Pressure effects on the intervalence-transfer electronic absorption band of the mixed-valence Creutz-Taube ion in various media

In this paper we present the results of a pressure tuning spectroscopy (PTS) study of the intervalence-transfer (IT) band of the (Cl₅·5H₂O) salt of the Creutz-Taube ion. We studied this compound in three media: (1) as a crystalline solid, (2) doped in the polyelectrolyte poly(styrenesulfonic acid) (PSS), and (3) in water (D₂O). In all cases the principal effect of pressure on the IT band was found to be a shift of the band to higher energy. For the microcrystalline sample this shift was ∼400 cm^-1 over a pressure range from 0.001 to 200 kbar (1 kbar = 986.9 atm). In this same pressure range the blue shift of the IT band for the PSS-doped sample was ∼600 cm^-1. In water (D₂O) the shift was ∼35 cm^-1 in a pressure range from 0.001 to 7 kbar. In addition to the Gaussian fits described above we have fit the spectra using a vibronic coupling model developed by Piepho, Krausz, and Schatz (PKS). When analyzed in terms of this simple small-polaron model, this peak shift corresponds primarily to an increase in the electronic coupling between the metal centers of the ion. The degree of vibronic coupling changes either only slightly or not at all with pressure.