Luminescence stability constants

Addition of benzene to a solution of / -CD8 causes a two-fold enhancement of Eu3+ luminescence intensity. The stability constant of the complex between / -CD8 and benzene ( 200) is comparable to that of the complex with the native /i-CD. In contrast, benzoic and naphthoic acids show much stronger associations with / -CD8 than with native / -CD because association is assisted by the interaction between the carboxylic groups and the metal ion. Moreover, the enhancement factor of the luminescence is larger. 

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. 

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. 

However, the lanthanide p-diketonates shown in scheme 4 cannot be used as labels, since there is no active binding group on the p-diketonate ligands and these complexes are not very stable with stability constants in the order of 103-106 only, therefore the complexes dissociate in highly diluted solutions and the luminescence intensity decreases. Recently, three chloro-sulfonylated tetradentate p-diketones were synthesized by Yuan and Matsumoto (1996,1997) and Yuan et al. (1998a, 1998b) (scheme 5). They differ from other p-diketones, because the emission intensity of their Eu3+ complexes is not weakened by the presence of the sulfonyl... 

The calix[4]azacrowns 41 and 42 (fig. 38), capped with aminopolyamide bridges bind Lnm ions and form both 1 1 and 1 2 (Ln L) complexes with stability constants in the ranges log/3i v 5-6 and log (h 10-11 in acetonitrile for 42, while the stability of the complexes with 41 is about two orders of magnitude smaller. Hydration numbers around 1 were found for the Eum and Tbm complexes, but the ability of the ligands to sensitize Lnm luminescence is veiy weak, except in the Tbm complex with 42. No Er111 luminescence could be evidenced, but some Ndm emission was recorded, which is 12 times larger with ligand 42 than with receptor 41 (Oueslati et al., 2006). 

Interaction of the nitrate ion with lanthanide(III) in acetonitrile solution was studied by conductivity, vibrational spectroscopy and luminescence spectroscopy. Bidentate nitrate with approximate C2V local symmetry was detected. FT-IR spectral evidence for the formation of [La(N03)5]2, where La = Nd, Eu, Tb and Er with coordination number 9.9 has been obtained [128]. Two inequivalent nitrate ions bound to lanthanides were detected by vibrational spectroscopy. The inequivalent nature varied with different lanthanides. For example three equivalent nitrate groups for La and Yb, one nitrate different from the other two for Eu ion were detected. Vibrational spectral data point towards strong La-NC>3 interaction in acetonitrile [129]. Stability constants for lanthanide nitrate complexes are given in Table 4.10. 

Absorption and emission spectra provide basic information (molar absorption coefficients, luminescence quantum yields), but also their changes upon association between two species can be used to determine the stoichiometry and stability constant of host-guest complexes. Moreover, evidence for the existence of photo-induced processes can be simply obtained in some cases from the fluorescence spectra. 

Pt acetylenes can also function as fluoresecent sensors for cations. Receptor 45 incorporates two 4-ethynylbenzo-15-crown-5 moieties with luminescent dimino Pt(ii) complexes.In acetonitrile solution, complex 45 is weakly emissive (excitation at 405 nm, Amax = 635, 0= 1.1 x 10 ). However, on addition of significant increase in the emission intensity and a blue shift in A ax to 555 nm were observed. At 40equiv. of Mg or Zn, the measured enhancement was 1,035- and 870-fold, respectively. Other cations, namely, K, Na, and Gd, resulted in a less than 10-fold emission enhancement. The binding stoichiometry was found to be 1 2 receptor cation, and the overall stability constants calculated for Na, Mg, and Zn were [3 = 7.9 x 10, 5.3 x 10, and 9.3 x 10 respectively. 

Complexes of lactic acid with a wide variety of metal ions are known and their stability constants have been determined. A variety of techniques has been used in the study of the species formed in the solutions of this hydroxy acid and inorganic ions. Electrochemical and spectrophoto-metric methods have been used with the main aim of determining stability constants of the complex ions. CD and optical rotatory dispersion (ORD) have also proved to be powerful methods in studying, for instance, Mo, Mo , Cu" and Co", as well as lanthanide complexes. Very recently, Brittain et used circularly polarized luminescence (CPL) techniques in the study... 

The control of pH in a solution containing a luminescent analyte is also of great importance for sensitive and reliable analysis. For aromatic compounds with acidic or basic substituents, excitation and emission wavelengths of the ionized and free forms are likely to differ. In the case of fluorescence from metal chelates, the pH must be controlled to ensure that the conditional stability constant for the complex is optimal for the particular analytical situation. 

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