Zn2+ selective luminescent ‘off–on’ probes derived from diaryl oxadiazole and aza-15-crown-5

New photo-induced electron transfer (PET) probes OMOX and OBOX, carrying an additional binding site in the form of ‘oxadiazole nitrogen’ have been designed to evaluate binding interactions with biologically significant Li+, Na+, K+, Ca2+, Mg2+, and Zn2+ including environmentally toxic Ba2+ and Cd2+ using optical spectral techniques. While Li+, Na+, and K+ did not appreciably perturb either the absorption or emission spectra, Ba2+, Ca2+, Mg2+, Zn2+, and Cd2+ induced slight red shifts (2–8 nm) in the UV–visible spectra as well as pronounced chelation induced enhanced fluorescence (CHEF). Both OMOX and OBOX exhibited the highest CHEF in contact with the zinc ion, whereas Ba2+, Ca2+, Mg2+, and Cd2+ induced relatively less emission enhancements. OBOX, which is a poorer emitter (Φf=0.0062) than OMOX (Φf=0.015), showed highly promising 160-fold emission enhancement in the presence of Zn2+. Potential, therefore is available in OBOX to function as a selective luminescent ‘off–on’ sensor for Zn2+ in the presence of coordinatively competing Ba2+, Ca2+, Mg2+, and Cd2+ ions.