Development of the charge-transfer-to-solvent process with increasing solvent fluid density: the effect of ion pairing

The study of the UV spectroscopic behaviour of alkali metal iodides dissolved in supercritical ammonia showed that two absorbing species contributed to the UV absorption of the solutions. The two species differed in the type of interaction of iodide with the cation, i.e. going from contact ion pairs to free iodide ion, the observed absorption band varied according to the species that prevailed as the solvent density (ρ1) changed. This experimental evidence was supplemented with molecular dynamics simulations and electronic structure calculations which showed that at very low ρ1 when the contact ion pair is the dominant species, a sudden change from the internal charge transfer photoexcitation route to a charge-transfer-to-solvent transition occurred. This finding emphasized the importance of solvation at very low ρ1 not only for the photoexcitation process, it also allows connecting the thermodynamic behaviour of the solutes in solution with that observed in their vapour phase. We have tried to draw a consistent picture of the available information of UV photoexcitation for iodides in vapour, in solution either forming contact ion pairs or present as free iodide ions, including their behaviour in small clusters of highly polar molecules. The importance of the cation has been clearly shown in this investigation. A relation between the photoexcited electron in contact ion pairs and the solvated electron of alkali metals in small NH3 clusters has been conjectured.