Photoexcitation of gold nanoparticles in their plasmon transition around 530 nm provides the means to carry high-energy reactions at room temperature. In the case of dicumyl peroxide (with activation energy of 34.3 kcal/mol) the reaction occurs in less than 1 min under 532 nm laser excitation. The results suggest that the peroxide is exposed to temperatures of ~500 °C for submicrosecond times, and provides a guide as to which type of organic reactions may benefit from plasmon-mediated energy delivery.

The swelling of microcrystalline cellulose by the use of polar protic solvents such as ethanol or methanol enables the penetration of benzophenone into submicroscopic pores of the natural polymer, while solvents such as benzene or dichloromethane do not open the polymer chains, thus not producing any entrapped benzophenone. Ground-state diffuse reflectance studies revealed a dramatic blue shift in the 350-nm absorption of benzophenone in the former case, in accordance with a strong interaction of the hydroxyl groups of cellulose with the ketone. Diffuse reflectance laser flash photolysis studies of benzophenone adsorbed on microcrystalline cellulose showed, in cases where benzophenone is entrapped in the polymer chain, the formation of a transient which decays nonexponentially and exhibits a maximum absorption at about 530 nm, assigned to triplet benzophenone. After ca. 25 ps, this transient generates another species with an absorption maximum at 545 nm. We assigned this new species to the diphenylketyl radical. In all cases where the solvent does not swell cellulose, a different behavior was observed typical for benzophenone microcrystals triplet decay. The ketyl radical formation is greatly reduced in this case. Triplet benzophenone decays by complex kinetics and lives about 10 ps when adsorbed onto microcrystalline cellulose, while the ketyl radical, when formed, lives 1 order of magnitude longer than the triplet. Samples which exhibit a high yield of ketyl radical formation also have a smaller phosphorescence emission in accordance with the fact that large amount of triplet molecules were consumed in the process of hydrogen abstraction from the matrix, involving hydrogens linked to carbons bearing a hydroxyl group.

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Surface plasmon excitation of aqueous colloidal gold nanoparticles with visible light in the presence of H2O2 led to rapid and selective oxidation of sec-phenethyl and benzyl alcohols to acetophenone and benzaldehyde, respectively. Laser drop, light emitting diode, and microwave irradiation have been used as energy sources. Interestingly, sec-phenethyl alcohol conversion was calculated to be 95% in 20 min when monochromatic 530 nm LEDs were used, being as good or better yield than the corresponding laser and microwave techniques. These results demonstrate the versatility of this inexpensive arrangement. Further attention was placed on the possible mechanism for Au nanoparticle plasmon-mediated alcohol oxidations in the presence of H2O2. We propose electron transfer with the nanoparticle surface, as well as the participation of peroxyl and ketyl free radicals as fundamental steps in the reaction pathway.

The ketone-photoinduced formation of Au, Ag, and Cu nanoparticles from their corresponding ions in solution has been carried out using benzoin photoinitiators. Ketones are good photosensitizers for nanoparticle synthesis not because of the energy they can absorb or deliver, but rather because of the reducing free radicals they can generate. Efficient photochemical nanoparticle generation thus requires a careful selection of substrates and experimental conditions such that free radical generation occurs with high quantum efficiency, where metal ion precursors do not inhibit radical formation. A key consideration to achieve nanoparticle synthesis with short exposure times is to minimize excited-state quenching by metal ions. Applications of nanostructures in catalysis require control of the nanoparticle characteristics, such as dimension, morphology, and surface properties. Part of this article describes the strategies to modify photochemically prepared particles. Finally, we illustrate some of the nanoparticle applications that interest us, with some emphasis on plasmon-mediated processes.

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Laser flash photolysis (LFP) studies, atoms in molecules (AIM) studies, and density functional theory (DFT) calculations have been performed in order to study the mechanism of the hydrogen abstraction by alpha-diketones in the presence of phenols. Laser irradiation of a degassed solution of 1,2-diketopyracene in acetonitrile resulted in the formation of a readily detectable transient with absorption at 610 nm, but with very low absorptivity. This transient decays with a lifetime of around 2 micros. The quenching rate constant for substituted phenols, kq, ranged from 1.10x10(8) L mol-1 s-1 (4-cyanophenol) to 3.87x10(9) L mol-1 s-1 (4-hydroxyphenol). The Hammett plot for the reaction of the triplet of 1,2-diketopyracene with phenols gave a reaction constant rho=-0.9. DFT calculations (UB3LYP/6-311++G**//UB3LYP/6-31G*) of the triplet complex ketone-phenol revealed that hydrogen transfer has predominantly occurred and that the reaction with alpha-diketones are generally 7 kcal/mol less endothermic than the respective reactions of the monoketones. These results together with the geometries obtained from the DFT calculations, natural bond order (NBO) analysis, and AIM results indicate that hydrogen abstraction for alpha-diketones is facilitated by the electrophilicity of the ketone, instead of neighboring group participation by the second carbonyl group.

. J. Chem. 68, 812 (1990).Diphenylmethyl radicals have been generated by 266 nm laser excitation of 1,1,3,3-tetraphenylacetone adsorbed on silica gel and included in NaX and Silicalite zeolites and have been studied using diffuse reflectance laser flash photolysis techniques. The spectrum for the radical shows A, , at -335 nm in all three supports and is similar to that in solution. The radicals decay over time scales that vary from hundreds of nanoseconds to minutes and there are indications that some radicals may be decaying on shorter time scales than we can monitor. The efficiency of oxygen quenching increases in going from Silicalite to NaX to silica gel, consistent with the greater accessibility of oxygen to silica gel pores as compared to the narrow channels in Silicalite. Laser dose and ketone loading effects were also examined for the various supports. Potential applications of a kinetic treatment of the data based on dispersive reaction kinetics are also discussed as a means of dealing with the problem of decay kinetics that occur over a wide range of time scales. On a produit des radicaux diphknylmtthyles en excitant, l'aide d'un laser optrant a 266 nm, de la 1,1,3,3-tktraphknylacttone adsorbte sur du gel de silice ou incluse dans des ztolithes de NaX et de Silicalite et on les a ttudits en faisant appel aux techniques de photolyse <> a l'aide de laser i rtflectance diffuse. Le spectre du radical prksente un A, , a environ 335 nm dans chacun des trois supports et il est semblable a celui observt en solution. Les radicaux se dtcomposent a des vitesses qui varient de quelques centikmes de nanosecondes a des minutes; certaines indications nous laissent croire que certains radicaux pourraient se dkcomposer a des vitesses plus rapides que celles que l'on peut mesurer. L'efficacitt du pikgeage par l'oxygkne augmente lorsqu'on passe de la Silicalite au NaX au gel de silice; cet ordre est en accord avec la plus grande disponibilitk de l'oxygkne sur les pores du gel de silice par comparaison avec les canaux Ctroits qui existent sur la Silicalite. Pour divers supports, on a aussi examink les effets de la dose du laser ainsi que de la quantitC de cttone utiliste. On discute aussi des applications potentielles d'un traitement cinttique des donntes bast sur la cinttique de rkactions dispersives cornrne moyen de rksoudre le problkme de la cinktique de dtcompositions qui se produisent a des vitesses trks diffkrentes.