Cyclen-based lanthanide complexes

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. 

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). 

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 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. ... 

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-... 

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]. 

The above examples demonstrate how simple anions such as halides, carboxylates and carbonates can bind direcdy to the lanthanide centres and concomitantly modulate the photophysical output of such complexes. In a similar manner, aryl-based anions can be employed, and the formation of luminescent ternary complexes can thus be achieved. Both Gunnlaugsson et al. [69] and Faulkner et al. [70] have demonstrated, for instance, that simple anionic antennas can be bound to Tb and Yb cyclen complexes (Scheme6.14a, b, c). The former two Tb examples (Scheme 6.14a, b) are based on the use of cationic complexes. 

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