Femtosecond dynamics of intramolecular photoinduced proton transfer in <Emphasis Type="Italic">N</Emphasis>-substituted 2-(2-aminophenyl)-4H-3,1-benzoxazin-4-ones

Femtosecond dynamics of processes in the excited state of 2-(2-aminophenyl)-4H-3,1-benzoxazin-4-ones has been studied by femtosecond absorption spectroscopy. The rate constants of intramolecular photoinduced proton transfer (IPPT) have been determined for the N-substituted derivatives (0.7–11 ps⁻¹). The IPPT rate constant depends on the inductive constant of the substituent and the potential barrier height, which was calculated by a quantum-chemical method (TDDFT). The multiexponential character of the kinetics of photoinduced absorption by the compounds with a low inductive constant of the N-substituent in the spectral region of the S₁ → S_N absorption and the stimulated emission of the IPPT product is explained by the rapid (∼10 ps⁻¹) relaxation process preceding the IPPT.     

Femtosecond dynamics of excited-state intramolecular proton transfer in o-tosylaminobenzoic and o-acetylaminobenzoic acids

The dynamics of excited-state intramolecular proton transfer (ESIPT) and of relaxation processes in o-tosylaminobenzoic acid (TAC) and o-acetylaminobenzoic acid (AAC) have been studied by femtosecond absorption spectroscopy with a time resolution of 30 fs. The ESIPT characteristic time in the TAC dimer and monomer and in AAC monomer is 50 fs. The excited product of photoinduced proton transfer in the monomer undergoes effective radiationless deactivation with a characteristic time of 30 ps, one of the channels of which is internal rotation followed by intersystem crossing and internal conversion. The product of ESIPT in the TAC dimer deactivates preferentially into the ground state via radiative transition with a time of 291 ps. ESIPT in the AAC dimer is thermodynamically unfavorable and occurs with a low yield.     

Excited-state proton transfer from pyranine to acetate in methanol

Excited-state proton transfer (ESPT) of pyranine (8-hydroxypyrene-1,3,6-trisulphonate, HPTS) to acetate in methanol has been studied by steady-state and time-resolved fluorescence spectroscopy. The rate constant of direct proton transfer from pyranine to acetate (k ₁) is calculated to be ∼1 × 10⁹ M⁻¹ s⁻¹. This is slower by about two orders of magnitude than that in bulk water (8 × 10¹⁰ M⁻¹ s⁻¹) at 4 M acetate.     

Kinetic and equilibrium study of the deprotonation of 4-nitrophenyl[bis(ethylsulphonyl)]methane by organic bases in acetonitrile in the presence of common cation BH<Superscript>+</Superscript> and tetrabutylammonium perchlorate

The results of kinetic and equilibrium experiments with the set of reaction of proton abstraction from 4-nitrophenyl[bis(ethylsulphonyl)]methane in acetonitrile are reported. Two strong organic bases are used: 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD). The rates of proton transfer reaction have been measured by T-jump method in the presence of perchlorate of the appropriate base as a common cation BH⁺ and supporting electrolyte-tetrabutylammonium perchlorate (TBAP) in the temperature range between 20–40°C are: k _H =1.32×10⁷−2.00×10⁷ and 2.82×10⁷−4.84×10⁷ dm ³mol⁻¹s⁻¹ for MTBD and TBD respectively. The enthalpies of activation ΔH _MTBD ^≠ =13.5 and ΔH _TBD ^≠ =18.1 kJmol⁻¹. The entropies of activation are negative: ΔS _MTBD ^≠ =−62.3 and ΔS _TBD ^≠ =−40.3 Jmol⁻¹K⁻¹. The change of the absorbance of the anion of 4-nitrophenyl[bis9ethylsulphonyl)]methane at the temperature 25°C in the presence of common cation BH⁺ gives the equilibrium constants K=705 and 906 M⁻¹ for MTBD and TBD respectively. Kinetic and equilibrium results are discussed. The possible mechanism of proton transfer reaction between 4-nitrophenyl[bis(ethylsulphonyl)]methane and cyclic organic bases: MTBD and TBD in acetonitrile is proposed.     

UV‐Light‐Induced Hydrogen Transfer in Guanosine–Guanosine Aggregates

Aggregates of a lipophilic guanine (G) derivative have been studied in n‐hexane by femtosecond‐to‐microsecond UV‐visible broadband transient absorption, stationary infrared and UV‐visible spectroscopy and by quantum chemical calculations. We report the first time‐resolved spectroscopic detection of hydrogen transfer in GG aggregates, which leads to (G?H) ^. radicals by means of G⁺G^? charge transfer followed by proton transfer. These radicals show a characteristic electronic spectrum in the range 300–550 nm. The calculated superimposed spectrum of the species that result from NH???N proton transfer agrees best with the experimental spectrum.     

A Time‐Dependent Picture of the Ultrafast Deactivation of keto‐Cytosine Including Three‐State Conical Intersections

Using mixed quantum–classical dynamics, the lowest part of the UV absorption spectrum and the first deactivation steps of keto‐cytosine have been investigated. The spectrum shows several strong peaks, which mainly come from the S₁ and S₂ states, with minor contributions from the S₃. The semiclassical trajectories, launched from these three states, clearly indicate that at least four states are involved in the relaxation of keto‐cytosine to the ground state. Non‐adiabatic transfer between the ππ* and nπ* excited states and deactivation via three‐state conical intersections is observed in the very early stage of the dynamics. In less than 100 fs, a large amount of population is deactivated to the ground state via several mechanisms; some population remains trapped in the S₂ state. The latter two events can be connected to the fs and ps transients observed experimentally.     

Fullerene C<Subscript>60</Subscript> as a superefficient quencher of singlet exited states of polycyclic aromatic hydrocarbons

Quenching of fluorescence of polycyclic aromatic hydrocarbons (PAH), namely, naphthalene, anthracene, 9,10-diphenylanthracene, 9,10-dibromoanthracene by C₆₀ fullerene in ethylbenzene at 293 K was found and investigated. The phenomenon is characterized by abnormally high values of bimolecular rate constants for quenching (k _bim = (0.18–6.78)·10¹² L mol⁻¹ s⁻¹) determined from the Stern—Volmer dependence of the PAH fluorescence intensity on the C₆₀ concentration and occurs through the inductive-resonance (dominant channel) and exchange-resonance (minor channel) energy transfer from ¹PAH* to C₆₀. The overlap integrals of the PAH fluorescence spectra with the C₆₀ absorption spectrum and the critical energy transfer distances were calculated. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 432–436, March, 2007.     

Determination of Kinetic Parameters in the Biosorption of Cr (VI) on Immobilized <Emphasis Type="Italic">Bacillus cereus</Emphasis> M<Superscript>1</Superscript><Subscript>16</Subscript> in a Continuous Packed Bed Column Reactor

Due to technological advancement, environment suffers from untreated toxic heavy metal bearing effluent coming from different industries. Chromium (VI) is one of those heavy metals having adverse impact on ecological balance, human, and plant health because of its carcinogenic properties. Biosorption is presented as an alternative to traditional technologies which are costly and inefficient for treatment of industrial wastes containing low amount of heavy metals. In this study, bioremediation of Cr (VI) ions by immobilized Bacillus cereus M¹ ₁₆ was investigated in a laboratory scale packed bed up-flow column reactor. The effect of important parameters, such as the inlet flow rate, influent concentration, and effective bed height, has been studied. External mass transfer, surface adsorption, and intrabead mass transfer were also studied to conclude the rate limiting step for removal of Cr (VI) and to determine the process parameters which are important for biosorption optimization. The external mass transfer coefficient was calculated at different flow rates (6.51 × 10⁻² to 7.58 × 10⁻² cm/min). Using the model, the surface adsorption rate constant (k _ad) and the intrabead mass transfer coefficient (k _i) were predicted as 0.0267 × 10⁻³ and 0.7465 × 10⁻³ l/g/min, respectively. Both are much lower than the external mass transfer coefficient (k _e). The surface adsorption phenomenon is acting as the rate-limiting step due to its high resistance for removal of Cr (VI).     

Dynamics of carbocation formation in the photolysis of 1,2,2,3-tetramethyl-1,2-dihydroquinoline in alcohols

Dynamics of the formation of the carbocation in the ground state as a result of photoinduced proton transfer from a solvent to the excited state of 1,2,2,3-tetramethyl-1,2-dihydroquinoline (3MDHQ) in MeOH and 2,2,2-trifluoroethanol (TFE) was registered by pump-probe laser photolysis (λ_pump = 310 nm) with femtosecond time resolution. The lifetimes of the excited singlet state of 3MDHQ τ = 115 and 780 ps were determined in TFE and MeOH, respectively. The transient species with absorption spectrum corre-sponding to the spectrum of the carbocation from 3MDHQ (λ_max = 480 nm) is generated at time delays lower than 500 fs from the unrelaxed excited singlet state.     

Synthesis,structure, and electric properties of oxygen-deficient perovskites Ba<Subscript>3</Subscript>In<Subscript>2</Subscript>ZrO<Subscript>8</Subscript> and Ba<Subscript>4</Subscript>In<Subscript>2</Subscript>Zr<Subscript>2</Subscript>O<Subscript>11</Subscript>

Perovskite phases Ba₃In₂ZrO₈ and Ba₄In₂Zr₂O₁₁ with the nominal concentration of structural oxygen vacancies 1/9 and 1/12, respectively, were synthesized by solid-phase and solution methods. X-ray diffraction showed cubic symmetry of both phases with the unit cell parameter a = 0.4193(2) and 0.4204(3) nm, respectively. The absence of superstructural lines resulted in the conclusion on statistical arrangement of oxygen vacancies. Thermogravimetry and mass spectrometry proved that both phases can reversibly absorb water from gas phase (pH₂O = 2 × 10⁻² atm) with observed correlation between the concentration of oxygen vacancies and amount of absorbed water. The total water amount was up to 0.9 mol per formula unit or, if recalculated for perovskite unit ABO₃, 0.3 and 0.23 mol H₂O, respectively. The temperature curves of coductivity in the atmosphere with various partial water vapor pressures (pH₂O = 3 × 10⁻⁵ and 2 × 10⁻² atm) showed significantly higher conductivity and lower activation energy (0.52 eV) in humid atmosphere due to proton transfer. The proton conductivity is up to 5 × 10⁻⁴ Ohm⁻¹ cm⁻¹ at 300°C for Ba₃In₂ZrO₈ specimen. IR spectrometry showed that protons in the structure exist primarily in OH-groups.     

Photolysis of <Emphasis Type="Italic">p</Emphasis>-benzoquinone and <Emphasis Type="Italic">p</Emphasis>-chloranil in aqueous sodium sulfite solution

Photosubstitution of the sulfo group for hydrogen (chlorine in chloranil) is observed under irradiation of sulfonated derivatives of hydroquinone formed upon dissolving of p-benzoquinone and p-chloranil in aqueous solutions of sodium sulfite. The quantum yield of the photochemical reaction is 0.18 ± 0.02 for p-benzoquinone. The rate constant of the thermal reaction of substitution is 5.5 × 10⁻⁴ l mol⁻¹ s⁻¹. The substitution reaction is sensitized by eosin upon irradiation with visible light. During the course of irradiation of sulfohydroquinones, the formation of a product with an absorption maximum at 235 nm was found. The product decays with a rate constant of 0.005 s⁻¹.     

Photophysics of Push–Pull Distyrylfurans,Thiophenes and Pyridines by Fast and Ultrafast Techniques

Time‐resolved transient absorption and fluorescence spectroscopy with nano‐ and femtosecond time resolution were used to investigate the deactivation pathways of the excited states of distyrylfuran, thiophene and pyridine derivatives in several organic solvents of different polarity in detail. The rate constant of the main decay processes (fluorescence, singlet–triplet intersystem crossing, isomerisation and internal conversion) are strongly affected by the nature [locally excited (LE) or charge transfer (CT)] and selective position of the lowest excited singlet states. In particular, the heteroaromatic central ring significantly enhances the intramolecular charge‐transfer process, which is operative even in a non‐polar solvent. Both the thiophene and pyridine moieties enhance the S₁→T₁ rate with respect to the furan one. This is due to the heavy‐atom effect (thiophene compounds) and to the ¹(π,π)*→³(n,π)* transition (pyridine compounds), which enhance the spin‐orbit coupling. Moreover, the solvent polarity also plays a significant role in the photophysical properties of these push–pull compounds: in fact, a particularly fast ¹LE*→¹CT* process was found for dimethylamino derivatives in the most polar solvents (time constant, τ≤400 fs), while it takes place in tens of picoseconds in non‐polar solvents. It was also shown that the CT character of the lowest excited singlet state decreased by replacing the dimethylamino side group with a methoxy one. The latter causes a decrease in the emissive decay and an enhancement of triplet‐state formation. The photoisomerisation mechanism (singlet/triplet) is also discussed.     

trans/cis‐Isomerization of Fluorene‐Bridged Azo Chromophore with Significant Two‐Photon Absorbability at Near‐Infrared Wavelength

Azo‐containing materials have been proven to possess second‐order nonlinear optical (NLO) properties, but their third‐order NLO properties, which involves two‐photon absorption (2PA), has rarely been reported. In this study, we demonstrate a significant 2PA behavior of the novel azo chromophore incorporated with bilateral diphenylaminofluorenes (DPAFs) as a π framework. The electron‐donating DPAF moieties cause a redshifted π–π* absorption band centered at 470 nm, thus allowing efficient blue‐light‐induced trans‐to‐cis photoisomerization with a rate constant of 2.04×10^?1 min^?1 at the photostationary state (PSS). The open‐aperture Z‐scan technique that adopted a femtosecond (fs) pulse laser as excitation source shows an appreciably higher 2PA cross‐section for the fluorene‐derived azo chromophore than that for common azobenzene dyes at near‐infrared wavelength (λ_ex=800 nm). Furthermore, the fs 2PA response is quite uniform regardless of the molecular geometry. On the basis of the computational modeling, the intramolecular charge‐transfer (ICT) process from peripheral diphenylamines to the central azo group through a fluorene π bridge is crucial to this remarkable 2PA behavior.     

Intensification of Biocatalytical Processes by Synergistic Substrate Conversion. Fungal Peroxidase Catalyzed <Emphasis Type="Italic">N</Emphasis>-Hydroxy Derivative Oxidation in Presence of 10-Propyl Sulfonic Acid Phenoxazine

Many industrial pollutants, xenobiotics, and industry-important compounds are known to be oxidized by peroxidases. It has been shown that highly efficient peroxidase substrates are able to enhance the oxidation of low reactive substrate by acting as mediators. To explore this effect, the oxidation of two N-hydroxy derivatives, i.e., N-hydroxy-N-phenyl-acetamide (HPA) and N-hydroxy-N-phenyl-carbamic acid methyl ester (HPCM) catalyzed by recombinant Coprinus cinereus (rCiP) peroxidase has been studied in presence of efficient substrate 3-(4a,10a-dihydro- phenoxazin-10-yl)-propane-1-sulfonic acid (PPSA) at pH 8.5. The bimolecular constant of PPSA cation radical reaction with HPA was estimated to be (2.5 ± 0.2)·10⁷ M⁻¹ s⁻¹ and for HPCM was even higher. The kinetic measurements show that rCiP-catalyzed oxidation of HPA and HPCM can increase up to 33,000 times and 5,500 times in the presence of equivalent concentration of high reactive substrate PPSA. The mathematical model of synergistic rCiP-catalyzed HPA–PPSA and HPCM–PPSA oxidation was proposed. Experimentally obtained rate constants were in good agreement with those calculated from the model confirming the synergistic scheme of the substrate oxidation. In order to explain the different reactivity of substrates, the docking of substrates in the active site of the enzyme was calculated. Molecular dynamic calculations show that the enzyme–substrate complexes are structurally stable. The high reactive PPSA exhibited higher affinity to enzyme active site than HPA and HPCM. Furthermore, the orientation of HPA and HPCM was not favorable for proton transfer to the distal histidine, and different substrate reactivity was explained by these diversities.     

Excited State Intramolecular Proton Transfer of 1-Hydroxyanthraquinone

The excited state intra-molecular proton transfer dynamics of 1-hydroxyanthraquinone in solution are investigated by femtosecond transient absorption spectroscopy and quantum chemistry calculations. Two characteristic bands of excited state absorption and stimu-lated emission are observed in transient absorption spectra with the excitation by the pump wavelength of 400 nm. From the delayed stimulated emission signal, the time scale of the intra-molecular proton transfer is determined to be about 32 fs. The quantum chemistry calculations show that the molecular orbits and the order of the S₂ and S₁ states are rever-sal and a conical intersection is demonstrated to exist along the proton transfer coordinate. After proton transfer, the second excited state of tautomer populated via the conical intersection undergoes the internal conversion with ～200 fs and the following intermolecular energy relaxation with ～16 ps. The longer component 300 ps can be explained in terms of the relaxation from excited-state tautomer to its ground state. From our observations, two proton transfer pathways via a conical intersection are proposed and the dominated one preserves the molecular orbits.     

Single‐Site Copper(II) Water Oxidation Electrocatalysis: Rate Enhancements with HPO42− as a Proton Acceptor at pH 8

The complex Cu^II(Py₃P) ( 1 ) is an electrocatalyst for water oxidation to dioxygen in H₂PO₄^?/HPO₄^2? buffered aqueous solutions. Controlled potential electrolysis experiments with 1 at pH 8.0 at an applied potential of 1.40 V versus the normal hydrogen electrode resulted in the formation of dioxygen (84 % Faradaic yield) through multiple catalyst turnovers with minimal catalyst deactivation. The results of an electrochemical kinetics study point to a single‐site mechanism for water oxidation catalysis with involvement of phosphate buffer anions either through atom–proton transfer in a rate‐limiting O? O bond‐forming step with HPO₄^2? as the acceptor base or by concerted electron–proton transfer with electron transfer to the electrode and proton transfer to the HPO₄^2? base.     

Spectroscopic and Thermodynamic Studies on Charge Transfer Complex Formation between 2-Aminopyridine and 2,5-Dihydroxy-<Emphasis Type="Italic">p</Emphasis>-benzoquinone

Charge transfer complex formation between 2-aminopyridine (2AP) as the electron donor with 2,5-dihydroxy-p-benzoquinone (AHBQ) as the π-electron acceptor has been investigated spectrophotometrically in acetonitrile (AN) and 50% acetonitrile + 50% 1,2-dichloroethane (V/V), (ANDC). The stoichiometry of the complex has been identified by Job’s method to be 1:1. The Benesi-Hildebrand equation has been applied to estimate the formation constant (K _CT) and molecular extinction coefficient (ε). It was found that the value of K _CT is larger in ANDC than in AN. The thermodynamic parameters are in agreement with the K _CT values in that the enthalpy of formation (−ΔH) has a larger value in ANDC than in AN, suggesting higher stability of the complex in ANDC. The complex formed between 2AP and DHBQ has been isolated as a solid and characterized using elemental analysis, FTIR, and ¹H NMR measurements. Moreover, it has been found that the formed complex involves proton transfer in addition to charge transfer.     

Electron transfer through single-crystal silver electrode/<Emphasis Type="Italic">n</Emphasis>-decanthiol monolayer/NaNO<Subscript>3</Subscript> solution interfaces

The kinetics of Ru[(NH₃)₆]³⁺ reduction in 1 M NaNO₃ solution at Ag(210) and Ag(111) singlecrystal electrodes modified by n-decanthiol monolayer is studied by electrochemical impedance spectroscopy and cyclic voltammetry. By using these two methods, standard rate constants of the redox reaction involving Ru[(NH₃)₆]^3+/2+ redox couple in the absence and in the presence of the n-decanthiol film were estimated. The equivalent circuit describing the experimental data in the presence of the self-assembled organic monolayer and in the absence of redox reaction is an electrical circuit comprising a large resistance (∼10⁶ Ω) connected in parallel with a capacitance (∼10⁻⁸ F). Analysis of kinetic data and extrapolation of Tafel lines resulted in the determination of the rate constant at unmodified Ag-electrode, which is characteristic of very fast heterogeneous electron transfer. The calculated rate constants for n-decanthiol-modified silver singlecrystal faces (210) and (111) in 1 M NaNO₃ solution (pH 6.3) equal 4.63 × 10⁻⁵ and 3.05 × 10⁻⁵ cm/s, respectively. The results are compared with the data at hand reported by different authors for gold electrodes in indifferent electrolyte solution in the absence and in the presence of self-assembled monolayer.     

The flash photolysis of aqueous solutions of rhodizonic and croconic acids

The flash photolysis of aqueous solutions of rhodizonic and croconic acids has been studied in the presence and absence of electron acceptors. No transient absorption which could be identified with an excited state was observed with either anion. The rate of recovery of the ground state in the absence of additives was a first-order process with both acids and gave rate constants for deactivation of the excited state, k_D, of 2.4 × 10⁵ s^?1 for rhodizonate and 2.8 × 10⁵ s^?1 for croconate. With croconate dianion in the presence of three acceptors, 4-nitrobenzylbromide, methylviologen, and biacetyl, a transient absorption was detected, with a maximum absorbance at 500 nm, and was tentatively identified with the monoanion radical, formed following electron transfer to the acceptor. From the rate of growth of the transient, rate constants for the rate of electron transfer to the acceptor were measured as follows: 4-nitrobenzylbromide: 2.8 × 10⁹ M^?1 s^?1; methyl viologen: 3.7 × 10¹⁰ M^?1 s^?1; and biacetyl: 2.0 × 10⁸ M^?1 s^?1. The significance of the measurements is discussed in relation to the mechanism proposed for the photochemical reactions of these dianions. © 1994 John Wiley & Sons, Inc.     

Oxygen Quenching of nπ* Triplet Phenyl Ketones: Local Excitation and Local Deactivation

We have studied the charge‐transfer‐induced deactivation of nπ* excited triplet states of benzophenone derivatives by O₂(³Σ), and the charge‐transfer‐induced deactivation of O₂(¹Δ_g) by ground‐state benzophenone derivatives in CH₂Cl₂ and CCl₄. The rate constants for both processes are described by Marcus electron‐transfer theory, and are compared with the respective data for a series of biphenyl and naphthalene derivatives, the triplet states of which have ππ* configuration. The results demonstrate that deactivation of the locally excited nπ* triplets occurs by local charge‐transfer and non‐charge‐transfer interactions of the oxygen molecule with the ketone carbonyl group. Relatively large intramolecular reorganization energies show that this quenching process involves large geometry changes in the benzophenone molecule, which are related to favorable Franck‐Condon factors for the deactivation of ketone‐oxygen complexes to the ground‐state molecules. This leads to large rate constants in the triplet channel, which are responsible for the low efficiencies of O₂(¹Δ_g) formation observed with nπ* excited ketones. Compared with the deactivation of ππ* triplets, the non‐charge‐transfer process is largely enhanced, and charge‐transfer interactions are less important. The deactivation of singlet oxygen by ground‐state benzophenone derivatives proceeds via interactions of O₂(¹Δ_g) with the Ph rings.