Phenanthroimidazole derivatives as a chemosensor for picric acid: a first realistic approach.

A series of phenanthroimidazole (PI) derivatives (M1–M3): 2-phenyl-1H-phenanthro [9,10-d]imidazole (M1), 2-anthryl-1H-phenanthro[9,10-d]imidazole (M2), and 2-pyrenyl-1H-phenanthro[9,10-d]imidazole (M3) were synthesized and characterized using various spectroscopic techniques. The fluorescence quenching studies or sensing properties of the molecules M1–M3 with picric acid (PA) were studied. The molecules (M1–M3) exhibit strong fluorescence with a peak emission wavelength at 386, 486 and 456 nm. This emission gets quenched with the addition of PA. The mechanism of fluorescence quenching involves proton transfer from PA to M in the ground state, followed by the formation of a complex between the cation (M⁺) and picrate anion (PA⁻). The proton transfer breaks the conjugation inside the imidazole ring which in turn leads to loss of fluorescence. The proof of proton transfer was evident from the single X-ray crystal structure of the co-crystals (M1⁺–PA⁻ and M3⁺–PA⁻) and their Hirshfeld surface analysis. Time-resolved fluorescence measurements reveal that quenching is static. The complex formation constant (K_s) and life-time for all the molecules are measured. [ABSTRACT FROM AUTHOR]