Specific chiral sensing of amino acids using induced circularly polarized luminescence of bis(diimine)dicarboxylic acid europium(III) complexes

The circularly polarized luminescence (CPL) from [Eu(pda)₂] ^- (pda = 1,10-phenanthroline-2,9-dicarboxylic acid) and [Eu(bda) ₂]^- (bda = 2,2-bipyridine-6,6-dicarboxylic acid) in aqueous solutions containing various amino acids was investigated. The europium(III) complexes exhibited bright-red luminescence assignable to the f-f transition of the Eu^III ion when irradiated with UV light. Although the luminescence was not circularly polarized in the solid state or in aqueous solutions, in accordance with the achiral crystal structure, the complexes exhibited detectable induced CPL (iCPL) in aqueous solutions containing chiral amino acids. In the presence of l-pyrrolidonecarboxylic acid, both [Eu(pda) ₂]^- and [Eu(bda)₂]^- showed similar iCPL intensity (g_lum ∼ 0.03 for the ⁵D₀ → ⁷F₁ transition at 1 mol·dm^-3 of the amino acid). On the other hand, in the presence of l-histidine or l-arginine, [Eu(pda)₂]^- exhibited intense CPL (g _lum ∼ 0.08 for the ⁵D₀ → ⁷F₁ transition at 0.10 mol·dm^-3 of the amino acid), whereas quite weak CPL was observed for [Eu(bda)₂] ^- under the same conditions (g_lum < 0.01). On the basis of analysis of the iCPL intensities in the presence of 12 amino acids, [Eu(pda)₂]^- was found to be a good chiral CPL probe with high sensitivity (about 10^-2 mol·dm^-3) and high selectivity for l-histidine at pH 3 and for l-arginine at pH 7. The mechanism of iCPL was evaluated by analysis of the fine structures in the luminescence spectra and the amino acid concentration dependence of g_lum. For the [Eu(pda)₂]^--histidine/arginine systems, the europium(III) complexes possess coordination structures similar to that in the crystal with slight distortion to form a chiral structure due to specific interaction with two zwitterionic amino acids. This mechanism was in stark contrast to that of the europium(III) complex-pyrrolidonecarboxylic acid system in which one amino acid coordinates to the Eu^III ion to yield an achiral coordination structure.