Detection of Molecular Chirality in Lanthanide Complexes by the Raman Optical Activity

Monika Krupová^+*, Valery Andrushchenko^*, and Petr Bouř^*

* Institute of Organic Chemistry and Biochemistry AV CR, Flemingovo nám. 2, Prague, Czech Republic

+ Charles University in Prague, Faculty of Sciences, Albertov 6, Prague, Czech Republic

Since conventional structural analysis offers rather limited means for the chirality detection, a series of lanthanide tris-(β-diketonates) was developed as effective receptors for a better chirality sensing in biomolecular substrates. These lanthanide complexes containing β-diketonate ligands are electrically neutral; they can further coordinate with various small organic molecules such as chiral alcohols, amino alcohols or amino acids in organic solvents and produce a strong chiral signal.

In the current study, interaction of the [Eu(FOD)₃] complex with (R)- and (S)- enantiomer of 1-PhenylEthanol in n-hexane was studied using Raman Optical Activity (ROA) and ultraviolet circular dichroism (UVCD) spectroscopies. Since resonance phenomenon in Raman scattering was observed in the studied systems due to the correspondence of Europium electronic transition energy to the laser excitation wavelength, about a 10²-fold enhancement if compared to non-resonant vibrational ROA. This enabled shorter detection times as well as lower sample concentrations.

The UVCD was used to support the results obtained by ROA measurements and determine the metal/ligand ratio in the complexes. Although lanthanide tris-(β-diketonates) are silent in the UVCD spectra, symmetric UVCD signals induced around 300 nm were observed for [Eu(FOD)₃] upon complexation with (R)- or (S)-1-PhenylEthanol. Further experiments with structural variations of lanthanide tris-(β-diketonate) complexes and theoretical modeling can further refine the chirality sensing systems for chiral recognition in biological substrates.