Electron-withdrawing Trifluoromethyl Research Articles

Theoretical Exploration of Energy Transfer and Single Electron Transfer Mechanisms to Understand the Generation of Triplet Nitrene and the C(sp3)-H Amidation with Photocatalysts.

Mechanistic explorations and kinetic evaluations were performed based on electronic structure calculations at the CASPT2//CASSCF level of theory, the Fermi's golden rule combined with the Dexter model, and the Marcus theory to unveil the key factors regulating the processes of photocatalytic C(sp3)-H amidation starting from the newly emerged nitrene precursor of hydroxamates. The highly reactive nitrene was found to be generated efficiently via a triplet-triplet energy transfer process and to be benefited from the advantages of hydroxamates with long-range charge-transfer (CT) excitation from the N-centered lone pair to the 3,5-bis(trifluoromethyl)benzoyl group. The properties of the metal-to-ligand charge-transfer (MLCT) state of photocatalysts, the functionalization of chemical moieties for substrates involved in the charge-transfer (CT) excitation, such as the electron-withdrawing trifluoromethyl group, and the energetic levels of singlet and triplet reaction pathways may regulate the reaction yield of C(sp3)-H amidation. Kinetic evaluations show that the triplet-triplet energy transfer is the main driving force of the reaction rather than the single electron transfer process. The effects of electronic coupling, molecular rigidity, and excitation energies on the energy transfer efficiency were further discussed. Finally, we investigated the inverted behavior of single-electron transfer, which is correlated unfavorably to the catalytic efficiency and amidation reaction. All theoretical explorations allow us to better understand the generation of nitrene with visible-light photocatalysts, to expand highly efficient substrate sources, and to broaden our scope of available photosensitizers for various cross-coupling reactions and the construction of N-heterocycles.

Highly Stable and Moisture-Immune Monocomponent White Perovskite Phosphor by Trifluoromethyl (-CF3) Regulation.

Halide perovskites are emerging as promising candidates for white light solid state lighting. Nevertheless, there are still challenges of a high water stability, a tunable color temperature, and a high photoluminescence quantum yield (PLQY). Herein, we report hydrophobic, electron-withdrawing trifluoromethyl (-CF3)-modified phenethylamine lead bromide (PEA2PbBr4) with ultrahigh stability in water for >2 months, and the broadband white light emission is illustrated by self-trapped excitons attributed to exciton-phonon coupling that coordinate molecular vibration, lattice distortion, and electrostatic interaction. In particular, by Mn2+ doping, the emission color can be tuned from cold (10237 K) to warm (2406 K), and a greatly enhanced PLQY of ≤87.93% can be achieved. Furthermore, the perovskites also possess an excellent color rendering index (the highest is 94). A monocomponent white light-emitting diode with amazing CIE 1931 coordinates of (0.33, 0.32) is further assembled, demonstrating a luminance of 471.5 cd m-2 at 50 mA and good long-term operation stability after >2 months. This study of highly efficient and stable perovskites with high-quality white light emission will open up new opportunities in solid state lighting.

Homoleptic Ir(III) Phosphors with 2-Phenyl-1,2,4-triazol-3-ylidene Chelates for Efficient Blue Organic Light-Emitting Diodes

In this report, we synthesized two series of deep-blue-emitting homoleptic iridium(III) phosphors bearing 1,2,4-triazol-3-ylidene and 5-(trifluoromethyl)-1,2,4-triazol-3-ylidene cyclometalate. Compared with reported synthetic routes using Ag2O as the promoter, herein, we adopted a different strategy to furnish these complexes in high yields. Also, the meridional to facial isomerization was executed in the presence of trifluoroacetic acid. These phosphors were examined using NMR spectroscopies, single-crystal X-ray diffraction studies, and photophysical methods. The results revealed that electron-withdrawing trifluoromethyl substitution on the N-heterocyclic carbene fragment only gave a minor variation of photoluminescence peak wavelengths and a decrease in radiative lifetime but notable reduction in thermal stabilities. The parent 1,2,4-triazol-3-ylidene complexes have been demonstrated to be suitable for use as deep-blue phosphors, with structured emission with the peak max. located at ∼420 nm and with photoluminescence quantum yields in a range of 34.8-42.5% in degassed THF solution at RT. Fabrication of both the phosphorescent organic light-emitting diodes (OLEDs) and phosphor-sensitized OLEDs (or hyperphosphorescence) was successfully conducted, from which the OLED device based on m-tz1 showed a max. external quantum efficiency (EQE) of 10% with CIEx,y coordinates of 0.15, 0.06, while the corresponding hyperphosphorescent OLED using m-tz2 as a sensitizer and t-DABNA as a terminal emitter afforded a significantly improved max. EQE of 19.7%, EL λmax of 468 nm, and FWHM of 31 nm with CIEx,y coordinates of 0.12, 0.13.

Rational construction of a reversible arylazo-based NIR probe for cycling hypoxia imaging in vivo

Reversible NIR luminescent probes with negligible photocytotoxicity are required for long-term tracking of cycling hypoxia in vivo. However, almost all of the reported organic fluorescent hypoxia probes reported until now were irreversible. Here we report a reversible arylazo-conjugated fluorescent probe (HDSF) for cycling hypoxia imaging. HDSF displays an off-on fluorescence switch at 705 nm in normoxia-hypoxia cycles. Mass spectroscopic and theoretical studies confirm that the reversible sensing behavior is attributed to the two electron-withdrawing trifluoromethyl groups, which stabilizes the reduction intermediate phenylhydrazine and blocks the further reductive decomposition. Cycling hypoxia monitoring in cells and zebrafish embryos is realized by HDSF using confocal imaging. Moreover, hypoxic solid tumors are visualized and the ischemia-reperfusion process in mice is monitored in real-time. This work provides an effective strategy to construct organic fluorescent probes for cycling hypoxia imaging and paves the way for the study of cycling hypoxia biology.

Hard and soft Lewis-base behavior for efficient and stable CsPbBr3 perovskite light-emitting diodes

Abstract All-inorganic CsPbBr3 perovskite is an attractive emission material for high-stability perovskite light-emitting diodes (PeLEDs), due to the high thermal and chemical stability. However, the external quantum efficiencies (EQEs) of CsPbBr3 based PeLEDs are still far behind their organic–inorganic congeners. Massive defect states on the surface of CsPbBr3 perovskite grains should be the main reason. Lewis base additives have been widely used to passivate surface defects. However, systematic investigations which relate to improving the passivation effect via rational molecule design are still lacking. Here, we demonstrate that the CsPbBr3 film’s optical and electrical properties can be significantly boosted by tailoring the hardness–softness of the Lewis base additives. Three carboxylate Lewis bases with different tail groups are selected to in-situ passivate CsPbBr3 perovskite films. Our research indicates that 4-(trifluoromethyl) benzoate acid anion (TBA−) with the powerful electron-withdrawing group trifluoromethyl and benzene ring possesses the softest COO− bonding head. TBA− thus acts as a soft Lewis base and possesses a robust combination with unsaturated lead atoms caused by halogen vacancies. Based on this, the all-inorganic CsPbBr3 PeLEDs with a maximum EQE up to 16.75% and a half-lifetime over 129 h at an initial brightness of 100 cd m−2 is thus delivered.

Triplet Excited States Modulated by Push-Pull Substituents in Monocyclometalated Iridium(III) Photosensitizers.

A series of novel monocyclometalated [Ir(tpy)(btp)Cl]+ complexes (Ir2-Ir5) were synthesized using 2,2':6',2″-terpyridine (tpy) and 2-(2-pyridyl)benzo[b]thiophene (btp) ligands, as well as their derivatives bearing electron-donating tert-butyl (t-Bu) and electron-withdrawing trifluoromethyl (CF3) groups. Ir2-Ir5 exhibited visible-light absorption stronger than that of the known complex [Ir(tpy)(ppy)Cl]+ (Ir1; ppy = 2-phenylpyridine). Spectroscopic and computational studies revealed that two triplet states were involved in the excited-state dynamics. One is a weakly emissive and short-lived ligand to ligand charge-transfer (LLCT) state originating from the charge transfer from the btp to the tpy ligand. The other is a highly emissive and long-lived ligand-centered (LC) state localized on the btp ligand. Interestingly, the excited state dominant with 3LLCT was completely changed to the 3LC state upon the introduction of substituents on both the tpy and btp ligands. For instance, the excited state of the parent complex Ir2 was weakly emissive (Φ = 2%) and short-lived (τ = 110 ns) in CH2Cl2; conversely, Ir5, fully furnished with t-Bu and CF3 groups, displayed intense phosphorescence with a prolonged lifetime (τ = 14 μs). This difference became increasingly prominent when the solvent was changed to aqueous CH3CN, most probably due to the 3LLCT stabilization. The predominant excited-state nature was switchable between the 3LLCT and 3LC states depending on the substituents employed; this was demonstrated through investigations of Ir3 and Ir4, bearing either the t-Bu or the CF3 group, where the complexes exhibited properties intermediate between those of Ir2 and Ir5. All of the Ir(III) complexes were tested as photosensitizers in photocatalytic H2 evolution over a Co molecular catalyst, and Ir5 outperformed the others, including Ir1, due to improvement in the following key properties: visible-light-absorption ability, excited-state lifetime, and reductive power of the one-electron-reduced species against the catalyst.

Curvature-regulated transmembrane anion transport by a trifluoromethylated bisbenzimidazole

In this paper, we demonstrate that modification of anion-transport active 1,3-bis(benzimidazol-2-yl)benzene with strongly electron-withdrawing trifluoromethyl and nitro groups leads to a dramatic increase in the anionophoric activity, and the activity may be greatly regulated by the curvatures of the liposomes used.

Synthesis of trifluoromethyl substituted nucleophilic glycine equivalents and the investigation of their potential for the preparation of α-amino acids

Abstract The synthetic preparation of several Ni(II) complexed Schiff Bases of glycine will be introduced, as well as investigations into their reactivity and utility. Key to these investigations is the incorporation of electron-withdrawing trifluoromethyl groups within the framework of the conjugated system that stabilizes the enolate derived from the glycine component. Reactivity was evaluated for each of the complexes under phase transfer catalyzed alkylations with hydroxide bases, as well as the DBU catalyzed Michael Additions of optically active 3′-substituted-2-oxazoladinone amides of unsaturated carboxylic acids. It was found that the trifluoromethyl containing nucleophilic glycine equivalents were more reactive than their non-trifluoromethyl analogues in both reaction types. Therefore, the application of these modified Ni(II) complexes of glycine Schiff Bases are useful for the preparation of α-amino acids through phase transfer catalyzed alkylation as well as the preparation of optically pure β-substituted pyroglutamic acid precursors.

The electron inductive effect of dual non-conjugated trifluoromethyl acceptors for highly efficient thermally activated delayed fluorescence OLEDs

Recently, pure organic twisted donor-acceptor (D-A) structured thermally activated delayed fluorescence (TADF) materials have been extensively demonstrated as a new generation emitter in organic light-emitting diodes (OLEDs) due to their low-cost and high-efficiency. Herein, we designed and synthesized a highly twisted TADF emitter 2,3,5,6-tetra(3,6-di-tert-butyl-9H-carbazol-9-yl)bis(trifluoromethyl)benzene (4tCzDCF3Ph) by a facile catalyst-free C-N coupling reaction. The two electron-withdrawing non-conjugated trifluoromethyl (CF3) moieties bearing strong electron inductive effects instead of other widely reported conjugation effects contributes for the necessary D-A interactions for TADF emission. 4tCzDCF3Ph exhibited an extremely small singlet-triplet energy gap (ΔEST) of 0.02 eV, fast kRISC rate of 7.02 × 105 s−1 and modest photoluminescence quantum yield (PLQY) of ~70%. A low turn-on voltage of 2.6 V, along with maximum current efficiency of 50.6 cd A−1, power efficiency of 40.7 lm W−1 and external quantum efficiency (EQE) of 16.2% could be facilely achieved for 4tCzDCF3Ph based green TADF OLEDs, which is significantly higher than <1% of 4CzCF3Ph and 9.5% of 5CzCF3Ph TADF emitters based on single trifluoromethyl acceptor.

Preparation and Characterization of Semi-alicyclic Polyimides Containing Trifluoromethyl Groups for Optoelectronic Application.

Transparent polyimides (PI) films with outstanding overall performance are attractive for next generation optoelectronic and microelectronic applications. Semi-alicyclic PIs derived from alicyclic dianhydrides and aromatic diamines have proved effective to prepare transparent PIs with high transmittance. To optimize the combined properties of semi-alicyclic PIs, incorporating bulky trifluoromethyl groups into the backbones is regarded as a powerful tool. However, the lack of fundamental understanding of structure–property relationships of fluorinated semi-alicyclic PIs constrains the design and engineering of advanced films for such challenging applications. Herein, a series of semi-alicyclic PIs derived from alicyclic dianhydrides and trifluoromethyl-containing aromatic diamines was synthesized by solution polycondensation at high temperature. The effects of alicyclic structures and bulky trifluoromethyl groups on thermal, dielectric and optical properties of PIs were investigated systematically. These PI films had excellent solubility, low water absorption and good mechanical property. They showed high heat resistance with Tg in the range of 294–390 °C. It is noted that tensile strength and thermal stability were greatly affected by the rigid linkages and alicyclic moieties, respectively. These films exhibited obviously low refractive indices and significantly reduced dielectric constants from 2.61 to 2.76, together with low optical birefringence and dielectric anisotropy. Highly transparent films exhibited cutoff wavelength even as low as 298 nm and transmittance at 500 nm over 85%, displaying almost colorless appearance with yellowness index (b*) below 4.2. The remarkable optical improvement should be mainly ascribed to both weak electron-accepting alicyclic units and bulky electron-withdrawing trifluoromethyl or sulfone groups. The present work provides an effective strategy to design molecular structures of optically transparent PIs for a trade-off between solution-processability, low water uptake, good toughness, high heat resistance, low dielectric constant and excellent optical transparency.

Light Emission in Betalains: From Fluorescent Flowers to Biotechnological Applications.

The discovery of visible fluorescence in the plant pigments betalains revealed the existence of fluorescent patterns in flowers of plants of the order Caryophyllales, where betalains substitute anthocyanins. The serendipitous initial discovery led to a systemized characterization of the role of different substructures on the photophysical phenomenon. Strong fluorescence is general to all members of the family of betaxanthins linked to the structural property that the betalamic acid moiety is connected to an amine group. This property has led to bioinspired tailor-made probes and to the development of novel biotechnological applications in screening techniques or microscopy labeling. Here, we comprehensively review the photophysics, photochemistry, and photobiology of betalain fluorescence and describe all current applications.

Highly efficient solid-state emission of diphenylfumaronitriles with full-color AIE, and application in explosive sensing, data storage and WLEDs

Luminescent materials with aggregation-induced emission (AIE) have attracted extensive attentions for their strong emission in aggregated states. Although all kinds of AIE luminogens have been recently developed, it is difficult to realize full-color AIE which is popular for the various applications in optoelectronic fields. Herein, a class of diphenylfumaronitrile core-based dyes emitting in a wide region from near ultraviolet (382 nm) to near-infrared (682 nm) have been synthesized and well characterized. It is found that the emission wavelength of the dyes severely depends on the peripheral substituent, including electron-withdrawing trifluoromethyl (1), methyl (2), electron-pushing methoxyl (3), and bulky aromatic amine (7–9), and that the substituting location imposes great effect on the emission intensity of multiple-substituted dyes (4–6). Compound 4 with two methyl substituting at 3,4-position gives a ultrahigh quantum yield of 93% for unique packing structure. 1–3 and 7 can be used in explosive detecting and data storage for their emission sensitive to picric acid and external stimuli, respectively. When 4 and 8 applied in white-light-emitting diodes (WLEDs), a pure white emission can be obtained with 90 of CRI and (0.32, 0.32) of CIE.

Synthesis of Quinoxaline-Based Small Molecules Possessing Multiple Electron-Withdrawing Moieties for Photovoltaic Applications

Three quinoxaline-based small molecules possessing multiple electron-withdrawing moieties were synthesized by the Suzuki coupling reaction for organic photovoltaic cells (OPVs). The electron-donating triarylamine units were linked to both ends of electron-accepting 2,3-diphenyl quinoxaline (DPQ) derivatives with strong electron-withdrawing trifluoromethyl (CF3) moieties to produce a reference D-A-D type small molecule of CF3Qx-0F. Furthermore, one and two fluorine atoms were additionally introduced to the 6,7-positions ofthe DPQ unit of CF3Qx-0F affording CF3Qx-1F and CF3Qx-2F, respectively. Owingto the significant contribution of the electron-withdrawing CF3 and fluorine units, all inverted-type OPVs based on three small molecules exhibited high open circuit voltages greater than 0.82 V. In addition, the power conversion efficiencies (PCEs) of the devices were gradually improved with increasing number of fluorine atoms. The highest PCE (2.82%) with a Voc of 0.88 V, a short-circuit current of 6.38 mA cm−2, and a fill factor of 50.6% was achieved from the device based on CF3Qx-2F.

Effect of multiple electron-withdrawing substituents on photovoltaic properties of quinoxaline-based polymers

Two quinoxaline-based polymers, in which the electron-donating 2-octyldodecyl modified benzodithiophene (BDT) group was connected to the electron-accepting 2,3-diphenyl quinoxaline (Qx) derivatives through a thiophene bridge, were synthesized by the Stille coupling reaction. Interestingly, the strong electron-withdrawing trifluoromethyl (CF3) substituents were introduced at para-position of the benzene ring at 2,3-positions Qx to yield PBDT-CF3Qx. Moreover, two fluorine atoms were incorporated at the 6,7-positions of Qx in PBDT-CF3Qx to produce PBDT-CF3QxF. This simple approach demonstrate the effect of multiple electron-withdrawing moieties on the optical, electrochemical, and photovoltaic properties of Qx-based polymers.

Enhanced photovoltaic performance of quinoxaline-based small molecules through incorporating trifluoromethyl substituents

Two D-A-D type quinoxaline-based small molecules have been synthesized by Suzuki coupling reaction for solution-processable organic photovoltaic cells (OPVs). The electron-donating triphenylamine was connected to both ends of an electron-withdrawing 2,3-diphenylquinoxaline core through thiophene bridge to produce QxTPA. In addition, QxCF3TPA was formed by introducing strong electron-withdrawing trifluoromethyl (CF3) groups into the para-position of the phenyl ring at the 2,3-position of quinoxaline core in QxTPA to explore their effect on the various properties of small molecules. The finding revealed the significant contributions of CF3 substituents in enhancing the photovoltaic performances of OPVs with QxCF3TPA compared to the reference case with QxTPA.

Electronically Divergent Triscyclometalated Iridium(III) 2-(1-naphthyl)pyridine Complexes and Their Application in Three-Component Methoxytrifluoromethylation of Styrene

A systematic study of the photophysical and electrochemical properties of triscyclometalated homoleptic iridium(III) complexes based on 2-(1-naphthyl)pyridine (npy) ligands is presented. A systematic investigation of ligand substitution patterns showed an influence on the lifetime of the excited state, with slight changes in the absorption and emission spectral features. Specifically, the emission lifetime of a complex of an npy ligand substituted with a strongly electron-withdrawing trifluoromethyl group was longer than that of the corresponding complex with the electronically nonperturbed ligand (3.7 μs versus 1.5 μs). Electronically complementary ligands and complexes with ­orthogonal configurations showed slightly shorter excited state lifetimes compared with unsubstituted npy (1.4–3.0 μs). All complexes displayed reversible or quasireversible redox-couple processes, with the complex of the trifluoromethylated ligand showing the highest ground-state oxidation potential E1/2 ox [Ir(III)/Ir(IV) = 0.95 V vs. SCE in CH2Cl2]. This study showed that these complexes can be used as efficient photoredox catalysts, as demonstrated by their application in a regioselective methoxytrifluoromethylation in which the npy complexes showed equal or better performance compared with the archetypical photo­redox catalyst tris[2-phenylpyridinato]iridium(III).

A Review on the Scope of TFDO-Mediated Oxidation in Organic Synthesis-- Reactivity and Selectivity

Dioxiranes are three-membered strained ring peroxides that are typical archetype examples of electrophilic entities. A dioxirane-based oxidant named 3-methyl(trifluoromethyl)dioxirane (TFDO) is a fluorinated analogue of the extremely valuable oxidant dimethyldioxirane (DMDO). Owing to the strained threemembered ring and presence of electron-withdrawing trifluoromethyl group, TFDO is several times more reactive than DMDO and acts as a significant chemical reagent. Moreover, TFDO exhibits high regio-, chemo- and stereo-selectivity even under unusual reaction conditions, i.e. at pH values close to neutrality and at subambient temperatures. The TFDO transfers an oxygen atom to “unactivated” carbon-hydrogen bonds of alkanes as well as to the double bonds of alkenes and also helps in oxidation of compounds containing heteroatoms having a lone pair of electrons, such as sulfides and amines. TFDO-mediated oxidation is considered to be one of the main procedures in the 21st century for the synthesis of oxygen-containing organic molecules. This review throws light on the applications of TFDO in organic syntheses to provide an insight into the future research and gives a comprehensive summary of the selective functionalization of activated and non-activated organic compounds.

Enhanced open-circuit voltages of trifluoromethylated quinoxaline-based polymer solar cells

Abstract Three quinoxaline-based conjugated polymers with donor-π-acceptor configurations have been synthesized by Stille coupling reaction. The electron-donating 2,3-dioctylthienyl- substituted benzodithiophene (BDT) unit was linked to the electron-accepting 2,3-diphenylquinoxaline (DPQ) group through a thiophene bridge, to produce a reference polymer PTBDT-Qx. Furthermore, the strong electron-withdrawing trifluoromethyl moieties were introduced at the para-position of the phenyl groups in the 2,3-positions of DPQ and 6,7-difluorinated DPQ, to afford PTBDT-QxCF3 and PTBDT-FQxCF3, respectively. Owing to the continuous reduction in their HOMO energy levels with increasing number of electron-withdrawing groups, the open-circuit voltage (Voc) in polymer solar cells (PSCs) shows a gradual improvement in the order of PTBDT-Qx

Synthesis of Trifluoromethylated Quinoxaline-Based Polymers for Photovoltaic Applications.

A series of quinoxaline-based conjugated polymers, in which the electron-donating benzodithiophene (BDT) unit is linked to the electron-accepting 6,7-difluorinated quinoxaline (DFQ) derivatives by a thiophene bridge, is synthesized. To investigate their effects on the intrinsic properties of polymers, strong electron-withdrawing trifluoromethyl (CF3 ) groups were incorporated into the meta-position of the phenyl ring at the 2,3-positions of the DFQ unit of the reference polymer, labelled PEhB-FQx, to yield the target polymer PEhB-FQxCF3. In addition, the 2-ethylhexyloxy substituents on the BDT donor in PEhB-FQxCF3 are changed to the more planar 2-ethylhexyl thiophene units to produce another target polymer PThB-FQxCF3. Owing to the significant contributions of the CF3 moiety, PEhB-FQxCF3 exhibits quite discernible optical and electrochemical properties along with significant enhancement in photovoltaic performances compared to the reference polymer PEhB-FQx. Furthermore, the incorporation of the alkylthienyl side chains on the BDT moiety confers on the resultant PThB-FQxCF3 to possess the maximum power conversion efficiency of 7.26% with an open circuit voltage of 0.88V, short-circuit current density of 12.20mA cm-2 , and fill factor of 67.80%.

Rhodium-catalyzed Enantioselective Synthesis and Properties of Silicon-stereogenic Benzofuranylmethylidene-benzoxasiloles

The enantioselective synthesis of silicon-stereogenic benzofuranylmethylidene-benzoxasiloles was achieved with moderate enantioselectivity by the cationic rhodium(I)/(S)-BINAP complex-catalyzed desymmetrization of symmetrical bis(2-ethynylphenol)silanes, possessing the tert-butyl and methyl groups on the silicon atom. This reaction involves 1,2-silicon/1,3-carbon migrations and oxycyclization, and the 1,2-silicon migration constructs the stereogenic silicon center. The thus obtained benzofuranylmethylidene-benzoxasiloles with the electron-donating methoxy, methyl, and phenyl groups on two benzene rings showed good fluorescence quantum yields in solution state, on the other hand, that with the electron-withdrawing trifluoromethyl groups on two benzene rings showed good fluorescence quantum yield in solid state.