Fluorescent benzimidazo[1,2-a]quinolines: synthesis, spectroscopic and computational studies of protonation equilibria and metal ion sensitivity†
We describe the UV-Vis and fluorescence spectroscopic characterization of newly synthesised amino and diamino substituted benzimidazo[1,2-a]quinolines and their various 1 : 1 metal complexes together with the related computational analysis of their acid/base properties and metal binding affinities. The work was performed in order to evaluate the photophysical features of these compounds and assess their potential chemosensor activity towards pH and metal ions in several polar and non-polar organic solvents. In addition, pH titrations and titration with metal chloride salts were carried out to determine the selectivity towards Co2+, Cu2+, Ni2+ and Zn2+ cations and explore their potential as chemosensors and pH probes. While all systems exhibit notable but impractical differences in sensitivities and spectral responses with the used metals, computational analysis aided in identifying benzimidazo[1,2-a]quinolines mono- and disubstituted with the piperazine fragment as very promising and efficient pH sensors in the acidic environment, particularly in the range of pH ≈ 3, which is extended to pH ≈ 6 upon the addition of metal cations. Their analytical features are significantly better than those involving the chain amino substituents, and their further development is strongly suggested.