Electron-withdrawing Units Research Articles

Near-infrared resonance stimulated Raman study of short-lived transients in PTB7 films

Low-bandgap conjugated polymers, which have an alternating structure of electron-donating and electron-withdrawing monomer units in their main chains, are expected to show high photovoltaic efficiency when the monomer units are appropriately designed. For understanding key factors of the photovoltaic efficiency, we have investigated the structure and dynamics of short-lived transients in films of a low-bandgap conjugated polymer with an alternating structure of thieno[3,4-b]thiophene and benzo[1,2-b:4,5-b′]dithiophene, PTB7, by time-resolved near-infrared stimulated Raman spectroscopy as well as time-resolved near-infrared absorption spectroscopy. Positive polarons of PTB7 are clearly distinguished from its singlet excitons in stimulated Raman spectra by the peak position of a CC stretch band of the thiophene rings in benzodithiophene and intensity of a CSC deformation band of them. These differences strongly suggest that the repeated structure of thiophene rings in the main chain predominantly contributes to the charge transfer characteristics of PTB7 in films. Time-resolved absorption and stimulated Raman spectra of PTB7 films blended with a fullerene derivative show that spectral changes associated with the initial charge separation complete within 1 ps while effects of conformational changes and intermediate species are absent. We suggest that alternating copolymers composed of electron-donating and electron-accepting units commonly undergo the initial charge separation process without conformational changes or intermediates.

Effect of electron donor and acceptor in dithienopyrrolobenzothiadiazole-based organic dyes for efficient quasi-solid-state dye-sensitized solar cells

Three new organic dyes H1-3 with dithienopyrrolobenzothiadiazole moiety as the π-bridge have been synthesized and applied in quasi-solid-state dye-sensitized solar cells. The influences of different electron acceptor of the sensitizers on the photo-physical, electrochemical and photovoltaic performances have been studied. The results show that the inserted electron-withdrawing unit benzothiadiazole in the acceptor part can obviously enhance the light-harvesting ability of the dyes. In addition, the introduced benzothiadiazole moiety in acceptor can increase the π-electron distribution in LUMO level, which may facilitate to the photogenergated electrons injection into TiO2 film. The effects of different electron donor on the solar cell performances have been evaluated. The dihexyloxy substituted triphenylamine is better to suppress the intermolecular aggregation and restrain electron recombination. Finally, the quasi-solid-state solar cell sensitized by dye H2 with 1 mM chenodeoxycholic acid as the co-adsorbent obtains a power conversion efficiency of 7.56%, which close to the level of dye N719 (7.66%) under the same condition.

Organic Room-Temperature Phosphorescence with Strong Circularly Polarized Luminescence Based on Paracyclophanes.

Pure organic materials with intrinsic room-temperature phosphorescence typically rely on heavy atoms or heteroatoms. Two different strategies towards constructing organic room-temperature phosphorescence (RTP) species based upon the through-space charge transfer (TSCT) unit of [2.2]paracyclophane (PCP) were demonstrated. Materials with bromine atoms, PCP-BrCz and PPCP-BrCz, exhibit RTP lifetime of around 100 ms. Modulating the PCP core with non-halogen-containing electron-withdrawing units, PCP-TNTCz and PCP-PyCNCz, successfully elongate the RTP lifetime to 313.59 and 528.00 ms, respectively, the afterglow of which is visible for several seconds under ambient conditions. The PCP-TNTCz and PCP-PyCNCz enantiomers display excellent circular polarized luminescence with dissymmetry factors as high as -1.2×10-2 in toluene solutions, and decent RTP lifetime of around 300 ms for PCP-TNTCz enantiomers in crystalline state.

Tuning the Photophysical Properties of Push-Pull Azaheterocyclic Chromophores by Protonation: A Brief Overview of a French-Spanish-Czech Project.

Conjugated push-pull molecules that incorporate nitrogen heterocycles as electron-withdrawing units are interesting materials because of their luminescence properties. These chromophores can be easily and reversibly protonated at the nitrogen atom of the heterocyclic ring and this can cause dramatic color changes. White and multi-color photoluminescence both in solution and in the solid state can be obtained by an accurate control of the amount of acid. Thus, with a suitable design these compounds have potential applications in the development of colorimetric pH sensors and the fabrication of OLEDs based on only one material. We provide here a brief overview of our collaborative efforts made in this area.

Synthesis and Photovoltaic Effect of Electron-Withdrawing Units for Low Band Gap Conjugated Polymers Bearing Bi(thienylenevinylene) Side Chains.

A novel (E)-5-(2-(5-alkylthiothiophen-2-yl)vinyl)thien-2-yl (TVT)-comprising benzo[1,2-b:4,5-b’]dithiophene (BDT) derivative (BDT-TVT) was designed and synthetized to compose two donor-acceptor (D-A) typed copolymers (PBDT-TVT-ID and PBDT-TVT-DTNT) with the electron-withdrawing unit isoindigo (ID) and naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NT), respectively. PBDT-TVT-ID and PBDT-TVT-DTNT showed good thermal stability (360 °C), an absorption spectrum from 300 nm to 760 nm and a relatively low lying energy level of Highest Occupied Molecular Orbital (EHOMO) (−5.36 to –5.45 eV), which could obtain a large open-circuit voltage (Voc) from photovoltaic devices with PBDT-TVT-ID or PBDT-TVT-DTNT. The photovoltaic devices with ITO/PFN/polymers: PC71BM/MoO3/Ag structure were assembled and exhibited a good photovoltaic performance with a power conversion efficiency (PCE) of 4.09% (PBDT-TVT-ID) and 5.44% (PBDT-TVT-DTNT), respectively. The best PCE of a PBDT-TVT-DTNT/PC71BM-based device mainly originated from its wider absorption, higher hole mobility and favorable photoactive layer morphology.

Effect of Nitrogen Cation as "Electron Trap" at π-Linker on Properties for p-Type Photosensitizers: DFT Study.

On the basis of thieno(3,2-b)thiophene and dithieno[3,2-b:2′,3′-d]thiophene (T2 and T3 moieties) as π-linker, the A, D and S series dyes were designed to investigate the effect of the introducing N+ as an “electron trap” into T2 and T3 on the properties of the dyes. The optimized structures, electronic and optical properties were investigated by the density functional theory (DFT) and time-dependent DFT (TD-DFT). The results show that the properties of the dyes are sensitive to the N+ position in π-linkers. D series dyes with electron-withdrawing units located near the donor have better properties than the corresponding A series with the electron-withdrawing units located near the acceptor. For A and D series, the N+ modified dye named T2N+1-d displays the largest red shift of the UV–vis absorption, the maximum integral values of the adsorption-wavelength curves over the visible light, the highest light harvesting efficiency (LHE, 0.996), and the strongest adsorption energy (−44.33 kcal/mol). T2N+1-d also has a large driving force of hole injection (ΔGinj, −0.74 eV), which results in a more efficient hole injection. Bearing a lengthier π-linker than T2N+1-d, the properties of T2N+1-s are further improved. T2N+1-d moiety or its increased conjugated derivatives may be a promising π-linker.

Synthesis of Quinoxaline-Based Small Molecules Possessing Multiple Electron-Withdrawing Moieties for Photovoltaic Applications

Three quinoxaline-based small molecules possessing multiple electron-withdrawing moieties were synthesized by the Suzuki coupling reaction for organic photovoltaic cells (OPVs). The electron-donating triarylamine units were linked to both ends of electron-accepting 2,3-diphenyl quinoxaline (DPQ) derivatives with strong electron-withdrawing trifluoromethyl (CF3) moieties to produce a reference D-A-D type small molecule of CF3Qx-0F. Furthermore, one and two fluorine atoms were additionally introduced to the 6,7-positions ofthe DPQ unit of CF3Qx-0F affording CF3Qx-1F and CF3Qx-2F, respectively. Owingto the significant contribution of the electron-withdrawing CF3 and fluorine units, all inverted-type OPVs based on three small molecules exhibited high open circuit voltages greater than 0.82 V. In addition, the power conversion efficiencies (PCEs) of the devices were gradually improved with increasing number of fluorine atoms. The highest PCE (2.82%) with a Voc of 0.88 V, a short-circuit current of 6.38 mA cm−2, and a fill factor of 50.6% was achieved from the device based on CF3Qx-2F.

An A−D−Aʹ−Dʹ strategy enables perylenediimide-based polymer dyes exhibiting enhanced electron transport characteristics

In this work, we develop two new A−D−Aʹ−Dʹ type polymer dyes, namely PPDIDTBT and PPDIDTBT-2F, containing perylenediimide (PDI), thiophene, and benzothiadiazole units. Both polymer dyes have high thermal stability and broad absorption throughout the UV–vis region. More importantly, the introduction of the second electron-withdrawing units, benzothiadiazoles, endows both polymer dyes with low-lying LUMO energy levels of −3.85 eV for PPDIDTBT and −3.88 eV for PPDIDTBT-2F. The charge transport properties of the two polymer dyes were examined by fabricating organic field-effect transistors (OFETs) with top-gate/bottom-contact configuration. Both polymer dyes exhibit enhanced electron transport characteristics with the highest electron mobility of 0.070 cm2 V−1 s−1, which is among the highest values for PDI-based polymer semiconductors reported to date. Our results highlight that the A−D−Aʹ−Dʹ strategy is a useful approach in tuning orbital energetic and charge transport properties of polymer dyes for developing high-performance OFETs.

Fast enantioselective determination of triadimefon in different matrices by supercritical fluid chromatography

A fast analytical method through supercritical fluid chromatography was developed for enantioselective determination of triadimefon in soil and apple samples. Effects of chiral stationary phases, co-solvents, column temperature, and back pressure on chiral separation of triadimefon were discussed in detail. Chiral stationary phases containing electron-donating units located at phenylcarbamate moieties showed better enanitonrecognition abilities in triadimefon than those with electron-withdrawing units. Then, the modified pretreatment procedure was applied in enantioselective analysis of triadimefon in two matrices. When using cellulose tris(3,5-dimethylphenylcarbamate)-coated silica gel as the chiral stationary phase and the CO2-methanol (95/5, v/v) mixture as the mobile phase, the LOD and LOQ were 0.38 mg/kg and 2.00 mg/kg, respectively, much lower than the others chiral stationary phases. Good linearity (R2 = 0.9999) and recoveries (92.90–105.56%，RSD ≤ 1.52%) for them were achieved.

One-shot triphenylamine/phenylketone hybrid as a bipolar host material for efficient red phosphorescent organic light-emitting diodes

Abstract A new bipolar host material, 2,2′,2′′-tris(diphenylketone)amine (N-DPK) was developed in a one-shot by Ullmann reaction, and characterized by spectroscopic techniques and spatial arrangement was ascertained by single X-ray crystal structure analysis. The compound is possessing C3-symmetry with three diphenylketone units connected to the central nitrogen atom. In this organic architecture, the electron-withdrawing units, phenylketone are incorporated to ortho-position of triphenylamine, electron-donating unit to generate rigidity, which is responsible to reduce the gap between single and triplet energy levels. The material possessed triplet energy (ET) at 2.53 eV, indicating that it could be used as a host material for red phosphorescent organic light-emitting diodes. The OLED device was fabricated with bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) (Ir(MDQ)2acac) and N-DPK, as guest and host, respectively, and 10 wt% doping concentration exhibited good device performance with maximum external quantum efficiency (EQE) of 16.1%.

Improving energy transfer efficiency of dye-sensitized solar cell by fine tuning of dye planarity

Two push–pull metal-free sensitizers with 5,11-dihydroindolo[3,2-b]carbazole derivatives as electron-donating groups and 4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic acid (BTZ) as electron-withdrawing unit, denoted by SK201 and SK202, were synthesized and used for fabrication of dye-sensitized solar cells (DSSCs). SK202 contains a thienyl group between the donor and acceptor, whereas in SK201 the donor and acceptor are connected directly by a single bond. Introduction of a thienyl group improved the planarity of the dye molecule, broadened the absorption spectrum, enhanced the molar extinction coefficient, increased the dye loading on TiO2, and accelerated interface electron transfer on TiO2. This fine tuning of dye structure improved the performances of DSSCs based on SK202 sensitizers and gave a power conversion efficiency (PCE) of 11.0% (Jsc 16.5 mA cm−2, Voc 932 mV, and fill factor 71.7%), compared with that of 7.2% for SK201, under standard AM1.5G solar irradiation (100 mW cm−2) with a Co(II/III) complex based redox couple.

Asymmetric indenothiophene-based nonfullerene acceptors for binary- and ternary-blend polymer solar cells

A series of nonfullerene acceptors (IOTC, IETC, IOPC and IEPC) have been designed and synthesized by using asymmetric indenothiophene as an electron-donating core. Different side chains (linear 4-n-octylphenyl and branched 4-(2-ethylhexyl)phenyl), and electron-withdrawing units (2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile and 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile) were used to tune the optoelectronic properties of the resulting indenothiophene-based nonfullerene acceptors. With a benchmark p-type copolymer of PBDB-T, the best-performance binary devices based on IOTC and IETC (both with the ending group of 2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile) exhibited power conversion efficiencies (PCEs) of 7.94% and 7.40%, respectively. Whereas, the devices based on IOPC and IEPC (both with the ending group of 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile) showed relatively low PCEs of 4.86% and 5.26%, respectively. The improved photovoltaic properties of IOTC- and IETC-based binary devices are mainly attributed to their higher hole- and electron-mobilities compared to those of IOPC- and IEPC-based counterparts. In order to improve the light-harvesting ability of the binary PBDB-T:IOTC in the short wavelength region, PC71BM was introduced as the second acceptor to form a ternary blend of PBDB-T:IOTC:PC71BM. After optimization, the ternary blend of PBDB-T:IOTC:PC71BM with a weight ratio of 1:1:0.5 achieved a best PCE of 9.85% with a short-circuit current density (Jsc) of 16.35 mA/cm2, and a fill factor (FF) of 64.90%, much higher than the corresponding binary blend based on either PBDB-T:IOTC or PBDB-T:PC71BM. We further demonstrated that the addition of PC71BM not only improved the light absorption but also enhanced the carrier mobilities of the ternary blend thereby increasing the final PCE.

Molecular Heterostructures of Covalent Triazine Frameworks for Enhanced Photocatalytic Hydrogen Production.

Conjugated polymers have emerged as promising candidates for photocatalytic H2 production owing to their structural designability and functional diversity. However, the fast recombination of photoexcited electrons and holes limits their H2 production rates. We have now designed molecular heterostructures of covalent triazine frameworks to facilitate charge-carrier separation and promote photocatalytic H2 production. Benzothiadiazole and thiophene moieties were selectively incorporated into the covalent triazine frameworks as electron-withdrawing and electron-donating units, respectively, by a sequential polymerization strategy. The resulting hybrids exhibited much improved charge-carrier-separation efficiency as evidenced by photophysical and electrochemical characterization. An H2 evolution rate of 6.6 mmol g-1 h-1 was measured for the optimal sample under visible-light irradiation (λ>420 nm), which is far superior to that of most reported conjugated-polymer photocatalysts.

How to Design Donor-Acceptor Based Heterocyclic Conjugated Polymers for Applications from Organic Electronics to Sensors.

Over the past few years, significant progress has been made in the design of organic semi-conducting conjugated polymers that readily transport holes or electrons and can result in light emission. The conjugated backbone consist mainly of electron-donating (donor) and electron-withdrawing (acceptor) units as alternating groups in a conjugated oligomer or polymer that can be regulated by physical properties such as π conjugation length, monomer alteration, inter/intramolecular interactions and energy levels. Certainly, it is notable today that the highest occupied molecular orbital level of the producing material is localized predominantly on the electron-donating moiety and lowest unoccupied molecular orbital level on the electron-accepting moiety. Conjugated oligomers or polymers are used in many detecting fields due to their exceptional ability to sense toxic chemicals, metal ions and biomolecules. The conjugated polymers have unique delocalized π-electronic "molecular wires" that can expand the fluorescence intensity considerably. The fluorescence intensity of polymers can be quenched by particular quenching molecules. In this review, the fluorescence intensity, detecting of multiple metal ions, solubility, photochemical stability and optoelectronic properties of these conjugated polymers, and how they can be regulated by different functional groups, are discussed in detail.

1,3-Dithiole-2-one-Fused Subphthalocyanine and Subporphyrazine: Synthesis and Properties Arising from the 1,3-Dithiole-2-one Units.

Subphthalocyanine (SubPc) and its benzo ring-removed analogue, subporphyrazine (SubPz), bearing 1,3-dithiole-2-one (S2CO) groups as a new class of substituents were synthesized. In addition to the perturbed optical properties due to the presence of electron-withdrawing S2CO units, the deep bowl-shaped structure of the SubPz derivative allowed concave-convex interaction to form a unique co-crystal structure with C60. Finally, using the reactivity of the peripheral S2CO units, S2CO-fused SubPc was successfully converted into tetrathiafulvalene (TTF)-annulated SubPc in a yield higher than that of the direct synthesis from a TTF-fused phthalonitrile.

Dependence of the surface-assisted fullerene-based complex structure on the template molecule design

Fullerene derivatives as a kind of star carbon materials have received intense investigation because of their three-dimensional shape, anisotropic electron mobility, and high electron affinity. Indeed, the cutting-edge developments of fullerene nanomaterials have had a tremendous impact on a wide range of applications, such as organic solar cells, field effect transistors, and photodetectors. To explore their full potential applications, research into fullerene-based multilevel nanostructures relying on hierarchical interactions from bottom to top is rapidly expanding. It is of great theoretical and practical significance to prepare multilevel fullerene nanostructures with structural and properties controlled by optimizing the influencing factors. This review would offer several aspects including the chemical structures of organic molecules and the nanostructures of the organic molecules and fullerene-organic complexes. Whether monolayers or multilayers, fullerene molecules tend to fall into a space of suitable size, in which the located positions are affected by the intermolecular interactions. For the covered surfaces, fullerenes are more likely to approach the electron-withdrawing units through the donor–acceptor and charge transfer interaction. Through the implementation of this review, an exhaustive analysis on the chemical modification, including the molecular backbone and substituents, preformed network synergies, and adsorption sites is presented. In addition, the relationship between the molecules and structures that illustrates the importance of the molecular design for the controlled fullerenes hybrid nanostructures can be further understood based on the results of the joined experimental and computational investigations.

Achieving Over 15% Efficiency in Organic Photovoltaic Cells via Copolymer Design.

Ternary blending and copolymerization strategies have proven advantageous in boosting the photovoltaic performance of organic solar cells. Here, 15% efficiency solar cells using copolymerization donors are demonstrated, where the electron-withdrawing unit, ester-substituted thiophene, is incorporated into a PBDB-TF polymer to downshift the molecular energy and broaden the absorption. Copolymer-based solar cells suitable for large-area devices can be fabricated by a blade-coating method from a nonhalogen and nonaromatic solvent mixture. Although ternary solar cells can achieve comparable efficiencies, they are not suitable for environment-friendly processing conditions and show relatively low photostability compared to copolymer-based devices. These results not only demonstrate high-efficiency organic photovoltaic cells via copolymerization strategies but also provide important insights into their applications in practical production.

Copolymers of Diketopyrrolopyrrole and Benzothiadiazole: Design and Function from Simulations with Experimental Support

Alternating block copolymers consisting of diketopyrrolopyrrole and benzothiadiazole electron acceptor units linked together via aromatic five-membered donor heterocycles are studied using a combination of computer simulation techniques and experiments. Four copolymers are modeled starting from their monomers to stacked macromolecules: with two different linkers—thiophene or furan, connecting electron-withdrawing core units—and two different alkyl substituents at lactam nitrogens of diketopyrrolopyrrole—linear dodecyl and branched 2-octyldodecyl chains. In our experiments, we aim at characterization of the optical and electrochemical properties of two copolymers with branched side chains differing in the linker, since as the literature survey shows the data published on these copolymers are very sparse. These properties can be easily interpreted and later compared with theoretical predictions. The results of simulations supported by experiments show that monomers of these polymers have very similar electr...

Molecular design and synthesis of D–π–A structured porphyrin dyes with various acceptor units for dye-sensitized solar cells

The large efficiency loss for the SGT-023-based device with a stronger electron-withdrawing pyridothiadiazole unit could be ascribed to a fast charge recombination rate caused by the tilted adsorption mode on the TiO2 surface.

An A2–π–A1–π–A2-type small molecule donor for high-performance organic solar cells

A new A2–π–A1–π–A2-type small-molecule donor using a strong electron-withdrawing unit as the central unit was synthesized and its photovoltaic performance was investigated.