Electron-deficient Unit Research Articles

Chiroptical Helices of N-Terminal Aryl Amino Acids through Orthogonal Noncovalent Interactions.

In the solid state, amino acids (alanine and phenylglycine) with appended pyrene segments self-assembled into α-helix-like structures by asymmetrical H-bonds between carboxylic acid and amide segments, further inducing supramolecular tilted chirality of the achiral pyrenes. These structures bind melamine and electron-deficient units through H-bond and charge-transfer interactions, respectively. Charge-transfer interactions enhance the dissymmetry g-factor of absorption (gabs ; up to 1.4×10-2 ) with an extended Cotton effect active region (from 250 to 600 nm). Incorporating melamine inverts the handedness of circularly polarized luminescence and boosts the dissymmetry g-factor (glum ). Melamine also induces macroscopic chirality at the nanoscale, whereby the 2D lamellar structures are transformed into 1D helices at the nanoscale, leading to giant tubular structures at the microscale.

Tetrathiafulvalene-Fused Heterabuckybowl: Protonation-Induced Electron Transfer and Self-Sensitized Photooxidation.

The electron donor tetrathiafulvalene (D1 ) was fused onto the electron-rich heterabuckybowl trichalcogenasumanene (D2 ) through an electron-deficient pyrazine unit (A) to give 1 c, 1 d, 2 c, and 2 d, featuring the D1 -A-D2 structure. Both D1 and D2 play a pivotal role in intramolecular charge-transfer (ICT) transitions, consequently 1 c, 2 d, 2 c, and 2 d show a broad ICT band at 450-720 nm in steady state. They exhibit two charge-separated transient states, CS1 and CS2 , that appear in sequence. CS1 has a short lifetime (542 fs), and the D1 moiety on CS1 is in the radical cation state with an absorption maximum (λmax ) at 889 nm. CS1 then converts into CS2 (λmax , 1105 nm) through an ICT between D1 .+ and D2 , affording D1 (1-δ).+ and D2 δ.+ . Compounds 1 c, 1 d, 2 c, and 2 d show protonation-induced intramolecular electron transfer that leads to absorption at λ=700-1300 nm. Owing to the existence of an electron-rich C=C bond on the D1 moiety and in situ generation of 1 O2 by the pyrazine-fused D2 moiety, compounds 1 c, 1 d, 2 c, and 2 d display self-sensitized photooxidation in 50 s.

Electron-Deficient and Quinoid Central Unit Engineering for Unfused Ring-Based A1 -D-A2 -D-A1 -Type Acceptor Enables High Performance Nonfullerene Polymer Solar Cells with High Voc and PCE Simultaneously.

Here, a pair of A1 -D-A2 -D-A1 unfused ring core-based nonfullerene small molecule acceptors (NF-SMAs), BO2FIDT-4Cl and BT2FIDT-4Cl is synthesized, which possess the same terminals (A1 ) and indacenodithiophene unit (D), coupling with different fluorinated electron-deficient central unit (difluorobenzoxadiazole or difluorobenzothiadiazole) (A2 ). BT2FIDT-4Cl exhibits a slightly smaller optical bandgap of 1.56 eV, upshifted highest occupied molecular orbital energy levels, much higher electron mobility, and slightly enhanced molecular packing order in neat thin films than that of BO2FIDT-4Cl. The polymer solar cells (PSCs) based on BT2FIDT-4Cl:PM7 yield the best power conversion efficiency (PCE) of 12.5% with a Voc of 0.97 V, which is higher than that of BO2FIDT-4Cl-based devices (PCE of 10.4%). The results demonstrate that the subtle modification of A2 unit would result in lower trap-assisted recombination, more favorable morphology features, and more balanced electron and hole mobility in the PM7:BT2FIDT-4Cl blend films. It is worth mentioning that the PCE of 12.5% is the highest value in nonfused ring NF-SMA-based binary PSCs with high Voc over 0.90 V. These results suggest that appropriate modulation of the quinoid electron-deficient central unit is an effective approach to construct highly efficient unfused ring NF-SMAs to boost PCE and Voc simultaneously.

Asymmetric indenothienothiophene-based unfused core for A-D-A type nonfullerene acceptors

In this work, we design and synthesize three asymmetric acceptor-donor-acceptor (A-D-A) type nonfullerene acceptors (ITOTC, ITUTC and ITUIC) using a novel indenothienothiophene-based unfused core together with two different electron-deficient terminal units (2-(6-oxo-5,6-dihydro-4H-cyclopenta[c]thiophen-4-ylidene)malononitrile for ITOTC and ITUTC, and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile for ITUIC). Alkyl chains with different lengths are incorporated in the indenothienothiophene-based unfused core to ensure the good solubility of the resulting nonfullerene acceptors (n-octyl for ITOTC, n-undecyl for ITUTC and ITUIC), and to tune their charge transporting properties as well. Photovoltaic properties of these nonfullerene acceptors are investigated by using a typical wide band-gap polymer of PBDB-T as the donor material. The best-performance polymer solar cell based on ITOTC: PBDB-T with annealing shows a power conversion efficiency (PCE) of 8.35%, in comparison with the counterpart based on ITUTC:PBDB-T which shows a PCE of 7.68%. However, 7.68% PCE is higher than that of the best-performance PSC based on ITUIC:PBDB-T (6.45% PCE). These results suggest that both side-chains and electron-deficient terminal groups can be optimized to improve the photovoltaic properties of A-D-A type nonfullerene acceptors based on the indenothienothiophene-based unfused core.

Effect of Spacer Length in Naphthobispyrazine-Based π-Conjugated Polymers on Properties, Thin Film Structures, and Photovoltaic Performances

Abstract Incorporation of an electron-deficient building unit based on a large π-system into the polymer backbone is a powerful strategy for the development of high-performance π-conjugated polymers. The naphthobispyrazine (NPz) tetracyclic π-electron system is a promising electron-deficient building unit but its use in π-conjugated polymers has seldom been investigated. Here, we synthesized four NPz-based π-conjugated polymers with different donor units, namely thiophene (P1), bithiophene (P2), terthiophene (P3), and quaterthiophene (P4), and systematically studied their structure-property relationships. The absorption range was significantly red-shifted when the spacer length was longer than bithiophene due to the decreased steric hindrance between the NPz moieties. Furthermore, the crystallinity in the polymer thin films was dramatically improved when the spacer length was longer than terthiophene. These results clearly affect the efficiency of polymer/fullerene-based solar cells, with the maximum efficiency of 4.4% obtained using P3. This study may be helpful for the design of NPz-based π-conjugated polymers with improved performance.

Achieving High-Performance Photothermal and Photodynamic Effects upon Combining D-A Structure and Nonplanar Conformation.

Various organic nanoagents have been developed for photothermal therapy (PTT) and photodynamic therapy (PDT) under near-infrared (NIR) irradiation. Among them, small molecule-based nanoagents are very attractive due to their advantages of well-defined chemical structures, high purity, good reproducibility, and easy processability. However, only a few small molecule-based nanoagents have been developed for PDT under NIR irradiation. Moreover, the mechanism of PDT under NIR is still elusive. Herein, a semiconducting small molecule (BTA) with donor-acceptor-donor structure and twisted conformation is developed for PDT/PTT under NIR irradiation. A large π-conjugated electron-deficient unit is used as the core to couple with two electron-donating units, ensuring the strong absorption under 808 nm. Moreover, the donor-acceptor structures and twisted conformation can reduce the energy gap between the singlet and triplet states (∆EST ) to afford effective intersystem crossing, beneficial for reactive oxygen species generation. The mechanism is probed by experimental and theoretical evidence. Moreover, the BTA nanoparticles exhibit excellent biocompatibility and PTT/PDT in vitro performance under NIR irradiation. This provides a strategy for designing highly efficient PDT/PTT molecular materials.

A Flexible Interpenetrated Zirconium-Based Metal-Organic Framework with High Affinity toward Ammonia.

Flexible metal-organic frameworks (MOFs) are highly attractive porous crystalline materials presenting structural changes when exposed to external stimuli, the mechanism of which is often difficult to glean, owing to their complex and dynamic nature. Herein, a flexible interpenetrated Zr-MOF, NU-1401, composed of rare 4-connected Zr6 nodes and tetratopic naphthalenediimide (NDI)-based carboxylate linkers, was designed. The intra-framework pore opening deformation and inter-framework motions, when subjected to different solvent molecules, were investigated by single-crystal XRD. The distance and overlap angle between the stacked NDI pairs in the entangled structure could be finely tuned, and the interactions between NDI and solvent molecules led to solvochromism. Furthermore, the presence of electron-deficient NDI units in the linker and acid sites on the node of the interpenetrated porous structure offered high density of adsorption sites for ammonia molecules, resulting in high uptake at low pressures.

Synthesis of Poly(bisisoindigo) Using a Metal-Free Aldol Polymerization for Thin-Film Transistor Applications.

Typical syntheses of conjugated polymers rely heavily on organometallic reagents and metal-catalyzed cross-coupling reactions. Here, we show that an environmentally benign aldol polymerization can be used to synthesize poly(bisisoindigo), an analog of polyisoindigo with a ring-fused structural repeat unit. Owing to its extended conjugation length, poly(bisisoindigo) absorbs across the UV/vis/NIR spectrum, with an absorption tail that reaches 1000 nm. Due to the four electron-deficient lactam units on each repeat unit, poly(bisoindigo) possesses a low-lying LUMO, which lies at -3.94 eV relative to vacuum. Incorporation of the ring-fused monomer unit also lowered the overall torsional strain in the polymer backbone (relative to polyisoindigo), and the polymer was successfully used in prototype unipolar n-channel organic thin-film transistors.

Ultra-low-band gap thienoisoindigo-based ambipolar type neutral green copolymers with ProDOT and thiophene units as NIR electrochromic materials

In this paper, thiophene and alkyl substituted propylenedioxythiophene (ProDOT) are taken as the donor units and the alkyl substituted thienoisoindigo (TIIG) unit is adopted as the electron deficient unit. The three donor-acceptor (D-A) type polymers were synthesized via Stille coupling polymerization at the molar feed ratio of 3:2:1, 2:1:1 and 3:1:2 (Thiophene:TIIG:ProDOT), namely as PTIIG-1, PTIIG-2, and PTIIG-3, respectively. The electrochemical band gaps are 0.96, 0.95 and 0.99 eV as the TIIG content decreases. The optical band gaps of the polymers are obtained by the UV–Vis–NIR analysis, and the data are 0.92, 0.94 and 0.98 eV as the TIIG content decreases, suggesting that all polymers belong to the ultra-low-band gap polymers. PTIIG-1 and PTIIG-2 showed neutral green color and oxidized blue color. Polymer PTIIG-3 is an exception, which exhibited a brown color in neutral state and a blue color in oxidized state. At the n-doping state, three polymers exhibited a greyish-white color. As electrochromic polymers, they not only have multicolor properties, but also have excellent kinetic characteristics. Three polymers all exhibited fast response time lower than 1 s in a 4 s time interval, stable optical contrast of 18–25% in 440–870 nm and 58–72% in NIR, large coloration efficiency of 102–254 cm 2 C -1 in 440–870 nm and 404–612 cm 2 C -1 in NIR. The results of electrochemical and optical curves, spectroelectrochemical and electrochromic behaviors showed that the three TIIG-based polymers had potential application in NIR electrochromic areas. • Three low band gap polymers were obtained based on thienoisoindigo, ProDOT and thiophene units. • They all possessed ambipolar property. • Three polymers all exhibited perfect switching performance, especially in the NIR region. • All polymers changed from neutral green to oxidized blue, also showed color changes upon n-doping.

Tunable Conjugated Organoborane Oligomers for Visible-Light-Driven Hydrogen Evolution

A series of linear conjugated organoborane oligomers are synthesized here by introducing electron-deficient borole units to the well-explored thiophene unit of variable length. It is found that the...

Relation between the Electronic Properties of Regioregular Donor-Acceptor Terpolymers and Their Binary Copolymers.

By analyzing the optical band gap and energy levels of seven different regioregular terpolymers in which two different electron-rich donor moieties are alternating with a common electron-deficient acceptor unit along the backbone, we establish a direct correlation with the properties of the corresponding binary copolymers in which one donor and one acceptor are combined. For this study, we use diketopyrrolopyrrole as the common acceptor and different π-conjugated aromatic oligomers as donors. We find that the optical band gap and frontier orbital energies of the terpolymers are the arithmetic average of those of the parent copolymers with remarkable accuracy. The same relationship is also found for the open-circuit voltage of the bulk heterojunction solar cells made with the ter- and copolymers in combination with [6,6]-phenyl-C71-butyric acid methyl ester. Comparison of these findings with data in the literature suggests that this is a universal rule that can be used as a tool when designing new π-conjugated polymers. The experimental results are supported by a semiempirical quantum chemical model that accurately describes the energy levels of the terpolymers after parametrization on the energy levels of the copolymers and also provides a theoretical explanation for the observed arithmetic relations.

Benzothiadiazole functionalized Co-doped MIL-53-NH2 with electron deficient units for enhanced photocatalytic degradation of bisphenol A and ofloxacin under visible light

Nowadays, designing highly active photocatalysts for pollutant degradation under visible light still remains a challenging problem. Herein, a novel benzothiadiazole functionalized Co-doped MOF-based photocatalyst with electron deficient unit was first synthesized via a feasible step-by-step assembly strategy. Benzothiadiazole, as typical electron deficient group, could effectively promote the separation and transfer of photoinduced charge carriers. The implantation of Co ion could be served as an effective mediator to further facilitate the charge transfer through a Co3+/Co2+ redox pathway. Interestingly, the as-synthesized Co-MIL-53-NH-BT exhibited significantly enhanced photocatalytic degradation capacity for BPA and OFX under visible light irradiation, with removal efficiency as high as 99.9 % and 99.8 % within 120 min. TOC analysis suggested that majority of contaminants had been degraded into CO2 and H2O. The important parameters influencing the photocatalytic activity were investigated, and the kinetics study was also conducted. The possible degradation pathways and the possible photocatalytic mechanism were proposed. More importantly, the as-synthesized Co-MIL-53-NH-BT showed good reusability, stability as well as universal applicability. To sum up, current work not only developed an efficient and visible-light active photocatalyst for treating organic-contaminated wastewater, but also afforded some novel insight into the utilization of benzothiadiazole in MOF-based photocatalyst towards improving photocatalytic activity.

In situ growth of benzothiadiazole functionalized UiO-66-NH2 on carboxyl modified g-C3N4 for enhanced photocatalytic degradation of sulfamethoxazole under visible light

A newly-constructed hybrid photocatalyst with electron deficient units and an n–n heterojunction was synthesized to degrade antibiotics under visible light.

Small-bandgap quinoid-based π-conjugated polymers

This review covers the progress in small-bandgap quinoid-based π-conjugated polymers, focusing on a variety of electron-deficient quinoid building units.

Highly Regioselective Direct C-H Arylation: Facile Construction of Symmetrical Dithienophthalimide-Based π-Conjugated Molecules for Optoelectronics.

Controllable direct C-H arylation with high regioselectivity is highly desirable yet remains a formidable challenge. Herein, a facile regioselective direct C-H arylation is developed for efficient construction of a variety of symmetrical dithienophthalimide-based π-conjugated molecules. The resulting methodology is applicable to a wide range of substrates, from electron-rich units to electron-deficient units with large steric end groups. Aryl halides have been confirmed to be able to couple with dithienophthalimide (DTI) via direct C-H arylation, showing high regioselectivity. Varying the functional end groups onto the DTI core has been demonstrated to fine tune the emission colors to cover most of the visible spectra. The results suggest a facile strategy towards highly selective direct C-H arylation, opening the prospects towards efficient construction of π-conjugated molecules for various potential optoelectronic applications.

Molecular origin of photostability for fluorene-based donor–acceptor type photovoltaic polymers

Quantitative analyses of photodegradation for three fluorene-based photovoltaic polymers, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (APFO3), polyfluorene (PFO), and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT), were conducted to understand the molecular origin of photostability for polymers. The Fourier transform infrared spectra of the polymer thin films varied with irradiating white light at 100 mW cm−2 irrespective of their molecular architectures. The absorption peaks corresponding to alkyl side chains in the fluorene unit decreased, whereas those for polymers that did not comprise carbonyl groups increased. This spectral variation indicates that alkyl side chains in the fluorene unit decompose when the molecular structure of fluorene changes to that of fluorenone. The reaction rate constant of the formation of C=O bonds for APFO3 was 1.64 × 10−5 s−1, lower than those for PFO (7.59 × 10−5 s−1) and F8BT (2.64 × 10−5 s−1), under light irradiation at 30 °C. The photostability of the polymers was improved by designing a donor–acceptor type molecular architecture incorporating photostable electron-deficient benzothiadiazole units with photo-unstable fluorene units.

Effects of Halogenation in B ← N Embedded Polymer Acceptors on Performance of All-Polymer Solar Cells.

Halogenation, for example, fluorination and chlorination, is an effective strategy to regulate the performance of organic photovoltaic materials. Although fluorination has been widely applied to polymer acceptors, systematic studies on the comparison of nonhalogenated, fluorinated, and chlorinated polymer acceptors have been a blank to now. Herein, a B ← N embedded electron-deficient unit (A), namely, BNIDT was copolymerized with three electron-rich units (D), that is, benzodithiophene (BDT), fluorinated BDT, and chlorinated BDT to obtain three D-A polymers of BN-BDT, BN-BDT-F, and BN-BDT-Cl, respectively. The three polymers exhibit similar LUMOs of ca. -3.77 eV, whereas the HOMOs are remarkably decreased from BN-BDT (-5.46 eV) to BN-BDT-F (-5.71 eV) and further slightly lowered to BN-BDT-Cl (-5.74 eV). All-polymer solar cells (all-PSCs) were fabricated using PBDB-T as the donor and the three B ← N-based polymers as the acceptors. The efficiencies of all-PSCs were significantly promoted from nonhalogenated BN-BDT (1.60%) to fluorinated BN-BDT-F (3.71%) and further elevated to chlorinated BN-BDT-Cl (4.23%). Device characterizations revealed that halogenation on the polymer acceptors leads to enhanced hole-transfer driving forces and better donor/acceptor miscibility, for example, smaller domain sizes and root-mean-square roughness (rms) values, which further gives rise to higher and more balanced hole/electron mobilities and efficient physical processes, for example, efficient exciton dissociation and collection and weaker recombination losses in halogenated devices. This work demonstrates that the photovoltaic performance of nonhalogenated polymer acceptors can be remarkably boosted by fluorination and further enhanced by chlorination. This is the first systematic study on the halogenated polymer acceptors by comprehensively comparing nonhalogenated, fluorinated, and chlorinated ones.

Wide-Band-Gap Phthalimide-Based D-π-A Polymers for Nonfullerene Organic Solar Cells: The Effect of Conjugated π-Bridge from Thiophene to Thieno[3,2-b]thiophene

Conjugated polymers with D-π-A backbone structures have been intensively investigated and have largely promoted the rapid progress of organic solar cells (OSCs). However, as one of the simplest electron-accepting (A) units, phthalimide (PhI) only attracts less attention to construct promising D-π-A photovoltaic polymers in the OSC community. Thus, the correlations among the chemical structure, optoelectronic properties, and photovoltaic performance need to be systematically investigated. Here, we combined the PhI moiety with the electron-donating (D) unit benzodithiophene (BDT) to synthesize two D-π-A copolymers PE80 and PE81, where the π-bridge corresponds to the thiophene (T) and thieno[3,2-b]thiophene (TT), respectively. When blended with a low-band-gap (Eg = 1.33 eV) nonfullerene acceptor Y6, PE81 achieved a power conversion efficiency (PCE) of 10.21% with an open-circuit voltage (VOC) of 0.90 V, which are much higher than those of the PE80:Y6 device (PCE = 4.11% and VOC = 0.88 V). Our results indicate that PhI is also a promising electron-deficient unit to construct photovoltaic polymers and using a TT π-bridge is a simple strategy to improve the photovoltaic performance of D-π-A polymers.

P3HT-Based Polymer Solar Cells with 8.25% Efficiency Enabled by a Matched Molecular Acceptor and Smart Green-Solvent Processing Technology.

A novel molecular acceptor of TrBTIC (2,7,12-tris((2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile-7-benzothiadiazole-2-)truxene) is designed by attaching the 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile-benzothiadiazole (BTIC) electron-deficient unit to an electron-rich truxene core. TrBTIC has excellent solubility in common solvents and features good energy level matching with poly(3-hexylthiophene) (P3HT). Interestingly, P3HT can be readily dissolved in warm 1,2,4-trimethylbenzene (TMB), a green solvent, but crystallizes slowly with long-term aging in TMB at room temperature. A prephase separation can thus occur before active blend film deposition, and the separation degree can be easily controlled by varying the aging time. After 40 min of aging, the resulting active blend has the most appropriate phase separation with uniform nanowires, which forms favorable interpenetrating networks for exciton dissociation and charge transport. As a result, the device performance is improved from 6.62% to 8.25%. Excitingly, 8.25% is a new record for P3HT-based solar cells. The study not only provides an efficient nonfullerene acceptor for matching P3HT donors but also develops a promising processing technology to realize high-performance P3HT-based polymer solar cells with an efficiency over 8%.

Hybrid Alkyl–Ethylene Glycol Side Chains Enhance Substrate Adhesion and Operational Stability in Accumulation Mode Organic Electrochemical Transistors

Donor-acceptor copolymers featuring electron-deficient isoindigo units and electron-rich 3,4-ethyl-E enedioxy (EDOT) groups are presented as new materials for accumulation mode organic electrochemi ...