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

Abstract

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.

Photoinduced intermolecular electron transfer between Coumarin dyes and electron donating solvents in cetyltrimethylammonium bromide (CTAB) micelles: evidence for Marcus inverted region

“…Coupling strength between donor and acceptor also plays an important role here. With increase in donor-acceptor distance, the coupling strength decreases several times because it is related exponentially to the distance between donor and acceptor as e ÀbR , where b ¼ 1A 0À1 [4,10,13,16]. Thus, we may conclude that the change in ET rate from neat solvent to micellar solution is primarily due to the increase in the donor-acceptor distance.…”

“…Most of these investigations, so far have been carried out are either in neat solvent where the solvent acts as a donor or under diffusive condition where the solvent is non-interacting and reactants have to diffuse before ET takes place [11][12][13]. While ET processes in the homogeneous medium are well known, only a limited works have been performed in organized media such as micelles [14][15][16][17]. It would be interesting to study ET in organized media because these systems resemblance with many biological and chemical systems in nature.…”

Ultrafast dynamics in a nanocage of enzymes: Solvation and fluorescence resonance energy transfer in reverse micelles

“…After certain time (up to 400 ps), the probe is expected to cross the aqueous phase and again face the nonpolar isooctane medium offering relatively homogeneous environments, as evidenced by the decay of the Γ (t). Taking the diffusion constant of an organic probe at the micellar surface to be 15.3 Å 2 /ns [23], the estimated distance traversed by the probe in 400 ps is 3.5 Å. This is much shorter than the diameter of the RM (40 Å) at w 0 = 10.…”

Photoinduced electron transfer in the head group region of sodium dodecyl sulfate micelles

“…The ODRB is anchored in the micelle by its long hydrocarbon tail with the RB chromophore located in the head group region because of its positive charge. The polarity of the DMNA also causes it to locate almost exclusively in the head group region [17]. Therefore, the dynamics of the electron transfer can be modeled as a donor interacting with a number of acceptors all of which are constrained to move in the essentially spherical head group region.…”