An AIE-active tetraphenylethylene-based cyclic urea as a selective and efficient optical and colorimetric chemosensor for fluoride ions

Abstract

Anion sensing is a very challenging and important issue in many diverse fields, including biological, medical and environmental sciences. In this paper, we report the serendipitous synthesis of a tetraphenylethylene (TPE) bearing a cyclic urea (coded as: 1 ). This easy-to-make, simple chemosensor shows high selectivity towards fluoride (F sup(-)) ions. Importantly, this work demonstrates two important factors: firstly, the use of cyclic urea for ion sensing and secondly, the use of aggregation induced emission (AIE)- TPE signalling units. This selective fluoride sensing can be observed by the naked eye, UV-vis absorption, fluorescence spectroscopy, and sup(1)H-NMR spectroscopy. Solutions of sensor 1 are fluorescent in DMSO with fluorescence quantum yield Phi=1.3 and upon addition of fluoride ions the emission is quenched with Phi sub(F)=0.07. Notably, other anions, such as CN sup(-), AcO sup(-), NO sub(3) sup(-), H sub(2)PO sub(4) sup(-), HSO sub(4) sup(-), I sup(-), OH sup(-), ClO sub(4) sup(-), SCN sup(-) and Cl sup(-), do not show any quenching effect. The binding constant (K) between sensor 1 and fluoride was found to be 4.90x10 sup(8) M sup(-2) with a detection limit of 30 Mu M. We also studied AIE-activity of 1 in THF/water mixture. As such the fluorescence quantum yield (Phi F ) of 1 in pure THF solution shown to be 0.05 percent which was enhanced 22-fold to 1.98 and 6.65 percent at f sub(w)=90 and 99 percent, respectively. This is clear confirmation that compound 1 is AIE active due to decreased solubility and increased molecular self-assembly as the water fraction is increased.