Synthesis and crystal structures of lanthanide 4-benzyloxy benzoates: influence of electron-withdrawing and electron-donating groups on luminescent properties.

Three new 4-benzyloxy benzoic acid derivatives [4-benzyloxy benzoic acid = HL1; 3-methoxy-4-benzyloxy benzoic acid = HL2; 3-nitro-4-benzyloxy benzoic acid = HL3] have been employed as ligands for the support of six lanthanide coordination compounds [Tb(3+) = 1-3; Eu(3+) = 4-6] with the aim of testing the influence of electron releasing (-OMe) or electron withdrawing (-NO(2)) substituents on the photophysical properties. The new complexes have been characterized by a variety of spectroscopic techniques and two of the Tb(3+) complexes [1 and 2] have been structurally authenticated by single-crystal X-ray diffraction. Compounds 1 and 2 crystallize in the monoclinic space group P21/n. The molecular structure of 1 consists of homodinuclear species that are bridged by two oxygen atoms from two benzoate ligands [corrected].In the case of 1, the carboxylate ligands coordinate to the central Tb(3+) ion in bidentate chelating and bidentate bridging modes. By contrast, the X-ray structure of 2 reveals that each Tb3+ ion is connected to two neighboring ions by four methoxy substituted benzoates via the carboxylate groups in bridging mode to form an infinite one-dimensional coordination polymer [corrected]. Examination of the packing diagrams for 1 and 2 revealed the presence of a one-dimensional molecular array that is held together by intermolecular hydrogen-bonding interactions. The incorporation of an electron-releasing substituent on position 3 of 4-benzyloxy benzoic acid increases the electron density of the ligand and consequently improves the photoluminescence of the Tb(3+) complexes. On the other hand, the presence of an electron-withdrawing group at this position dramatically decreases the overall sensitization efficiency of the Tb(3+)-centered luminescence due to dissipation of the excitation energy by means of a pi*-n transition of the NO(2) substituent along with the participation of the ILCT bands. The weaker photoluminescence of the Eu(3+) complexes is attributable to the poor match of the triplet energy levels of the 4-benzyloxy benzoic acid derivatives with that of the emitting level of the central metal ion.