Effect of a halogenide substituent on the stability and photophysical properties of lanthanide triple-stranded helicates with ditopic ligands derived from bis(benzimidazolyl)pyridine

Abstract

Bis{1-ethyl-2-[6-(N,N-diethylcarbamoyl)-4-halogenopyridin-2-yl]benzimidazol-5-yl}methane (halogeno = chloro, L^E; bromo, L^F) have been synthesized as ditopic receptors for the development of lanthanide-containing helicates able to couple with biological material and to test the influence of the halogeno substituent on the wrapping process, the structure of the resulting dimetallic edifices, and the photophysical properties of the encapsulated ions. The stability of the [Eu₂(L)₃]⁶⁺ helicates, as determined by NMR competitive titrations, decreases by respectively one (L^F) and three (L^E) orders of magnitude compared to the value found for the unsubstituted ligand (L^B) although it remains large, log β₂₃ = 23.8 (L^F) and 21.8 (L^E) in acetonitrile. The [Ln₂(L^E)₃]⁶⁺ helicates are shown to be isostructural in acetonitrile over the lanthanide series (Pr to Yb) and the crystal structure of [Tb₂(L^B)₃]⁶⁺ appears to be a good model for their solution structure, as demonstrated by paramagnetic NMR measurements (lanthanide induced shift method) and relaxation time determination. Ligand L^E appears to be a fair sensitiser of Eu^III, the quantum yield of [Eu₂(L^E)₃]⁶⁺ being 25% larger than that found for [Eu₂(L^B)₃]⁶⁺, but the ligand ³ππ* state and Tb(⁵D₄) excited level are in resonance, which limits the sensitisation of Tb^III. High resolution luminescence spectra of [Eu₂(L^E)₃]⁶⁺, both in solution and in the solid state, are presented and discussed in terms of site symmetry and vibronic coupling mechanisms.