Electron-deficient Heterocycles Research Articles

Tetraazaisoindigos:Serpentine Syntheses and Their Expected and Unexpected Photophysical and Electronic Properties.

Isoindigo, anelectron-withdrawing building block for polymeric field-effect transistors, has long been considered to be non-fluorescent. Moreover, using electron-deficient heterocycle to replace the phenyl ring in the isoindigo core for better electron transport behaviour is synthetically challenging. Here we report the syntheses of a series of tetraazaisoindigos, including pyrazinoisoindigo (PyrII), pyrimidoisoindigo (PymII) and their hybrid (PyrPymII), and the investigation on their photophysical and electric properties. Proper flanking groups need to be chosen to stabilize these highly electron-deficient bislactams. Both PyrIIandPymIIderivatives showlower LUMO energy levels than that of naphthalene bisimide (NDI). Interestingly, PyrII is instinctively unstable and can be easily reduced, while both PymII derivatives are stable. More surprisingly, PymII derivatives are highly fluorescent and their photoluminescence quantum yields are around 40%, 133 times higher than that of reported isoindigo derivatives. UV-vis spectroscopic results and theoretical calculations show that strong intramolecular hydrogen-bond exists in PymII, whichprohibits it from non-radiative decay and accounts for its fluorescent behaviour. PymII deriviatives are n-type semiconductors, while Ph-PyrII and the hybrid show balanced ambipolar charge transport behaviour, all among the best isoindigo derivatives. Our study not only discloses the structure-property relationship of tetraazaisoindigos, but also provideselectron-deficient monomers for conjugated polymers.

The Synthesis and Crystallographic Characterization of Emissive Pt(II) and Au(I) Compounds Exploiting the 2-Ethynylpyrimidine Ligand

The luminescent properties of Au(I) and Pt(II) compounds are commonly tuned by exploiting the alkynyl ligand with varying electron density. Herein, we describe the synthesis of three new emissive transition metal compounds, tbpyPt(C2pym)2, Ph3PAuC2pym, and Cy3PAuC2pym (where HC2pym = 2-ethynylpyrimidine), verified by 1H-NMR, EA, and a single-crystal X-ray diffraction analysis. The tbpyPt(C2pym)2 complex crystallized as an Et2O solvate in the orthorhombic space group Pbca with Z = 24 with three unique Pt(II) species within the unit cell. The Cy3PAuC2pym species crystallizes in a monoclinic space group with one unique complex in the asymmetric unit. Changing the identity of the phosphine from Cy3P to Ph3P influences interactions within the unit cell. Ph3PAuC2pym, which also crystalizes in a monoclinic space group, has an aurophilic bonding interaction Au–Au distance of 3.0722(2) Å, which is not present in crystalline Cy3PAuC2pym. Regarding optical properties, the use of an electron-deficient heterocycle provides an alternate approach to blue-shifting the emission of Pt(II) transition metals’ compounds, where the aryl moiety is made more electron-deficient by exploiting nitrogen within this moiety instead of the typical strategy of decorating the aryl ring with electron withdrawing substituents (e.g., fluorines). This is indicated by the blue-shift in emission that occurs in tbpyPt(C2pym)2 (λmax, emission = 512 nm) compared to the previously reported tbpyPt(C22-py)2 (where HC22-py = 2-ethynylpyridine) complex (λmax, emission = 520 nm).

Modification of the N-methyl-4,5-diazacarbazole D-π-A with a π-conjugated bridge to enhance excited-state properties for superior utilization in green energy applications: A theoretical approach

In organic photovoltaic technology, tailoring the acceptor or donor unit to enhance intramolecular charge transfer is a well-known approach, however, the effect of modifying conjugated bridge units is an underdeveloped phenomenon. In this study, this approach has been focused, where a series of conjugated bridges including, a) electron-excessive heterocycles, b) electron-deficient heterocycles, and c) short-chain unsaturated hydrocarbons have been substituted from thiophene bridge-containing D-π-A; N-methyl-4,5-diazacarbazole (donor) and bay-annulated indigo (acceptor) molecule. This quantum chemical study aims to investigate the effect of various types of π-conjugated bridges on the electronic structure and solar cell activity of already reported material. The selected π-conjugated bridges include electron-excessive (furan, pyrrole, & thiophene) and electro-deficient (diazine, tetrazine, & pyridine) heterocycles, and short-chain hydrocarbons (ethylene & ethyne). The optimized structural parameters e.g., dihedral angles (θ), distortion energies (Edis), & bond distances (d), and electronic behaviour i.e., Frontier Molecular Orbitals (FMO), Electrostatic Potentials (ESP), Density of States (DOS) spectra, dipole moments (μ), and absorption spectra of the hypothetically designed molecules are estimated using density functional theory calculations. The solar cell proficiency is investigated with transition dipole moment (μT), reorganization energy (RE), Transition density matrix (TDM), exciton binding energy (Eb), and open circuit voltage (Voc). Our finding encompassed that our designed D-π-A molecules show significant improvement in their electronic behaviour, especially when the thiophene bridge of the reference molecule is substituted with the excess electron heterocycles, especially pyrrole. The HOMO-LUMO gap of this particular molecule (MPy) is reduced to 2.07 eV, resulting in the significant charge transfer between donor and acceptor units (0.15 e-). Moreover, the results of RE, TDM, Eb, and Voc also support the high solar cell efficiency of pyrrole-containing molecules.

Parallel Minisci Reaction of gem-Difluorocycloalkyl Building Blocks.

Parallel Minisci reactions of nonfluorinated and gem-difluorinated C4-C7 cycloalkyl building blocks (trifluoroborates and carboxylic acids) with a series of electron-deficient heterocycles were studied. A comparison of the reaction's outcome revealed better product yields in the case of carboxylic acids as the radical precursors in most cases, albeit these reagents were used with three-fold excess under optimized conditions. The nature of the heterocyclic core was found to be important for successful incorporation of the cycloalkyl fragment. The impact of the CF2 moiety on the oxidation potential of fluorinated cycloalkyl trifluoroborates and the reaction outcome, in general, was also evaluated.

Dual-Ligand Catalyst for the Nondirected C-H Olefination of Heteroarenes.

Pd(II)-catalyzed nondirected C-H functionalization of heteroarenes is a significant challenge for the following reasons: poor reactivity of electron-deficient heterocycles and the unproductive coordination of Lewis basic nitrogen atoms. Existing methodologies using palladium catalysis often employ a large excess of heterocycle substrates to overcome these hurdles. Despite recent advances in nondirected functionalization of arenes that allow them to be used as limiting reagents, the reaction conditions are incompatible with electron-deficient heteroarenes. Herein we report a dual-ligand catalyst that enables Pd(II)-catalyzed nondirected C-H olefination of heteroarenes without using a large excess of substrate. In general, the use of 1-2 equiv of substrates was sufficient to obtain synthetically useful yields. The reactivity was rationalized by the synergy between two types of ligands: a bidentate pyridine-pyridone ligand promotes C-H cleavage; the monodentate heterocycle substrate acts as a second ligand to form a cationic Pd(II) complex that has high affinity for arenes. The proposed dual-ligand cooperation is supported by a combination of X-ray, kinetics, and control experiments.

C3-Functionalization of indoles with α-heteroaryl-substituted methyl alcohols.

The transition metal-free Cs2CO3/Oxone®-mediated C3-alkylation of indoles proceeds in moderate to high yields with a variety of C4-C7 functionalized indoles and is applicable to 2-, 3- and 4-hydroxymethyl pyridines and related electron-deficient heterocycles, permitting novel late-stage drug functionalizations. Preliminary mechanistic studies support a hydrogen autotransfer-type chain process starting with an initial oxidation of the alcohol to the corresponding aldehyde, followed by a subsequent condensation onto indole and reduction/hydride delivery from another equivalent of the primary alcohol.

2-(Pyridin-2-ylmethylene)indan-1-ones: The first study of E/Z isomerization of cyclic chalcones with electron-deficient hetaryl moiety

The products of condensation of pyridine-2-carbaldehyde with 1-indanone (4) 5,6-dimethoxy-1-indanone (5), and 7-acetamido-1-indanone (6) were synthesized and studied as derivatives of cyclic chalcones with electron-deficient heterocycle (pyridine) as distinct from the earlier studied analogues with electron-rich heterocycles (azoles). All products are formed as the E-isomers. The structure of compound 4 was proved by X-ray analysis. The UV-induced E/Z isomerization of compounds 4–6, as well as the transformation occurring upon protonation, were studied experimentally by NMR and IR spectroscopy and theoretically by DFT calculations. The rate of isomerization for compound 4 is much higher than those of 5 and 6. All possible isomers relative to the exocyclic CC bond and rotamers about the C–C bond with the pyridine ring were calculated. General scheme of transformation of compounds 4–6 upon UV irradiation and protonation is proposed.

Rational design of ZL003-based organic dyes for highly efficient dye-sensitized solar cells: Influence of alkynyl group and π-spacers on photovoltaic performance

Dye-sensitized solar cells (DSSCs) have displayed huge potential in inexpensive, efficient, and clean solar energy-technology. To achieve highly efficient DSSCs, a series of D-π-A organic dyes were designed and their optoelectrical properties were calculated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Compared with ZL003, ZS1 designed by introducing the triple bond between donor and π bridge, has higher conjugation, narrower HOMO-LUMO bandgap, and extended absorption spectra. Then five new organic dyes were designed by substituting benzothiadiazole part of ZS1 with five electron-deficient heterocycles. The designed dyes have more conjugated structure, which leads to stronger light harvesting ability with the absorption spectra expanding to the near infrared region, the maximum absorption wavelength red shifting 78 nm∼448 nm and light harvesting efficiency (LHE) curve broadening obviously. The intermolecular electron transfer (IET) rate of the design dye is much faster than that of ZL003, over 85.3% of electrons in the design dye have been injected into TiO2 within 200 fs. Especially, the power conversion efficiency (PCE) of BBTD, BTTD and BZOD is more than 25.7%, mainly reflected in the obvious jump of the theoretical short current density (Jscpred.) by more than 68%. Therefore, the design dyes have a good application prospect. We believe that this work could provide theoretical guidance for the design of potential organic sensitizers in highly efficient DSSCs.

Role of Electron-Deficient Imidazoles in Ion Transport and Conductivity in Solid-State Polymer Electrolytes

Solid-state polymer electrolytes offer a safer alternative to traditional lithium-ion batteries based on organic electrolytes. The focus is here on imidazole(Im)-functionalized polymer electrolytes, where the Im ligand promotes salt dissolution, while its functionalization allows tuning the dynamic interactions between the cations in solution and the Im ligand tethered to the polymer backbone. Although careful choice of a polymer backbone and Im linker functionality have resulted in polymer electrolytes with increased total ionic conductivities and a boost in the Li+ ion contribution, improvements in performance through modifications of the Im heterocycle remain underexplored. In this work, we systematically investigate poly(methylsiloxane) polymers functionalized with a series of halogen-substituted imidazoles and show that Li+ ion conduction can be tuned by an electron-deficient heterocycle ligand. When the number of halogen substituents increases, the Li+ ion mobility also increases as measured by pulsed-field-gradient nuclear magnetic resonance (NMR) and NMR relaxometry. Although beneficial for Li+ transport, electron-deficient Im ligands result in clustering, as indicated by wide-angle X-ray scattering, and poor salt dissolution, which in turn impedes the overall ionic transport. This work highlights the importance of synthetic design and the necessity for high salt solvation and weak cation–polymer binding to obtain high Li+ transport numbers.

Crystal engineering of heterocyclic arylene(ethynylene) oligomers through programmed aromatic stacking

Electron-rich heteroarenes stack with perfluoroarene side chains and twist arylene-ethynylene conjugated oligomers, while electron deficient heterocycles do not. These trends correlate with X-ray structures, optical spectra, and computations.

Buchwald-Hartwig Amination of Coordinating Heterocycles Enabled by Large-but-Flexible Pd-BIAN-NHC Catalysts.

A new class of large-but-flexible Pd-BIAN-NHC catalysts (BIAN=acenaphthoimidazolylidene, NHC=N-heterocyclic carbene) has been rationally designed to enable the challenging Buchwald-Hartwig amination of coordinating heterocycles. This robust class of BIAN-NHC catalysts permits cross-coupling under practical aerobic conditions of a variety of heterocycles with aryl, alkyl, and heteroarylamines, including historically challenging oxazoles and thiazoles as well as electron-deficient heterocycles containing multiple heteroatoms with BIAN-INon (N,N'-bis(2,6-di(4-heptyl)phenyl)-7H-acenaphtho[1,2-d]imidazol-8-ylidene) as the most effective ligand. Studies on the ligand structure and electronic properties of the carbene center are reported. The study should facilitate the discovery of even more active catalyst systems based on the unique BIAN-NHC scaffold.

Precision Synthesis of Conjugated Polymers Using the Kumada Methodology

Since the discovery of conductive poly(acetylene), the study of conjugated polymers has remained an active and interdisciplinary frontier between polymer chemistry, polymer physics, computation, and device engineering. One of the ultimate goals of polymer science is to reliably synthesize structures, similar to small molecule synthesis. Kumada catalyst-transfer polymerization (KCTP) is a powerful tool for synthesizing conjugated polymers with predictable molecular weights, narrow dispersities, specific end groups, and complex backbone architectures. However, expanding the monomer scope beyond the well-studied 3-alkylthiophenes to include electron-deficient and complex heterocycles has been difficult. Revisiting the successful applications of KCTP can help us gain new insight into the CTP mechanisms and thus inspire breakthroughs in the controlled polymerization of challenging π-conjugated monomers.In this Account, we highlight our efforts over the past decade to achieve controlled synthesis of homopolymers (p-type and n-type), copolymers (diblock and statistical), and monodisperse high oligomers. We first give a brief introduction of the mechanism and state-of-the-art of KCTP. Since the extent of polymerization control is determined by steric and electronic effects of both the catalyst and monomer, the polymerization can be optimized by modifying monomer and catalyst structures, as well as finding a well-matched monomer-catalyst system. We discuss the effects of side-chain steric hindrance and halogens in the context of heavy atom substituted monomers. By moving the side-chain branch point one carbon atom away from the heterocycle to alleviate steric crowding and stabilize the catalyst resting state, we were able to successfully control the polymerization of new tellurophene monomers. Inspired by innocent role of the sterically encumbered 2-transmetalated 3-alkylthiophene monomer, we introduce the treatment of hygroscopic monomers with a bulky Grignard compound as a water-scavenger for the improved synthesis of water-soluble conjugated polymers. For challenging electron-deficient monomers, we discuss the design of new Ni(II)diimine catalysts with electron-donating character which enhance the stability of the association complex between the catalyst and the growing polymer chain, resulting in the quasi-living synthesis of n-type polymers. Beyond n-type homopolymers, the Ni(II)diimine catalysts are also capable of producing electron-rich and electron-deficient diblock and statistical copolymers. We discuss how density functional theory (DFT) calculations elucidate the role of catalyst steric and electronic effects in controlling the synthesis of π-conjugated polymers. Moreover, we demonstrate the synthesis of monodisperse high oligomers by temperature cycling, which takes full advantage of the unique character of KCTP in that it proceeds through distinct intermediates that are not reactive. The insight we gained thus far leads to the first example of isolated living conjugated polymer chains prepared by a standard KCTP procedure, with general applicability to different monomers and catalytic systems. In summarizing a decade of innovation in KCTP, we hope this Account will inspire future development in the field to overcome key challenges including the controlled synthesis of electron-deficient heterocycles, complex and high-performance systems, and degradable and recyclable materials as well as cutting-edge catalyst design.

Efficient construction of a redox responsive thin polymer layer on glassy carbon and gold surfaces for voltage-gated delivery applications

The notoriously non-selective aryl-diazonium surface-functionnalization turned selective in the presence of the electron-deficient heterocycle. The electro-activation of the probe allowed the rapid and nearly quantitative liberation of covalently tethered ligands.

Asymmetric Borylative Coupling of Vinylazaarenes and Ketones Catalyzed by a Copper(I) Complex

Azaarenes are attractive structural units widely found in chiral pharmaceuticals, agrochemicals, and biologically active natural products. An ongoing strategy has been the construction of new chira...

The effect of protonation on the excited state dynamics of pyrimidine chromophores

The effect of protonation on the photophysics and especially on the excited state dynamics of two pyrimidine chromophores, bearing the A-(π-D)2 and A-(π-D)3 structure, is studied by means of fs-ps and ns time resolved fluorescence spectroscopy. Three different acids, namely camphorsulphonic (CSA), acetic (AcOH) and trifluoroacetic acid (TFA) were used. The chromophores bear the pyrimidine electron deficient heterocycle as electron-withdrawing group, used as protonation site, as well as diphenylamino electron donors. Protonation is revealed through the emergence of red-shifted absorption and fluorescence bands accompanied by a quenching of the fluorescence of the neutral molecules. Time-resolved dynamics reveal that protonation with CSA and TFA do not influence the excited state lifetime of the chromophores, pointing to a static quenching process. On the other hand, the lifetime is decreased upon protonation with AcOH. Further investigation based on the Stern-Volmer plots showed that addition of AcOH leads to both dynamic and static quenching.

Nickel-catalyzed electrochemical Minisci acylation of aromatic N-heterocycles with α-keto acids via ligand-to-metal electron transfer pathway

A nickel-catalyzed electrochemical methodology for the Minisci acylation of aromatic electron-deficient heterocycles with α-keto acids has been developed. The reaction is performed in an undivided cell under constant current conditions, featuring broad scope of substrates and avoiding the conventional utilization of silver-based catalysts in conjunction with excess amount of oxidants. Cyclic voltammetric analysis disclosed that a ligand-to-metal electron transfer process may be involved in the generation of the key acyl radicals.

A Modular Approach to Phosphorescent π-Extended Heteroacenes.

A modular route to previously inaccessible classes of ring-fused π-extended heteroacenes bearing the heavy inorganic element tellurium (Te) is presented. These new materials can be viewed as n-doped analogs of molecular graphene subunits that exhibit color tunable visible light phosphorescence in the solid state and in the presence of air. The general mechanism of phosphorescence in these systems was probed experimentally and computationally via time-dependent density functional theory (TD-DFT). The incorporation of Te into π-extended oligoacene frameworks was achieved by an efficient Zr/Te transmetalation protocol; related zirconium-element exchange reactions have been used to prepare both electron-rich and electron-deficient heterocycles containing different elements from throughout the p-block. Therefore, the current study provides a clear path to incorporate inorganic elements into heteroacenes of greater complexity and side group selectivity compared to existing synthetic routes.

BF3·SMe2 for Thiomethylation, Nitro Reduction and Tandem Reduction/SMe Insertion of Nitrogen Heterocycles

Herein, a general, solvent‐free and straightforward thiomethylation of electron deficient heterocycles using BF3·SMe2 as a dual thiomethyl source and Lewis acidic activator is presented. A range of heterocycles including pyrimidine, pyrazine, pyridazine, thiazole and purine derivatives were successfully substituted using this method. An unexpected reductive property of BF3·SMe2 towards nitropyridines was also discovered including an intriguing tandem reduction/SMe insertion process in certain substrates. Notable features of the present work include its convenience and use of a non‐malodorous reagent while the discovery of novel chemical transformations using BF3·SMe2 provides fundamental new insights into the reactivity of this commonly employed reagent.

Influence of heteroatom substitution in benzene rings on structural features and spectral properties of subphthalocyanine dyes

Peculiarities of the molecular geometry, electronic structure and spectral properties of heterocyclic subphthalocyanines analogues containing electron-deficient heterocycles have been considered using DFT and TD DFT methods. Subporphyrazine with annulated 1,2,5-selenadiazole rings was prepared and characterised for the first time and molecular parameters of the 1,2,5-thiadiazole analogue have been determined by single crystal X-ray diffraction. The theoretical results are compared with the experimental data and the effect of heteroatom substitution in benzene rings of subphthalocyanines on spectral-luminescence properties was revealed. Fusion of pyrazine, 1,2,5-thiadiazole or 1,2,5-selenadiazole rings instead benzene fragments leads to considerable stabilization of the frontier π-molecular orbitals of subporphyrazine macrocycle. The HOMO-LUMO gap is increased when pyrazine or 1,2,5-thiadiazole rings are fused, but becomes narrower in the presence of 1,2,5-selenadiazole rings. The intense Q band observed in the visible region (530–575 nm) originates from the lowest ππ* transition (2a2→1e*) and its energy follows the changes in HOMO-LUMO gap. Two transitions (2a1→1e* and 1a1→1e*) contribute predominantly to closely lying excited states (B1, B2) responsible for the intense Soret band in the UV-region (305–310 nm) of subphthalocyanine and its pyrazine and 1,2,5-thiadiazole analogues. For the Se-analogue the energy of the 2a1→1e* transition is decreased and B1 band is shifted bathochromically to 342 nm. Fusion of π-electron-deficient 1,2,5-chalcogenadiazole rings leads to appearance of the less intense band at 390 (S) or 408 nm (Se) due to electronic transition from HOMO 2a2 to 2e* orbital localized predominantly on peripheral heterocyclic fragments (CT band). The heavy atom effect of chalcogens decreases the fluorescence quantum yield in heterocyclic subphthalocyanines analogues, but promotes the intersystem crossing and strongly enhances the singlet oxygen quantum yield for 1,2,5-selenadiazole fused subporphyrazine (ΦΔ = 0.98 in CH2Cl2).

Electrochemical Minisci-type trifluoromethylation of electron-deficient heterocycles mediated by bromide ions

An electrochemical methodology for the Minisci-type trifluoromethylation of electron-deficient heterocycles mediated by cheap and easily available bromide ions has been developed.