Conformational Control of Intramolecular Electron Transfer in Calix[4]diquinones and Their Cationic Complexes

Abstract

The synthesis is described of a photoactive molecular dyad comprising a luminescent ruthenium(II) tris(2,2‘-bipyridyl) fragment covalently attached, via a methylene bridge, to a calix[4]arene receptor in which two of the four walls are benzoquinone functions. A distribution of interconverting conformers is expected for the calix[4]arene platform, including cone, partial cone, and 1,3-alternated forms, according to molecular dynamics simulations and NMR studies. Luminescence from the metal complex is quenched because of light-induced electron transfer from the triplet state to a nearby quinone. The MD simulations indicate that the redox-active subunits approach within orbital contact of each other and that the noncone conformations adopt coparallel arrangements of the redox partners that appear to be highly favorable for electron transfer. Cations, such as barium(II), are held at the lower rim of the receptor by coordination to the four O atoms and by two N atoms provided by an ancillary 2,2‘-bipyridine that is appended to the receptor. Cation complexation increases slightly the thermodynamic driving force for light-induced electron transfer but restores luminescence from the ruthenium(II) tris(2,2‘-bipyridyl) fragment. This apparent contradiction is explained in terms of the cation inducing a major structural modification of the supermolecule. Both MD and NMR studies indicate that the cation forces the luminophore away from the quinone and, by favoring the cone conformation of the calix[4]arene, destabilizes cofacial orientations between closely spaced reactants. Consequently, the system functions as a sensitive photoionic detector.

Use of an Osmium Complex as a Universal Luminescent Probe for Enzymatic Reactions

“…[11] A [Ru-A C H T U N G T R E N N U N G (bpy) 3 ] 2 + fragment covalently attached to a calix [4]arene receptor varied its luminescence in response to cations such as barium(II). [12] Chemical oxygen sensors based on luminescence quenching of osmium complexes with 2,2,2-terpyridine [13] and a-diimine ligands [14] have also been reported. Owing to their electrochemical reversibility and stability in both their oxidised and reduced states, osmium complexes have also been employed as redox mediators in biocatalytic processes.…”

Noncovalently Linked Zinc Porphyrin-Ru(bpy)3Dyad Assembled via Axial Coordination

“…Some porphyrin derivatives linked covalently to ruthenium polypyridyl complexes have also been reported. [10][11][12][13][14][15][16][17][18] Recently, a number of covalently linked donor-acceptor systems have been extensively studied, but sometimes require time-consuming laborious synthetic works. Fortunately, a variety of donor-acceptor systems can be also assembled easily through noncovalent interactions such as ion-pairing, 19 H-bonding in DNA base-pairing, 20 or ligand binding on transition metal.…”

Through-Space Exciton Coupling and Multimodal Na+/K+ Sensing Properties of Calix[4]arenecrowns with the Thienylene Analogue ofpara-Terphenoquinone as Chromophore

“…[1] In the nineties, chromogenic recognition by chromophore-derived calix [4]arenes [2] and electrochemical recognition by quinone-derived calix [4]arenes [3] of alkali metal cations were reported. However, as far as we know, there are no reports on through-space interactions between two chromophores incorporated in a calixarene skeleton, although a paper dealing with intramolecular electron transfer in calix [4]arenediquinone derivatives appeared quite recently, [4] and multifunctional ion-sensing calixarenes that combine chromogenic and redox-active properties in one molecule are unknown. The combination of these properties could offer a promising route towards novel sensing materials.…”

“…We have now designed and synthesized the novel calix [4]arenecrowns 1 and 2, together with their reference compounds 3 and 4, since the thienylene analogue of p-terphenoquinone as chromophore shows a strong absorption maximum in the visible region and affords a very stable radical anion and radical cation in electrochemical reactions. [5] We found that the two diametrically arranged chromophores of 1 and 2 undergo through-space exciton coupling on absorption of visible light.…”

“…[7] Scheme 1. The calix [4]arenecrowns 1 and 2 and the reference compounds 3 and 4. Formation of 1-Na and 2-Na .…”