Dansyl-based fluorescent chemosensors for selective responses of Cr( iii )

Three chemosensors DN1, DN2 and DN3, incorporating dansyl dye as a fluorophore and various carboxhydrazones as ionophores, are reported. DN1 contains a pyridine-carboxhydrazone tridentate coordination site, forming a 2:1 stoichiometric complexation species with Cr3+, and exhibits selectivity for Cr3+ over alkali and alkaline earth metals and most first-row transition metals in aqueous media. DN1 displays high quantum yield (Φ = 0.86) and fluorescence enhancement following Cr3+ coordination within a wide pH range. DN2 contains a 8-hydroxyquinoline-carboxhydrazone tetradentate metal-binding moiety, forming a 1:1 complex with Cr3+. DN2 also exhibits significantly fluorescence enhancement but a much lower quantum yield following Cr3+ binding in aqueous solution. In contrast, their congener DN3, having a salicyl-carboxhydrazone tridentate binding site, does not exhibit any significant fluorescence variation following Cr3+ binding. These results demonstrate that the hydroxyl group is an important factor influencing the fluorescence response to Cr3+ in aqueous media.