Abstract
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.