Luminescent, Cu2+-binding chromophore-spacer-receptor conjugates derived from ruthenium-substituted lysine

Two luminescent chromophore-spacer-receptor conjugates, [H(2)cyclenAc-Lys{Ru(bipy)(2)m}-OH](PF6)(4) (3) and [m-Lys{Ru(bipy)(2)m}-OMe](PF6)(2) (4) (m = 4-carbonyl-4-methyl-2,2'-bipyridyl; bipy = 2,2-bipyridyl) were synthesized. Both compounds bind copper(II) ions in aqueous solution with concomitant quenching of their ruthenium-based emission. A pentadentate acylated cyclen ligand (cyclen = 1,4,7,10-tetraazacyclododecane) is the metal binding receptor site in 3. The conjugate 4 contains a bidentate 2.2'-bipyridyl subunit. Both compounds were prepared starting with the synthesis of the spacer-receptor subunits and subsequent coupling with the chromophore [Ru(bipy)(2)(m-OSu)](PF6)(2) (8, HO-Su = N-hydroxysuccinimide). The emission properties of 3 in aqueous solution are almost independent of the pH. Compound 4 shows a significant quenching below pH 5 which is most likely due to an electron transfer from Ru2+ to the protonated bipyridyl ligand. Both ligands, 3 and 4, form 1:1 complexes with copper(II) ions. Higher aggregates are only formed at substoichiometric metal ion concentrations. The complex 3-Cu2+ is stable over a range of pH 2-12. In contrast, the complex 4-Cu2+ dissociates under basic conditions above approximately pH 10. Copper(II) is rebound upon acidification making 4 the first example for a receptor which exhibits reversibility at high pH. (C) 2003 Elsevier Science B.V. All rights reserved.