Correlations of optical and thermal charge transfer

Abstract

The relationships between the observed charge transfer absorption maxima and the independently measured free energy parameters of the constituent donor–acceptor (D/A) couples are contrasted for systems in which the D/A coupling is weak (as in ion pairs) to those in which it is strong (as in metal-to-ligand charge transfer systems). The general relationship between the measured parameters can be expressed as, hν max= FΔ E 1/2+ χ reorg− RT ln K DA, where Δ E 1/2 is the difference between the half-wave potentials and χ reorg is the reorganizational free energy of the constituent D/A couples, and K DA is an equilibrium constant for the electron transfer process that relates the ground state, the excited state and the constituent D/A couples. For ion pair systems, Δ E 1/2 can be measured directly, χ reorg is often available from kinetic studies of electron transfer self-exchange reactions ( χ reorg ex=4Δ G ‡) and K DA can be replaced by the ratio of ground state and excited state ion pair association constants. For strongly coupled systems, RT ln K DA can be replaced by the difference of the ground state stabilization and excited state destabilization energies that result from configurational mixing between the ground and excited state. Values of χ reorg are not usually known in strongly coupled systems, but plausible estimates can be based on simpler electron transfer systems. For polypyridine ligands, symmetry issues and configurational mixing between ligand components, and between different polypyridine ligands can contribute to the spectroscopy, but do not appreciably complicate the absorption maximum/electrochemical correlations. The solvent reorganizational free energy contribution to hν max is expected to be small for the electrochemical correlations involving the strongly coupled systems, in part due to charge delocalization and in part due to the correction ( K DA) for electrochemical measurements on the constituent redox couples. Experimental data indicate that reorganizational parameters are small and only very weakly solvent dependent for the strongly coupled systems: (1) metal-to-metal charge transfer transitions in CN-bridged Ru II/Ru III complexes; and (2) metal-to-ligand charge transfer transitions in simple Ru II–polypyridine complexes.