Solvent Dependent Fluorescent Properties of a 1,2,3-Triazole Linked 8-Hydroxyquinoline Chemosensor: Tunable Detection from Zinc(II) to Iron(III) in the CH3CN/H2O System

A triazole-containing 8-hydroxyquinoline (8-HQ) ether 2 was efficiently synthesized in two steps from the "click" strategy. Compound 2 gave a strong fluorescence (Phi = 0.21) in nonprotic solvent like CH3CN, and a weak fluorescence (Phi = 0.06) in protic solvent like water. In water, a more than 100 nm red shift of the fluorescence maximum was observed for compound 2 in comparison with that in CH3CN. This fluorescence difference may be attributed to the intermolecular photoinduced proton transfer (PPT) process involving the protic solvent water molecules. Similarly, this intermolecular PPT process was also observed in the high-water-content CH3CN aqueous solution (e.g., CH3CN/H2O = 5/95; v/v); The water content in the CH3CN/H2O binary solvent mixture greatly affected the fluorescence intensity (e.g., Phi = 0.06 and 0.25 when CH3CN/H2O = 5/95 and 95/5, v/v, respectively) and emission wavelength. Using this interesting property, by simple variation of the water content in the CH3CN aqueous solution, compound 2 was tuned from a selective "turn-on" fluorescent sensor for Zn2+ (CH3CN/H2O = 5/95, v/v) to a ratiometric one for Zn2+ and a selective "turn-off" one for Fe3+ (CH3CN/H2O = 95/5, v/v) over a wide range of pH value. In high-water-content (CH3CN/H2O = 5/95, v/v) aqueous solution compound 2 shows a selective "turn-on" response toward Zn2+, with a 10-fold enhancement in the fluorescence intensity at 428 nm and a 62 nm blue shift of the emission maximum (490 to 428 nm) due to the inhibition of intermolecular PPT process upon chelating with Zn2+. However, in a less polar solvent (CH3CN/H2O = 95/5, v/v) in which compound 2 has high fluorescence (quantum yield =0.25), it shows a ratiometric response toward Zn2+, with a continuous decrease of the fluorescence intensity at 399 nm and an increase at 423 nm. More interestingly, in this case, it also exhibits a very sensitive, selective, and ratiometric fluorescence quenching in the presence of Fe3+, with an 81 nm red shift of the emission maximum (399 to 480 nm) in a wide range of pH through a metal ligand charge transfer (MLCT) effect.