Entropic Barriers Determine Adiabatic Electron Transfer Equilibrium

A thermodynamic analysis of the forward and reverse rate constants for adiabatic and nonadiabatic electron transfer equilibria over an 80 K temperature range is reported. The kinetic data were acquired by a spectroscopic approach that utilized excited state injection into TiO2 by sensitizers with two redox active groups linked through aromatic bridges that allow for intramolecular adiabatic (bridge = phenyl) or nonadiabatic (bridge = xylyl) electron transfer. Two impactful results were garnered from this analysis: (1) entropic barriers controlled the adiabatic electron transfer kinetics, and (2) the free energy barriers were unaffected by the degree of electronic coupling within experimental uncertainty. The second result stands in contrast to the common expectation that enhanced electronic coupling lowers the free energy barrier. This analysis provides new insights into how electronic coupling influences the free energy and barriers for electron transfer reactions.