Structural effect on the central cavity of a pendent 12N3 macrocycle on bonding and photophysical properties of Eu3+ and Tb3+ complexes: Experimental and theoretical study

The effect of the ring size of the central cavity of a triaza macrocycle on coordination, bonding and properties is described through synthesis, characterization and detailed DFT studies. For this, two new Eu3+ and Tb3+ neutral complexes based on new 12N3 macrocyclic chelator bearing three 8-hydroxyquinoline pendants, [Ln(12N3Me5Ox)((HO)-O-2)(n)].xH(2)O (Ln = Eu3+ and Tb3+; 12N3Me5Ox = 1,5,9-tris-{(5-methylene-8-hydroxyquinoline)-1,5,9-triazacyclononane} (x = 4 for Eu and x = 5.3 for Tb; n = 2, 3) were synthesized and characterized by elemental analysis, FT-IR, FT-NMR, ESI-mass spectrometry, molar conductance and thermal analysis data. The spectral and analytical data reveal that both the complexes have different coordination number and geometry as compared to its lower homologue. The europium complex is eight coordinate with stoichiometry [Eu(12N3Me5Ox)(H2O)(2)].4H(2)O with twist square antiprismatic geometry while [Tb(12N3Me5Ox)(H2O)(3)].5.3H(2)O has nine coordination with distorted mono-capped square antiprismatic geometry. The FTIR and TGA analyses confirmed the presence of coordinated waters. The molecular modeling studies suggest that the metal ion can be easily encapsulated in the central cavity of the ligand. The geometries of the complexes are distorted due to parallel pi-pi and parallel-displaced pi-pi stacking arrangement between three pendants of 8HQ with inter planner distance are 3.318 and 3.821 angstrom. The (HNMR)-H-1 spectra of Eu3+ complex spectra were much sharper than that of Tb3+ complex. The photoluminescence spectra showed different behaviour in solid as compared to solution state. In the solid state, and at 77 K, the complexes showed characteristic lanthanide emission in addition to the ligand emission peaks. However, in solution, the lanthanide-centred emissions of the complexes were overlapped with the ligand emissions. A weak antenna effect was observed only in the powder form of the complexes. The vibrational data calculated from the DFT [GGA (B3LY-D3)] optimized structures showed good agreement with the experimental results. The excitation and emission behaviour of the ligand and the complexes were established by molecular orbital analysis of the ground state as well as on the excited state optimized geometry at DFT level. The nature of bonding between the lanthanide ions and the 12N3Me5Ox(3-), interpreted by means of the natural bond orbital (NBO) and Morokuma Ziegler energy decomposition analysis (ETS-NOCV) scheme, suggest that the Ln-L bonds are more electrostatic (similar to 73%) than covalent (similar to 27%). The covalent character of the complexes increases n the order Tb > Eu. The Slater Condon parameters and spin-orbital coupling constants were calculated from LF-DFT (ligand field density functional theory) calculation. The theoretical Nephelauxetic parameter was also compared with experimental data. (C) 2019 Elsevier B.V. All rights reserved.