Design of luminescent building blocks for supramolecular triple-helical lanthanide complexes

Abstract

The ligand 2,6-bis(1′-ethyl-5′-methylbenzimidazol-2′-yl)pyridine (L⁵) reacts with lanthanide perchlorate in acetonitrile to give the mononuclear triple-helical complexes [Ln(L⁵)₃]³⁺(Ln = Eu, Gd or Tb). The crystal structure of [Eu(L⁵)₃][ClO₄]₃·4MeCN has been determined, which shows three unco-ordinated perchlorate anions and an [Eu(L⁵)₃]³⁺ cation where the three tridentate ligands are wrapped around a pseudo-C₃ axis. The co-ordination sphere around Eu^III may be best described as a slightly distorted trigonal-tricapped prism where the six benzimidazole nitrogen atoms occupy the vertices of the prism and the three pyridine nitrogen atoms occupy the capping positions. A detailed geometrical analysis showed that the ethyl groups in L⁵ produce a slide of the strands which is responsible for the distortion of the triple-helical structure as exemplified by the low symmetry for the Eu^III site in the luminescence spectra of [Eu(L⁵)₃]³⁺. Proton NMR spectra in acetonitrile indicate that the triple-helical structure is maintained for [Ln(Lⁱ)₃]³⁺{Ln = Eu or Tb; L = 2,6-bis(1′-R-benzimidazol-2′-yl)pyridine [R = Me L¹, Et L², Pr L³ or CH₂C₆H₃(OMe)₂-3,5 L⁴] or L⁵} on the NMR time-scale, but the stability of the complexes together with the structural arrangement of the ligands depend on the size of the substituents bound to the benzimidazole nitrogen atoms. Photophysical studies of [Eu(Lⁱ)₃]³⁺ show that these steric effects affect the quantum yield in solution and that methyl groups bound to the 5 positions of the benzimidazole rings in L⁵ shift the π→π* transitions centred on the ligand, but do not strongly modify the emission properties of [Eu(L⁵)₃]³⁺. Extended Hückel calculations give a qualitative insight into the factor controlling the π→π* transitions of the ligands and complexes.