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

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 pK_a 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 K_d 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.