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The effect of 4-halogenobenzoate ligands on luminescent and structural properties of lanthanide complexes: experimental and theoretical approaches
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The ligands 4-fluorobenzoate (4-fba), 4-chlorobenzoate (4-cba), 4-bromobenzoate (4-bba) and 4-iodobenzoate (4-iba) were chosen in order to synthesize europium(III), gadolinium(III) and terbium(III) complexes and compare the effect of halogens on their physical chemistry and luminescent properties. The homobimetallic complex [Eu(4-iba)3(H2O)(dmf)]2 crystallizes in the monoclinic P21/c space group with unit cell parameters a = 8.3987(9) Å, b = 25.314(3) Å, c = 14.1255(17) Å, and β = 105.347(2)°. FTIR spectroscopy indicates that the bidentate bridging mode of the carboxylato ligand was present in all complexes while bidentate chelate and a mixture of bidentate bridging and chelate modes were also found. According to emission spectra profiles and the Judd–Ofelt parameters the halogen of ligand molecules modifies the chemical environment symmetry around the europium(III) ion in their respective complexes. The complexes [Eu(4-fba)3(H2O)2] and [Eu(4-iba)3(H2O)2] have the highest symmetry around the europium(III) while the complexes [Eu(4-cba)3]·2H2O, [Eu(4-bba)3]·5/2H2O and [Eu(4-iba)3(H2O)(dmf)]2 have the lowest. The different halogens at the para position do not change the covalence degree of Eu–O bonds significantly, however they play a role in the ligand to metal charge transfer energies. The highest non-radiative energy transfer rates from ligand to europium(III) were found for the complexes [Eu(4-cba)3]·2H2O and [Eu(4-bba)3]·5/2H2O.