A kinetically inert and optically active Cr-III partner in thermodynamically self-assembled heterodimetallic non-covalent d-f podates

Stoichiometric mixing of the segmental ligand 2-{6-[N,N-diethylcarboxamido]pyridin-2-yl}-1,1'-dimethyl-5,5'-methylene-2'-(5-methylpyridin-2-yl) bis[1H-benzimidazole] (L) with Ln(CF3SO3)(3) (Ln = La-Lu) and Cr(CF3SO3)(2) under an inert atmosphere produces quantitatively the self-assembled triple-stranded non-covalent podates (HHH)-[LnCr(II)L(3)](5+). Air oxidation of the low-spin Cr-II complexes gives selectively the head-to-head-to-head podates (HHH)-[LnCr(III)L(3)](6+) into which inert Cr-III has been incorporated. The X-ray crystal structures of [LnCr(III)(L)(3)](CF3SO3)(6)(CH3CN)(4) (Ln = Eu, 7; Ln = Lu, 8) confirm the formation of regular triple-helical cations (HHH)-[LnCr(III)L(3)](6+) whose structure is maintained in acetonitrile according to ESI-MS, spectrophotometry and NMR data. Photophysical studies evidence efficient sensitization of both Eu-III and Cr-III through ligand excitation at low temperature, while a subsequent intramolecular Eu-III-->Cr-III energy transfer (eta = 70%) limits Eu-centred luminescence and induces directional light-conversion along the three-fold axis, resulting in Cr-III emission. For (HHH)-[(TbCrL3)-L-III](6+), the better spectral overlap between the emission spectrum of Tb-III (D-5(4) --> F-7(J)) and the absorption spectrum of Cr-III ((4)A(2)-->T-4(2)) provides a quantitative Tb-III Cr-III energy transfer (eta greater than or equal to 99%) and long-range intermetallic communication. De-complexation of Ln(III) with water or EDTA(4-) gives the first inert and optically active-Cr-III-containing triple-helical nonadentate receptor (HHH)-[(CrL3)-L-III](3+).