Effect Of Perfluorination Research Articles

Effect of perfluorination at the 3-ethyl group in chlorophyll-a derivatives on physical properties in solution

Methyl 3-perfluoroethyl-pyropheophorbide-a exhibited a redmost visible (Qy) absorption maximum in solution at a longer wavelength than that of the non-fluorinated counterpart, while the former Qy band was sharper and more intense than the latter. In addition, the other visible bands of the fluorinated compound were red-shifted from those of the non-fluorinated. The perfluoroethylated molecule was more fluorescent at a longer wavelength with a longer lifetime, comparing with the ethylated analog. Similar results as in the above free bases were obtained for the corresponding nickel, copper, and zinc complexes. Additionally, the nickel and copper complexes exhibited similar absorption behaviors. This observation was due to the electron-withdrawing perfluoroethyl group bonding directly to the core chlorin π-system.

Synthesis, crystal structure and Hirshfeld surface analysis of 2-(perfluoro-phen-yl)acetamide in comparison with some related compounds.

The mol-ecular and crystal structures of the title compound, C8H4F5NO, were examined by single-crystal X-ray diffraction and Hirshfeld surface analysis. The title compound was synthesized by a new method at the interface of aqueous solutions of LiOH and penta-fluoro-phenyl-aceto-nitrile. In the crystal, hydrogen bonds and π-halogen inter-actions connect the mol-ecules into double layers. Analysis of the Hirshfeld surface showed that the most important contributions to the crystal packing are made by F⋯F (30.4%), C⋯F/F⋯C (22.9%), O⋯H/H⋯O (14.9%), H⋯F/F⋯H (14.0%) and H⋯H (10.2%) contacts. The Hirshfeld surfaces of analogues of the title compound were compared and the effect of perfluorination on the crystal packing was shown.

Structural, optical, and electronic characterization of perfluorinated sexithiophene films and mixed films with sexithiophene

Abstract

The hapticity of octafluorocyclooctatetraene in its first-row mononuclear transition metal carbonyl complexes: effect of perfluorination

The iron tricarbonyl complex of octafluorocyclooctatetraene was synthesized by Hughes and co-workers and shown by X-ray crystallography to have a trihapto–monohapto structure (η3,1-C8F8)Fe(CO)3 in contrast to the tetrahapto structure (η4-C8H8)Fe(CO)3 formed by the non-fluorinated cyclooctatetraene. This difference has stimulated a comprehensive density functional theoretical study of the octafluorocyclooctatetraene metal carbonyl complexes (C8F8)M(CO)n (n = 4, 3, 2, 1 for M = Ti, V, Cr, Mn, and Fe; n = 3, 2, 1 for M = Co, Ni) for comparison with their hydrogen analogues (C8H8)M(CO)n. In most such systems, the substitution of fluorine for hydrogen leads to relatively small changes in the preferred structures. However, for the iron carbonyl derivatives (C8X8)Fe(CO)3 (X = H, F), the difference observed experimentally has been confirmed by theory with (η3,1-C8F8)Fe(CO)3 and (η4-C8H8)Fe(CO)3 being the lowest energy structures by 4 and 14 kcal/mol, respectively. The ligand exchange reactions C8H8 + (C8F8)M(CO)n → C8F8 + (C8H8)M(CO)n are predicted to be exothermic for almost all of the systems considered, with the (η3,1-C8X8)Fe(CO)3 system being the main exception. This suggests that the C8F8 ligand generally bonds more weakly to transition metals than the C8H8 ligand in accord with the electron-withdrawing effect of the ligand fluorine atoms.

Modulating the electronic properties of asymmetric push–pull and symmetric Zn(II)-diarylporphyrinates with para substituted phenylethynyl moieties in 5,15 meso positions: A combined electrochemical and spectroscopic investigation

Push–pull Zn(II)-porphyrinates have recently shown attracting performances as light harvesting systems in dye-sensitized solar cells (DSSCs). To fully exploit their intrinsically high efficiency it is important to finely tune their HOMO and LUMO levels, which can be achieved by proper choice of the push and pull substituents. Of course such target-oriented molecular design requires the availability of reliable relationships between molecular structure and electronic properties; therefore we have carried out a combined electrochemical, spectroscopic and computational investigation on a wide, systematic range of Zn(II)-porphyrinates 5,15 meso substituted with phenylethynyl linkers, including a first symmetric series carrying on the opposite terminals the same substituent (N(CH3)2, OCH3, COOCH3, COOH, NO2); and a second push–pull one, with the terminal positions carrying one donor and one acceptor group belonging to the series above. Moreover, two suitably modified porphyrins allowed evaluation of the effects of (i) the presence or absence of the phenyl group in the linker between the porphyrin core and the acceptor group, and (ii) the effect of perfluorination on the same phenyl group. A rationalization scheme is proposed encompassing the whole porphyrin set, affording inter alia interesting clues on the different localization of the redox centres and effective conjugation between the porphyrin core and the side chains as a function of the molecular design.

Electron scattering from hexafluoroacetone molecules: cross section measurements and calculations

Absolute total cross section (TCS) for electron scattering from hexafluoroacetone (HFA: (CF3)2CO) molecules has been measured from 1 to 400 eV, employing a linear electron-transmission technique. The TCS energy function for the electron–HFA collision has two broad enhancements: the first, visible between 2 and 28 eV, can be partly attributed to resonant processes; the second is much broader, centred around 50 eV, and is characteristic of perfluorinated targets. The low-energy enhancement is superimposed with weak features perceptible near 3.6, 8.5 and 17 eV. Calculations were carried out for HFA and for acetone to obtain the integral elastic and ionization cross sections at intermediate and high incident energies, using the independent atom approximation and binary-encounter-Bethe approach, respectively. The computed total (elastic plus ionization) cross sections are above 70 eV in reasonable agreement with the experimental TCSs. A comparison is made between cross sections for HFA and its per-hydrogenated counterpart, acetone ((CH3)2CO); the effect of perfluorination is indicated.

Effects of Perfluorination on Thiophene and Pyrrole Oligomers

The effect of perfluorination on thiophene and pyrrole oligomers in neutral, cationic, and anionic states was investigated with density functional theory at the (TD)B3P86-30%/6-31G* level. For the title compounds fluorination leads to planarization. For pyrroles a band gap reduction of 0.58 eV results, as unsubstituted pyrroles are nonplanar and disordered in the solid state. For thiophene the band gap is slightly increased as long thiophene oligomers are almost planar. Ionization energies and electron affinities increase upon fluorination by 0.65 and 0.60 eV for polythiophene and by 0.45 and 0.90 eV for polypyrrole. Conduction band widths increase by 0.5 for polythiophene and by 0.7 eV for polypyrrole. Spectra of charged (doped) forms are almost identical to those of the parent systems. Like parent systems, fluorinated oligomers with chain lengths of more than six rings develop a third UV absorption that increases in strength and decreases in energy upon chain length increase.

The effects of perfluorination on carbohydrate–π interactions: computational studies of the interaction of benzene and hexafluorobenzene with fucose and cyclodextrin

The effect of benzene fluorination on C-H...pi interactions is studied using a number of computational methods applied to a range of intermolecular complexes. High level wavefunction methods (CCSD(T)) predict a slightly greater interaction energy for complexes of benzene with methane or fucose, compared to corresponding complexes involving hexafluorobenzene. A number of more approximate treatments, DFT with the M06-2X functional, PM3-D* and MM methods, give interaction energies within 1 kcal mol(-1) of the high level values, and also correctly predict that the interaction energy is slightly greater for benzene compared to hexafluorobenzene. However, the DFT-D model used here predicts that the interaction energy is slightly greater for hexafluorobenzene. Molecular dynamics simulations, employing the GLYCAM-06 force field, validated here, are used to model the complexes of benzene and hexafluorobenzene with beta-cyclodextrin in aqueous solution. We predict the binding free energies of the complexes to be within 0.5 kcal mol(-1), and suggest that the different chemical shifts of the H5 protons observed in the two complexes arise from their slightly different structures, rather than from different binding energies.

The Effect of Perfluorination on the Aromaticity of Benzene and Heterocyclic Six-Membered Rings

Despite having six highly electronegative F's, perfluorobenzene C(6)F(6) is as aromatic as benzene. Ab initio block-localized wave function (BLW) computations reveal that both C(6)F(6) and benzene have essentially the same extra cyclic resonance energies (ECREs). Localized molecular orbital (LMO)-nucleus-independent chemical shifts (NICS) grids demonstrates that the F's induce only local paratropic contributions that are not related to aromaticity. Thus, all of the fluorinated benzenes (C(6)F(n)H((6-n)), n = 1-6) have similar ring-LMO-NICS(pi zz) values. However, 1,3-difluorobenzene 2b and 1,3,5-trifluorobenzene 3c are slightly less aromatic than their isomers due to a greater degree of ring charge alternation. Isoelectronic C(5)H(5)Y heterocycles (Y = BH(-), N, NH(+)) are as aromatic as benzene, based on their ring-LMO-NICS(pi zz) and ECRE values, unless extremely electronegative heteroatoms (e.g., Y = O(+)) are involved.

Electron scattering from C4H6 andC4F6molecules

Absolute electron-impact total cross sections (TCSs) for C4H6isomers (1, 3-butadiene and 2-butyne) and for hexafluoro-2-butyne (C4F6)have been measured in a linear electron-beam transmission experiment as afunction of impact energy, between 0.5 and 370 eV. For the C4H6molecule distinct differences in the shape and magnitude of TCS energydependences of both isomers are apparent at low energies. In the 1, 3-butadieneTCS curve, the enhancement below 1.2 eV, the narrow maximum centred around3.2 eV and the dominant broad peak at 9–9.5 eV are easily distinguishable; a veryweak shoulder is located near 8 eV. The TCS energy function of 2-butynedemonstrates a remarkable resonant-like maximum located near 3.6 eV andmuch broader enhancement centred at 8 eV. Above 50 eV both C4H6curves merge together. The hexafluoro-2-butyne TCS has the maximum near 8 eVand very broad enhancement spanning between 20 and 80 eV; weak shouldersnear 3 and 6 eV are also noticeable. The effect of perfluorination is indicated.

An exceptional red shift of emission maxima upon fluorine substitution.

The effect of perfluorination on photophysical properties was investigated through synthesis and photophysical characterization of two isostructural donor-acceptor-donor dye molecules. The synthesis of two versatile fluorinated benzene compounds, 1,4-difluoro-2,5-diperfluorooctylbenzene (1) and 1,4-dibromo-2,5-difluoro-3,6-diperfluorooctylbenzene (2), is presented. The X-ray structure of 2 has been determined and shows that the perfluorinated octyl chains segregate from the benzene rings in the solid state, giving rise to a layered structure. The further synthesis through Suzuki coupling reactions using 4-formylbenzeneboronic acid with (2) and 1,4-dibromo-2,5-dioctylbenzene (3) gave, respectively, 1,4' '-diformyl-2',5'-difluoro-3',6'-diperfluorooctyl-p-terphenylene (4) and 1,4' '-diformyl-2',5'-dioctyl-p-terphenylene (5). The condensation of the dialdehydes 4 and 5 with 9,10-phenanthrenequinone and ammoniumbicarbonate in glacial acetic acid gave the dye molecules 1,4' '-bis(1H-phenanthro[9,10-d]imidazol-2-yl)-2',5'-difluoro-3',6'-diperfluorooctyl-p-terphenylene (6) and 1,4' '-bis(1H-phenanthro[9,10-d]imidazol-2-yl)-2',5'-dioctyl-p-terphenylene (7), respectively. The UV-vis spectra of the two molecules are nearly identical, whereas the fluorescence spectra are very different. Compound 7 shows blue fluorescence with little solvent dependence (lambda(emission) = 410 nm in THF, CH2Cl2, and hexane), whereas compound 6 shows a highly solvent-dependent emission wavelength (lambda(emission) = 583 nm in THF, lambda(emission) = 560 nm in CH2Cl2, and lambda(emission) = 450 nm in hexane). The fluorescence red shift of compound 6 in a series of solvents with different polarity is discussed using the Lippert-Mataga equation. Fluorescence lifetime and quantum yields were also determined. Ultraviolet photoelectron spectroscopy (UPS) was performed on thin films of compound 6 and 7 on a gold substrate. The observed ionization potential was 6.15 eV for 6 and 5.85 eV for 7" [correction].

On the role of the electron impact ionisation and electron scattering cross-section in the breakdown strength of dielectric gases

Discusses the role of the electron impact ionisation cross-section sigma i( epsilon ) and the electron scattering cross-section sigma sc( epsilon ) as a function of electron energy epsilon in the breakdown strength of gases/mixtures. Four gases (Ne, Ar, N2 and SF6) were chosen for which sigma i( epsilon ) and sigma sc( epsilon ) at low energies are known. Direct-current breakdown strength measurements on Ne, Ar, (Ne+SF6), (Ar+SF6), (N2+Ne+SF6) and (N2+Ar+SF6) were made and are reported, using sphere-on-sphere to sphere and square-rod to plane electrode geometries. On the basis of these measurements and the data on sigma sc( epsilon ) and sigma i( epsilon ) it was concluded that the magnitude of sigma sc( epsilon ) at subionisation ( epsilon <I) and especially at subexcitation ( epsilon <energy of lowest excited electronic state) energies is much more significant in effecting a high dielectric strength than is sigma i( epsilon ). The effect of perfluorination and static polarisability on sigma i( epsilon ) is also discussed.

The Far U. V. Spectra of Hexafluoro 1,3-Butadiene and 1,3-Butadiene

Abstract The absorption spectra of hexafluoro 1,3-butadiene and 1,3-butadiene were measured from 2500A to 1150A. The effects of perfluorination on the spectra are discussed in the light of previous studies and recent calculations.