An ESR study of the photolysis of methanol adsorbed on silica gel

ESR has been used to study the photolysis of adsorbed methanol on various specimens of silica gel by light of wavelength 365 nm, which is found to detach -hydrogen from the OCH₃ group of the surface to give the radical SiOCH₂, which is stabilized in the pores. The alcohols CD₃OH and CH₃OD are also examined.     

Photochemistry of 1,3,5-trithianes in solution: Steady-state and laser flash photolysis studies

In this work, we characterized the direct photochemistry of a set of five structurally-related 1,3,5-trithianes. The compounds were 1,3,5-trithiane, the α- and β-isomers of the 2,4,6-trimethyl derivatives, and the α- and β-isomers of the 2,4,6-triphenyl derivatives. Under steady-state, 254-nm irradiation of acetonitrile solutions of all five trithianes, dithioesters of the form RC( = S)SCH(R)SCH₂R were identified and shown to be primary photoproducts (R = H, CH₃, or C₆H₅). Shorter dithioesters, RC( = S)SCH₂R, were also identified and shown to be secondary products. The presence of the dithioesters could be monitored by their strong absorption bands in the region of 310 nm. This same band was evident following the laser flash photolysis of the five trithianes. The laser-induced transient spectra showed another absorbing species (I) in all five trithianes. This species was not stable and showed a complementary decay that matched the growth of the stable photoproducts at 310 nm. This suggested that the intermediates (I) are the precursors of the corresponding dithioesters, RC( = S)SCH(R)SCH₂R. These correlated processes were related to monophotonic events. However, in the laser flash photolysis experiments in the triphenyl derivatives, there was an additional pathway for the formation of the dithioesters, and this was biphotonic. When the biphotonic formation of products was compensated for, RC( = S)SCH(R)SCH₂R formation quantum yields from steady-state and laser flash photolysis matched within experimental error. The absorption band of (I) varied systematically with substituents, 320 nm in 1,3,5-trithiane, 340 nm in the 2,4,6-trimethyl derivatives, and 420 nm in the 2,4,6-triphenyl derivatives. The nature of these intermediates (I) were discussed as resulting from CS bond cleavage, probably heterolytic.     

Photochemistry of 5-allyloxy-tetrazoles: steady-state and laser flash photolysis study

The photochemistry of three 5-allyloxy-tetrazoles, in methanol, acetonitrile and cyclohexane was studied by product analysis and laser flash photolysis. The exclusive primary photochemical process identified was molecular nitrogen elimination, with formation of 1,3-oxazines. These compounds were isolated in reasonable yields by column chromatography on silica gel and were fully characterized. DFT(B3LYP)/6-31G(d,p) calculations predict that these 1,3-oxazines can adopt two tautomeric forms (i) with the NH group acting as a bridge connecting the oxazine and phenyl rings and (ii) with the -N=bridge and the proton shifted to the oxazine ring. Both tautomeric forms are relevant in the photolysis of oxazines in solution. Secondary reactions were observed, leading to the production of phenyl vinyl-hydrazines, enamines, aniline and phenyl-isocyanate. Transient absorption, detected by laser flash photolysis, is attributed to the formation of triplet 1,3-biradicals generated from the excited 5-allyloxy-tetrazoles. The 1,3-biradicals are converted to 1,6-biradicals by proton transfer, which, after intersystem crossing, decay to generate the products. Solvent effects on the photoproduct distribution and rate of decomposition are negligible.     

Photochemistry of tetrasulfur tetranitride: laser flash photolysis and quantum chemical study

The photochemistry of tetrasulfur tetranitride (1) was studied in hexane solution by the laser flash photolysis technique (LFP). The experimental findings were interpreted using the results of previous matrix isolation studies (Pritchina, E.A.; Gritsan, N.P.; Bally, T.; Zibarev, A.V. Inorg. Chem. 2009, 48, 4075) and high-level quantum chemical calculations. LFP produces two primary intermediates, one of which is the boat-shaped 8-membered cyclic compound (2) and the other is the 6-membered S(3)N(3) cyclic compound carrying an exocyclic (S)-N═S group (3). The primary products 2 and 3 absorb a second photon and undergo transformation to the 6-membered S(3)N(3) cycle carrying an exocyclic (N)-S≡N group (4), which is very unstable and converts back to intermediate 3. The quantum yield of the primary product formation is close to unity even though the quantum yield of photodegradation of 1 is low (~0.01). Thus, 1 is a photochromic compound undergoing in solution the thermally reversible photochemical isomerization. The mechanism of the photochromic process was established, and the rate constants of the elementary reactions were measured.     

Laser flash photolysis study of electron transfer processes of adsorbed anthracene on titania–silica surfaces

We have studied the photophysics of anthracene adsorbed to photocatalytic silica–titania mixed oxide systems prepared by two different methods: a sol–gel synthesis and an impregnation route. The observed photophysics depend upon the method of synthesis, with the sol–gel prepared samples resulting in enhanced radical cation formation via static excited singlet state quenching. This mechanism, whilst operative, does not give rise to long-lived charge separation in the case of the impregnated samples. These results are discussed in terms of differing sample morphologies.     

Photochemistry of carbonyl photoinitiators. Photopolymerisation,flash photolysis and spectroscopic study

The photochemistry of eight aromatic carbonyl based photopolymerisation initiators is examined and compared using u.v. absorption, phosphorescence, micro-second flash photolysis, steady-state photolysis and photopolymerisation rates for n-butyl methacrylate. In the latter case, conversion rates are significantly greater in the presence of a tertiary amine (diethylmethylamine). In both the presence and absence of the amine, the photofragmenting type initiators are more efficient than the hydrogen abstracting benzophenone. Benzil exhibits the most notable conversion rates from being the least efficient in the absence of the amine to the most efficient in its presence. There is no definite correlation between ε_max and phosphorescence quantum yields with photopolymerisation rates, although it is significant that overall the phosphorescence quantum yields are enhanced to varying extents by the tertiary amine. End-of-pulse transient absorption spectra between 300 and 380 nm (λ_{max at} ∼ 340 nm) for the photo-fragmenting initiators in 2-propanol are tentatively assigned to the benzoyl radical, whereas benzophenone gives the typical ketyl radical formed via hydrogen atom abstraction at ∼ 500 nm. In the former case, transient formation is reduced by up to 50% in the presence of the tertiary amine except for benzoin and Irgacure 184. In the case of benzophenone, ketyl radical formation is enhanced significantly. All transients are strongly quenched by oxygen indicating that the triplet state is directly involved in free radical formation. Transient formation from benzil appeared to be the least affected by oxygen and this effect may well account for its high efficiency during polymerisation. Addition of the hydrogen donors benzhydrol and a secondary amine (diethylamine) enhances ketyl radical formation in the case of benzophenone in 2-propanol, confirming the mechanism of hydrogen atom abstraction. In the cases of benzil and benzoin, transient formation is not affected indicating that photofragmentation is the primary step in free radical formation. A steady-state photolysis study in ethyl acetate in the absence and presence of the hydrogen atom donors appears to support this conclusion. The application of these results in terms of current mechanisms is discussed.     

Sorption of traces of strontium ions on silica gel containing crown ether

This paper is concerned with the synthesis of silica gel sorbents by the sol method. A sorbent selective for trace amounts of Sr ions has been prepared. This method of synthesis is shown to be preferable to the tradition of impregnating the carrier.L. V. Pisarzhevskii Institute of Physical Chemistry, Ukrainian Academy of Sciences, Ukraine, 252039 Kiev, Science Prosp., 31. Translated from Teoreticheskiya i Éksperimental'naya Khimii, Vol.32, No. 4, pp. 258–260, July–August, 1996. Original article submitted October 17, 1995.     

Diffuse-reflectance laser flash photolysis of 16-(1-pyrene)-hexadecanoic acid adsorbed on silica

The absorption and fluorescence of 16-(1-pyrene)-hexadecanoicacid adsorbed on silica have been investigated. Time-resolved transient diffuse reflectance spectra were recorded following pulsed nanosecond laser excitation at 355 nm of pyrene, 1-methylpyrene and 16-(1-pyrene)-hexadecanoicacid adsorbed on silica. In addition to a rapidly decaying transient, absorbing at 420 nm assigned as the triplet state, and of the radical cation, absorbing at 460 nm, another long living transient species absorbing at 420 nm was observed for 16-(1-pyrene)-hexadecanoic acid. The decay is reversible but complete recovery takes several hours. Although no definitive assignment could be made for this transient several possibilities are discussed. The radical cations of the investigated molecules are formed by a biphotonic process. The non-exponential decay of the radical cations could be analyzed in the framework of a Gaussian distribution of free energy barriers.     

Photochemistry and photopolymerization activity of novel perester derivatives of fluorenone: a conventional and laser flash photolysis study

The photochemistry of three novel t-butylperester derivatives of fluorenone was examined and compared with unsubstituted fluorenone and a mono-t-butylperester of benzophenone using both conventional microsecond and nanosecond laser flash photolysis. On conventional microsecond flash photolysis in 2-propanol, all four fluorenone compounds gave transient absorption in the region 300–400 nm due to a ketyl radical formed from the abstraction of a hydrogen atom from the solvent by the upper excited triplet n—π* state of the fluorenone chromophore. This assignment was confirmed by a pH-dependent study on the transient absorption spectra. The nitro-t-butylperester derivative of fluorenone gave additional absorption above 400 nm due to species associated with the nitro group. No evidence for benzoyloxy radical formation could be found in non-hydrogen-atom-donating solvents with microsecond flash photolysis which is associated with homolysis of the perester groups. On nanosecond laser flash photolysis of the fluorenone compounds at 355 nm excitation in acetonitrile and hexa-fluorobenzene, transient absorptions were observed in the region 320–640 nm due to the corresponding triplet states. All the t-butylperester derivatives showed residual absorbances at longer time delays which were tentatively assigned to the corresponding benzoyloxy radicals produced by homolysis of the perester groups. In contrast, the mono-t-butylperester of benzophenone, included for comparison only, showed very weak transient absorption in the region 320–640 nm compared with that of the strong triplet of benzophenone under the same excitation conditions. The triplet absorptions and lifetimes of the fluorenone compounds were correlated with their photopolymerization activities in bulk methylmethacrylate monomer. In oxygenated solutions, the triplet absorptions of fluorenone and benzophenone were effectively quenched; however, long-lived transient growths were observed for all the t-butylperester derivatives. The intensities of these novel transient absorptions appear to correlate with the total number of t-butylperester groups in the fluorenone molecule and tentative assignments are discussed.     

Reaction of hydroxyl radical with aromatic hydrocarbons in nonaqueous solutions: A laser flash photolysis study in acetonitrile

Laser flash photolysis (LFP) of acetonitrile solutions of N-hydroxypyridin-2-thione in the presence of trans-stilbene generates a transient absorbance at 392 nm, attributed to the addition of hydroxyl radical to stilbene. The observed transient absorbance was used in competitive LFP experiments to determine relative rates of reaction for hydroxyl radical with a range of aromatic hydrocarbons in acetonitrile. Structure-reactivity relationships for the reaction of hydroxyl radical with arenes are derived. With these aromatic hydrocarbons, we observe a good correlation between the rates of hydroxyl-radical reaction and the ionization potential of the arene. Kinetic isotope effects are consistent with hydroxyl-radical addition being the dominant reaction pathway with the arene.     

Visible radiation effects on flavocytochrome b2 in dilute aqueous solution: a steady-state and laser flash photolysis study

Irradiation of flavocytochrome b2 by visible radiation at 450 nm in dilute aqueous solution is found to have a devastating effect not only on its activity but also on the important flavin mononucleotide (FMN) constituents. The active site and the substrate binding site are also found to be largely modified on exposure to visible radiation. This has a telling effect on the constituent aromatic amino acids, tryptophan and tyrosine, and therefore justifies the role of FMN as a very potent photosensitizer. Partial unfolding of the irradiated enzyme molecule is also observed. Damage is much greater in deaerated conditions, which indicates that molecular oxygen plays a protecting role in this particular system. The inactivation is mediated through rapid electron transfer from tryptophan and tyrosine to excited flavin, forming flavin semiquinone and tryptophanyl and tyrosinyl radicals, which in turn cause permanent damage at the molecular level.     

Ozonolysis of olfefins and acetylenes adsorbed on silica gel

Ozonation of phenyl ethylenes adsorbed on untreated silica gel results in aromatic aldehydes and on dried silica gel in ozonides and aldehydes. On the other hand ozonation of alkyl ethylenes on both types of silica gel results in a mixture of ozonides or polymeric peroxides similar to that obtained in aprotic solvents.     

Chiral separation of amino acids using a chiral crown ether by impregnation on a polymeric support and monoamine modified silica gel

A chiral monoaza-15-crown-5 ether derivative was prepared from l-Leucinol and used as a chiral stationary phase. The new chiral stationary phases CSP-1 and CSP-2 were employed in separating the enantiomers of the sodium and potassium salts of amino acids. The sodium and potassium salt of the d-enantiomers of all amino acids (PhyAlaNa, PhyAlaK and PhyGlyNa, PhyGlyK, and TrpNa, TrpK) show higher selectivity than the l-enantiomers for both CSP-1 and CSP-2.     

Etoposide的氧化还原反应: 脉冲辐解和激光光解研究

首次用脉冲辐解时间分辨方法研究了etoposide(VP16)在水溶液中与N~3^.,(SCN)~2^.^-和e~a~q^-之间发生的单电子氧化还原反应,测定了VP16的阴离子自由基、脱质子中性自由基的特征吸收谱;测得VP16与e~a~q^-,N~3^.,(SCN)~2^.^-的绝对反应速率常数分别为2.7×10^9,3.2×10^9和2.5×10^8dm^3.mol^-^1.s^-^1。研究表明,水溶液中的VP16可为248nm激光光电离,光电离的瞬态产物为阳离子自由基及脱质子中性自由基,并且测定了其酸碱电离的pK值。测得SO~4^.^-自由基单电子氧化VP16的反应速率常数为2.8×10^9dm^3.mol^-^1.s^-^1。     

Photochemistry of carboxylic acids containing the phenyl and thioether groups: Steady-state and laser flash photolysis studies

The mechanisms for the direct photolysis of phenylthioacetic acid (PTAA) and S-benzylthioglycolic acid (SBTGA) in acetonitrile were investigated using steady-state and laser flash photolysis. For both compounds, a variety of stable photoproducts were found under steady-state, 254 nm irradiation of acetonitrile solutions. The products from the direct photolysis of PTAA included carbon dioxide (photodecarboxylation), acetic acid, diphenyl disulfide, diphenyl sulfide, thiophenol, thioanisole, di(phenylthio) methane, and S-phenyl benzenethiosulfate. The products from the direct photolysis of SBTGA included carbon dioxide, toluene, dibenzyl, dibenzyl sulfide, dibenzyl disulfide, thioglycolic acid, benzyl mercaptan, benzyl alcohol, and benzaldehyde. These stable photoproducts were identified and characterized using HPLC, GC, GC–MS, and UV–vis methods. Quantum yields were determined for the formation of the various stable products following steady-state irradiations in the absence and in the presence of oxygen. In laser flash photolysis (266 nm Nd:YAG laser) experiments, a variety of transients (e.g., phenylthiyl radical, benzyl radical, etc.) was found. For both substrates (PTAA and SBTGA), photoinduced CS bond cleavage was the main primary process. It was also found that for both acids, photoinduced CC bond cleavage occurred, but as a minor process relative to CS bond cleavage. Detailed mechanisms of the primary and secondary processes are proposed and discussed. The validity of these proposed mechanisms was supported by an analysis of the quantum yields of stable products and their transient precursors. Supplementary observations on reactions between the radicals (e.g., C₆H₅–S, C₆H₅–CH₂) and oxygen are also consistent with the proposed mechanisms.     

Effect of aggregation on bacteriochlorin a triplet-state formation: a laser flash photolysis study

Bacteriochlorin a (BCA) is a potential photosensitizer for photodynamic therapy of cancer. It has been shown previously that the photoefficiency of the dye is mainly dependent on singlet oxygen (1O2) generation. Nanosecond laser flash photolysis was used to produce and to investigate the excited triplet state of the dye in methanol, phosphate buffer and dimiristoyl-L-alpha-phosphatidylcholine (DMPC) liposomes. The transients were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yield of singlet-triplet intercrossing. The lifetime of the BCA triplet state was measured at room temperature. The triplet-state quantum yield is quite high in methanol (0.7) and in DMPC (0.4) but only 0.095 in phosphate buffer. In the last case, BCA is in a monomer-dimer equilibrium, and the low value of the quantum yield observed was ascribed to the fact the triplet state is only formed by the monomers.     

EPR and fluorescence study of molecular dynamics of lipids in bilayers adsorbed on porous silica gel

Lipid bilayers immobilized on highly porous silica gel monoliths (SLBs) have been proposed as templates for chemosensors. Procedures for preparation and characterization of SLBs have been developed and applied. Rotational dynamics and orientational ordering at different depths in a SLB membrane and, for comparison, in bilayer liposomes have been studied for the first time using n-PC phospholipid spin labels.     

Laser flash photolysis evidence for styryl radical cation cyclization in the SET-induced photorearrangement of a p-methoxy-substituted 2-phenylallyl phosphite

The SET-induced photorearrangement of dimethyl 2-(4-methoxyphenyl)allyl phosphite, 9 (UV light, uranium glass filter, 9,10-dicyanoanthracene (DCA), biphenyl), gives phosphonate 12 in 83% isolated yield. Laser flash irradiation at 355 nm of oxygen saturated solutions of phosphite 9 containing DCA and biphenyl generates the transient UV spectrum of the biphenyl radical cation that is quenched by electron transfer from phosphite 9 (k(q) = 8.9 x 10(9) M(-1) s(-1) at 20 degrees C) to form the 4-methoxystyryl cation 10. The UV spectrum of 10 decays by a measured first-order rate constant of 8.0 x 10(6) s(-1), presumably to generate the cyclic distonic radical cation 11. Intermediate 10 was further characterized by measurement of the second-order rate constants for its reaction with azide, chloride, and bromide ions and with the neutral nucleophile trimethyl phosphite. This study provides the first spectroscopic evidence regarding the proposed mechanism (Schemes 1 and 2) for the SET-induced photorearrangements of dimethyl 2-arylallyl phosphites to the corresponding 2-arylallylphosphonates. Moreover, absolute rate constants for the intramolecular trapping of alkene radical cations have seldom been measured. The removal of the electron from the styryl moiety of phosphite 9, rather than from phosphorus, and the detectability of 10 arise from the stabilizing effect of the 4-methoxy substituent. These results, however, do not allow conclusions to be made concerning the site of removal of an electron in the SET-induced photorearrangement of dimethyl 2-phenylallyl phosphite 1 to phosphonate 6.     

Ultraviolet-visible spectroscopic study of ion-pairs of sodium and potassium monoethyl benzeneazophosphonates and their crown ether complexes in acetonitrile

The u.v.-vis spectroscopic study of ion-pairs of the sodium and potassium monoethyl ester of [α-(4-benzeneazoanilino)-N-benzyl] phosphoric acid, [α-(4-benzeneazoanilino)-N-4-hydroxybenzyl] phosphonic acid and [α-(4-benzeneazoanilino)-N-4-methoxybenzyl] phosphonic acid and their complexes with dibenzo-18-crown-6 and dibenzo-24-crown-8 is described. While the free sodium salts of monoethyl benzeneazophosphonic acids, with an extremely low solubility in acetonitrile, may be classified as tight ion-pair salts, the correspondingly more soluble potassium salts exist as solvated loose ion-pairs in acetonitrile solutions. The interaction of crown ether either with sodium or potassium monoethyl benzeneazophosphonates produces crown separated ion-pairs.     

Photosensitized reactions of oxime ethers: a steady-state and laser flash photolysis study

The mechanistic aspects of the photosensitized reactions of a series of oxime ethers were studied by steady-state (product studies) and laser flash photolysis methods. Nanosecond laser flash photolysis studies have shown that chloranil-sensitized reactions of the oxime ethers result in the formation of the corresponding radical cations. The radical cation species react with nucleophiles such as MeOH by clean second-order kinetics with rate constants of (0.7-1.4) x 10(6) M(-1) s(-1). Only a small steric effect is observed in these reactions, which is taken as an indication that the reaction center is not the O-alkyl moiety, but rather somewhere else in the molecule. Product studies in a polar nonnucleophilic solvent (MeCN) revealed that in order for the oxime ether radical cation to react more readily, alpha-protons must be available on the alkyl group. The O-methyl (1), O-ethyl (2), and O-benzyl (3) acetophenone oximes all reacted readily to give acetophenone oxime as the major product (as well as an aldehyde derived from the O-alkyl group), whereas O-tert-butyl acetophenone oxime (4) did not. The product formation can be explained by a mechanism that involves electron transfer followed by proton transfer (alpha to the oxygen) and subsequent beta-cleavage. When using 3 in MeOH, a change in the product formation is observed, the most important difference being the presence of benzyl alcohol rather than benzaldehyde as the major product. On the basis of the data from LFP and steady-state experiments, it is suggested that the competing mechanism under these conditions involves electron transfer, followed by a nucleophilic attack on the nitrogen, a MeOH-assisted [1,3]-proton transfer, and subsequent loss of benzyl alcohol. This mechanism is supported by DFT (B3LYP/6-31G) and AM1 calculations.