Charge transfer aided selective sensing and capture of picric acid by triphenylbenzenes

A fluorescent chemo-sensor, 1,3,5-tris(4'-(N,N-dimethylamino)phenyl)benzene was synthesized by substituting the N-H protons of 1,3,5-tris(4'-aminophenyl)benzene with methyl groups. The chemo-sensor shows highly selective and remarkable fluorescence quenching in the presence of picric acid with a detection limit of 1.5 ppm. The origin of the selectivity was investigated using absorption, fluorescence emission and H-1 NMR spectroscopic techniques. The solid state structure of 1,3,5-tris(4'-(N,N-dimethylamino)phenyl)benzene and its picric acid complex reveals multiple hydrogen bonds (N-H center dot center dot center dot O and C-H center dot center dot center dot O), pi-pi interactions and electrostatic interactions between 1,3,5-tris(4'-(N,N-dimethylamino) phenyl) benzene and picric acid. The proton transfer process from picric acid to 1,3,5-tris(4'-(N,N-dimethylamino)phenyl)benzene results in the formation of picrate anions and the triply protonated 1,3,5-tris(4'-(N,N-dimethylamino)phenyl)benzene species containing dimethylammonium (-NHMe2+) groups.