Hydrogen Peroxide Sensors Based on Fluorescence Quenching of the 2-AminobenzimidazoleFluorophore

The fluorescence emission of the parent 2-aminobenzimidazole (ABZ, 1), the mono- and disubstituted derivatives (2, 3), 2-aminonaphthoimidazole (4), and 4-amino dinaphthodiazepine 5 (λ_em = 315-400 nm) is strongly quenched in the presence of aqueous hydrogen peroxide. The quenching process is dual: for diazepine 5, quenching is dynamic at lower H₂O₂ concentrations with linear reduction of the fluorescence lifetime from 4.3 to 2.6 ns. At higher H₂O₂ concentrations, a second species appears in the absorption and emission spectra with fluorescence lifetimes of 1.3 ns, indicating the formation of a new (ground-state) hydrogen-bonded ABZ-H₂O₂ complex (static quenching). Sensors 1 and 2 show also dual quenching that fits with a static 1:1 and 1:2 model with K_1:1 = 8(11) M^-1 and K_1:2 = 21(147) M^-1 for 1(2). The formation of a 1:2 complex (1:(H₂O₂)₂) is also supported by density functional theory (DFT) calculations and spectra simulations.