Benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxylic acid based fluorescent sensors for pH and Fe3+

• We synthesized two new probers which have similar structures, however their optical properties are very great differences.
• The structure of the sensor L1 is original for the detection of pH and Fe3+ simultaneously.
• A new structure formed from compound L2 upon acidification and result in the generation of the isosbestic point.
• We have proved the structure change of L2 upon acidification by absorption spectra, 1H NMR and DFT/TDDFT calculation.

Photophysical properties of two new naphthalimide derivatives, 1-benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxyl-4-pyridin-2-ylpiperazine (L1) and 1-benzimidazo[2,1-a]benz[de]isoquinoline-7-one-12-carboxyl-4-pyridin-4-ylpiperazine (L2), as sensors for pH and Fe3+ were investigated. L1 and L2 could act as reversible switches by repeated protonation and deprotonation due to the photoinduced electron transfer (PET) between the fluorophore (benzimidazo[2,1-a]benz[de]isoquinoline-7-one) and the donor (pyridine). The sigmoid responses cover the acidic pH range from 5.80 to 1.80 of L1 and from 9.10 to 4.50 of L2. The protonation of L1 and L2 induced more than 110-fold and 20-fold increase of fluorescence intensity and the corresponding pKa are 3.52 and 6.88, respectively. In addition, the sensor L1 was also employed as a turn-on fluorescent probe for the detection of Fe3+ in Tris–HCl buffer solution and Kd was obtained as 9.3 × 10−5. Additionally, the detection limit of L1 for Fe3+ reached 2.5 × 10−6 M. The mechanism of fluorescence sensing as well as the geometrical structure and electronic structure of the sensors were rationalized using DFT/TDDFT calculations.

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