMS-Co was constructed by linking the borate ester covalent heterocyclic (B₃O₃) structure and terpyridine (tpy) metal coordination motif (TMT) (Fig. 1).Open in a separate windowFig. 1(a) Synthesis route of the semiconducting 2D COMS. (b) Design principles of COMS via the condensation of boronic acid. (c) The calculated cross-sectional view of 2D COMS-Co.In order to explore the extended construction of 2D COMS, the self-condensation ligand 4-phenylborate-2,2′; 6′,2′′-terpyridine (L¹) integrated with CoCl₂ as the Co²⁺ carrier was employed through an anhydrous and oxygen-free hydrothermal reaction at 120 °C under argon, and the typical synthesis process is depicted in Fig. 1a (for details, please see ESI^†). Starting from the condensation reaction of boronic acid (Fig. 1b), H₂O could be generated and played a key role in dissolving CoCl₂ and further releasing Co²⁺ ions (CoCl₂ is practically insoluble in mesitylene and dioxane). This was followed by coordination between the terpyridine motifs and the released Co²⁺ ions to obtain 2D COMS. This novel strategy could achieve crystalline 2D materials by self-controlling the release of Co²⁺ through the one-step condensation reaction of boronic acid. The calculated cross-sectional view of 2D COMS-Co indicates that the thickness of the monolayer 2D COMS-Co is 0.69 nm (Fig. 1c). The dual synergistic connecting effects of covalence and coordination in 2D COMS-Co provided nanosheet-like morphology with significant improvement in the semiconducting performance. The hole mobility of the 2D COMS-Co-based FET device could reach to 5.7 × 10⁻⁵ cm² V⁻¹ s⁻¹ with an ON–OFF ratio of 221, which demonstrated the potential application of 2D COMS-Co in semiconducting devices.The scanning electron microscopy (SEM) image (Fig. 2a) reveals that 2D COMS-Co has a uniform micron-sized nanosheet-like structure. Furthermore, the transmission electron microscopy (TEM) image indicates that the as-synthesized 2D COMS-Co consists of few multi-layers. Fig. 2c shows an enlarged image of the selected area visualized with high-resolution TEM (HRTEM), where the section enclosed by white lines depicts lattice fringes. Moreover, the selected area electron diffraction (SAED, inset of Fig. 2c) image shows a circular ring pattern that reveals the well-defined crystalline structure of 2D COMS-Co; these results are in agreement with our X-ray diffraction data (Fig. S2, ESI^†). Most interestingly, the atomic force microscopy (AFM) image (Fig. 2d) clearly demonstrates that the selected thickness of 2D COMS-Co is 2.07 nm, and the resultant thickness is three times less than the calculated value of 0.69 nm (Fig. 1c), indicating its uniform three-layer stacked structure. The above results collectively provide strong evidence of typical 2D features for COMS-Co. Moreover, the high-angle annular dark-field (HAADF) image and the scanning transmission electron microscopy (STEM) elemental mapping further revealed that the C (red), B (cyan), N (green), O (blue) and Co (pink) elements are homogeneously distributed on the selected area of 2D COMS-Co (Fig. 2e–k). The electronic structure of 2D COMS-Co was further characterized by four-probe scanning tunnel microscopy (STM) (please see details in ESI^†). The high-resolution STM imaging revealed that the 2D COMS-Co flake has a layered flake structure with uniformly distributed hexagonal hole morphology and the resultant holes have a diameter of 4.72 nm (Fig. 2l); this observation is in accordance with the proposed structure of 2D COMS-Co (Fig. S8, ESI^†).Open in a separate windowFig. 2Morphological characterization of 2D COMS-Co. (a) SEM image of powder. (b) TEM image. (c) HRTEM image (d) AFM image. (e) HAADF image. (f–k) STEM elemental mapping. (l) STM image.Due to the covalence and coordination effects, the absorption peak of 2D COMS-Co obtained from the UV-vis measurements (Fig. 3a) is red-shifted in comparison to that for ligand L¹, and the optical band gap is calculated to be 1.85 eV. Furthermore, density functional theory (DFT) calculations revealed that the highest occupied molecular orbital (HOMO) of 2D COMS-Co was dominated by the π orbital of the B₃O₃ linkage and the phenyl ring, whereas the π* orbital of TMT was responsible for the lowest unoccupied molecular orbital (LUMO). Additionally, the calculated electronic band gap of 2D COMS-Co was 1.89 eV, which was in accordance with the optical band gap, confirming the successful synthesis of 2D COMS-Co. Fig. 3b displays the X-ray photoelectron spectroscopy (XPS) spectrum of 2D COMS-Co, in which the appearance of the binding energy peak at 192.2 eV is significantly different from that for L¹ (191.1 eV), which can be ascribed to the –C–B–O₂ structure.^18,19 Compared to the observation for L¹, the N 1s peak for 2D COMS-Co shifted to a higher binding energy value (399.2 eV for COMS-Co and 397.5 eV for L¹). The skewing of the peaks toward higher binding energy was induced by the contribution of N atoms in terpyridine to the metal center. Therefore, the XPS results clearly provide evidence of the existence of both covalent and coordination effects in the as-prepared 2D COMS-Co. The Fourier transform infrared (FT-IR) spectra in Fig. 3c show that the –B(OH)₂ bands (around 3500 cm⁻¹) of the ligand L¹ are significantly attenuated compared to that observed for 2D COMS-Co. In addition, the B–O stretching vibrations at 1351 cm⁻¹ can be observed for 2D COMS-Co but not for L¹.^13,20–22 Thus, the above results show that the expected B₃O₃ rings for 2D COMS-Co have indeed been formed. Notably, the nitrogen adsorption–desorption isotherms reveal that the Brunauer–Emmett–Teller (BET) specific surface area of 2D COMS-Co is as high as 598 m² g⁻¹ (Fig. 3d); it surpasses those of other layered materials, including the reported graphene oxide paper (10 m² g⁻¹), clays (10 to 100 m² g⁻¹), and pillared clays (50 to 300 m² g⁻¹), and is in the range of the values of the most porous zeolites and many porous carbons.^13,23 This drastic enhancement can be assigned to the uniform porous structure of the as-synthesized 2D COMS-Co.Open in a separate windowFig. 3(a) UV-vis spectra of L¹ and 2D COMS-Co (left), electron distribution on HOMO/LUMO and the orbital energy gap (right). (b) XPS B 1s and N 1s spectra of L¹ and 2D COMS-Co. (c) FT-IR spectra of L¹ and 2D COMS-Co. (d) N₂ adsorption and desorption isotherms of 2D COMS-Co (inset: stacking porous view).The above results show that the 2D COMS-Co units arranged in periodic strips can provide conducting pathways for charge carrier transport through TMT and central cobalt ions. 2D COMS-Co was employed to function as the active semiconducting channel in a field-effect transistor (FET) device. As shown in Fig. 4a, the individual 2D COMS-Co sample is deposited on an Si wafer as the semiconducting layer to construct the FET device. The 2D COMS-Co-based FET device delivered hole mobility of 5.7 × 10⁻⁵ cm² V⁻¹ S⁻¹ and an ON/OFF ratio of 221, which were comparable with those of previously reported 2D MOFs (Table S1^†). Therefore, these results collectively demonstrate the potential application of 2D COMS-Co in semiconducting devices.Open in a separate windowFig. 4(a) Schematic of the FET device employing 2D COMS-Co as the semiconducting layer. (b) Transfer curve of the 2D COMS-Co-based FET device with the inset showing an optical and model diagram of the device; V_G is the gate-source voltage and I_D is the drain current.In summary, a novel two-dimensional covalent organometallic nanosheet (COMS) was designed and successfully synthesized through the strategy of one-step conjunction of a TMT sandwich coordinate motif with B₃O₃ linkage. This facile strategy could achieve crystalline 2D COMS-Co by self-controlling the release of Co²⁺ through a one-step condensation reaction of boronic acid. The systematic characterization demonstrated the successful preparation of porous and crystalline 2D COMS-Co. Particularly, the as-prepared 2D COMS-Co-based FET device presented hole mobility of 5.7 × 10⁻⁵ cm² V⁻¹ S⁻¹ and an ON/OFF ratio of 221, which demonstrated the potential application of 2D COMS-Co in semiconducting devices. Therefore, we envision that this work will open a new avenue for the synthesis of two-dimensional semiconducting covalent organometallic materials by using covalence and coordination dual synergistic connection effects via a one-step facile strategy.     

Synthesis and Photovoltaic Properties of a D–A Copolymer Based on the 2,3‐Di(5‐hexylthio­phen‐2‐yl)quinoxaline Acceptor Unit

A new D–A copolymer ( PBDT‐DTQx) based on the 2,3‐di(5‐hexylthiophen‐2‐yl)quinoxaline acceptor unit and a bithienyl‐substituted benzodithiophene (BDT) donor unit is designed and synthesized for application as the donor material in polymer solar cells (PSCs). The polymer film shows a broad absorption band covering the wavelength range from 300 to 720 nm and a low highest occupied molecular orbital (HOMO) energy level at ?5.35 eV. A device based on PBDT‐DTQx :PC₇₀BM ([6,6]‐phenyl‐C71‐butyric acid methyl ester) (1:2.5, w/w) with chloronaphthalene as a solvent additive displays a power conversion efficiency (PCE) of 3.15%. With methanol treatment, the PCE of the PSCs is further improved to 3.90% with a significant increase of the short‐circuit current density, J_sc, from 10.10 mA cm^?2 for the device without the methanol treatment to 11.71 mA cm^?2 for the device with the methanol treatment.

     

Photoinduced transformations of stiff-stilbene-based discrete metallacycles to metallosupramolecular polymers

Control over structural transformations in supramolecular entities by external stimuli is critical for the development of adaptable and functional soft materials. Herein, we have designed and synthesized a dipyridyl donor containing a central Z-configured stiff-stilbene unit that self-assembles in the presence of two 180° di-Pt(II) acceptors to produce size-controllable discrete organoplatinum(II) metallacycles with high efficiency by means of the directional-bonding approach. These discrete metallacycles undergo transformation into extended metallosupramolecular polymers upon the conformational switching of the dipyridyl ligand from Z-configured (0°) to E-configured (180°) when photoirradiated. This transformation is accompanied by interesting morphological changes at nanoscopic length scales. The discrete metallacycles aggregate to spherical nanoparticles that evolve into long nanofibers upon polymer formation. These fibers can be reversibly converted to cyclic oligomers by changing the wavelength of irradiation, which reintroduces Z-configured building blocks owing to the reversible nature of stiff-stilbene photoisomerization. The design strategy defined here represents a novel self-assembly pathway to deliver advanced supramolecular assemblies by means of photocontrol.Natural systems provide many examples of self-assembled biosupramolecules that respond to external stimuli through conformational changes that ultimately play a role in carrying out their various biological functions. Mimicking this stimuli-responsive behavior in artificial systems is a promising route toward obtaining sophisticated molecular-based architectures with functional and structural tunability (1–3). Using the absorption of photons as a trigger is particularly attractive in that light-induced transformations maintain high spatial and temporal resolution without producing waste products even during multiple reversible switching sequences (4). In materials science, one of the most appealing characteristics of photochromic molecules is the direct conversion of light into mechanical energy based on their photo-reversible structural transformations (5). Among such chromophores, a stiff-stilbene moiety (1,1′-biindane) is useful owing to its unique characteristics (6). First, stiff stilbene can adopt either a cis or trans configuration with respect to its central double bond. Second, the high activation barrier between the two isomers (∼43 kcal⋅mol⁻¹, corresponding to a half-life of ∼10⁹ y at 300 K) makes thermal E/Z isomerization negligible at temperatures of 420 K and lower. Third, the quantum yield for the photoisomerization of either isomer is high (50%). Fourth, the stiff-stilbene core is readily substituted using well-established synthetic methods. Owing to these promising characteristics, Boulatov and coworkers (7) constructed a molecular force probe by integrating the moiety into a stretched polymer to mimic the strain generated in diverse functional groups. Yang and coworkers (8) reported hydrogen-bonded supramolecular polymers and studied their polymerization mechanisms and physical properties based on the photoisomerization of the stiff-stilbene units. Nevertheless, stiff-stilbene-based supramolecular entities are underexplored despite exhibiting properties that make the functionality potentially useful in the construction of photoresponsive supramolecular materials.Coordination-driven self-assembly is a powerful method of constructing supramolecular coordination complexes (SCCs) by the spontaneous formation of metal–ligand bonds that draws inspiration from the design principles of natural systems (9–20). This approach organizes metal acceptors and organic donors to prepare well-defined cavity-cored 2D metallacycles and 3D metallacages, which can be functionalized on their interior or exterior vertices for applications in host–guest chemistry (21, 22), catalysis (23), molecular flasks (24), bioengineering (25), amphiphilic self-assembly (26), and so on. The versatility of coordination-driven self-assembly can be enhanced by designs that allow for post-self-assembly modifications that in some cases result in complete structural transformations. For example, the Stang group previously demonstrated the transformation of self-assembled polygons by changing the angle between the bonding sites of a ligand from 180° to 120° upon treatment of Co₂(CO)₆ with an acetylene moiety (27). Yang and coworkers (28) reported the construction of multibisthienylethene hexagons capable of reversible supramolecule-to-supramolecule conversions induced by ring-open and ring-closed conformational changes of the bisthienylethene units. Herein we expand upon the transformations established by the systems described above designing SCCs capable of evolving from discrete metallacycles into infinite constructs using external stimuli.Supramolecular polymers can be defined as dynamically reversible polymeric arrays (29–37) that form from the explicit manipulation of noncovalent forces between monomeric units (38–43). Supramolecular polymer chemistry can readily complement coordination-driven self-assembly, as exemplified by our efforts to design hierarchical supramolecular polymerizations of discrete organoplatinum(II) metallacycles, thus accessing novel supramolecular polymeric materials, such as macroscopic hexagonal supramolecular polymer fibers (44), dendronized organoplatinum(II) metallacyclic polymers (45), and a responsive, cavity-cored supramolecular polymer network metallogel (46). Herein, we report photoresponsive supramolecular transformations between discrete organoplatinum(II) metallacycles and infinite metallosupramolecular polymers induced by a cis/trans conformational transition of a stiff-stilbene-based dipyridyl ligand. The self-assembly behaviors, physical properties, topologies, and morphologies of these SCCs can be regulated by photoisomerization, demonstrating this powerful approach to prepare advanced supramolecular coordination complexes.