Electron-deficient Groups Research Articles

Unsymmetrical squaraine dye containing dithieno[3,2-b:2′,3′-d]pyrrole as a π-spacer: A potential photosensitizer for dye-sensitized solar cells

Designed with electron-rich and electron-deficient groups as π-linkers, a series of novel unsymmetrical squaraine dyes (SQ) for dye-sensitized solar cells (DSSCs) have been simulated using density functional theory (DFT) and time-dependent DFT (TDDFT) methods. Dye/TiO2 complexes were also calculated to explore various properties on the interface. The results show that, compared with the prototype dye of WCH-SQ11, the modified WCH-SQ11-1, obtained by replacing the 3,4-ethylenedioxythiophene (EDOT) unit in WCH-SQ11 with dithieno[3,2-b:2′,3′-d]pyrrole (DTP), keeps a good balance in various key properties including light harvesting efficiency (LHE), driving force for electron injection (ΔGinject), charge transfer (CT) character, and vertical dipole moment (μnormal). Further investigation of the adsorbed WCH-SQ11-1 (i.e. the WCH-SQ11-1/(TiO2)38 complex) reveals a red-shifted absorption spectrum compared to the free dye. The analysis of the molecular orbitals responsible for the maximum absorption indicate a direct electron injection mechanism for the WCH-SQ11-1/(TiO2)38 system.

Molecular design toward highly efficient photovoltaic polymers based on two-dimensional conjugated benzodithiophene.

As researchers continue to develop new organic materials for solar cells, benzo[1,2-b:4,5-b']dithiophene (BDT)-based polymers have come to the fore. To improve the photovoltaic properties of BDT-based polymers, researchers have developed and applied various strategies leading to the successful molecular design of highly efficient photovoltaic polymers. Novel polymer materials composed of two-dimensional conjugated BDT (2D-conjugated BDT) have boosted the power conversion efficiency of polymer solar cells (PSCs) to levels that exceed 9%. In this Account, we summarize recent progress related to the design and synthesis of 2D-conjugated BDT-based polymers and discuss their applications in highly efficient photovoltaic devices. We introduce the basic considerations for the construction of 2D-conjugated BDT-based polymers and systematic molecular design guidelines. For example, simply modifying an alkoxyl-substituted BDT to form an alkylthienyl-substituted BDT can improve the polymer hole mobilities substantially with little effect on their molecular energy level. Secondly, the addition of a variety of chemical moieties to the polymer can produce a 2D-conjugated BDT unit with more functions. For example, the introduction of a conjugated side chain with electron deficient groups (such as para-alkyl-phenyl, meta-alkoxyl-phenyl, and 2-alkyl-3-fluoro-thienyl) allowed us to modulate the molecular energy levels of 2D-conjugated BDT-based polymers. Through the rational design of BDT analogues such as dithienobenzodithiophene (DTBDT) or the insertion of larger π bridges, we can tune the backbone conformations of these polymers and modulate their photovoltaic properties. We also discuss the influence of 2D-conjugated BDT on polymer morphology and the blends of these polymers with phenyl-C61 (or C71)-butyric acid methyl ester (PCBM). Finally, we summarize the various applications of the 2D-conjugated BDT-based polymers in highly efficient PSC devices. Overall, this Account correlates the molecular structures of the 2D-conjugated BDT-based polymers with their photovoltaic properties. As a result, this Account can guide the molecular design of organic photovoltaic materials and the development of organic materials for other types of optoelectronic devices.

Copper-catalyzed N-arylation of 2-arylindoles with aryl halides

10 mol% CuI combined with the DMEDA ligand can efficiently catalyze the N-arylation of 2-arylindoles with aryl iodides and aryl bromides in good to excellent yields. The aryl halides bearing electron-rich or electron-deficient functional groups can be well tolerated under this mild reaction conditions.

Conjugated donor-acceptor copolymers from dicyanated naphthalene diimide

A series of donor–acceptor (D–A) alternating copolymers including P(2CNNDI-T), P(2CNNDI-TT), P(2CNNDI-BT), and P(2CNNDI-BDT) based on dicyanated naphthalene diimide (2CN-NDI) in backbones were presented and characterized in this contribution. Their structures were unambiguously identified by NMR spectra. Optical and electrochemical measurements revealed that their LUMO energy levels are substantially lowered at about −4.4 eV by the substitution of electron-deficient cyano groups on the backbone of as-synthesized polymers. And the HOMO levels varied from −6.15 eV to −5.70 eV as a reflection of electron-donating ability of combined thiophene derivatives. The moderate electron-transporting characteristics under ambient conditions based on these dicyanated polymers indicated their promising applications as air-stable electron transporting semiconductors in future electronics.

α‐Halogenoacetanilides as Hydrogen‐Bonding Organocatalysts that Activate Carbonyl Bonds: Fluorine versus Chlorine and Bromine

α-Halogenoacetanilides (X=F, Cl, Br) were examined as H-bonding organocatalysts designed for the double activation of CO bonds through NH and CH donor groups. Depending on the halide substituents, the double H-bond involved a nonconventional CH⋅⋅⋅O interaction with either a HCXn (n=1-2, X=Cl, Br) or a HCAr bond (X=F), as shown in the solid-state crystal structures and by molecular modeling. In addition, the catalytic properties of α-halogenoacetanilides were evaluated in the ring-opening polymerization of lactide, in the presence of a tertiary amine as cocatalyst. The α-dichloro- and α-dibromoacetanilides containing electron-deficient aromatic groups afforded the most attractive double H-bonding properties towards CO bonds, with a NH⋅⋅⋅O⋅⋅⋅HCX2 interaction.

Development of Polymer Acceptors for Organic Photovoltaic Cells

This review provides a current status report of the various n-type polymer acceptors for use as active materials in organic photovoltaic cells (OPVs). The polymer acceptors are divided into four categories. The first section of this review focuses on rylene diimide-based polymers, including perylene diimide, naphthalene diimide, and dithienocoronene diimide-based polymers. The high electron mobility and good stability of rylene diimides make them suitable for use as polymer acceptors in OPVs. The second section deals with fluorene and benzothiadiazole-based polymers such as poly(9,9’-dioctylfluorene-co-benzothiadiazole), and the ensuing section focuses on the cyano-substituted polymer acceptors. Cyano-poly(phenylenevinylene) and poly(3-cyano-4-hexylthiophene) have been used as acceptors in OPVs and exhibit high electron affinity arising from the electron-withdrawing cyano groups in the vinylene group of poly(phenylenevinylene) or the thiophene ring of polythiophene. Lastly, a number of other electron-deficient groups such as thiazole, diketopyrrolopyrrole, and oxadiazole have also been introduced onto polymer backbones to induce n-type characteristics in the polymer. Since the first report on all-polymer solar cells in 1995, the best power conversion efficiency obtained with these devices to date has been 3.45%. The overall trend in the development of n-type polymer acceptors is presented in this review.

Design, Synthesis, and Structure–Property Relationships of Isoindigo-Based Conjugated Polymers

Conjugated polymers have developed rapidly due to their promising applications in low-cost, lightweight, and flexible electronics. The development of the third-generation donor-acceptor (D-A) polymers greatly improved the device performance in organic solar cells (OSCs) and field-effect transistors (FETs). However, for further improvement of device performance, scientists need to develop new building blocks, in particular electron-deficient aromatics, and gain an in-depth understanding of the structure-property relationships. Recently, isoindigo has been used as a new acceptor of D-A conjugated polymers. An isomer of indigo, isoindigo is a less well-known dye and can be isolated as a by-product from certain biological processes. It has two lactam rings and exhibits strong electron-withdrawing character. This electron deficiency gives isoindigo-based polymers intriguing properties, such as broad absorption and high open circuit voltage in OSCs, as well as high mobility and good ambient stability in FETs. In this Account, we review our recent progress on the design, synthesis, and structure-property relationship study of isoindigo-based polymers for FETs. Starting with some discussion on carrier transport in polymer films, we provide some basic strategies towards high-performance polymer FETs. We discuss the stability issue of devices, the impediment of the alkyl side chains, and the choice of the donor part of conjugated polymers. We demonstrate that introducing the isoindigo core effectively lowers the HOMO levels of polymers and provides FETs with long-time stability. In addition, we have found that when we use inappropriate alkyl side chains or non-centrosymmetric donors, the device performance of isoindigo polymers suffers. To further improve device performance and ambient stability, we propose several design strategies, such as using farther branched alkyl chains, modulating polymer energy levels, and extending π-conjugated backbones. We have found that using farther branched alkyl chains can effectively decrease interchain π-π stacking distance and improve carrier mobility. When we introduce electron-deficient functional groups on the isoindigo core, the LUMO levels of the polymers markedly decrease, which significantly improves the electron mobility and device stability. In addition, we present a new polymer system called BDOPV, which is based on the concept of π-extended isoindigo. By application of some strategies successfully used in isoindigo-based polymers, BDOPV-based polymers exhibit high mobility and good stability both in n-type and in ambipolar FETs. We believe that a synergy of molecular engineering strategies towards the isoindigo core, donor units, and side chains may further improve the performance and broaden the application of isoindigo-based polymers.

Synthesis and characterization of low band gap thiadiazolobenzo-imidazole chromophores

Thiadiazolobenzoimidazole (TBI) acceptor was designed in order to increase the electron-withdrawing ability of 2,1,3-benzothiadiazole (BT). The key feature of TBI is the three electron deficient imine groups, rendering it a stronger acceptor than BT. Three TBI-containing compounds TBI-a to TBI-c were synthesized by reaction between 5,6-diamino-2,1,3-benzothiadiazole analogue and the corresponding anhydride and exhibit maximum absorption at 600nm. The band gaps of TBI-a to TBI-c are in between that of BT-containing compounds and benzo(1,2-c:4,5-c′)bis[1,2,5]thiadiazole-containing compounds, ranging from 1.60 to 1.88eV, which make them potentially useful for organic solar cell application. Moreover, all the compounds are electrochromic and become absorbing at the telecommunication wavelengths upon electrochemical switching.

Photochemical Generation of 9H‐Fluorenyl Radicals

A series of 9-substituted fluorenols and 9,9'-disubstituted-9,9'-bifluorenyls were irradiated to give products derived from fluorenyl radicals. Product distribution was solvent dependent. A TEMPO adduct was isolated from the photoexcitation of 9-fluorenol. An unusual unsymmetrical 3,9'-bifluorenyl was observed from the photolysis of 9-trifluoromethylfluorenol and 9,9'-di(trifluoromethyl)-9,9'-bifluorenyl in more polar or hydrogen-bonding solvents. The electronic nature of 9-substituted fluorenyl radicals was probed using theoretical calculations showing the dipolar character of species with electron-deficient groups. These constitute the first examples of "doubly destabilized" radicals.

Solid state anion–π interactions involving polyhalides

The stabilization of polyhalides in the solid state with the support of electron-deficient pentafluorophenyl groups is described. Furthermore, a synthetic approach towards the sensitive tetraiodide dianion is described and ESI mass spectrometric evidence for its presence in solution is reported.

Effects of π-conjugated side chains on properties and performances of photovoltaic copolymers

Poly[(benzodithiophene-2,6-diyl)(2,5-thienylene)] (P1) and its derivatives with π-conjugated side-chains (P2: thienylethenyl and P3: thienylcyanoethenyl) were designed and synthesized in order to study the effects of the conjugated side-chains on the optical, electrochemical, and photovoltaic properties of the copolymers. It was found that the electronic properties and energy levels of the copolymers can be effectively tuned through changing a conjugated side-chain attached to a polymer backbone. Polymer solar cells based on copolymer/PC61BM exhibited power conversion efficiencies of 2.85%, 3.44%, and 4.49%, respectively, for P1, P2, and P3 under a simulated solar light (AM 1.5G, 100mW/cm2), compared with 4.17% for a main-chain type copolymer corresponding to P3. The results reveal that the introduction of the conjugated side-chain with an electron-deficient group to a polymer backbone is an effective approach for improving the performance of the photovoltaic materials.

Synthesis of Zwitterionic Group 14 Centered Complexes: Traditional Coordination and Unusual Insertion Chemistry

Novel cationic E–Cl (E = Ge, Sn) fragments stabilized by a bis(phosphino)borate ligand (2E) were synthesized by a 1:1 stoichiometric addition of ECl2 and [Tl][(Ph2PCH2)2BPh2]. The metrical parameters are consistent with dative bonds between the phosphorus atoms and the electron-deficient group 14 element, which is in contrast to the traditionally used aryl- and nitrogen-based ligands, which are always covalently bound. The reaction of a second equivalent of bis(phosphino)borate results in the unexpected insertion of the main group center into the aliphatic B–C bond of the ligand backbone to form 3E, in addition to phosphine-borane dimer (Ph2PCH2BPh2)2 (4). The pendant phosphine on 3E was shown to possess donor ability in the coordination of BH3 (5E).

Perfluorophenylcalix[4]arenes: prospective hosts for nucleophilic guests. Synthesis, structure and quantum chemical calculations

Abstract Calixarenes substituted at the upper rim by electron-deficient pentafluorophenyl groups have been synthesized by reaction of hexafluorobenzene with organolithium derivatives of calixarenes. The single crystal structures indicate stacking interactions between the two perfluorinated aromatic rings. The thermodynamic characteristics of the flattened cone-flattened cone inversion process have been studied using dynamic NMR and quantum chemical methods. The calculated activation free energy for tetra(pentafluorophenyl)-substituted species is higher than that derived for tetraphenyl calixarene. The calixarenes substituted at the upper rim by electron-deficient pentafluorophenyl groups are potential hosts for anion recognition.

Direct (Hetero)Arylation: A New Tool for Polymer Chemists

The coupling of aryl halides with catalytically activated aryl C-H bonds provides a desirable and atom-economical alternative to standard cross-coupling reactions for the construction of new C-C bonds. The reaction, termed direct (hetero)arylation, is believed to follow a base-assisted, concerted metalation-deprotonation (CMD) pathway. During this process, carboxylate or carbonate anions coordinate to the metal center, typically palladium, in situ and assist in the deprotonation transition state. Researchers have employed this methodology with numerous arenes and heteroarenes, including substituted benzenes, perfluorinated benzenes, and thiophenes. Thiophene substrates have demonstrated high reactivity toward C-H bond activation when appropriately substituted with electron-rich and/or electron-deficient groups. Because of the pervasive use of thiophenes in materials for organic electronics, researchers have used this chemistry to modularly prepare conjugated small molecules and, more recently, conjugated polymers. Although optimization of reaction conditions such as solvent system, phosphine ligand, carboxylate additives, temperature, and time is necessary for efficient C-H bond reactivity of each monomer, direct (hetero)arylation polymerization (DHAP) can afford high yielding polymeric materials with elevated molecular weights. The properties of these materials often rival those of polymers prepared by traditional methods. Moreover, DHAP provides a facile means for the synthesis of polymers that were previously inaccessible or difficult to prepare due to the instability of organometallic monomers. The major downfall of direct (hetero)arylation, however, is the lack of C-H bond selectivity, particularly for thiophene substrates, which can result in cross-linked material during polymerization reactions. Further fine-tuning of reaction conditions such as temperature and reaction time may suppress these unwanted side reactions. Alternatively, new monomers can be designed where other reactive bonds are blocked, either sterically or by substitution with unreactive alkyl or halogen groups. In this Account, we illustrate these methods and present examples of DHAP reactions that involve the preparation of common homopolymers used in organic electronics (P3HT, PEDOT, PProDOT), copolymers formed by activation of electron-rich (bithiophene, fused bithiophenes) and electron-deficient monomers (TPD, 1,2,4,5-tetrafluorobenzene, 2,2'-bithiazole). Our group is optimizing these reactions and developing ways to make DHAP a common atom-economical synthetic tool for polymer chemists.

Benzodifuran-alt-thienothiophene based low band gap copolymers: substituent effects on their molecular energy levels and photovoltaic properties

A series of low band gap photovoltaic copolymers containing benzodifuran-alt-thieno[3,4-b]thiophene (BDF-alt-TT) were designed and synthesized. Functional groups with different electron-withdrawing abilities such as carbonyl, ester, fluorine, sulfuryl and cyano were introduced to the thieno[3,4-b]thiophene unit to finely tune the molecular energy levels of these copolymers. All these copolymers are characterized by UV-Vis spectroscopy and electrochemical cyclic voltammetry (CV) to evaluate the effects of these electron-deficient functional groups. According to their electrochemical behaviours, the electron-deficient abilities of these functional groups can be arranged in the order carbonyl < fluorine + ester < fluorine + carbonyl < sulfuryl < cyano + ester. It was found the sulfuryl group and cyano substituted copolymers show blue-shift in absorption spectra compared to the other three copolymers, indicating that sulfuryl and cyano groups may cause stronger steric hindrance to the conjugated backbones. The photovoltaic properties of these copolymers were also investigated by making polymer solar cell (PSC) devices. Interestingly, although the copolymers have different molecular structures, the optimal D/A (polymer/PC71BM, wt/wt) ratios of all of the devices are 1 : 1.5. 1,8-Diiodooctane (DIO) was used to further improve the photovoltaic performance of the devices. The champion efficiency of 5.23% with an open circuit voltage (Voc) of 0.63 V was achieved from PBDFTT-CF/PC71BM based devices. The device based on the cyano-substituted copolymer, PBDFTT-ECN, shows a rather high Voc of 0.87 V, but the short current density (Jsc) is significant lower than the device based on PBDFTT-CF, and its seems that the offset between the LUMO levels of PBDFTT-ECN and PC71BM is too small to achieve efficient charge separation. In conclusion, the molecular energy level of the copolymers with the alternating aromatic–quinoid (A–Q) structure, BDF-alt-TT, can be tuned effectively by introducing electron-deficient groups. Furthermore, according to the photovoltaic results of PBDFTT-S and PBDFTT-CNF, a LUMO level of −3.70 eV may reach the threshold of getting efficient charge separation of the polymer/PC71BM blend.

Dithia[3.3]paracyclophane-based monometal ruthenium acetylide complexes: synthesis, characterization and substituent effects

A series of dithia[3.3]metaparacyclophane-based monometal ruthenium acetylide complexes have been synthesized. All of the complexes have been fully characterised by NMR spectrometry, X-ray crystallography and elemental analyses. At the same time, their basic optical properties, such as UV/Vis absorption spectra, and electrochemical properties have been determined. (1)H NMR and X-ray crystal structure studies reveal that there are intramolecular C-H···π interactions in these ruthenium complexes, in both solution and solid states. Electrochemical studies reveal that the substituted groups on the dithia[3.3]paracyclophane ring can clearly affect the oxidation activities of the ruthenium center by way of the intramolecular C-H···π interaction. In addition, electron-donating groups facilitate the oxidation of the ruthenium center compared with electron-deficient groups. UV/Vis absorption and IR spectra of some complexes in neutral and oxidized states also have been studied. IR spectra studies indicated that the substituents in the cyclophane have some effects on the ν(C≡C) bands of these complexes in the neutral state and little effect on ν(C≡C) of these complexes in the oxidized state.

CuI/TBAB as a novel efficient catalytic system for Heck reaction in water

A novel efficient ligand-free CuI/TBAB catalytic system has been developed for Heck reaction in water without the protection of an inert atmosphere. Both aryl iodides and aryl bromides containing electron-deficient and electron-donating groups reacted smoothly and afforded the corresponding products in moderate to good yields.

Adjustment of charge trap number and depth in molecular backbone to achieve tunable multilevel data storage performance

A series of conjugated donor–acceptor (D–A) molecules with the electron-rich unit triphenylamine (TPA) as donor and azobenzene chromophore and/or cyano group as acceptors were successfully synthesized. Acceptors varying in number and electron-withdrawing strength were incorporated into the molecular backbone to investigate the impact on device switching behavior. It is found that the data storage states and memory effects are highly dependent on the number and strength of the electron-deficient groups. The devices based on the D–π–A1–π–A2 molecule with two electron-withdrawing groups exhibited excellent ternary memory behavior, while those based on the D–π–A1 or D–π–A2 molecule containing one electron-accepting moiety showed binary memory characteristics. In addition, the bistable memory effects of TPAVC 2 and TPAAH 3-based data storage devices varied from WORM to flash when the acceptor changed from a cyano group to an azobenzene chromophore. Therefore, tunable multilevel memory device performance has been achieved by adjusting the number and electron-withdrawing strength of acceptor moieties in the molecular backbone, offering guidance for the rational design of superior D–A molecules for high density data storage (HDDS) applications.

Synthesis and Photovoltaic Properties of a Polythiophene Derivative with Triphenylamine-Vinylene Conjugated Side Chain Attaching Carbonyl end Group

A new two-dimensional conjugated (2D conjugated) and side chain isolated polythiophene derivative with a dodecane carbonyl group terminated triphenylamine-vinylene (CTPA) conjugated side chain and unsubstituted terthiophene spacer, PT4-CTPA, was designed and synthesized. Compared to its polymer analogue of PT2-TPA (with only one thiophene spacer and without the pendant electron-deficient group), the 2D-conjugated and side chain isolated PT4-CTPA with the electron-withdrawing carbonyl group on the conjugated side chain shows a lower lying HOMO (the highest occupied molecular orbital) energy level at –5.24 eV and enhanced absorption of the polymer backbone. A bulk heterojunction polymer solar cell with the polymer as a donor and (6,6)-phenyl C71-butyric acid methyl ester as an acceptor demonstrated a power conversion efficiency (PCE) of 1.56% with an open circuit voltage of 0.7 V under AM1.5G, 100 mW/cm2. PCE is more than seven times higher than that of PT2-TPA (0.21%, under same experimental conditions). © 2012 Wiley Periodicals, Inc. Adv Polym Techn 32: E822–E831, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.21324

Synthesis and characterization of phenathrothiadiazole-based conjugated polymer for photovoltaic device

A new accepter with phenanthro[9,10-c][1,2,5]thiadiazole unit was synthesized and utilized for the synthesis of the conjugated polymers, PCPDTDTPT and PCDTPT, containing electron donor–acceptor pairs for OPV devices. This novel phenanthro[9,10-c][1,2,5]thiadiazole unit with phenanthene group can supply planarity and possess electron deficient thiadiazole group. The solid films of PCPDTDTPT and PCDTPT show absorption bands with maximum peaks at 492 and 394nm and the absorption onsets at 620 and 519nm, corresponding to band gaps of 2.00 and 2.39eV, respectively. The HOMO and LUMO levels of PCPDTDTPT and PCDTPT were estimated to be (−5.54, −3.63) and (5.81, 3.65)eV, respectively. The PCDTPT with carbazole moiety shows lower HOMO levels as compared to PCPDTDTPT caused by strong electron donating ability of nitrogen atom to provide higher Voc. The devices with PCPDTDTPT: PC71BM with TiOx layer showed a VOC of 0.77V, a JSC of 4.71mA/cm2, and a FF of 0.36, giving a power conversion efficiency of 1.24%.