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 pKa 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.

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