Molecular Geometry Dependent Electronic Coupling and Reorganization Energy for Electron Transfer between Dye Molecule Adsorbed on TiO2 Electrode and Co Complex in Electrolyte Solutions

Electron transfer kinetics between donor and acceptor molecules in electrolytes has been described by Marcus theory using reorganization energy (lambda), electronic coupling (H), and free energy difference (Delta G degrees). In solution, the molecules can collide freely, while collision occurs only at the exposed area of the molecules when the donors or the acceptors are anchored onto an electrode, altering the values of lambda and H. To date, these structural effects of electrodebound molecules have not been considered in detail. To study geometrical effects, we fabricate TiO2 electrodes with nine different donor-(pi-bridge)-acceptor type molecules and measure the kinetics of electron transfer from five different Co complexes in electrolytes. For densely adsorbed electrodes, the molecules with larger donor moieties have faster reduction kinetics and the kinetics are independent of the length of the pi-bridge. When the amount of the adsorbed molecules is reduced, the kinetics become faster and the kinetics depend on the pi-bridge length. These phenomena can be partially correlated to the increased exposed area of the molecules to the electrolyte. By fitting the data, we obtain lower lambda values for lower dye-loading conditions, which is not expected if only the effect of solvent molecules is considered. Obtained H values with various geometries suggest that it is important not only to increase the exposed area but also to expose the point giving high H values to increase the kinetics. One example found is designing molecules with small molecular orbitals to increase H values, though this would also give large lambda values.