Design, synthesis and photophysical properties of 8-hydroxyquinoline-functionalized tripodal molecular switch as a highly selective sequential pH sensor in aqueous solution

The development and photophysical properties of a biomimetic analogue of microbial siderophore from quinolobactin have been described. The putative analogue, 5,5'-(2(((8-hydroxyquinolin-5-yl)-methylamino) methyl) 2-methylpropane-1,3-diyl)-bis(azanediyl)-bis(methylene) diquinolin-8-ol (TAME5OX), was synthesized with three bidentate 8-hydroxyquinoline (8HQ) groups, connected to a 1,1,1-tris( aminomethyl) ethane framework, and was selected for its potential applications in chemical and biological fields. A combination of absorption and emission spectrophotometry, potentiometry, electrospray mass spectrometry, NMR, IR and theoretical investigation was used to fully characterize TAME5OX. The intense fluorescence from TAME5OX is quenched intermittently under acidic and basic conditions due to the photoinduced intramolecular electron transfer from excited N-pyridyl to hydroxyl moiety of 8HQ units. This renders the ligand an OFF-ON-OFF type of pH-dependent fluorescent sensor. DFT was employed for optimization and evaluation of vibrational modes, excitation and emission properties of the protonated, neutral, deprotonated states of the analogue. Anomalous enhancement observed in the fluorescence spectra of the neutral form of the sensor can be attributed to subtle structural differences from its cationic and anionic forms. Plausible explanations for low fluorescence of the acidic as well as basic form are provided.