Ion-pairing of octyl viologen diiodide in low-polar solvents: an experimental and computational study

We have investigated the ion-pairing and solvent effect on the NMR and UV/vis spectra of 1,1'-di- n-octyl-4,4'-bipyridinium diiodide in various solvents. A strikingly different behavior is observed in the low polar solvent dichloromethane. A large deshielding of the meta bipyridinium core resonance occurs and charge transfer (CT) transitions are observed in the visible region due to the formation of ion-pairs. The CT bands show a marked blue-shift as the polarity of the solvent is increased. Experimental data have been compared with the results of DFT calculations of proton's chemical shifts and TD-DFT calculations of the vertical electronic transitions of model ion-pairs (using the smaller methyl viologen dication) in the gas phase and after the inclusion of the solvent reaction field by means of the PCM scheme. Different geometrical arrangements of the ion-pairs have been investigated, and the direct and indirect solvent effect has been elucidated. A good agreement is obtained which allows one to get insights concerning the CT transitions of this system and the geometry of the ion-pairs in solution of low-polar solvents.