Photoinduced Intermolecular Electron Transfer in Micelles: Dielectric and Structural Properties of Micelle Headgroup Regions

Photoinduced intermolecular (donor/acceptor) electron transfer is studied both experimentally and theoretically for donors and acceptors located in the headgroup region of micelles. Fluorescence up-conversion and fluorescence yield measurements were performed to characterize photoinduced electron transfer from N,Ndimethylaniline (DMA) and N,N-dimethyl-1-naphthylamine (DMNA) to octadecylrhodamine B (ODRB) in three types of aqueous micelle solutions: dodecyl-, tetradecyl-, and cetyltrimethylammonium bromide (DTAB, TTAB, and CTAB, respectively). The data were analyzed with a detailed theory that assumes a Marcus distance-dependent rate constant. Because DMA, DMNA, and ODRB reside in the headgroup region of the micelles, the theory includes diffusion of the molecules in this region of the micelles. The micelles are modeled as a spherical core of low dielectric constant surrounded by a spherical shell headgroup region with intermediate dielectric properties, which in turn is surrounded by water. An analytical theory, which accounts for geometrical and dielectric properties of the three-region micelle environment, is used to calculate the solvent reorganization energy and free energy of transfer. To fit the data, the three-region dielectric model is necessary, and the dielectric constant of the micelle headgroup region of each micelle can be approximately determined. In addition, including local structure is required to fit the data, yielding some information about molecular organization in the headgroup region.