Spectroscopic investigations of Eu3+-complexes with ligands containing multiple diglycolamide pendant arms in a room temperature ionic liquid

Fluorescence spectroscopy was used to study the complexation of different ligands containing multiple diglycolamide (DGA) pendent arms such as T-DGA (tripodal diglycolamide), C4DGA (upper rim functionalized calix [4] arene with four DGA moieties), and C8DGA (both side functionalized calix [4] arene with eight DGA moieties) with Eu3+ in ionic liquids such as C4mimNTf2 and C8mimNTf2 (1-alkyl-3-methylimidazolium bis(trifluoromethylsulphonyl)imide). Mainly five sets of emission profiles were observed in all the spectra (1st set at~580 nm assigned as the 5D0→7F0 transition, 2nd set at~593 nm assigned as the 5D0→7F1 transition, 3rd set at~613 nm assigned as the 5D0→7F2 transition, 4th set at~650 nm assigned as the 5D0→7F3 transition, and 5th set at~700 nm assigned as the 5D0→7F4 transition). The difference in the spectral features revealed that the nature of the complexes differs significantly from one another. Judd-Offelt constant Ω2, which is a measure of the covalency of the metal-ligand bond, follows the order Eu3+-C4DGA (C8mimNTf2)>Eu3+-C8DGA (C8mimNTf2)>Eu3+-TODGA (C4mimNTf2)>Eu3+-T-DGA (C4mimNTf2)>Eu3+aq. The high intensity of the 5D0→7F2 transitions in all complexes suggests that Eu3+ resides in a highly asymmetric environment. From the splitting patterns of the transitions, the local site symmetry around Eu3+ was found to be C2V for the Eu3+-T-DGA complex in both C4mimNTf2 and a 1:5 acetonitrile-water mixture, C4V for Eu3+-C8DGA and Eu3+-TODGA complexes, and either C1, C2, or CS for the Eu3+-C4DGA complex. A linear relationship between the 5D0-7F1 splitting vs E(5D0-7F0) and E(5D0-7F0) vs E(5D0-7F1) was observed. The crystal field parameters in the different complexes follow the trend Eu3+-C4DGA>Eu3+-TODGA>Eu3+-T-DGA in C4mimNTf2~Eu3+-C8DGA>Eu3+-T-DGA in 5:1 acetonitrile-water mixture.