Cyclen-lanthanide complexes

Dickins and coworkers have developed a series of cyclen-lanthanide complexes as luminescent anion receptors working in aqueous solutions. Cyclens 50 functionahzed with three coordinative pendant arms were confirmed to provide 7-coordination to the Eu and Tb cations, while two sites were available for external anions [129,130]. Displacement of the two coordinated water molecules by the guest anions rapidly occurred. In the pH range of 5.5-6.5, these complexes exhibited changes in the luminescent properties in response to acetate, F , S04 , citrate, lactate, or malonate anion, while Cl , Br , r, and NOs anions gave no significant response. 

Fig. 21 Stereochemistry of cyclen-lanthanide complex with four pendant arms. Reprinted with permission from [119], (2002), American Chemical Society...

Several heterocycle-lanthanide complexes provided attractive, imaginative, and suggestive approaches to anion sensing, in which Ugand architecture based on heterocyclic chemistry played important roles. Cyclen-lanthanide complexes have several advantages of facile design and systematic synthe-... 

Zucchi, G. Scopelliti, R. Biinzli, J.-C. G. Importance of the chromophore orientation to the ligand-to-metal energy transfer in lanthanide complexes with pendant-arm fitted cyclen derivatives. J. Chem. Soc., Dalton Trans. 2001, (13), 1975-1985. 

In principle, lanthanide complexes of alkyl- (phosphinates) or alkoxy- (phosphonate esters) DOTP derivatives may give rise to 32 stereoisomers, existing as 16 enantiomeric pairs, which are indistinguishable by NMR spectroscopy. The isomers originate from chiral elements inherent in these complexes, including the R or S configuration at each phosphorus and the helicity defined by the pendant arm orientations (AIA). Various Ln3+ complexes of phosphinate and phosphonate ester ligands derived from 1,4,7,10-tetraazacyclododecane (cyclen) have been described in the literature [104-107]. 

These systems are promising as potential labels due to high emission quantum yields and excited-state lifetimes that can be as long as several tenths of a millisecond (108). A cyclen (12-ane-N4) unit connected to a phenanthridine moiety in fluorophore-spacer-receptor conhguration (Fig. 26) exhibit strong Tb(III) based luminescence (109) in the absence of protons and oxygen. Few other luminescent lanthanide complexes are available in the literature (110,... 

The photophysics of lanthanide complexes has drawn considerable attention in recent years, in part because of the potential applications of lanthanides (sensors, electroluminescent displays, etc.) and several recent reviews highlighting applications of luminescent lanthanide complexes have appeared. A discussion of infrared f-f luminescence of Yb, Nd, and Er in complexes having macrocyclic ligands such as porphyrins, cyclen derivatives, and calixarenes was published by Korovin and Rusakova. " In addition, DaSilva and co-workers describe the development of highly luminescent lanthanide complexes and their application as light-conversion molecular devices. ... 

The similarities in chemical behaviour across the lanthanides mean that kinetically stable lanthanide complexes offer a degree of control that cannot readily be achieved through selective self-assembly of small building blocks. Azamacrocycle complexes derived from cyclen amino-carboxylates are particularly suited to this, and ternary complexes can be used to study the photophysical behaviour of lanthanide complexes. We initially used such an approach to probe the effectiveness of chromophores as sensitizers for lanthanides without recourse to extensive synthesis (as illustrated in Fig. 7). This approach can be used with simple chromophores to probe the mechanism of energy transfer. For instance, tetrathiafulvalene... 

Table 2 Cyclen-based lanthanide complexes applied as molecular probes according to scheme 1...

The last examples discussed in this chapter are examples of the binding of anions to cyclen-based lanthanide complexes, where the binding occurs directly onto the metal center. This gives rise to significant changes, particularly for Lu(III) complexes, in the hypersensitive AJ = 2 transition, which is sensitive to the local environment of the metal ion. 

The above account has given some insight into the use of lanthanide complexes for sensing. This is a very active area of research and is fast growing, as can be clearly seen from the few examples listed, which have, with a few exceptions, been based on the cyclen structure. As for many of the transition metal ion examples shown above, these have also been used for imaging proposes, particularly within cells, an area recently reviewed by one of the leaders in the field. ... 

The cyclen/cyclam architecture also allows uncoordinated chromophores, additional probing groups, and even biological materials to be easily grafted on a lanthanide complex. Such systems are therefore, and also due to their higher stability, often chosen for biological applications. 

Neutral N-derivatized octadentate ligands based on cyclen (1,4,7,10-tetraazacy-clododecane) form tripositive cationic complexes with the trivalent lanthanides [116-124]. The N-substituted tetraamide derivatives have proven useful in understanding the relationship between the solution structure of the Ln3+ complex and its water exchange rate, a critical issue in attaining optimal relaxation efficiency of CA s [125-131]. 

Lowe et al. [7] and Frias et al. [8] described complexes of cyclene-based molecules with lanthanoids. A gadolinium complex which would exhibit pH-dependent relaxivity thanks to a switch in hydration state was prepared. [7] Cyclene bore a sulphonamide substituent in order to achieve a variation of the coordination environment of the lanthanide centre as a function of pH (Scheme 7). 

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