AIE-active ferrocene conjugated imidazole fluorescent chromophores for “turn on/off” detection of hydrogen peroxide in mixed aqueous media and bio-imaging applications

ABSTRACT We have synthesized ferrocene-appended Y-shaped methoxyphenyl-substituted imidazole derivatives 2-ferrocenyl-4,5-bis(4-methoxyphenyl)-1H-imidazole ( 1 ) and 2-ferrocenyl-4,5-bis((E)-4-methoxystyryl)-1H-imidazole ( 2 ), and characterized them using analytical and spectroscopic techniques ( 1 H, 13 C NMR, FT-IR, and HR-Mass). Optical properties, including absorption, emission, quantum yield, and aggregation-induced emission (AIE), were demonstrated. The emission, ranging from weak to enhanced, was achieved through the AIE process in CH 3 CN/H 2 O mixtures by restricting intramolecular rotation (RIR). In the aggregated state, the quantum efficiency of chromophores 1 and 2 increases threefold and twofold, respectively, compared to the pure CH 3 CN solution. Utilizing the AIE state, these chromophores were further employed to detect hydrogen peroxide (H 2 O 2 ) in a mixed aqueous medium via a fluorescence turn-on/off approach. This was conducted with water fractions ( f w ) of 80% for chromophore 1 and 70% for chromophore 2 , revealing a highly specific and sensitive fluorescence quenching response to H 2 O 2 in the aqueous mixture. The response is rapid within a linear range of 0-110 µM and 0-80 µM, with detection thresholds of 24.7 nM for chromophore 1 and 30.9 nM for chromophore 2 , respectively. The limited detection capability is attributed to the absence of an alkene group in the chromophore, leading to low sensitivity due to the chromophore's high molecular rigidity, which impedes molecular motion. Moreover, chromophores 1 and 2 are non-cytotoxic (cell viability above 80%), enabling them to detect intracellular H 2 O 2 in human cervical carcinoma (HeLa) cells through fluorescence bio-imaging. Notably, the significant bright green fluorescence is quenched upon the addition of H 2 O 2 . Additionally, DFT/B3LYP calculations were used to explore the HOMO and LUMO energy levels and charge distribution of the optimized structure. The calculated HOMO and LUMO energies were compared with the experimentally obtained redox potential values.