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

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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 EuIII 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 EuIII 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.