(50a) Using Oxidation State-Dependent Self-Assembly of Ferrocenyl Surfactants to Enhance Efficiency of Light Energy Harvesting

We report an investigation of the influence of reversible self-assembly of redox-active surfactants on charge transfer at chemically functionalized electrodes. Specifically, we employed (11-ferrocenylundecyl)-trimethylammonium bromide (FTMA) as a model self-assembling redox mediator and alkanethiol-modified gold films as hydrophobic electrodes. By performing cyclic voltammetry in aqueous solutions containing FTMA above its critical micellar concentration, we measured substantial current rectification (I_a/I_c= 17, where I_a and I_c are peak anodic and cathodic currents, respectively) at the hydrophobic electrodes. In contrast, hydroxymethyl ferrocene (a non-self-assembling redox mediator) at hydrophobic electrodes and FTMA at bare gold electrodes yielded relatively low levels of rectification (I_a/I_c= 1.7 and 2.3, respectively). Scan-rate dependent measurements revealed I_a of FTMA to arise largely from diffusion of FTMA from bulk solution to the hydrophobic electrode whereas I_c was dominated by adsorbed FTMA, leading to the proposal that current rectification observed with FTMA is mediated by interfacial assemblies of reduced FTMA that block access of oxidized FTMA to the hydrophobic electrode. Support for this proposal was obtained by using atomic force microscopy and quartz crystal microbalance measurements. Additional characterization of a mixed surfactant system containing FTMA and dodecyltrimethylammonium bromide (DTAB) revealed that interfacial assemblies of DTAB also block access of oxidized FTMA to hydrophobic electrodes; this system exhibited I_a/I_c>80. These results and others suggest that current rectification occurs because oxidized FTMA does not mix with interfacial assemblies of reduced FTMA or DTAB formed at hydrophobic electrodes. More broadly, these results show that self-assembling redox mediators offer the basis of new principles for controlling charge transfer at electrode/solution interfaces. The use of self-assembling redox mediators to reduce recombination reactions within a dye-sensitized solar cell will be presented.