Judd-Ofelt parameter of diglycolamide-functionalized calix[4]arene Eu3+ complexes in room temperature ionic liquid for structural analysis: effects of solvents and ligand stereochemistry

The complexation behavior of several diglycolamide (DGA)-functionalized calix [4]arene ligands (C4DGA) with Eu3+ in C8mimNTf2 (1-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) was studied by time resolved laser fluorescence spectroscopy (TRLFS). The high intensity emission peaks of Eu3+ at 614 and 619 nm, which are due to the 5D0→7F2 transition, sensitive to the local environment around Eu3+, suggest that Eu3+ sits in a highly asymmetric environment. The metal-ligand stoichiometry of the complexes is 1:1, while there was no inner sphere water molecule around Eu3+. The splitting pattern of the individual transitions of Eu3+ indicates that the local site symmetry around Eu3+ is C4V. Though the emission characteristics of the complexes in C8mimNTf2 and a 1:5 acetonitrile-water mixture were similar, the smaller linewidth in C8mimNTf2 can be attributed to the restricted motion of the Eu3+ complex. Judd and Ofelt parameters (Ω2, Ω4) were calculated for all Eu3+–C4DGA complexes. Steric crowding around the ligating sites decreases the proximity of Eu3+ with the carbonyl oxygens. This is evident from a decrease in the Ω2 value, which is a signature of the increase in the bond length and concomitant decrease in the covalent interactions. Higher transition probabilities (Amd, A2ed, A4ed) of the Eu3+–C4DGA complexes in C8mim·NTf2 compared to a 1:5 acetonitrile-water mixture point to more mixing of the metal f orbitals with the ligand orbitals. The transition probability of the electric-dipole transition of 5D0→7F2 is three times more than the magnetic-dipole transition probability and twice that of the electric-dipole transition of 5D0→7F4. Alkyl substituents on the amidic nitrogen atoms increase the quantum efficiency as it decreases the additional non-radiative pathway.