Luminescence, anion binding

Dickins, R. S. Gunnlaugsson, T. Parker, D. Peacock, R. D. Reversible anion binding in aqueous solution at a cationic heptacoordinate lanthanide centre selective bicarbonate sensing by time-delayed luminescence. Chem. Commun. 1998, 16, 1643-1644. 

Other groups have subsequently reported anion receptors that work on the same principle. For instance, an Eu(III) complex of the bis-bipyridinephen-ylphosphine oxide ligand 86 made by Ziessel and co-workers is able to sense anions by luminescence enhancement in acetonitrile, with stability constants which follow the trend fluoride>acetate>chloride>nitrate [61]. Tsukube and co-workers have investigated the properties of the Eu(III) and Tb(III) complexes of the chiral ligand 87 [62]. Anion binding was assessed by profiling luminescence enhancement in acetonitrile, and it was found that the different metal centres provided different selectivities. The emission at 548 nm of the Tb(III) complex was increased by 5.5 times in the presence of 3 equivalents of chloride compared to 2.2 for nitrate and 1.1 for acetate. Conversely the emission at 618 nm of the Eu(III) complex was increased 8.3 times by 3 equivalents of nitrate, 2.5 times for chloride and 1.0 times for acetate. Stability constants were not reported. 

Some chemical substances have been used in the past as switches to control ion transport. Several new molecular sensory devices, which are responsive crown ethers used for the dynamic control of cation and anion binding induced by changes in pH, redox potential, temperature, light, and magnetic and electrical field, have been developed. These new ligands possess chromophores or fluorophores linked to the macrocycle, and display drastic variations in their photochemical and/or luminescent properties upon cation complexation. 

The luminescent emission of Ru (bpy)3 is also very useful for signalling anion binding. In the emission spectra of 37-40 both a blue shift (of up to 16 nm for 40) and an increase in intensity of the Amax of the MLCT emission band was observed on addition of dihydrogenphosphate. It has been proposed that the conformational flexibility of the receptors is decreased by complexation of the anion guest thus reducing the rate of non-radiative decay through vibrational and rotational relaxation. Similarly, macrocyclic complexes 41-44 and 77-79 were also found to sense chloride by luminescence enhancement. 

In Other examples anion binding can cause quenching of the Ru (bpy)3 MLCT emission. In aqueous solution polyaza receptors 80-82 bind phosphate and ATP anions, producing up to a 15% reduction in the emission intensity of max at 605 nm [16]. Similarly, 76 shows up to a 40% reduction in the intensity of the luminescent emission at 630 nm in the presence of dihydrogenphosphate in DMSO solution. 

The Ru°bipyridylcalix[4]diquinone receptor 83 selectively binds and senses acetate anions (from NMR titrations in DMSO-de solution K = 9990 M ) [54]. This receptor is only weakly luminescent because the Ru (bpy)3 MLCT emission is partially quenched by oxidative electron transfer to the electron-poor calix[4]diquinone. Addition of acetate to acetonitrile solutions of 83 resulted in a five-fold increase in liuninescence intensity (60% for chloride) concomitant with a slight blue shift of the emission maximum. Anion binding causes this increase in emission intensity by interrupting the electron transfer pathway from the Ru°(bpy)3 to the calix[4]diquinone, thus reducing its quenching effect. 

An obvious approach to anion sensing would be to use cationic species as the anion-binding sites. In addition, some metal ions have well-characterized luminescent properties that can be utilized for sensing purposes. For... 

Brace, J.L Dickins, R.S. Govenlock, L.J. Gunnlaugsson. T. Lopinski. S. Lowe. M.P. Parker. D. Peaeoek. R.D. Perry. J.J.B. Aime, S. Botta. M. The seleetivity of reversible oxy-anion binding in aqueous solution at a chiral europium and terbium center Signaling of earbonate chelation by changes in the form and eireular polarization of luminescence emission. J. Am. Chem. Soc. 2000, 122 (40), 9674-9684. 

Beer, P.D. Graydon, A.R. Sutton, L.R. Luminescent anion recognition Selective induced emission by binding of dihydrogenphosphate. Polyhedron 1996, 15, 2457. 40. 

Bruce JI, Dickins RS, Govenlock LJ, Gunnlaugsson T, Lopinski S, Lowe MP, et al. The Selectivity of Reversible Oxy-anion Binding in Aqueous Solution at a Chiral Europium and Terbium Center Signaling of Carbonate Chelation by Changes in the Form and Circular Polarization of Luminescence Emission. J Am Chem Soc 2000 122 9674-9684. 

Halides are negatively charged spherical structures which normally bind to anion receptors in a non-directional fashion. One of the first examples of the use of halides for binding to lanthanide complexes, by direct metal coordination within a coordinatively unsaturated environment, was that of Charbonniere et al. [55] who developed lanthanide complexes of a bis-bipyridine-phosphine oxide ligand (Scheme 6.9a), as luminescent anion sensors for halides. In this design, the bipyridine (bpy) units were expected to coordinate to the... 

Luminescence Studies of Anion Binding in Catalysis and Sensing 8.4.1 Phosphate Ester Binding and Cleavage... 

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