Electron-withdrawing Units Research Articles

Azo-linked covalent triazine-based framework as organic cathodes for ultrastable capacitor-type lithium-ion batteries

A highly stable 3D π-conjugation covalent triazine-cored framework (Azo-CTF) with triazine as the electron-rich center bridged by azo redox-active linkers is proposed and prepared as cathode materials for improving both rate performance and cycle stability of LIBs, two critical issues addressed in organic electrodes. The synchronous and orderly introduction of electron-withdrawing units and redox active units in conjugated polymer framework can effectively tune electronic structure and redox potential of materials, contributing to the improvement of electrical and electrochemical behavior. Attributed to abundant redox azo sites, resilient and accessible pores network, and good intramolecular and interfacial electron transfer ability, Azo-CTF cathode shows a large and reversible capacity output of 205.6 mAh g−1, a ultralong cycle life (89.1% capacity retention after 5000 cycles), and a high power density of 4253 W kg−1 even at an energy density of 258 Wh kg−1, much better than many reported organic cathodes and even better than some commercial inorganic cathodes. In addition, in-situ Raman spectroscopy and theoretical calculations were done and further verified the redox process of azo active sites and the optimization of molecular orbital in covalent framework for better battery cathode, indicating the availability of molecular-level electronic structure tuning.

Comparative study of the optoelectronic properties of diketopyrrolopyrrole based polymers obtained by direct C-H arylation

Donor–acceptor (D-A) type of new constituted diketopyrrolopyrrole-based polymers, PDPP-EDOT (poly A), PDPP-TP (poly B), and PDPP-EBTE (poly C), were synthesized by one-pot direct C–H heteroarylation polymerization methodology supported by Pd(OAc)2/Bu4NBr catalytic system. C–H heteroarylation coupling of highly selective sites avoided the difficulty of the synthesis using organometallic reagent and gave highly pure polymers in good yields. The targeted polymers were evaluated using NMR, TGA, DSC, UV–VIS absorption, GPC, and electrochemical cyclic voltammetry. The polymers are soluble in most of organic solvents, and TGA studies show their high thermally stability. Polymers bandgaps are tuned by alternative copolymerizing with different electron-donating and/or electron-withdrawing units. UV–VIS absorption spectra demonstrated that the polymers show excellent light absorption, with narrow optical bandgaps, around 1.20 eV, confirmed by cyclic voltammetry parameters. All of these promising properties indicated that they might be excellent polymeric materials for polymer solar cells, organic field effect transistors, and so on.

Utilization of conformation change and charge trapping to achieve binary/ternary rewritable memory performance of carbazole-based organic molecules

To better understand the relationship between the molecular structure and memory characteristics, two carbazole-based organic compounds (Cz-2Ph3F 6FDA and Cz-2TPA 6FDA) with different ratios of electron-donating and electron-withdrawing units were designed and synthesized.

Constructing Donor-Resonance-Donor Molecules for Acceptor-Free Bipolar Organic Semiconductors.

Organic semiconductors with bipolar transporting character are highly attractive as they offer the possibility to achieve high optoelectronic performance in simple device structures. However, the continual efforts in preparing bipolar materials are focusing on donor-acceptor (D-A) architectures by introducing both electron-donating and electron-withdrawing units into one molecule in static molecular design principles. Here, we report a dynamic approach to construct bipolar materials using only electron-donating carbazoles connected by N-P=X resonance linkages in a donor-resonance-donor (D-r-D) structure. By facilitating the stimuli-responsive resonance variation, these D-r-D molecules exhibit extraordinary bipolar properties by positively charging one donor of carbazole in enantiotropic N+=P-X- canonical forms for electron transport without the involvement of any acceptors. With thus realized efficient and balanced charge transport, blue and deep-blue phosphorescent organic light emitting diodes hosted by these D-r-D molecules show high external quantum efficiencies up to 16.2% and 18.3% in vacuum-deposited and spin-coated devices, respectively. These results via the D-r-D molecular design strategy represent an important concept advance in constructing bipolar organic optoelectronic semiconductors dynamically for high-performance device applications.

Wide bandgap donor polymers containing carbonyl groups for efficient non-fullerene polymer solar cells

Two wide bandgap conjugated polymers (PTCO8 and PTCO1) with different side chains, composed of alkylcarbonyl-substituted thiophene (TOC) as the electron-withdrawing unit and two-dimension benzodithiophene (BDT) as the electron-donating unit, are developed for non-fullerene polymer solar cells (PSCs). Relative to non-carbonyl-substituted control polymer PTC8, PTCO8 and PTCO1 exhibit a red-shifted absorption and deeper-lying highest occupied molecular orbital (HOMO) level. When blended with ITIC, the as-cast PTCO1-based devices yield a power conversion efficiency of up to 9.33% with an open-circuit voltage of 0.95 V, a short-circuit current density of 16.91 mA cm−2, and a fill factor of 57.9%, outperforming those of PTC8-based devices (4.04%) and PTCO8-based devices (5.91%). This work demonstrates that the carbonyl group is a promising electron-deficient moiety to construct wide-bandgap polymers for non-fullerene PSCs. Adjusting the length of side chains is a feasible strategy to improve PSC performance further.

Asymmetric Host Molecule Bearing Pyridine Core for Highly Efficient Blue Thermally Activated Delayed Fluorescence OLEDs.

In this study, two host materials, pCzBzbCz and pCzPybCz, are synthesized to achieve a high efficiency and long lifetime of blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs). The molecular design strategy involves the introduction of a pyridine group into the core structure of pCzPybCz as an electron-withdrawing unit, and an electron-donating phenyl group into the structure of pCzBzbCz. These host materials demonstrate good thermal stability and high triplet energy (T1 =3.07 eV for pCzBzbCz and 3.06 eV for pCzPybCz) for the fabrication of blue TADF-OLEDs. In particular, pCzPybCz-based OLED devices demonstrate an external quantum efficiency (EQE) of 22.7 % and an operational lifetime of 24 h (LT90 , time to attain 90 % of initial luminance) at an initial luminance of 1000 cd m-2 . This superior lifetime could be explained by the C-N bond dissociation energy (BDE) in the host molecular structure. Furthermore, a mixed-host system using the electron-deficient 2,4-bis(dibenzo[b,d]furan-2-yl)-6-phenyl-1,3,5-triazine (DDBFT) is proposed to inhibit the formation of the anion state of our host materials. In short, the device operational lifetime is further improved by applying DDBFT. The carbazole-based asymmetric host molecule containing a pyridine core realizes a high-efficiency blue TADF-OLED showing a positive effect on the operating lifetime, and can provide useful strategies for designing new host materials.

Miscibility‐Controlled Phase Separation in Double‐Cable Conjugated Polymers for Single‐Component Organic Solar Cells with Efficiencies over 8 %

A record power conversion efficiency of 8.40 % was obtained in single-component organic solar cells (SCOSCs) based on double-cable conjugated polymers. This is realized based on exciton separation playing the same role as charge transport in SCOSCs. Two double-cable conjugated polymers were designed with almost identical conjugated backbones and electron-withdrawing side units, but extra Cl atoms had different positions on the conjugated backbones. When Cl atoms were positioned at the main chains, the polymer formed the twist backbones, enabling better miscibility with the naphthalene diimide side units. This improves the interface contact between conjugated backbones and side units, resulting in efficient conversion of excitons into free charges. These findings reveal the importance of charge generation process in SCOSCs and suggest a strategy to improve this process: controlling miscibility between conjugated backbones and aromatic side units in double-cable conjugated polymers.

An A-π-D-π-A-Type Organic Semiconductor Based Optoelectrical Device With Photo Response and Optical Memory Behaviors

A novel photoactive semiconductor (named as IDTOT-4F) with an A-π-D-π-A-type configuration is synthesized. It contains an electron-donating fused ring (D) as the core flanked with two π-spacers and is end-capped with two electron-withdrawing units (A). The intramolecular charge transfer effect endows IDTOT-4F with strong and broad light absorption and a relatively narrow band gap (1.46 eV). Thin-film optoelectrical devices based on IDTOT-4F exhibit both n-type and p-type switching behaviors. Besides, the p-channel device shows significantly photoresponsive performance with the maximum P (photo/dark current ratio), R (photoresponsivity), and D* (detectivity) values of around 60, 0.07 A W−1, and 2.5 × 1010 Jones, respectively. Further, IDTOT-4F based optoelectrical devices exhibit good optical memory characteristics with a time constant τ1 of 4.6 h, indicating its applicability to nonvolatile optical memory devices. The results provide new insights into the photoresponsive behavior of fused-ring semiconductors and pave the way for the design of nonvolatile optical memory devices.

Panchromatic Organoboron Molecules with Tunable Absorption Spectra.

Panchromatic molecules, e. g. organic small molecules with wide absorption spectra, are very desirable for solar energy-related applications. Here, we report the development of a series of organoboron compounds composed of an organoboron core unit, two π-bridging units and two electron-withdrawing end-capping units. All seven molecules have the HOMO localized on the core unit and the LUMO delocalized on the whole conjugated backbone. They exhibit wide absorption spectra consisting of two strong absorption bands with the full width at half maximum of ca. 280 nm. These panchromatic compounds can be used as electron acceptors in organic solar cells. We elucidate the relationship between the chemical structures and opto-electronic properties of these organoboron panchromatic compounds. Increasing the electron-withdrawing capability of the core units results in a downshifted HOMO level as well as blueshifted long-wavelength absorption band with increased extinction coefficient. Extending the π-bridging units causes an increased HOMO level and blueshifted long-wavelength absorption band with increased extinction coefficients. Weakening the electron-withdrawing capability of the end-capping units leads to an upshifted LUMO level and blueshifted long-wavelength absorption peak with decreased extinction coefficient. This work provides insight into the absorption spectrum manipulation of panchromatic molecules and would pave the way for the development of solar energy-related applications.

Effective structural modifications enabled by end-capped effects based on fluorene-core donor, with high open-circuit voltage in organic photovoltaic devices

End-capped effects are important in modifying the structures of donor materials, with the purpose of regulating material properties and achieving high open-circuit voltage (VOC). A series of acceptor-π-donor-π-acceptor (A-π-D-π-A) type small-molecule donor (SMD) materials for bulk-heterojunction (BHJ) organic solar cells with PC71BM as electron acceptor have been designed and synthesized. Three novel small molecules are denoted as flu(3TRD)2, flu(3TCN)2 and flu(3TIN)2, with dioctyl-substituted fluorene as core, 3,3″-dioctyl-2,2':5′,2″-terthiophene as π-bridge, terminated by rhodanine, malononitrile and 1,3-indanedione, respectively. Improvements of photovoltaic performance have been successfully realized via tuning terminal electron-withdrawing units. The power conversion efficiency (PCE) of the devices as-cast prepared from flu(3TRD)2, flu(3TCN)2 and flu(3TIN)2 increases in turn, which is ascribed to incremental optical absorption thereby promoting short-circuit current density (JSC). Particularly, flu(3TCN)2 shows deeper LUMO and HOMO energy levels than flu(3TRD)2 and flu(3TIN)2 due to the strong electron-withdrawing character of malononitrile. As expected, the device as-cast based on flu(3TCN)2 exhibits a reasonable high VOC of 1.07 V. In addition, a satisfactory PCE of 7.06% has been obtained for flu(3TCN)2 based device with dichloromethane vapor treatment.

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.

ThDione: A Powerful Electron-Withdrawing Moiety for Push-Pull Molecules.

A series of new push-pull chromophores based on a combined cyclopenta[c]thiophene-4,6-dione (ThDione) acceptor, N,N-dimethylaniline, N-piperidinylthiophene or ferrocene donors, and ethylene or buta-1,3-dienylene π-linkers has been designed and synthesized. Utilizing one or two ThDione acceptors afforded linear or branched push-pull molecules. Experimental and theoretical study of their fundamental properties revealed thermal robustness up to 260 °C, a electrochemical/optical HOMO-LUMO gap that is tunable within the range of 1.47-2.19/1.99-2.39 eV, and thorough elucidation of structure-property relationships. Compared to currently available portfolio of heterocyclic electron-withdrawing units, ThDione proved to be a powerful and versatile acceptor unit. It imparts significant intramolecular charge transfer and polarizes the π-system, which results in enhanced (non)linear optical response.

Investigation on Y-shaped tri-fluoromethyl substituted quinoxalines: synthesis, optical and morphological studies

Y-shaped tri-fluoromethyl substituted quinoxaline derivatives 2,3-bis(4-methoxyphenyl)-6-(trifluoromethyl)quinoxaline (1) and 2,3-bis((E)-4-methoxystryryl)-6-(trifluoromethyl)quinoxaline (2) have been synthesized and characterized using various analytical and spectroscopic techniques (1H,13C, 19F NMR, FT-IR and HR-Mass). Optical properties such as absorption, emission, quantum yield, effect of solvent polarities and Aggregation Induced Emission (AIE) have been illustrated. The compound 1 exhibited negative solvatochromism and compound 2 positive solvatochromism which are in correlation with π* values by Kamlet and Taft. Compounds 1 and 2 showed large Stoke’s shifts attributed to more polar excited state of the compounds as compared to that of polar ground state, due to the presence of electron withdrawing trifluoromethyl group. Both the compounds displayed fluorescence in solid state and AIE state due to the restricted intramolecular rotation. The compounds 1 and 2 exhibited low quantum yields because of ICT (Intramolecular charge transfer) attributed to the presence of electron-donating methoxy phenyl unit and an electron-withdrawing quinoxaline unit. AIE exhibited by both the compounds in THF/water mixture were due to the formation of nano-aggregates, it is further characterised by DLS and determined the particle size to be 196.8 nm for compound 1 and 197.3 nm for compound 2. Compound 1 appeared as ice flakes and compound 2 as a cluster consisting of various micro crystals like projections in the images determined by scanning electron microscope (SEM).

Investigation of Ferrocene Linkers in β-Substituted Porphyrins.

A series of β-ferrocene-modified zinc porphyrins, with various electron-withdrawing units appended to the ferrocene, were synthesized, and their electronic properties were investigated. The ferrocene was able to be modified with the substituents, with its oxidation potential increased by up to 0.3 V, without significantly perturbing the porphyrin core. A small red-shift of the strongest absorption band (B band) occurred upon the addition of the electron-withdrawing substituents (270 cm-1), occurring alongside a broadening of the band. The singlet state is unaffected by the ferrocene substitution; however, the triplet state lifetimes are decreased by 10.4-10.6 μs from that of the unsubstituted ferrocene porphyrin (18.1 μs). Computational studies showed that the changes in the optical properties are due to a loss of degeneracy of the porphyrin lowest unoccupied molecular orbitals; this is supported by resonance Raman spectroscopy studies, which show different enhancement patterns when probing the high- and low-energy edges of the B band.

Structural isomers of 9-(pyridin-2-yl)-9H-carbazole in combination with 9′H-9,3':6′,9″-tercarbazole and their application to high efficiency solution processed green TADF OLEDs

Two host materials, CzPy2TCz and CzPy3TCz, were designed as structural isomers and synthesized to achieve high efficiency thermally activated delayed fluorescence-organic light emitting diodes (TADF-OLEDs). The design strategy involved introducing a pyridine group into the core structure as an electron-withdrawing unit and varying the substitution position of tercarbazole (TCz). To realize green TADF-OLED, the two host materials synthesized in this study have excellent thermal stability and high excited triplet energy (T1 = 2.95–2.98 eV). The maximum external quantum efficiency and current efficiency values for CzPy2TCz were 23.81% and 80.2 cd/A, respectively and the respective values for CzPy3TCz were 20.27% and 70.1 cd/A, respectively. Structural isomers with carbazole (Cz) and TCz units at the 2,6-position of the pyridine core effectuate better device performance. Consequently, we found that the host materials introduced in this study play an important role in implementing high performing solution-processed green TADF-OLED.

Exploring thieno[3,4-c]pyrrole-4,6-dione combined thiophene as π-bridge to construct non-fullerene acceptors with high VOC beyond 1.0 V

Thieno[3,4-c]pyrrole-4,6-dione (TPD) unit has been widely used to construct a great deal of D-π-A p-type copolymers due to its strong electron-deficient property. However, there are limited reports on the TPD-based n-type materials, particularly small molecule non-fullerene acceptors (NFAs), mainly due to the time-consuming synthetic routes. Herein, two new TPD-based NFAs named as TPD-Th1 and TPD-Th3 were successfully obtained, adopting indacenodithiophene (IDT) as the central core and rhodanine (R) or 2-(1,1-dicyanomethylene)rhodanine (RCN) as the terminal electron-withdrawing unit. Both TPD-Th1 and TPD-Th3 exhibit highly planar structure due to the intramolecular noncovalently conformational locking between the O atom on the TPD unit and H atoms of two neighboring thiophene ring. The optimized PTB7-Th:TPD-Th3 photovoltaic devices show a much higher power conversion efficiency (PCE) of 5.53% in comparison with PTB7-Th:TPD-Th1 (PCE = 0.62%). The latter lower PCE could be attributed to the unmatched LUMO energy levels. Our results open a unique way to utilize TPD to construct small molecule NFAs to achieve a relatively high VOC and PCE.

Effect of fused triphenylamine core in star-shaped donor-π-acceptor molecules on their physicochemical properties and performance in bulk heterojunction organic solar cells

In this work, the synthesis of a novel star-shaped donor-acceptor molecule I with acridine-based core being a fused derivative of triphenylamine and linked through π-conjugated terthiophene spacers and terminal hexyldicyanovinyl electron-withdrawing units is reported. Its physicochemical and photovoltaic properties were comprehensively studied and compared to those of molecule II being a structural analog but with a pristine propeller-like triphenylamine core. The novel molecule shows combination of the higher crystallinity, solubility and thermal stability. As compared to II, bulk heterojunction organic solar cells based on I as a donor and PC71BM as an acceptor showed the three times higher hole mobility, 50% larger external quantum efficiency, which resulted in up to twice higher power conversion efficiency reaching 6.14%. This work demonstrates that the triphenylamine core fused by p-tolylmethylene groups in the star-shaped donor acceptor molecules is a promising building block to design highly soluble and crystalline materials for organic optoelectronic devices.

Molecular Engineering of Fully Conjugated sp2 Carbon-Linked Polymers for High-Efficiency Photocatalytic Hydrogen Evolution.

The diverse nature of organic precursors offers a versatile platform for precisely tailoring the electronic properties of semiconducting polymers. In this study, three fully conjugated sp2 carbon-linked polymers have been designed and synthesized for photocatalytic hydrogen evolution under visible-light illumination, by copolymerizing different C3 -symmetric aromatic aldehydes as knots with the 1,4-phenylene diacetonitrile (PDAN) linker through a C=C condensation reaction. The hydrogen evolution (HER) is achieved at a maximum rate of 30.2 mmol g-1 h-1 over a polymer based on 2,4,6-triphenyl-1,3,5-triazine units linked by cyano-substituted phenylene, with an apparent quantum yield (AQY) of 7.20 % at 420 nm. Increasing the degree of conjugation and planarity not only extends visible-light absorption, but also stabilizes the fully conjugated sp2 -carbon-linked donor-acceptor (D-A) polymer. Incorporating additional electron-withdrawing triazine units into the D-A polymer to form multiple electron donors and acceptors can greatly promote exciton separation and charge transfer, thus significantly enhancing the photocatalytic activity.

High performance conjugated terpolymers as electron donors in nonfullerene organic solar cells

Three pi-conjugated terpolymers based on the nonconventional molecular design strategy D1–D2–D1–A comprising two different multi-fused ladder-type arene electron-donating units and an electron-withdrawing unit are synthesized for organic photovoltaics.

Efficient organic solar cells based on a new "Y-series" non-fullerene acceptor with an asymmetric electron-deficient-core.

Herein, a new "Y-series" non-fullerene acceptor, Y21, bearing an asymmetric electron-deficient-core (DA'D) and fluorinated dicyanomethylene derivatives as flanking groups, was designed and synthesized for organic solar cell applications. Rather than being a perfect C2 symmetric traditional "Y-series" acceptor, Y21 possesses an electron-withdrawing unit (A') shifted from the center of DA'D, turning into an asymmetric molecular geometry. Photovoltaic devices based on PM6:Y21 can realize a high Jsc of 24.9 mA cm-2 and a PCE of 15.4%. Our work demonstrates a new way to tune the photoelectronic properties of the "Y-series" NFAs.