Effect Of Fluorine Substitution Research Articles

Triple decker sandwiches and related compounds of the first row transition metals with cyclopentadienyl and hexafluorobenzene rings: remarkable effects of fluorine substitution.

The complete series of Cp2M2(μ-C6F6) (M = Ti, V, Cr, Mn, Fe, Co, Ni) structures have been examined theoretically for comparison with their unsubstituted Cp2M2(μ-C6H6) analogues. The singlet triple decker sandwich titanium complex Cp2Ti2(η(6),η(6)-C6F6) with a closed shell electronic structure and a non-planar C6F6 ring is preferred energetically by a wide margin (>20 kcal mol(-1)) over other isomers and spin states. This is in contrast to the hydrogen analogue for which related triplet spin state structures are clearly preferred. A similar low-energy triple-decker sandwich Cp2V2(η(6),η(6)-C6F6) structure is found for vanadium but with a quintet spin state. The later transition metals from Cr to Ni energetically prefer the so-called "rice-ball" cis-Cp2M2(μ-C6F6) structures with varying hapticities of metal-ring bonding, a range of formal orders of metal-metal bonding, and varying spin states depending on the metal atom. Thus the lowest energy Cp2Cr2(μ-C6F6) structures are triplet and quintet structures with pentahapto-trihapto η(5),η(3)-μ-C6F6 rings and formal Cr=Cr double bonds. This contrasts with the structure of Cp2Cr2(μ-C6H6) having a bis(tetrahapto) η(4),η(4)-C6H6 ring and a formal Cr-Cr quadruple bond. The lowest energy Cp2Mn2(μ-C6F6) structures are trans and cis quintet spin state structures. This contrasts with Cp2Mn2(μ-C6H6) for which a closed-shell singlet triple decker sandwich structure is preferred. The lowest energy Cp2Fe2(μ-C6F6) structure is a triplet cis structure with a tetrahapto-dihapto η(4),η(2)-μ-C6F6 ring and a formal Fe-Fe single bond. The lowest energy Cp2Co2(μ-C6F6) structures are singlet spin state structures with formal M-M single bonds and either bridging bis(trihapto) η(3),η(3)-C6F6 or tetrahapto-dihapto η(4),η(2)-C6F6 rings. For Cp2Ni2(μ-C6F6) low energy singlet cis and trans structures are both found. The singlet cis-Cp2Ni2(μ-C6F6) structure has a Ni-Ni single bond of length ∼2.5 Å and a bridging bis(dihapto) η(2),η(2)-C6F6 ligand with an uncomplexed C=C double bond. The singlet trans-Cp2Ni2(μ-C6F6) structure has a bis(trihapto) η(3),η(3)-C6F6 ligand.

Aerobic degradation study of three fluoroanilines and microbial community analysis: the effects of increased fluorine substitution.

The fate of fluorinated compounds in the environment, especially polyfluorinated aromatics, is a matter of great concern. In this work, 4-Fluoroaniline (4-FA), 2,4-Difluoroanilines (2,4-DFA), and 2,3,4-Trifluoroanilines (2,3,4-TFA), were chosen as the target pollutants to study their biodegradability under aerobic conditions. The required enriched time of the mixed bacterial culture for degrading 4-FA, 2,4-DFA, and 2,3,4-TFA was 26, 51, and 165 days, respectively, which suggested that the longer enrichment time was required with the increase of fluorine substitution. At the initial concentrations of 100-200 mg L(-1), the 4-FA, 2,4-DFA, and 2,3,4-TFA could be degraded completely by the mixed bacterial culture. The maximum specific degradation rates of 4-FA, 2,4-DFA, and 2,3,4-TFA were 22.48 ± 0.55, 15.27 ± 2.04, and 8.84 ± 0.93 mg FA (g VSS h)(-1), respectively. Also, the three FAs enriched cultures showed certain potential of degrading other two FAs. The results from enzyme assay suggested the expression of meta-cleavage pathways during three FAs degradation. The denaturing gradient gel electrophoresis analysis revealed that unique bacterial communities were formed after FAs enrichment and these were principally composed of β-Proteobacteria, Oscillatoriophycideae, δ-Proteobacteria, α-Proteobacteria, Thermales, Xanthomonadales, Deinococci, Flavobacteriia, and Actinobacteridae. The Shannon-Wiener indexes in three FAs enriched culture decreased with the increase of fluorine substitution, indicating the significant effect of fluorine substitution on the microbial diversity. These findings supply important information on the fate of three FAs under aerobic environment, and the bacterial communities in their degradation systems.

Effect of fluorination on electronic properties of polythienothiophene-co-benzodithiophenes and their fullerene complexes.

Fluorination of conjugated polymers is a popular way of designing new electron donors for the bulk heterojunction (BHJ) based organic solar cells (OSCs). However, not all fluorinated polymers observed experimentally enhance the power conversion efficiency of OSCs, and the fundamental understanding of the effect of fluorination is not yet fully uncovered. Herein, we report the effect of fluorine substitution on the electronic properties of polythienothiophene-co-benzodithiophenes as well as their complexes with fullerene, using density functional theory (DFT) and time-dependent DFT methods at the molecular level. Systematic computations of energy gaps (E(g)(opt) and E(g)(hl)), ionization potentials (IP), electron affinities (EA), molecular electrostatic potential (MEP) surfaces, and dipole moments (μ) are carried out for these systems. We found that the fluorination of the thienothiophene unit favors lower E(g)(opt), E(g)(hl), IP, and EA as well as stronger μ compared to the fluorination of the benzodithiophene unit, suggesting that efficient exciton dissociation and charge carriers formation may take place efficiently for the former case. These results support recent experimental findings that the performance of polythienothiophene-co-benzodithiophene-based organic solar cells enhances when thienothiophene unit is fluorinated. The present results highlight that more efficient conjugated polymers for OSC can be designed if the gap engineering is carried out by focusing on the low IP, low EA, and high dipole moment.

Decamethyl- and octamethyl-ferrocenium salts of F1- and F2-TCNQ: Effects of fluorine substitution on the crystal structures and magnetic interactions

Decamethyl- and octamethyl-ferrocenium salts of FnTCNQ (n=1, 2) were prepared and their crystal structures characterized. The 2,5-F2TCNQ salts [Fe(C5Me5)2](2,5-F2TCNQ) (1) and [Fe(C5Me4H)2](2,5-F2TCNQ) (2) exhibit one-dimensional …[D]+[A]−[D]+[A]−… mixed-stack structures, while the salts with polar acceptors [Fe(C5Me5)2][A] (A=F1TCNQ (3), 2,3-F2TCNQ (4), 2,6-F2TCNQ (5)) and [Fe(C5Me4H)2][A] (A=2,3-F2TCNQ (6), 2,6-F2TCNQ (7)) consist of [D]+[A2]2−[D]+ units involving a diamagnetic dimer of the acceptors. These salts are isomorphous to the corresponding TCNQ salts. 1 and 2 exhibit small ferromagnetic interactions at low temperatures. 1 undergoes an antiferromagnetic phase transition at TN=3.9K, which is higher than TN=2.1K of the metamagnetic polymorph of [Fe(C5Me5)2](TCNQ).

The mechanism and dynamic studies for the reactions of OH radical with CH3−nFnOCF2CHFCl (n = 0, 1, and 2)

The mechanism and dynamic properties for the multi-channel reactions of CH3OCF2CHFCl+OH (R1), CH2FOCF2CHFCl+OH (R2), and CHF2OCF2CHFCl+OH (R3) were carried out theoretically. The geometric parameters were optimized at the BMK/6-311+G(d,p) level. Subsequently, the energies were refined at the BMC-CCSD level. Based on the information of partial potential energy surface, the rate constants were evaluated by using the canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) method. For every reaction, there are two possible H-abstraction positions, i.e., CH3−nFn (n=0, 1, and 2) group and CHFCl group. The major reaction channel and the effect of fluorine substitution on the reactivity are explored. Both questions are elucidated by analyzing the reaction energy, barrier height, bond dissociation energy, and rate constants.

Low-Bandgap Conjugated Polymers for Bulk Heterojunction Solar Cells

Polymer solar cells (PSCs) have drawn much attention for applications in renewable energy because of their potential for low cost, lightweight, and large-area processability. On the basis of bulk heterojunction (BHJ) concept using blends of p-type semiconducting polymers and n-type semiconducting fullerenes, device performances have been improved rapidly. Low-bandgap conjugated polymers have been developed intensively in recent years with the aim of better matching between the absorption and solar spectra, reaching a power conversion efficiency (PCE) more than 7%. On the other hand, [6,6]-phenyl-C61-butyric acid methyl ester ([60]PCBM) and its corresponding C70 derivative ([70]PCBM) are exclusively employed as an acceptor in the highly efficient PSCs. A facile method to control HOMO−LUMO levels of polymers by combining electron-rich (donor) and electron-deficient (acceptor) moieties in their repeating units, forming internal donor−acceptor (D−A) structures, has been intensively explored. Among the numerous building blocks, carbazole is one of the most prevalent electron-rich units owing to its fully aromatic fused ring with sufficient chemical and environmental stability. Various electron-deficient fused-heteroarenes were systematically copolymerized with 2,7-carbazole toward rational design of photovoltaic conjugated polymers by Leclerc et al. in 2008. They found that the HOMO and LUMO energy levels are mainly correlated to carbazole and acceptor units, respectively, and symmetric acceptor units allow for a higher degree of crystalline order. In their study, poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)] (PCDTBT) with a relatively low HOMO level (−5.5 eV) had superior potential for applications in PSCs. The initial PCE reached 3.6% in a typical BHJ device and has been subsequently improved, exceeding 6% by thickness optimization and nanomorphology control of the BHJ layer. However, the bandgap of PCDTBT is still larger than the value (1.5 – 1.7 eV) of the ideal polymers for PCE values exceeding 10%. Recently, to lower the bandgap of PCDTBT without sacrificing the low HOMO level, further different electron-accepting monomers including phenylbenzotriazole, naphthothiadiazole, dialkoxylated benzothiadiazole, and quinoxaline-based units have been incorporated into 2,7-carbazole based copolymers. In addition, ladder-typed analogues of carbazole with forced planarity were copolymerized with benzothiadiazole unit. However, the PCE values of the PSCs with these polymers still remain at most comparable to PCDTBT-based devices.To investigate the effect of fluorine substitution on molecular and film structures, optical, electrochemical, and photovoltaic properties of a low-bandgap polymer, PCDTBT with deep HOMO energy level, a fluorinated analogue of PCDTBT (i.e., PCDTBT-F) has been developed for the first time by replacing two hydrogen atoms on benzothiadiazole (BT) units with two fluorine atoms. An analogous polymer, PCBBBT-F with additional hexylthiophenes between the thiophene and carbazole of PCDTBT-F has also been prepared to overcome the poor solubility of PCDTBT-F. The PCBBBT-F film showed wide absorption bands in UV and visible regions with an optical bandgap of 1.82 eV that is smaller than that of PCDTBT (1.89 eV), whereas the film of PCDTBT-F exhibited blue-shifted absorption with a bandgap of 1.96 eV due to the low molecular weight arising from the deficient solubility. The HOMO energy level of PCDTBT-F is lower than that of PCDTBT owing to the electron-withdrawing fluorination of the BT unit, whereas PCBBBT-F exhibited a higher HOMO level than PCDTBT, implying that the additional incorporation of electron-donating hexylthiophenes negated the fluorination effect. A BHJ-PSC that employed PCDTBT-F or PCBBBT-F as an electron donor and a fullerene derivative [70]PCBM as an electron acceptor yielded lower PCEs of 1.29% and 1.98%, respectively, than that of PCDTBT (6.16%) due to the unfavorable film structures of PCDTBT-F:[70]PCBM resulted from the poor solubility and low molecular weight as well as low crystallinities and limited exciton lifetimes of the fluorinated polymers. These results provide valuable information on the elaborate design of PCDTBT-based polymers for the PSC applications. I will also talk about other examples of low-bandgap conjugated poymers, especially phophole-based ones for BHJ solar cells.1-6 [1] T. Umeyama, K. Hirose, K. Noda, K. Matsushige, T. Shishido, Y. Matano, N. Ono, and H. Imahori, J. Phys. Chem. C, 116, 1256 (2012).[2] T. Umeyama, K. Hirose, K. Noda, K. Matsushige, T. Shishido,. H. Saarenpää, N. V. Tkachenko,. H. Lemmetyinen,. N. Ono, and H. Imahori, J. Phys. Chem. C, 116, 17414 (2012).[3] T. Umeyama, Y. Watanabe, M. Oodoi, D. Evgenia, T. Shishido, and H. Imahori, J. Mater. Chem., 22, 24394 (2012).[4] V. Manninen, M. Niskanen, T. I. Hukka, F. Pasker, S. Claus, S. Höger, J. Baek, T. Umeyama, H. Imahori, and H. Lemmetyinen, J. Mater. Chem. A, 1, 7451 (2013).[5] T. Umeyama, Y. Watanabe, E. Douvogianni, and H. Imahori, J. Phys. Chem. C, in press.[6] Y. Matano, H. Ohkubo, T. Miyata, Y. Watanabe, T. Umeyama, and H. Imahori, Eur. J. Inorg. Chem., in press.

Effect of Fluorine Substitution on the Reaction between Methylene and Acetone: A DFT Study

A theoretical study of the kinetics and mechanism of the reaction between methylene and acetone in the gas phase was performed using the density functional theory method at UB3LYP levels of theory using a 6-311G** basis set. Equilibrium molecular geometries and harmonic vibrational frequencies of the reactants, transition states, intermediates and product were calculated. These calculations showed that the reaction proceeds through an asynchronous concerted mechanism. The effect of fluorine substitution on the reaction rate was investigated: the computed activation barrier exhibits a fairly high sensitivity to the nature of substituents at the C7- and C8-positions. Gibbs free energies of activation for all structures were computed.

Effects of metastability on hydrogen sorption in fluorine substituted hydrides

In this work ab initio calculations and Calphad modelling have been coupled to describe the effect of fluorine substitution on the thermodynamics of hydrogenation–dehydrogenation in simple hydrides (NaH, AlH3 and CaH2). These example systems have been used to discuss the conditions required for the formation of a stable hydride–fluoride solid solution necessary to obtain a reversible hydrogenation reaction.

The effect of fluorine substitution in alcohol-amine complexes.

The effect of fluorine substitution on the hydrogen bond strength in alcohol-amine molecular complexes was investigated, with a combination of vapour phase infrared spectroscopy and theoretical calculations. The complexes were combined from methanol (MeOH), ethanol (EtOH) and trifluoroethanol (TFE) as the hydrogen bond donor, and either dimethylamine (DMA) or trimethylamine (TMA) as the acceptor. The fundamental OH-stretching vibration involved in hydrogen bonding was measured for all complexes, as well as the weak second NH-stretching overtone for the DMA complexes. Equilibrium constants for complex formation were determined by combining a calculated intensity and the measured integrated absorbance. The observation of two transitions in the alcohol-DMA complexes provides an opportunity for two independent determinations of the equilibrium constants. Molecular interactions between the monomers were elucidated by Natural Bond Orbital, Atoms in Molecules and Non-covalent Interactions analysis. We find that the alcohol-amine complexes with TFE as the hydrogen bond donor form stronger hydrogen bonds and that secondary interactions between the monomers increase from MeOH to EtOH to TFE. TFE is a stronger acid than EtOH and MeOH making the OH bond weaker, and the OH-stretching frequency is redshifted in TFE relative to EtOH. This redshift is small in the monomers but significantly enhanced upon complexation.

Effect of fluorine substitution on structures and reactivity of Keggin-Al13in aqueous solution: an exploration of the fluorine substitution mechanism

The structures and reactivity of fluoridated Keggin-aluminum tridecamers (K-Al13) in aqueous solution were studied using density functional theory (DFT) in order to explore the fluorine substitution process and the influence of F(-) on the chemical behavior of Keggin polynuclear Al species. The structures of fluoridated K-Al13 were optimized with the consideration of both explicit and bulk solvent effects. The obtained optimization parameters indicate that the fluorine substitution on distinct sites results in the different structural changes. The consistency between the computational and experimental (19)F and (27)Al NMR chemical shifts validates the suitable computational method for the present clusters. According to the computed energy barriers of water exchange reactions, it was found that the reactivity of η-OH2 decreases as a function of fluorine bridges. The appearance of fluorine bridges does not change the dissociative activated mechanism, but the larger dissociative trend was observed. Combining the present computational results with previous experimental observations, we reasonably put forward the fluorine substitution process and attempt to interpret the experimental phenomenon concerned at the molecular scale and further reveal the mechanism of F(-) promoting dissolution of mineral.

Semiexperimental Equilibrium Structures for cis,cis- and trans,trans-1,4-Difluorobutadiene by the Mixed Estimation Method and Definitive Relative Energies of the Isomers

Equilibrium molecular structures accurate to 0.001 Å and 0.2° have been determined for cis,cis- and trans,trans-1,4-difluorobutadiene by the semiexperimental mixed estimation method. In this method, structures are fitted concurrently to equilibrium rotational constants and bond parameters obtained from an intermediate level of electronic structure theory. The effect of fluorine substitution on the carbon backbone of butadiene is surprisingly small. Definitive energy differences for the ground states were computed, employing the focal-point analysis (FPA) technique, between the trans,trans and cis,cis isomers (ΔH°0 = 5.6(3) kJ mol(-1)) and the cis,trans and cis,cis isomers (ΔH°0 = 3.2(2) kJ mol(-1)) of 1,4-difluorobutadiene. These differences confirm the exceptional relationship that the trans,trans isomer has the highest energy and the cis,cis isomer the lowest energy, endorsing what was reported earlier on the basis of experimental observations in benzene solution.

Effects of fluorine substitution on the electrochemical performance of layered Li-excess nickel manganese oxides cathode materials for lithium-ion batteries

Li[Li1/6Ni1/4Mn7/12]O2−xFx (x = 0, 0.025, 0.05, 0.075, 0.1) as the cathode materials for rechargeable lithium batteries have been synthesized via the co-precipitation method followed by a high-temperature solid-state reaction. Field emission scanning electron microscopy images exhibit that fluorine substitution catalyzes the growth of the primary particles. Although the initial discharge capacity decreases as the fluorine content increasing, the fluorine substituted materials present significant improvement in the cycling performance. Among the synthesized materials, Li[Li1/6Ni1/4Mn7/12]O1.95F0.05 exhibits excellent high temperature (50 °C) cycling performance with a capacity retention of 93.7% after 30 cycles while the bare Li[Li1/6Ni1/4Mn7/12]O2 cathode exhibited only 73.7%.

Effect of Fluorine Substitution on Photovoltaic Properties of Benzothiadiazole–Carbazole Alternating Copolymers

To investigate the effect of fluorine substitution on molecular and film structures, optical, electrochemical, and photovoltaic properties of a moderate bandgap polymer, poly(2,7-carbazole-alt-dithienylbenzothiadiazole) (PCDTBT) with deep HOMO energy level, a fluorinated analogue of PCDTBT (i.e., PCDTBT-F) has been developed for the first time by replacing two hydrogen atoms on benzothiadiazole (BT) units with two fluorine atoms. An analogous polymer, PCBBBT-F with additional hexylthiophenes between the thiophene and carbazole of PCDTBT-F, has also been prepared to overcome the poor solubility of PCDTBT-F. The PCBBBT-F film showed wide absorption bands in UV and visible regions with an optical bandgap of 1.82 eV that is smaller than that of PCDTBT (1.89 eV), whereas the film of PCDTBT-F exhibited blue-shifted absorption with a bandgap of 1.96 eV due to the low molecular weight arising from the deficient solubility. The HOMO energy level of PCDTBT-F is lower than that of PCDTBT, owing to the electron-withd...

Direct dynamics study on the hydrogen abstraction reaction of fluoromethanes with vinyl radical

A direct dynamics method has been employed to predict the kinetic feature of the hydrogen abstraction reactions of vinyl radical (C2H3) with different fluoromethanes (CH2F2 and CHF3). The required potential energy surface information for the kinetic calculation was obtained at the QCISD(T)/6-311++G(d,p)//B3LYP/6-311G(d,p) dual-level. The calculated reaction energies are −8.3kcal/mol for the hydrogen abstraction reaction of CH2F2 with C2H3, −3.2kcal/mol for the abstraction of CHF3 with C2H3, and the two potential barriers are 9.4 and 10.4kcal/mol, respectively. The rate constants were evaluated by means of the conventional transition-state theory (TST), the canonical variational transition-state theory (CVT) with or without small-curvature tunneling (SCT) correction over the temperature range of 200–3000K. The dynamics calculations indicate that both the rate constants of the two hydrogen abstraction reactions exhibit a positive temperature dependence, in which the variational effect is negligible, whereas the tunneling effect is only considerable in the lower temperature range. Agreement between the calculated and documentary data available is good. In addition, the effects of fluorine substitution on the potential barriers and the rate constants for this class of reactions were examined.

Fluorine Substitution Effects on Flexibility and Tunneling Pathways: The Rotational Spectrum of 2‐Fluorobenzylamine

The effect of ring fluorination on the structural and dynamical properties of the flexible model molecule 2-fluorobenzylamine has been studied by rotational spectroscopy in free-jet expansion and quantum chemical methods. The complete potential energy surface originating from the flexibility of the aminic side chain has been calculated at the B3LYP/6-311++G** level of theory and the stable geometries were also characterized with MP2/6-311++G**. The rotational spectra show the presence of two of the predicted four stable conformers: the global minimum (I), in which the side chain's dihedral angle with the phenyl plane is almost perpendicular, is stabilized by an intramolecular hydrogen bond between the fluorine atom and one hydrogen of the aminic group; and a second conformer II (EII -EI ≈5 kJ mol(-1)) in which the dihedral angle is smaller and the amino group points towards the aromatic ortho hydrogen atom. This conformation is characterized by a tunneling motion between two equivalent positions of the amino group with respect to the phenyl plane, which splits the rotational transition. The ortho fluorination increases, with respect to benzylamine, the tunneling splitting of this motion by four orders of magnitude. The motion is analyzed with a one-dimensional flexible model, which allows estimation of the energy barrier for the transition state as approximately 8.0 kJ mol(-1).

Defect induced distortion of armchair and zigzag graphene and boron nitride nanoribbons

Density functional theory is used to calculate the minimum energy structures of zigzag and armchair graphene and boron nitride nanoribbons having a single vacancy and more than 100 atoms. The effect of fluorine substitution on the structure of graphene nanoribbons is also investigated. The frequencies of the structures have been calculated to verify the absence of imaginary frequencies indicating the structures are at a minimum on the potential energy surface. The results show the presence of a vacancy or substitution of a different kind of atom in the nanoribbons causes significant distortion of the ribbons from two dimensions.

Photodissociation Dynamics of C6F5Br at 234 nm: Fluorination Effects on Br/Br*Formation Pathways

Photodissociation dynamics of organic halides has been extensively investigated due to its potential to the stratosphere ozone depletion and relevant environmental problems. For example, alkyl bromide as the simplest organic halides has been used for model of photodissociation dynamics. The A-band of alkyl bromide arises from the C–Br bond localized transition and consists of three overlapping transitions to repulsive states (Q1, Q0, and Q1: on ascending order of excitation energy). The dominant transition in the A-band is to the Q0+ state which correlates to the Br(P1/2) products, suggesting the spin-orbit ground Br(P3/2) atom formation arises as a results of nonadiabatic coupling between the Q0+ and Q1 PESs via a conical intersection along the C–Br bond coordinate. Compared to the photodissociation of alkyl bromide, the photodissociation dynamics of aryl halides show more complicated because more electronic states are involved and thus making multiple dissociation pathways probable. Multiple dissociation pathways include the predissociation between excited electronic states, internal conversion processes from vibrational excited states, and dissociation processes from repulsive states. It has been unraveled by numerous studies that dissociation pathways are substantially dependent on the type of halogen, atomic substituents, and excitation wavelengths. In the early work on the bromobenzene (C6H5Br) photodissociation near 266 nm, 7-12,14 the main dissociation channel is an indirect dissociation involving the bound (π, π) and repulsive (π, σ) states. In the short wavelength, significant UV absorption of bromobenzene is caused by and transition; the mixing between these states becomes more probable and additional routes can be observed as in the case of photodissociation of iodobezene. With the purpose of elucidation of dissociation dynamics of bromobenzene at short-wavelengths, we have previously investigated 234 nm dissociation dynamics of bromobenzene using velocity ion map imaging. Observed trimodal translational distributions for Br/Br formation channels have been attributed to the direct and indirect dissociation mechanisms originating from the initially excited (π, π) state. This is indicative that dissociation mechanisms of bromobenzene at 234 nm are more complicated than those near 270 nm. In this regards, more systematic studies focused on effects of atomic substituents such as fluorine, bromine and chlorine on phenyl group are required to have a comprehensive understanding of the photodissociation dynamics of bromobenzene near 234 nm. The objective of the present work is to investigate the fluorine substitution effects in bromobenzene near 234 nm further. In this note, as continuing efforts for this purpose, we present ion-imaging study for photodissociation dynamics of pentafluorobromobenzen (C6F5Br) at 234 nm using velocity map imaging coupled with [2+1] resonance enhanced multiphoton ionization (REMPI) scheme. With aid of ab initio calculations, detailed photodissocation pathways are discussed. The velocity map imaging apparatus, similar to that used previously, consists of molecular beam chamber and main interaction chamber with TOF spectrometer. Liquid samples of C6F5Br were used without further purification. Vapors of liquid C6F5Br (10-20 torr, 98.5%) were carried by helium gas at 1.5 atm through the pulsed value operating synchronously with the pulse laser at 10 Hz. Output laser pulses from the Nd:YAG pumped dye laser (HD500, Lumonics) were frequency-doubled to emit at 234 nm with an energy range of 50-150 μJ and subsequently focused onto the skimmed molecular beam with a 150 mm focal lens. C6F5Br molecules were dissociated and Br fragments were simultaneously ionized using [2+1] REMPI schemes. With a set of ionoptics for the velocity map ion-imaging, the Br were expanded, accelerated by nonhomogeneous electric field and projected onto a 40 mm Chevron-type dual MCP coupled to phosphor screen. The transient image from the phosphor screen were taken by a charge-coupled device (CCD) camera. All timings of the pulsed value, Nd:YAG laser, and the gating pulse for MCP were manipulated by a multichannel delay generator. (SRS, DG535). Upon irradiation at 234 nm, C6F5Br dissociates to release the Br/Br photofragments. The resultant Br and Br images resulting from 234 nm photolysis of C6F5Br are presented in Figures 1(a) and 1(b), respectively. Because each image is a 2D projection of 3D distribution of photofragments with cylindrical symmetry around laser polarization axis, the shape of an image is dependent on the speed (energy) and angular distributions of fragments. From the measured images for Br and Br channels, the speed distributions (P(ν)) were extracted by integrating the 3D speed distribution over all angles at each speed, and subsequently the total translation σ n ←

The effect of fluorine substitution on chiral recognition: interplay of CH⋯π, OH⋯π and CH⋯F interactions in gas-phase complexes of 1-aryl-1-ethanol with butan-2-ol

The molecular diastereomeric complexes between R-1-phenyl-1-ethanol, S-1-(4-fluorophenyl)ethanol and S-1-(2-fluorophenyl)ethanol and R and S-butan-2-ol, isolated under molecular beam conditions in the gas phase, have been investigated by mass-selective resonant two-photon ionization (R2PI) and infrared depleted R2PI (IR-R2PI). The comparison of the three systems allowed us to highlight the significance of specific intermolecular interactions in the chiral discrimination process. The interpretation of the results is based on theoretical predictions mainly at the D-B3LYP/6-31++G** level of theory. The homo and heterochiral complexes are endowed with fine differences in intermolecular interactions, namely strong OH···O, and weaker CH···π, OH···π, CH···F as well as repulsive interactions. The presence of a fluorine atom in the para position of the aromatic ring does not influence the overall geometry of the complex whilst it affects the electron density in the π system and the strength of CH···π and OH···π interactions. The role and the importance of CH···F intermolecular interactions are evident in the complexes with fluorine substitution in the ortho position. While the ortho hetero complex is structurally analogous to the hetero para and non-fluorinated structures, butan-2-ol in the ortho homo adduct adopts a different conformation in order to establish a CH···F intermolecular interaction.

Structural variation of donor–acceptor copolymers containing benzodithiophene with bithienyl substituents to achieve high open circuit voltage in bulk heterojunction solar cells

Three new donor–acceptor copolymers P1, P2, and P3 were synthesized with benzodithiophene with bithienyl substituents as the donor and 5,6-difluorobenzo[c][1,2,5]thiadiazole, 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole, and 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole as the acceptors, respectively. The insertion of thiophene spacer between the donor and the acceptor broadened the absorption of the polymers P2 and P3 and resulted in a red shift of ∼30 nm as compared to that of the polymer P1. However, the inclusion of fluorine atoms on the polymer had detrimental effects on the photovoltaic properties of the polymers. The synthesized donor–acceptor polymers were tested in bulk heterojunction (BHJ) solar cells with [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) acceptor. Polymer P2 gave a PCE of 3.52% with PC71BM in which the active layer was prepared in chloroform with 3% v/v 1,8-diiodooctane (DIO) additive. The effect of fluorine substitution and thiophene group insertion on the UV/Vis absorbance, photovoltaic performances, morphology, and charge carrier mobilities for the polymers are discussed.

Effect of Fluorine Substitution on the Aromaticity of Polycyclic Hydrocarbons

The effect of fluorine substitution on the aromaticity of polycyclic hydrocarbons (PAH) is investigated. Magnetically induced current densities, current pathways, and current strengths, which can be used to assess molecular aromaticity, are calculated using the gauge-including magnetically induced current method (GIMIC). The degree of aromaticity of the individual rings is compared to those obtained using calculated nucleus-independent chemical shifts at the ring centers (NICS(0) and NICS(0)(zz)). Calculations of explicitly integrated current strengths for selected bonds show that the aromatic character of the investigated polycyclic hydrocarbons is weakened upon fluorination. In contrast, the NICS(0) values for the fluorinated benzenes increase noteworthy upon fluorination, predicting a strong strengthening of the aromatic character of the arene rings. The integrated current strengths also yield explicit current pathways for the studied molecules. The current pathways of the investigated linear polyacenes, pyrene, anthanthrene, coronene, ovalene, and phenanthro-ovalene are not significantly affected by fluorination. NISC(0) and NICS(0)(zz) calculations provide contradictory degrees of aromaticity of the fused individual ring. Obtained NICS values do not correlate with the current strengths circling around the individual rings.