Water Molecules Gating a Photoinduced One‐Electron Two‐Protons Transfer in a Tyrosine/Histidine (Tyr/His) Model of Photosystem II

We investigate a biomimetic model of a Tyr_Z/His₁₉₀ pair, a hydrogen‐bonded phenol/imidazole covalently attached to a porphyrin sensitizer. Laser flash photolysis in the presence of an external electron acceptor reveals the need for water molecules to unlock the light‐induced oxidation of the phenol through an intramolecular pathway. Kinetics monitoring encompasses two fast phases with distinct spectral properties. The first phase is related to a one‐electron transfer from the phenol to the porphyrin radical cation coupled with a domino two‐proton transfer leading to the ejection of a proton from the imidazole–phenol pair. The second phase concerns conveying the released proton to the porphyrin N₄ coordinating cavity. Our study provides an unprecedented example of a light‐induced electron‐transfer process in a Tyr_Z/His₁₉₀ model of photosystem II, evidencing the movement of both the phenol and imidazole protons along an isoenergetic pathway.     

激光诱导的亚硝酸与二苯醚的反应机理

355 nm光照下利用瞬态吸收光谱技术进行了有氧、无氧条件下二苯醚与亚硝酸体系的反应机理研究, 考察了其中瞬态物种的衰减行为, 并对其光解产物进行了GC-MS分析. 研究表明, HNO₂在355 nm紫外光的照射下产生的OH自由基和二苯醚反应生成C₁₂H₁₀O-OH 加合物, N₂条件下C₁₂H₁₀O-OH衰减的速率常数为(1.86±0.14)×10⁵ s^－1, 在有氧条件下, C₁₂H₁₀O-OH可转化为C₁₂H₁₀O-OHO₂, 衰减的速率常数为(6.6±0.4)×10⁶ s^－1. N₂条件下最终产物为苯酚、2-羟基二苯醚、4-羟基二苯醚、4-硝基二苯醚.     

Photochemical reactions of quaternary ammonium dithiocarbamates as photobase generators and their use in the photoinitiated thermal crosslinking of poly(glycidyl methacrylate)

The photochemical behavior of quaternary ammonium salts (QA salts) with N,N‐dimethyldithiocarbamate as photobase generators and the photoinitiated thermal crosslinking of poly(glycidyl methacrylate) (PGMA) with the QA salts were investigated. The formation of basic compounds in the photolysis of 1‐phenacyl‐(1‐azonia‐4‐azabicyclo[2,2,2]octane) N,N‐dimethyldithiocarbamate was ascertained by the color change of phenol red as an acid–base indicator. ¹H NMR spectra of photoproducts in CDCl₃ under N₂ showed that the photolysis of 1‐naphthoylmethyl‐(1‐azonia‐4‐azabicyclo[2,2,2]octane) N,N‐dimethyldithiocarbamate resulted in the quantitative formation of triethylenediamine and a dithiocarbamate derivative. The presence of oxygen in the photolysis decreased the photolysis rate. The amine was also detected in its photolysis in polystyrene films. The effects of ammonio groups and counteranions of QA salts on the photoinitiated thermal crosslinking of PGMA films were also investigated. Quaternary ammonium dithiocarbamates acted as excellent photobase generators and effective photoinitiated thermal crosslinkers for PGMA. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1329–1341, 2001     

The absorption spectra of the phenoxyl radical in solid argon

Vacuum ultraviolet photolysis of phenol, phenol-d₆ and anisole during condensation with excess argon at 20 K has produced and trapped the phenoxyl radical as evidenced by structured absorptions at 397.2 and 628.1 nm. A broad photosensitive 416 ± 2 nm band is tentatively assigned to the phenol cation.     

The reaction of trichloromethyl radicals with hydrogen chlorid and a new estimation of the rate of combination of trichloromethyl radicals

The effect of the addition of hydrogen chloride on the photolysis of carbon tetrachloride in the presence of cyclohexane has been investigated in a companion paper. The data enable the rate constant ratio k₈/(k₅)^1/2 to be determined. Since k_?8 is well established, k₅ can be estimated from known thermochemical data. The validity of the thermochemical derivation is checked by applying it to trifluoromethyl radicals. The photolysis of bromotrichloromethane and carbon tetrachloride in the presence of hydrogen chloride has been investigated over a range of temperatures. From these results and assuming reaction (5) has no activation energy, Arrhenius parameters for reaction (8) have been determined: The activation energies for the reaction of methyl, trichloromethyl, and trifluoromethyl radicals with hydrogen chloride are compared, and at first sight surprising results are rationalized in terms of relative electronegativity.     

Solvent‐Free One‐Step Photochemical Hydroxylation of Benzene Derivatives by the Singlet Excited State of 2,3‐Dichloro‐5,6‐dicyano‐p‐benzoquinone Acting as a Super Oxidant

Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible‐light irradiation of 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent‐free hydroxylation of benzene derivatives with electron‐withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3‐dichloro‐5,6‐dicyanohydroquinone (DDQH₂) in the presence of water under deaerated conditions. In the presence of dioxygen and tert‐butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto‐ and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH‐adduct radicals. On the other hand, DDQ ^{. ?} produced by the photoinduced electron transfer from benzene derivatives reacts with the OH‐adduct radicals to yield the corresponding phenol derivatives and DDQH₂. DDQ is recovered by the reaction of DDQH₂ with tert‐butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.     

Transformation of 2,4-dichlorophenol by H2O2/UV-C, Fenton and photo-Fenton processes: Oxidation products and toxicity evolution

In the present study, H₂O₂/UV-C, Fenton and photo-Fenton treatment of 2,4-dichlorophenol was compared in terms of oxidation products and acute toxicity. The oxidation products were identified by gas chromatography-mass spectroscopy, high performance liquid chromatography and ion chromatography, whereas changes in acute toxicity were evaluated by the Vibrio fischeri luminescence inhibition assay. H₂O₂/UV-C and photo-Fenton processes ensured complete 2,4-dichlorophenolremoval, detoxification and significant mineralization. Hydroquinone and formic acid were identified as the common oxidation products of the studied advanced oxidation processes investigated. 3,5-dichloro-2-hydroxybenzaldehyde, phenol, 4-chlorophenol and 2,5-dichlorohydroquinone were identified as the additional H₂O₂/UV-C oxidation products of 2,4-dichlorophenol. Acute toxicity decreased with decreasing 2,4-dichlorophenol and increasing chloride release.     

Photolysis of 2-mercaptobenzothiazole in the presence of surfactants

The quantum yield of the photolysis of 2-mercaptobenzothiazole (MBT) in the molecular form increases by a factor of 20 in the presence of the surfactants cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPCl) in comparison with aqueous solution. In the photolysis of the MBT ionic form, the quantum yield does not change in the presence of CTAB and decreases in the presence of CPCl. The maximums of the MBT absorption spectra shift with an increase in the surfactant concentration for both the molecular and ionic forms. Simultaneously, the MBT photolysis quantum yield increases. It has been shown that the quantum yield of MBT photolysis in water increases in the presence of compounds containing heavy atoms.     

Significant roles of oxygen and unbound <Superscript>•</Superscript>OH radical in phenol formation during photo-catalytic degradation of benzene on TiO<Subscript>2</Subscript> suspension in aqueous system

Studies on photo-catalytic degradation of benzene using TiO₂ photo-catalyst as a suspension in water is reported. Degradation studies have been carried out using 350 nm UV light. Phenol, a photo-catalytic product of benzene, was monitored under varying experimental conditions such as amount of TiO₂, concentration of benzene, photolysis time, ambient (air, O₂, Ar, N₂O and N₂O–O₂ mixture), etc. The phenol yields in both aerated and O₂-purged systems increased with the photolysis time. In contrast to oxygenated systems, the yields of phenol in deoxygenated (viz. Ar-purged and N₂O-purged) systems were quite low (~30 μM) and remained steady. H₂O₂ yields in all these systems were also monitored, and found lower than an order of magnitude as compared to phenol yields for the respective systems. The rate of phenol production in aerated 1 mM benzene solution containing 0.05% TiO₂ suspension was evaluated at 12.3 μM min⁻¹ which is lower than the rate obtained in an O₂-saturated system (22.4 μM min⁻¹). The low yields of phenol in both Ar- and N₂O-purged systems, and also the increasing trends in oxygenated systems, together reveal that, for the phenol formation with an enhanced rate, oxygen is essential. In the present study, it is implied that the photo-generated hole, which is mainly an ^•OH radical, is either freely available in the aqueous phase or migrates to the aqueous phase from the catalyst surface, to react with benzene to produce HO-adduct radical. Later, following reaction with oxygen, the adduct produces phenol. On the other hand, h⁺ and surface adsorbed ^•OH radical, being trapped/bonded due to rigid association with the catalyst surface, were not able to generate phenol under similar experimental conditions. The mechanism of phenol formation with TiO₂ photolysis in an aqueous system is rechecked, on the basis of present results on h⁺/surface adsorbed ^•OH radical/unbound ^•OH radical scavenging by benzene, collectively with previous reports on various systems.     

Comparative Actions of NiO and TiO<Subscript>2</Subscript> Catalysts on the Destruction of Phenol and its Derivatives in a Dielectric Barrier Discharge

This study examined processes of decomposing phenol and its derivatives (resorcin, pyrocatechol and hydroquinone) in aqueous solutions under the action of an atmospheric pressure oxygen dielectric barrier discharge in the presence or absence of catalysts in the plasma zone. Two types of catalysts were tested, NiO and TiO₂. It was found that both materials exhibited catalytic properties. The action of NiO accelerated the step of phenol destruction while the action of TiO₂ catalyst resulted in a more preferable composition of decomposition products and provided a higher degree of carboxylic acid conversion into carbon dioxide than the NiO catalyst.     

Methane generated during photocatalytic redox reaction of alcohols on TiO2 suspension in aqueous solutions

Studies on the photo-catalytic redox reaction of C1–C3 alcohols such as methanol, ethanol and 2-propanol were carried out in aqueous solution containing TiO₂ photocatalyst (0.1% w/v) as suspension using 350 nm light. Other hydrocarbons such as ethane and ethene in the case of ethanol, and propene in the case of 2-propanol with low yields were produced along with the major photolytic products methane and carbon dioxide. The yields of methane and CO₂ were found to be dependent on the light exposure time and ambient conditions. Methane yields were higher in 2-propanol and ethanol systems than in methanol system, showing their better hole-scavenging properties. In the aerated condition, methane was produced during photolysis of all alcohols in the presence of TiO₂ and the yield was comparable to those observed in the corresponding CO₂-saturated systems. The overall results reveal that the surface adsorbed, as well as in-situ-generated CO₂ from photo-oxidation of alcohols are equally responsible for methane formation through photo-reduction in presence of TiO₂. In the O₂-saturated system, the methane yield was lower as compared to that in aerated system, in contrast to the CO₂ yield. In N₂O-and N₂-purged systems, the yield of methane was observed to be low, inferring that the methane generation has not taken place through photodecomposition/photodissociation of alcohols. Again, photolysis of alcohols without TiO₂ did not generate any methane.     

Kinetic modeling of aqueous phenol degradation by UV/H2O2 process

A dynamic kinetic model for the oxidation of phenol in water by an UV/H₂O₂ process is developed. The model is based on the elementary chemical and photochemical reactions, initiated by the photolysis of hydrogen peroxide into hydroxyl radicals. The model is validated by using experimental data obtained from the open literature for an actual UV/H₂O₂ process. Using those data and the developed kinetic model, kinetic rate constants for phenol intermediates, catechol and hydroquinone, are estimated. Moreover, the optimum initial hydrogen peroxide concentration is estimated by means of the validated model. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 40: 34–43, 2008     

Oxidation of phenol on RuO2–TiO2/Ti anodes

The oxidation of phenol on the RuO₂–TiO₂/Ti electrode has been studied by cyclic voltammetry, polarization measurements, electrochemical impedance spectroscopy and potentiostatic transients in H₂SO₄ and NaCl aqueous solutions. A reaction path with polymerization as the main reaction and side reactions after the initial step, similar to the reaction path on other electrode materials, is suggested. The formation of a phenoxy radical in a diffusion-controlled irreversible process is the initial step. The polymerization of phenoxy radicals leads to the formation of porous polyoxyphenylene film, strongly adherent to the electrode surface. The cyclic voltammetry measurements indicate side products, which could be, according to the literature, of quinone-like structure. Polyoxyphenylene film inhibits further oxidation of phenol, although complete electrode passivation was not observed. The presence of polyoxyphenylene film does not influence the pseudocapacitive behaviour of the electrode to a great extent, since the polyoxyphenylene film covers dominantly the coating surface, while active sites placed within coating cracks remain uncovered. The film seems to be permeable for hydrogen ions and water molecules.     

Preparation and photochemistry of macromolecular bound ruthenium(II) complexes in aqueous solutions

Ruthenium(II) polypyridyl complexes with macromolecular ligands poly(methylolacrylamide-co-vinylpyridine) and poly (acrylamide-co-vinylpyridine) have been synthesized. The macromolecular ruthenium (II) complexes which are soluble in water have been characterized and their absorption and emission properties have been studied in aqueous solution. Photolysis of the complex in aqueous solution leads to photoaquation reactions with release of coordinated pyridines of the polymer. In the case of monomeric complex, cis-[Ru(bpy)₂(py)₂]Cl₂, photolysis in water in presence of Cl^? ions produces only the substitution of the pyridine by water whereas in the polymeric complexes, [Ru(bpy)₂(MAAM-co-VP)₂]Cl₂ photolysis in the presence of chloride produces [Ru(bpy)₂(MAAM-co-VP)Cl]Cl and [Ru(bpy)₂(AM-co-VP)Cl]Cl, respectively. Quantum yields for the photosubstitution reactions have been determined and mechanistic details are outlined.     

TiO_2光催化失活复活特性与表面NO_3~-浓度的关系

以乙醛作为活性标的,通过人为控制表面附着NO3-浓度,检测TiO2对乙醛的吸附能力和光催化降解能力,得出了TiO2光催化失活复活特性和表面附着NO3-浓度的定量关系。结果表明,表面附着NO3-浓度达到4.24wt%时,光催化降解能力降低50%;表面附着NO3-浓度达到10.50wt%时,TiO2的吸附能力降低50%;通过溢流状态下水洗2h,可除去98%的表面附着NO3-,使TiO2光催化活性复活。     

Solvent-Free One-Pot Synthesis of Sulfonephthaleins from Saccharin and Phenols

Sulfonephthaleins can be synthesized in a single pot from saccharin and phenol via the in situ formation of 2-sulfobenzoic anhydride, followed by its reaction with phenol using H₂SO₄ as the condensing agent, in the absence of any solvent. This solvent-free synthesis is more economical and environmentally benign.     

Replacement of an oxygen atom by chlorine atoms in the reaction of pentafluorobenzaldehyde with CCl3-substituted compounds in the presence of AlCl3

The interaction of pentafluorobenzaldehyde with RCCl₃ (R = Cl, Ph, C₆F₅) in the presence of threefold molar excess of AlCl₃ proceeds with replacing the oxygen atom of the aldehyde group by two chlorine atoms from the CCl₃ group and results in the formation of pentafluorobenzylidene chloride. The electrophilic mechanism of the reaction is proposed.     

Fluorescence quenching and laser photolysis of dipyrrolylbenzenes in the presence of chloromethanes

Fluorescence quenching of 1,4-bis(1H-pyrrol-1-yl)benzene, 1-(1H-pyrrol-2-yl)-1-(1-vinyl-1H-pyrrol-1-yl)benzene, and 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene with chloromethanes (methylene chloride, chloroform, and carbon tetrachloride) in solvents with different polarities follows electron-transfer mechanism. The occurrence of an electron-transfer step is confirmed by formation of short-lived pyrrolylbenzene radical cations. An exception is quenching of fluorescence of 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene in n-hexane in the presence of CCl₄ and CHCl₃ and in pure CCl₄. In this case, neutral 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene^·-Cl radical is formed via recombination of 1,4-bis(1-vinyl-1H-pyrrol-2-yl)benzene radical cation and chloride anion. A relation was found between the nature of the short-lived species detected by laser photolysis and stable product obtained by stationary photolysis.     

核壳结构TiO2@SiO2催化碳酸二甲酯与苯酚酯交换合成碳酸二苯酯

以反相微乳液法和沉淀法相结合制备了核壳结构TiO₂@SiO₂,首次用于碳酸二甲酯与苯酚酯交换合成碳酸二苯酯反应,显示较好的催化活性. 采用200 ℃焙烧的TiO₂@SiO₂,用量0.20 g,反应9 h,苯酚转化率达41.8%,酯交换选择性为100%. 透射电镜显示TiO₂@SiO₂核厚壳薄,TiO₂核直径220-300 nm,SiO₂壳厚度40-60 nm,具有介孔结构. TiO₂@SiO₂对碳酸二甲酯与苯酚酯交换反应有好的重复使用性,使用4次苯酚转化率仍保持在40%以上. TiO₂与SiO₂发生相互作用,Ti进入骨架形成Ti-O-Si键,骨架Ti的形成提高了TiO₂@SiO₂的催化性能.