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

Abstract

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 P2 1 /n and their molecular structures consist of homodinuclear species that are bridged by two oxygen atoms from two benzoate ligands. In the case of 1, the carboxylate ligands coordinate to the central Tb 3+ ion in bidentate chelating and bidentate bridging modes. By contrast, three different coordination modes (bidentate chelating, bidentate bridging and monodentate) are observed in the case of compound 2. 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 p*-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.

Synthesis, crystal structure and photophysical properties of lanthanide coordination polymers of 4-[4-(9H-carbazol-9-yl)butoxy]benzoate: the effect of bidentate nitrogen donors on luminescence

“…However, the overall quantum yields and the sensitization efficiency noted for both the lanthanide compounds in the current study are found to be somewhat inferior as compared with various lanthanide benzoate complexes reported earlier in our laboratory. [3][4][5] The fact that both the lanthanide complexes derived from 4-[4-(9H-carbazol-9-yl)butoxy]benzoate (1 and 3) exhibit residual ligand emission in the 375-475 nm region, as can be noted from the respective emission spectra (Fig. 6 and 8), indicates the modest efficiency of energy transfer from the ligand to the Ln 3+ center.…”

“…Eu(L) 3 (tmphen) (5). Elemental analysis (%): calcd for C 85 H 76 EuN 5 O 9 (1463.48): C, 69.76;H, 5.23;N, 4.79;found C, 69.81;H, 5.28;N, 4.83.…”

“…where A MD,0 = 14.65 s −1 represents the spontaneous emission probability of the magnetic dipole 5 Table 3 summarizes the Φ overall , Φ Ln , Φ sen , radiative (A RAD ) and non-radiative (A NR ) decay rates. It is evident from the table that the europium coordination polymer displays a poor quantum yield (Φ overall = 0.11%) as compared to the terbium analogue (Φ overall = 2.0%), which can be explained on the basis of a larger −1 ) complex 3 shows an improved quantum yield.…”

“…By contrast, the introduction of a nitro group in the 3 position dramatically diminishes the photoluminescence efficiency of the Tb 3+ -3-nitro-4-benzyloxy benzoate complex due to the existence of a pathway that permits dissipation of the excitation energy via the π*-n transition of the nitro group in conjunction with the ILCT band. 5 Studies by de Bettencourt-Dias et al 6 suggest that derivatization of benzoic acid analogues with thiophene has a beneficial tuning effect on the triplet state of the antenna which results in a higher emission quantum yield. This is a consequence of improved matching of the ligand and lanthanide ion excited states.…”

Preparation, Crystal structure and Luminescence Properties of Lanthanide Complexes with 2,4,6-tri(pyridin-2-yl)-1,3,5-triazine and Organic Carboxylic Acid

“…Among the five emission bands, the electric dipole transition 5 D0→ 7 F2 around 616 nm is the strongest. In addition, the ratio of electric dipole transition and magnetic dipole transition ( 5 D0→ 7 F1) of Complexes 1 and 2 are 3.31 and 4.28, respectively, which indicates that the coordination environment of Eu 3+ complexes are not center symmetric [50][51][52][53][54]. The luminescence intensities of electric dipole transition of Complexes 1 and 2 are 0.74 × 10 5 and 1.4 × 10 5 , respectively.…”