Synthesis and photophysical and magnetic studies of ternary lanthanide(iii) complexes of naphthyl chromophore functionalized imidazo[4,5-f][1,10]phenanthroline and dibenzoylmethane

Abstract

The luminescent ternary lanthanide(III) complexes [Ln(DBM)₃(L1)] and [Ln(DBM)₃(L2)] (Ln = Eu and Tb) of dibenzoylmethane (DBM) and 2-(2-naphthyl)-1H-imidazo[4,5-f][1,10]phenanthroline (L1) and 2-(2-hydroxy-1-naphthyl)-1H-imidazo[4,5-f][1,10]phenanthroline (L2) are reported in this work. The naphthyl chromophore functionalized imidazophenanthroline ligands L1 and L2 are structurally characterized by using single crystal X-ray diffraction: (L1), a = 14.933(4) Å, b = 7.4965(13) Å, c = 15.999(4) Å, α = γ = 90°, β = 102.670(7)°, V = 1747.4(6) ų, Z = 4, monoclinic, P2₁/c and (L2), a = 7.8555(11) Å, b = 26.216(4) Å, c = 10.0748(17) Å, α = γ = 90°, β = 96.799(7)°, V = 2060.2(6) ų, Z = 4, monoclinic, P2₁/n. A photophysical study reveals that the ligands L1 and L2 sensitize europium(III) emission. The complexes [Eu(DBM)₃(L1)] and [Eu(DBM)₃(L2)] exhibit the characteristic emission of the Eu(III) ion at 298 and 77 K. The terbium(III) complexes [Tb(DBM)₃(L1)] and [Tb(DBM)₃(L2)] exhibit only ligand centred fluorescence at room temperature, while metal centred emission is observed at 77 K. The non-luminescence of the Tb(III) center at room temperature is due to the proximity of the triplet states of the ligands to the ⁵D₄ level of the Tb(III) ion. DC magnetic susceptibility studies have been conducted on the Tb(III) complexes in a 1 T field over the temperature range 20–300 K. The complexes display room temperature (300 K) χ_MT values of 9.69 and 10.21 cm³ mol⁻¹ K, respectively. A gradual decrease in the χ_MT values is observed on reducing the temperature and reached the value of 7.80 and 6.60 cm³ mol⁻¹ K, respectively, at 20 K. The decrease in the χ_MT values as the temperature is lowered could be attributed to the depopulation of the Zeeman split crystal field levels.