Showing papers in "Journal of Photochemistry and Photobiology A-chemistry in 1991"
TL;DR: In this paper, the effects of three nonporous powder samples of TiO2, ZrO2 and MoO3, with surface areas between 30 and 60 m2 g−1, on the kinetics of disappearance of 4-chlorophenol (4-CP) in water, under illumination at λ ⩾ 340 nm and 293 K, were determined.
Abstract: The effects of three non-porous powder samples of TiO2, ZrO2 and MoO3, with surface areas between 30 and 60 m2 g−1, on the kinetics of disappearance of 4-chlorophenol (4-CP) in water, under illumination at λ ⩾ 340 nm (to avoid the direct photolysis of 4-CP) and 293 K, were determined. The TiO2 sample was by far the most active with an initial quantum yield of ca. 0.01 for an initial 4-CP concentration C0 = 0.155 mM in the pH range 3–6. The photocatalytic character of the 4-CP disappearance in its presence was shown by (i) the effects of the catalyst mass and radiant flux on the initial rate r0, (ii) the Langmuirian-type relationship between r0 and C0, (iii) the very small activation energy of 4-CP disappearance, (iv) the number of 4-CP molecules degraded per unit of surface area, (v) the stability of TiO2, and (vi) the quasi-suppression of the activity by substitutional doping with Cr3+ ions which behave as recombination centres of the photoproduced charges. Hydroquinone (HQ), the main intermediate in the degradation, corresponds to para hydroxylation and its maximum concentration was ca. 100 times smaller than C0 for C0 = 0.155 mM. 1,4-Benzoquinone, formed from HQ, and 4-chloro-1,2-dihydroxybenzene were also detected. None of the toxic biphenyl derivatives produced by direct photolysis was found in the presence of TiO2. Complete mineralization of 4-CP was eventually obtained. Without TiO2, at λ ⩾ 290 nm, the degradation produced more numerous and more stable aromatic intermediates.
297 citations
TL;DR: In this paper, the effect of the deposition of metal particles onto a powdered semiconductor oxide on its photocatalytic activity has been determined for group VIII transition metals only, and the results showed that a small amount of silver caused the catalytic oxygen-free oxidation of 2-propanol, although the rate was much lower than that with platinum which possesses dehydrogenating properties.
Abstract: Until now, the effect of the deposition of metal particles onto a powdered semiconductor oxide on its photocatalytic activity has been determined for group VIII transition metals only. In this study, AGTiO2 samples were prepared by photodeposition, characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) and tested, under band-gap illumination, using the oxidation of 2-propanol (neat liquid phase or mixed with water) in the absence or presence of molecular oxygen. The silver deposits were found to be metallic both before and after use, but the size of the particles and their distribution on the support were markedly influenced by the type of TiO2 sample. A small amount of silver caused the catalytic oxygen-free oxidation of 2-propanol, although the rate was much lower than that with platinum which possesses dehydrogenating properties. This effect is thought to be due to the electron-attracting role of the metal particles. For oxidation of 2-propanol by molecular oxygen, an optimum silver content was observed (corresponding to a rate improvement of approximately 50%). This was interpreted to be due to the beneficial effects of silver on the trapping of electrons and/or the reduction of oxygen. At higher than optimum silver contents there was a detrimental increase in the probability of recombination of charges at the numerous silver particles. The optimum was shifted to lower values in the presence of water, which is tentatively explained by competitive hole trapping by 2-propanol, water and silver.
179 citations
TL;DR: In this paper, the influence of Fe3+, Fe2+ and Ag+ on phenol photodegradation performed in aqueous polycrystalline TiO2 (anatase) and TiO 2 (rutile) dispersions was investigated.
Abstract: The influence of Fe3+, Fe2+ and Ag+ on phenol photodegradation performed in aqueous polycrystalline TiO2 (anatase) and TiO2 (rutile) dispersions was investigated. These ions can react very easily with peroxo species produced on the catalyst surface and/or in the solution. Experimental conditions for the continuous photoproduction of Fenton reactive can be achieved in this way. Maximum photoactivity was observed for TiO2 (anatase) in the presence of oxygen and [Fe3+] = 5 × 10−4 M. The behaviour of Fe2+ was similar to Fe3+ for the same experimental conditions. Anatase photoactivity was beneficially influenced in the presence of oxygen and [Ag+] = 1 × 10−4 M.
157 citations
TL;DR: In this paper, the excitation of NO2−, HNO2 or NO3− in the presence of nitrophenol induces hydroxylations in the ortho or para positions with respect to the phenol function.
Abstract: The excitation of NO2−, HNO2 or NO3− in the presence of nitrophenol induces hydroxylations in the ortho or para positions with respect to the phenol function. No nitration is observed in contrast with phenol and hydroxybiphenyls. The reaction can be attributed to hydroxyl radicals formed in the photolysis of NO2−, HNO2 and NO3−. With nitrite, the reaction is more efficient at 253.7 nm than at longer wavelengths. This effect is not observed with HNO2. When NO2− or HNO2 are excited in the presence of nitrophenol, the formation of dihydroxynitrobenzenes competes with the scavenging of hydroxyl radicals by nitrite ions (or HNO2) and the efficiency of the reaction is significantly influenced by the relative concentrations.
89 citations
71 citations
TL;DR: A detailed study of the phenomenon was performed with 4-biphenylcarboxylic acid as discussed by the authors, revealing a multitude of emitting species (the acid and its anion, either adsorbed on the silica cage surface or “dissolved” in solvent-rich cages).
Abstract: Room temperature phosphorescence (RTP) is obtained from a wide variety of organic molecules when trapped in silica matrices prepared by the “sol—gel” polymerization process of tetramethoxysilane. Examples include polycyclic hydrocarbons, aromatic acids, an aromatic base (quinine) and an organic dye (eosin-y). Lifetimes of up to several seconds are observed. Conditions for observing RTP vary. In some cases co-trapping of a heavy atom is needed, whereas in others RTP is observed even in wet gels without a heavy atom. A detailed study of the phenomenon was performed with 4-biphenylcarboxylic acid. It RTP and delayed fluorescence properties were studied as a function of base concentration, reaction time and temperature (Arrhenius analysis), revealing a multitude of emitting species (the acid and its anion, either adsorbed on the silica cage surface or “dissolved” in solvent-rich cages).
64 citations
TL;DR: In this article, the reaction rate of photo-oxidation of carboxylic acids in the presence of these catalysts is investigated and shown to be zeroth order with respect both to acetic acid and to dissolved oxygen.
Abstract: Acetic acid is oxidized by oxygen in the presence of UV light, TiO2 and dissolved copper ions being the catalysis. The reaction rate is zeroth order with respect both to acetic acid and to dissolved oxygen, and is proportional to the absorbed light intensity, with a quantum yield of 0.06. The variation in reaction rate with the concentrations of the different copper species present in the solution, i.e. free ions, Cu(CH3COO)+ and Cu(CH3COO)2, leads us to propose the second of these as the reactive species adsorbed on the TiO2 surface, the diacetato complex (or neutral salt) also being adsorbed on the TiO2 surface, but as a poison. These results extend those already obtained for formic acid and are a part of a general scheme of photo-oxidation of carboxylic acids in the presence of these catalysts.
63 citations
TL;DR: In this article, the UV-visible absorption spectrum of methylglyoxal was measured by conventional UV spectroscopic techniques (220-490 nm) and in separate experiments in which gas phase methylglyoxide was produced is situ by the chlorine-atom-initiated oxidation of acetol (390-470 nm).
Abstract: The UV-visible absorption spectrum of methylglyoxal was measured by conventional UV spectroscopic techniques (220–490 nm) and in separate experiments in which gas phase methylglyoxal was produced is situ by the chlorine-atom-initiated oxidation of acetol (390–470 nm). In the region where the two sets of results overlap, reasonably good agreement is observed. These results indicate considerably high cross-sections than those previously reported in the literature.
55 citations
TL;DR: In this paper, the accuracy of room temperature adsorption of benzyl alcohol onto TiO2 (P25) from acetonitrile and aqueous solutions was reported.
Abstract: Tests are reported of the accuracy with which room temperature adsorption of benzyl alcohol onto TiO2 (P25) from acetonitrile and aqueous solutions can be fitted to a Langmuir-type adsorption isotherm modelled by competitive adsorption—desorption of solute and solvent in a monomolecular mixed-solution layer at the TiO2 surface. Better agreement, and a considerably larger adsorption equilibrium constant Kads, is shown for the acetonitrile suspensions. Initial rates of TiO2-sensitized photo-oxidation of benzyl alcohol to benzaldehyde have also been compared. These too are larger for the acetonitrile solutions and their concentration dependence shows better agreement with predictions arrived at by combining Kads values from adsorption studies with k′LH values deduced from Langmuir—Hinshelwood-type analysis of TiO2-sensitized photodehydrogenation rates. Possible reasons for the failure of data from the aqueous suspensions to give comparable agreement with such predictions are considered. However, kinetic tests of these possibilities are shown not to a differentiate unequivocally between mechanisms which would restrict reaction events to the monomolecular surface layer and others which would allow photogenerated species to escape from that layer and react in the aqueous solution.
54 citations
TL;DR: In this article, a new method involving the use of flash photolysis followed by rapid high performance liquid chromatography analysis is described and applied to the examination of the photodegradation of 4-chlorophenol (bdI) in aerated aqueous solution (6.3 × 10 −4 M).
Abstract: A new method involving the use of flash photolysis followed by rapid high performance liquid chromatography analysis is described and applied to the examination of the photodegradation of 4-chlorophenol (bdI) in aerated aqueous solution (6.3 × 10 −4 M). Analysis after a single flash indicates that p -benzoquinone ( II ) is the only observed photoproduct. Irradiation with a more intense flash or with multiple flashes yields other products (principally hydroxy- p -benzoquinone ( III ) and hydroquinone ( IV ), which are known photoproducts of the photolysis of ( II ). The photoreaction of ( I ) is relatively independent of pH, except that above pH 7 the product distribution changes in accord with the known dark chemistry of ( II) at high pH. Oxygen is a reagent in the photoreaction of ( I ), as bubbling with argon significantly decreases the yield of ( II ). This new method should be particularly useful for studying photochemical reactions in dilute solutions, since photolysis of intermediate products can be avoided.
53 citations
TL;DR: In this article, stable state and time-resolved fluorometric investigations of the excited state proton transfer reactions of carbazole and 2-naphthylamine in the presence of β-cyclodextrin are reported.
Abstract: Steady state and time-resolved fluorometric investigations of the excited state proton transfer reactions of carbazole and 2-naphthylamine in the presence of β-cyclodextrin are reported in this paper. The results, together with earlier reports, reveal that the prototropic reaction depends not only on the microenvironment of the molecule, which is imposed by cyclodextrin, but also on the nature of the molecule itself. Thus, in comparison with the bare chromophore, the deprotonation rate of the cyclodextrin inclusion complex is enhanced when the guest molecule is carbazole, whereas it is decreased for guests like naphthylamine or naphthols.
TL;DR: The quantum yield of 2-nitrophenol is very low, but increases at short wavelengths (in a degassed solution at pH 8.2, φ254 nm = 1.3 × 10 −3 and φ365 nm = 2.2 × 10−6) as discussed by the authors.
Abstract: The direct phototransformation of 2-nitrophenol is dependent on wavelength. The quantum yield is very low, but increases at short wavelengths (in a degassed solution at pH 8.2, φ254 nm = 1.3 × 10−3 and φ365 nm = 2.2 × 10−6). The main photoproducts have been identified as catechol, cyclopentadiene carboxylic acids, 2-nitrosophenol, nitrohydroquinone, 3-nitrocatechol and 2-aminophenoxazone. In acidic solution, photohydrolysis into catechol and nitrous acid is the main primary photoreaction. Photoreduction into nitrosophenol and secondary hydroxylations into dihydroxynitrobenzenes are also observed. The latter reaction is attributed to the excitation of nitrous acid (or nitrite ions) formed by photohydrolysis. In basic solution, the ring photocontraction is the major primary reaction.
TL;DR: In this paper, the photophysical properties of rhodamine 6G and of the cationic and the zwitterion molecular forms were investigated in different water-ethanol mixtures.
Abstract: Absorption and emission (fluorescence spectra and radiative decay curves) spectroscopy techniques are applied to study the photophysical properties of rhodamine 6G and of the cationic and the zwitterion molecular forms of rhodamine 19 in different water—ethanol mixtures. The present results show the influence of the specific solute-solvent interactions on these properties, mainly on the rate constant of the non-radiative deactivation of rhodamines. The internal conversion mechanism of rhodamines with monoethylamino groups is discussed.
TL;DR: In this article, the number of photons is related to other energy quanta, e.g. electron-hole pairs or chemical excitations, allowing the treatment of these processes by analogy with ordinary chemistry: equilibrium is reached when the sum of the chemical potentials including that of photons, is zero.
Abstract: Light is treated as heat in traditional thermodynamics. Photons are assigned a temperature but zero chemical potential. In processes such as photovoltaic or photochemical absorption, electroluminescence and chemiluminescence, the number of photons is related to that of other energy quanta, e.g. electron—hole pairs or chemical excitations. This leads to a non-zero chemical potential of photons allowing the treatment of these processes by analogy with ordinary chemistry: equilibrium is reached when the sum of the chemical potentials, including that of photons, is zero.
TL;DR: In this paper, a simple method to evaluate fluorescence quantum yields based on corrected fluorescence emission spectra of dyes in dilute and strongly absorbing solutions using different excitation wavelengths is presented.
Abstract: A simple method to evaluate fluorescence quantum yields based on corrected fluorescence emission spectra of dyes in dilute and strongly absorbing solutions using different excitation wavelengths is presented. The method is supported by a detailed knowledge of the apparatus geometry and energy profile of excitation. Several recommended quantum counters were used (9,10-diphenylanthracene, rhodamines 101 and B, cresyl, violet, oxazine 1 and 1-ethyl-4-(4-p-dimethylaminophenyl)-1,3 butadienyl)-quinolinium perchlorate, (LDS 798) to cover the emission range from the UV to the visible and near-IR A curve of correction factor f vs. the optical density of the samples was obtained enabling an accurate determination to be made of the fluorescence quantum yield as a function of concentration and excitation wavelength. A case study of two squaraines (bis[4-(dimethylamino)phenyl]squaraine (HSQ) and bis[4-(dimethylamino)-2-methylphenyl]squaraine (MeSQ)) is presented. The quantum yields of these squaraines were obtained in saturated solutions in dichloromethane and in dilute samples at room temperature. The quantum yields determined are the same for high and low concentrations of the compounds indicating that no aggregation effects occur.
TL;DR: In this paper, the authors show that DBMBF 2 shows two fluorescence peaks at 398 and 416 nm in the concentration region [DBMBF2 ] > 1 × 10 −2 M.
Abstract: In aprotic organic non-aromatic solvents, dibenzoylmethanatoboron difluoride (DBMBF 2 ) shows in addition to the monomeric fluorescence peaks, a broad fluorescence band at 522 nm in the concentration region [DBMBF 2 ] > 1 × 10 −2 M; the broad emission is ascribed to a DBMBF 2 excimer which is formed by a dynamic process. In the [DBMBF 2 ] = 10 −7 to 10 −3 M region, DBMBF 2 shows two fluorescence peaks at 398 and 416 nm; the latter peak becomes stronger at the expense of the former at higher concentrations and is the only emission peak at 10 −3 M. The two fluorescence peaks are ascribed to two conformers of DBMBF 2 in solution. DBMBF 2 forms a stable ground state complex with benzene, but relatively unstable complexes with electron-rich olefins, dienes and tertiary amines. In contrast with the strong exciplex fluorescence emission in the presence of benzene, DBMBF 2 fluorescence is quenched by trans -anethole and tertiary amines with no new emission.
TL;DR: In this article, the dye-sensitized photo-oxidation of dihydroxybenzenes and chlorinated derivatives was studied as a function of solvent polarity and substitution pattern of the reactant.
Abstract: The dye- sensitized photo-oxidation of dihydroxybenzenes and chlorinated derivatives was studied as a function of solvent polarity. Rate constants for the disappearance of singlet molecular oxygen (O 2 ( 1 Δ g )), determined by time-resolved phosphorescence at λ ⪢ 1050 nm, are in the range 5 × 10 5 to 3 × 10 8 M −1 s −1 depending on the solvent polarity and substitution pattern of the reactant. The ratio between the rate constant for reactive quenching k r and the total quenching constant k t is also strongly dependent on substitution pattern and solvent. The values of k r indicate that the O 2 ( 1 Δ g )-mediated photo-oxidation of the dihydroxybenzenes is a viable possibility for their degradation in an aqueous environment. A charge-transfer mechanism is suggested for the photo-oxygenation. Dihydroxybenzenes are fairly good O 2 ( 1 Δ g ) self-sensitizers of photo-oxidation on irradiation at 308 nm, with O 2 ( 1 Δ g ) quantum yields of approximately 0.2 in the most favourable case. For hydroquinones, this presumably occurs through the excitation of the corresponding quinone derivatives which are always present in traces in the aerated solutions.
TL;DR: In this paper, the photochemical activity of undoped and iron-doped TiO2 particles was studied by laser flash photolysis, where a short duration laser pulse was used to produce electron-hole pairs; methyl viologen was employed as electron scavenger.
Abstract: The photochemical activity of undoped and iron-doped TiO2 particles was studied by laser flash photolysis. A short duration laser pulse was used to produce electron-hole pairs; methyl viologen was employed as electron scavenger. The presence of small amounts of iron ions (less than 0.5%) in TiO2 matrices is beneficial to the photoreduction of methyl viologen (MV2+); by contrast, an increase in the amount of iron doping in TiO2 samples (from 0.5% to 5%) sharply reduces the MV+ yield to a value similar to that when pure TiO2 is used. The implications of these results to charge transfer photoreactions, such as dinitrogen reduction to ammonia, are discussed.
TL;DR: In this paper, a suspension of chlorophyll-a in synthetic seawater was irradiated with solar light until bleaching, and several isoprenoid photoproducts were identified.
Abstract: A suspension of chlorophyll- a in synthetic seawater was irradiated with solar light until bleaching. After irradiation, methyl ethyl maleimide and several isoprenoid photoproducts were identified by gas chromatography and gas chromatography/mass spectrometry analyses. In order to explain the formation of these isoprenoid compounds, various mechanisms involving the initial addition of singlet oxygen on the double bond of the chlorophyll- a phytyl chain were proposed. The photosensitized oxidation of phytyl propionate in the presence of haematoporphyrin confirmed these mechanisms.
TL;DR: The fluorescence of paper produced using high yield pulp increases on bleaching with hydrogen peroxide as mentioned in this paper, probably as a result of the photoreduction of groups such as quinones.
Abstract: The fluorescence of paper produced using high yield pulp increases on bleaching with hydrogen peroxide. The new blue fluorescent species help to offset the natural yellow colour of the paper. Irradiation of the paper in the presence of reducing agents leads to further bleaching and a further increase in fluorescence, probably as a result of the photoreduction of groups such as quinones.
TL;DR: The photo-induced electron transfer reactions of the triplet state of rose bengal (RB) and several electron donors were investigated by the complementary techniques of steady state and time-resolved electron paramagnetic resonance (EPR) and laser flash photolysis (LFP) as mentioned in this paper.
Abstract: The photoinduced electron transfer reactions of the triplet state of rose bengal (RB) and several electron donors were investigated by the complementary techniques of steady state and time-resolved electron paramagnetic resonance (EPR) and laser flash photolysis (LFP). The yield of radicals varied with the light fluence rate, RB concentration and, in particular, the electron donor used. Thus for L-dopa (dopa, dihydroxyphenylalanine) only 10% of RB anion radical (RB −) was produced, with double the yield observed with NADH (NAD, nicotinamide adenine dinucleotide) as quencher and more than three times the yield observed with ascorbate as quencher. Quenching of the RB triplet was both reactive and physical with total quenching rate constants of 4 × 10 8 mol −1 dm 3 s −1 and 8.5 × 10 8 mol −1 dm 3 s −1 for ascorbate and NADH respectively. The rate constant for the photoinduced electron transfer from ascorbate to RB triplet was 1.4 × 10 8 mol −1 dm 3 s −1 as determined by Fourier transform EPR (FT EPR). FT EPR spectra were spin polarized in emission at early times indicating a radical pair mechanism for the chemically induced dynamic electron polarization. Subsequent to the initial electron transfer production of radicals, a complex series of reactions was observed, which were dominated by processes such as recombination, disproportionation and secondary (bleaching) reactions. It was observed that back electron transfer reactions could be prevented by mild oxidants such as ferric compounds and duroquinone, which were efficiently reduced by RB −.
TL;DR: In this paper, the absorption, emission and excited state decay behavior of Ru(bpy)2(HAT)2+ (bpy,2,2′-bipyridyl; HAT, 1,4,5,8,9,12-hexaazatriphenylene) were studied in various solvents, low temperature ethanol, perfluorinated ionomer membranes (Nafion), in the presence of DNA or synthetic polynucleotides and in a solution of sodium polystyrenesulphonate (PSSS
Abstract: In order to assess its usefulness as a probe for medium properties, the absorption, emission and excited state decay behaviour of Ru(bpy)2(HAT)2+ (bpy,2,2′-bipyridyl; HAT, 1,4,5,8,9,12-hexaazatriphenylene) were studied in various solvents, low temperature ethanol—methanol glasses, perfluorinated ionomer membranes (Nafion), in the presence of DNA or synthetic polynucleotides and in a solution of sodium polystyrenesulphonate (PSSS). The emission intensity and lifetime are quenched in acid solution and in protic solvents, increasing in the order H2O
TL;DR: In this paper, the photocatalytic hydroxylation of benzoic acid in aqueous TiO2 suspension is suggested as evidence for the generation of hyroxyl radicals in the H2O(l)-TiO2(s) system.
Abstract: The photocatalytic hydroxylation of benzoic acid in aqueous TiO2 suspension is suggested as evidence for the generation of hydroxyl radicals in the H2O(l)—TiO2(s) system. The hydroxylation reaction was studied by UV spectroscopy and high performance liquid chromatography (HPLC). The experimental results are discussed in terms of the formal kinetic analysis and the aromatic carbon charges calculated by the INDO method.
TL;DR: In this article, the photodecomposition of the rhodamine 6G (R6G) dye dissolved in n -alcohols and aqueous sodium dodecylsulphate (SDS) micellar solution was investigated under continuous UV-visible illumination.
Abstract: The photodecomposition of the rhodamine 6G (R6G) dye dissolved in n -alcohols and aqueous sodium dodecylsulphate (SDS) micellar solution was investigated under continuous UV-visible illumination. The results indicate that in alcohol solution the photodecomposition efficiency increases with the dye concentration. It also depends on the nature of the alcohol the lowest efficiency being observed in ethyleneglycol. The mechanism involves the electron transfer reaction of the triplet excited state with a ground state dye molecule (T + S 0 → R + + R − . In aqueous SDS micellar solution, the R6G cation is embedded in the hydrophobic interior of the micelle close to the negatively charged sulphate end groups. We show that a rather high concentration (3 × 10 −3 M) of R6G can be dissolved in 10 −1 M SDS aqueous solution, protecting R6G from the electron transfer reaction and secondary decomposition reactions.
TL;DR: In this paper, a specific reagent, sodium 1,3-cyclohexadiene-1,4-diethanoate, was used to determine if singlet oxygen is involved in photocatalytic transformations in aqueous ZnO suspensions.
Abstract: A specific reagent, sodium 1,3-cyclohexadiene-1,4-diethanoate, was used to determine if singlet oxygen is involved in photocatalytic transformations in aqueous ZnO suspensions. No formation of endoperoxide was observed and it can be concluded that O2*(1Δg) is not involved in the photocatalytic transformation of this substrate. From the inhibiting effect of 2-propanol, which is much more efficient at pH 11 than at pH 7, it can deduced that the oxidation by hydroxyl radicals, which is the main route in basic solution, competes with one or several pathways in neutral solution. Oxidation by positive holes is probably one of the competing pathways.
TL;DR: In this article, the electronic properties of 2,7-dimethoxy-N-methyl carbazole (DMNMC) and DMC in various polar and non-polar environments at room temperature were compared with those of N-methylcarbazole.
Abstract: The electronic spectroscopic and photophysical properties of 2,7-dimethoxy-N-methyl carbazole (DMNMC) and 2,7-dimethoxycarbazole (DMC) in various polar and non-polar environments at room temperature were compared with those of N-methylcarbazole (NMC). The synthesis and characterization of these compounds are described. The first two electronic transitions 1Lb ← 1A and 1La ← 1A which are well separated and characterized in NMC and carbazole are mixed in the absorption and fluorescence spectra of DMNMC and DMC. This “proximity effect” is responsible for the unidentified first absorption band system observed in solvents at room temperature, for a marked decrease in both the fluorescence quantum yields and lifetimes (NMC, τF = 15.1 ns; DMNMC, τF = 8.4 ns; DMC, τF = 3.9 ns in 3-methylpentane) and for a loss of the mirror-image between the absorption and the fluorescence spectra of these dimethoxy derivatives. The importance of the vibronic interactions between these two states is shown to be controlled (reduced) by N-methylation and/or the intermolecular hydrogen bonding formation in a proton-acceptor solvent such as ethanol.
TL;DR: The laser flash photolysis (excitation at 248 nm) of benzophenone in aqueous solution was investigated using absorption and conductivity measurements in this paper, where the authors showed that triplets undergo triplet-triplet (T-T) annihilation partly by an electron transfer reaction leading to cation and anion radicals and a second ion-forming process is two-photon photoionization which leads to radical cations and hydrated electrons.
Abstract: The laser flash photolysis (excitation at 248 nm) of benzophenone in aqueous solution was investigated using absorption and conductivity measurements. In addition to intersystem crossing to the ground state, benzophenone triplets undergo triplet—triplet (T—T) annihilation partly by an electron transfer reaction leading to cation and anion radicals. A second ion-forming process is two-photon photoionization which leads to radical cations and hydrated electrons. Secondary reactions, involving the protolytic equilibrium between ketyl radicals and anion radicals, are observed and discussed.
TL;DR: In this paper, the dynamics of the photodissociation of NO2 at the excimer wavelength of 248 nm were investigated using a two-laser pump-probe technique.
Abstract: The dynamics of the photodissociation of NO2 at the excimer wavelength of 248 nm were investigated using a two-laser pump-probe technique. The unrelaxed NO photofragment was probed in vibrational levels 0–9 by (1 + 1) resonantly enhanced multiphoton ionization (REMPI), using the γ bands of NO in the A2Σ+ ← X2Π transition. From the spectra, the relative populations of the ro-vibrational states of NO were determined. The resulting vibrational distribution was broad and inverted, peaking in υ = 5,and also exhibiting a large population in υ = 0. The rotational distributions were non-thermal; a clearly bimodal distribution was observed in υ = 2. The spin states of NO were equally populated, but a preference was observed for the II(A′) over the II(A″) λ doublet state. The dynamics of the process, dominated by the deposition of vibrational energy in the NO photofragment, could be qualitatively fitted by a Franck-Condon model, the shape of the vibrational distribution being attributed to the overlap of the vibrational wavefunction of NO in NO2 with that of the NO product.
TL;DR: In this paper, the photochemistry of 7-dehydrocholesterol yielding previtamin D, the precursor of vitamin D, is reinvestigated with respect to the optimum wavelength at which the best selectivity for prev vitamin D vs. side products is obtained.
Abstract: The photochemistry of 7-dehydrocholesterol yielding previtamin D, the precursor of vitamin D, is reinvestigated with respect to the optimum wavelength at which the best selectivity for previtamin D vs. side products is obtained. Long irradiation periods lead to a steady state whose wavelength-dependent composition can be calculated by a kinetic model employing the absorption cross-sections of the components and the quantum yields of the reactions. The maximum stationary concentration of the desired isomer (greater than 60%) is obtained at 296 nm. Experimental results at several wavelengths show good agreement with the calculated kinetics. The investigations show that the process is more selective and more economic if the irradiation is stopped before the steady state is reached. The possible light sources are discussed. Lasers can easily generate the desired wavelengths, e.g. a KrF laser twice Raman shifted in deuterium (292 nm). However, their photons are still too expensive for production of vitamin D itself. Recently, a Br2 “excimer” lamp has been demonstrated. With its emission maximum at 292 nm and its efficiency it is probably ideal for vitamin D production.
TL;DR: On UV irradiation of TiO 2 powder at 293 K, paramagnetic centres are generated as indicated by electron paramagnetic resonance (anisotropic singlet at a g value of 2.009) as discussed by the authors.
Abstract: On UV irradiation of TiO 2 powder at 293 K, paramagnetic centres are generated as indicated by electron paramagnetic resonance (anisotropic singlet at a g value of 2.009). The formal rate constant of formation of paramagnetic centres during irradiation is 0.038 s −1 and their rate of decay after irradiation is 0.011 s −1 . On irradiation of an aqueous TiO 2 suspension saturated with air or oxygen, the formation of hydroxyl radicals is observed using the spin trapping technique (5,5-dimethyl-1-pyrroline- N -oxide as spin trap). No radical formation is observed in argon-saturated suspensions. Oxygen acts as an electron acceptor, separating the photogenerated charge carriers, and enabling the formation of hydroxyl radicals.