Interstate coupling and dynamics of excited singlet states of isolated diphenylbutadiene

In this paper, we report on absolute fluorescence quantum yields from photoselected vibrational states of jet-cooled 1,4-diphenylbutadiene for excess vibrational energies, E_v = 0−7500 cm⁻¹, above the apparent electronic origin of the S₁(2A_g) state. The pure radiative lifetimes, τ_r, of the strongly scrambled S₂(1B_u)—S₁(2A_g) molecular eigenstates (E_v = 1050−1800 cm⁻¹) show a marked dilution effect, (τ_r/τ_r(S₂) ≈ 40), being practically identical with the τ_r values from the S₁(2A_g) manifold (E_v = 0–900 cm⁻¹), which is affected by near-resonant vibronic coupling to S₂(1B_u) and exhibiting the dynamic manifestations of the intermediate level structure. Isomerization rates in the isolated molecule, which do not exhibit vibrational mode selectivity, were recorded over the energy range 0–6600 cm⁻¹ above the threshold.     

Analysis and physical significancy of the kinetic parameters associated with albumin adsorption onto glassy carbon obtained by electrochemical impedance measurements

The kinetics of albumin adsorption onto a glassy carbon rotating disk electrode in a phosphate buffer, is reported from the time variations of the double-layer capacitance C_d, of the charge transfer resistance R_t and of the Tafel coefficient of the electrochemical reaction b; these three electrical quantities are determined by electrochemical impedance and faradaic current I measurements. The variations of C_d, 1/R_t, b  (R_tI)⁻¹ and I can be written under the form: α₀ + α₁ e^−t/τ1 + α₂ e^−t/τ2, where α₀, α₁, α₂, τ₁, τ₂ characterize a given electrical quantity. We demonstrate that this type of variation validates a two-step adsorption mechanism already proposed, starting from simply taking into account C_d(t). We analyze and discuss comprehensively various possibilities of interpreting the shifts between the time-constants associated with thevarious quantities as well as the correlations between these constants and those associated with physical adsorption phenomena.     

Characterisation of electron transport and back reaction in dye-sensitised nanocrystalline solar cells by small amplitude laser pulse excitation

The characterisation of the transport and interfacial reaction of electrons in dye-sensitised nanocrystalline solar cells is complicated by the non-linearity of these processes. This problem has been overcome by superimposing small amplitude pulsed laser excitation on steady background illumination. The laser perturbation of the photostationary state is sufficiently small that the photocurrent and photovoltage responses can be fitted using constant values of the electron diffusion coefficient D_n and electron lifetime τ_n. Analytical and finite difference solutions of the continuity equation have been used to analyse the experimental photocurrent, photocharge and photovoltage transients, and the intensity dependence of D_n and of τ_n has been established by varying the bias illumination level, and hence the dc photocurrent density, j_dc. The intensity dependence of D_n (D_n∝j_dc^0.68) is attributed to trapping/detrapping involving a distribution of trapping levels. The intensity dependence of τ_n (τ_n∝j_dc^−0.62) may indicate that the back reaction of electrons with I₃⁻ is not first order in electron concentration. Other possible explanations are that the interfacial electron transfer rate constant depends on trap occupancy or on the rate of surface or bulk electron diffusion.     

New expansion of the Boys function

We propose a new expansion for the Boys function ∫₀¹t^2jexp(−r²t²) dt appearing in the calculation of molecular two-electron matrix elements if Gaussian basis sets are employed. This expansion involves a power series involving the terms C_{i, j}(τ) (r²−R²)ⁱ multiplied by exp(−τr²), where τ is an optimized parameter τ∈[0, 1]. The performances of the introduced expansion are discussed and illustrated by some numerical experiments. It appears that the proposed expansion is considerably shorter than the customary Taylor series, which in turn is the special case for τ=0. This is of some importance, particularly for higher j values. Further, the proposed expansion enables a single expression for calculating erf(x) for the whole range of variable x. The recursive relations for the expansion coefficients are derived and the truncation errors are estimated. A new method for calculating the Boys function by means of asymptotic series is represented too. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 68: 305–315, 1998     

Interpretation of differences in temperature and pressure dependences of density and concentration fluctuations in amorphous poly(phenylmethyl siloxane)

Differences in the temperature and pressure dependences of the relaxation times of a slow diffusional process and the α structural relaxation pose an interesting problem. This feature, observed by dynamic light scattering in amorphous poly(phenylmethyl siloxane), is related to another basic feature of lack of thermorheological simplicity discovered by Plazek in polystyrene, poly(vinyl acetate), and amorphous polypropylene. A quantitative explanation based on the predictions of a general coupling theory of relaxations has been found. The coupling theory also predicts the Kohlrausch fractional exponential time correlation function exp[?(tτ^*)^1?n] at long times, as observed by photon correlation spectroscopy, and crossover to an exponential time dependence exp–(t/τ₀) at short times, as frequently assumed in Brillouin scattering. An additional relation between τ^* and τ₀ predicted by the theory is confirmed also by the experimental data.     

Nuclear relaxation and molecular motion of 1,3,5-trifluorobenzene-d3 in liquid solutions

The ²D and ¹⁹F spin-lattice relaxation times of 1,3,5-trifluorobenzene-d₃ in 0.15 mol fraction solutions in various solvents have been measured over the temperature range 270–400 K. The relationship between the reorientational correlation times, τ_θ, and the angular momentum correlation times, τ_J, obtained from the nuclear relaxation data has been compared with the theoretical relationships predicted by the J-diffusion limit of the extended diffusion (EDJ) model and the Fokker-Planck-Langevin (FPL) model for symmetric top molecules. It was found that the rotational motion of 1,3,5-trifluorobenzene-d₃ in most solutions was better described by the FPL model with frictional anisotropy τ₆/τ_· in the range 1–2. (τ₆ and τ_· are the correlation times for the angular velocity components along the axes parallel and perpendicular to the molecular symmetry axis.) The viscosity and temperature dependence of τ_θ has been analyzed in terms of a modified Debye equation and values for the anisotropic interaction parameter, κ, in each solvent are reported. The variation in κ with solvent is attributed mainly to the variation in the dipole moment of the solvent molecules. The frictional anisotropy (τ₆/τ_·) is found to decrease as κ increases. This is interpreted in terms of the effects of molecular shape and electrostatic interactions between CF bond dipole moment and solvent dipole moments.     

Kinetics of the Fe(II) reduction of trans-halogenopyridinebis(dimethylglyoximato)Co(III) complexes

The kinetics of the Fe(II) reduction of trans-chloro, bromo and iodopyridinebis(dimethylglyoximato)Co(III) have been studied at 30.0±0.1°C and I = 1.0 mol. dm⁻³(NaClO₄) in the [H⁺] range 0.0043–0.115 mol. dm⁻³. The reaction showed an inverse dependence on [H⁺]. The second order rate constant could be expressed in the form k_II = k₁ + k₂(1 + K_B[H⁺])^{−1. The kinetic data were found to be:} Co(DH)₂(py)Cl−k₁ = 0.051 ±0.003 dm³ mol⁻¹s⁻¹, 0.051±0.003 dm³ mol⁻¹ s⁻¹,k₂ = 0.76±0.04 dm³ mol⁻¹ s⁻¹ K_B = 325±8 dm³ mol⁻¹;Co(DH)₂(py)Br-k₁ = 0.071±0.004 dm³mol⁻¹ s⁻¹,k₂ = 1.21±0.04 dm³ mol⁻¹ s⁻¹ K_B = 460±15 dm³ mol⁻¹; Co(DH)₂(py)I-k₁ = 0.075±0.006 dm³ mol⁻¹ s⁻¹,k₂ = 1.91±0.09 dm³ mol⁻¹ s⁻¹ K_B = 625±30 dm³ mol⁻¹. The inverse dependence on [H⁺] suggests an inner-sphere mechanism involving protonated and unprotonated species of the complex. The order of rates for the three complexes was found to be Co(DH)₂(py)I > Co(DH)₂(py)Br > Co(DH)₂(py)Cl.     

Thermal relaxation of poled non-linear optical sidechain polymers: A new,semi-empirical model

Electro-Optic relaxation of a poled, Non-Linear Optical sidechain polymer with T_g 140°C, containing 4-dimethylamino-4′-nitrostilbene (“DANS”) in the sidechains, has been studied at 120°C with and without annealing at the same temperature. The time-dependence of the decaying EO coefficients r(t) shows a strong departure from the classical single-exponential Debye model, especially in the unannealed samples. This departure is attributed to physical ageing, slowing down the orientational relaxation of the sidechains. The Debye model with r(t)-r(0). exp -t/τ] is modified semi-empirically by introducing a time-dependent characteristic Debye relaxation time τ(t). Of several trial expressions, one is selected which fits the relaxation data. This is τ(τ)-τ_i+C.t^b     

Excited state proton transfer in β-carboline

Steady state and time-resolved fluorescence techniques were used to investigate the excited state acid—base equilibria behaviour of β-carboline and 2-methyl-β-carboline in aqueous solutions. In acid conditions the β-carboline cation is responsible for light absorption and fluorescence (λ_max = 450 nm; τ_F = 22.0 ns). However, in alkaline solutions excitation of the neutral species leads to the formation of the β-carboline cation by a rapid proton transfer with the solvent. A diffusion-controlled quenching of the excited state cation by OH⁻ ions produces a zwitterion species which fluoresces with λ_max at 510 nm (τ_F = 1.6 ns). The zwitterion is also formed after excitation of the ground state anion at pH 14. pK values for the ground state and excited state acid—base equilibria are reported.     

Temperature dependence of radiationless processes. Isoquinoline in solution

The temperature dependence of the fluorescence quantum yield φ_f, the fluorescence lifetime τ_f, and the oscillator strength f(S₀→S₁) of isoquinoline in solution has been measured between room temperature and 77 K. Following an Arrhenius type expression, φ_f in ethanol increases from 0.012±0.002 at 295 K to 0.61±0.03 at 77 K paralleled by an increase of τ_f from 0.25±0.10 ns to 9.0±0.2 ns. Over the same temperature range f(S₀→S₁) and the radiative fluorescence lifetime remain constant. By analyzing the temperature dependent data, it is shown that a spin-allowed internal conversion process with an activation energy of ～1500 cm^?1 is responsible for the observed temperature effect. A mechanism is proposed based upon a thermally activated depopulation of the S₁(ππ^*) state of isoquinoline via a slightly higher state, presumably the S₂(ππ^*) singlet state. An extremenly fast process involving the dissociation of the hydrogen bond deactivates this latter state, by possing S₁.     

Decreasing current ramp technique for the study of fast electrode reactions

A new technique for measuring rate constants of moderately fast electrode reactions is proposed. The input is a decreasing current ramp of the form i(t) = Δi(1 ? t/τ). The overpotential transient-response expected for the activation/diffusion model is confirmed through experiments on the system K₄Fe(CN)₆+K₃Fe(CN)₆/Pt. The rate constant and the transfer coefficient are obtaine presence of several interesting features (a maximum, a zero crossing etc.) in the η?t profile makes this technique almost unique.     

Structural changes in low temperature water

Previous experiments on the neutron structure factor S(q) for heavy water near 11.2°C have been extended so that the next term in the Taylor expansion may be examined. An improved result for the isochoric temperature derivative of S(q) at 11.2°C is obtained. These data and those for the (ΔT)² coefficient in the Taylor expansion may be explained on a simple basis: namely that OH and HH distances between neighbouring molecules vary as (T  t₀)⁻ⁿ where (T  T₀) ≈ 1200n for T ≈ 300 K.     

Three-body association of CO+ and N2. The case for a chemical bond

Hamilton, Bierbaum and Leone recently reported a very efficient vibrational quenching of CO⁺ (v) by N₂, only about six collisions being required for quenching at 300 K. This is an exceedingly efficient quenching process in the light of recent systematic studies of diatomic molecular ion vibrational quenching. To probe further the CO⁺−N₂ interaction, the rate coefficient for three-body association of CO⁺ and N₂ with He as third body was measured and is very large, being 2.1 × 10⁻²⁹ cm⁶ s⁻¹ at 298 K and 1.8 × 10⁻²⁸ cm⁶ s⁻¹ at 80 K. Both the efficient quenching and the rapid association imply a strong interaction between CO⁺ and N₂, much stronger than the expected ≈ 0.2 eV electrostatic potential well depth. These experiments indicate a “chemical bonding” interaction between CO⁺ and N₂, with a well depth as large as 1 eV.     

Electrode processes of Cr(II) in non-complexing and complexing media

The formal potentials and the kinetics parameters for the electrode process: Cr²⁺+2 e⁻ = Cr^o occurring at a mercury electrode in solutions of NaClO₄, NaCI, NaBr, and NaSCN, were determined from the analysis of irreversible anodic and cathodic chronocoulometric waves. The interaction of Cr(II) with Cl⁻ was found to be negligible (equilibrium constant K <1) whereas the interaction with Br⁻ and SCN⁻ was weak (K₁(Br⁻)=1 M⁻¹ and β₂(SCN⁻) = 25 M⁻²). The results of the analysis of the formal rate constant of this and other amalgam forming reactions suggested that the formation of amalgam was the most important step in the whole process.     

Exciplex type emission from anthracene doped aggregates of phenanthrene formed in n-hexane frozen solutions

Fluorescence, absorption and fluorescence excitation spectra of concentrated (3 × 10⁻⁴ M) solutions of phenanthrene in frozen n-hexane in the presence of anthracene have been investigated at 77 and 4 K. Decay time analysis of the anthracene sensitized emission reveals the existence of at least two types of superposed emissions. One emission (τ_M ≈ 9 ns) is similar to that from solid solutions of anthracene in phenanthrene crystal while the other is structureless and anthracene-concentration dependent (τ_M ≈ 30–65 ns). The latter has been assigned to an exciplex type emission from particular anthracene-phenanthrene configurations which occur in very small size phenanthrene pseudocrystallites, imbedded in an n-alkane matrix.     

Time-resolved spectral investigations of laser light induced microplasma

The dynamical and spectral properties of an optical breakdown microplasma created by pulses of different lasers on surfaces of insulators (KCI), metals (Cu) and semiconductors (V₂O₅), have been investigated. Experiments were carried out in air and vacuum using different wavelengths (λ = 0.694μm, type OGM-20,λ = 1.06μm with a home-made laser based on neodymium glass crystal, and λ = 10.6μm, similarly home-made) and pulse durations (Q-switched and free-running regimes). To follow the integral, dynamical and spectral characteristics of the luminous spot of microplasma we have used fast cameras (SFR-2M, IMACON-HADLAND), a high speed spectral camera (AGAT-2) and a spectrograph (STE-1). It has been shown that the microplasma consists of two parts: fast front (peak) with τ≈100 ns and slow front (tail) with τ≈1μs durations. The detonation front speed is of the order of ≈10⁵ cm s⁻¹ and follows the temporal dependence of to t^0.4. It depends on the composition of the surrounding gas and its pressure and could be connected with quick evaporation of the material investigated (peak) and optical breakdown of the ambient gaseous atmosphere (tail). From the delay in appearance of different characteristic spectral lines of the target material and its gaseous surrounding we have shown that the evolution of the microplasma involves evaporation and ionization of the atoms of the parent material followed by optical breakdown due to the incident and absorbed laser light, together with microplasma expansion.     

Electron transfer mechanism for the oxidation of ternary N-(2-acetamido)iminodiacetatocobaltate(II) complexes involving succinate and maleate as secondary ligands by periodate

The kinetics of oxidation of the ternary complexes [Co^II(ADA)(Su)(H₂O)]^2? and [Co^II(ADA)(Ma)(H₂O)]^2? (ADA?=?N-(2-acetamido)iminodiacetate, Su?=?succinate and Ma?=?maleate) by periodate have been investigated spectrophotometrically at 580?nm under pseudo-first-order conditions in aqueous medium over 30?C50?°C range, pH 3.72?C4.99, and I?=?0.2?mol?dm^?3. The kinetics of the oxidation of [Co^II(ADA)(Su)(H₂O)]^2? obeyed the rate law d[Co^III]/dt?=?[Co^II(ADA)(Su)(H₂O)]^2?[H₅IO₆] {k ₄ K ₅?+?(k ₅ K ₆ K ₂/[H⁺)}, and the kinetics oxidation of [Co^II(ADA)(Ma)(H₂O)]^2? obeyed the rate law d[Co^III]/dt?=?k ₁ K ₂[Co^II] _T [I^VII] _T /{1?+?([H⁺]/K ₇)?+?K ₂[I^VII] _T }. The pseudo-first-order rate constant, k _obs, increased with increasing pH, indicating that the hydroxo form of maleate complex, [Co^II(ADA)(Ma)(OH)]^3?, is the reactive species. The initial Co(III) products were slowly converted to the final products, fitting an inner-sphere mechanism. Thermodynamic activation parameters were calculated using the transition state theory equation. The initial cobalt(II) complexes were characterized by physicochemical and spectroscopic methods.     

Binding of the bioflavonoid robinetin with model membranes and hemoglobin: Inhibition of lipid peroxidation and protein glycosylation

Recent years have witnessed burgeoning interest in plant flavonoids as novel therapeutic drugs targeting cellular membranes and proteins. Motivated by this scenario, we explored the binding of robinetin (3,7,3′,4′,5′-pentahydroxyflavone, a bioflavonoid with remarkable ‘two color’ intrinsic fluorescence properties), with egg yolk phosphatidylcholine (EYPC) liposomes and normal human hemoglobin (HbA), using steady state and time resolved fluorescence spectroscopy. Distinctive fluorescence signatures obtained for robinetin indicate its partitioning (K_p = 8.65 × 10⁴) into the hydrophobic core of the membrane lipid bilayer. HbA–robinetin interaction was examined using both robinetin fluorescence and flavonoid-induced quenching of the protein tryptophan fluorescence. Specific interaction with HbA was confirmed from three lines of evidence: (a) bimolecular quenching constant K_q ? diffusion controlled limit; (b) closely matched values of Stern–Volmer quenching constant and binding constant; (c) τ₀/τ = 1 (where τ₀ and τ are the unquenched and quenched tryptophan fluorescence lifetimes, respectively). Absorption spectrophotometric assays reveal that robinetin inhibits EYPC membrane lipid peroxidation and HbA glycosylation with high efficiency.     

The thermodynamic properties of DyBr3(s) and DyI3(s) from T=5 K to their melting temperatures

Low-temperature calorimetric measurements have been performed on DyBr₃(s) in the temperature range (5.5 to 420 K ) and on DyI₃(s) from T=4 K to T=420 K. The data reveal enhanced heat capacities below T=10 K, consisting of a magnetic and an electronic contribution. From the experimental data on DyBr₃(s) a C⁰_p,m (298.15 K) of (102.2±0.2) J·K⁻¹·mol⁻¹ and a value for {S⁰_m (298.15 K) − S⁰_m (5.5 K)} of (205.5±0.5) J·K⁻¹·mol⁻¹, have been obtained. For DyI₃(s), {S⁰_m (298.15 K) − S⁰_m (4 K)} and C⁰_p,m (298.15 K) have been determined as (226.9±0.5) J·K⁻¹·mol⁻¹ and (103.4±0.2) J·K⁻¹·mol⁻¹, respectively. The values for {S⁰_m (5.5 K) − S⁰_m (0)} for DyBr₃(s) and {S⁰_m (4 K) − S⁰_m (0)} for DyI₃(s) have been calculated, giving S⁰_m (298.15 K)=(212.3±0.9) J·K⁻¹·mol⁻¹ in case of DyBr₃(s) and S⁰_m (298.15 K) =(233.1±0.7) J·K⁻¹·mol⁻¹ for DyI₃(s). The high-temperature enthalpy increment has been measured for DyBr₃(s) in the temperature range (525 to 799 K) and for DyI₃(s) in the temperature range (525 to 627 K). From the results obtained and enthalpies of formation from the literature, thermodynamic functions for DyBr₃(s) and DyI₃(s) have been calculated from T→0 to their melting temperatures at 1151.0 K and 1251.5 K, respectively.     

Temperature dependence of the reaction NO + NO3 → 2NO2

The flash photolysis-visible absorption technique has been used to measure rate constants for the reaction NO + NO₃ → 2NO₂ (1) over the temperature range 224–328 K. The temperature dependence of the rate constant is given by the expression k₁(T) = (1.59 ± 0.32) × 10⁻¹¹exp(122/T) cm³ molecule⁻¹ s⁻¹ where the stated uncertainties refer to the ± 2σ limits from both random and systematic errors.