Spectroscopic Evidence of Nanodomains in THF/RTIL Mixtures: Spectroelectrochemical and Voltammetric Study of Nickel Porphyrins

The presence and effect of RTIL nanodomains in molecular solvent/RTIL mixture were investigated by studying the spectroelectrochemistry and voltammetry of nickel octaethylporphyrin (Ni(OEP)) and nickel octaethylp orphinone (Ni( OEP one)). Two oxidation and 2-3 reduction redox couples were observed, and the UV-visible spectra of all stable products in THF and RTIL mixtures were obtained. The E degrees values for the reduction couples that were studied were linearly correlated with the Gutmann acceptor number, as well as the difference in the E degrees values between the first two waves (Delta E-12 degrees = vertical bar E-1 degrees - E-2 degrees vertical bar). The E-12 degrees for the reduction was much more sensitive to the %RTIL in the mixture than the oxidation, indicating a strong interaction between the RTIL and the anion or dianion. The shifts in the E degrees values were significantly different between Ni(OEP) and Ni(OEPone). For Ni(OEP), the E-1 degrees values were less sensitive to the %RTIL than were observed for Ni(OEPone). Variations in the diffusion coefficients of Ni(OEP) and Ni(OEPone) as a function of %RTIL were also investigated, and the results were interpreted in terms of RTIL nanodomains. To observe the effect of solvation on the metalloporphyrin, Ni(OEPone) was chosen because it contains a carbonyl group that can be easily observed in infrared spectroelectrochemistry. It was found that the nu(co) band was very sensitive to the solvent environment, and two carbonyl bands were observed for Ni(OEPone)(-) in mixed THF/RTIL solutions. The higher energy band was attributed to the reduced product in THF, and the lower energy band attributed to the reduced product in the RTLL nanophase. The second band could be observed with as little as 5% of the RTIL. No partitioning of Ni(OEPone)(+) into the RTIL nanodomain was observed. DFT calculations were carried out to characterize the product of the first reduction. These results provide strong direct evidence of the presence of nanodomains in molecular solvent/RTIL mixtures.