Selective receptor for Fe(III) ion with a fluorescence-ON pyrene motif in semi-aqueous solution

A novel fluorescent sensor for the selective and sensitive detection of Fe3+ based on fluorescent enhancement has been developed in the present study. The sensor 3-(4-bromo-2-hydroxyphenyl)-1-(pyren-1-yl)prop-2-en-1-one (ABS) was synthesized by Claisen-Schmidt reaction between acetyl pyrene and 5-bromosalicylaldehyde. It was characterized by FT-IR, HR-MS, H-1 and C-13 NMR. The sensor exhibited weak fluorescence due to C = C isomerization and intramolecular charge transfer (ICT) process but upon binding with Fe (3+) it exhibited increase in fluorescence due to blockage of ICT and C = C isomerization. While adding above 50 mu M Fe3+, the probe exhibited decrease in fluorescence, as against the saturation curve. The decrease was ascribed to ligand to metal charge transfer (LMCT). This phenomenon of electron transfer from the fluorophore to the bound metal ion resulted in decrease of the fluorescence via non-radiative energy transfer. The binding constant was found to be 2 x 10(3) M-1. The binding mechanism was established based on theoretical calculations whereas the fluorescence enhancement and decrease were confirmed by quantum yield and life-time studies. The binding mode of ABS-Fe3+ complex was ascertained by using H-1 NMR and DFT studies. When increasing Fe3+ concentration from 50 nM to 1 mu M, the emission intensity was linearly increased against the concentration of Fe3+ with a correlation coefficient of 0.999 and LOD was found to be 11.2 nM (3 sigma/S). Finally, the probe was successfully employed to determine Fe3+ in real samples and cell imaging studies.