Dual electronic effects achieving a high-performance Ni(II) pincer catalyst for CO2 photoreduction in a noble-metal-free system.

A carbazolide-bis(NHC) NiII catalyst (1; NHC, N-heterocyclic carbene) for selective CO2 photoreduction was designed herein by a one-stone-two-birds strategy. The extended π-conjugation and the strong σ/π electron-donation characteristics (two birds) of the carbazolide fragment (one stone) lead to significantly enhanced activity for photoreduction of CO2 to CO. The turnover number (TON) and turnover frequency (TOF) of 1 were ninefold and eightfold higher than those of the reported pyridinol-bis(NHC) NiII complex at the same catalyst concentration using an identical Ir photosensitizer, respectively, with a selectivity of ∼100%. More importantly, an organic dye was applied to displace the Ir photosensitizer to develop a noble-metal-free photocatalytic system, which maintained excellent performance and obtained an outstanding quantum yield of 11.2%. Detailed investigations combining experimental and computational studies revealed the catalytic mechanism, which highlights the potential of the one-stone-two-birds effect.

A Small-Molecule Organic Cathode with Extended Conjugation toward Enhancing Na+ Migration Kinetics for Advanced Sodium-Ion Batteries.

Organic cathode materials show excellent prospects for sodium-ion batteries (SIBs) owing to their high theoretical capacity. However, the high solubility and low electrical conductivity of organic compounds result in inferior cycle stability and rate performance. Herein, an extended conjugated organic small molecule is reported that combines electroactive quinone with piperazine by the structural designability of organic materials, 2,3,7,8-tetraamino-5,10-dihydrophenazine-1,4,6,9-tetraone (TDT). Through intermolecular condensation reaction, many redox-active groups C═O and extended conjugated structures are introduced without sacrificing the specific capacity, which ensures the high capacity of the electrode and enhances rate performance. The abundant NH2 groups can form intermolecular hydrogen bonds with the C═O groups to enhance the intermolecular interactions, resulting in lower solubility and higher stability. The TDT cathode delivers a high initial capacity of 293 mAh g-1 at 500 mA g-1 and maintains 90 mAh g-1 at an extremely high current density of 70 A g-1. The TDT || Na-intercalated hard carbon (Na-HC) full cells provide an average capacity of 210 mAh g-1 during 100 cycles at 500 mA g-1 and deliver a capacity of 120 mAh g-1 at 8 A g-1.

The Development of Visible-Light Organic Photocatalysts for Atom Transfer Radical Polymerization via Conjugation Extension.

This work aimed to develop organic photocatalysts (PCs) that could mediate organocatalytic atom transfer radical polymerization (O-ATRP) under visible light. Through the core-modification of known chromophoric structures and ring-locking to reach a conjugation extension, annulated N-aryl benzo[kl]acridines were identified as effective visible light-responsive photocatalysts. The corresponding selenium-doped structure showed excellent performance in the O-ATRP of methacrylates, which could afford polymer products with controlled molecular weights and low dispersities under the irradiation of visible light at a 100 ppm catalyst loading.

Near-IR Emissive B-N Lewis Pair-Functionalized Anthracenes via Selective LUMO Extension in Conjugated Dimer and Polymer.

Acenes are attractive as building blocks for low gap organic materials with applications, for example, in organic light emitting diodes, solar cells, bioimaging and diagnostics. Previously, we have shown that modification of dipyridylanthracene via B-N Lewis pair fusion (BDPA) strongly redshifts the emission, while facilitating self-sensitized reactivity toward O2 to reversibly generate the corresponding endoperoxides. Herein, we report on the further expansion of the π-system of BDPA to a vinyl-substituted monomer, vinylene-bridged dimer, and a polymer with an average of 20 chromophores. The extension of π-conjugation results in largely reduced band gaps of 1.8 eV for the dimer and 1.7 eV for the polymer, the latter giving rise to NIR emission with a maximum at 731 nm and an appreciable quantum yield of 7 %. Electrochemical and computational studies reveal efficient delocalization of the lowest unoccupied molecular orbital (LUMO) along the pyridyl-anthracene-pyridyl axis, which results in effective electronic communication between BDPA units, selectively lowers the LUMO, and ultimately narrows the band gap. Time-resolved emission and transient absorption (TA) measurements offer insights into the pertinent photophysical processes. Extension of π-conjugation also slows down the self-sensitized formation of endoperoxides, while significantly accelerating the thermal release of singlet oxygen to regenerate the parent acenes.

End-extended Conjugation Strategy to Reduce the Efficiency-Stability-Mechanical Robustness Gap in Binary All-Polymer Solar Cells.

Concurrently achieving high efficiency, mechanical robustness and thermal stability is critical for the commercialization of all-polymer solar cells (APSCs). However, APSCs usually demonstrate complicated morphology, primarily attributed to the polymer chain entanglement which has a detrimental effect on their fill factors (FF) and morphology stability. To address these concerns, an end-group extended polymer acceptor, PY-NFT, was synthesized and studied. The morphology analysis showed a tightly ordered molecular packing mode and a favorable phase separation was formed. The PM6:PY-NFT-based device achieved an exceptional PCE of 19.12% (certified as 18.45%), outperforming the control PM6:PY-FT devices (17.14%). This significant improvement highlights the record-high PCE for binary APSCs. The thermal aging study revealed that the PM6:PY-NFT blend exhibited excellent morphological stability, thereby achieving superior device stability, retaining 90% of initial efficiency after enduring thermal stress (65 °C) for 1500 hours. More importantly, the PM6:PY-NFT blend film exhibited outstanding mechanical ductility with a crack onset strain of 24.1%. Overall, rational chemical structure innovation, especially the conjugation extension strategy to trigger appropriate phase separation and stable morphology, is the key to achieving high efficiency, improved thermal stability and robust mechanical stability of APSCs.

Double π-Extended Undecabenzo[7]helicene.

This work reports the first double π-extended undecabenzo[7]helicene 1, which is a large chiral nanographene, composed of 65 fused rings and 186 conjugated carbon atoms. The molecular identity of 1 has been confirmed by single crystal X-ray diffraction. A wine coloured solution of 1 in dichloromethane absorbs light from ultraviolet to the near infrared, featuring an extremely large molar absorption coefficient of 844 000 M-1 cm-1 at 573 nm. Optically pure 1 shows a record high electronic circular dichroism intensity in the visible spectral range (|Δϵ|=1375 M-1 cm-1 at 430 nm) known for any discrete polycyclic aromatic hydrocarbon. These unusual photophysical properties of 1 contrast sharply with those of a mono-undecabenzo[7]helicene derivative 2.

Synthesis of extended conjugated indolyl chalcones as potent anti-breast cancer, anti-inflammatory and antioxidant agents.

In the present investigation, synthesis of a series of extended conjugated δ-chloro-α-cyano substituted indolyl chalcones (5a-p) was accomplished by reacting 3-cyanoacetylindole 2 with 3-chloro-3-phenyl-propenal 4 in the presence of piperidine. The structural interpretations of newly synthesized compounds were based on chemical and spectroscopic evidences. Anti-tumor evaluation of the synthesized compounds in vitro against MCF-7 (breast carcinoma) cell line revealed that they possess high anti-tumor activities. Among them, compound 5e and 5a demonstrated excellent activity against breast carcinoma (GI50 <0.1 and 4µM respectively) as good as adriamycin (GI50 <0.1µM). The compounds were also screened against the normal Vero monkey cell line, which showed moderate selectivity against inhibition of cancer cells. The effect of extended conjugation on activity authenticated by comparing activity profile of compound 5a, 5i and 5m with their simple analogues. Among the synthesized compounds, 5i and 5l were found to be active anti-inflammatory agents in addition to having noteworthy antioxidant potential. These results suggest the possible use of these compounds for the design and development of novel anti-breast cancer agents.

The synthesis of lactone-bridged 1,3,5-triphenylbenzene derivatives as pi-expanded coumarin triskelions.

Two triply lactone-bridged 1,3,5-triphenylbenzene derivatives with solubilizing moieties have been synthesized in five and six steps from commercially available starting materials. Compounds containing the 1,3,5-triphenylbenzene core with two atom bridges are relatively unknown. This new class of pi-expanded coumarins contain triskelion architectures and X-ray crystallographic studies of one of the triskelions indicates that the 1,3,5-triphenylbenzene core adopts a near-planar geometry. This is the only known example of a two atom-bridged 1,3,5-triphenylbenzene derivative to adopt a planar structure.

Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts.

Linear poly(p-phenylene)s are modestly active UV photocatalysts for hydrogen production in the presence of a sacrificial electron donor. Introduction of planarized fluorene, carbazole, dibenzo[b,d]thiophene or dibenzo[b,d]thiophene sulfone units greatly enhances the H2 evolution rate. The most active dibenzo[b,d]thiophene sulfone co-polymer has a UV photocatalytic activity that rivals TiO2, but is much more active under visible light. The dibenzo[b,d]thiophene sulfone co-polymer has an apparent quantum yield of 2.3 % at 420 nm, as compared to 0.1 % for platinized commercial pristine carbon nitride.

Biaxially Conjugated Materials for Organic Solar Cells.

Organic solar cells (OSCs) represent one of the most important emerging photovoltaic technologies that can implement solar energy conversion efficiently. The chemical structure of organic semiconductors deployed in the active layer of OSCs plays a critical role in the photovoltaic performance and chemical/physical stability of relevant devices. With the structure innovation of organic semiconductors, especially nonfullerene acceptors (NFAs), the performance of OSCs have been promoted rapidly in recent years, with state-of-the-art power conversion efficiencies (PCEs) exceeding 19.5%. Compared with other photovoltaics like perovskite, the shortcoming of OSCs mainly lies in the high nonradiative recombination loss. However, the photocurrent density is superior in OSCs owing to the easy modulation of the NFA band gap toward the near-infrared region. In these regards, the effort to further boost the PCE of OSCs to achieve a milestone >21% should be devoted to reducing the nonradiative loss while further broadening the absorption band. Developing organic semiconductors with biaxially extended conjugated structures has provided a potential solution to achieve these goals. Herein, we summarize the design rules and performance progress of biaxially extended conjugated materials for OSCs. The descriptions are divided into two major categories, i.e., polymers and NFAs. For p-type polymers, we focus on the biaxial conjugation on some representative building blocks, e.g., polythiophene, triphenylamine, and quinoxaline. Whereas for n-type polymers, some structures with large conjugated planes in the normal direction are presented. We also elaborate on the biaxial conjugation strategies in NFAs with modification site at either the π-core or side-group. The general structure-property relationships are further retrieved within these materials, with focus on the short-wavelength absorption and nonradiative energy loss. Finally, we provide an outlook for the further structure modification strategies of biaxially conjugated materials toward highly efficient, stable, and industry-compatible OSCs.

Synthesis of ITIC Derivatives with Extended π-Conjugation as Non-Fullerene Acceptors for Organic Solar Cells.

The realization of printed organic solar cells (OSCs) as a commercial technology is dependent on the development of high-performance photovoltaic materials suitable for large-scale device manufacture. In this study, the design, synthesis, and characterization of a series of A-D-A'-D-A-type molecular acceptors based on indacenodithienothiophene (IDTT) and thiophene-flanked 2,1,3-benzothiadiazole (DTBT) are reported. The synthesized molecular acceptors showed broader absorption ranges and narrower band gap energies than those of well-known 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno [2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']dithiophene (ITIC)-based molecular acceptors. Furthermore, the synthesized acceptors could tune the frontier molecular orbital energy levels, dipole moments, and their crystallinities by introducing fluorine (F) atoms and cyano (CN) groups on DTBT as a core A' unit. The cyano-substituted DTBT-based molecular acceptor (CNDTBT-IDTT-FINCN) showed a strong molar absorptivity and dipole moment, high hole/electron charge mobilities, and a favorable face-on orientation using films blended with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)] (PBDB-T). An inverted organic photovoltaic (OPV) device using CNDTBT-IDTT-FINCN exhibits a power conversion efficiency (PCE) of 9.13% when using PBDB-T as a donor material in small cells (0.12 cm2). Sub-module devices with an active area of 55.45 cm2 are fabricated using bar-coating and exhibit PCEs of up to 7.50%. This demonstration of a high-efficiency large-area device makes CNDTBT-IDTT-FINCN a suitable and promising candidate for printed OPV devices.

Single-junction polymer solar cells with over 10% efficiency by a novel two-dimensional donor-acceptor conjugated copolymer.

Recent advances in bulk heterojunction (BHJ) polymer solar cell (PSC) performance have resulted from compressing the band gap to enhance the short-circuit current density (JSC) while lowering the highest occupied molecular orbital to increase the open-circuit voltage (VOC) and consequently enhance the power conversion efficiencies (PCEs). However, PCEs of PSCs are still constrained by a low JSC, small VOC, and low fill factor (FF). In this study, we report 10.12% PCE from single-junction PSCs based on a novel two-dimensional (2D) conjugated copolymer. By introduction of conjugated 5-alkylthiophene-2-yl side chains to substitute nonconjugated alkoxy side chains in one-dimensional (1D) poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), a novel 2D donor-acceptor low-band-gap conjugated copolymer, poly[[4,8-bis[(5-ethylhexyl)thienyl]benzo[1,2-b;3,3-b]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7-DT), is developed. 2D PTB7-DT is further systematically investigated by absorption spectroscopy, cyclic voltammetry, charge carrier mobility measurement, thin film morphology, and wide-angle X-ray diffraction and compared with 1D PTB7. In comparison with 1D PTB7, 2D PTB7-DT possesses a narrower band gap, tighter π-π stacking, and higher charge carrier mobility. These results are consistent with the observation from first-principle calculations. Consequently, the single-junction PSCs based on 2D PTB7-DT exhibit a PCE of 10.12% with a high JSC, larger VOC, and high FF in comparison with the PSCs based on 1D PTB7.