Speciation and Luminescence of a Binuclear Lanthanide Complex Bearing an Aminophenolate Chromophore

Lanthanide(III) complexes show diverse speciation in solution. Here, the photophysical properties and speciation in solution of binuclear lanthanide complexes bearing an aminophenolate chromophore that reversibly bind to both metal centers were determined. The aminophenolate can reversibly bind to both lanthanide ions in a pH dependent manner. Above pH 3 the phenolate moiety remains deprotonated and the lanthanide complexes exhibit very high kinetic stability, however, protonation of the amino group modulates the luminescence intensity without changing the luminescence lifetime of the lanthanide. Below pH 3, protonation of the phenol results in a flexible, open structure, leading to reversible demetallation of the lanthanide complex as the system comes under thermodynamic control. This study highlights the importance of a detailed understanding of both the speciation and pH responsive behavior of highly luminescent binuclear lanthanide complexes for their development as responsive probes. These results confirm the promise of phenolate derived binuclear complexes as highly luminescent responsive probes, modulated by changes in pH.