Lanthanide triple-stranded helical complexes with a substituted 2,6-pyridinedicarboxylateElectronic supplementary information (ESI) available: analytical, IR and luminescence data, contact and dipolar shifts, calculated Cartesian co-ordinate for the EuIII complex, plots of chemical shifts for protons H8, H9 versus the chemical shift of proton H2. See http://www.rsc.org/suppdata/dt/b1/b104448f/

Abstract

A 4-aminophenylethynyl substituent has been grafted into the 4-position of the pyridine ring in dipicolinic acid to yield ligand L4 the pKa's of which are 8.77(5), 3.54(3), 2.58(1) and 1.01(3). Ligand L4 reacts with trivalent lanthanide ions to give stable triple-stranded helical complexes in water (logβ13 = 13.7(1) for Eu). The separation of the contact and pseudocontact contributions to the NMR paramagnetic shifts according to the crystal field independent method points to a single axial structure for [Ln(L4 − 2H)3]3− along the series Nd–Yb. The solution structure generated by using the dipolar shifts compares well with that reported for the EuIII tris(dipicolinate), with a co-ordination polyhedron which can be described as a distorted tricapped trigonal prism. This is confirmed by a photophysical study of the EuIII complex both in solution and in the solid state. The triplet state of co-ordinated L4 lies at low energy (around 18 000 cm−1 in water) resulting in a relatively poor sensitisation of EuIII (quantum yield ca. 0.1% in methanol, lower in water) but in a sizable energy transfer to YbIII.