Photosensitized electron-transfer reactions and hydrogen evolution in organized microheterogeneous environments: separation of ground-state xanthene-bipyridinium complexes by silica colloids

Enhanced photoinduced electron transfer at the surface of charged lipid bilayers

Photocatalytic systems often suffer from poor quantum yields due to fast charge recombination: The energy-wasting annihilation of the photochemically created charge-separated state. In this report, we show that the efficiency of photoinduced electron transfer from a sacrificial electron donor to positively charged methyl viologen, or to negatively charged 5,5'-dithiobis(2-nitrobenzoate), increases dramatically upon addition of charged phospholipid vesicles if the charge of the lipids is of the same sign as that of the electron acceptor. Centrifugation and UV/Vis titration experiments showed that the charged photosensitizers adsorb at the liposome surface, that is, where the photocatalytic reaction takes place. The increased photoelectron transfer efficiency in the presence of charged liposomes has been ascribed to preferential electrostatic interactions between the photosensitizer and the membrane, which prevents the formation of photosensitizer-electron-acceptor complexes that are inactive towards photoreduction. Furthermore, it is shown that the addition of liposomes results in a decrease in photoproduct inhibition, which is caused by repulsion of the reduced electron acceptor by the photocatalytic site. Thus, liposomes can be used as a support to perform efficient photocatalysis; the charged photoproducts are pushed away from the liposomes and represent "soluble electrons" that can be physically separated from the place where they were generated.

Analysis of association constant for ground state dye-electron acceptor complex of photoinitiator systems and the association constant effect on the kinetics of visible-light-induced polymerizations

We investigated the formation of ground state donor/acceptor complexes between xanthene dyes (rose Bengal (RB) and fluorescein (FL)) and a diphenyl iodonium salt (DPI) which is dissolved in 2-hydroxyethyl methacrylate (HEMA) monomer. To characterize the association constant of the complex, we have suggested a new analysis model based upon the Benesi-Hildebrand model. Because the assumption of the original Benesi-Hildebrand model is that the absorption bands are due only to the presence of the complex and that the absorption by the free component is negligible; the model cannot be applied to our systems, which is a dye-based initiator system. For each dye, the molar absorptivity of the ground state complex was evaluated as a function of wavelength and this analysis confirmed the validity of the modified Benesi-Hildebrand model. In addition, we observed the RB/DPI photoinitiator system failed to produce a perceptible polymerization rate but the FL/DPI photoinitiator system provided very high rates of polymerization. Based upon the association constant for these complexes, we concluded that the observed kinetic differences arise from the different association constant values of the ground state dye-acceptor complex, resulting in back electron transfer reaction.

Interfacial and solution properties of tetraalkylammonium bromides and their sodium dodecyl sulfate interacted products: a detailed physicochemical study

The adsorption and solution behaviors of symmetrical tetramethyl-, tetraethyl-, tetrapropyl-, and tetrabutylammonium bromides (TMAB, TEAB, TPAB, and TBAB, respectively) were studied at the air/water interface and in the bulk aqueous environments. Their salts were prepared by reacting tetraalkylammonium bromide (TAAB) with sodium dodecyl sulfate (SDS) in a solution from which the products of the higher two homologues (tetrapropylammonium dodecyl sulfate (TPADS) and tetrabutylammonium dodecyl sulfate (TBADS)) could only be isolated as solids and for which detailed characterization has been performed. The interfacial behaviors of 1:1 molar mixtures of TAAB and SDS and the prepared TPADS and TBADS were examined. Micellization of the 1:1 mixtures along with the isolated species were studied in the presence and absence of NaBr salt. The energetics of the micellization process and the counterion binding of the micelles were evaluated. The interaction of the TAABs with SDS micelles was examined, and the results were evaluated in terms of single- and two-site binding interaction models. Of the formed tetraalkylammonium dodecyl sulfates (TAADSs), only TBADS evidenced clouding, which was investigated in detail along with 1:1 molar mixtures of TBAB and SDS in aqueous solution in the presence of additives such as NaBr, SDS, and TBAB. The solution behaviors of the TAADS and the clouding of TBADS have been rationalized in terms of a mixed micellar model.

Self-Assembly of Nonionic Surfactants into Lyotropic Liquid Crystals in Ethylammonium Nitrate, a Room-Temperature Ionic Liquid

The stability of a variety of lyotropic liquid crystals formed by a number of polyoxyethylene nonionic surfactants in the room-temperature ionic liquid ethylammonium nitrate (EAN) is surveyed and reported. The pattern of self-assembly behaviour and mesophase formation is strikingly similar to that observed in water, even including the existence of a lower consolute boundary or cloud point. The only quantitative difference from water is that longer alkyl chains are necessary to drive the formation of liquid crystalline mesophases in EAN, suggesting that a rich pattern of "solvophobic" self-assembly should exist in this solvent.

Photochemical reactions in organized and semiorganized media

The article describes organic photochemical reactions in heterogeneous fields. The first part of the article includes an introduction of miscellaneous electrostatic fields adsorbing photoactive species and the second part summarizes the types of photochemical reactions in their fields. Photochemical reactions carried out in various heterogeneous fields, inorganic as well as organic, were classified by their reaction type, that is, unimolecular reactions, bimolecular reactions, energy transfer reactions, and electron transfer reactions.