Naphthaldehyde-Urea/Thiourea Conjugates as Turn-On Fluorescent Probes for Al3+ Based on Restricted C=N Isomerization

We have synthesized naphthaldehydecarbonohydrazone (NC) and naphthaldehydethiocarbonohydrazone (NTC) Schiff base ligands by the condensation reaction of 2-hydroxy naphthaldehyde with urea and thiourea, respectively. Both these Schiff base ligands show blue turn-on fluorescence in the presence of aluminum(III), as a result of a restricted C=N isomerization mechanism. Aluminum(III) can be detected in the presence of most of the competing metal ions by using NC and NTC. The association constant (log?Ka) of NC and NTC for Al3+ in acetonitrile was calculated to be 14.5 and 14.1, respectively.