A kinetically inert and optically active CrIII partner in thermodynamically self-assembled heterodimetallic non-covalent d–f podates†
Abstract
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)-[LnCrIIL3]5+. Air oxidation of the low-spin CrII complexes gives selectively the head-to-head-to-head podates (HHH)-[LnCrIIIL3]6+ into which inert CrIII has been incorporated. The X-ray crystal structures of [LnCrIII(L)3](CF3SO3)6(CH3CN)4 (Ln = Eu, 7; Ln = Lu, 8) confirm the formation of regular triple-helical cations (HHH)-[LnCrIIIL3]6+ whose structure is maintained in acetonitrile according to ESI–MS, spectrophotometry and NMR data. Photophysical studies evidence efficient sensitization of both EuIII and CrIII through ligand excitation at low temperature, while a subsequent intramolecular EuIII → CrIII energy transfer (η = 70%) limits Eu-centred luminescence and induces directional light-conversion along the three-fold axis, resulting in CrIII emission. For (HHH)-[TbCrIIIL3]6+, the better spectral overlap between the emission spectrum of TbIII (5D4 → 7FJ) and the absorption spectrum of CrIII (4A2 → 4T2) provides a quantitative TbIII → CrIII energy transfer (η ≥ 99%) and long-range intermetallic communication. De-complexation of LnIII with water or EDTA4− gives the first inert and optically active-CrIII-containing triple-helical nonadentate receptor (HHH)-[CrIIIL3]3+.