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Quenching Mechanisms and Diffusional Pathways in Micellar Systems Unravelled by Time‐Resolved Magnetic‐Field Effects
Author(s) -
Goez Martin,
Henbest Kevin B.,
Windham Emma G.,
Maeda Kiminori,
Timmel Christiane R.
Publication year - 2009
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200802502
Subject(s) - dabco , micelle , chemistry , quenching (fluorescence) , cationic polymerization , photochemistry , octane , micellar solutions , radical ion , radical , ion , sodium dodecyl sulfate , inorganic chemistry , aqueous solution , organic chemistry , physics , quantum mechanics , fluorescence
Radical observation : Time‐resolved magnetic‐field effects yield a very detailed picture of electron‐transfer quenching in micelles and of the fate of the resulting radical‐ion pairs. The system xanthone/DABCO (A/D, see figure) permits a separation of the different static and dynamic quenching pathways and a distinction between bulk and surface diffusion.Magnetic‐field effects (MFEs) are used to investigate the photoreaction of xanthone (A) and DABCO (D) in anionic (SDS) or cationic (DTAC) micelles at high pH (DABCO=1,4‐diazabicyclo[2.2.2]octane, SDS=sodium dodecyl sulfate, DTAC=dodecyl trimethyl ammonium chloride). From MFE experiments with nanosecond time resolution, the radical anion A . − can be observed without any interference from the much more strongly absorbing triplet 3 A*, the different quenching processes can be separated and their rates can be measured. Triplet 3 A* is quenched dynamically both by the SDS micelle ( k 1 =5.0×10 5 s −1 ) and by DABCO approaching from the aqueous phase ( k 2 =2.0×10 9 M −1 s −1 ). Static quenching by solubilised DABCO (association constant with the SDS micelles, 1.5 M −1 ) also participates at high DABCO concentrations, but is chemically nonproductive and does not lead to MFE generation. The MFEs stemming from the radical ion pairs ${\overline {{\rm{A}}^{ \bullet - } \,{\rm{D}}^{ \bullet + } } }$ are about 40 times larger in the anionic micelles than in the cationic ones despite a higher yield of free radicals in the latter case. This can be rationalised by different diffusional dynamics: Because of the location of their precursors, A . − and D . + are formed at opposite sides of the micelle boundary. Subsequently, the negatively charged Stern layer of the SDS micelle traps the radical cation, which then undergoes surface diffusion, so both the recombination probability and the spin mixing are high; in contrast, the positive surface charge of the DTAC micelle forces the radical cation into the bulk of the solution, thus efficiently blocking a recombination.