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pH and metal sensing properties of benzimidazole based Schiff bases

Intramolecular charge-transfer (ICT) organic systems based on electron donor-π-electron acceptor structures (D-π-A) are widely used as fluorescent chemosensors. Benzimidazole moiety is an attractive building block of such molecular structures due to multifunctionality of its heteroaromatic conjugated planar structure. Schiff bases are one of the most investigated classes of organic compounds as a result of their excellent chelating ability. They are frequently described as optical chemical sensors [1] or chemodosimeters [2] due to their desirable optical properties, where benzimidazole is often incorporated as a functional or electron accepting unit in a respective D-π-A system [3]. Benzimidazole based Schiff bases exhibit structural similarity with biomolecules which offers an interesting platform for development of novel optically responsive sensor molecules. The interactions of benzimdazole based Schiff bases 1-3 with different metal ions and effects of solvents and pH on their response have been studied by absorption and fluorescence spectroscopies. Basic photophysical properties are described in different solvents. Schiff bases exhibited naked-eye visible colour change upon protonation and pKa values around 4. The pKa is significantly increased upon immobilisation in bulk membrane (apparent pKa = 6.7). All compounds indicated good sensing properties for metal ions. Compound 2 is presented as colourimetric sensor for Al3+ and fluorimetric sensor for Cu2+ (Fig 1.) ion.

benzimidazoles ; Schiff bases ; chemosensors ; pH sensors

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