Intramicellar electron transfer

Fig. 4.7. Kinetic features of elementary processes in the intramicellar electron transfer reaction...

The photoinduced reduction of some quinones by zinc porphyrin and also by its tetraphenyl derivative has been studied in micellar systems. The mean time for intramicellar electron transfer has been established as 0.2 ps, and for duroquinone the rates of entry and exit from the micelle have been found to be 5 x 10 m s and6 x 10 m s respectively. Quinones possessing long chains are less mobile and partial charge separation could be achieved. Irradiation of anthraquinone in aqueous sodium dodecyl sulphate leads to anthraquinol and the surfactant-anthrahydroquinone ether as major products via the triplet state of the anthraquinone. ... 

Fig. 3. The dependence of intramicellar electron transfer rate constant on the difference of the electron acceptor reduction potential and electron donor oxidation potential [104]...

The significance of this functional organization becomes evident also when the back reaction of Cu+ and DI is considered. Previous studies have shown that the intramicellar electron transfer from Cu " to DI", though thermodynamically highly favourable, cannot compete kinetically with the escape of the cuprous ion from its native... 

Electron transfer can be accomplished by quenching of a micelle trapped chromophore by ions capable of ion pairing with the micelle surface. For example, excited N-methylphenothiazine in sodium dodecylsulfate (SDS) micelles can exchange electrons with Cu(II). The photogenerated Cu(I) is rapidly displaced by Cu(II) from the aqueous phase so that intramicellar recombination is averted, Fig. 5 (266). Similarly, the quantum yield for formation of the pyrene radical cation via electron transfer to Cu(II) increases with micellar complexation from 0.25 at 0.05 M SDS to 0.60 at 0.8 M SDS (267). The electron transfer quenching of triplet thionine by aniline is also accelerated in reverse micelles by this mechanism (268). 

A new probe of solvent accessibility of bound sensitizers has been described and tested for the particular case of a series of Ru" and Os photosensitizers bound to sodium lauryl sulphate micelles. The method depends upon the large solvent deuterium effect on excited-state lifetimes, and a correlation has been established between accessibility of bound complexes and hydrophobicity of the ligands. Luminescence properties of amphiphilic annelide-type complexes of ruthenium in micellar phases have been described. In the case of [4,4 -bis(nonadecyl)-2,2 -bipyridyl]bis-[4,4 -di-(10,13,16-trioxaundecyl)-2,2 -bipyridyl]ruthenium dichloride, intramicellar self-quenching effects have an influence on the excited-state lifetime, and the mechanism of self-quenching has been determined. Deactivation of [Ru(bipy)3] by [Co(EDTA)] has been studied in a micellar environment and found to occur by electron transfer at diffusion-controlled rates a stereoselective effect has been observed. ... 

CT complexes are formed and electron transfer occurs from excited molecules of anthracene derivatives to methylviologen in aqueous micellar media. Methylene blue quenches pyrene fluorescence by electron transfer in SDS micelles . E.lectron transfer between anthraquinone sulphonate radicals and duroquinone in SDS micellar solution occurs in the aqueous phase there is no evidence of intramicellar transfer. Photoionisation of... 

Photochemically-induced electron transfer from N-methylphenothiazine in anionic micelles to cupric ions occurs in the Stern layer. This process is complete within the 15 ns duration of the 347 nm ruby laser excitation. The primary photochemical event is formation of the Tj state of N-methylphenothiazine and both this and the subsequent electron transfer occur with unit quantum yield completely suppressing competitive photochemical pathways. Added Co and Ni deactivate the triplet state. A related system in which Eu " replaced Cu was examined, and electron transfer was again observed. In this case back reaction occurs in the ground state and intermicellar events may be kinetically distinguished from the much faster intramicellar events. ... 

The effect of the electrochemical and hydrophobic properties of the reactants on the intramicellar charge separation rate constant for model systems of ruthenium-tris-bipyridile, tris-phenanthroline and tris-diphenylphenanthroline complex quenching by organic cations in SDS micelles was studied by Miyashita et al. [124,125]. The plot of vs. the free energy of the electron transfer reaction calculated from electrochemical and spectral properties of the reactant (Fig. 4) differs from the well-known Rehm-Weller and Marcus plots for homogeneous solutions. 

Fig. 4. The dependence of intramicellar quenching rate constant on Gibbs energy changes of electron transfer for quenching of ruthenium-tris-4,7-diphenyl-l, 10-phenanthrolin by substituted pyridinium cations in SDS micellar solution [124]...

Electron transfer rates between adrenaline and related benzene diols and complexes of iron(III) with some substituted 1,10-phenanthrolines have been reported [67] in surfactant systems. In cationic systems the reactions take place in the aqueous phase and reaction rates are lower than they are in simple aqueous systems, but in anionic surfactant systems the reaction rates are enhanced, reactions probably taking place at the micellar interface. The rates of exit and entrance of aromatic compounds from and into micelles have recently been studied using phosphorescence decay measurements [68] exit rate constants of aromatic hydrocarbons are of the order of 10 to 10 s " S whereas values of 10 to 10 (moll ) s have been reported for intramicellar energy transfer processes. Release of aromatic phosphorescence probes from micelles followed by their deactivation in the aqueous phase is hence expected to be an important mode of deactivation of the triplet state [69]. Kinetic schemes for triplets that are partitioned between aqueous and micellar phases are considered for the cases of single occupancy and double occupancy of the micellar units. 

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