Strong Electron-withdrawing Group Research Articles

Construction of Carbon Dots with Color-Tunable Aggregation-Induced Emission by Nitrogen-Induced Intramolecular Charge Transfer.

As one of the most promising fluorescent nanomaterials, the fluorescence of carbon dots (CDs) in solution is extensively studied. Nevertheless, the synthesis of multicolor solid-state fluorescence (SSF) CDs is rarely reported. Herein, CDs with multicolor aggregation-induced emission are prepared using amine molecules, all of them exhibiting dual fluorescence emission at 480nm (Em-1) and 580-620nm (Em-2), which is related to the SS bonds of dithiosalicylic acid and the conjugated structure attached to CO/CN bonds, respectively. As a strong electron-withdrawing group, the increase of CN content makes dual-fluorescent groups on the surface of CDs produce push and pull electrons, which determines intramolecular charge transfer (ICT) between the double emission. With the increase in CN content from 35.6% to 58.4%, the ICT efficiency increases from 8.71% to 45.94%, changing the fluorescence of CDs from green to red. The increase of ICT efficiency causes fluorescence quantum yield enhancement by nearly five times and redshift of the fluorescence peak. Finally, based on the multicolor luminescence properties induced by the aggregation of CDs, pattern encryption and white-LED devices are realized. Based on the fat solubility and strong ultraviolet absorption characteristics of CDs, fingerprint detection and leaf anti-UV hazards are applied.

Solution-processable silicon naphthalocyanine tetraimides as near infrared electron acceptors in organic solar cells

Two silicon naphthalocyanine electron acceptors, CH3O-SiNcTI and 3BSO-SiNcTI, with four strong electron-withdrawing imide groups, were designed, synthesized, and applied in bulk heterojunction organic solar cells (BHJ OSCs). They exhibited strong near-infrared absorption in 300–1100 nm, maximum extinction coefficients of up to 5.2 × 105 M−1 cm−1, deep lowest unoccupied molecular orbital energy levels of −3.92 eV, and good photothermal stability. The power conversion efficiency of BHJ OSCs based on acceptor of 3BSO-SiNcTI reached 4.22%, which is the highest value among solution-processable phthalocyanine or naphthalocyanine derivatives. The measurements of space-charge-limited current, charge recombination, charge collection capability, atomic force microscopy (AFM), and transmission electron microscopy (TEM) were carried out. It is found that BHJ OSCs based on PTB7-Th:3BSO-SiNcTI, have higher and balanced carrier mobility, less charge recombination, better charge transport, and favorable film morphology, which lead to their higher photovoltaic performance. This study indicates the great potential of SiNcTIs as near-infrared chromophores for high-performance electron acceptors in BHJ OSCs.

Self-compatibilization effect of phosphonate with cyano group on flame retardancy and mechanical properties of epoxy

A phosphonate flame retardant containing cyano group (CPDPO) was synthesized by using diphenylphosphinyl chloride and 4-cyanophenol and used to modify the flame retardancy epoxy (EP). The curing process, flame retardant performance and mechanism of EP/CPDPO were investigated. The cyano is a strong electron-withdrawing group which makes some CPDPO to react with EP via transesterification, thus about 17% diphenyl phosphine oxide (DPO) and 4-cyanophenol group will be grafted onto the chain of EP. Therefore CPDPO plays a self-compatibilization role in the system. The EP with 6 wt% CPDPO achieves the UL-94 V-0 rating and a LOI value of 36.6%. The peak of heat release rate and total heat release of EP with 6 wt% CPDPO are decreased by 47% and 24% compared with EP, respectively. The DPO grafted on the EP network tends to work in the condensed phase, while the free CPDPO is easy to work in the gas phase. The latter is far more than the former. Therefore, the effect of gas phase is obviously greater than that of condensed phase, which matches the structure of EP which has good charring ability itself. Moreover, the EP/CPDPO has better mechanical properties than that of EP and transparency equivalent to itself. This work suggests that the phosphonate containing strong electron-withdrawing group not only has self-compatibilization effect, but also regulates the properties and flame retardant mechanism.

Acceptor Modulation Strategies for Improving the Electron Transport in High-Performance Organic Field-Effect Transistors.

High-performance ambipolar and electronic type semiconducting polymers are essential for fabricating various organic optoelectronic devices and complementary circuits. This review summarizes the strategies of improving the electron transport of semiconducting polymers via acceptor modulation strategies, which include the use of single, dual, triple, multiple, and all acceptors as well as the fusion of multiple identical acceptors to obtain new heterocyclic acceptors. To further improve the electron transport of semiconducting polymers, the introduction of strong electron-withdrawing groups can enhance the electron-withdrawing ability of donors and acceptors, thereby facilitating electron injection and suppressing hole accumulation. In addition, the relationships between the molecular structure, frontier molecular orbital energy levels, thin film morphology, microstructure, processing conditions, and device performances are also comprehensively discussed. Finally, the challenges encountered in this research area are proposed and the future outlook is presented.

Incorporating Cyano Groups to a Conjugated Polymer Based on Double B←N-Bridged Bipyridine Units for Unipolar n-Type Organic Field-Effect Transistors

The development of n-type semiconductors lags far behind that of their p-type counterparts, demonstrating the exploration of exclusive n-type π-conjugated polymers is significant. The double B←N-bridged bipyridine (BNBP)-based polymers P-BNBP-TVT containing (E)-1,2-di(thiophen-2-yl)ethene (TVT) previously reported exhibits ambipolar character because of the electron-rich nature. Herein, we incorporated strong electron-withdrawing cyano groups into the 3,3′-positions of the TVT moiety to a copolymer P-BNBP-2CNTVT to develop n-type π-conjugated polymers. The LUMO/HOMO energy levels of P-BNBP-2CNTVT are −3.80/−5.95 eV, respectively, which are ~0.4 eV lower than that of P-BNBP-TVT without cyano groups. Unsurprisingly, compared with ambipolar P-BNBP-TVT, the organic field-effect transistors (OFETs) based on P-BNBP-2CNTVT showed unipolar n-type characteristics with an electron mobility of 0.026 cm2 · V−1 · s−1 and a lower threshold voltage of ~25 V as well as high I on/I off of ~105. This study demonstrates that organoboron π-conjugated polymers could be regarded as a tool for constructing exclusive n-type semiconducting polymers used in OFETs.

Effectively controlling the ESIPT behavior and fluorescence feature of 2-(2′-hydroxyphenyl)-4-chloromethylthiazole by changing its π-conjugation: A theoretical exploration

Based on density functional theory (DFT) and time-dependent DFT (TD-DFT), the effects of π-conjugation on the excited-state intramolecular proton transfer (ESIPT) process and photophysical properties of 2-(2′-hydroxyphenyl)-4-chloromethylthiazole (HCT) have been studied. The strong electron acceptor with π-conjugated structure (MN1, MN2) have been chosen to analyze the changes of intramolecular H-bond, the frontier molecular orbital, the absorption/fluorescence spectrum and the energy barrier of ESIPT process. Our results show that the strong electron-withdrawing groups (MN1, MN2) weaken the intramolecular H-bond in the S1 state. The substitution effect on the H-bond strength in the S0 state is opposite. For MN1-HCT and MN2-HCT, both the forward and backward ESIPT reactions can occur. The normal and tautomer form can exist simultaneously in the S1 state. The absorption and fluorescent peaks of MN1-HCT and MN2-HCT all red-shift more than 150 nm due to the extended π-conjugated chain.

Synthesis of 1-Amino-2,2,2-trifluoroalkylphosphonates from Alkene-Tethered Trifluoroacetimidoyl Chlorides.

The reaction of alkene-tethered trifluoroacetimidoyl chlorides with trialkyl phosphites furnishes 1-amino-2,2,2-trifluoroalkylphosphonates. The products were generated in moderate to good yields, and the scalability of this process was showcased. Partial hydrolysis of the phosphonate moiety was achieved. The cyclization is proposed to occur via formation of an imidoyl phosphonate intermediate that becomes susceptible to nucleophilic attack at nitrogen through the strong electron-withdrawing groups at the imidoyl carbon.

A DFT study of the double (3 + 2) cycloaddition of nitrile oxides and allenoates for the formation of spirobiisoxazolines

The molecular mechanism of the double (3 + 2) cycloaddition (32CA) reaction between nitrile oxides and allenoates has been studied using density functional theory at the M06-2X/6-311G (d,p) level of theory. In the first 32CA, the nitrile oxide adds chemo- and regio-selectively to the C–C double bond of the allenoate closest to the carboxylate group followed by a subsequent regioselective addition to the olefinic bond of the isoxazoline intermediate. The rate constant for the preferred pathway (formation of 4-methylene-2-isoxazoline intermediate) in the reaction of ethyl substituted allenoate and mesitonitrile oxide is 5.3 × 102 s−1 in THF which is about 13 times faster than the closest competing step (formation of its regioisomer 5-methylene-2-isoxazoline intermediate) which has a rate constant of 4.4 × 101 s−1. Strong electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) decrease activation barriers and hence increase the reaction rate. Also, the dimerization of nitrile oxide to form furaxon is found to be kinetically unfavored.

Tracking the Transition from Pericyclic to Pseudopericyclic Reaction Mechanisms Using Multicenter Electron Delocalization Analysis: The [1,3] Sigmatropic Rearrangement.

Herein, the power of multicenter electron delocalization analysis to elucidate the intricacies of concerted reaction mechanisms is brought to light by tracking the transition of [1,3] sigmatropic rearrangements from the high-barrier pericyclic mechanism in 1-butene to the barrierless pseudopericyclic mechanism in 1,2-diamino-1-nitrosooxyethane. This transition has been progressively achieved by substituting the migrating group, changing the donor and acceptor atoms, and functionalizing the alkene unit with weak and strong electron-donating and electron-withdrawing groups. Fourteen [1,3] sigmatropic reactions with electronic energy barriers ranging from 1 to 89 kcal/mol have been investigated. A very good correlation has been found between the barrier and the four-center electron delocalization at the transition state, the latter calculated for the atoms involved in the four-centered ring adduct formed along the reaction path. Surprisingly, the barrier has been found to be independent of the bond strength between the migrating group and the donor atom so that only the changes induced in the multicenter bonding control the kinetics of the reaction. Additional insights into the effect of atom substitution and group functionalization have also been extracted from the analysis of the multicenter electron delocalization profiles along the reaction path and qualitatively supported by the topological analysis of the electron density.

Accelerated Dimerization of α,β-Unsaturated D-Xylo-Hexofurano-5-ulose Derivatives through Asynchronous Hetero-Diels-Alder Reaction.

While some hetero-Diels-Alder (HDA) reactions are accelerated by either carbonyl or phosphate groups attached directly to the heterodiene moiety, the alkyl or aryl groups, on the other hand, have minimal influence. However, in this article, we demonstrate that aryl groups have a significant effect on the spontaneous dimerization reaction of α,β-unsaturated D-xylo-hexofurano-5-ulose derivatives to their respective pyrano adducts via intermolecular HDA reaction. Experimental and computational studies provide strong evidence that dimerization follows the Woodward-Katz two-stage mechanism reaction (asynchronous process), from which the aryl/aryl π-stacking interaction is mainly responsible for the rate-determining step (RDS) and electrostatic interaction for the second bond formation. Since the latter interaction is highly affected by dipolar moment, 5-ulose derivative having a strong electron-withdrawing group (R = CN; μ = 14.3 D) is spontaneously dimerized more than 15 times faster than 5-ulose that possesses an electron-donating group (R = OMe; μ = 2.1 D).

Twistarenes with eleven linearly-annulated aromatic rings: synthesis, photophysics and radical characteristics

<p indent=0mm>Organic open-shell conjugated systems are of great importance because of their fascinating properties and potential applications in organic electronics and biological fields. Inspired by these features, we herein reported the synthesis and photophysics of two novel <italic>s-</italic>indacene-based twistarene derivatives (<bold>7a</bold> and <bold>7b</bold>) with linearly-annulated eleven aromatic rings. Both of them exhibit the blue-shift of the maximum absorption peaks of <sc>683 nm</sc> for <bold>7a</bold> and <sc>678 nm</sc> for <bold>7b</bold> compared with 6,8,10,17,19,21-hexaphenyl-1.22,4.5,11.12,15.16-tetrabenzonona twistacene <sc>(739 nm),</sc> which might be attributed to the incorporation of cyclopenta units that destroy the π-conjugation to a certain extent. In addition, molecule <bold>7b</bold> including two strong electron-withdrawing pentafluorophenyl groups displays better stability than <bold>7a</bold>. The radical cations were conveniently generated upon the addition of silver hexafluoroantimonate (AgSbF₆) to <bold>7</bold>, which was verified by NIR-UV-Vis spectra, electron spin resonance (ESR) spectroscopy and density fumctional theory (DFT) calculation. In addition, molecules <bold>7a</bold> and <bold>7b</bold> can also show obvious electron paramagnetic signals after heating, resulting in stable free radicals.

Investigating the site-, regio-, and stereo-selectivities of the reactions between organic azide and 7-heteronorbornadiene: a DFT mechanistic study.

The site-, regio-, and stereo-selectivities of the title reactions have been studied using density functional theory (DFT) at the M06/6-311G(d,p) level of theory. The effects of substituents on both the three-atom component (TAC) and norbornadiene derivatives have been investigated with a focus on the site-selectivity. The reaction of benzylazide with (1S,4R)-2-tosyl-7-oxabicyclo[2.2.1]hepta-2,5-diene and (1R,4S)-2-bromo-3-tosyl-7-oxabicyclo[2.2.1]hepta-2,5-diene proceeds via addition across the substituted olefinic bond of the two norbornadiene derivatives. Substituents on the TAC do not affect the selectivity of the reaction while substituents on the norbornadiene significantly affect the selectivity of the reaction. Benzylazide preferentially adds across the substituted olefinic bond of the norbornadiene derivative when strong electron-withdrawing group (EWGs) and electron-releasing group (ERGs) substituents are on the norbornadiene while weak ERGs and EWGs on the norbornadiene significantly decreases the site-selectivity such that addition across either double is no longer favored over the other. The formation of exo-cycloadducts is generally favored over the endo-cycloadducts. The reaction of benzylazide and norbornadiene derivatives is a highly irreversible exergonic reaction. The direction of electron density flux is dependent on the nature of the substituent on the reactants. Global reactivity indices and Parr function calculations are in good agreement with the activation barriers and the selectivity of the reactions.

Tuning the optoelectronic properties of triphenylamine (TPA) based small molecules by modifying central core for photovoltaic applications.

Small donor molecules based on fused ring acceptors exhibit encouraging photovoltaic properties and expeditious advancement in organic solar cells. Central core modification of non-fullerene acceptor materials is a favorable methodology to enhance electronic properties and efficiency for OSCs. Herein, four new donor molecules, namely, BDTM1, PYRM2, ANTM3, and NM4 are designed with a strong donor moiety triphenylamine, tetracyanobutadiene as acceptor unit, and thiophene as spacer linked to a modified central core. Geometric parameters, optical, electrical properties, effect of central core modification on tailored molecules BDTM1-NM4 are investigated and compared with reference DPPR. DFT together with TDDFT approaches using MPW1PW91 functional is used to study key parameters like absorption maximum (λmax), frontier molecular approach, ionization potential, electron affinity, the density of states, transition density matrix along with open-circuit voltage (VOC), dipole moment and reorganization energy. Among all these molecules, BDTM1 shows maximum calculated absorption λmax (817nm) and the lowest band gap (2.54eV). This bathochromic shift in BDTM1 is due to the presence of 4,8-dimethoxy-2,6-di-2-thienylbenzodithiophene as a strong electron-withdrawing group. Computed reorganization energies (RE) shows that BDTM1 has the highest hole and electron mobility among all designed molecules. Combination of BDTM1 donor and PC61BM acceptor further verifies charge transfer and their interaction. The results illustrate that designed donor molecules (BDTM1-NM4) are better in performance and are recommended for experimentation to develop efficient OSCs. Four new donor molecules, namely, BDTM1, PYRM2, ANTM3, and NM4 are designed with a strong donor moiety triphenylamine, tetracyanobutadiene as acceptor unit and thiophene as spacer linked to a modified central core. Geometric parameters, optical, electrical properties, effect of central core modification on tailored molecules BDTM1-NM4 are investigated and compared with reference DPPR.

Dramatically Enhanced and Red-shifted Photoluminescence Achieved by Introducing an Electron-withdrawing Group into a Non-traditional Luminescent Small Organic Compound.

Small organic compounds without any traditional fluorescent chromophores are generally non-emissive, and only very few are reported to emit weak blue fluorescence. Here we synthesized a non-traditional luminescent small organic compound N-(2,2,2-trifluoroethyl)acrylamide (TFAM) with dramatically enhanced and red-shifted photoluminescence by introducing a strong electron-withdrawing group into acrylamide (AM). Very impressively, TFAM emits cyan (472 nm) and yellow-green (560 nm) fluorescence in solutions and solid state, respectively. TFAM also shows aggregation-induced emission enhancement (AIEE) and excitation-dependent fluorescence (EDF) characteristics, as well as temperature and metal cations-responsive fluorescence. Theoretical calculations show that the introduction of electron-withdrawing group leads to a lower energy gap between the HOMO-LUMO energy levels in TFAM than in AM. And strong cooperative hydrogen bonds are formed in TFAM molecules, resulting in rigidification of molecular conformations. The study provides a strategy for preparing non-traditional luminescent compounds with enhanced and red-shifted photoluminescence.

C^C* Platinum(II) Complexes with Electron-Withdrawing Groups and Beneficial Auxiliary Ligands: Efficient Blue Phosphorescent Emission

The combination of strong electron-withdrawing groups in cyclometalated N-heterocyclic carbene ligands (C^C*) with known beneficial auxiliary ligands in phosphorescent platinum(II) complexes leads to efficient light-to-deep-blue emission with quantum yields of up to 92%. All compounds were characterized and investigated regarding their photophysical, electrochemical, and thermal properties, and three complexes could additionally be characterized by solid-state structures. Density functional theory calculations (PBE0/6-311G* with dispersion correction) are reported.

Synthesis of Dihydroisoquinoline and Dihydropyridine Derivatives via Asymmetric Dearomative Three-Component Reaction

Synthesis of Dihydroisoquinoline and Dihydropyridine Derivatives via Asymmetric Dearomative Three-Component Reaction

Novel indole-based photosensitizers coupled with PEG-HEC quasi-solid-state electrolyte to improve energy conversion and stability of organic dyes based-dye sensitized solar cells

A series of novel Indole based organic dyes were synthesized and coupled with a Poly-ethylene glycol-Hydroxy ethyl cellulose quasi-solid-state-electrolyte (PEG-HEC QSSE) polymer blended iodine electrolyte to obtain a metal-free dye-based dye-sensitized solar cell (DSSCs) with better stability. The newly synthesized organic dyes were confirmed through 1H, 13C NMR, CHN analysis. The optical and electrochemical properties of the dyes were determined using UV-visible spectrometry and cyclic voltammetry techniques. A quasi-solid state (QSS) blended polymer PEG-HEC with iodide redox couple as electrolyte was characterized via Field emission Scanning Electron microscope (FE-SEM), UV-visible spectroscopy, XRD and DSC. A foremost comparative study was analysed on DSSC devices with the newly synthesised organic dyes (DPC, DTC, DPCY, DTCY, DPTCY and DTTCY) coupled with the QSSE. The electrochemical impedance spectrum was discussed for all the six dyes coupled with PEG-HEC QSSE, among that DTTCY sensitized device exhibits good ionic conductivity (1.13 × 10−3 S cm−1), high chemical capacitance (Cµ=17.01 µF), high recombination resistance (53.97 Ω) and lower charge transfer resistance (2.35 Ω) due to the presence of a strong electron-withdrawing group in the organic dye and also QSS behaviour of electrolyte. The longer electron lifetime (τn-58.51 ms) and better diffusion coefficient (4.25 × 10−7 cm2 s−1) of the device prove the short circuit current (Jsc= 16.39 mA cm−2) and enhancement of power conversion efficiency (PCE) of 6.24 % with better stability under AM 1.5 G illumination (100 mW cm−2). The combination of new Indole based organic dyes with PEG-HEC QSSE paved a way to improve the PCE and stability of organic dyes based DSSCs.

Preparation and characterization of organic soluble polyimides with low dielectric constant containing trifluoromethyl for optoelectronic application

Polyimide (PI) films have excellent comprehensive properties and bright prospects in the next generation of optoelectronic and microelectronic applications. PIs were synthesized by aromatic diamines and alicyclic dianhydrides considered to be the most effective method for preparing polyimides with high light transmittance and low dielectric constant. In order to improve the overall performance of polyimide, the introduction of a large trifluoromethyl group into the main chain is also considered a powerful strategy. In this article a novel diamine containing 6 trifluoromethyl groups has been designed and synthesized by Suzuki reaction. Afterwards, PIs were synthesized with fluorinated aromatic dianhydride and alicyclic dianhydride through a two-step method. The two PIs have excellent solubility: PI-a and PI-b exhibited excellent solubility in common polar solvents. Such as DMF, DMAc, NMP and DMSO at room temperature, and PI-a was soluble in low-boiling solvents such as ethyl acetate, CH2Cl2 and CHCl3 at room temperature. PI-b was soluble in low-boiling solvents acetone and CHCl3 at room temperature, which was rarely seen in other aromatic polyimides. Soluble in common organic solvents, especially low boiling point solvents, made these polymers easy to process and suitable for processing technology in solution. This was an important modification for PIs. They also showed good high temperature resistance: the decomposition temperature (T5%) of two PIs were 520 °C and 475 °C. In addition, the residual rate at 790 °C was 50% and 49% respectively. Two PI films had a very low dielectric constant 2.26 and 2.24, respectively, which was attributed to the hyperbranched structure and strong electron-withdrawing fluorine-containing groups. PI-a and PI-b also had low water absorption of 0.66% and 0.72%, respectively. The transmittance of PI-a and PI-b high transparent films at 450 nm were 74% and 82%, respectively. The significant optical modification was mainly due to the weak electron-accepting alicyclic unit and the large trifluoromethyl electron withdrawing group. This work mainly provided an effective strategy for synthesizing comprehensive excellent performance PIs with high transmittance, excellent solubility, low water absorption, high heat resistance and low dielectric constant.

Rational design of AIE-based fluorescent probes for hypochlorite detection in real water samples and live cell imaging

As one kind of important disinfectant and reactive oxygen species (ROS), hypochlorite (ClO-), plays vital roles in both water treatment and cell homeostasis. In this work, by decorating a series of groups with different electron donating and withdrawing properties on tetraphenylethene (TPE), four aggregation-induced emission (AIE)-based fluorescent probes containing C˭C double bonds as the potential reaction sites named Probe A, B, C and D were constructed, and their sensing performance for ClO- was systematically studied. The results showed that the substituents can not only effectively tune the photophysical properties of the probes, but also make a significant impact on their sensing performance for ClO-. Combined with the theoretical calculation results, it can be inferred that the reactivity of the probes for ClO- can be greatly enhanced with the increase of electron cloud density on the C˭C double bonds by the introduction of strong electron-donating group (EDG) and electron-withdrawing group (EWG) adjacent to the double bonds. Finally, the best performing Probe D was selected and then successfully applied to ClO- detection in real water samples and live cell imaging.

Highly efficient deep-red TADF organic light-emitting diodes via increasing the acceptor strength of fused polycyclic aromatics

Deep-red and near-infrared organic light-emitting diodes (DR/NIR-OLEDs) showed a wide range of applications such as healthcare and night-vision. However, red emitters generally demonstrated severe non-radiative emission due to energy gap law, and the development of efficient deep-red and near-infrared thermally activated delayed fluorescence (DR/NIR-TADF) materials was still under-researched by now. Herein, we designed and synthesized a series of efficient red/DR TADF emitters. Their emission wavelengths were rationally regulated into DR region via introducing strong electron-withdrawing groups from pyridinyl (PY) to trifluoromethyl (CF3) and to cyano (CN). The internal conversion (IC) process of these emitters was suppressed greatly via the incorporation of fused polycyclic aromatics (dibenzo[a,c]phenazine). Furthermore, the structural relaxation of their excited states was also reduced due to the restricted σ bond rotation between donor (D) and acceptor (A) units. As a result, these red/DR emitters showed high photoluminescence quantum yields (PLQYs) and excellent electrofluorescence performance. Particularly, OLEDs based on CN-TPA showed remarkably high maximum external quantum efficiencies (EQEs) of 22.80% at 668 nm, 18.41% at 688 nm and 15.05% at 698 nm, which were among the best values for reported TADF emitters. This result may provide a straightforward design strategy for efficient DR/NIR TADFs through optimizing the acceptor strength of rigidified polyaromatic frameworks.