Mixed Valence Isomers.

The article reports the direct spectroscopic observation of mixed valence isomers, the two alternate charge distributions of a mixed valence complex. In the normal two-state Marcus-Hush description of a symmetric mixed valence complex, electron exchange is a degenerate process, and the isomers, differing only in the position of the charge, cannot be distinguished. By introducing a slight asymmetry into a mixed valence complex, it is possible in principle to introduce a small energy difference between the two sides of the Marcus-Hush potential energy surface such that substantial Boltzmann populations of the two mixed valence isomers can exist in equilibrium.