Photochemical Ion Receptor Based on a Structurally Distorted Ruthenium(II) Complex Having a Crown-Ether Moiety at the 3,3′-Positions on the 2,2′-Bipyridine Ligand

Abstract

Ru(bpy)2(CE-bpy)2+ was prepared where bpy and CE-bpy were 2,2′-bipyridine and bpy having a crown-ether moiety at the 3,3′-positions, respectively. Although Ru(bpy)2(CE-bpy) 2+ showed only very weak emission in acetonitrile, recognition of Na + , Li + , or K + by the crown-ether moiety in CE-bpy resulted in increases in both the emission intensity and the lifetime of the complex, demonstrating that it acted as a photoreceptor. The results were discussed in terms of a steric hindrance between the 3,3′-substituents on CE-bpy and structural changes in both CE-bpy and the complex upon ion recognition, as studied by variable-temperature

Conformational Control of Excited-State Dynamics in Highly Distorted Ru(II) Polypyridyl Complexes

“…An increasing amplitude of a new component, as observed here, is expected for a simple titration of binding sites by a reagent (that is, metal ions). These observations and interpretation differ from other reports in which a single lifetime was said to lengthen steadily in proportion to the amount of metal present. , We have not yet been able to account for this discrepancy, although we note that it requires quite good data over a range of conditions before the introduction of additional fitting parameters is justified. In addition, there is reason to believe that relatively small changes in temperature near ambient could mask the presence of the intermediate lifetime state (vide infra).…”

“…Steric repulsion between substituents at the 3,3‘-positions of a bipyridyl ligand has been shown to produce a species that is much less emissive than the parent Ru(bpy) 3 2+ . This reduced emission has been ascribed, at least in part, to variation in the coordination geometry of the modified bipyridine ligands. − Such variation affects the energy of the emissive triplet metal-to-ligand charge transfer state, 3 MLCT, and its coupling to other states, thereby modulating the luminescence efficiency. The degree of control that can be achieved by this approach has been demonstrated with modified bipyridines whose geometry is covalently restricted.…”

“…The degree of control that can be achieved by this approach has been demonstrated with modified bipyridines whose geometry is covalently restricted. In a series of tris-chelated homoleptic complexes containing 2−4 carbon-bridged bipyridines, Thummel et al demonstrated quantum yields spanning 2 orders of magnitude and emission energies varying by 77 nm (≈1900 cm -1 ). , In other complexes containing bipyridine ligands substituted in the 3,3‘-positions, steric interactions have also been postulated to cause reduced emission at room temperature. − ,− Emission quantum yield is determined by nonradiative processes that occur in competition with radiative decay from 3 MLCT states. The two nonradiative processes of primary concern in Ru(bpy) 3 2+ derivatives are 3 MLCT → 3 MC internal conversion (IC) and 3 MLCT → S 0 intersystem crossing (ISC).…”

Synthesis of Crown-Ester-Bipyridines and Crown-Ester-Viologens

“…10 If the 2,2-bipyridyltype of metal complex is fluorescent, then the conformational changes cause remarkable changes in the photophysical properties of these metal complexes, and thus offer the opportunity for the design of luminescence sensors. 11 Another approach to use fluorescence as an analytical tool consists in the binding of rare earth cations within calixcrowns to form highly luminescent devices which can be used as efficient labels for bioaffinity assays. 12 In this work, we have directed our attention toward the synthesis of crown-ester -modified 4,4¢-bipyridyl-deriva-tives, which can serve as versatile electron relays.…”

Unusually Short-Lived Solvent-Dependent Excited State in a Half-Sandwich Ru(II) Complex Induced by Low-Lying 3MC States

“…Such analysis reveals that a stronger distortion of a complex causes the energy difference between the 3 MLCT and the 3 dd-MC states to shrinkconsequently, the excited-state lifetime decreases. − This observation is in line with ligand field theory, which describes a metal-centered σ M * orbital of generally higher energy than the respective ligand associated molecules but can be accessed at higher temperatures or in distorted complexes, when the orbital overlap of the metal and the ligand is diminished . This structurally sensitive luminescence was utilized to design molecular sensors based on crown ether-extended bipyridine ligands coordinated to a Ru­(II) ion: Complexation of a metal ion by the crown ether leads to structural stiffening of the ligand, and consequently the luminescence of the complex is switched on. − Despite their critical importance in determining the lifetime of excited states in widely used photoactive Ru­(II) complexes, 3 dd-MC states have eluded a direct observation by means of time-resolved spectroscopy for a long time. Their lifetime has been considered short as the thermal activation of the 3 dd-MC states from the lower lying long-lived 3 MLCT states is considered the rate-limiting step of excited-state decay in Ru­(II)–polypyridine complexes.…”