Novel fluorescent sensors based on benzimidazo [2,1-a]benz[de]isoquinoline-7-one-12-carboxylic acid for Cu²⁺

Fluorescent sensors of N-(4-methylpiperazin-1-yl)benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylamide (C1) and its corresponding quaternary ammonium salt (C2) were prepared. C1 showed 45-fold fluorescence turn-on response towards Cu²⁺ with a detection limit of 5.7 × 10^-8 mol L^-1 in acetonitrile-H₂O (9:1) buffer solution and C2 showed 18-fold fluorescence enhancement towards Cu²⁺ with a detection limit of 3.4 × 10^-7 mol L^-1 in the same condition. The Cu²⁺ sensing of C1 and C2 were both based on the photoinduced electron transfer (PET) process. Such behaviors confirmed that the benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylic acid based C1 and C2 could be utilized as fluorescent sensors for Cu²⁺. The mechanism of fluorescence enhancement of C1 towards Cu²⁺ was verified by DFT/TDDFT calculation using Gaussian 03. In addition, an obvious color change was observed when an aqueous solution of C2 was treated with aqueous NaOH. Such behavior confirms that C2 could be used as a fluorescent OH^- sensor in water.