The application of chiroptical spectroscopy (circular dichroism) in quantifying binding events in lanthanide directed synthesis of chiral luminescent self-assembly structures

Abstract

The binding of asymmetrical and optically pure tridentate ligands (L = 1(S) and 1(R)) containing one carboxylic group and 2-naphthyl as an antenna to lanthanide ions (M = La(III) and Eu(III)) was studied in CH₃CN, showing the successive formation of M:L, M:L₂ and M:L₃ stoichiometric species in solution. The europium complexes EuL₃ were also synthesised, structurally characterised and their photophysical properties probed in CH₃OH and CH₃CN. The changes in the chiroptical properties of both 1(S) and 1(R) were used (by circular dichroism (CD) spectroscopy) to monitor the formation of these chiral self-assemblies in solution. While circularly polarised luminescence (CPL) showed the formation of Eu(1(S))₃ and Eu(1(R))₃ as enantiomers, with high luminescence dissymmetry factors (g_lum), fitting the CD changes allowed for binding constants to be determined that were comparable to those seen in the analyses of absorbance and luminescence changes.