Selective detection of multicarboxylate anions based on "turn on" electron transfer by self-assembled molecular rectangles.

Two new large molecular rectangles (4 and 5) were obtained by the reaction of two different dinuclear arene ruthenium complexes [Ru(2)(arene)(2)(OOOO)(2)Cl(2)] (arene = p-cymene; OOOO = 2,5-dihydroxy-1,4-benzoquinonato (2), 6,11-dihydroxy-5,12-naphthacene dionato (3)) with the unsymmetrical amide NN (N-[4-(pyridin-4-ylethynyl)phenyl]isonicotinamide) donor ligand 1 in methanol in the presence of AgO(3)SCF(3), forming tetranuclear cations of the general formula Ru(4)(arene)(4)(NN)(2)(OOO O)(2). Both rectangles were isolated in good yields as triflate salts and were characterized by multinuclear NMR, ESI-MS, UV/Vis, and photoluminescence spectroscopy. The crystal structure of 5 was determined by X-ray diffraction. Luminescent rectangle 5 was used for anion sensing with an amide ligand as a hydrogen-bond donor and an arene-ruthenium acceptor as a signaling unit. Rectangle 5 strongly bound multicarboxylate anions, such as oxalate, tartrate, and citrate, in UV/Vis titration experiments in 1:1 ratios, in contrast to monoanions, such as F(-), Cl(-), NO(3)(-), PF(6)(-), CH(3)COO(-), and C(6)H(5)COO(-). The fluorescence titration experiment showed a large fluorescence enhancement of 5 upon binding to multicarboxylate anions, which could be attributed to blocking of the photoinduced electron transfer process from the arene-ruthenium moiety to the amidic donor in 5; this was likely to be a result of hydrogen bonding between the ligand and the anion. On the other hand, rectangle 5 was not selective towards any other anions. To the naked eye, multicarboxylate anions in a solution of 5 in methanol appear greenish upon irradiation with UV light.