Multiple Mechanisms for Electron Transfer at Metal/Self-Assembled Monolayer/Room-Temperature Ionic Liquid Junctions: Dynamical Arrest versus Frictional Control and Non-Adiabaticity

Electrochemical devices consisting of gold electrodes coated by electronically well-behaved self-assembled alkanethiol monolayers of variable thickness, a ferrocene/ferrocenium redox probe and a typical room-temperature ionic liquid (RTIL) [bmim][NTf2] (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) as a unique reaction medium with an exceptionally broad spectrum of relaxational modes (probed under variable temperature and pressure conditions), have been used to vary the intrinsic electron-transfer (ET) rate constant over eight orders of magnitude (from 0.1 to 3 x 10(7) s(-1)) by further tuning of the overvoltage. A remarkable interplay of ET mechanisms was observed, which was accompanied by the stepwise drop in the reorganisation free energy of the medium from 1.0 to 0.1 eV. The first mechanistic changeover to the dynamically arrested regime, with a locking ultra-slow relaxation time of approximately 50 mu s, occurred at donor-acceptor separations below 20 angstrom. Another mechanistic changeover to the full solvent friction regime, controlled by a medium relaxation process of approximately 100 ns, emerged for ET distances smaller than 8 angstrom.