Effect of temperature on the dynamics of electron transfer in heterogeneous medium: Evidence for apparent Marcus inversion

Temperature effect on electron transfer (ET) rates in coumarin–aromatic amine systems have been investigated in TX-100 micellar solution using fluorescence quenching measurements. Interesting observation in the present systems is the appearance of Marcus inversion in ET rates at unexpectedly lower exergonicity, which we explain on the basis of the two-dimensional ET (2DET) theory. Temperature dependent results in the present systems indicate substantial activation barrier, not only in the normal Marcus region, but also in the inverted region as well as at the maximum of the Marcus correlation curve. Because of the involvement of the nuclear tunneling via the participation of high frequency vibrational modes, temperature effects are usually not expected in the true inverted region. It is inferred from the present results that the Marcus inversion observed in the present systems is actually an apparent inversion, arising at exergonicities much lower than the true barrierless situation, and consequently show strong temperature effect. On the basis of the 2DET theory, the apparent inversion in these systems is understood to occur due to persistent nonequilibrium solvation for the reactant state along solvation axis during the progress of the ET reaction along intramolecular coordinate. Observed temperature effect supports the applicability of 2DET theory and also justifies the observed inversion in the ET rates at unexpectedly lower exergonicities. Present results demonstrate the observation of apparent Marcus inversion even when the ET reactions are in the normal exergonicity region (−ΔG0 < λs).