PH dependent photophysical studies of new europium and terbium complexes of tripodal ligand: Experimental and semiempirical approach

Abstract

The photophysical properties of adduct of a novel nonadentate tripodal ligand, 5,5′-(2-(((8-hydroxyquinolin-5-yl)methylamino)methyl)-2-methylpropane-1,3-diyl)bis(azanediyl)bis(methylene diquinolin-8-ol, (TAME5OX), with Eu3+ and Tb3+ metal ions have been probed for photonics applications. The absorption spectroscopy of these complexes show remarkable spectral changes due to characteristic lanthanide transitions, which support the use of TAME5OX as a sensitive optical pH based sensor to detect Ln3+ metal ions in biological systems. In addition, these complexes have also been shown to exhibit strong green fluorescence allowing simultaneous sensing within the visible region under physiological pH in competitive medium for both Eu3+ and Tb3+ ions. The intense fluorescence from these compounds were revealed to intermittently get quenched under acidic as well as basic conditions due to the photoinduced intramolecular electron transfer from excited 8-hydroxyquinoline (8-HQ) moiety to metal ion, just an opposite process. This renders these compounds the OFF–ON–OFF type of pH-dependent fluorescent sensor. The thermodynamic stability and aqueous coordination chemistry of the chelator with the said lanthanide ions have also been probed by potentiometric, UV–visible and fluorescence spectrophotometric method. TAME5OX has been found to form two protonated complexes [Ln(H5L)]5+ and [Ln(H4L)]4+ below pH 2.5 with both metal ions, which consecutively deprotonates through one proton process with rise of pH. The formation constants (logβ11n) of neutral complexes have been determined to be 33.51 and 32.16 with pLn (pLn=−log[Ln3+]) values of 16.14 and 19.48 for Eu3+ and Tb3+ ions, respectively, calculated at pH 7.4, indicating TAME5OX is a good lanthanide synthetic chelator. The emission lifetimes of the Eu3+ and Tb3+ complexes recorded in D2O and H2O suggest the presence of water molecules in the first coordination sphere of the metal ions. NMR titrations were carried out to determine the stoichiometry of chelates. The complexe's coordination geometries were optimized by using PM7 as sparkle/PM7 model. The theoretical electronic behavior was evaluated to support the experimental findings, based on ZINDO/S methodology at configuration interaction with single excitations (CIS) level. These results emphasize the capability of the use of the theoretical models in prediction of geometries and all other calculations of compounds containing lanthanide ions and create new interesting possibilities for the design in-silico of novel and highly efficient lanthanide–organic edifice.