Synthesis, Cation-Binding and Optical Spectral Studies of Photoemmitive Benzothiazole Crown Ethers

Crown ethers 1–4, encompassing a photoemittive benzothiazole chromophore have been synthesised using standard protocols. Alkali metal picrate extraction profiles reveal that compared to the known crown ethers, benzothiazole benzo-15-crown-5 1 and benzothiazole dibenzo-18-crown-6 3 exhibit relatively higher % extraction for K⁺ than Na⁺ ions. The UV-visible and fluorescence spectra of 1–4, in comparison to their neutral forms were found to be red shifted on protonation due to enhanced intramolecular charge transfer transition. The ortho-substituted benzothiazole crowns 2–4 showed higher Stokes shifts compared to the para analog 1 in the presence of CF₃CO₂H, presumably due to H-bond assisted conformational restriction. No changes were noticeable in the absorption spectra in the presence of alkali metal ions. Even, fluorescence properties of 1–4 were not found to be drastically perturbed by these ions. While 1 exhibited slight quenching at alkali metal ion concentration over 10-folds with respect to that of 1, interestingly, 2–4 showed a slight enhancement of fluorescent intensity at least up to 10-fold concentration of metal ions over those of 2–4. Further increase of metal ion concentrations produces quenching effects. This behavior has been tentatively explained by invoking electrostatic interaction between these cations and the benzothiazole nitrogen ligand.