Electron-deficient Unit Research Articles

Effects of including electron-withdrawing atoms on the physical and photovoltaic properties of indacenodithieno[3,2-b]thiophene-based donor–acceptor polymers: towards an acceptor design for efficient polymer solar cells

Three new D–A polymers using indacenodithieno[3,2-b]thiophene (IDTT) as an electron-rich unit and benzoxadiazole (BO), benzodiathiazole (BT) or difluorobenzothiadiazole (FBT) as an electron-deficient unit were synthesized and applied in solar cells.

Bis(2-oxo-7-azaindolin-3-ylidene)benzodifuran-dione-based donor–acceptor polymers for high-performance n-type field-effect transistors

Two donor–acceptor (D–A) conjugated polymers were synthesized using a strongly electron-deficient unit as the acceptor, and dithiophene and (E)-2-(2-(thiophen-2-yl)vinyl)thiophene as the donor units.

Electronically tuned sulfonamide-based probes with ultra-sensitivity for Ga3+ or Al3+ detection in aqueous solution

Three electronically tuned fluorescent probes (1–3) were synthesized by conjugating a fluorescent unit to N,N-bis-(hydroxyethyl)ethylenediamine. Probe 1 bearing an electron-deficient naphthalenedimide unit did not give a fluorescence response to the presence of various metal ions including monovalent metal ions (Na+, K+, and Ag+), divalent metal ions (Ca2+, Cd2+, Co2+, Ni2+, Cu2+, Hg2+, Pb2+, and Zn2+) and trivalent metal ions (Al3+, Ga3+, Fe3+, and Cr3+) in an aqueous solution. By contrast, probes 2 and 3 possessing 1,8-naphthalimide and pyrene fluorophores, respectively, exhibited selective fluorescent “OFF-ON” behaviors as a result of Ga3+/Al3+ binding among the diverse metal ions, suggesting the importance of fluorophore electronic character with regard to metal ion sensing. The ethylenediamine analog of probe 3, corresponding to probe 4, was unable to yield a significant change in fluorescence intensity in the presence of any metal ions tested here, revealing the essential role of two hydroxyl groups for metal ion binding. A high association constant of Ka = 2.99 × 105 M−1 was obtained for probe 3 with Ga3+, with a limit of detection (LOD) of 10 nM. This LOD is the lowest value known for Ga3+ detection using chemical sensors. Along with an increase in aggregate sizes, PET suppression of probes upon metal ion binding was the primary contributor to the enhancement in fluorescence emission necessary for the sensitive detection of the target ions. The probe-metal ion complexes were fully characterized via TEM, FE-SEM, 1H NMR, fluorescence spectroscopy techniques and DFT calculations.

Metal-free photosensitizers based on benzodithienothiophene as π-conjugated spacer for dye-sensitized solar cells

A novel series of metal-free organic photosensitizers based on D-π-A or D-A1-π-A structure motif namely, BD-n, (where n = 1–5) bearing a high hole-transporting benzodithienothiophene as the π-bridge for the first time, carbazole or triarylamine as the electron-donor and benzothiadiazole or quinoxaline as the auxiliary electron-deficient unit was synthesized and investigated for DSSC applications. DSSCs based on these five photosensitizers were also fabricated which exhibited efficiencies ranging from 3.92% to 5.46%. Among these DSSCs, devices based on D-A1-π-A type dye, BD-3 and bidenate dye, BD-5 as a photosensitizer afforded the best efficiencies of 5.34%–5.46%. Our results suggested that an incorporation of a proper auxiliary accepting group (A1) into the D-π-A conjugation system to enhance light harvesting and charge transfer as well as the use of V-shaped di-anchoring strategy to suppress aggregation on the TiO2 surface were found to be useful and effective strategies to enhance the photovoltaic performance of a DSSC. These devices exhibited relatively long electron lifetimes suggesting the slow charge recombination kinetics and thus better DSSC performance. Our findings also suggest that benzodithienothiophene is a promising π-bridge to construct organic photosensitizers for high-efficiency DSSCs.

Syntheses and photovoltaic properties of 6-(2-thienyl)-4H-thieno[3,2-b]indole based conjugated polymers containing fluorinated benzothiadiazole

In this report, a series of copolymers based on 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) as an electron-rich unit and fluorinated DTBT as an electron-deficient unit were synthesized, namely PTTIF1 and PTTIF2, and applied to photovoltaic devices. TTI unit was coupled with fluorinated DTBT to utilize the merit of introduction of fluorine atom leading to the lowering of the HOMO energy level while keeping high planarity of the conjugated backbone.The synthesized copolymers show a noticeable change in HOMO energy levels as compared with non-fluorinated polymer (PTTIDTBT-h). Optimized photovoltaic devices of PTTIF2 exhibited power conversion efficiency of 4.36% with decent JSC and FF values, which can be explained by the higher charge transporting ability of PTTIF2 with preferable face-on crystallite population than PTTIF1 in grazing incident wide-angle X-ray scattering (GIWAXS).

Influence of the additional electron-withdrawing unit in β-functionalized porphyrin sensitizers on the photovoltaic performance of dye-sensitized solar cells

The β-functionalized porphyrin containing an additional electron-withdrawing unit, 2,3-diphenylquinoxaline(DPQ) for LP-5 or 2,1,3-benzothiadiazole (BTD) for LP-6 with different electron-withdrawing abilities, between the porphyrin core and the anchoring group and the reference porphyrin dye (LP-4) have been designed and synthesized for DSCs. The influence of the additional electron-withdrawing units on molecular properties as well as photovoltaic performance of the corresponding DSCs was investigated systematically. Compared with LP-4, the introduction of additional electron-deficient unit at the porphyrin β π-linker in LP-5 and LP-6 decreases the lowest unoccupied molecular orbital (LUMO) energy levels, resulting in the broader absorption spectra and significantly improved IPCE spectra in the region 350–500 nm, which ensures the better light-harvesting properties and the higher short-circuit current density (Jsc). On the other hand, the introduction of additional acceptors of LP-5 and LP-6 induces dye aggregation and reduces the lifetime of the charge–separated states, which decreases the open–circuit voltage (Voc). Interestingly, the loss in Voc is overcompensated by the improvement in Jsc. The study provides not only an alternative approach to design novel porphyrin sensitizers, but also an insight into how to manipulate the LUMO energy levels of porphyrin sensitizers via the β-linker modifications for the optimal photovoltaic applications.

Self-quenched semiconducting polymer nanoparticles for amplified invivo photoacoustic imaging.

Development of photoacoustic (PA) imaging agents provides opportunities for advancing PA imaging in fundamental biology and medicine. Despite the promise of semiconducting polymer nanoparticles (SPNs) for PA imaging, the molecular guidelines to enhance their imaging performance are limited. In this study, semiconducting polymers (SPs) with self-quenched fluorescence are synthesized and transformed into SPNs for amplified PA imaging in living mice. The self-quenched process is induced by the incorporation of an electron-deficient structure unit into the backbone of SPs, which in turn promotes the nonradiative decay and enhances the heat generation. Such a simple chemical alteration of SP eventually leads to 1.7-fold PA amplification for the corresponding SPN. By virtue of the targeting capability of cyclic-RGD, the amplified SPN can effectively delineate tumor in living mice and increase the PA intensity of tumor by 4.7-fold after systemic administration. Our study thus provides an effective molecular guideline to amplify the PA brightness of organic imaging agents for invivo PA imaging.

Synthesis and properties of thiophene- and quinoxaline-based random copolymers for organic photovoltaics

In this study, a series of polymers (denoted as PTQxFs) were prepared from random copolymers—poly[(5,8-(6,7-difluoro-2,3-dihexylquinoxaline))-alt-(2,5-thiophene)]-ran-[(2,5-(3-dodecylthiophene))-alt-(2,5-thiophene)]—via Pd(0)-catalyzed Stille polymerization using thiophene as the electron-rich unit and 6,7-difluoro-2,3-dihexylquinoxaline as an electron-deficient unit, which is newly synthesized by our group. This series of polymers were utilized for organic photovoltaics, and their absorption spectra, energy levels, and photovoltaic properties were investigated. The two fluorine atoms in the 6,7-difluoro-2,3-dihexylquinoxaline unit, exhibiting strong electron-withdrawing properties, enhanced the open-circuit voltage of solar cell devices due to the decrease of HOMO energy levels. With increasing number of 6,7-difluoro-2,3-dihexylquinoxaline units in the random copolymers, the HOMO energy levels decreased from −5.33 eV for PTQxF1 to −5.50 eV for PTQxF9, resulting in increased open-circuit voltage.

Solution and Solid-State Studies on the Halide Binding Affinity of Perfluorophenyl-Armed Uranyl-Salophen Receptors Enhanced by Anion-π Interactions.

The enhancement of the binding between halide anions and a Lewis acidic uranyl-salophen receptor has been achieved by the introduction of pendant electron-deficient arene units into the receptor skeleton. The association and the occurrence of the elusive anion-π interaction with halide anions (as tetrabutylammonium salts) have been demonstrated in solution and in the solid state, providing unambiguous evidence on the interplay of the concerted interactions responsible for the anion binding.

Covalent Functionalization of GaP(110) Surfaces via a Staudinger-Type Reaction with Perfluorophenyl Azide

Despite the markedly low chemical reactivity of the nonpolar (110) surfaces of III–V semiconductors, the covalent functionalization of GaP(110) surfaces with perfluorophenyl azide (PFPA) molecules by a Staudinger-type reaction occurs only slightly above room temperature (325 K). Scanning tunneling microscopy observations, combined with density functional theory calculations, support the formation of stable, covalent perfluorophenyl nitride (PFPN) molecule–surface bonds, which can be described as Lewis acidic Ga-stabilized phosphine imides. π–π stacking between aromatic, electron-deficient PFPN units results in compact, commensurate 2D molecular assembly at the surface. PFPA deposition on GaP(110) at room temperature with no additional annealing leads to an intermediate phase consistent with an alternating 1D array of physisorbed and chemisorbed molecular units. This work provides a new route for covalently bonding molecular linkages to the (110) surfaces of III–V semiconductors.

Photovoltaic Small Molecules of TPA(FxBT-T-Cz)3: Tuning Open-Circuit Voltage over 1.0 V for Their Organic Solar Cells by Increasing Fluorine Substitution.

To simultaneously improve both open-circuit voltage (Voc) and short-circuit current density (Jsc) for organic solar cells, a novel D(A-π-Ar)3 type of photovoltaic small molecules of TPA(FxBT-T-3Cz)3 was designed and synthesized, which contain central triphenylamine (TPA), terminal carbazole (Cz), armed fluorine-substituted benzothiadiazole (FxBT, where x = 1 or 2), and bridged thiophene (T) units. A narrowed ultraviolet-visible absorption and a decreasing highest occupied molecular orbital energy level were observed from TPA(F1BT-T-3Cz)3 to TPA(F2BT-T-3Cz)3 with increasing fluorine substitution. However, the TPA(F2BT-T-3Cz)3/PC71BM-based solar devices showed a rising Voc of 1.01 V and an enhanced Jsc of 10.84 mA cm-2 as well as a comparable power conversion efficiency of 4.81% in comparison to the TPA(F1BT-T-3Cz)3/PC71BM-based devices. Furthermore, in comparison to the parent TPA(BT-T-3Cz)3 molecule without fluorine substitution, the fluorine-substituted TPA(FxBT-T-3Cz)3 molecules exhibited significantly incremental Voc and Jsc values in their bulk heterojunction organic solar cells, owing to fluorine incorporation in the electron-deficient benzothiadiazole unit.

Strategy for Improved Photoconversion Efficiency in Thin Photoelectrode Films by Controlling π-Spacer Dihedral Angle

Benzo[c][1,2,5]thiadiazole (BT) has been used in dye-sensitized solar cells (DSCs) for its light-harvesting abilities. However, as a strongly electron deficient unit, BT causes rapid back electron transfer (BET), which in turn lowers the photoconversion efficiency (PCE) of devices. Herein, we report a powerful strategy for retarding BET by controlling both the photoelectrode thickness and π-spacer dihedral angle. To achieve this, we introduced planar (BT-T) or twisted π-spacers (BT-P, BT-MP, and BT-HT) between BT units and anchoring groups and used different photoelectrode thicknesses between 1.8 and 10 μm. Computational and experimental results show that twisted π-spacers were more efficient at retarding BET than the planar π-spacer. However, BET was found to be less important than expected, and light harvesting efficiency (LHE) played a critical role as the thickness of the photoelectrode decreased because charge collection efficiency was enhanced. The planar dye BT-T obtained the highest LHE, this valu...

Comparison of p-type sensitizers with different electron-induced effects in dye-sensitized solar cells: A theoretical investigation

Two classical p-type photosensitizers with modified conjugation spacers by the electron-rich unit (thiophene) and electron-deficient unit (pyrimidine) are calculated and investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). Creatively, regeneration reactions between reduced sensitizers and I3− as well as I2− are explored theoretically, reappearing the regeneration process that electrons are transferred from reduced dyes to I3− or I2− in p-type dye-sensitized solar cells (p-type DSSCs) intuitively, and uncovering the difference of π-spacers with different electron-induced effects in the process of dye regeneration. Besides, calculation also indicates that thiophene-based dye (PS) has better light capture efficiency but unfavorable performance of charge transfer when compared with the pyrimidine-based dye (PN). From another perspective, both sensitizers have efficient driving forces of injection. In addition, dyes absorbing on the (NiO)9 semiconductor are also considered. Our comparison discussion of two sensitizers with different π-spacers provides great assistance for deeply investigating p-type photosensitizes with higher efficiencies and their regeneration processes, contributing to the potential tandem DSSCs.

Syntheses of PCDTBT containing tetrafluorobenzene as electron-withdrawing group with deep HOMO energy level and applications for photovoltaics

Abstract Conjugated polymers using tetrafluorophenylene unit were synthesized and deployed for photovoltaics. The tetrafluorophenylene unit which has the strong electron-withdrawing ability, was utilized for the copolymers of push-pull type with low bandgaps for organic solar cells. 9H-Carbazole as electron-rich unit and di-2-thienyl-2,1,3-benzothiadiazole (DTBT) or tetrafluorophenylene unit as electron-deficient unit were utilized for the syntheses of donor-acceptor (D-A) types of conjugated polymers with different ratios of tetrafluorophenylene unit (PCDTBTF-1, PCDTBTF-3 and PCDTBTF-10). By introduction of tetrafluorophenylene unit in the backbone of PCDTBT, the energy level of the highest occupied molecular orbital (HOMO) and the bandgap were decreased as compared to those of PCDTBT. The higher amount of tetrafluorophenylene unit, the lower energy levels of the HOMO of the polymers were observed. The device of PCDTBTF-1:PC71BM (1:1) with additive of diphenyl ether (DPE) showed an open-circuit voltage (VOC) of 0.82 V, a short circuit current (JSC) of 9.66 mA/cm2, and a fill factor (FF) of 0.44, which yields PCE of 3.44%.

Synthesis, characterization and photovoltaic properties of low band gap donor-acceptor polymers containing benzodithiophene donor unit with fluorenylthiophene as 2D-conjugated side for organic solar cell application

ABSTRACTTwo donor−acceptor low band gap polymers P1 (octyl as solubilizing group) and P2 (ethylhexyl as solubilizing group) containing fluorenylthiophene-substituted benzoditihiophene as an electron-rich unit and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione as an electron-deficient unit are designed and synthesized for polymer solar cells application. Compared with P2 based on ethyl hexyl group, P1 with octyl group displays well resolved vibronic shoulder peak in absorption spectra, stronger intermolecular interactions, and higher hole mobility. Polymer solar cells based on P1 and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) exhibit a maximum power conversion efficiency of 1.78% under AM 1.5G illumination (100 mW/cm2).

Influence of donor-acceptor conjugation between thiophene-phthalazinone structures containing Sp3 CN bond on the frontier orbital levels and optic-electronic properties

Herein, an electron-deficient phthalazinone unit containing a unique Sp3 hybrid nitrogen atom as an acceptor to cut off the ether bond was presented and three Donor–Acceptor (D–A) conjugated fluoropolymers via Classical Aromatic Nucleophilic Displacement Polymerization (CANDP) method were successfully synthesized. These polymers exhibit excellent thermostability. The 5% weight loss temperature of PDT2TF under nitrogen atmosphere is high, up to 470 °C. This is due to that the D–A conjugation between thiophene-phthalazinone units affects its resonance energy and stabilizes the thiophene-phthalazinone structures. These three fluoropolymers show strong absorptions and fluorescence in visible light region both in solution and thin film states. The band gaps (Eg) of these polymers are narrower than that of their corresponding di-NH capped monomers, owing to the good electronic communication properties of the Sp3 nitrogen atom in the phthalazinone unit. The Eg value of PDT3TF is decreased to 2.03 eV. These results indicate that phthalazinone unit is an efficient acceptor and could exhibit strong D–A effect with thiophene unit. Also, the Sp3 nitrogen atom in the phthalazinone shows good electronic wave function and charge transport properties. This facile CANDP synthetic method combined with Sp3 CN bond linked electron-deficient phthalazinone unit affords polymers with controllable photoelectric properties. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 3470–3483

Annulated Thienyl-Vinylene-Thienyl Building Blocks for π-Conjugated Copolymers: Ring Dimensions and Isomeric Structure Effects on π-Conjugation Length and Charge Transport

A series of annulated thienyl-vinylene-thienyl (ATVT) building blocks having varied ring sizes, isomeric structures, and substituents was synthesized and characterized by spectroscopic, electrochemical, quantum chemical, and crystallographic methods. It is found that ATVT ring size and isomeric structure critically affect the planarity, structural rigidity, optical absorption, and redox properties of these new π-units. Various solubilizing substituents can be introduced on the annulated hydrocarbon fragments, preserving the ATVT planarity and redox properties. The corresponding π-conjugated copolymers comprising ATVT units and electron-deficient units were also synthesized and characterized. The solubility, redox properties, and carrier transport behavior of these copolymers also depend remarkably on the annulated ring size and the ATVT unit isomeric structure. One of the copolymers composed of an ATVT with five-membered rings (1), (E)-4,4′,5,5′-tetrahydro-6,6′-bi(cyclopenta[b]thiophenylidene), and a naph...

Very Small Bandgap π-Conjugated Polymers with Extended Thienoquinoids.

The introduction of quinoidal character to π-conjugated polymers is one of the effective approaches to reducing the bandgap. Here we synthesized new π-conjugated polymers (PBTD4T and PBDTD4T) incorporating thienoquinoids 2,2'-bithiophene-5,5'-dione (BTD) and benzo[1,2-b:4,5-b']dithiophene-2,6-dione (BDTD) as strong electron-deficient (acceptor) units. PBTD4T showed a deep LUMO energy level of -3.77 eV and a small bandgap of 1.28 eV, which are similar to those of the analog using thieno[3,2-b]thiophene-2,5-dione (TTD) (PTTD4T). PBDTD4T had a much deeper LUMO energy level of -4.04 eV and a significantly smaller bandgap of 0.88 eV compared to those of the other two polymers. Interestingly, PBDTD4T showed high transparency in the visible region. The very small bandgap of PBDTD4T can be rationalized by the enhanced contribution of the resonance backbone structure in which the p-benzoquinodimethane skeleton in the BDTD unit plays a crucial role. PBTD4T and PBDTD4T exhibited ambipolar charge transport with more balanced mobilities between the hole and the electron than PTTD4T. We believe that the very small bandgap, i.e., the high near-infrared activity, as well as the well-balanced ambipolar property of the π-conjugated polymers based on these units would be of particular interest in the fabrication of next-generation organic devices.

Improved photovoltaic properties of the copolymers based on diketopyrrolopyrrole with broad absorption and high open-circuit voltage

A series of donor–acceptor conjugated polymers based on the alkylthienyl-benzodithiophene (BDTT) as an electron-rich unit, and diketopyrrolopyrrole (DPP) incorporated in backbone or side chain as electron-deficient units have been synthesized and applied in polymer solar cells. Compared with the binary copolymers, random terpolymers exhibit a broader absorption range. By controlling the ratio of the electron-deficient units, both photophysical, electrochemical properties, lamellar distance, π−π* stacking distance and the photovoltaic properties of the random terpolymers were changed dramatically. Bulk heterojunction polymer solar cells based on the copolymers as the electron donors and (6,6)-phenyl-C61-butyric acid methyl ester as the acceptor were fabricated. The power conversion efficiency of 4.46% was obtained from the terpolymer (PBDTT-DPP-TTDPP). Optimization via solution vapor annealing, the binary copolymer (PBDTT-TTDPP) showed the best power conversion efficiency of 4.74% with a short-circuit current of 10.63 mA cm−2, a high open-circuit voltage of 0.88 V and a fill factor of 51.0%.

Synthesis and photovoltaic properties of benzimidazole-based copolymer with fluorine atom

A new accepter unit, 5,6-difluoro-2,2-dimethyl-2H-benzo[d]imidazole, was prepared and utilized for the synthesis of the conjugated polymers containing electron donor–acceptor pair for OPVs. Difluoro-dimethyl-2H-benzoimidazole unit was designed to introduce electron-withdrawing F atom in dimethyl-2H-benzimidazole unit to improve deeper HOMO and higher V OC of the polymer. New semiconducting copolymer with dioctyloxybenzodithiophene, as the electron-rich unit, and difluoro-dimethyl-2H-benzoimidazole, as the electron-deficient unit, was synthesized by Stille polymerization.