Electron photodetachment from iodide in ionic liquids through charge-transfer-to-solvent band excitation

Solvation of iodide and electrons in an ionic liquid (N,N,N-trimethyl-n-propylammonium bis(trifluoromethanesulfonyl)imide; TMPA-TFSI) was studied through the absorption spectra of the charge-transfer-to-solvent (CTTS) state of iodide and of solvated electrons. The interaction between the TMPA cation and iodide was strong, whereas electrons were weakly solvated in TMPA-TFSI. We followed electron photodetachment from iodide to the ionic liquid and formation of the solvated electrons by observing absorption in the visible and near-infrared regions using a nanosecond laser flash photolysis method. The quantum yield of the photodetachment in TMPA-TFSI was estimated to be 0.34, which is much higher than that in a high-concentration aqueous salt solution previously reported. We also examined a reaction of the solvated electrons with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (Bmim-TFSI) as a solute in TMPA-TFSI. The reaction rate was determined to be 5.3 x 10(8) M(-1) s(-1). The electrons before full solvation (dry electrons) reacted with Bmim cations efficiently. These observations suggest that the electrons in TMPA-TFSI can move easily before solvation.     

Electron crystallography and organic materials with non-linear optical properties

Results of electron microscopic studies of crystal structures of a number of bis-benzylidene cyclohexanones are presented. It is shown that some of these compounds are efficient crystalline non-linear optically active (NLO) chromophores with second harmonic generation (SHG) properties. Appropriately functionalized chromophores of this type can be used as a polycondensation comonomer to produce partly crystalline main-chain NLO-active polymers. Electron diffraction crystal structural data, obtained for very small crystals, allowed us to get reasonable estimations of macroscopic crystal NLO-coefficients, relating quantum-chemically calculated molecular first hyperpolarizability components to crystal axes.     

Electron emission from laser-heated fullerene dianions: probing the repulsive Coulomb barrier

Electron detachment from fullerene dianions Cm2- (m=76,84) stored in a room temperature Penning trap was probed upon pulsed laser excitation at wavelengths of 355 and 532 nm. The fraction of Cm(2-) surviving trapping times exceeding tens of milliseconds under UHV conditions, as well as the fraction of singly charged anions Cm(-) generated were recorded as a function of the laser fluence. Analysis by means of Poisson statistics yields absolute absorption cross sections and the number of photons necessary to induce the detachment. The cross sections obtained are in good agreement with the literature values. By describing the electron detachment as a statistical unimolecular process, we deduce effective activation energies from the number of photons required. These energies are compared to the sum of the second electron affinity and the Coulomb barrier height as calculated from an electrostatic charging model.     

Electron photodetachment from aqueous anions. 3. Dynamics of Geminate pairs derived from photoexcitation of mono- vs polyatomic anions

Photostimulated electron detachment from aqueous inorganic anions is the simplest example of solvent-mediated electron transfer reaction. As such, this photoreaction became the subject of many ultrafast studies. Most of these studied focused on the behavior of halide anions, in particular, iodide, that is readily accessible in the UV. In this study, we contrast the behavior of these halide anions with that of small polyatomic anions, such as pseudohalide anions (e.g., HS(-)) and common polyvalent anions (e.g., SO(3)(2-)). Geminate recombination dynamics of hydrated electrons generated by 200 nm photoexcitation of aqueous anions (I(-), Br(-), OH(-), HS(-), CNS(-), CO(3)(2-), SO(3)(2-), and Fe(CN)(6)(4-)) have been studied. Prompt quantum yields for the formation of solvated, thermalized electrons and quantum yields for free electrons were determined. Pump-probe kinetics for 200 nm photoexcitation were compared with kinetics obtained at lower photoexcitation energy (225 or 242 nm) for the same anions, where possible. Free diffusion and mean force potential models of geminate recombination dynamics were used to analyze these kinetics. These analyses suggest that for polyatomic anions (including all polyvalent anions studied) the initial electron distribution has a broad component, even at relatively low photoexcitation energy. There seems to be no well-defined threshold energy below which the broadening of this electron distribution does not occur, as is the case for halide anions. The constancy of (near-unity) prompt quantum yields vs the excitation energy as the latter is scanned across the lowest charge-transfer-to-solvent band of the anion is observed for halide anions but not for other anions: the prompt quantum yields are considerably less than unity and depend strongly on the excitation energy. Our study suggests that halide anions are in the class of their own; electron photodetachment from polyatomic, especially polyvalent, anions exhibits qualitatively different behavior.     

Two-photon absorption and nonlinear optical properties of octupolar molecules.

Two-photon absorption (TPA) cross sections of four representative series of octupolar molecules are theoretically investigated. The general structure--TPA-property relationship is described by using the effective four-state valence-bond three-charge-transfer model. As the charge-transfer character of the ground electronic state increases due to the strong donors or acceptors, (i) the transition dipole matrix elements between the ground and 2-fold degenerate excited states increase, (ii) the energy gap decreases, and consequently (iii) the TPA transition amplitude monotonically increases. Thus, the design strategy to maximize the TPA cross section of the octupolar molecule is established. On the basis of the four-state model, the first hyperpolarizability of the octupolar molecule is found to be linearly proportional to the TPA cross section. This theoretical relationship is confirmed by using the ab initio calculation results. The Hammett correlation analysis of the TPA cross section and first hyperpolarizability is also presented.     

Electron photodetachment dissociation of DNA anions with covalently or noncovalently bound chromophores

Double stranded DNA multiply charged anions coupled to chromophores were subjected to UV-Vis photoactivation in a quadrupole ion trap mass spectrometer. The chromophores included noncovalently bound minor groove binders (activated in the near UV), noncovalently bound intercalators (activated with visible light), and covalently linked fluorophores and quenchers (activated at their maximum absorption wavelength). We found that the activation of only chromophores having long fluorescence lifetimes did result in efficient electron photodetachment from the DNA complexes. In the case of ethidium-dsDNA complex excited at 500 nm, photodetachment is a multiphoton process. The MS³ fragmentation of radicals produced by photodetachment at λ = 260 nm (DNA excitation) and by photodetachment at λ > 300 nm (chromophore excitation) were compared. The radicals keep no memory of the way they were produced. A weakly bound noncovalent ligand (m-amsacrine) allowed probing experimentally that a fraction of the electronic internal energy was converted into vibrational internal energy. This fragmentation channel was used to demonstrate that excitation of the quencher DABSYL resulted in internal conversion, unlike the fluorophore 6-FAM. Altogether, photodetachment of the DNA complexes upon chromophore excitation can be interpreted by the following mechanism: (1) ligands with sufficiently long excited-state lifetime undergo resonant two-photon excitation to reach the level of the DNA excited states, then (2) the excited-state must be coupled to the DNA excited states for photodetachment to occur. Our experiments also pave the way towards photodissociation probes of biomolecule conformation in the gas-phase by Förster resonance energy transfer (FRET).     

Electron paramagnetic resonance and optical absorption spectral studies on chalcocite

A chalcocite mineral sample of Shaha, Congo is used in the present study. An electron paramagnetic resonance (EPR) study on powdered sample confirms the presence of Mn(II), Fe(III) and Cu(II). Optical absorption spectrum indicates that Fe(III) impurity is present in octahedral structure whereas Cu(II) is present in rhombically distorted octahedral environment. Mid-infrared results are due to water and sulphate fundamentals.     

Electron photodetachment spectroscopy of solvated anions. A probe of structure and energetics

Electron photodetachment of solvated anions of the form ROHF^- has been undertaken. Observation or non-observation of photodetachment provides information about the location of the bridging proton, i.e. whether the complex has the structure ROH·F^- or RO^-·HF. Where photodetachment is observed, the experimental detachment threshold energy yields the solvation energy for the corresponding free anion.     

Electron paramagnetic resonance and optical absorption spectral studies on covellite mineral

A covellite mineral sample from Coquimbo region, Chile is used in the present study. An electron paramagnetic resonance (EPR) study on powdered sample confirms the presence of Mn(II) and Cu(II). Optical absorption spectrum indicates that Fe(II) and Cu(II) impurities are present in octahedral structure. Bands in the near-infrared from 7,000 to 5,000 cm(-1) result from the overtones of the first fundamental OH-stretching modes.     

New approaches on gas phase molecular absorption spectrometry detection in gas chromatography

A hyphenated technique for coupling a spectrophotometric detector to a gas chromatograph with a packed column is described. The packed column was connected directly to the flow cell (path length 1 cm) which was placed in a UV-visible molecular absorption spectrophotometer with diode array detector. This detection system was employed for separating and determining benzene, toluene, 1,4-xylene, 1,2-xylene and mesitylene. Molecular absorption spectra of all the separated compounds have also been obtained. A new possibility for coupling capillary gas chromatography and gas phase molecular absorption spectrometry is currently being tested. In this new case, two fibre optics are used and the measurement is performed in the capillary column.     

New approaches on gas phase molecular absorption spectrometry detection in gas chromatography

A hyphenated technique for coupling a spectrophotometric detector to a gas chromatograph with a packed column is described. The packed column was connected directly to the flow cell (path length 1 cm) which was placed in a UV-visible molecular absorption spectrophotometer with diode array detector. This detection system was employed for separating and determining benzene, toluene, 1,4-xylene, 1,2-xylene and mesitylene. Molecular absorption spectra of all the separated compounds have also been obtained. A new possibility for coupling capillary gas chromatography and gas phase molecular absorption spectrometry is currently being tested. In this new case, two fibre optics are used and the measurement is performed in the capillary column.     

Improvement of the wetting and absorption properties of lignocellulosic fibers by means of gas phase ozonation

Gas phase ozonation was done on sheets made from chemical thermomechanical pulp in order to improve the wetting properties of the lignocellulosic fibers. The degree of modification was varied by letting the reaction continue for different lengths of time, ranging from 1 to 60 min. Changes in the chemistry of the fibers after ozone exposure were investigated using Fourier transform infrared (FT-IR) spectroscopy and electron spectroscopy for chemical analysis (ESCA). The evolution of a carbonyl signal and the decrease of aromatic absorption over time was observed with FT-IR spectroscopy. The carbonyl peak grew in intensity as the reaction continued throughout the whole range of treatment times. The ESCA showed that carbonyl and carboxyl functionalities were introduced after 10 min of ozone exposure and that the intensity of the peak from the aliphatic and aromatic carbons decreased. However, an ozone treatment longer than 15 min did not affect the chemical surface composition, as analyzed by ESCA. The single-fiber contact angle with water, measured using a Cahn balance, decreased with extended ozonation. Measuring the time required for the sheet to absorb a water droplet with a high speed camera showed that even a very short ozone exposure (1 min) dramatically affected the absorption behavior. The rate of absorption dramatically increased after as little as 1 min of ozone exposure. This improvement in absorption rate was most likely due to the formation of low molecular weight degradation products, acting as wetting agents, created during the ozonation.     

Electronic absorption spectrum of pyridine N-oxide in the gas phase. General characterization

The full electronic absorption spectrum of pyridine N-oxide vapor in the near ultraviolet region has been obtained. The vibronic structure has been analyzed in detail. The four absorption bands, which are observed at 341, 290, 228, and 217 nm, correspond to four 0–0 vibronic transitions. The spectrum is interpreted in terms of the CNDO/S method. The symmetry of the vibrations that exhibit activity due to the Herzberg-Teller effect in different electronic states has been studied. N. G. Chernyshevskii Saratov State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 345–349, March–April, 1995. Translated by I. Izvekova     

Controlled formation of peptide bonds in the gas phase

Photoexcitation (using 157 nm vacuum ultraviolet radiation) of proton-bound peptide complexes leads to water elimination and the formation of longer amino acid chains. Thus, it appears that proton-bound dimers are long-lived intermediates along the pathway to peptide formation. Product specificity can be controlled by selection of specific complexes and the incorporation of blocking groups at the N- or C-termini. The product peptide sequences are confirmed using collision-induced dissociation.     

Ammonium determination in wine by gas phase molecular absorption spectrometry

A procedure for the determination of ammonium is described, based on its transformation into ammonia gas and subsequent measurement by UV-visible molecular absorption spectrometry at 194 nm. Two different procedures for generating the ammonia are proposed, using NaOH solution and solid NaOH, respectively. After generation conditions for both procedures had been optimized, better sensitivity was observed with solid NaOH. In these conditions, the method showed two ranges of linear response: from 1 to 100 g and from 100 to 400 g of ammonium. Finally, we determined ammonium in wine samples. Preliminary studies indicated that the only observable interfering species was ethanol, which could be eliminated by subjecting the sample to vacuum evaporation until nearly dry. This process also increased the sensitivity of the method by as much as ten times. The values obtained from three different wine samples yielded results within the range of the concentration obtained by other authors. The recovery of ammonium added to the wine samples was around 95%.     

Stepwise solvatochromism of ketyl anions in the gas phase: photodetachment excitation spectroscopy of benzophenone and acetophenone radical anions microsolvated with methanol

Electronic absorption spectra of bare and methanol-solvated radical anions of benzophenone ((C6H5)2CO) and acetophenone ((C6H5)CH3CO) were measured by monitoring the photodetachment efficiency in the gas phase. Strong absorption bands due to autodetachment after transitions to bound excited states were observed. Stepwise spectral shifts approaching the limit of the condensed phase spectra were found to occur as the cluster size increases. In the case of benzophenone radical anion, the solvation of two methanol molecules exhibits the near convergence to the limit, representing the full coordination with the solvent molecules around the carbonyl group. For the acetophenone case, the coordination number was not apparently determined because of their relatively small shifts. Relationships between hydrogen bonding and electronic structure are analyzed for the spectral shifts with the aid of calculations based on density functional theory. The calculational results show that the coordination angle of the solvent molecule is affected mostly by steric hindrance around the carbonyl group, and that there is no evidence for reorientation due to specific hydrogen bonding interaction with the singly occupied orbital, which has been formerly persisted for an interpretation of the transient absorption following pulse radiolysis in alcoholic solutions. An alternative possibility involving deformation with respect to intramolecular coordinates is discussed.     

Nonlinear optical absorption properties of two multisubstituted p-dimethylaminophenylethenyl pyridiniums

Two multisubstituted D-pi-A type pyridinium salts, N-methyl-2,6-di[(E)-p-dimethylaminophenylethenyl]pyridinium iodide (1) and N-methyl-2,4,6-tri[(E)-p-dimethylaminophenylethenyl]pyridinium iodide (2), were synthesized with good yields by a modified method, and the structure of 1 was confirmed by X-ray diffraction. The nonlinear optical absorption (NOA) properties of the two compounds in the solution state were investigated by the Z-scan technique. Compounds 1 and 2 both exhibited very strong saturated absorption at picosecond pulse, and 1 displayed reverse saturated absorption, while 2 revealed saturated absorption properties at the nanosecond pulse. The electron-donating ability, number of side chains, and molecular coplanar characteristics of the compounds have a significant impact on their NOA properties.     

IR absorption spectra of some aminoalcohols in the gas phase

The IR spectra of aminoalcohols in the gas phase show that in practically all instances (with the exception of N-allylaminoethanol) they are present mainly as monomers, with or without intramolecular H-bonds (IHB), the former predominating in the vicinal compounds. In 1,4-aminoalcohols the concentrations of molecules with and without IHB are practically equalized. The strongest IHB occur in the seven-membered intramolecular rings which are formed in 4-aminobutan-1-o1, for which the greatest difference is observed between the frequencies of the free OH groups and those bound in IHB. Analysis of the effect of an increase in temperature on the intensities of the OH bands of the free OH groups and of those bound in IHB shows that the partial rupture of the IHB occurs to a greater extent in the unsubstituted aminoalcohols of the vicinal series than in aminoalcohols with two alkyl substituents on the amino group. The IHB exhibit less tendency to break with rise in temperature in the vicinal aminoalcohols than in the 1,3- and 1,4-aminoalcohols.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1323–1333, June, 1990.