New “Off-On” Responsible Fluoroionophores for Alkaline Earth Metal Ions Based on Benzo-15-crown-5 Bearing 9-Phenanthreneacetamide

The photochemical behaviors of a series of 9-phenanthreneacetamide (PA) derivatives were investigated using UV, fluorescence, and 1H-NMR spectroscopy in acetonitrile solutions. To apply the electron transfer (ET) action of PA as a detecting moiety of chemosensors, 4′-(9-phenanthreneacetamido)-benzo-15-crown-5 (1) and 3′-methyl-4′-(9-phenanthreneacetamido)-benzo-15-crown-5 (2) were synthesized. After metal ion complex formation, the fluorescence intensity increased with increased concentration of the guest alkaline earth metal ions. The photochemical responses of the fluorescence intensity, as defined by the fluorescence intensity ratio (Imax/I0) between free and complex of 1 for Ca2+, was determined to be 4.76. However, the addition of guest ions in a solution of 2 greatly enhanced the fluorescence intensity of 2. The Imax/I0 of 2 was 16.6 for Ca2+. The efficiency of ET of PA can be tuned by adding an electron-donating group onto a suitable position; fluorescent 2 was able to read out metal ions as an “Off-On” fluorescence signal.