High-Pressure Testing of Heterogeneous Charge Transfer in a Room-Temperature Ionic Liquid:  Evidence for Solvent Dynamic Control

Abstract

We report the first application of a high-pressure electrochemical strategy to study heterogeneous charge transfer (CT) in a room-temperature ionic liquid, [BMIM][BTA]. High-pressure kinetic studies on electron exchange for two redox couples of different charge type, viz. [Fe(bipy)3]3+/2+ and [Fe(cp)2]+/0, at bare Au electrodes within the range of 0.1-150 MPa, revealed large positive volumes of activation that were found to be virtually the same for the two redox couples in terms of the CT rate constants and diffusion coefficients, despite the reactant's charge type. Independent viscosity (fluidity) studies at elevated pressure (up to 175 MPa), were also performed and revealed a pressure coefficient closely resembling the former ones. Complementary temperature-dependent kinetic studies within the range of 298-358 K also revealed the virtual similarity in activation enthalpies for the same kinetic and diffusion processes, as well as the viscosity of [BMIM][BTA]. A rigorous analysis of the complete variety of obtained results strongly indicates that dynamic (frictional) control of CT is operative by way of the full adiabatic mechanism. The contribution of the Franck-Condon term to the activation free energy of the kinetic process seems almost diminished because of the high value of electronic coupling and freezing out of the outer-sphere reorganization energy. Further analyses indicate that frictional control most probably takes place through slow translational modes (implying "minimal volume" cooperative dislocations) of constituent ions. This kind of motion seems further slowed down within the vicinity of the active site presumably located within the diffusive-like zone situated next to the compact (first) part of the metal/ionic liquid junction.

Fundamental signatures of short- and long-range electron transfer for the blue copper protein azurin at Au/SAM junctions

“…3, 4), the relaxation component ν eff is a thermally activated process as well. Hence, in this case, the value of ΔG aðEXPÞ (or ΔH aðEXPÞ , as its constituent part) should contain an additional component, namely, ΔG aðηÞ (or, respectively ΔH aðηÞ ), related to the protein friction (24,27,28,34,41,53). For short-range ET (n ¼ 4), the value of ΔH aðEXPÞ is twice that of the long-range (n ¼ 15) ET; see Table 1.…”

“…Electrochemical measurements were carried out with an Autolab Electrochemical Analyzer PGSTAT 30, equipped with the General Purpose Electrochemical System software for Windows (Version 4.9). The pressure vessel and electrochemical cell have been described elsewhere (27,28,34,56). The Autolab software was used for the primary data analysis (finding peak positions, midpoint potentials, etc.)…”

“…As a decisive development of the preceding work (19,20,25,26), we offer unique kinetic data obtained through temperature-and pressure-variation and the mechanistic analysis through a unique global fitting procedure accounting for ET at different SAM thickness, temperature, and pressure conditions that provided the variation of the reduced ET rate constant over almost five orders of magnitude. Importantly, our previous work demonstrated that Au-deposited SAMs can withstand pressure-related stress within 5 to 150 MPa (27,28,34).…”

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

“…However, there is relatively little experimental data available that demonstrates this pattern. [5][6][7] In the ergodically normal ET regime, typical for sufficiently fluid liquid-phase media, for particular heterogeneous (electrode-implicated) processes, proper unimolecular rate constants can be presented by the unified expression in Equation (1): [8][9][10][11][12][13][14][15][16][17] Abstract: 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) […”

Charge-Transfer Patterns for [Ru(NH3)6]3+/2+ at SAM Modified Gold Electrodes: Impact of the Permeability of a Redox Probe

“…According to contemporary theoretical models [3][4][5][6][7][8][9] and recent experimental studies (performed in molecular and/or ionic liquids exploiting chemical redox markers or redoxactive proteins) [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27], heterogeneous charge transfer mechanism may display two different intrinsic mechanisms: adiabatic (solvent friction) -at short distances, and nonadiabatic (tunneling) -at long distances, as well as intermediate regime. In all cases the unimolecular rate constant (k o et ) of charge transfer processes, according to the resent theoretical update, can be presented by the eq.…”