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A novel synthesized nitroxide amide-BODIPY prefluorescent probe was used to study cellular redox balance that modulates nitroxide/hydroxylamine ratio in cultured human fibroblasts. FLIM quantitatively differentiated between nitroxide states of the cytoplasm-localized probe imaged by TIRF, monitoring nitroxide depletion by hydrogen peroxide; eluding incorrect interpretation if only fluorescence intensity is considered.

Gold nanoparticles (AuNP) supported on hydrotalcite, γ-Al2O3 and TiO2 P25 have been prepared photochemically under mild conditions using ketyl radicals as the primary reducing agent; these nanocomposites were prepared directly in the solid phase. Such dry, solventless methods are attractive from an environmental perspective. The composite materials were successfully characterized using diffuse reflectance UV–visible spectroscopy, SEM, TEM, X-ray diffraction spectroscopy (XRD) and X-ray photoelectron spectroscopy (XPS). The supported AuNP were predominantly spherical and ranged in size from 20–140 nm depending on the support and the percent Au loading, indicating the potential applications of these particles as photocatalysts. The catalytic activity of the supported AuNP was evaluated using the well-studied reduction of 4-nitrophenol. UV–visible spectroscopy was used to monitor the reaction and illustrated the ability of these easily prepared AuNP composites to act has heterogeneous catalysts. The role of percent Au loading and type of support on the catalytic activity of supported AuNP was also investigated. Composites with relatively large AuNP may hold promise as efficient catalysts in plasmon-mediated light-driven reactions.

The photobehavior of merocyanine 540 (MC) was studied in homogeneous media (ethanol, buffer and glycerol), and in microheterogenous systems (Triton X-100 micelles and in the presence of human serum albumin) using stationary and time-resolved techniques. Merocyanine 540 in aqueous solution mostly forms aggregates, which in the presence of Triton X-100 or HSA are disaggregated. The extent of binding to HSA and its characteristics were estimated from dye absorption and fluorescence changes following protein addition; the Trp-214 fluorescence quenching was also employed to assess the extent of dye association, and physical separation was employed to evaluate the dye's apparent association constant. These results showed that dye adsorption on HSA takes place at both main protein-binding sites (I and II). This adsorption leads to dye monomerization, changing its photobehavior remarkably, with a noticeable increase in fluorescence and triplet lifetimes. These slower decays can be ascribed to a reduction of the dye photoisomerization rate. In addition, the adsorption of the dye partially protects it from the oxygen present in solution, thus reducing the apparent dye triplet-quenching rate constant. However, singlet oxygen and MC triplet quantum yields remain very low in all the systems tested. Finally, we found that the photoconsumption of merocyanine bound to HSA takes place predominantly by a type I mechanism, being more than seven times more efficient than that taking place in ethanol.

Cucurbiturils (CBs) of the appropriate size (CB[7] and CB[8]) form strong guest-host complexes in phosphate buffer solution (PBS) with acridine orange (AO) and pyronine Y (PYY) with 1 : 1 and 2 : 1 stoichiometries for CB[7] and CB[8] complexes, respectively. Binding constants in the range 0.87-1.60 x 10(6) M(-1) and 5.2-6.3 x 10(13) M(-2) were determined by titration with fluorescence spectroscopy for 1 : 1 and 2 : 1 complexes, respectively. These binding constants in PBS and the eight-fold excess of CBs minimize the presence of free dye in solution and also stabilize the host-guest complex in the culture medium. Images showing that the CB complexes can cross the cell membrane of 3T3 cells have been acquired using fluorescence microscopy. Given the current importance of supramolecular CB complexes and the search for new drug delivery systems, the present findings open avenues for the use of CBs as nanocapsules to transport drugs into the cells.

2-Azaxanthone, a nitrogenated derivative of the well-studied organic chromophore xanthone, has been covalently bound through 2-(ethylthio)ethylamido linkers to the carboxylic acid groups of short, soluble single-walled carbon nanotubes (CNTs) of 450 nm average length, and the resulting azaxanthylium-functionalized CNTs (AZX-CNT, 8.5 wt % AZX content) characterized by solution (1)H NMR, Raman and IR spectroscopy and thermogravimetric analysis. Comparison of the quenching of the triplet excited state of AZX (steady-state and time-resolved) and of the transient optical spectra of CNTs and AZX-CNT shows that the covalent linkage boosts the interaction between the azaxanthylium moiety and the short CNT units. The triplet excited state of the azaxanthylium derivative is quenched by CNT with and without covalent bonding, but when it is covalently bonded, the singular transient spectrum is compatible with the photogeneration of electron holes through electron transfer from CNT to excited azaxanthylium units.

Pulsed laser irradiation of pyrene in thin films through lithographic masks leads to blue fluorescent images characteristic of pyrene excimer emission. The images are stable at room temperature and are readily observable using fluorescence microscopy. For a constant energy dose, the images are stronger and better resolved at the shorter laser wavelengths and cannot be recorded with continuous wavelength (CW) light sources. Images are easier to obtain with high Tg films, while polymers with Tg below room temperature image only when the laser also causes polymer cross-linking. It is proposed that laser excitation generates excited singlet pyrene and simultaneously causes a temperature change that facilitates diffusion and thus excimer formation. Following excited state (monomer and excimer) decay, the polymer matrix cools down and prevents the separation of the pyrene pairs, that are preserved by the rigid polymer matrix. Some of the images can be thermally erased by annealing above Tg. The method may find applications in imaging and information storage. © 2009 American Chemical Society.

Dioxo iron phthalocyanine tetrasulphonate (FeO2Pc) has been incorporated inside the intergallery space of a layered double oxide (LDH). Evidence for the inclusion of the FeO2Pc inside the intergallery spaces of LDH was obtained from the expansion of the basal layer distance of the host. The resulting material (0.10–0.13 mmol FeO2Pc × g−1) exhibits the optical spectrum characteristic of isolated metal phthalocyanine. Laser flash photolysis of the solid allows detecting the triplet excited state of the metal complex as a long-lived transient species whose decay is not complete in 2 ms. The solid is able to act as an effective photocatalyst for the degradation of aqueous phenol.

The effect of methylene blue (MB) encapsulation in cucurbit[7]uril (CB[7]) on triplet excited-state behavior and singlet oxygen (1O2) generation has been studied by using laser flash photolysis (LFP) and time-resolved near-IR luminescence spectroscopy. The lifetime of the triplet excited state of MB is longer in the CB[7] cavity (140 micros for MB-CB[7] vs 79.5 micros for aqueous MB). Cucurbituril also protects the dye triplets from quenching by oxygen, reducing the quenching rate constant [kq(O2)] from 2.6x10(9) M(-1) s(-1) to 0.2x10(9) M(-1) s(-1). The quantum yield of 1O2 production in the air-equilibrated D2O solutions is similar for free MB and for MB-CB[7], and the singlet oxygen lifetime is approximately 70 micros, suggesting its decay occurs in the aqueous (D2O) phase. The generation of singlet oxygen is delayed by CB[7]; this is attributed to the time required for oxygen to access the CB[7] nanocavity and react with the MB triplet. Thus, the rate-limiting step for sensitization is the entry of oxygen into the CB[7] cavity. Encapsulation inside CB[7] increases the relative efficiency of photoinduced MB2+* dication-radical generation, for which a modest yield is observed.

The photodegradation of nonsteroidal anti-inflammatory drugs (NSAIDs), a class of medications that includes aspirin and ibuprofen, has generated considerable interest since the 1990s, largely because of the phototoxic and photoallergic effects that frequently accompany their therapeutic use. Among NSAIDs, ketoprofen, which contains a benzophenone chromophore, has been extensively studied, reflecting both its notorious adverse effects and the fascination that photochemists have with benzophenone. The photochemistry of ketoprofen involves the intermediacy of an easily detectable carbanion with a remarkable lifetime of 200 ns in water; its life expectancy can in fact be extended to minutes under carefully controlled anhydrous conditions. Over the past decade, we have used some key properties of the ketoprofen carbanion to conduct mechanistic studies on carbanions under various conditions. In particular, its ease of photogeneration provides the temporal control required for kinetic studies, which, combined with its long lifetime and readily detectable visible absorption, have enabled extensive laser flash photolysis work. These studies have led to an intimate understanding of the reaction dynamics for carbanions in solution, including the determination of absolute rate constants for protonation, S(N)2, and elimination reactions. Together they provide excellent exemplars of reactivity patterns that today are part of all introductory curricula in organic chemistry and illustrate the fundamentals of nucleophilic substitution paradigms. More recently, we have begun to exploit the photochemistry of ketoprofenate and have developed the ketoprofenate photocage, a valuable tool for the photocontrolled cleavage of protecting groups and concomitant drug release. The photorelease has been illustrated with ibuprofen, among many other molecules. These photocages have been further improved with the use of the xanthone chromophore; the goal is the release of antiviral agents taking advantage of the improved UVA absorption of xanthone (xanthonate photocages). In this Account, we survey our work of the past few years on the photochemistry of ketoprofen and related chromophores. Beginning with studies on the phototoxicity of ketoprofen, we have made the journey to new prodrug candidates, unraveling mechanistic elements of aroyl-substituted benzyl carbanions along the way.

2,4,6-Triarylpyrylium units (TP+) have been attached covalently to the walls of short, water-soluble single-wall carbon nanotubes (sSWNT) to give TP-sSWNT sample (6.8 wt % on TP+). The high loading achieved in this manner has allowed characterization of the material by a wide range of techniques, including AFM (about 1 μm length), thermogravimetric analyses (absence of inorganic particles in the sample), Raman spectroscopy (radial breathing vibration mode at 180 cm-1), and solution 1H NMR spectroscopy (up to 0.5 ppm variation in the δ of the protons upon attachment to the graphene walls). Optical spectroscopy reveals the absence of the van Hove singularities in the TP-sSWNT sample. Fluorescence spectroscopy indicates sSWNT quenching of the otherwise intense TP+ emission. Laser flash photolysis has allowed detecting a transient species characterized by a broad absorption from 400 to 700 nm that decays with a fast (-3 quantum yield.

Different metallophthalocyanines and 2,4,6-triphenylpyrylium (TP +) ions were entrapped in different inorganic supports such as Y zeolite, mesoporous MCM-41, TiO2-SiO2 and SiO2 following a specific protocol. The resulting supramolecular structures were characterized by chemical analysis and diffuse reflectance UV-Vis measurements. The determination of the band gap on the basis of UV-Vis measurements showed that the host is not a spectator in this process and an electronic interaction occurs that lowers the band gap of the support. The XPS measurements were performed with an in situ X-ray photoelectron spectrometer, which can be operated at pressures of up to 1 mbar at the sample. They indicated that the formation of the supramolecular structure generates a stable environment, in which the oxidation state of the metal can still be influenced by reaction of the metal center with gas molecules. The photocatalytic tests carried out in photodecomposition of dipropyl sulfide showed a good correlation between the band gap values and the photocatalytic activity for metallophthalocyanine complexes. It is remarkable that the triphenylpyrylium ion follows the same trend observed for metallophthalocyanines, although it is a different type of photosensitizer.

Treatment of CdSe quantum dots with a solution of elemental sulfur, followed by UVA or visible photoactivation, leads to as much as a 500% luminescence enhancement and stabilization as a result of the modification of surface sites by short sulfur oligomers. These modified quantum dots show excellent response to solution quenchers and are promising materials for the development of quantum dot-based sensors.

— The effect of an alternating current (AC) magnetic field (MF) on radical behavior is identical to that exerted by a direct current (DC) field of the same instantaneous strength provided that the frequency is low enough in comparison with radical pair dynamics. This criterion is easily met by environmental fields. In general, combined AC/DC fields will lead to increased radical concentrations and oscillating free radical concentrations. Interestingly, the frequency of oscillation for radical concentration seldom follows exactly the pattern of the external AC component of the MF. Even the simple case of an AC-only field at 60 Hz can lead to oscillations in radical concentrations at 120 Hz. The concentration time dependence patterns can be even more complex when the singlet and triplet levels of the radical pair are not degenerate. Further, the effects can change dramatically depending upon the absolute and relative values of the AC and DC components, thus providing a possible explanation for MF windows for certain effects reported experimentally. Effects on the average radical concentration are probably relevant only for fields higher than about 0.1 gauss. Oscillating radical concentrations may influence signal transduction processes or other cellular mechanisms; at the present time there is not enough knowledge available to establish a reasonable threshold for these types of effects. This contribution aims at providing a solid foundation to guide the experimentalist in the design of meaningful experiments on the possible role of MF perturbations of radical chemistry on biological response.

Laser photolysis of benzoyl radical precursors (such as PhCOC[OH]R2) in oxygen-containing solutions leads to a laser-specific cascade of oxidative events in which benzoyl peroxyl radicals undergo self-reaction, ultimately leading to the sequential formation of benzoyloxyl, phenyl and phenyl peroxyl radicals. Thus, photoreactions producing benzoyl (PhCO) radicals lead to PhOO under conditions where benzoyl radicals do not decarbonylate. These reactions require the high-intensity conditions that can only be achieved under pulsed laser excitation; yet, they do not involve any multiphoton processes. Type I mechanisms of this type may be of importance as new laser-based therapeutic technologies are developed.

The direct irradiation of trimethylolpropane propoxylate triacrylate (PPO-TMPTA) – a good chromophore at 193 nm – in thin films of polymethylsilsesquioxanes (PMSSQs), leads to efficient photobleaching at that wavelength with high contrast. The photobleaching is due to loss of the acrylate vinyl C C bond, as confirmed by FTIR and NMR. Other chromophores, such as non-conjugated dienes and dialkyldiazenes, were evaluated and the photobleaching was found to be less efficient compared to acrylates. We analyzed the effect of the film composition (matrix, acrylate, photoinitiator and plasticizers) as well as the processing atmosphere on the final contrast and speed of the photobleaching. The PMSSQ matrix was superior to polyacrylates and the best results were obtained with acrylates containing long flexible arms such as PPO-TMPTA. The addition of radical photoinitiators increases slightly the photobleaching speed, but not as much as addition of plasticizers. The presence of oxygen slows down the process without stopping it. A mechanism is proposed in which the acrylate triplet state can add or couple to other acrylates, generating 1,4-diradicals. Alternatively photoionization can also trigger a cascade of events leading to C C loss.

Two new sensors for the detection of reactive oxygen species have been synthesized and characterized; they contain the 4-amino-7-nitrobenzofurazan (ABF) moiety covalently tethered to a phenol. Comparison of sensors ABFhd and dABFhd (the latter containing two ortho-methyl groups) shows that introduction of steric bulk leads to an improvement of fluorescent sensor performance, thus confirming our previous predictions based on computational chemistry. ABFhd and dABFhd are new tools for biological applications involving reactive oxygen species, in particular oxygen-centered radicals.

Dibenzoylmethane and avobenzone photochemistry involves the formation of transient enol isomers (Z and E). Conjugation of the OH group with the carbonyl group in these transient isomers reduces the OH bond energy. A fast reduction of the pre-fluorescent probe (C343T) was observed after addition to photolysed DBM samples in nonpolar solvents. This can be attributed to a hydrogen transfer reaction from the transient E accumulated. While no quenching for Z by TEMPO was detected in the laser flash photolysis timescale. Theoretical calculations of spin densities distribution of the radical formed from Z and E showed a more delocalized distribution that would indicate a low reactivity towards oxygen. Our results suggest that DBM and avobenzone can effectively behave as photoantioxidants or photoactivated antioxidants under conditions where its enol isomers can be accumulated.

We report a novel method of identifying damage to DNA leading to the loss of intercalation sites. Thiazole orange (TO), an intercalating cyanine dye, fluoresces strongly when intercalated in DNA, but not free in solution. Upon UVC-induced damage to DNA, the change in TO fluorescence is greater than the change in any of the other spectral or biochemical indicators (absorbance, circular dichroism and agarose gel electrophoresis), thus providing a fast screening method to identify damage to DNA. The method is geared toward high levels of damage, such as those that may result during radiation treatment of food products.

The quenching of core and core-shell CdSe quantum dots by TEMPO and 4-amino-TEMPO has been examined using steady state fluorescence spectroscopy. The efficiency of quenching is strongly dependent on the nanoparticle size, the binding properties of the nitroxide, and the presence or not of a protective shell, ZnS in our systems. The shell reduces the quenching efficiency significantly only in the case of binding nitroxides, such as 4-amino-TEMPO. Downward quenching plots revealing bimodal quenching characterize the Stern-Volmer plots obtained for 4-amino-TEMPO. The slope characteristic of the low concentrations regime depends strongly on the presence of a shell. For example, for particles with a 2.4 nm core, emitting at 525 nm the concentrations of 4-amino-TEMPO required to reduce the emission to one half are 0.65 microM and 0.08 mM for core and core-shell nanoparticles, respectively. Surprisingly, in the high concentration regime, a single Stern-Volmer slope of about 4000 M-1 seems to accommodate all systems. We speculate that this value is characteristic of the exchange of TOPO ligands by 4-amino-TEMPO.

Laser excitation of aqueous solutions of fenofibric acid (FA) at pH 7.4 show the formation of two reaction intermediates, the triplet state and the hydrated electron. The former is longer lived in water than in acetonitrile; its anionic form decays irreversibly by decarboxylation to give a carbanion that protonates before or after rearrangement. Several spectroscopic and quenching studies suggest that in aqueous media the triplet state of FA has a π,π* character, in comparison with an n,π* character in organic media. Further, the known chemistry of the triplet, including decarboxylation and hydrogen abstraction, occurs predominantly from the n,π* state, and as a consequence, activation energies are higher when the lowest triplet has a π,π* character. Photoionization is more important in aqueous than in organic media and involves a biphotonic process. Hydrated electrons are trapped by FA, leading to the corresponding ketyl radical after protonation.

Fluorescein (2-(6-hydroxy-3-oxo-(3H)-xanthen-9-yl)benzoic acid) has been prepared inside the pores of zeolite-Y via ship-in-a-bottle synthesis. Fluorescein, whose dimensions prevent it from entering through the ∼7 A windows of the faujasite zeolite used, was prepared by the acid-catalyzed reaction of resorcinol and phthalic anhydride. In this article we report initial spectroscopic data as well as an example of the usefulness of these fluorescence-labeled nanoparticles for imaging applications such as confocal fluorescence microscopy. Encapsulated fluorescein shows a remarkable increase in photostability.

Methyl triazone (4,4′,4″-[1,3,5-triazine-2,4,6-triyltriimino]tris-trimethyl benzoate) has been included in mesoporous MCM-41 (Mobil's composition of matter-41) silica, and its fluorescence emission has been compared in solution and in the solid state. Although inclusion does not affect significantly the absorption properties, a fluorescence emission shift and a behavior similar to the solid state has been observed by increasing the loading. It is believed that this observation reflects molecular aggregation inside the MCM-41 channels. The potential of formulations of this type for sunscreen applications is discussed.

The phototoxic and photoallergic effects of the once popular UV sunscreen p-aminobanzoic acid are related, in part, to its ability to sensitize the formation of singlet oxygen as well as other reactive oxygen species. In this work we demonstrate that the sunscreen-photoinduced inactivation of a model protein, horseradish peroxidase, is reduced by approximately a factor of three when the sunscreen is encaspsulated in zeolite sodium Y. These results provide evidence that using the technology of zeolite encapsulation to prepare a supramolecular sunscreen that minimizes the skin contact of active ingredients may reduce the adverse effects of "naked" sunscreens on biological systems. These radiation-induced effects, unfortunately, frequently accompany the desirable UV-screening role of these products. These results provide an important benchmark for the use of zeolite encapsulation as a means of improving the safety of UV sunscreens for topical application.

The micellar photoreduction of benzophenone in the presence of Ag+ leads to very rapid and efficient formation of silver nanoperticles. External magnetic fields can be used to control the rate of formation and properties of silver nanoparticles generated by reaction of ketyl radicals formed in the photoreduction of benzophenone in surfactant micelles. The effect is attributed to Zeeman splitting of the triplet sublevels of the confined radical pair that causes a reduction in the rate of geminate processes and increases the availability of ketyl radicals (through escape processes) for metal ion reduction.

Exposure of thin polymer films to laser excitation at 157 nm leads to surface ablation that in the case of fluorinated polymers is accompanied by the photogeneration of HF, a possible problem in lithographic and imaging applications of these compounds. Fluorescence spectroscopy, fluorescence microscopy and atomic force microscopy have been used to examine the polymer films following laser exposure and to compare fluorinated and nonfluorinated films.

Texto completo: acesso restrito. p.620–624 Submitted by Edileide Reis (leyde-landy@hotmail.com) on 2013-08-28T15:34:56Z No. of bitstreams: 1 ef050267p.pdf: 167516 bytes, checksum: 79320d484955c126069761ce99be4190 (MD5) Made available in DSpace on 2013-08-28T15:34:56Z (GMT). No. of bitstreams: 1 ef050267p.pdf: 167516 bytes, checksum: 79320d484955c126069761ce99be4190 (MD5) Previous issue date: 2006 Pipelines for heavy crude oils have recurrent clogging situations due to paraffin deposition that may lower production rates and be responsible for leakages. This work evaluates the effect of three polymers on crude oil wettability and on paraffin deposition inhibition: polypropylene (PP), high-density polyethylene (HDPE), and a vinyl acetate copolymer with 28% oxygen content (EVA28). Under static conditions, the interfacial tension between the crude oil and the linings was determined by the contact angle being 12% and 17% higher for EVA28 and HDPE, respectively, than for that of PP. As the crude oil inside the pipelines is flowing at high rates, the dynamic interfacial tension was also observed for high flow rates when molecular effects overtake hydrodynamic ones. For this, the perturbation of intermolecular orientation within the flow, caused by the beginning of deposit formation and by the wettability, was determined using depolarization of the fluorescence induced by laser. Under flow, EVA28 and HDPE caused an increase of interaction with the crude oil of 79% and 43%, respectively, compared to that of PP. HDPE yielded globular wax deposits. The higher tendency of HDPE than PP to form deposits was attributed to the absence of methyl branches in the first. The EVA28 tendency to form deposits was attributed to the oxygen atoms on the surface as well as to its high polycyclic aromatic hydrocarbon sorption. Thus, under a high flow rate, PP is better suited to inhibit wax deposition than HDPE and EVA28.

[reaction: see text] Irradiation of 2- and 4-xanthone acetic acid in aqueous buffer (pH 7.4) leads to efficient (Phi = 0.67 and 0.64, respectively) photodecarboxylation to give the corresponding methyl products, consistent with an intermediate benzylic carbanion. Fluorescence and laser flash photolysis (LFP) studies suggest singlet state reactivity, which is unusual for aromatic ketones. 3-Xanthone acetic acid is photoinert under the same conditions. The results suggest that the reactive xanthone acetic acids are promising precursors for carbanion-mediated photocages.

The potential performance of Photonic Crystal Fibers (PCF) in the construction of microdevices, particularly sensors, has not been fully explored. One of the principal requirements for new PCF applications is the incorporation of additional functionality. We have explored the possibility of filling the PCF microchannels with liquids containing fluorescent probes such as coumarin 6 (C6) or nanocrystal semiconductors (CdSe quantum dots). Fluorescent probes can undergo significant changes in their emission properties in response to pH variations, or in the presence of other materials, such as paramagnetic species. In the case of quantum dots incorporated by capillarity in PCF channels an interesting size dependence of their luminescent properties on the presence of nitroxide radicals is reported. TEMPO (2,2,6,6-tetramethylpiperidine-N-oxide free radical) quenches the fluorescence of green quantum dots (2.4 nm) followed by spontaneous emission-recovery within minutes. However, for red quantum dots (6.7 nm) longer times are necessary to observe partial fluorescence recovery. This behavior agrees with a strong non-linearity of the quenching observed for the smaller quantum dots in solution compared to a closer-to-linear dependence for the bigger nanocrystals. In the case of 4-amino-TEMPO quenching occurs at much lower concentrations and fluorescence recovery is minimal. The mechanisms involved are discussed.

The photophysical properties of an intercalating unsymmetrical monomethine cyanine dye and single-stranded DNA homopolymers show strong association for poly(dA) and poly(dG), but not for poly(dC) and poly(dT), as determined by several spectroscopic techniques and molecular dynamics calculations. While poly(dA) and poly(dG) appear to bind the dye as a monomer (with dramatic increase in fluorescence), poly(dC) and poly(dT) bind only very weakly, and seem to promote dye aggregation. Only in the case of poly(dA) there seems to be a unique, well defined form of intercalation, that molecular dynamics calculations suggest involve the quinoline ring between two bases, in an arrangement that should favor pi-stacking; consistently with this, the decay of the fluorescence shows a single exponential, the absorption spectrum shows a shift in the dye maximum, the fluorescence is strong, and the induced circular dichroism follows a simple pattern.

[reaction: see text] A new synthetic methodology is reported that takes advantage of the persistent free radical effect (PFRE), where clean products can be obtained in good yields from radical cross-combination reactions, despite their reputation for being of little synthetic value and for resulting in complex mixtures; these problems can be avoided when the PFRE is used as a synthetic tool.

Acid-catalyzed deprotection of tert-butoxycarbonyl (t-Boc) pendant groups present in a polymer film is one of the most common chemical reactions in photolithography. We present here a novel method to determine the catalytic chain length of this reaction under different development conditions. We demonstrate this in model PMMA thin films containing triphenylsulphonium triflate as a photoacid generator, acid sensor coumarin 6, t-Boc protected coumarin 4, and base 2-piperidin-1-yl-ethanol (2-PE). Deprotection of the t-Boc group, catalyzed by the photogenerated acid, during the post-exposure bake leads to formation of coumarin 4, a strongly fluorescent molecule whose concentration is monitored using fluorescence. We take advantage of the change in the emission spectra of coumarin 6 upon protonation to monitor acid formation. To quantify the amount of acid, an acid–base titration is done in the polymer film using 2-PE. Knowing the amount of deprotected probe and the amount of acid, we are able to calculate the number of moles of deprotected C4-t-Boc per mol of acid, which is the actual catalytic chain length.Key words: photolithography, chemical amplification, catalytic chain length, fluorescent sensors, photoacid generator.

Fluorescence sensors have been developed to monitor radical reactions, DNA damage, acid-catalyzed processes, detection of electron-deficient molecules, and zeolite applications. The examples presented emphasize applications in polymer and supramolecular systems.

The photochemistry of 1-(4-tert-butylphenyl)-tetrahydro-thiopyranium triflate (1), an arylcycloalkylsulfonium salt, was investigated in acetonitrile and methanol at low conversion in order to understand the reaction mechanism and its efficiency as photoacid generator. Both types of C-S bond in 1 are cleaved from the excited state. The heterolytic cleavage of the methylene C-S bond produces 4-t-BuC(6)H(4)S(CH(2))(4)CH(2)(+) by ring opening. The carbocation generates acid and arylalkenylsufides by elimination or 1,2 hydride shift and elimination. The predominantly homolytic cleavage of the aryl C-S bond yields 4-t-BuC(6)H(4)* and c-C(5)H(10)S(+)* as the fragmentation products. The radicals react with the solvent forming acid, pentamethylene sulfide and tert-butylbenzene. In methanol, the formation of 4-tert-butylanisole indicates a contribution of solvolysis in the excited state of 1 or a competing formation of free aryl cation by heterolytic fragmentation. The acid generation efficiency of 1 in solution (acetonitrile or methanol) is lower than that corresponding to triphenylsulfonium triflate (TPS OTf) under the same conditions. This suggests a pathway for the regeneration of 1 after photocleavage. The photochemistry of 1 is discussed in terms of the contribution of fragmentation and ring opening reaction paths to its overall acid generation efficiency, a key property in terms of its applications in resist formulations.

Zeolite Y containing Ru(bpy)32+ and TP+ has been obtained by the stepwise ship-in-a-bottle synthesis of the two components. The identity of the two ions has been demonstrated by diffuse reflectance...

Fluorescent images that illustrate acid-catalyzed tert-butoxycarbonyl (tBoc) deprotection patterns in polymer films were obtained using fluorescent sensors based on 7-hydroxycoumarin dyes. Three commercial 7-hydroxycoumarins, which are highly fluorescent, become practically nonemissive upon protection of the 7-hydroxyl position with tBoc. In thin polymer films, the protected prefluorescent probes can return to their deprotected, fluorescent states by reaction with catalytic amounts of photogenerated acid and mild heating.

Prefluorescent probe dansyl-TEMPO was synthesized within NaY supercages by means of a base-catalyzed ship-in-a-bottle synthesis to yield the supramolecular assembly DT@NaY, providing the first example of base-catalyzed synthesis in the normally acidic zeolite cavities. This material was successfully employed in the solid state as a sensor to monitor intercavity diffusion of carbon-centered radicals produced by AIBN thermolysis and in a slurry to study the interpore diffusion of carbon-centered radicals produced in solution from a suitable precursor (3,3‘-diphenyl-3H,3‘H-[3,3‘]bibenzofuranyl-2,2‘-dione, 6). The dynamics of hydrogen atom transfer from a good donor, such as the synthetic lactone antioxidant 3-phenyl-2-coumaranone, to DT@NaY were also investigated in benzene slurry.

A series of porous titanosilicates (Ti-preferrierite, Ti-ferrierite, Ti/ITQ-6, Ti/ITQ-2, and silylated Ti/MCM-41), all of them active as alkene epoxidation catalysts, have been submitted to diffuse reflectance laser flash photolysis. The transient spectra in the microsecond time domain exhibit characteristic absorption bands at 350 nm (sharp) and 450−800 nm (featureless) attributable to triplet ligand-to-metal charge-transfer state and ejected electrons, respectively. It is proposed that the relative intensity of these two bands is a reflection of the presence of isolated, tetrahedral Ti atoms versus Ti aggregates, the former being considered the most active titanium sites in olefin epoxidation. Our study shows the applicability of laser flash photolysis to the characterization of titanium atoms in titanosilicates.

[reaction: see text]. Rate constants for hydrogen abstraction from phenols by a prefluorescent-TEMPO probe are reported. The nitroxide is employed as a potential model of peroxyl radicals. The probe works by nitroxide suppression of the fluorescence of the chromophore. The fluorescence is restored when the nitroxide abstracts a hydrogen atom to produce the diamagnetic hydroxylamine. The phenols studied in this project exhibited rate constants between 0.003 and 0.2 M(-1) s(-1). A deuterium isotope effect of 10 for TROLOX confirms that the mechanism is dominated by hydrogen transfer.

The photostability of the widely used UVB sunscreen agents 2′-ethylhexyl-2-cyano-3-phenylcinnamate (1), 2-hydroxy-4-methoxybenzophenone (2), octyl salicilate (3), and 2′-ethylhexyl-4-methoxycinnamate (4) has been investigated under UVA irradiation in the absence and presence of TiO2, an inorganic filter commonly employed in combination with organic filters in sunscreen preparations. In the absence of TiO2, 1–3 are photostable and 4 undergoes the expected E–Z isomerization; the presence of TiO2 caused mineralization of the organic filters and, surprisingly, the process is noticeably faster in the presence of surfactant than in sunscreen and water suspensions. The results indicate that in water suspension, mineralization is likely to occur on or near the TiO2 particle surface; when the organic sunscreens are segregated in the micelle core, reactive radicals, produced during TiO2-promoted degradation of the micellar system, may participate in sunscreen degradation. In addition, a pre-fluorescent probe for carbon-centered radical detection, 4-(3-hydroxy-2-methyl-4-quinolineoxy)-2,2,6,6,-tetramethylpiperidine-1-oxyl free radical or QT (5), was employed to demonstrate that carbon-centered radicals are evolved during micelle degradation and may participate in the mineralization of sunscreens.

The photochemical and photophysical properties of 4-(3-hydroxy-2-methyl-4-quinolinoyloxy)- 2,2,6,6-tetramethylpiperidine-1-oxyl free radical (QT) have been studied as a prefluorescent probe to monitor free radical processes in polymer films. This methodology takes advantage of the efficient intramolecular quenching of the fluorescence of quinoline by the paramagnetic nitroxide, which is disabled when TEMPO reacts with carbon-centered radicals. The fluorescence intensity-time profile observed in the thermal decomposition of 2,2¢-azobis(isobutyronitrile) (AIBN) in poly(methyl methacrylate) (PMMA) films showed initial increments in the fluorescence with time, according to the trapping of carbon-centered radicals by QT in the polymer films. Comparison of data under nitrogen and oxygen saturation conditions suggests that oxygen trapping of the carbon-centered radicals at 90 °C is about 20 times faster than reaction with nitroxides. The activation energy for AIBN decomposition in PMMA was measured as 34.1 kcal/mol. Analysis of the fluorescence lifetime distribution establishes the involvement of both static and dynamic fluorescence quenching of the diamagnetic reaction product by AIBN, The financial support by the Natural Sciences and Engineering Research Council of Canada and postdoctoral grants from Fundación Andes, Chile (A.A.), and Comunidad Autónoma de Madrid of Spain (O.G.) are gratefully acknowledged.

Resonance-stabilized radicals containing indane, indene, and fluorenyl moieties exhibit attenuated reactivity toward oxygen. Rate constants of approximately 10(5) M(-1) s(-1) were observed for the most stabilized radicals. The dependence of k(OX) (rate constant for radical trapping by oxygen) on the corresponding bond dissociation energies revealed that stereoelectronic effects are more important than steric effects in determining the low radical reactivity with oxygen. Scavenging by the nitroxide TEMPO was also examined, and revealed that in this case steric effects are more important than in the case of oxygen. The rate constants for the hydrogen abstraction by cumyloxyl and tert-butoxyl radicals generated thermally and photochemically have been determined in benzene, and were in the range of ca. (1-13) x 10(6) M(-1) s(-1), showing that benzylic stabilization has a modest effect on substrate reactivity as a hydrogen donor toward alkoxyl radicals.

Direct irradiation of fenvalerate (FV) in aqueous solution (4 x 10(-3) M) gives rise to 2,3-diarylisohexanitrile through the homolytic CO-CH(i-Pr) bond cleavage, followed by a rapid decarboxylation of the primary carbonyloxy radical, and radical recombination. The unusual exclusive formation of the cross combination product suggests that the persistent free radical effect is operative in this system and controls product formation. Spectroscopic evidence of the intermediacy of benzylic radicals decaying in microsecond time scale has been obtained by laser flash photolysis. These studies also reveal a remarkable attenuation of radical reactivity toward oxygen resulting from cyano substitution at the radical center. TPY photocatalyzed degradation of FV also leads to decarboxylation products accompanied by oxidative mineralization (38%). The TPY-photocatalyzed degradation can occur through two operating mechanisms involving the generation of OH(*) radicals and/or FV(*+) radical cations. Evidence for the latter intermediate has been obtained by laser flash photolysis in acetonitrile solution (detection of TP(*) radical) and by the quenching by FV of the emission from the TPY solid.

Cyclobis-(N,N‘-paraquat-p-phenylene) macrocycle (24+), the electron acceptor ring present in many [2]-catenanes and rotaxanes, has been obtained encapsulated within the supercages of zeolite Y by ship-in-a-bottle synthesis from the open precursor (12+) and 1,4-bis(bromomethyl)benzene. Upon 266 nm laser excitation, the corresponding 1•+ and 2•(3+) radical cations living hundreds of μs are generated within the zeolite cages in line with the photochemical behavior reported for the parent methyl viologen included within zeolites. Addition of 1,4-dimethoxybenzene (DMB) to 12+@Y and 24+@Y leads to the formation of the corresponding charge transfer (CT) complexes characterized by a broad CT band at λmax 475 nm. Laser flash photolysis at 266 nm of 12+@Y and 24+@Y containing DMB shows a dramatic reduction of the signal of the 1•+ and 2•(3+) radical cations due to the electron-transfer quenching of 12+ and 24+ excited states by DMB. In contrast, 532 nm selective excitation of the 12+-DMB or 24+-DMB CT complexes all...

It is well established that α-tocopherol, TocH, is an outstanding lipid-soluble, peroxyl radical trapping antioxidant in homogeneous systems. It is also well established that TocH functions as a prooxidant in human low-density lipoprotein, LDL, subjected to attack by peroxyl radicals generated in the aqueous phase by, for example, thermal decomposition of the azo compound, ABAP. This tocopherol-mediated peroxidation, TMP, of LDL involves a three-step chain reaction, one step being hydrogen atom abstraction from the LDL lipids by the tocopheroxyl radical, Toc•. The occurrence of TMP has been attributed to three factors, (i) translocation by TocH of radical character from the aqueous phase into LDL lipid, (ii) isolation of the water-insoluble Toc• in the LDL particle in which it is formed for times sufficient to permit it to react with the lipid, and (iii) the small lipid volume of LDL which ensures that no particle can contain more than a single radical for a significant length of time. This consensus view...

Samples of zeolite Y containing 2,4,6-triphenylpyrylium or tris(bipyridyl)ruthenium(II) have been prepared and their photophysical and photochemical properties compared to those of the same samples containing encapsulated nanoscopic TiO2 clusters as relays. The emission of encapsulated Ru(bpy)32+ and TP+ photolumophores undergoes static quenching in the presence of codoped TiO2 clusters. This is the expected behavior for systems, where photosensitizer and relay lack diffusional mobility. Time-resolved diffuse reflectance laser flash photolysis shows the transient generation of TP• pyranyl radical and Ru(bpy)33+, upon irradiation of TP+ and Ru(bpy)32+ in the presence of TiO2, respectively. This provides firm support for the occurrence of photoinduced electron transfer between excited TP+ as acceptor and TiO2 clusters as donors and from Ru(bpy)32+ to the conduction band of TiO2. Using the photoinactivation of horseradish peroxidase as a test reaction in aqueous medium, a synergism has been found with respec...

A series of Y, Beta, mordenite and ZSM-5 zeolites containing different amounts of nanosized TiO2 clusters have been prepared by TiO2+ exchange followed by condensation. The presence of TiO2+ and its subsequent oligomerization can be followed in Raman spectroscopy by the appearance of the band at ca. 530 cm−1 and its replacement by others at 394, 460 and 637 nm. X-ray diffractograms of the solids show a considerable intensity decrease as the amount of TiO2 present increases. High-resolution transmission electron microscopy (HRTEM) shows that no isolated TiO2 particles are present in the case of zeolite Y, while the sample of mordenite at the highest Ti loading contains isolated TiO2 particles distinguishable by HRTEM. Diffuse reflectance UV-Vis spectroscopy indicates that the bandgap and the apparent extinction coefficient depend remarkably on the Ti content as well as on the zeolite crystal structure. Quantum chemical calculations at the semiempirical level also predict large variations in the HOMO–LUMO energies as a function of the number of Ti atoms. It has been found that the photoactivity of TiO2@Y and TiO2@mordenite is higher than that of commercial anatase for the photooxygenation of thianthrene to thianthrene oxide (λex>200 nm), but smaller for the hoseradish peroxidase photodeactivation in aqueous buffer (λex=350 nm). These variations in the photocatalytic activity of TiO2 illustrate the potential that inclusion of TiO2 nanoclusters within zeolites has to modulate its photoactivity.

Dibenzotropylium ion (DT+) has been generated as an indefinitely persistent species within the channels of ZSM-5 in its H+-form. Diffuse reflectance laser flash photolysis has allowed detection of a transient (two bands: 300 nm, sharp and ∼440 nm broad) decaying in the μs time-scale that has been assigned to the corresponding triplet excited state. The tight fit of DT+ within the straight channels ( 5.2×5.7 A 2 ) of ZSM-5 and the presence of coadsorbed water explain why the DT+ triplet excited state is not quenched by oxygen but interacts with triethylamine, which is highly water-soluble. In the latter case, formation of a new transient compatible with DT (λmax=270 and 350 nm) through electron transfer from the amine to DT+ triplet is observed.

A novel approach employing a prefluorescent paramagnetic sensor has been employed to determine the absolute kinetics for end-cap cleavage in TEMPO-capped polystyrene obtained by “living” free radical polymerization. This new methodology takes advantage of the suppression of coumarin fluorescence when this chromophore is tethered to a paramagnetic nitroxide. This coumarin−nitroxide is an excellent free radical trap for carbon-centered radicals; upon radical trapping, the resulting diamagnetic alkoxyamine is strongly fluorescent. Thus, fluorescence buildup is a direct measure of free radical formation and can be employed to quantify their formation or to study their kinetics. Studies of the temperature dependence of the process can be employed to determine activation parameters and bond dissociation energies. This simple technique can be employed to study the dynamics in the actual polymer systems, overcoming the frequent need to resort to model compounds.