Unusual specificity of a receptor for Nd3+ among other lanthanide ions for selective colorimetric recognition

A rare example of a reversible recognition of Nd(3+) by a newly synthesized molecular receptor (L(1)), having a diazo group as the reporter functionality, is reported. Studies revealed that this receptor eventually forms a [Nd(3+)](2)L(1) type complex, through the formation of the intermediate complex [Nd(3+)] x L(1) following a two-step equilibrium process. Respective equilibrium constants for two successive processes were evaluated based on data obtained from the systematic fluorescence titration. Formation of [Nd(3+)](2)L(1) caused a detectable change in color, and associated affinity constants were also evaluated from spectrophotometric titration data. A rather unusual binding mode of L(1) for Nd(3+) ion is established by various spectroscopic studies. The remarkable specificity of L(1) for Nd(3+), constitutes a rare example of a highly selective receptor for this ion in the presence of excess of all other lanthanide ions.