Intramolecular Luminescence Quenching in Amino Acid Bridged Ruthenium(II)-Copper(II) Complexes with Negligible Ground-State Coupling

Abstract

A series of heterobimetallic complexes [{Ru(bipy)2}m-AA{Cu(phen)(H2O)}](PF6)3 (m-AA = Nω-(4-carbonyl-4′-methyl-2,2′-bipyridyl)-L-α,ω-diamino acid; diamino acid (AA): 1b, L-2,3-diaminopropionic acid; 2b, L-2,4-diaminobutyric acid; 3b, L-ornithine, 4b, L-lysine) has been synthesized and characterized starting from the 2,2′-bipyridineruthenium(II)-substituted amino acids [{Ru(bipy)2}m-AA-αN-tBoc]2+ (αN-tBoc-1a–4a, αN-tBoc = Nα-tert-butoxycarbonyl). The metal centers in 1b–4b are bridged by insulating alkyl chains. Consequently, spectroscopic and electrochemical investi-gations confirmed the absence of any appreciable ground-state coupling between ruthenium(II) and copper(II). However, the luminescence of 1b–4b is significantly quenched compared to that of the parent compounds 1a–4a. Quenching was found to proceed only intramolecularly and to be most efficient in 1b with the shortest alkyl spacer. Equilibrium constants for the formation of 1b–4b were determined from luminescence titrations of the respective ruthenium-substituted amino acid with [Cu(phen)(NO3)2]. The complexes 2b–4b (K = 1.61–2.08 × 107M–1) were found to be significantly more stable than 1b (K = 5.7 × 106M –1). This is explained by a stronger electrostatic repulsion between the two metal centers in the latter compound.