2,2 -bipyridyl luminescence

Examples include luminescence from anthracene crystals subjected to alternating electric current (159), luminescence from electron recombination with the carbazole free radical produced by photolysis of potassium carba2ole in a fro2en glass matrix (160), reactions of free radicals with solvated electrons (155), and reduction of mtheiiium(III)tris(bipyridyl) with the hydrated electron (161). Other examples include the oxidation of aromatic radical anions with such oxidants as chlorine or ben2oyl peroxide (162,163), and the reduction of 9,10-dichloro-9,10-diphenyl-9,10-dihydroanthracene with the 9,10-diphenylanthracene radical anion (162,164). Many other examples of electron-transfer chemiluminescence have been reported (156,165). 

Luminescence spectroscopy is one of the most sensitive techniques for identification of impurities in dyes. The most commonly observed impurities in to-bipyridyl complexes of the type [RuL2X2] are the homoleptic tris-bipyridyl species [RuL3]2+. Since the emission quantum yields of the [RuL3]2+ complexes are significantly higher than those of the [RuL2X2] complexes, one can identify the homoleptic impurities at a level of less than 1%. This does depend, however, on the relative quantum yields, and position of the emission spectral maxima, for the complexes and impurities involved. 

Beer PD, Timoshenko V, Maestri M et al (1999) Anion recognition and luminescent sensing by new ruthenium(II) and rhenium(I) bipyridyl calix[4]diquinone receptors. Chem Commun... 

R. Grigg and W. D. J. A. Norbert, Luminescent pH sensors based on di(2,2-bipyridyl) (5,5-diami-nomethyl-2,2 -bipyridyl)ruthenium (II) complexes, J. Chem. Soc. Chem. Commun. 1300-1302... 

Complexes containing the 3,5-dinitrosalicylate ion, e.g. [Ln2(C2H202N2)3],- H20 (u = 7 -> 15), and methylsalicylate (MesaP ), e.g. [Ln(Mesal)2(OH)-(H2O)] (Ln = La, Pr, Nd, Sm, Gd, Dy, Er, Yb and have been reported. Tris-salicylaldehydato (said ) complexes, Ln(sald)3 (La, Pr, Nd, Sm, Eu, or Tb) form 1 1 adducts with o-phenanthroline (o-phen), aa -bipyridyl, quinoline, and pyridine. The luminescence spectrum of the Eu " complexes showed that, in the solid state, the symmetrically forbidden electric dipole transition intensity was much enhanced for the o-phen adduct when compared to its salicylate analogue. The simple said" complexes were very poor emitters. 

The stability of the polypyridyl rhenium(I) compounds mentioned above stimulated applications of this coordination chemistry. Thus, new heterotopic bis(calix[4]arene)rhenium(I) bipyridyl receptor molecules have been prepared and shown to bind a variety of anions at the upper rim and alkali metal cations at the lower rim. A cyclodextrin dimer, which was obtained by connecting two permethylated /3-cyclodextrins with a bipy ligand, was used for the preparation of a luminescent rhenium(I) complex. The system is discussed as a model conipound to study the energy transfer between active metal centers and a bound ditopic substrate. The fluorescence behavior of rhenium(I) complexes containing functionalized bipy ligands has been applied for the recognition of glucose. ... 

While many metal centers can be reversibly cycled between two (or more) oxidation states, few organic moieties can match such reversibility especially in protic media. Nevertheless, the first supramolecular example of an electroswitch-able luminescent device involved the benzoquinone-hydroquinone couple. The luminescence of 55 " is switched off due to PET in the benzoquinone state of the redox couple. Electrochemical or chemical reduction of the benzoquinone under protic conditions to hydroquinone recovers the luminescence of the tris(2,2 -bipyridyl) Ru(II) unit. It is noted that the luminescence of tris(2,2 -bipyridyl) Ru(Il) itself is electroswitchable. Indeed tris(2,2 -bipyridyl) Ru(II) came to fame as a solar energy material from more humble beginnings as a luminescent redox indicator. However 55 achieves the same switching at a lower magnitude of reduction potential. Here lies the advantage of the supramolecular design. Like tris(2,2 -bipyridyl) Ru(II), many lumophores show electroswitchable luminescence. An... 

The bipyridyl chromophore has been extensively used in lanthanide coordination chemistry. In addition to those based on the Lehn cryptand (see Section IV.B.4), a number of acyclic ligands have also employed this group. One such ligand is L17, which binds to lanthanide ions such that one face of the ligand is left open (Scheme 3) (60). As expected, luminescence is extremely weak in water and methanol, but stronger in acetonitrile ( = 0.30, 0.14 for europium and terbium, respectively). In addition, the nature of the counter ion can... 

Alteration of the chromophore by conversion of some of the bipyridyl groups to their N-oxide derivative leads to a significant improvement in the luminescence performance (88). Although the lifetimes of the europium complexes of ligands L54 and L55 were still less than a millisecond, the quantum yields of 0.15 and 0.20 respectively are significantly better. 

Recently, two cryptands and their lanthanide complexes have been synthesized which include either a bipyridyl (L56) or pyridyl (L57) chromophore (89). These have proved effective at populating the lanthanide excited states. Aqueous luminescence lifetimes of up to... 

Ruthenium(II) bipyridyl and Cr(III) aquo complexes luminesce strongly when photostimulated. The emission of light can be quenched effectively by such species as oxygen, paraquat, Fe(II) aquo complexes, Ru(II) complexes and Cr(NCS)i (Sutin [15]). Pfeil [16] found that the quenching rate coefficients are typically a third to a half of the value which might be predicted from the Smoluchowski theory [3]. 

Left Orange-red luminescence from 5 pM (bipyridyl)jRuC12 in methanol after most of the air was removed by bubbling with Dry Ice. Right Luminescence is quenched (decreased) after bubbling 02 through the solution for 30 s. 

One should not gain the impression from the foregoing that redox excitation is restricted to reactions between aromatic radical ions in aprotic solvents. Studies of such reactions have indeed dominated research because the optical and electrochemical properties of many aromatics are well known, but there are numerous cases of redox excitation outside this chemical domain. For example, singlet oxygen seems to arise from oxidations of superoxide ion in acetonitrile [94]. Similarly, luminescent tris(2,2 -bipyridyl)ruthenium(II) can arise in at least three ways (1) from a kind of ion annihilation in CH3CN [95] or DMF,... 

Ruthenium complexes have been applied successfully to the luminescent detection of proteins on blotting membranes like nitrocellulose [160]. The bipyridyl and phenanthroline complexes modified with aminoreactive NHS-ester or isothiocyanate groups are commercially available [161]. An even higher sensitivity and lower detection limit can be obtained by encapsulating... 

Beer and coworkers have modified their approach by derivatizing the upper rim of calix[4]arene with two (48, Tos = tosylate) and four (49) Ru(II) tris-bipyridyl reporter sites. The amide linkers of 48 and 49 form a hydrogen-bonding cleft for anion occupation. A preliminary report [385] indicates that MLCT luminescence from the Ru(II) centers is sensitive to anion association. 

The 1,2-dithienylethene unit has also been used to link together metal tris(bipyridyl) moieties including an unsymmetrical Ru(II)/Os(II) complex. In the open form luminescence from the MLCT state is observed with efficient energy transfer from ruthenium to osmium. However, the emission is quenched upon conversion to the closed form because of energy transfer to the photo-chromic 1,2-dithienylethene orbitals.55... 

Nakamaru, K. (1982) Synthesis, luminescence quantum yields, and lifetimes of trischelated ruthenium(II) mixed-ligand complexes including 3,3 -dimethyl-2,2 -bipyridyl, Bull. Chem. Soc. Jpn. 55, 2697-2705. 

A bimetallic Os(II) complex of the macrocyclic ligand MAC1 (Scheme 2) was prepared and characterized by Venturi et al. Each Os(II) center of the complex has a [(bpy)20s(II)] fragment attached to one of the bipyridyl units of MAC1. The complex has absorption and luminescence behavior reasonably similar to that of [Os(bpy)3]2+ with an absorption maximum of 488 nm and emission with a maximum of 740 nm (trt = 59 ns, 0rt = 0.003). The complex may be used in the development of supramolecular assemblies [23]. 

The photostability of Os(II) bipyridyl complexes with covalently attached hydro quinone was investigated by Keyes and coworkers. In the complex [Os(bpy)2(bbhq)]+ (Scheme 1) the Os(II) center is coordinated to one ligand nitrogen and one of the oxygens of the central hydro quinone. The complex has an Os to bpy MLCT absorption with a maximum of 493 nm, but is non-luminescent. Photolysis in acetonitrile in the presence of excess Cl" did not yield any measurable photodegradation [32]. 

Hyslop and coworkers reported an Os(II) bipyridyl carbonyl complex (Fig. 7) covalently linked to both free base and Zn(II) tetraphenylpor-phyrin [45]. The Os(II) chromophore, having a CO ligand, has a luminescence maximum of 589 nm in CH2CI2 and excitation of the complex results in energy transfer to the free base porphyrin. In the mixed Os(II)/Zn(II) complex,... 

The luminescence spectra of all receptors in CH3CN were found to be dramatically affected by the addition of acetate or chloride. While compound 19 exhibits an emission decrease, the other receptors 17,18 and 20 show a remarkable intensity increase (up to 500%) with a slight concomitant blue shift of the emission maximum (660 nm for 17). The anion-induced enhancement of luminescence intensity in the case of 17 is clearly due to the decrease of the electron transfer between the ruthenium(II) bipyridyl centre and the quinone moieties. Alternatively, receptors bearing ruthenium or rhenium complexes on the upper rim were also described [20]. 

For instance 59, the Re(I) bipyridyl analogue of receptor 43, also selectively senses acetate anions [37]. The lack of an electrostatic interaction accounts for a significantly lower stability constant for acetate (from H NMR titrations K= 1,790 M 1 in deuterated DMSO solution) and hence a smaller luminescence response than its [Ru(bpy)3]2+ counterpart. 

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