ABSORPTION-BAND SHAPE-ANALYSIS FOR PHOTOINDUCED INTERMOLECULAR ELECTRON-TRANSFER IN SOLUTION - APPLICATION TO A NEW NEAR-INFRARED TRANSITION BETWEEN FREE HEXACYANOFERRATE(II) AND HEXACYANOFERRATE(III)

A mixture of high concentrations of [Fe(CN)6]3- and [Fe(CN)6]4- in water and D2O exhibits a weak absorption band at 800 nm under conditions where binuclear mixed-valence complexes or hydration products cannot be detected. At constant cation concentration the band follows the Lambert-Beer law with respect to both components. The absorption increases on alkali halide addition, with small but notable variations along the alkali-metal series, but decreases with increasing temperature. It is suggested that this band is caused by photoinduced electron transfer in ion-paired outer-sphere mixed-valence configurations. Band shape analysis shows that extensive solvent and ion-pair reorganization occurs on electron transfer. From the band shape parameters it is concluded that the corresponding thermal process is close to the adiabatic limit. Rate constants calculated from the optical parameters are higher than reported values probably due to inhomogeneous band broadening.