The influence of europium(III) and terbium(III) on the electronic system of impudent tripodal ligand: Binding, spectrophotometric and theoretical investigations

The aqueous coordination chemistry of a novel hexadentate tripodal ligand, 5-[[3-[(8-hydroxy-5-quinolyl)methoxy]-2-[(8-hydroxy-5-quinolyl)methoxymethyl]-2-methyl propoxy]methyl]quinolin-8-ol (TMOM5OX) towards Eu3+ and Tb3+ metal ions have been investigated by potentiometric, UV–vis and fluorescence spectrophotometric methods. TMOM5OX has been found to form two protonated complexes [Eu(H5L)]5+ and [Tb(H4L)]4+ below pH 2.3, which consecutively deprotonates through one and two-proton processes with rise of pH. The formation constants (log β11n) of neutral complexes have been determined to be 35.24 and 35.23, and the pLn (pLn = −log[Ln3+]) values of 16.63 and 18.57 for Eu3+ and Tb3+ ions, respectively, calculated at pH 7.4, indicating TMOM5OX is a good lanthanide synthetic chelator. The photophysical properties of novel quinoline-lanthanide systems were also scrutinized for photonics applications. The absorption spectroscopy of these complexes shows marked spectral variations due to characteristic lanthanide transitions, which support the use of TMOM5OX 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 Eu(III) and Tb(III) ions. The intense fluorescence from these compounds were revealed to intermittently get quenched under acidic and 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 complexes coordination geometries were optimized using the sparkle/PM6 model and the theoretical spectrophotometric studies were carried out in order to validate the experimental findings, based on ZINDO/S methodology at configuration interaction with single excitations (CIS) level. These results clearly attest for the efficacy of the theoretical models employed in all calculations and create new interesting possibilities for the design in silico of novel and highly efficient lanthanide-organic frameworks.