Redox and Photophysical Behaviour of Complexes of NpO2+ Ions with Carbomyl methyl phosphine oxide in 1-Hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide Ionic Liquid

Abstract

Room temperature ionic liquids (RTIL) have attracted considerable attention due to their many remarkable properties useful for technological applications not only involving formation of metal ion complexes but also as electrolyte. For understanding the role of ionic liquid in such applications requires the experimental and theoretical studies on the metal ion complexes. Therefore, an electrochemical, theoretical and luminescence studies have been carried out in the present work to correlate redox behavior, complexation, and photophysical characteristics NpO2+ complexes with carbomyl methyl phosphine oxide (CMPO) ligand in ionic liquid, which is important for the actinide partitioning. The electrochemical behavior of NpO2+extracted by CMPO in 1-Hexyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (HmimNTf2) (IL) was studied at glassy carbon (GC) electrode. The redox reaction of NpO2+ to NpO22+and vice versa in HmimNTf2 is quasi-reversible and controlled by diffusion as well as charge transfer kinetics. The cyclic voltammograms obtained in ionic liquid were much broader with reduced cathodic and anodic peak current densities (jpc and jpa) as compared to the one obtained with aqueous solution. The transport properties and kinetics of Np(V) were deduced from electrochemical calculations. The complexation behaviour of CMPO with Np(V) in aqueous solution as well as in HmimNTf2 was studied using density function theory (DFT). Based on these calculations NpO2+forms [Np(CMPO)2(NO3)2]1− type of complex in aqueous solution whereas [Np(CMPO)2(NO3)2]1−.(IL)2complex in IL. Oxidation state of neptunium was further confirmed using Time resolved fluorescence spectroscopy (TRFS) and UV–vis absorption spectroscopy. Moreover TRFS data depicted the stabilization of Np as NpO2+ in both aqueous as well as in RTIL phases. The emission spectroscopic measurement also pin pointed the fact that emission peaks of Np-CMPO complex in RTIL is blue shifted with respect to aqueous solution, which is attributed to the change in equatorial bonding/coordination of the NpO2+in the basal plane. From Photoluminescence (PL) spectroscopy and its kinetics; it was also observed that emission intensity and average lifetime are more in case of Np-CMPO complex in RTIL as compared to aqueous phase because of presence of relatively higher concentration OH group in aqueous phase which are known to be fluorescence quencher. This is a rare study where electrochemical measurements established the redox behavior, DFT calculations confirms the complexation behavior and TRFS brings out the interesting optical features of NpO2+in aqueous and ionic liquid media.