Showing papers in "Journal of Photochemistry in 1985"

New molecular energy storage systems

Abstract: In order to find an outstanding molecular energy storage system A/B composed of a cycle of reactions including a direct sunlight-induced endoergic process and an energy-releasing reverse process, the photochemical valence isomerization of new organic compounds leading to highly strained systems was investigated on the basis of new concepts. Metal complex catalysis for the energy-releasing process was also studied using homogeneous and heterogeneous catalysts. Consequently we found an excellent system (“DONAC”). A bench-scale test plant was constructed to examine the practical application of the energy-releasing process from donor-acceptor quadricyclane to donor-acceptor norbornadiene.

Thermal-lensing measurements of singlet molecular oxygen (1Δg): quantum yields of formation and lifetimes

Abstract: Absolute quantum yields φΔ for singlet molecular oxygen (1O2(1Δg)) production were determined by time-resolved thermal-lensing (TL) using several sensitizers. The φΔ value is smaller than the quantum yield φisc for intersystem crossing for tetraphenylporphyrin, zinc tetraphenylporphyrin and anthracene is air-saturated benzene solutions. For anthracene, the values φisc = 0.78 ± 0.08 and φΔ/ = 0.78 ± 0.16 were measured using this method. The lifetimes of 1O2, determined by time-resolved TL in several solvents at concentrations similar to those detected by emission, are in good agreement with literature data for values up to about 100 μs. For longer lifetimes the values are smaller than those in the literature. Various possible reasons for the discrepancies are discussed.

Photoisomerization and thermoisomerization I: Unusual photochromism of N-(3,5-di-tert-butyl-salicylidene) amines

Abstract: Unusually photosensitive Schiff bases were prepared from 3,5-di-tert-butylsalicylaldehyde and amines; the aniline derivative exhibits both normal and reverse photochromism on irradiation with UV light followed by either storage in the dark or irradiation with visible light. From a consistent kinetics explanation of the thermal back reaction, two courses for the photo-bleaching and the thermal fading of the photochromer, which is assigned to a quinoid amine form, are illustrated.

Intramolecular monomer and excimer fluorescence with dipyrenylpropanes: double-exponential versus triple-exponential decays

Abstract: The fluorescence decays of 1,3-di(1-pyrenyl)propane undergoing intramolecular excimer formation can be fitted to a sum of three exponentials, whereas only two exponentials are needed for 1,3-di(2-pyrenyl)propane. It is concluded, from an analysis of the decay parameters, that one monomer and two excimers are involved in the excimer formation for 1,3-di(1-pyrenyl)-propane, in contrast with that for 1,3-di(2-pyrenyl)propane where only one excimer and one monomer are needed in the kinetic scheme. Kinetic and thermodynamic data are presented for both molecules. The significance of the various cases (double and higher) of multi-exponential decay is discussed.

Chemical actinometry in the UV by azobenzene in concentrated solution: A convenient method

Abstract: The advantages of concentrated solutions of azobenzene in actinometry are discussed. The experimental procedure and the correct method of evaluation are given in detail. The partial photochemical quantum yields for both photoisomerization steps were calculated by means of kinetic simulation and were checked using liquid chromatography. They differ for the π,π* and the n,π* transitions.

The kinetics of formation of twisted intramolecular charge transfer (TICT) states in p-substituted dialkylanilines: consequences of conical intersections along the reaction coordinate

Abstract: The dual fluorescence spectra of a series of dialkylaminobenzonitriles and dialkylaminobenzoesters measured at room and low temperature in polar and non-polar solvents are compared. The esters always show a higher ratio of long-wavelength to short-wavelength fluorescence. This ratio strongly depends on the type of dialkylamino substituent. In non-polar solvents it is highest for the compound with the lowest donor ionization potential (D ≡ 2,5-dimethylpyrrolidino). The fluorescence spectra show clear evidence of dual fluorescence (twisted intramolecular charge transfer (TICT) emission) even for the corresponding nitrile in alkane solvents. This is rationalized by invoking thermodynamic control of TICT state formation. In the case of polar solvents the order of the rate constants for TICT state formation is changed such that for rotating parts of similar size it correlates with the initial Franck-Condon twist angle, indicating kinetic control. Low temperature quantum yield measurements and direct laser kinetic data show that in n -butyl chloride at low temperatures the rate of formation of the TICT states of the esters is a factor of 2 – 13 greater than that of the nitriles. This is ascribed to the presence of a conical intersection along the reaction coordinate in the case of the nitriles. Microviscosity theory is used to discuss some effets which depend on the size of the dialkylamino substituents and on the nature of the solvent in terms of the available free volume for rotation.

Structures and functions of thin metal layers on semiconductor electrodes

Abstract: Metal-coated semiconductor electrodes such as Au/n-TiO 2 and Au/n-GaP show photovoltaic effect that cannot be explained by the conventional potential barrier model for metal—semiconductor contact. From experimental and theoretical investigations it has been concluded that the microscopically discontinuous structure of the metal layer is responsible for the anomalous photovoltaic effect. Several theoretical conclusions which are interesting from the point of view of solar energy conversion are derived. (1) Metal-coated semiconductor electrodes in electrolyte solutions generate high photovoltages at the metal—semiconductor interface in cases where the metal layer forms islands approximately 5 nm in diameter and separated by more than 20 nm from each other provided that the potential of the electrode is controlled so as to give band bending in the metal-free part of the surface. The maximum photovoltage can increase up to the equivalent of the band gap of the semiconductor in ideal cases. (2) The metal—semiconductor contact becomes ohmic when the potential of the electrode approaches the flat-band potential for the bare electrode in cases where the metal layer either has cracks or forms islands with gaps wider than 20 nm, say. Changes in the barrier height at the metal—semiconductor interface are not assumed in the theory, which can be applied to any metal—semiconductor pair. The conclusions provide a thoeretical basis for the explanation of the mechanisms of interesting properties of metal-coated semiconductor photoelectrodes and photocatalysts, such as enhanced hydrogen photoevolution on platinum-coated p-type semiconductor electrodes or on platinum-coated semiconductor particles in solution.

Substituent effects in the quenching of acetophenone and benzophenone triplets by oxygen and the di-tert-butylnitroxy radical, and the efficiency of singlet oxygen photogeneration

Abstract: The behavior of the triplets of a number of p-substituted acetophenones and benzophenones with respect to quenching by oxygen and the di-tert-butylnitroxy radical (DTBN) has been examined in benzene and acetonitrile using 3371 nm laser flash photolysis In several cases the efficiency φΔ of singlet oxygen (1O2*) generation in the course of triplet quenching by oxygen has been determined using 1,3-diphenylisobenzofuran as the trapping agent for 1O2* The variation of the rate constants for triplet quenching with respect to the substituent nature suggests that the aromatic carbonyl triplets act as donors and acceptors in the quenching interaction with oxygen and DTBN respectively Relative to the rate constant for quenching by oxygen, the substituent effect on φΔ is less pronounced, particularly in the case of benzophenones; the φΔ are limited to the range 03 − 07 in the two solvents Fast equilibration among the various spin configurations of the encounter complex responsible for the nearly substituent-independent φΔ for benzophenones and back electron transfer in charge-transfer-derived ion pairs leading to 1O2* formation are suggested

Photochemical production of hydrogen from water

Abstract: The quantum yields φ( 1 2 H2) of hydrogen production were optimized as a function of the pH and the concentrations of the components of the Ru(bpy)32+/MV2+/edta/colloidal platinum model system (bpy ≡ 2,2′-bipyridine; MV2+ ≡ methylviologen; edta ≡ ethylenediaminetetraacetic acid) irradiated at 453 nm. An optimum quantum yield φ( 1 2 H2) of 0.171 ± 0.020 was found for the following optimized parameters: pH 5; [Ru(bpy)32+] = 5.65 × 10−5 M; [MV2+] = 3 × 10−3 M; [edta] = 0.1 M; concentration of chemically prepared colloidal platinum, 1.92 × 10−5 M. The quantum yields of the methylviologen radical cation (MV+·) were determined under the same conditions, but without platinum, and an optimum value φ(MV+·) = 0.181 ± 0.02 was obtained. The hydrogen and MV+· yields are thus closely related throughout the MV2+ concentration range investigated which supports the fact that colloidal platinum is operating with an efficiency close to 100%. Various types of heterogeneous catalysts (radiolytically prepared colloidal metals, metal deposited onto semiconductor powders, metal and metal oxide powders) were tested and compared under optimized experimental conditions. The relative catalytic efficiency of metal hydrosols for hydrogen production was as follows: iridium, platinum, osmium > ruthenium > rhodium > cobalt, nickel, palladium, silver, gold > copper, cadmium, lead. The highest φ( 1 2 H2) was observed for colloidal iridium (φ( 1 2 H2) = 0.173 ± 0.020). PtTiO2 was found to be the most efficient of the supported metals (φ′( built1 2 H2) = 0.160 ± 0.020). Hydrogen production from water was studied as a function of the nickel content (0.5 – 13.8 wt.%) for NiTiO2 and an optimum yield φ′( 1 2 H2 = 0.108 ± 0.02 was found for a nickel content of about 5 wt.%. RuO2 and IrO2 codeposited on zeolite gave the highest yields of the metal oxides (φ′( 1 2 H2) = 0.102 ± 0.02). The efficiencies of low cost catalysts such as nickel powder, TiO2, Fe2O3, Sm2O3, CeO2, MnO2 and ZnO were also examined.

Photophysical properties of quaternary salts of 4-dialkylamino-4'-azastilbenes and their quinolinium analogues in solution. IX

Abstract: The decay processes of excited quaternary salts of trans-1-alkyl-4-[4-dialkylaminostyrl]-pyridinium and trans-1-alkyl-4-[4-dialkylaminostyryl]-quinolinium (At+X−; X− ≡ I− or ClO4− were studied by pulse and steady state methods as a function of temperature, quenchers (ferrocene and azulene) and solvent polarity. Introduction of the dialkylamino group markedly reduces the quantum yield Ot→c of trans → cis photoisomerization and slightly enhances the quantum yield Of of fluorescence. In polar solvents deactivation of the first excited singlet 1*At+ predominantly by internal conversion via an activated step is suggested by the temperature dependence of Of and a low yield for intersystem crossing. In solvents of moderate polarity (e.g. dichloromethane) a major short-lived transient (TrT) (λmax ≈ 400 nm, ⪆ 700 nm; τT ≈ 1 – 10 μs) and a minor long-lived transient (TrR) (λmax ≈ 400 nm; t 1 2 > 10 μs) were observed for the iodides by laser flash photolysis. Pulse radiolysis studies permit the assignment of TrR to a radical A . TrT, which was also observed for iodides and perchlorates in polar solvents either on addition of excess I− or on sensitized excitation, is assigned to the lowest triplet state 3*At+ of the trans configuration. On the basis of results from energy transfer experiments with a range of sensitizers, the energy of 3*At+ is estimated to be 180 kJ mol− for the pyridinium and quinolinium salts respectively. In solvents of moderate polarity TrT is assigned to the triplet state (3*At+ … I−) of the ion pair, the population of which is enhanced by the heavy atom effect.

Photoluminescence of 2-(o-aminophenyl)benzimidazole

Abstract: The absorption and fluorescence spectra of 2-( o -aminophenyl)benzimidazole in different solvents have shown the existence of two structures formed by intramolecular hydrogen bonding. Both are equally favourable in non-polar media whereas one is more stable in polar solvents. pH effects have also been discussed with the evaluation of the ground and excited state equilibrium constants of the various equilibria.

β-Carboline as a fluorescence standard

Abstract: Fluorescence measurements of β-carboline in 1 N H2SO4 suggest its use as a fluorescence reference compound superior to the widely used quinine bisulphate. The emission spectrum (λmax = 450 nm) and quantum yield (φF = 0.60) of the β-carboline cation are similar to those of quinine. However, in contrast to quinine, the fluorescence decay of β-carboline is an excellent fit to a single-exponential lifetime across the emission band with τ = 22.03 ± 0.12 ns. The experimental conditions required for the application of β-carboline as a fluorescence standard are reported.

A synthetic system mimicking the energy transfer and charge separation of natural photosynthesis

Abstract: A synthetic molecular triad consisting of a porphyrin P linked to both a quinone Q and a carotenoid polyene C has been prepared as a mimic of natural photosynthesis for solar energy conversion purposes. Laser flash excitation of the porphyrin moiety yields a charge-separated state C +· —P—Q −· within 100 ps with a quantum yield of more than 0.25. This charge-separated state has a lifetime on the microsecond time scale in suitable solvents. The triad also models photosynthetic antenna function and photo-protection from singlet oxygen damage. The successful biomimicry of photo-synthetic charge separation is in part the result of multistep electron transfers which rapidly separate the charges and leave the system at high potential, but with a considerable barrier to recombination.

Photocatalysis of irradiated semiconductor surfaces: its application to water splitting and some organic reactions☆

Abstract: Hydrogen production from organic compounds and water was investigated using powdered semiconductor photocatalysts. The complete decomposition observed for several organic compounds demonstrated that water is involved in the reactions as an oxidizing agent. Photocatalyses of dyes and semiconductors were found to be applicable to amino acid synthesis. The quantum yields of photocatalytic amino acid synthesis using visible light are about 20% – 40% in the absence of a metal catalyst such as platinum. Moreover the reactions are highly selective and depend strongly on the type of semiconductor. This method was applied to the asymmetric synthesis of amino acids using asymmetric catalysts. Rather high optical yields of 50% were achieved for the synthesis of L-phenylalanine.

Roles of organized molecular assemblies in artificial photosyntheses

Abstract: As has been verified in biological photosynthesis, organized molecular assemblies (OMAs) are capable of controlling photochemical energy conversion. Charge separation of photoactivated species has bee attempted using type I OMAs such as micelles, microemulsions, bilayer membranes and polysoaps. The charge separation is enhanced by molecular diffusion which is aided by local electric field and segregation effects due to microenvironmental compartmentalization. The transport of electrons across the membrane wall of vesicles has been studied in an effort to achieve up-hill energy conversion. The attempts were successful in principle, but in practice considerable improvement of the membrane is required. However, new synthetic amphiphiles have been developed which form bilayers, and stable membranes with variable thicknesses (of he order of 50 A - 10 μm) have become available. Geometric structures of the membrane may also be modified easily. Both energy transfer and electron transport systems can be constructed using the new bilayer membranes with appropriately immobilized elements. Linked donor—acceptor systems, as represented by porphyrin—quinone, may serve as good photoreaction centres when they are installed on these type II OMAs with immobilized elements. The development of type III OMAs is required for efficient coupling of elementary processes in artificial photo-synthesis. The construction of charge pools in OMAs seems to be one of the solutions, as has been demonstrated in several examples such as the cooperative activation of two photoredox systems by the use of OMAs with electron pools.

Excited state proton transfer: A new feature in the fluorescence of methyl 5-chlorosalicylate and methyl 5-methoxysalicylate☆

Abstract: The fluorescence of methyl 5-methoxysalicylate and, to a lesser extent, methyl 5-chlorosalicylate departs appreciably from the usual emission of salicylic acid derivatives. The emission spectra, quantum yields and lifetimes of these compounds are compared here with those of the 4-methoxy, 3-chloro and 4-chloro derivatives which show dual emission and excitation spectra similar to those of the parent compound methyl salicylate. Moreover, the photophysics of the 3- and 4-derivatives can be understood in terms of a conformational equilibrium in the ground state and an excited state proton transfer reaction. In contrast, a new emission process with the same excitation spectrum as the proton transfer band is needed to explain the fluorescence of the 5-derivatives. It is speculated that the new flourescence originates in those excited species which fail to undergo the proton transfer reaction despite having the appropriate intramolecular hydrogen bond in the ground state; this explanation has already been considered by Weller in the excited state equilibrium hypothesis.

Behaviour of the laser dye 4-dicyanomethylene-2-methyl-6-dimethylaminostryryl-4H-pyran in the excited singlet state

Abstract: The spectral and temporal behaviour of the excited singlet state of 4-dicyanomethylene-2-methyl-6-dimethylaminostyryl-4 H -pyran (DCM) in a variety of solvents was studied The results obtained enable us to propose a possible mechanism for the behaviour of DCM in the excited singlet state

Interfacial processes involving strong electronic interactions in solar energy conversion and storage

Abstract: It has been known for a long time that strong chemical bonding of intermediate species is necessary for efficient electrocatalysis, particularly of electron transfer mechanisms. This important aspect has been neglected in previous investigations of materials for fuel-producing and energy-storing photoelectrochemical reactions. In order to emphasize this statement the progress of experiments with two groups of photoelectrodes which undergo strong interaction with reactants in the electrolyte is discussed. The first group consists of d band semiconductors (e.g. PtS2 and RuS2) in which the photogeneration of holes in the valence band derived from transition metal d states leads to the formation of interfacial coordination complexes with electron donors (e.g OH−). The second group consists of electrodes exhibiting combined electronic and ionic conduction which are able to photo-intercalate guest species (e.g. Cu+ into Cu6−xPS5I or Cu3PS4). The behaviour of anodically polarized RuS2 (ΔEG = 1.2 eV) as a stable photoelectrode which is able to liberate oxygen from water with a high quantum efficiency during illumination with visible and near-IR light confirms that complicated energy conversion reactions can take place in the presence of strong interactions even with thermodynamically unstable materials. By comparing the performances of various ruthenium dichalcogenides it is shown that the photoelectrochemical reaction path depends strongly on the d state density in the upper region of the valence band. Energy losses due to kinetic inhibition (unfavourable energetic position of the intermediate states) can be reduced by using semiconductors containing transition metal pairs or clusters in their crystal structure. Semiconductor materials which are able to photo-intercalate or photo-insert cations as a result of strong electron transfer interactions could be developed for light-powered ion pumps, intercalation batteries which can be charged by solar energy and systems capable of storing optical information. Photoelectrode systems which undergo strong interactions with redox systems open up new perspectives for photoelectrochemical energy conversion and storage but also demand major research efforts. New materials with specific electronic and interfacial properties for which simplified electron transfer models are not adequate need to be developed. It will also be necessary to deal with high concentrations of surface states.

Photoelectrochemistry in colloidal systems: Interfacial electron transfer between colloidal TiO2 and thionine in acetonitrile

Abstract: When adsorbed on a colloidal TiO2 semiconductor suspension, the thiazine dye thionine exhibited a red shift in its absorption and emission spectra. The apparent association constant for the association of TiO2 colloid and thionine in acetonitrile was determined to be 2.75 × 105 M−1. Reversible photoelectrochemical reduction of thionine was observed when the colloidal TiO2 suspension in acetonitrile containing thionine was subjected to band gap illumination (λ < 380 nm). The interfacial electron transfer between colloidal TiO2 and thionine in acetonitrile was investigated by means of steady photolysis and nanosecond laser flash photolysis techniques. The quantum yield for the photoelectrochemical reduction of thionine to semithionine was determined to be 0.1. The dependence of the photoelectrochemical production of semithionine on thionine concentration, the acidity of the medium and the intensity of excitation was examined.

Polymer-modified electrodes, catalysis and water-splitting reactions

Abstract: Highlights of results on surface modified CdS semiconductors in the presence and absence of an externally applied anodic bias are presented. At zero applied bias, the surface modification involved coating the faces of CdS with electrically conductive polypyrrole and catalytic dispersions of transition metals (Pt, Rh, RuO2) immobilized in polystyrene films. The combination of polypyrrole and catalytic polystyrene films prevented photo-induced dissolution of CdS and catalyzed H2 and O2 evolution. The amount and composition of metal dispersed in the polystyrene coating and the hydrophobic nature of the films were important in achieving good catalytic activity and stability of the CdS semiconductor. Sorption processes, the electronic contact between the polymer and the semiconductor and the particular transition metal catalyst selected also influenced the net yield of H2.

Luminescence of porphyrins and metalloporphyrins IX: dimerization of meso-Tetrakis(N-methyl-4-pyridyl)-porphine

Abstract: meso -Tetrakis( N -methyl-4-pyridyl)porphine (H 2 TMPyP(4) 4+ ) undergoes dimerization in aqueous solution. The dimer is believed to possess a stacked face-to-face configuration with the two porphyrin rings held about 1 nm apart. Fluorescence from the dimer is red shifted relative to the monomer but it is not quenched. The other isomeric H 2 TMPyP 4+ compounds together with metal chelates and the conjugate diacids exist only as monomeric species in aqueous solution (below 10 −4 mol dm −3 ).

Optimization of the photocatalytic properties of titanium dioxide

Abstract: The preparation of titanium dioxide for use as a photocatalyst in the decomposition of water, acetic acid and propan-2-ol is described. Samples having the highest reactivity were obtained by heating X-ray amorphous titanium dioxide (prepared by precipitation from titanium tetraisopropoxide) at 350 °C for between 1 and 4 h. The catalysts were shown by X-ray and UV-visible absorption measurements to contain anatase. The anatase crystallites were very small (about 80 A). Platinization of the catalysts increased their reactivity in every case.

Electrode-confined catalyst systems for use in optical-to-chemical energy conversion☆

Abstract: Catalysis of multiple-electron transfer fuel-forming redox reactions at illuminated semiconductor—liquid electrolyte interfaces is important in achieving efficient optical energy conversion. The characterization of redox polymer—noble metal combinations as catalysts for the reduction of H 2 O and aqueous CO 2 is described in this article. Emphasis is on the demonstration of rational synthetic methods applied to interfaces and the correlation of surface structure and function.

Metalloporphyrin-photosensitized formation of hydrogen from organic and inorganic substrates☆

Abstract: In outgassed aqueous solution porphyrins can be photoreduced to give a π radical anion, a phlorin or a chlorin. The type of product formed depends on the nature of the porphyrin, the central metal ion and the solution pH. Porphyrins have been identified that form stable π radical anions and phlorins. These species, unlike the chlorins, are oxidized back to the original porphyrin by air and, depending on the pH, they can reduce water to hydrogen in the presence of a platinum catalyst. Thus irradiation of these porphyrins in aqueous solution containing an electron donor and colloidal platinum leads to hydrogen formation. Various electron donors have been used, including H 2 S, hydrogenated nicotinamide adenine dinucleotide (NADH), formic acid, hydroxylamine and ethanol. The efficiency of hydrogen production depends markedly on the type of donor used and NADH is particularly effective. Possible ways of using NADH/NAD + as a reversible redox couple are considered.

Relationship between the quantum yields of electron photoejection and fluorescence of aromatic carboxylate anions in aqueous solution

Abstract: The quantum yield O(e aq ) of photoejected electrons from aromatic carboxylate anions in aqueous solutions (pH 9.5 – 12.3) appreciably increases with increasing excitation energy; for example, O(e aq ) for benzoate is 0.0025 for the S 1 state (excitation at 254 nm) and 0.0048 for the S 2 state (excitation at 214 nm). The O(e aq ) value also increases with the number of CH 2 groups separating the phenyl and carboxylate groups; for example, for phenylbutyrate O(e aq ) is 0.05 for the S 1 state and 0.010 for the S 2 state. Furthermore, increases of O(e aq ) are linked with decreases in the quantum yield of fluorescence. In order to evaluate a possible consumption of the photoejected electrons by the ground state carboxylates, the rate constants k (e aq + substrate) for the reaction of solvated electrons with the corresponding substrates were determined.

Photocatalytic multielectron photoreduction of 18 tungstodiphosphate in the presence of organic compounds Production of hydrogen

Abstract: [P2W18O62]6− (“W186−”) undergoes stepwise multielectron photoreduction with near-visible and UV light in the presence of a great variety of organic compounds. Photolysis with high intensity light at low pH produces a reduction by one, one, two, two and four electrons followed by decomposition, as is the case with chemical and electrochemical reactions. The reduction proceeds via steps to the extent that the reduction of hydrogen is thermodynamically allowed. This takes place both with and without a platinum catalyst. At a higher pH, i.e. as the reduction potential for hydrogen evolution becomes more negative, only the higher reduction products produce hydrogen. The rate constants for hydrogen evolution are a function of the reduction step and the pH and are of the orders of 10−4s−1.

Photoinduced electron transfer quenching of rhodamine B in polymer films

Abstract: Photoinduced electron transfer reactions of rhodamine B with several redox quenchers in solution-cast films of poly(N-vinylpyrrolidone) have been investigated using fluorescence spectroscopy. The quenching of the rhodamine B fluorescence has been used to examine the efficiency of photo-induced electron transfer in a rigid amorphous polymer matrix. The data obtained show that the electron transfer efficiency is a function of the free energy of the transfer and depends on the redox potential of the quencher. The data are analyzed using theories of electron transfer in solid media and are correlated with the redox potentials of the quenchers to derive the distance dependence of the excited state transfer. Photoinduced electron transfer between rhodamine B and both donor and acceptor quenchers in the cast polymer films is observed to occur at distances of up to about 15 A for this system.

Micellar structure and micellar control of photochemical reactions

Abstract: In view of the vast potential of micellar systems as media in which reactions may be conducted, a clear understanding of the structure of micelles is essential. The unique features of micelles and how these have been utilized to catalyse and control photochemical reactivity are briefly surveyed here. Micellar media, when used for chemical reactions, exhibit features that are completely different from those of ordinary non-aqueous solvents. A thermal or photochemical reaction conducted in micellar media is influenced by the effects of the micellar environment which result in control and/or modification of reactivity. The salient features of micelles that influence the photochemical reactivity are cage and microviscosity effects, localization and compartmentalization effects, pre-orientational, polarity and counterion effects.

The fluorescence spectra of dianions of α- and β-naphthylamines

Abstract: The fluorescence spectra of the dianions of α- and β-naphthylamines were observed at high OH - concentrations and the fluorescence quantum yields were calculated. The p K a * values were determined for the respective equilibria.

Spectroscopic and kinetic study of the photoinduced methoxy substitution of 3-nitroanisole and 3,5-dinitroanisole

Abstract: The photoinduced nucleophilic substitution reactions of 3-nitroanisole (3-NA) in the presence of OH − in mixtures of H 2 O and CH 3 CN and in the presence of OCH 3 − in CH 3 OH were investigated by nanosecond time-resolved absorption spectroscopy. The absorption spectra of the solution recorded just after the triplet state of 3-NA has disappeared reveal that the formation of the substitution product is complete at that stage. We conclude that the addition of the nucleophile to the ring carbon atom C-1 is the rate-determining step in the methoxy substitution reactions of 3-NA in H 2 O and CH 3 OH. As well as the addition to C-1 there is at least one competing reaction of the nucleophile with the triplet state. This reaction, which reduces the overall quantum yield of the substitution reaction, leads to the formation of long-lived species (λ max = 370 nm). Analysis of previously reported results for the photosubstitution reactions of 3,5-dinitroanisole in the presence of OH − and triethylamine shows that they can also be explained by the mechanism for the substitution reactions of 3-NA.