Inversion in usual excitation intensities from solid state phosphor and improved fluorescence of Eu3+ ion in type (IV) deep eutectic solvent

Abstract

To resolve the issue of aggregation induced quenching (AIQ), poor adhesion/dispersion on film and fiber, limited dispersability/solubility and non biocompatibility issue; phosphor powder particles are generally casted into solution form. It was found in most cases that lanthanide luminescence and lifetime reduced substantially in aqueous medium and ionic liquid. Deep eutectic solvent (DES) has been found to be an alternate and more efficient solvent for lanthanide luminescence. With similar intention we have synthesized europium type (IV) DES (EuDES) via mechanochemical synthesis employing europium nitrate and urea. Fourier transformed infrared spectroscopy (FTIR) studies confirms that in DES, the Eu3+ ion is surrounded by bidentate nitrate ions and urea is coordinated to europium centre by CO group. There is no water molecule in primary coordination sphere of Eu3+ ion. Interestingly with rigid H-bonding and strong chemical interaction offered by DES; Eu3+ CTB is quenched whereas very high intensity of Eu3+ intra f-f transition appears which was reversal from usual trend. Moreover asymmetry ratio was found to be more than 3 with predominantly high intensity of electric dipole transition (EDT) suggesting very asymmetric environment around Eu3+ in DES. But the distribution of europium was found to be very homogenous with luminescence lifetime of ~3.7 ms and point group symmetry ~ C6v. Judd-Ofelt analysis suggested very low non-radiative channels present in DES resulting in extremely high quantum yield ~76% and very high color purity of 94.72% which was also assisted by absence of any water molecule as suggested by FTIR measurement.