Electron-deficient Unit Research Articles

Modulating the nanoscale morphology on carboxylate-pyrazine containing terpolymer toward 17.8% efficiency organic solar cells with enhanced thermal stability

It had been commonly accepted in the organic photovoltaic (OPV) community that subtle variations in the molecular structure of active layer materials would cause profound impacts on their aggregating structure and blend morphology and therefore the performance of such polymer solar cells (PSCs). Herein, we employed an electron-deficient building block 3,6-dithiophenyl-2-carboxylate pyrazine (DTCPz) for constructing one series of promising donor terpolymers of PMZ1, PMZ2, and PMZ3, respectively, gaining their relatively lower-lying highest occupied molecular orbital (HOMO) energy levels, more closed π-π stacking and enhanced crystallinity in thin films, and lower miscibility with acceptor Y6, in comparison with their parent polymer counterpart (namely PM6). Reaching DTCPz moieties up to 20% (mol/mol%) in its terpolymer composition, the resulting polymer (PMZ2) achieved more favorable phase separation with improved exciton dissociation, and charge transport and extraction. As a result, an outstanding fill factor of 77.2% and a promising power conversion efficiency of 17.8 % was achieved. Moreover, the corresponding device shows better thermal stability over the PM6-based one. This work suggests a facile method for significantly improving the thin film morphology of the active-layer materials via fine-tuning the chemical structure of electron-deficient units on the backbone of the wide bandgap donor polymer, therefore achieving enhanced photovoltaic performance and thermal stability for practical applications.

ANew Diazabenzo[k]fluoranthene-BasedD-A Conjugated Polymer Donor for Efficient Organic Solar Cells.

The development of wide-bandgap polymer donors having complementary absorption and compatible energy levels with near-infrared (NIR) absorbing nonfullerene acceptors is highly important for realizing high-performance organic solar cells (OSCs). Herein, a new thiophene-fused diazabenzo[k]fluoranthene derivative is successfully synthesized as the electron-deficient unit to construct an efficient donor-acceptor (D-A) type alternating copolymer donor, namely, PABF-Cl, using the chlorinated benzo[1,2-b:4,5-b']dithiophene as the copolymerization unit. PABF-Cl exhibits a wide optical bandgap of 1.93eV, a deep highest occupied molecular level of -5.36eV, and efficient hole transport. As a result, OSCs with the best power conversion efficiency of 11.8% are successfully obtained by using PABF-Cl as the donor to blend with a NIR absorbing BTP-eC9 acceptor. This work provides a new design of electron-deficient unit for constructing high-performance D-A type polymer donors.

Conjugated microporous polymers incorporating Thiazolo[5,4-d]thiazole moieties for Sunlight-Driven hydrogen production from water

In this paper, we describe efficient, inexpensive, and donor–acceptor (D–A) systems based on conjugated microporous polymers incorporating thiazolo[5,4-d]thiazole (ThTh) linkages for the extraction of H2 from H2O under visible light in the presence of ascorbic acid (AA) as a sacrificial electron donor without additional noble metals as co-catalyst. The integration of electron-rich pyrene (Py) and electron-deficient ThTh units in Py-ThTh-CMP resulted in a D–A system that provided an H2 evolution rate (HER) of 1874 µmol g–1h−1; this value was greater than those of the other tested CMPs prepared with and without the ThTh acceptor moiety. Notably, Py-ThTh-CMP also exceeded the HERs of many other reported materials for photocatalytic H2O reduction, including co-catalysts based on covalent organic frameworks, CMPs, and graphitic carbon nitride. The DFT and TD-DFT suggested the incorporation of ThTh moiety in the CMPs backbone to enhance the charge transfer via S1 ↔ T1 and contribute to the H2 formation. This study presents a new approach for preparing highly efficient photocatalysts for the practical development of cost-effective H2 evolution.

Fluorinated phenanthrenequinoxaline-based D-A type copolymers for non-fullerene polymer solar cells

Three polymers with indacenodithiophene (IDT) as the electron-rich unit and phenanthrenequinoxaline (phanQ) or fluorine substituted phenanthrenequinoxaline (FphanQ, 2FphanQ) as the electron-deficient units have been synthesized. The band gaps of PIDT-phanQ, PIDT-FphanQ, PIDT-2FphanQ were estimated to be 1.68, 1.68 and 1.71 eV with a HOMO energy level of −5.28, −5.33 and −5.41 eV, respectively. The photovoltaic performance of the three polymers in non-fullerene polymer solar cells with Y6 as the acceptor material was carefully investigated. The corresponding devices showed improved V oc as the fluorine atom increased in the phanQ unit, but the champion PCE was achieved in the PIDT-FphanQ:Y6 based device due to its higher J sc and FF. The best PCE was recorded to be 11.6% with a V oc of 0.79 V, a J sc of 21.4 mA cm −2 and a FF of 68.7% for the PIDT-FphanQ:Y6 blend without using additives. The results indicate that the proper design in donor and acceptor units of D-A conjugated polymer, i.e. introducing the moderate fluorine substitutes, will be able to simplify the processing for the efficient PSCs applications. Three donor polymers based on IDT unit and Phenanthrenequinoxaline derivatives were developed. The photovoltaic performance was improved by tuning the number of fluorine on the phenanthrenequinoxaline moiety. High efficiency was obtained without the use of additives, thus simplifying processing of efficient polymer solar cells. • Three polymers based on indacenodithiophene and phenanthrenequinoxaline units were designed for photovoltaic applications. • The mono-fluorinated polymer PIDT-FphanQ delivers a best PCE of 11.6% due to higher J sc and FF. • The high photovoltaic performance was achieved without solvent additives, simplifying the process for efficient PSC.

Aggregation-induced emission enhancement (AIEE)-active donor-acceptor type fluorophores based on the 1,3-diaryl pyrazole unit: Conspicuous mechanochromic fluorescence and biosensing of iodide ions

Two type luminophore 1 and 2, with the same triphenylamine (TPA) and malonitrile-functionalized 1,3-diaryl pyrazole (MDP) units but with different π-conjugated spacer, were designed and synthesized. The crystals solid-state emission behavior and mechanical stimuli-responsive fluorescence characteristics of 1 and 2 were investigated. Interestingly, the fluorophores 1 and 2 exhibited yellow-green and yellow crystals solid-state fluorescent, respectively. Furthermore, 1 and 2 also exhibited obvious mechanochromic fluorescence behavior with repeated four cycles. Compared to 1 with an 80 nm red-shift, 2 is more sensitive to mechanical stimuli and exhibits a large red-shift emission of 85 nm. The powder XRD experiments demonstrated that the mechanochromic fluorescence behavior of 1 and 2 were related to the morphology transformation from crystalline state to amorphous state. In addition, based on the MDP moiety as I− receptor, 1 and 2 were qualified for sensing of I− with low limit of detection (89.0 nM, 149.9 nM). The biosensing of I− based on the better detection performance of the probe 1 was achieved in A549 cells and zebrafish. These results certify the great potential of MDP as a strong electron-deficient unit for designing D-A type mechanochromic fluorescence materials and biosensors for the detection of I− in organism.

Thiazolothiazole based donor-π-acceptor fluorophore: Protonation/deprotonation triggered molecular switch, sensing and bio-imaging applications

Stimuli modulated photophysical properties of a new asymmetric multifunctional molecular probe, 2-(pyren-1-yl)-5-(pyridin-4-yl)thiazolo [5,4-d]thiazole (PYTZ-P) bearing electron-rich pyrene and electron-deficient pyridine units linked through a π-conjugated thiazolo [5,4-d]thiazole (TTz) bridge are reported. Its sensitivity towards protons (TFA) in solution is manifested in the form of bathochromically shifted emission that spreads all over the visible region, and is related to the increased acceptor strength of pyridine upon protonation and subsequent enhanced magnitude of intramolecular charge-transfer in the probe. Similar modulation of the luminescence behaviour of the probe was also observed in the solid state. These observations are well rationalised by the theoretical calculations. The applicability of the sensitivity of the probe towards TFA has been demonstrated by developing the ready-to-use portable paper strips. Furthermore, PYTZ-P showed different acid/base induced absorption/emission switching property. Interestingly, when the response of the probe was noted in the presence of several cations, modulation of the electronic properties was observed only in the presence of Hg2+ ions, with the lowest detection limit of 8.43 × 10−8 M. Additionally, the sensitivity of the probe towards TFA and Hg2+ ions in solution and solid states is convincingly mimicked in the fluorescence imaging of HeLa and A375 cancer cells.

Design and photovoltaic properties of conjugated polymers based on quinoxaline and diketopyrrolopyrrole for OSCs

We report the synthesis of low bandgap polymers containing a difluoroquinoxaline unit by Stille polymerization for use in organic solar cells (OSCs). A new series of copolymers were synthesized with 2, 3- didodecyl-6, 7- difluoro-quinoxaline and pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione as electron-deficient units and thiophene or bithiophene as the electron-rich unit. The bithiophene moiety in one of the polymers was associated with reduced electron density because of a weak push-pull effect and an increase in the highest occupied molecular orbital (HOMO) energy level. Efficient polymer solar cells were fabricated by blending the synthesized copolymers with the fullerene PC71BM, and the PCE was measured to be 3.41% under 100 mW/cm2 AM 1.5 G illumination. These results demonstrate that the careful design of a balanced chemical structure is crucial for high performance polymer solar cells.

Leverage of Pyridine Isomer on Phenothiazine Core: Organic Semiconductors as Selective Layers in Perovskite Solar Cells.

To drive the development of perovskite solar cells (PSCs), hole-transporting materials are imperative. In this context, pyridine derivatives are being probed as small molecules-based hole-transporting materials due to their Lewis base and electron-deficient unit. Herein, we focused our investigation on pyridine isomer molecules 4,4'-(10-(pyridin-x-yl)-10H-phenothiazine-3,7-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (x = 2, 3, or 4), in which the pyridine nitrogen heteroatom is located at the 2, 3, and 4 positions, named as 2PyPTPDAn, 3PyPTPDAn, and 4PyPTPDAn, respectively. We decipher the structure-properties-device performance relationship impacted by the different N-atom positions in pyridine. In the case of 3PyPTPDAn, the partial orbital overlap between highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) favors the generation of neutral excitons and hole transport, as well as improves the film-formation ability, and this induces efficient hole extraction as compared to their 2,4 analogues. The solar cells fabricated with 3PyPTPDAn gave on-par photovoltaic performance as that of typical Spiro-OMeTAD, and higher performance than those of 2PyPTPDAn and 4PyPTPDAn. The hydrophobicity and homogeneous film properties of 3PyPTPDAn add merits to the stability. This work emphasizes the guidelines to develop small molecules for organic solar cells, organic light-emitting diodes, and thermally activated delayed fluorescence.

Facile construction of fully sp2-carbon conjugated two-dimensional covalent organic frameworks containing benzobisthiazole units

Developing a facile strategy for the construction of vinylene-linked fully π-conjugated covalent organic frameworks (COFs) remains a huge challenge. Here, a versatile condition of Knoevenagel polycondensation for constructing vinylene-linked 2D COFs was explored. Three new examples of vinylene-linked 2D COFs (BTH-1, 2, 3) containing benzobisthiazoles units as functional groups were successfully prepared under this versatile and mild condition. The electron-deficient benzobisthiazole units and cyano-vinylene linkages were both integrated into the π conjugated COFs skeleton and acted as acceptor moieties. Interestingly, we found the construction of a highly ordered and conjugated D-A system is favorable for photocatalytic activity. BTH-3 with benzotrithiophene as the donor with a strong D-A effect exhibited an attractive photocatalytic HER of 15.1 mmol h−1g−1 under visible light irradiation.

A class of electron-deficient units: fluorenone imide and its electron-withdrawing group-functionalized derivatives.

This study describes the development of a series of strong electron-deficient acceptor units, namely, fluorenone imide (FOI) and its derivatives, functionalized with various electron-withdrawing groups, which are developed via functionalization of the fluorenone unit with an imide group and then systematic variation of the attached electron-withdrawing groups at the bridgehead benzylic position.

A novel class of rigid-rod perylene diimides and isoindigo semiconducting polymers

Three novel rigid-rod semiconducting polymers containing fused electron-deficient PDI and IID units have been synthesized through aldol polymerization. Their unique opto-electronic properties have been investigated systematically.

M-Phenylene linked macrocycle composed of electron-rich dithienogermole and electron-deficient tricoordinate boron units

Incorporation of non-conventional elements into π-conjugated systems is a powerful tool to control the electronic structures. In this work, electron-deficient tricoordinate boron units were combined with electron-rich dithienogermole units to provide an element-hybrid conjugated macrocycle with m-phenylene linkers. In contrast to the absorption spectra which were unaffected by solvent polarity, the element-hybrid macrocycle exhibited clear solvatochromism in the fluorescence spectra originating from the charge transfer (CT) transition between the dithienogermole and tricoordinate boron units. The addition of fluoride to the solution of the macrocycle resulted in the stoichiometric formation of tetracoordinate borate species. Interestingly, the fluorescence was intensified by the addition of fluoride, and the mechanism was confirmed by fluorescence spectroscopy and TD-DFT calculations.

Synthesis, Structures, and Properties of Helically Fused Anthraquinones with Unusually Close Carbonyl-Carbonyl Contacts.

Electron-deficient aromatic ketones consisting of three fused anthraquinone units were synthesized by oxidation of the corresponding fused anthracenes. X-ray analysis revealed that these compounds had nonplanar helical structures with unusual contacts, C=O⋅⋅⋅C=O 2.467 Å, between the inner carbonyl groups. The role of n⋅⋅⋅π* interactions in the short contacts was evaluated using a noncovalent interaction plot and natural bond orbital analysis. The dynamic process involving helical inversion was observed by the variable temperature 1 H NMR measurement of a derivative with 2,4,6-trimethylphenyl groups, and the barrier was estimated to be 77 kJ mol-1 . DFT calculations indicated that the helical inversion proceeded via a multistep mechanism. The characteristic spectroscopic and electrochemical data due to the electron-deficient anthraquinone units and the sterically congested carbonyl groups are discussed with the aid of DFT calculations.

Rational Design and Characterization of Symmetry-Breaking Organic Semiconductors in Polymer Solar Cells: A Theory Insight of the Asymmetric Advantage.

Asymmetric molecule strategy is considered an effective method to achieve high power conversion efficiency (PCE) of polymer solar cells (PSCs). In this paper, nine oligomers are designed by combining three new electron-deficient units (unitA)—n1, n2, and n3—and three electron-donating units (unitD)—D, E, and F—with their π-conjugation area extended. The relationships between symmetric/asymmetric molecule structure and the performance of the oligomers are investigated using the density functional theory (DFT) and time-dependent density functional theory (TD–DFT) calculations. The results indicate that asymmetry molecule PEn2 has the minimum dihedral angle in the angle between two planes of unitD and unitA among all the molecules, which exhibited the advantages of asymmetric structures in molecular stacking. The relationship of the values of ionization potentials (IP) and electron affinities (EA) along with the unitD/unitA π-extend are revealed. The calculated reorganization energy results also demonstrate that the asymmetric molecules PDn2 and PEn2 could better charge the extraction of the PSCs than other molecules for their lower reorganization energy of 0.180 eV and 0.181 eV, respectively.

Structure influence of alkyl chains of thienothiophene-porphyrins on the performance of organic solar cells

Two new A-D-A porphyrin derivatives, denoted as XLP-I and XLP-II, were prepared through extending the π-conjugation of thienothiophene-porphyrin center with phenylethynyl bridges and electron-deficient ethylrhodanine terminal units, and varying the structures of alkyl chain (linear vs branched) on peripheral thienothiophene substitutions of porphyrin rings. Both molecules show strong absorption in UV–visible–near-infrared region, good thermal stability, suitable energy levels, and ordered molecular packing in solid state. In organic solar cells, PC 71 BM was used as electron acceptor, and porphyrin small molecules were used as electron donors. The device based on XLP-I exhibits a power conversion efficiency (PCE) of 8.30%, an open circuit voltage ( V oc ) of 0.894 ​eV, and a fill factor ( FF ) of 62.1%. In contrast, the device based on XLP-II presents an inferior performance with a PCE of 3.14%, a V oc of 0.847 ​eV, and a FF of 49.3%. The better performance of XLP-I based device is mainly attributed to its optimized film morphology, excellent absorption, and well-balanced charge transport properties. ● Two new A-D-A porphyrin derivatives, denoted as XLP-I and XLP-II, have been prepared. ● Spectroscopic, thermal, electronic and charge transport properties and structural characterization were carried out. ● The performance of organic solar cells using these porphyrin derivatives were fabricated, it exhibits as a power conversion efficiency of 8.30%.

Multifunctional Viologen-Derived Supramolecular Network with Photo/Vapochromic and Proton Conduction Properties.

A supramolecular network [H4bdcbpy(NO3)2·H2O] (H4bdcbpy = 1,1′-Bis(3,5-dicarboxybenzyl)-4,4′-bipyridinium) (1) was prepared by a zwitterionic viologen carboxylate ligand in hydrothermal synthesis conditions. The as-synthesized (1) has been well characterized by means of single-crystal/powder X-ray diffraction, elemental analysis, thermogravimetric analysis and infrared and UV-vis spectroscopy. This compound possesses a three-dimensional supramolecular structure, formed by the hydrogen bond and π–π interaction between the organic ligands. This compound shows photochromic properties under UV light, as well as vapochromic behavior upon exposure to volatile amines and ammonia, in which the electron transfer from electron-rich parts to the electron-deficient viologen unit gives rise to colored radicals. Moreover, the intensive intermolecular H-bonding networks in 1 endows it with a proton conductivity of 1.06 × 10−3 S cm−1 in water at 90 °C.

Novel polymer donors based on simple A1-π-A2 structure for non-halogen solvent-processed organic solar cells

Two low band gap conjugated polymers based on acceptor1-π-acceptor2 (A1-π-A2) and donor-π-acceptor (D-π-A) type architecture were synthesized for organic solar cells. The novel A1-π-A2 type polymer contains a fluorine substituted benzotriazole (FTAZ) as acceptor1 block unit (A1), tetrazine (TTz) as acceptor2 unit (A2), and thiophene as the π-bridge, namely PBTZ-TTz. The compared D-π-A type polymer contains a benzodithiophene (BDT) as rich electronic unit, TTz as electron-deficient unit and thiophene as the π-bridge, namely PBDT-TTz. Although mounting more thiophene π bridge units for better solubility and backbone geometry, the novel “A1-π-A2” type copolymer PBTZ-TTz achieved a deep HOMO energy level of −5.55 eV, just slightly higher than that of D-π-A type polymer PBDT-TTz. Moreover, the polymer PBTZ-TTz obtained a wider and stronger UV–vis absorption, better coplanarity and π-π stacking than PBDT-TTz. After processed by non-halogen solvent 1,2,4- trimethylbenzene (TMB), the PBTZ-TTz:IT-4F based device achieved a decent open circuit voltage (Voc) of 0.88 V, a lower power conversion efficiency (PCE) of 5.9%, while PBDT-TTz based device reached the PCE of 7.4% with Voc of 0.95 V. The unexpected lower PCE of PBTZ-TTz:IT-4F device is mainly due to the holes cannot transfer efficiently from acceptor to donor, the lower external quantum efficiency (EQE) spectra responding, as well as the poor phase separation morphology of the blend film. It is worth noting that both the Voc achieved are in outstanding high levels among IT-4F based devices. These results reveal that the A1-π-A2 type copolymers can enrich non-halogen solvent-processed donor materials through simple synthesis, which is beneficial for future environmentally friendly processing.

A dithienobenzothiadiazole-quaterthiophene wide bandgap polymer enables non-fullerene based polymer solar cells with over 15% efficiency

In this work, a wide bandgap (WBG) polymer donor PDTfBT-4T based on dithienobenzothiadiazole (DTfBT) and quaterthiophene (4T) was synthesized for the application in non-fullerene polymer solar cells (PSCs). Polymer PDTfBT-4T showed film absorption peak at 554 nm and optical band gap of 1.93 eV. Complementary absorption and sufficiently energetic offsets could be achieved between the PDTfBT-4T and a benzotriazole-fused Y-series non-fullerene acceptor Y14. The PDTfBT-4T:Y14 active layer could deliver an open-circuit voltage of 0.83 V, a short-circuit current density of 25.42 mA cm−2, and a fill factor of 72.0%, corresponding to a power conversion efficiency (PCE) of 15.19%. This is the highest efficiency among WBG polymer donors based on an electron-deficient unit and an oligothiophene, which is also significantly higher than those of the fluorinated or esterified polythiophenes. The results indicate that DTfBT is a powerful electron-deficient unit, which is promising in the construction of simple WBG polymer donors for efficient NFA based PSCs.

Hymecromone naphthoquinone ethers as probes for hydrogen sulfide detection

Hydrogen sulfide (H2S), as one of the important gasotransmitters, plays an essential role in a many physiological and pathological processes. Commonly used methods for real-time detection and quantification of H2S are rather complicated. This report presents a simple fluorescence and colorimetry dual-mode assay based on ethers (Nq-Cm and Nq-2Cm) derived from hymecromone (7-hydroxy-4-methylcoumarin, Cm) and naphthoquinone (Nq). In these compounds an electron-deficient naphthoquinone unit was used as a suitable reagent for nucleophilic aromatic substitution (SNAr) reaction. Thiolysis of Nq-Cm and Nq-2Cm at physiological pH releases Cm and causes an significant fluorescence enhancement at 448 nm (with excitation at 320 nm). At the same time, the resulting naphthoquinone derivative Nq-2SH causes the solution a purple color, which allows naked-eye detection. Thus Nq-Cm and Nq-2Cm are colorimetrically selective for H2S. The efficacy of the Nq-Cm and Nq-2Cm were demonstrated in buffer with associated submicromolar detection limit as low as 130 nM and 150 nM, respectively. Presented results suggest that both probes are excellent quantitative detection tool for H2S.

The design of quinoxaline based unfused non-fullerene acceptors for high performance and stable organic solar cells

Unfused non-fullerene acceptors with the merits of easy synthesis, high yields and low cost have attracted much attention in recent years. And the development of new unfused acceptors with high performance and stability is urgently demanded. In this work, we designed and synthesized two unfused non-fullerene acceptors (UF-Qx-2F and UF-Qx-2Cl) with an electron-deficient quinoxaline (Qx) as the central unit linking with the electron-donating cyclopentadithiophene (CPDT) as the conjugated backbone. With different end groups, the crystallinity and packing behaviors of molecules were finely tuned. UF-Qx-2Cl, which is capped with 2-(5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC-2Cl), possesses more red-shifted absorption, higher crystallinity and more ordered molecular packing. The J52:UF-Qx-Cl based devices achieved a higher power conversion efficiency of 10.81% and better thermal and air stability than those of J52:UF-Qx-2F based devices. Our results indicate that the use of electron-deficient core units (Qx) is a promising way to design unfused NFAs, and the unfused non-fullerene acceptors hold a bright future to carry out high performance and stable organic solar cells with low cost for commercial application.