Luminescence coordination sites

If dendrimers contain both luminescent units and coordination sites, they can perform as luminescent ligands for metal ions [10]. Coupling luminescence with metal coordination can indeed be exploited for a variety of purposes that include investigation of dendrimer structures [11], encapsulated metal nanoparticles [12],... 

In most cases, metal ion coordination by a dendrimer takes place by units that are present along the dendrimer branches (e.g., amine, imine, or amide groups) or appended at the dendrimer periphery (e.g., terpyridine, cathecolamide ligands). When multiple identical coordinating units are present, dendrimers give rise to metal complexes of variable stoichiometry and unknown structures. Luminescent dendrimers with a well defined metal-coordinating site have been reported so far [16, 17], and the most used coordination site is 1,4,8,11-tetraazacyclotetradecane (cyclam). 

Dynamic quenching of the MLCT excited state of [Ru(phen)2(dppz)] " " by H" " transfer in MeCN solution occurs for proton donors with pAa values in the range 4.7-15.7. Comparisons of the quenching have been made in the presence and absence of DNA. " The addition of Cu " " to DNA-bound [Ru(bpy)2L] " " (L is the phenazine derivative (167)) leads to luminescence quenching. This is explained in terms of complexation of Cu " " with the vacant coordination site of L in [Ru(bpy)2L] " ". Formation of the [Ru(bpy)2L] " "/Cu " " complex in the presence of DNA is proposed to place one metal center in the major groove and one in the minor groove. " ... 

Eu(Tc)(cit) is 1 1 2 in this case. The fluorescence intensity of the 615-nm emission line of [Eu(Tc)(cit)2 ] is 22 times stronger than that of [Eu(Tc)]. Citrate, as a polydentate ligand, can chelate the Eu3+ ion via the oxygen atoms of its carboxy and hydroxy groups. It is assumed that citrate displaces water molecules which ligate to the 8- and/or 9-coordination sites of the Eu3+ ion and quench its fluorescence. Table 5 summarizes the luminescence properties of the [Eu(Tc)] complex (1 1 stoichiometry) and the corresponding chelate complexes with the intermediates of the citrate cycle. Citrate concentrations can be imaged with this luminescent probe by means of the RLI method (Fig. 15) [107]. 

The detection of aromatic carboxylates via the formation of ternary complexes using lanthanide ion complexes of functionalised diaza-crown ethers 30 and 31 has been demonstrated [134]. Like the previous examples, these complexes contained vacant coordination sites but the use of carboxylic acid arms resulted in overall cationic 2+ or 1+ complexes. Furthermore, the formation of luminescent ternary complexes was possible with both Tb(III) and Eu(III). A number of antennae were tested including picolinate, phthalate benzoate and dibenzoylmethide. The formations of these ternary complexes were studied by both luminescence and mass spectroscopy. In the case of Eu-30 and Tb-30, the 1 1 ternary complexes were identified. When the Tb(III) and Eu(III) complexes of 30 were titrated with picolinic acid, luminescent enhancements of 250- and 170-fold, respectively, were recorded. The higher values obtained for Tb(III) was explained because there was a better match between the triplet energy of the antenna and a charge transfer deactivation pathway compared to the Eu(III) complex. 

Parker and co-workers have reported a novel method for the selective detection of carboxy anions by time-delayed luminescence using the Eu(III) and Tb(III) complexes of 85 [59]. Luminescence measurements on the coordina-tively unsaturated complexes in aqueous solution showed significant increases in lifetime and emission intensity in the presence of anions. This behaviour is consistent with the anions displacing water from the non-ligated coordination sites at the metal centre. Studies allowed the number of water molecules (q) remaining coordinated in the presence of added anions to be estimated. For the triflate salt of the Eu(III) receptor q=2.14, whereas with hydrogencarbonate... 

Other examples are lanthanide(m) complexes with heptadentate ligands (derivatives of D03A) where two coordination sites are available for next ligand binding. The complexes were used for molecular recognition of some anions by means of NMR or luminescence spectroscopies <2002JA12697>. 

Dendrimer-type ligand (32) serves as a lanthanide container to exhibit on-off switchable luminescence upon lanthanide complexation in response to external anions [56]. Because of the presence of two classes of coordination sites for the lanthanide cations at the inner and outer spheres, the dendrimer 32 exhibits two different binding modes to afford on-off lanthanide luminescence, in which outer complexation at the tetradentate tripod site offers the on luminescence state upon quinoline excitation whereas, inner complexation at the multidentate core site corresponds to the off luminescence state. Upon complexation of 32 with Yb(CF3 SO3 )3, the quite weak NIR luminescence from the Yb(III) center suggests that the Yb(III) ion is most probably located at the inner coordination sites and apart from the excited quinoline moieties. Nevertheless, addition of SCN anion to the 32-Yb(CF3803)3 system induced remarkable spectral changes around the quinoline absorption band and about ninefold enhancement in luminescence intensity at around 980 nm. As the intense Yb luminescence appeared upon quinoline excitation, the employed SCN anion promoted the tripod-Yb +... 

The PL spectra were actually very sensitive to the overall surface structure and this allowed the study of the behavior of each type of luminescence center upon thermal treatment or adsorption of probe molecules. These studies have also shown that, by the way of an energy-transfer process, emission can arise from surface sites that are not necessarily those that absorbed light in the first step of the PL phenomenon. For instance, at 300 K, the energy absorbed by 5- and 4-coordinated sites is transferred to the 3-coordinated ones, whilst at 77 K, the energy-transfer process is largely suppressed and the original emission profiles of 4- and 5-coordinated centers can be observed [41]. 

An interesting possible further extension is the functionalization of bispidine ligands with hydrophobic groups, for example, for metal ion selective extractions (69, 339). biopolymers for nuclear medicinal applications (340), solids for heterogeneous catalysis and sensors, or additional coordination sites for the synthesis of heterodinuclear complexes with applications in biomimetic chemistry, catalysis, and as luminescence sensors. There is a variety of possible sites for ligand modification. Of particular interest is the C9 position, which has been selectively and stereospecifically reduced to an alcohol (190), and the two hydrolyzed C1,C5 ester groups (167). 

So, it can be concluded that the experimental results show that a mptt-modified silica gel surface can be successfully used as a substrate for luminescent lanthanide compounds. Furthermore, one could suppose that, controlling the total amount of available coordination sites in the surface, as well as the external pressure suffered by the doped samples, a complete control of the intensity and lifetime of the emitted light could be achieved. Furthermore, comparison with experimental data for M-[3 (trimethoxysilyl)propyl]-ethylenediamine-modified silica gel surfaces indicates the prominent role of the chemical composition of the silica-modified surface on the spectroscopic properties of the adsorbed complex. 

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