Novel fluorescent and colorimetric pH sensors derived from benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylic acid

Fluorescent sensors N-[2-(dimethylamino)ethyl]benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylamide (L1) and its corresponding quaternary ammonium salt L2 were prepared. Probe L1 exhibited fluorescence 'turn-on' response toward H⁺ in acetonitrile/H₂O (9:1) solution based on photoinduced electron transfer (PET) process. The fluorescence intensity of L1 gradually increased about 45-fold from pH 9.2 to 4.2 with a pK_a value of 6.67. L2 exhibited colorimetric response toward OH^- in water. The solution color of L2 changed from yellow to colorless under visible light and changed from yellow to blue under UV light from pH 8.9 to 11.5. ¹H NMR spectroscopy studies confirmed hydration of the carbonyl group (CO) in fluorophore of L2 occurred under basic condition. The reason of fluorescence enhancement of L1 toward H⁺ and color changes of L2 toward OH^- were demonstrated by density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations with 6-31G(d) basis sets using a suite of Gaussian 03 programs.