Micelles photoionization yields

The ability of micelles to enhance photoionization yields of hydrophobic molecules was demonstrated in the early 1970s. Thus, the photoionization yields of pyrene [59], phenothiazine [60] and tetramethylbenzidine [61] cations increased when these molecules were encapsulated in anionic micelles. The effect was attributed to efficient escape of electrons from the geminate charge-separated species formed within the micelle, which is accelerated by the anionic interface. The negative micellar surface imposes an electrostatic barrier between the cations, which remain with the micelle, and the aqueous electron in the bulk water phase, thus increasing the lifetimes of the photoredox products. 

Other notable micellar catalysed reactions are the attack of oxygen on an enolate anion, elimination reactions that have more carbanion-like transition states compared with non-micellar conditions and the selective hydroxymercuration of non-conjugated dienes. There has been considerable interest in the effect of micelles on photochemical and electron-transfer reactions. Micelles can increase yields in photoinduced electron transfer and lower the ionization threshold in photoionization. ... 

Fig. 2. The dependence of perylene photoionization relative quantum yields on the excitation wavelength in methanol (1), in Triton XlOO micelles (2), in SDS micelles (3) [101]...

The photoionization of phenothiazine and its derivatives in SDS micelles, which is not observed in homogeneous solutions and the premicellar region was studied by Gratzel et al. [106-109]. They suppose that the anionic micelles decrease the ionization potential of the phenothiazine derivatives and the photoionization includes an electron tunnelling from the micelle into the solution through the Stem layer. However, there are no direct data for these systems to give a value of the photoionization threshold. A drastic increase of the photoionization quantum yield at the transition to anionic micelles could also be caused by a decrease of the role of the geminate recombination. 

The illumination of TMB in the presence of electron acceptors with different j-values yields TMB radical cations with different quantum yields and consumption of the spin label. The highest quantum yield was obtained with j = 10 and especially with j = 16. This was explained in terms of TMB location in the hydrophobic region of the micelle asymmetrically to the electron acceptor molecule. However, another interpretation is possible as the stearic acid molecule does not coincide in the number of carbon atoms with the micelle-forming surfactant, one can assume that the carbon atom C-13 is located near the end of the methyl groups of the surfactant, i.e. near the centre of the micelle. In this case the results of Bales and Kevan show that TMB molecules are located inside the micelle symmetrically with the photoionization probabihty maximum near the third carbon atom from the micelle centre. Anyway, TMB molecules are located in hydrophobic nuclei of the micelles. The important... 

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