Photochemistry of dicyanopyridines

The photochemistry of a variety of dicyanopyridines (2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-dicyanopyridine) in solution at room temperature was investigated. Pulsed UV (308 nm) laser irradiation in deoxygenated acetonitrile yields the triplet state with lifetimes between 4 and 10 μs and absorption bands in the 400 and 320 nm regions. In the presence of added HCl an air-insensitive transient (τ ≈ 10–12 μs, λ_max ≈ 360–380 nm) was observed, suggesting the formation of a protonated excited state.

Irradiation in the presence of amines resulted in the production of the pyridyl radical anion (τ ≈ 40–80 μs, air sensitive, λ_max ≈ 360–380 nm) formed by electron transfer from the amine to the pyridine triplet excited state. Stern-Volmer analysis gave electron transfer rate constants in the range (1–8) × 10⁻⁸ M⁻¹ s⁻¹.

In methanol solvent, irradiation yielded an air-insensitive transient assigned as the neutral pyridyl radical (τ ≈ 30–200 μs, λ_max ≈ 370–385 nm). The formation of these transients is discussed in the context of previous photochemical electron spin resonance and product studies.     

The triple fluorescence of benzanilide and the dielectric medium modulation of its competitive excitation

The fluorescence of the benzanilide molecule at 298 K is inferred to consist of three independent electronic transitions associated with the single ground-state molecular species. F₁ (λ_max340 nm), the normal fluorescence is observed weakly and is ascribed to an n,π*,-π,π* mixed state. F′₂ is ascribed to the proton-transfer imidol tautomer fluorescence (previously reported) with unresolved λ_max (inferred at ≈460 nm). F″₂ is ascribed to a charge-transfer state fluorescence to the ground state, and occurs as a resolved CT transition in tetrahydrofuran at λ_max 520 nm. Comparison of the spectra of N-methylbenzanilide exhibiting only F₁ and F″₂ (CT) permitted the analysis of the benzanilide spectra.     

Dramatic persistence (minutes) of the triplet excited state and efficient singlet oxygen generation for C60 incorporated in Y zeolite and MCM-41 silicate

The triplet excited state of C₆₀ (λ_max=780 nm) lives minutes and can be monitored by conventional spectrophotometers when this fullerene is incorporated inside LiY, as opposed to C₆₀@HY and C₆₀@MCM-41 wherein C₆₀ triplet lives in the submillisecond time scale. C₆₀ adsorbed in LiY or MCM-41 efficiently generates ¹O₂ that was detected by its characteristic NIR emission (λ_em=1270 nm).     

Transients and cooperative action of β-carotene, vitamin E and C in biological systems in vitro under irradiation

Using N^•₃ species as specific electron acceptor a defined ascorbate radical: AH^•↔A^•−+H⁺ (λ_max=360 nm, =3400 dm³ mol⁻¹ cm⁻¹) is observed. The attack of DMSO^•+ on vit.E results in a vit.E^• radical (k=1×10⁹ dm³ mol⁻¹ s⁻¹; λ_max=425 nm, =2400 dm³ mol⁻¹ cm⁻¹; 2k=4.7×10⁸ dm³ mol⁻¹ s⁻¹). Vit.E-acetate leads to the formation of a radical cation (vit.E-ac^•+). β-carotene reacts also with DMSO^•+ forming a radical cation, β-car^•+ (k=1.75×10⁸ dm³ mol⁻¹ s⁻¹; λ_max=942 nm, =14 600 dm³ mol⁻¹ cm⁻¹), which probably leads to the formation of a dimer radical cation, (β-car)^•+₂ (k=2.5×10⁷ dm³ mol⁻¹ s⁻¹).

Using E.coli bacteria (AB1157) as a model system in vitro it was found that all three vitamins are rather efficient radiation protecting agents. They can also increase the activity of cytostatica, e.g., mitomycin C (MMC), by electron transfer process. The mixture of vit.E-ac and β-car acts contradictory, but adding vit.C to it a strong cooperative enhancement of the MMC activity is observed once again. A relationship between the pulse radiolysis and the radiation biological data is found and discussed. A possible explanation of the previously reported trials concerning the role of vit.E and β-car on the increased occurence of lung and other types of cancer in smokers and drinkers is presented.     

Solvated electrons in dimethylsulphoxide

The optical absorption spectrum obtained by pulse radiolysis of pure liquid dimethylsulphoxide includes a broad intense band in the near IR with λ_max 1500 nm. This band is assigned to solvated electrons with a half-life of 15 ± 2 nsec. The λ_max of the solvated electron correlates better with dimethylsulphoxide's inability to solvate negative ions than with its dielectric constant, which is 48.     

Circularly polarized emission from a narrow bandwidth dye doped into a chiral nematic liquid crystal

We report on circularly polarized light emitted from a chiral nematic liquid crystal doped with a luminescent organolanthanide dye. The organolanthanide emission displays an extremely narrow spectral bandwidth of ΔλE =8 nm. This is considerably narrower than the CNLC selective reflection bandwidth ΔλR =60 nm. When conventional dyes with broader emission bandwidths are dissolved into CNLCs, the average degree of circular polarization g of emitted light is reduced from the maximum degree g MAX ; this is due to the overlap of the emission band with the reflection band edges, and spectral regions outside the reflection band. Here, however, we can place the entire emission band inside the reflection band and achieve g ≈g MAX =1.27. Furthermore, a high degree of circular polarization is maintained under off-axis viewing up to a viewing angle of ≈30° to the normal.     

Redox reactions of 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) in aqueous solution

Pulse radiolysis technique has been employed to study the reactions of oxidizing (^√OH, N₃^√) and reducing radicals (e⁻_aq, CO₂^√−, acetone ketyl radical) with 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) at different pH. Hydroxyl radicals react mostly by addition reaction forming radical adducts (λ_max=420 nm) and the oxidation is only a minor process even in the alkaline region. The reaction with azide radicals produced phenoxyl radicals (λ_max=340 nm), which are formed on fast deprotonation of solute radical cation. Using PMZ^√+/PMZ and ABTS^√−/ABTS²⁻ as the reference couple, different methods are employed to determine the one-electron reduction potential of o-vanillin and the average value is estimated to be 1.076±0.004 V vs. NHE at pH 6. The phenoxyl radicals of o-vanillin were able to oxidize ABTS²⁻ quantitatively. The e_aq⁻ is observed to react with o-vanillin with rate constant value of 2×10¹⁰ dm³ mol⁻¹ s⁻¹. CO₂^√− and acetone ketyl radical are also observed to react with o-vanillin by electron transfer mechanism and showed the formation of transient absorption bands with λ_max at 350 and 390 nm at pH 4.5 and 9.7, respectively. The pK_a of the one-electron reduced species was determined to be 8.1. The results indicate that the aldehydic group is the most preferred site for electron addition.     

Pulse radiolysis studies of 4,7-phenanthroline(II)

The one-electron reduction of 4,7-phenanthroline (P) in aqueous solutions at neutral pH has been further studied by pulse radiolysis. The spectral and kinetic properties of the transient formed due to the reaction of 4,7-phenanthroline with hydrated electron were investigated. The transient absorption spectrum obtained 5μs after the pulse exhibits a broad band with a λ_max at 420 nm. The λ_max is 10 nm blue shift compared with the absorption spectrum obtained at pH 2.9 where the reactant was the protonated form. The bimolecular'rate constant of the reaction of 4,7-phenanthroline with hydrated electron was 0etermined to be (2.2±0.1)×10¹⁰ dm³ mol⁻¹ s⁻¹. It was found that the decay of the transient was mainly following a first-order kinetics. The first-order decay rate constant was determined to be (1.25±0.1)×10⁴s⁻¹.     

Photochemical redox titrations with tungsten(V)

A titration method has been developed for the determination of micromolar quantities of dichromate, vanadate and hexacyanoferrate(III) with in situ photochemical generation of tungsten(V) as the titrant. Precision of 1–3% was obtained. Spectrophotometric end-points were utilized because of the intense blue color of the titrant (λ_max 770nm; _max7.10³). Because the titrant reacts with oxygen, a closed, argon-flushed, circulating photolysis apparatus was constructed. Optimal solution condi- tions were investigated ; tungsten reagent must be added as the solid just before titra-tion. An induced reaction between dichromate and glucose was found, but satisfactory linear calibration curves were obtained. With reduced photolytic intensity, 0.2 μeq of dichromate can be determined.     

Sulfoxides as chiral complexation agents. Conformational enantiomer resolution and induced circular dichroism of bilirubins

Bilirubin-IX, the end product of porphyrin metabolism in mammals and the neurotoxic yellow-orange pigment of jaundice, exhibits a strong tendency to fold like a book into either of two interconverting enantiomeric conformations, which are further stabilized by intramolecular hydrogen bonding. Bilirubin exhibits optical activity in R-(-)-ethylmethylsulfoxide solvent, as seen by moderately strong bisignate circular dichroism Cotton effects (Δε^max₄₅₂=+10.9,Δε^max₄₀₄=-4.5), and in dichloromethane solution in the presence of 2M R-(+)-methyl-p-tolylsulfoxide its circular dichroism spectrum (Δε₄₆₃^max=+11.2,Δε₄₁₂^max=-7.1) is comparably strong. As observed earlier for chiral recognition of bilirubin by optically active amines and serum albumins, the optically active sulfoxide acts as a chiral complexation agent to induce an asymmetric transformation of bilirubin, whose bisignate circular dichroism spectra are characteristic of an exciton splitting arising from interaction of the two component dipyrrinone chromophores.     

The Ericksen number and Deborah number cascades in sheared polymeric nematics

Samples of liquid crystalline poly(γ-benzyl-glutamate) solutions are sheared between glass surfaces with gaps, d = 10-500 μ, and shearing velocities, V = 0·05-10 000 μs^-1 so that the Ericksen number E ≡ Vdγ₁/K is varied over a large range, E ≈ 1-10⁷. Here γ₁ is the rotational viscosity and K₁ is the Frank splay constant, with γ₁/K₁ estimated to be approximately 1 s μ^-2 for our samples. We observe by polarizing microscopy a sequence of transitions with increasing Ericksen number analogous to that observed in small molecule tumbling nematics: namely rotation of the director out of the shearing plane and into the vorticity direction at Vd ≈ 25 μ² s^-1, and formation of roll cells at Vd ≈ 50 μ² s^-1. The roll cells become finer with increased Vd in accord with predictions of linear stability theory using the Leslie-Ericksen equations, and at Vd ≳ 500 μ² s^-1, the cells become very irregular, producing director turbulence. The turbulence becomes finer in scale as Vd increases, reaching sub-micron, and possibly molecular scales when Vd ≧ 10⁵ μ² s^-1. At the highest velocities, transitions in orientation and texture are controlled by the Deborah number De≡λV/d, where λ is the molecular relaxation time, and uniform texture-free samples are obtained when De ≳ 5.     

The peculiarities of the spectral luminescence properties of N-anthryl-substituted pyridinium cations

A structureless fluorescence band λ^max_f ≈ 530 nm) in liquid solution and a fluorescence band with vibrational structure (λ = 376–480 nm) in rigid media (ethanol, 77 K and poly(methyl methacrylate) (PMMA), 296 K) were observed for N-anthryl-substituted pyridinium cations. In liquid solution the long-wave fluorescence is attributed to the biradical structure resulting from the photoinduced electron transfer (PET) from anthracene to the pyridinium ring, and the long-wave, low intensity absorption (and fluorescence excitation) band (λ=465 nm) is attributed to this interfragment PET which is weakly allowed by the torsional vibrations. In rigid media the PET is strongly forbidden (in the strict orthogonal pyridinium-anthracene structure) and radiative deactivation of the anthracene fragment is observed.     

Electronic structures and electronic spectra of the linkage isomers NSO and SNO

The relative stabilities and electronic structures of the linkage isomers NSO⁻ and SNO⁻ have been determined by the MNDO and ab initio Hartree—Fock—Slater methods. Both approaches predict a higher stability for SNO⁻ by ca. 100 kcal mol⁻¹, but an overlap population analysis indicates substantially higher bond orders for NSO⁻ compared to SNO⁻. The calculations also reveal a low energy pathway with a barrier of ca. 6 kcal mol⁻¹ for the isomerization process NSO⁻ → SNO⁻. Good agreement was found between the observed UV-visible absorption bands for NSO⁻ (λ_max 379 nm) and SNO⁻ (λ_max 340 nm) and calculated values of the electronic transition energies.     

Substituent effects on some physicochemical properties of chalcones and their vinylogues

Carbonyl stretching frequencies (ν_CO), dipole moments (μ), longwave maxima (λ_max), half-wave potentials (°_1/2), and relative intensities (Z/Z₀) of benzoyl-ion in mass-spectra oftrans chalcones and their vinylogues of the general type PhCO(CH=CH)_nC₆H₄R-p (I) were measured. Contrary to the previously investigated polyenes of the type R(CH=CH)_nR′ (R′ = CHO or COOEt) variation of the substituent R in compounds I has little influence on properties of I in the ground state and a satisfactory linear relationship between (ν_CO) or μ and σ constants exists only in the case of I (n = 0, 1); the best correlation with σ⁺ indicates that the mesomeric mechanism plays a great part in the transmission of electronic effects. λ_max.,E_1/2, orZ/Z₀ of all series I (n = 0–3) correlate with σ_p or σ⁰ constants, and the separation of mesomeric and inductive effects by multiparametric correlation reveals that in this case inductive effect is of great importance in the transmission of electronic effects. These features were accounted for by the non-planar structure of molecules of I, which was confirmed by X-ray analysis of I (R = Br). The Pariser-Parr-Pople method has also been applied to the calculation of π-electronic density in molecules I and it was found that the introduction of various substituents in para position of I exercises a very little influence on the electronic distribution in compounds I.     

The octant rule: XXIII. antioctant effects in γ,δ -unsaturated ketones.

The and -benzyl derivatives (1 and 2, respectively) of (+)-camphor have been synthesized and are found to exert a strong influence on the circular dichroism n→π^* Cotton effects: 1: Δε³⁰¹_max -0.36 (n- heptane) and 2: Δε³⁰²_max +3.22, relative to camphor: Δε³⁰⁴_max +1.8 (n-heptane). Evidence for electric dipole transition moment coupling in these γ, δ -unsaturated systems is found in the n→π^* UV: 1: ε²⁹¹_max 84 (n-heptane) and 2: ε²⁸⁵_max 303, relative to camphor: ε²⁹⁰_max 25.     

On the multiple peaks in charge-transfer probability versus laser frequency in the theory of laser assisted surface ion neutralization

Some aspects of the theory of LASIN (laser assisted surface ion neutralization) are discussed, with emphasis on the physical origins of the so-called double-peak structures found in some calculations of the charge-transfer (neutralization) probability, P, as a function of the laser frequency η. These two peaks have been called the first peak at η ≈ η_m = _O − _m (in a.u.), where _o(_m) is the electronic energy level of the ion/atom (middle of the solid's valence band) and the second peak, a much larger peak at η ≈ 1.3 η_m, respectively.

We show that these double-peak structures are all special cases of multiple-peak structures which result from quantum interference effects, and that, in fact, the second peak is to be regarded as the main resonance peak. This result is interesting in itself, because it is the first peak which has heretofore been considered the main resonance peak.

To simplify the discussion, a two-level model is adapted, which represents the solid valence band by a single level at _m. Clarification of the physical reason for the multiple peaks is based on the semiclassical theory of nonadiabatic transitions, in which the peaks are due to the phase difference between the two adiabatic paths that arise from the diagonalization of the two-level hamiltonian.

With the electronic hopping potential modelled by V(t) = V_osech(λt), and the laser potential by W(t) = W_osech(λt) cos(πt + δ), in the usual notation, an approximate analytical expression for P(η) is presented for the case W_o/V_o < 1, which covers most of the previous treatments, and is in good agreement with the exact results.     

Far-UV laser flash photolysis in solution. A study of the chemistry of 1,1-dimethylsilene in hydrocarbon solvents

The far-UV (193 nm) laser flash photolysis of nitrogen-saturated isooctane solutions of 1,1-dimethylsiletane allows the direct detection of 1,1-dimethylsilene as a transient species, which (at low laser intensities) decays with pseudo-first-order kinetics (τ 10 μs) and exhibits a UV absorption spectrum with λ_max 255 nm. Characteristic rapid quenching is observed for the silene with methanol (k_McOH = (4.9 ± 0.2) × 10⁹ M⁻¹ s⁻¹), tert-butanol (k_BuOH = (1.8 ± 0.1) × 10⁹ M⁻¹ s⁻¹) and oxygen (k_O2 = (2.0 ± 0.5) × 10⁸ M⁻¹ s⁻¹). The Arrhenius activation parameters for the reaction with methanol have been determined to be E_a = −2.6 ± 0.6 kcal mol⁻¹ and log A = 7.7 ± 0.3.     

Extraction and sensitive spectrophotometric determination of ytterbium with 2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol

The operating conditions for the absorptiometric determination of Yb(III) with the reagent 2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminophenol (3,5-diClDMPAP) by a liquid—liquid extraction technique are presented. The complex yields a molar absorptivity of 1.54 × 10⁵l.mole⁻¹.cm⁻¹ (λ_max = 588 nm) and an optimum concentration range of 0.025–1.360n mg/l., at pH = 10. The method developed has been applied to the determination of Yb(III) in synthetic and concrete mixtures.     

Kinetische und mechanistische untersuchungen von Übergangsmetall-komplex-reaktionen : III. Abhängigkeit des gleichgewichts der addition von tributylphosphan an pentacarbonyl(arylmethoxycarben)wolfram vom substituenten am ring

The reaction of pentacarbonyl(arylmethoxycarbene)tungsten, (CO)₅W[C(OCH₃(p-C₆H₄R)] [R = OCH₃ (a), CH₃ (b), H (c), Br (d), CF₃ (e)], with tributylphosphane at low temperatures results in a reversible addition of the phosphane to the carbene carbon atom. The addition—dissoziation equilibrium is not only dependent on the temperature but also to a strong degree on the nature of the substituent R. ΔG, ΔH and the equilibrium constant K increase in the series from R = OCH₃ to R = CF₃. With the exception of R = OCH₃ the substituents b to e form an isentropic class. For all substituents (a to e) a linear dependency from Jaffés σ-constants was observed for ΔH. Good linear correlation for the substituents b to e was also found for log K and σ as well as for log K and the CO-force constants k_cis and k_trans.     

Fluorescence properties of domperidone and its determination in pharmaceutical preparations

The native fluorescence characteristics of domperidone in various solvents and at different pH values are reported. The fluorophore is the benzo-1,3-diazolin-2-one group which is also present in the analogous benperidol, droperidol and pimozide molecules from which domperidone is derived; the substituent may therefore be considered as a relatively strong fluorophore; its excitation and emission maxima are little influenced by pH variation. Domperidone can be determined in pharmaceutical preparations by its native fluorescence in ethanol (λ_ex = 283 nm ;λ_em = 324 nm) and in 0.01 M HCl (λ_ex = 284 nm;λ_em = 329 mn) with a detection limit of 0.01 μg ml^-1.