pH dependent photophysical studies of new europium and terbium complexes of tripodal ligand: Experimental and semiempirical approach Rifat Akbar a , Minati Baral b , B K Kanungo a,n a Department of Chemistry, Sant Longowal Institute of Engineering & Technology, Longowal, Punjab 148106, India b Department of Chemistry, National Institute of Technology Kurukshetra, Haryana 136119, India article info Article history: Received 31 July 2014 Received in revised form 11 May 2015 Accepted 21 May 2015 Available online 10 June 2015 Keywords: Lanthanide Formation constant Fluorescence OFF–ON–OFF sensor Sparkle PM7 abstract The photophysical properties of adduct of a novel nonadentate tripodal ligand, 5,5′-(2-(((8-hydro- xyquinolin-5-yl)methylamino)methyl)-2-methylpropane-1,3-diyl)bis(azanediyl)bis(methylene diquino- lin-8-ol, (TAME5OX), with Eu 3 þ and Tb 3 þ 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 Ln 3 þ 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 phy- siological pH in competitive medium for both Eu 3 þ and Tb 3 þ 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 fluor- escence spectrophotometric method. TAME5OX has been found to form two protonated complexes [Ln(H 5 L)] 5 þ and [Ln(H 4 L)] 4 þ below pH 2.5 with both metal ions, which consecutively deprotonates through one proton process with rise of pH. The formation constants (log β 11n ) of neutral complexes have been determined to be 33.51 and 32.16 with pLn (pLn ¼log[Ln 3 þ ]) values of 16.14 and 19.48 for Eu 3 þ and Tb 3 þ ions, respectively, calculated at pH 7.4, indicating TAME5OX is a good lanthanide synthetic chelator. The emission lifetimes of the Eu 3 þ and Tb 3 þ complexes recorded in D 2 O and H 2 O 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. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Luminescence from lanthanides has attracted extensive inter- est, due to their fascinating optical properties and a host of applications, such as biomedical analysis, medical diagnosis, cell imaging, light-emitting diodes, and as ideal probes in the studies of biological systems [1–5]. These ions with their characteristic magnetic and spectral properties are the potential probes for the study of natural system. However, they exhibit low luminescence intensity due to Laporte forbidden f–f transitions [6], which are inefficient to populate lanthanide energy levels. One of the most useful strategies that has been employed to overcome this draw- back is the f states are trivially populated indirectly through coordinated organic ligands (antenna effect), which act as metal Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2015.05.038 0022-2313/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ91 1672 253166 (O), þ91 1672 253167 (R); fax: þ91 1672 280057. E-mail address: b.kanungo@gmail.com (B. Kanungo). Journal of Luminescence 167 (2015) 27–44