Aza-crown ether tethered with benzothiazole : Synthesis and optical spectral studies

Synthesis of a new fluoroionophore 2 encompassing azaphenyl-crown and benzothiazole is described. The UV-Vis spectra ol'2 are not altered with change in solvent polarity indicating the absence of solvalochromism. However, the fluorescence spectra are progressively red shifted in more polar solvents on account of increasing charge transfer character of the excited state. In the presence of trifluoroacetic acid (TFAA), protonation of benzothiazole nitrogen results in the red shift of the absorption spectra. Contrary to expectation, the emission spectra suffers blue shift in the presence of TFAA. Presumably, the emission for the case 2+H + occurs from higher vibrational energy levels which accounts for the blue shift. Unfortunately, selected alkali or alkaline earth metals ions examined for metal sensing applications have failed to induce significant perturbations either in the absorption or emission spectra. This may be attributed to poor electronic communication between the crown-bound metal ions and the benzothiazole fluorophore due to steric encumbrance between the ortho-substitutcd benzothiazole chromophore and the phenyl aza-crown moiety.