Cholesterol appended pyridinium ureas: a case of gel making and breaking for selective visual readout of F−

Cholesterol appended pyridinium ureas 1 and 2 have been designed and synthesized. The unsymmetrical urea-based chemosensor 1 fluorimetrically distinguishes F(-) from the other anions examined in both CH(3)CN and DMSO with appreciable binding constant values. The pyridinium - based symmetrical compound 2 acts as a low molecular weight gelator (LMWG) in CHCl(3) and is capable of detecting F(-) visually by breaking the gel. On the contrary, the chemosensor 1 in DMSO in the presence of tetrabutylammonium fluoride undergoes a change from sol to gel state that produces an unambiguous visual readout of F(-).