Effect Of Fluorine Substitution Research Articles

Effect of fluorine substitution on the electronic states and conductance of CuPc on Cu(100)

The electronic states and conductance of CuPc, F8CuPc, and F16CuPc on Cu(100) were studied by using scanning tunneling microscopy (STM) and the density functional theory (DFT) calculations. For all molecules, the lowest unoccupied molecular orbital (LUMO) was observed around the Fermi level (EF), regardless of the difference in the original LUMO levels of the free molecules. This indicates that the strong interaction causes the pinning of EF at LUMO levels. Meanwhile, the differential conductance at EF is drastically affected by the substitution: it increases and decreases for CuPc and F16CuPc, respectively, suggesting that the Fano effect plays a major role in the conductance. This finding provides a new strategy for the control of molecular conductance by chemical substitution.

Engineering D-A polymer polarity in heterostructure photocatalyst for improved interfacial charge transfer efficiency

Conventional inorganic semiconducting photocatalysts are facing insufficient light harvesting as well as severe charge recombination problems. Constructing heterostructures is a well-known strategy for accelerating charge separation, especially when hybridizing with donor–acceptor (D-A) conjugated polymers, which features tunable bandgap and high absorption coefficient, these problems can be simultaneously eased. However, the singlet exciton binding energy of D-A polymers is too high for efficient exciton dissociation. In this work, we designed fluorine substituted D-A polymer PxF-T (x = 1, 2, 4) and studied polarity effect on the interfacial charge transfer in CdS/PxF-T. By adjusting the numbers of F substitutions in the acceptor unit, the electron-withdrawing ability of A site can be improved while the polarity of the whole organic molecules is tuned. Among 1, 2, 4 substitutions, the photocatalytic hydrogen production results show that the single F substitution induces the most efficient hydrogen production activity of 6327.7 μmol·g−1·h−1, which is 30.1 times higher than that of pristine CdS. In addition, electrochemical test and time-resolved transient absorption study further confirmed the improved charge transfer activity. Density functional theory calculations reveal that the F introduction induced polarity change is responsible for the improved interfacial charge transfer efficiency, however the increased number of substitutions is not necessarily favorable for improved polarity. This work uncovers the effect of fluorine substitution on the intramolecular polarity of D-A semiconductor for the fabrication of efficient hybrid photocatalysts.

Effect of Fluorine Substitution on Absorption and Emission Properties of Figure-eight-shaped [5]Helicene Dimer : A Computational Study at RI-MP2/RI-ADC(2) Level.

Chirality is a very important characteristic of optically active molecules and polyaromatics with helical structures, and plays a vital role in various applications in material science. In the present work, we show the effects of fluorine substitution at various positions in a figure-8-shaped [5]helicene dimer on the ground and excited state $g$-factors. Calculations for the ground and excited states are performed at the MP2 and ADC(2) levels of theory, respectively. The results reveal that fluorination has a large effect on the excited state structures. The values of the excited state dissymmetry factors for the molecules with fluorinations at both ends of the figure-8 systems are smaller than that of the parent system. On the other hand, fluorinations only in the stacked-phenyl region results in an increase in the value of |gcpl|. The perfluorinated system shows the smallest |gcpl|.

Phase Transition Induced Enhanced Performance of Sodium-Rich Na1.2Mn0.8O2–yFy (y = 0–0.5) Cathodes

Increasing capacity and cycle stability is key to successfully commercializing sodium battery technologies. Currently, cation-rich cathodes are well suited for this trend owing to superior overall performance through fluorine substitution in the cation. However, the exact effects and synergy due to fluorine substitution as anions are still unknown. Understanding such synergistic effects can significantly facilitate increasing the capacity, leading to maximum performance at optimum conditions. In this work, the systematic addition of F into sodium-layered oxide and its corresponding changes in the crystal structure or phase formation are extensively studied. More specifically, the effect of fluorine substitution in the anion site on Na diffusion and the Na–Tm polyhedral are extensively studied using X-ray diffraction and subsequent Rietveld analysis. In addition, the effects of F on transition metals (i.e., Mn) and its valence states are also analyzed using X-ray absorption spectroscopy. Na1.2Mn0.8O1.5F0.5 not only showed superior capacity of approximately 172 mAh g–1 and capacity retention but also remarkable cycling stability for up to 300 cycles at higher current densities. Improvements in the performance due to distortion induced by F substitution are also presented. This study provides insight into the transition of cathode material properties from F doping (y = 0–0.2) to F substitution (y ≥ 0.3) and the associated structural changes and capacity improvements.

Efficient Quasi-2D Perovskite Light-Emitting Diodes Enabled by Regulating Phase Distribution with a Fluorinated Organic Cation.

Metal halide perovskites have become a research highlight in the optoelectronic field due to their excellent properties. The perovskite light-emitting diodes (PeLEDs) have achieved great improvement in performance in recent years, and the construction of quasi-2D perovskites by incorporating large-size organic cations is an effective strategy for fabricating efficient PeLEDs. Here, we incorporate the fluorine meta-substituted phenethylammonium bromide (m-FPEABr) into CsPbBr3 to prepare quasi-2D perovskite films for efficient PeLEDs, and study the effect of fluorine substitution on regulating the crystallization kinetics and phase distribution of the quasi-2D perovskites. It is found that m-FPEABr allows the transformation of low-n phases to high-n phases during the annealing process, leading to the suppression of n = 1 phase and increasing higher-n phases with improved crystallinity. The rational phase distribution results in the formation of multiple quantum wells (MQWs) in the m-FPEABr based films. The carrier dynamics study reveals that the resultant MQWs enable rapid energy funneling from low-n phases to emission centers. As a result, the green PeLEDs achieve a peak external quantum efficiency of 16.66% at the luminance of 1279 cd m−2. Our study demonstrates that the fluorinated organic cations would provide a facile and effective approach to developing high-performance PeLEDs.

Methylhydrazinium]2PbCl4, a Two-Dimensional Perovskite with Polar and Modulated Phases

Two-dimensional (2D) lead halide perovskites are a family of materials at the heart of solar cell, light-emitting diode, and photodetector technologies. This perspective leads to a number of synthetic efforts toward materials of this class, including those with prescribed polar architectures. The methylhydrazinium (MHy+) cation was recently presumed to have an unusual capacity to generate non-centrosymmetric perovskite phases, despite its intrinsically nonchiral structure. Here, we witness this effect once again in the case of the Ruddlesden–Popper perovskite phase of formula MHy2PbCl4. MHy2PbCl4 features three temperature-dependent crystal phases, with two first-order phase transitions at T1 = 338.2 K (331.8 K) and T2 = 224.0 K (205.2 K) observed in the heating (cooling) modes, respectively. Observed transitions involve a transformation from high-temperature orthorhombic phase I, with the centrosymmetric space group Pmmn, through the room-temperature modulated phase II, with the average structure being isostructural to I, to the low-temperature monoclinic phase III, with non-centrosymmetric space group P21. The intermediate phase II is a rare example of a modulated structure in 2D perovskites, with Pmmn(00γ)s00 superspace symmetry and modulation vector q ≅ 0.25c*. MHy2PbCl4 beats the previous record of MHy2PbBr4 in terms of the shortest inorganic interlayer distance in 2D perovskites (8.79 Å at 350 K vs 8.66 Å at 295 K, respectively). The characteristics of phase transitions are explored with differential scanning calorimetry, dielectric, and Raman spectroscopies. The non-centrosymmetry of phase III is confirmed with second harmonic generation (SHG) measurements, and polarity is demonstrated by the pyroelectric effect. MHy2PbCl4 also exhibits thermochromism, with the photoluminescence (PL) color changing from purplish-blue at 80 K to bluish-green at 230 K. The demonstration of polar characteristics for one more member of the methylhydrazinium perovskites settles a debate about whether this approach can present value for the crystal engineering of acentric solids similar to that which was recently adopted by a so-called fluorine substitution effect.

Effect of Fluorine Substitution on Magnetoresistance Pinning Energy and Irreversibility of (Cu, Tl)-1223 Phase

Effect of Fluorine Substitution on Magnetoresistance Pinning Energy and Irreversibility of (Cu, Tl)-1223 Phase

Enhancement in the inherent photostability of small molecule-based BHJ device by molecular architecturing

Three small molecules of D-A-D architecture were synthesized with benzothiadiazole as basic acceptor unit. It has been showed previously that the fluorination on the benzothiadiazole acceptor moiety and incorporation of fused thienothiophene spacer have increased the photovoltaic properties [1]. Hence the same strategies have applied here on small molecules to study both the photovoltaic and to tailor the inherent photostability of the material. The first small molecule, TBT (4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) was designed with benzothiadiazole- acceptor, and thiophene moiety as the donor unit. The effect of fluorine substitution (TDFBT- 5,6-difluoro-4,7-di(thiophen-2-yl) benzo [c][1,2,5]thiadiazole) and incorporation of fused donor moiety (TTBTT- 4,7-bis(thieno[3,2-b] thiophene-2-yl) benzo [c][1,2,5]thiadiazole) on the optical electrochemical, photovoltaic and photostability of the molecules were evaluated. The photostability study on the structurally modified molecules and blend with PCBM shows promising results. Structural modifications of the p-type molecules exhibit an improved photostability of neat films by significantly. The photostability of the active layer comprised of small molecule- PCBM blend has increased by >13.8 times on incorporation of fused thienothiophene (TTBTT) and by a factor of >5.2 on fluorination of acceptor (TDFBT). The results suggest that the structural modification p-type materials not only improve the optical, electrochemical and photovoltaic properties, but also the photostability of BHJ active layer. A careful selection of p-type materials with increased electron affinity with a sensible balancing with ambient stability and band edge matching with that of the n-type material would lead to stable photovoltaic devices.

Effect of fluorine substitution on properties of hole-transporting materials for perovskite solar cells

In photovoltaic technologies, hole-transporting materials (HTMs) play a critical role in realizing highly efficient performance of perovskite solar cells (PSCs). Herein, a series of fluorine substituted small molecules based on triphenylamine backbone are designed by controlling fluorine atoms number and position. A comprehensive study on the structural modification is carried out to reveal the effect of fluorine substitution of HTMs. The energy level alignment, reorganization energy and the charge-transport properties are explored based on Quantum-chemical calculations. It is found that all difluorinated molecules have lower HOMO levels, which indicates that perovskite solar cells with two fluorine substituted hole-transporting materials may have higher open-circuit voltages. Moreover, the UV–vis absorption spectra of the studied molecules are slightly red-shifted with the increase of fluorine atoms. To further understand the effect of fluorine substitution on the hole transport properties, the hole mobilities of all HTMs are evaluated quantitatively using the Marcus theory and Einstein relation. The calculated results show that difluorinated molecules TPB-m-2F exhibit relatively higher hole mobility owing to efficient intermolecular interactions. As for monofluorinated molecules TPB-1F-a and TPB-1F-b, TPB-1F-a with fluorine atoms substituted on the inside position exhibit higher hole-transporting performance. Overall, we conclude that suitable modification of fluorine substitution is expected to improve the electrochemical and hole transport property of HTMs. We hope our theoretical investigation provide useful information for further improving the photovoltaic performance of PSCs.

Effect of fluorine substitution on the structure and spectral property of fluorotellurite glass for upconversion luminescence thermometry

A series of TeO2–ZnO–ZnF2–La2O3–Yb2O3–Er2O3 glasses (TZL) with different amount of ZnF2 were prepared by the melt quenching method. The effects of fluorine concentration on the density, refractive index, thermal behavior, and vibrational characteristic of TZL glasses were analyzed. The Judd-Ofelt (J-O) theory was employed to study the spectroscopic properties of Er3+ ions. By using the J-O intensity parameters the spontaneous radiative transition probabilities, fluorescence branching ratios, and radiative lifetimes of some concerned transitions of Er3+ were predicted. The potential of TZL glasses for the gain around 1.5 μm was briefly evaluated. Moreover, the upconversion luminescence was investigated under the excitation of 980 nm laser. The dependence of luminescence intensity on pump power revealed that the upconversion luminescence of Er3+ in TZL glasses are contributed by two-photon involved energy transfer processes. Furthermore, the adjacent upconversion green emissions of Er3+ ions were used to temperature sensing based on the fluorescence intensity ratio (FIR) technique. High absolute sensitivity and resolution have been obtained, which indicate that the TZL glasses are potential materials for optical fiber temperature sensors.

Preparation and characterization of fluorine-substituted LiFeBO3 with carbon coating to enhance electrochemical performance and stability as a cathode material for Li-ion batteries

The fluorine-substituted LiFeBO3 with carbon coating (LFeBF/C) has been successfully demonstrated via a solid-state reaction to enhance its electrochemical properties and stability in air. Transmission electron microscopy characterization reveals that the primary LFeBF particles are fully covered by a thin layer of amorphous carbon. The formation of vonsenite-type LFeBF/C and the distribution of Li ions into its octahedral sites due to the fluorine substitution are confirmed using X-ray diffraction and solid-state nuclear magnetic resonance measurements, respectively. The stability of the LFeBF/C in air is promoted by the carbon layer. After two months of open-air storage, no significant changes in X-ray diffraction patterns can be observed. The synergistic effects of fluorine substitution and carbon coating enhance the overall electrochemical performance of the LFeBF/C as achieving a superior initial discharge capacity of 367.0 mAh g−1 at the rate of 0.05C and 231.7 mAh g−1 at 1.0C. In addition, the cyclic stability of the LFeBF/C is improved, reaching 197.5 mAh g−1 at the rate of 1.0C after 50 cycles, which corresponds to a retention of 85% of its initial capacity.

Effect of fluorine substitution on the photobiological and electronic properties of resveratrol crystal structure: A first- principles study

The electrical and optical properties of pure resveratrol (RS) and RS derivatives from substituted fluorine atom at hydrogen sites (fluorine is bonded to an oxygen or a carbon atom) are investigated by first-principles calculations. It is shown that RS is a semiconductor with an indirect bandgap of 2.47 eV. The substitution of hydrogen atoms by fluorine atoms that is bonded to oxygen atom changes the bandgap value and affect significantly on optical properties of RS. For other derivatives, the hydrogen substitution by fluorine does not change the bandgap value sensibility. The nature of band gap change from indirect to direct at F3 structure. Optical spectra of F3 to F8 derivatives are very similar to RS. Birefringenceis observed in all RS derivatives. The results show that fluorine atoms play key role in photo biological applications of RS derivatives.

Charge Photogeneration and Recombination in Fluorine-Substituted Polymer Solar Cells.

In this contribution, we studied the effect of fluorine substitution on photogenerated charge generation, transport, and recombination in polymer solar cells. Two conjugated polymer materials, PBDTTT-E (fluorine free) and PTB7 (one fluorine substitution), were compared thoroughly. Meanwhile, various characterization techniques, including atomic force microscopy, steady-state spectroscopy, transient absorption spectroscopy, spectroelectrochemistry, and electrical measurements, were employed to analyse the correlation between molecular structure and device performance. The results showed that the influence of fluorine substitution on both the exciton binding energy of the polymer and the carrier recombination dynamics in the ultrafast timescale on the polymer was weak. However, we found that the fluorine substitution could enhance the exciton lifetime in neat polymer film, and it also could increase the mobility of photogenerated charge. Moreover, it was found that the SOMO energy level distribution of the donor in a PTB7:PC71BM solar cell could facilitate hole transport from the donor/acceptor interface to the inner of the donor phase, showing a better advantage than the PBDTTT-E:PC71BM solar cell. Therefore, fluorine substitution played a critical role for high-efficiency polymer solar cells.

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Recent Advances in Molecular Design of Organic Thermoelectric Materials

Effect of Fluorine Substitution on the Electrochemical Property and Structural Stability of a Lithium-Excess Cation Disordered Rock-Salt CathodeSupported by National Natural Science Foundation of China (Grant Nos. 51602191 and 52072233), and the Beijing National Laboratory for Condensed Matter Physics.

Lithium-excess cation disordered rock-salt materials have received much attention because of their high-capacity as a candidate for cathodes for lithium-ion batteries. The ultra-high specific capacity comes from the coordinated charge compensation of both transition metal and lattice oxygen. However, the oxygen redox at high voltage usually leads to irreversible oxygen release, thereby degrading the structure stability and electrochemical performance. Lithium-excess Li1.14Ni0.57+0.5 x Ti0.19 – 0.5 x Mo0.10O2 – x F x (x = 0, 0.05, 0.10, 0.15, and 0.20) with different amounts of fluorine substitution were synthesized. Among them, Li1.14Ni0.620Ti0.140Mo0.10O1.85F0.15 exhibits a lower capacity decline, better rate performance, and lower structure damage. The effects of fluorine substitution on the electrochemical property and structural stability were systematic studied by x-ray photoelectron spectroscopy and in situ XRD etc. Results show that fluorine substitution reduces the average valence of the anion, allowing a larger proportion of low-valent redox active transition metals, increasing the transition metal redox capacity, inhibiting irreversible oxygen release and side reaction. Fluorine substitution further improves the structural stability and suppresses lattice deformation of the material.

Understanding the Effects of Fluorine Substitution in Lithium Salt on Photovoltaic Properties and Stability of Perovskite Solar Cells

Understanding the Effects of Fluorine Substitution in Lithium Salt on Photovoltaic Properties and Stability of Perovskite Solar Cells

Initial Stages of Oxidation Reactions of Ethylene Carbonate and Fluoroethylene Carbonate on LixCoO2 Surfaces: A DFT Study

Electrolytes based on fluorinated carbonate have been developed for improving the high−voltage performance of lithium ion batteries (LIBs). However, the functioning mechanism of fluorinated carbonate is still controversial. In this work, density functional theory (DFT) calculations are applied to investigate the initial oxidation reaction processes of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) on LixCoO2 () (x = 1, and 0.67) surfaces. The spin states of cobalt ions are examined for maintaining the comparability. It is demonstrated for the first time that FEC oxidation is more favorable than EC along with proton transfer thermodynamically and kinetically, which contrasts with the oxidation stability predicted based on isolated molecule model. These results can be attributed to stronger adsorption of fluorinated molecular fragment on the LixCoO2 surfaces. The fluorinated molecular fragment derived from FEC subsequently may facilitate the formation of a more stabilized/protective CEI layer, which contributes to the enhanced cathode interfacial stability and the improved high voltage cycling performance. The fundamental understanding as obtained in this work provides new insight on thinking about the effect of fluorine substitution on the oxidation stability of electrolyte solvents/additives, for developing high voltage, high energy density LIB systems.

Optical properties of novel oxyfluoride glasses on the systems of LaF3–LaO3/2–NbO5/2 and LaF3–LaO3/2–NbO5/2–AlO3/2

AbstractOxyfluoride glasses of xLaF3–(60 − x)LaO3/2–40NbO5/2 (x = 0, 5, 10, 35) and xLaF3–(60 − x)LaO3/2–30NbO5/2–10AlO3/2 (x = 0, 10, 20, 30) were prepared using a levitation technique. Both the glass‐transition temperature, Tg, and onset crystallization temperature, Tc, were lowered by substituting a part of the oxygen with fluorine in the glasses. An appropriate amount of fluorine maximized the difference between the temperatures, ΔT (= Tc − Tg), indicating the improvement in the glass‐forming ability. The atomic packing densities of the glasses were approximately 60%, which gradually increased with the fluorine content. The absorption edge of the glasses shifted toward the shorter wavelength region in the ultraviolet spectra and toward the longer region in the infrared spectra by fluorine substitution. In addition, in one of the oxyfluoride glasses, a wide transparency from 307 nm to 9.2 µm was realized. Furthermore, the glass exhibited superior optical properties, with a combination of a high refractive index, nd, of 2.020 and low wavelength dispersion, vd, of 30.1. The effect of fluorine substitution on the nd and its vd was analyzed using the Lorentz–Lorenz dispersion formula.

Reactivity of an air-stable dihydrobenzoimidazole n-dopant with organic semiconductor molecules

Summary 1,3-Dimethyl-2-(4-(dimethylamino)phenyl)-2,4-dihydro-1H-benzoimidazole, N-DMBI-H, is widely used as an air-stable n-dopant for organic semiconductors. Here, its reactivity is investigated with a variety of imide- and amide-containing semiconductor molecules that have reduction potentials in the range −0.54 to −1.10 V versus ferrocene. Reaction rates correlate poorly with these potentials. The more reactive of the imides form the corresponding radical anions cleanly, but kinetic isotope studies using N-DMBI-D indicate that the reaction proceeds via an initial hydride- or hydrogen-transfer step. For an amide- and ester-rigidified bis(styryl)benzene derivative the hydride-reduced product is stable under an inert atmosphere and can be observed directly; the radical anion can only be obtained in sub-stoichiometric yield and under certain reaction conditions. On the other hand, (N-DMBI)2 rapidly reduces all the imides and amides examined to the corresponding radical anions. The implications of these findings for dopant selection and use are discussed.

Effect of uniaxial strain on structural, electronic and optical properties of Sr2RuO4-xFx: A DFT study

• The first-principal calculations suggest a metallic ground state for S r 2 R u O 4 . • The introduction of fluorine in S r 2 R u O 4 opens the band gap. • The external uniaxial strain drastically alters the optical properties of S r 2 R u O x - 4 F x quaternary alloy. The effects of fluorine substitution and uniaxial strain on structural electronic and optical properties of superconductor S r 2 R u O 4 are investigated using the first-principal calculations. Band structure simulation suggests a metallic ground state for S r 2 R u O 4 , whereas the introduction of fluorine opens the band gap of aforementioned material. Results revealed that S r 2 R u O 4 - x F x (x = 2) quaternary alloy is an indirect-semiconductor with an energy gap of 1.519 eV. The electronic properties of S r 2 R u O 2 F 2 can be tuned by applying an external compressive and tensile strain (CTS) and also the absorption ability is improved under pressure. Other optical properties such as optical band gap, Urbach energy and optical conductivity in both x and z directions are studied. The obtained results might be useful for visible-light photoelectrical device applications.