Showing papers in "Journal of Photochemistry and Photobiology A-chemistry in 1989"

Photosensitized reduction of benzil by heteroatom-containing anthracene dyes

Abstract: Benzil in methanol was quantitatively reduced to benzoin by triethylamine in the presence of some heteroatom-containing anthracene dyes under visible irradiation. The thermodynamics, kinetics, and various quantum yields were investigated. The transient intermediate of the reaction was detected by flash photolysis. The relationship between the photosensitized reduction and the fading of the dyes was studied. The requirements for the dyes and the mechanism of these reactions were clarified.

Environmental photochemistry: is iron oxide (hematite) an active photocatalyst? A comparative study: α-Fe2O3, ZnO, TiO2

Abstract: The photocatalytic activity of α-Fe2_O_3 colloids is compared to the activities of colloids and suspensions of ZnO and TiO_2. The formation of H_2O_2 is investigated and the oxidation of organic molecules is studied with high sensitivity. While ZnO and TiO_2 are found to be quite active photocatalysts in the formation of hydrogen peroxide and in the degradation of chlorinated hydrocarbon molecules, only negligible photocatalytic activity is found for α-Fe_2O_3. Upper limits of quantum yields for several photoreactions of α-Fe_2O_3 are given.

Photophysics and kinetics of two photochromic indolinospirooxazines and one indolinospironaphthopyran

Abstract: The photochromic reactions of an indolinospironaphthooxazine, an indolinospiroquinolinooxazine and an indolinospironaphthopyran were investigated in toluene and in methanol solutions at 298 K using laser flash photolysis The CO bond rupture of the closed spiro compounds followed by the formation of several trans isomers occurs via the first excited singlet state during the laser pulse (2 ns) On the ns-μs time scale, structural evolution of these trans isomers leads to the most stable trans isomer, the photomerocyanine The quantum yields of photomerocyanine formation were determined in toluene and in methanol for all three compounds The relatively low values obtained (01 – 02) imply efficient internal conversion towards the ground state of the closed form The photomerocyanines of the three compounds studied show positive solvatochromism and the rate constants of their thermal ring closure increase when going from non-polar to polar solvents These spectral and kinetic characteristics of the photomerocyanines support the conclusion of a predominant quinoidal structure in contrast with the photomerocyanines of nitro-substituted spiropyrans in which the structure is essentially zwitterionic The photochromic behaviour of spirooxazines is found to be similar to that of the indolinospironaphthopyran and of other non-nitro-substituted spiropyrans

Photocatalysed transformation of chloroaromatic derivatives on zinc oxide II: Dichlorobenzenes

Abstract: The oxidative transformation of dichlorobenzenes photocatalysed by ZnO in aqueous solution has been studied using high performance liquid chromatography (HPLC) and gas chromatography—mass spectrometry (GC—MS). Many photoproducts were detected and their formation was quantitatively evaluated. With 1,3-dichlorobenzene an ortho or para hydroxylation is primarily observed and no dechlorination occurs. With 1,2- and 1,4-dichlorobenzene hydroxylation can occur with or without dechlorination. It is concluded that the reaction is influenced by adsorption and desorption phenomena. Self-inhibition is observed, which is explained by the reaction of OH radicals with the adsorbed primary products and by the formation of non-desorbing dimers, whereas the formation of H2O2 occurs with a constant rate. The total inhibition by ethanol is attributed to the scavenging of OH radicals involved in the first step of the reaction.

Triplet energy transfer sensitized fluorescence in 3,9-dibromoperylene

Abstract: Sensitized fluorescence of 3,9-dibromoperylene (DBP) was obtained from triplet energy transfer with ketones as donors. The maximum efficiency of this process (TETSF) in acetonitrile measured with time-correlated single-photon counting was QTS = 0.33 ± 0.08. Because of the high quantum yield of fluorescence (QF = 0.75 ± 0.10) for analytical applications of TETSF DBP is superior to the acceptor molecules investigated earlier. Particular attention was paid to a detailed elucidation of the energy transfer processes involved in TETSF. The analysis of the excess energy dependence provided evidence for the participation of a higher excited triplet state (Tn) of DBP.

Non-radiative deactivation channels of molecular rotors

Abstract: On the basis of the fluorescence properties of a series of intramolecular charge-transfer complexes of varied structure (p-(N,N-dialkylamino) benzylidene malononitriles and coumarin laser dyes) in an inert solvent (ethyl acetate), a model of non-radiative decay is proposed. The model specifically describes the relative contributions of the possible internal relaxation channels to non-radiative energy dissipation of the S1, ct states via the free rotor mechanism. The results are consistent with the theoretical predictions based on the calculated electron density distributions and the bond-order alternation in the S1, ct state.

The photochemistry of 3,5,3',5'-tetrasubstituted stilbenes : formation of an equilibrium mixture of isomeric 4,4'-dihydrophenanthrene derivatives

Abstract: Tetramethoxystilbene undergoes reversible trans [rlhar2] cis photoisomerization (PI). The extent of PI and the fluorescence yield vary only slightly with the temperature. Photocyclization of the cis isomer leads to two 4,4′-dihydrophenanthrene-like products (DHPs), only one of which is converted into cis by visible light. Above −100 °C a thermal equilibrium exists between the two DHPs, DHP 1 [rhlar2] DHP 2 . UV irradiation in the presence of iodine results mainly in photocyclodehydrogenation to (OCH 3 ) 4− phenanthrene. Tetrafluoro- and tetra( t -butyl)-stilbene photoisomerize to an extent decreasing on cooling, with the flourescence yield increasing concurrently. Photocyclization of tetrafluorostilbene, though less efficient than with tetramethoxy, again leads to two coloured modifications behaving as observed with tetramethoxy. In tetramethylstilbene, the corresponding DHP undergoes a two-stage decay, providing indirect evidence for the presence of two modifications. In tetra( t -butyl)-stilbene photocyclization is very inefficient.

Efficient photocatalytic degradation of chloral hydrate in aqueous semiconductor suspension

Abstract: Chloral hydrate was degraded in the presence of suspended TiO2 by illumination with a super-high-pressure mercury lamp through the Pyrex glass wall of a cell. The addition of H2O2 enhanced the degradation rate 3.4 times. The degradation rate increases with pH in the absence of H2O2 and exhibits a maximum at pH 5.5 in the presence of H2O2. Of the semiconductors tested ZnO was the most efficient catalyst. The effect of H2O2 was observed only with TiO2 and WO3.

S2 → S0 fluorescence and transient Sn ← S1 absorption of all-rans-β-carotene in solid and liquid solutions

Abstract: The spectral and luminescence characteristics of all-rans-β-carotene* were studied at 77 and 4.2 K in isopentane. The fluorescence spectra exhibit a vibronic structure; their 00 band halfwidths are 770 cm−1 at 77 K and 380 cm−1 at 4.2 K. Fluorescence quantum yields ϱ at 4.2 and 77 K are (8 ± 3) × 10−5 and (4 ± 2) × 10−5 respectively. The lifetimes of the observed fluorescence, estimated from the fluoroescence quantum yields ϱ and the natural fluorescence lifetimes, appear to be (8 ± 3) × 10−14 s at 4.2 K and (4 ± 2) × 10−14 s at 77 K. The fluorescence observed originates from the S2(1 1B+u state; the extremely short lifetime of the latter is due to its electronic relaxation to the lowest S1(2 1A−g) state. The S1 state of β-carotene was studied at 293 K using picosecond laser spectroscopy (λexc = 528 and 352 nm). The Sn ← S1 absorption spectrum in n-hexane has a maximum at 555 nm (18 000 cm−1). In toluene and chinoline, bathochromic shifts of 400 ± 100 and 800 ± 100 cm−1 are observed relative to the maximum in n-hexane. The Sn ← S1 transition to the higher excited state Sn can take place; the spectrum of the Sn ← S0 absorption to this state has a maximum at 275 nm (36 500 cm−1 in n-hexane. The lifetime of the S1 state of β-carotene in solvents with different viscosities and polarities (n-hexane, toluene, chinoline and vaseline oil) is 10 ± 2 ps.

Selective photoreduction of carbon dioxide to methanol with hydrous cuprous oxide

Abstract: An aqueous suspension of hydrous cuprous oxide (Cu2O·xH2O) is found to photoreduce carbonic acid selectively to methanol, but is itself sacrificed. The high yield and selectivity is attributed to strong chemisorption of carbon dioxide, highly negative flat-band potential and multielectron transfer.

Photodegradation of surfactants. V. Photocatalytic degradation of surfactants in the presence of semiconductor particles by solar exposure

Abstract: Anionic sodium dodecylbenzenesulphonate (DBS), cationic benzyl-dodecyldimethyl ammonium chloride (BDDAC) and non-ionic p-nonyl-phenyl poly(oxyethylene). (NPE-n) surfactants can be photodegraded in aqueous suspensions of titanium dioxide powder by solar exposure. Vertical and horizontal type solar reactors were employed out of doors. The photodegradation of linear and iso-DBS were compared by surface tension, UV absorption and dye extraction measurements. The degradation processes of DBS and NPE-9 were examined by IR and nuclear magnetic resonance (NMR) spectroscopy. The aromatic moiety in the surfactants is decomposed more easily than the alkyl and/or ethoxylate groups. This photodegradation system may be practical for treatment of wastewater containing surfactants.

Time-resolved laser spectroscopy of the sensitized photolysis of iodonium salts

Abstract: Laser flash photolysis of thioxanthone derivatives in the presence of substituted iodonium salts gives rise to an electron transfer process. Rate constants are measured and the cation radical of the thioxanthone is determined. Other results obtained with anthracene, pyrene, benzophenone and ruthenium tris-2,2′-bipyridine are discussed.

Study of the long-lived intermediate formed in the photoreduction of benzophenone by isopropyl alcohol

Abstract: The formation, structure and decay kinetics of the long-lived intermediate formed in the photoreduction of benzophenone by isopropyl alcohol in acetonitrile has been investigated. Laser flash photolysis and transient absorption measurements carried out with the benzophenone—isopropyl alcohol—acetonitrile and benzophenone—benzhydrol—acetonitrile systems indicated that the intermediate, generally called a “light-absorbing transient” (LAT), is formed by coupling of a diphenylketyl with a dimethylketyl radical. Dimerization of diphenylketyl radicals appeared to contribute very little to LAT formation. Study of the decay kinetics of the intermediate revealed that LAT actually consisted of a mixture of an ortho and a para isomer for which absorption spectra could be obtained. The ortho-LAT isomer decomposed faster, mainly by a benzophenone-enhanced process, and absorbed at longer wavelengths with a maximum at 358 nm, while para-LAT isomer decayed more slowly and had an absorption maximum at 317 nm. Additional investigations supplied absolute spectra for short-lived transients, i.e. for benzophenone and acetone triplet states and diphenylketyl and dimethylketyl radicals.

Determination of rate constants of 1O2 consumption by 1O2 acceptors in weakly deactivating solvents

Abstract: The rate constants k d obs of 1 O 2 consumption were measured directly in CS 2 for eight 1 O 2 acceptors using the time-resolved technique. The values obtained agree well with the values of k 1 obs in toluene and benzene which were estimated indirectly from the reactivity parameter β previously determined in self-sensitized or sensitized photo-oxygenation experiments. Only the measurements of the self-sensitized photo-oxygenation of 9,10-diphenyl-anthracene (DPA) and 9,10-dimethylanthracene (DMA) in CS 2 yield values of β which are consistent with a long lifetime of 1 O 2 of τ Δ = 34 ms in this solvent. The determination of β from measurements of the self-sensitized photo-oxygenation in CS 2 results in β values which are too high for the rest of the 1 O 2 acceptors, indicating that the lifetime of 1 O 2 in CS 2 is strongly reduced in these experiments. The same behaviour is observed for the self-sensitized photo-oxygenation of DPA and DMA in Freon 113.

Light-induced polymer and polymerization reactions XXXIII: direct photoinitiation of methyl methacrylate polymerization by excited states of ketones

Abstract: The S 1 and T 1 excited states of several aromatic ketones (benzophenone, xanthone, fluorenone, thioxanthone, acridone, N -methylacridone) are quenched by methyl methacrylate (MMA). The rate constants of T 1 quenching ( k 3 q ) measured by means of flash experiments show a linear log k 3 q vs. E T relationship. Results of kinetic treatment agree with the formation of 1,4-diradicals which decompose mainly into the starting materials, whilst some initiate the polymerization of MMA. Data of the kinetic experiments of polymer formation (monomer exponents, light intensity exponents) accord with such a mechanism. The polymers obtained contain the corresponding ketone moiety, identified by absorption and fluorescence measurements.

Photophysics and photochemistry of phenosafranin dye in aqueous and acetonitrile solutions

Abstract: The singlet and triplet excited state properties of phenosafranin dye were characterized in aqueous and acetonitrile solutions. The singlet excited state exhibited an absorption maximum at ∼ 430 nm and a lifetime of 930 ps in aqueous medium. The transient absorption spectra of the protonated and unprotonated forms of excited triplet were investigated by the laser flash photolysis technique. The protonated phenosafranin triplet had absorption maxima at 380 nm (ϵ = 11 000 M −1 cm −1 ), 615 nm (ϵ = 9700 M −1 cm −1 ) and 665 nm (ϵ = 10 800 M −1 ) and the unprotonated phenosafranin triplet had absorption maxima at 410 nm (ϵ = 8800 M −1 cm −1 ), 700 nm (ϵ = 6900 M −1 cm −1 ) and 790 nm (ϵ = 11 600 M −1 cm −1 ) in aqueous medium. The p K a of the protonated triplet phenosafranin was found to be 7.25. Diffusion-controlled quenching of the excited phenosafranin triplet by amines led to the reduction of the dye. The absorption spectra of the semireduced phenosafranin, which was characterized in acetonitrile, exhibited absorption maxima at 440 nm (ϵ = 29 000 M −1 cm −1 ) and 870 nm (ϵ = 19 000 M −1 cm −1 ).

Fluorescence properties and excited state interactions of 7-hydroxy-4-methylcoumarin laser dye

Abstract: The effect of solvent on some photophysical properties of 7-hydroxy-4-methylcoumarin (4-methylumbelliferone) is examined. Light absorption results in the population of a locally excited (LE) first singlet state (S 1 , ππ*) mixed with nπ*. This is characterized by extensive non-radiative energy dissipation on the subnanosecond time scale, which shows a sensitivity to environmental factors (solvent polarity and hydrogen-bonding ability) that may be exploited profitably in various systems of industrial and biological importance. Moreover, excellent linear correlations are established between some photophysical properties of 7-hydroxy-4-methylcoumarin and some solvent properties. The results of discussed on the basis of quantum chemical data obtained by application of the CNDO method. The fluorescence spectra in alcoholic solvents and water show complex formation via hydrogen-bonding interactions. The equilibrium constants K for complex formation (dye—water followed by ultrafast proton transfer) are determined at various temperatures. Small K values in the range 13 - 19 1 mol −1 are obtained. Some thermodynamic parameters are also determined. The results indicate an associative excited state complex with a dissociative ground state. The weak excited state complex formed with water is characterized by an extended strong blue-green fluorescence which can be attributed to tautomer formation via intramolecular proton transfer.

Photochemistry and photopolymerization study on 2-acetoxy and methyl-2-acetoxy derivatives of thioxanthone as photoinitiators

Abstract: The initiation efficiencies of 2-acetoxy thioxanthone and its methyl derivatives (in the 1, 3 and 4 positions) were studied by photocalorimetry when they were used as photoinitiators in the polymerization of butyl acrylate in bulk. The behaviour of the excited states of these new thioxanthones has been studied by spectroscopic measurements. As initiation results from the interaction of the thioxanthone molecule with tertiary amines, their photoreduction in the presence of a tertiary amine 2-N,N-diethylamino ethanol has also been investigated. Generally, most of the compounds exhibit high phosphorescence and triplet lifetimes at 77 K in addition to high photopolymerization efficiencies except 1-substituted derivatives where intramolecular hydrogen bonding is involved which enhances the rate of radiationless conversions.

Mechanistic aspects of polymer photostabilization

Abstract: A short account of modern theories in polymer degradation under oxidative conditions is followed by a detailed description of the different mechanistic aspects of polymer photostabilization. The discussion concentrates on the compounds presently known as photoantioxidants, which are believed to be the most efficient inhibitors of polymer photo-oxidation. The behaviour of multicomponent systems, i.e. stabilization mixtures, which is of prime practical importance is also reviewed.

Quenching of ketone triplet states by silanes

Abstract: Triplet state quenching of the ketones benzophenone, p -methylbenzophenone, p -phenylbenzophenone and benzil by silanes containing an SiH bond was investigated in a 9:1 (v/v) mixture of acetonitrile and acetone. It was found that both the n,π* and n,π*−π,π* triplet states are quenched if their energies are above 280 kJ mol −1 . Quenching rate constants k H were determined using steady state phosphorescence quenching and the time-resolved measurement of the triplet—triplet absorption. From an analysis of the behaviour of the triplet—triplet absorption of benzophenone as a function of time at variable silane concentrations, a reaction scheme was derived which includes the combination reactions of diphenyl ketyl, solvent and silyl radicals and the addition of silyl radical to the ground state of benzophenone. k H values obtained in this way agree with the phosphorescence quenching data and are of the order of 3 × 10 6 to 3.4 × 10 8 dm 3 mol −1 s −1 . Furthermore, it was found that the log k H values depend on the magnitude of the wavenumber of the SiH vibration of the silanes.

An excited state proton transfer reaction in micellar media

Abstract: The excited state proton transfer reaction was studied for carbazole (CAZL) in non-ionic (Triton X-100), cationic (cetyltrimethylammonium bromide (CTAB)) and anionic (sodium dodecylsulphate (SDS) micelles. The micelles strongly affect the equilibrium quotient of the process. Taking the excited state proton transfer reaction as a probe, the critical micelle concentrations (CMCs) for the three micellar solutions were estimated. Furthermore, a phase transition was observed at high ionic strengths for Triton X-100.

N-substituted 1,8-naphthalimide derivatives as high efficiency laser dyes

Abstract: A study of the photophysical properties of novel N -substituted 1,8-naphthalimide derivatives was performed in our laboratory. Some derivatives with a methoxy group in position 4 of the naphthalic ring have a high fluorescence quantum yield. The lasing efficiency of these compounds in acetonitrile solution, using an N 2 laser as the pumping system, is reported.

Radicals of 4,4′-bipyridyl and trans-1,2-dipyridylethylenes in organic solvents formed by laser flash photolysis

Abstract: Reactions of trans -di-( n -pyridyl)ethylenes ( n,n′ -DPEs, n,n′ = 2,3,4) and 4,4′-bipyridyl (BPY) following excitation by UV laser pulses were studied at room temperature in organic solvents. Upon direct laser excitation a triplet was observed for BPY (λ max = 340 nm) and 2,2′-DPE (λ max = 360 nm) but not for 3,3′-DPE and 4,4′-DPE. The transient of 4.4′-DPE in 2-propanol (λ max = 455 nm) and a second transient of BPY (λ max = 370 nm) are assigned to radicals (half-life ⩾ 0.1 ms) of the type H-DPE • and H-BPY • respectively. The structure of H-DPE • is that of a hydrogen atom adduct to one of the nitrogens. The radicals are formed by hydrogen atom abstraction from the solvent involving the excited n,π* singlet state of trans -4,4′-DPE and the n,π* triplet state of BPY.

Solvent effects on the photophysical properties of N-substituted 1,8-naphthalimide derivatives

Abstract: The photophysical properties of two N -substituted 1,8-naphthalimide derivatives were studied in various solvents (dichloromethane, benzene, dimethyl sulphoxide and water). The variations in the fluorescence spectra, absorption spectra, quantum yield and fluorescence lifetime were analysed. The results obtained are slightly different from the conclusions of previous investigations.

Solvatochromic effects in the fluorescence of a few diamino aromatic compounds

Abstract: Solvatochromic effects of 14 solvents and various dioxane-water mixtures on the fluorescence spectra of three isomeric phenylenediamines, 2, 3-diaminonaphthalene (2,3-DAN) and 2,7-diaminofluorene (2, 7-DAF) have revealed the following: (i) the Bilot-Kawski expression of polarizability provides a better specific analysis of solvatochromic effects than the Lippert expression or the Dimroth parameter; (ii) no expression correlating the solvent polarity and Stokes shift expresses the effect of charge transfer interactions in exciplex formation; (iii) the two amino groups in 2,3-DAN are twisted with respect to the plane of the naphthalene ring.

Thioxanthone photopolymers I: A kinetic study of methyl methacrylate polymerization using 2-benzyloxy thioxanthone, free and polymer bound, as photoinitiator

Abstract: The general photochemistry and photopolymerization activity of free and polymer-bound 2-benzyloxy thioxanthone were studied. The polymeric photoinitiator was prepared by chemical modification of an atactic polystyrene (Mn = 97 500). The spectroscopic properties of free and polymer-bound 2-benzyloxy thioxanthone were examined in various solvents and the data were compared with their behaviour as photoinitiators of polymerization. Quantitative steady state and microsecond flash photolysis studies in the absence and presence of a tertiary amine were also related to their photopolymerization activity. The dependence of the photopolymerization rates and quantum yield of polymerization of methyl methacrylate on the concentration of amine, photoinitiator and monomer was studied by photodilatometry. Furthermore, all the polymers obtained were characterized by gel permeation chromatography and the ratio of the propagation constant to the square root of the termination rate constant kp/kt12 was obtained.

Concerning interconversion of rotamers of trans-1,2-diarylethylenes in the triplet state

Abstract: The applicability of the NEER (non-equilibration of excited rotamers) hypothesis to the lowest triplet state of trans -1,2-diarylethylenes is investigated theoretically in the light of recent triplet-related experimental findings. CS-INDO CI calculations indicate that in the T 1 state the barrier hindering interconversion of rotamers is rather high, 11 – 13 kcal mol −1 , for stilbene and its naphthyl analogues, but decreases to about 6 kcal mol −1 in anthryl analogues, such as 2-styrylanthracene (2-StA), in consequence of considerable localization of the triplet excitation on the anthracene moiety. This suggests that, in fluid solutions, triplet rotamers of 2-StA and related compounds may interconvert during their fairly long lifetime. In highly viscous solutions the viscosity barrier to interconversion is much higher, impeding such an interconversion. The existence of solvent viscous drag, in addition to the theoretically predicted barriers in the triplet state, explains the experimental results in a wide range of temperatures and viscosities.

Dual amplified spontaneous emission from 7-amino-4-methyl coumarin dye

Abstract: This paper reports on the dual amplified spontaneous emission observed from solutions of 7-amino-4-methyl coumarin dye (coumarin 440) in certain solvents such as n -butyl acetate, dioxane etc. when exposed to high power nitrogen laser excitation. The results suggest that twisted intramolecular charge transfer coumarin photoisomers which form exciplexes with the solvent molecules have enough gain to produce amplified spontaneous emission even where there is apparently no detectable fluorescence.

Novel photoinduced electron transfer reactions between naphthalene and triphenylsulphonium salts

Abstract: Irradiation of solutions of naphthalene and triphenylsulphonium trifluoromethanesulphonate initiates an electron transfer reaction which gives 1- and 2-phenylnaphthalenes, 2- and 4-phenylthiobiphenyls, and diphenyl sulphide. The phenylnaphthalenes are formed by an in-cage reaction between phenyl radical and naphthalene radical cation, whereas diphenyl sulphide is an escape product. A second cage process, oxidation of diphenylsulphide by naphthalene radical cation, generates diphenylsulphinyl radical cation; reaction of this intermediate with phenyl radical gives the phenylthiobiphenyl isomers. However, irradiation of anthracene and triphenylsulphonium triflate yields only phenylated anthracenes and diphenylsulphide, because anthracene radical cation cannot oxidize diphenylsulphide.

Photochemical properties of ultrathin TiO2 films prepared by chemical vapor deposition

Abstract: Thin films of TiO 2 were prepared on fused silica and titanium substrates by chemical vapor deposition (CVD) using Ti[OCH(CH 3 ) 2 ] 4 under conditions of anatase formation. The UV—visible transmission spectrum of the film deposited on the fused silica substrate showed an absorption edge 30 nm shorter than that of anatase powder. This blue shift may be attributed to size quantization, i.e. the film may consist of extremely small TiO 2 particles. The absorption edge shifted to slightly longer wavelength upon heating the film in air at high temperatures. The photocatalytic property of the film was examined using the photodeposition reaction of silver from aqueous AgNO 3 . The silver photodeposition rate increased with the calcination temperature, maximizing at around 400°C and then decreasing at higher temperature. To compare the photocatalytic property with the photoelectrochemical property, photocurrents were measured in a KOH solution using the film deposited on the titanium substrate. They decreased after calcination at temperatures above 400°C. Compared to bulk TiO 2 , the wavelength dependence of the current was shifted to considerably shorter wavelengths, in agreement with the absorption spectrum.