Pressure Effects on the Intervalence Transfer Electronic Absorption Band of the Mixed-Valence Bis(fulvalene)diiron Monocation in Various Media

The pressure dependence of the two intervalence transfer (IT) electronic absorption bands of the mixed-valence bis(fulvalene)diiron cation is examined in various media. In general, a blue shift with increasing pressure is seen for both IT bands in all media. The blue shifting is attributable to an intramolecular effect, where an increase in pressure leads to an increase in the energy separation between the bonding ground and antibonding excited states of the IT transition. At high pressures and to variable degrees in the different media (crystal lattice, PMMA polymer, intercalated into zeolite 13X) compression leads to intermolecular effects on the energy of the two IT bands. Data obtained for the zeolite-intercalated bis(fulvalene)diiron cation are analyzed in terms of the PKS vibronic model. Values for the electronic and vibronic coupling parameters are obtained from spectra measured in the range of 11–138 kbar. The value of the PKS electronic coupling factor e was shown to increase by a factor of ~ 1.5 over this pressure range. The degree of valence localization in the bis(fulvalene)diiron cation can be changed appreciably with pressure. From and the PKS vibronic coupling factor X evaluated for the low-energy IT band it was calculated that| e|/X², a ratio that characterizes the valence localization, increases continuously from 0.59 at 11 kbar to 0.78 at 89 kbar.