Luminescent cryptand

The dimeric cryptand 19, containing four 2,2 -bipyridine subunits, was synthesized from the ferrocene ft/s-acyl chloride with the corresponding azamacrocycle this redox-active luminescent cryptand was ased to complex cations <97CC2195>. 

Until very recently, studies of the use of luminescent lanthanide complexes as biological probes concentrated on the use of terbium and europium complexes. These have emission lines in the visible region of the spectrum, and have long-lived (millisecond timescale) metal-centered emission. The first examples to be studied in detail were complexes of the Lehn cryptand (complexes (20) and (26) in Figure 7),48,50,88 whose luminescence properties have also been applied to bioassay (vide infra). In this case, the europium and terbium ions both have two water molecules... 

Related work is dedicated to compounds with (L)AuC=C-functions attached to crown ether and cryptand-type units, following the idea that the luminescence properties of the chromophores will be influenced by complexation of cations in the polyether groups.87 Scheme 15 presents two examples of the devices probed in these highly successful studies. 

Cu(II) is one of the best examples of a redox active guest, but apparently not when it is imprisoned in a cryptand such as 53. In this case, the Cu(II) is silent over a wide potential range during cyclic voltammetry. System 53 is designed as a lumophore-spacer-receptor system such as 28-30 and 33-34 in Section 1 with multiple lumophores. It also shows similar luminescence off-on switching with and even with Cu(II). The possibility of Cu(II) induced production of from moisture appears to have been ruled out. The absence of EET is a mystery which can only be dispelled by further studies on this interesting system. 

Various approaches have been taken to the synthesis of effective luminescent materials, using a variety of large encapsulating antenna-containing ligands, including podands, calixarenes, macrocycles, and macrobicycles (cryptands). These have been divided into acyclic (sub-section A) and cyclic (sub-section B). Representative ligands and complexes will be presented and discussed. 

The bipyridyl chromophore has been extensively used in lanthanide coordination chemistry. In addition to those based on the Lehn cryptand (see Section IV.B.4), a number of acyclic ligands have also employed this group. One such ligand is L17, which binds to lanthanide ions such that one face of the ligand is left open (Scheme 3) (60). As expected, luminescence is extremely weak in water and methanol, but stronger in acetonitrile ( = 0.30, 0.14 for europium and terbium, respectively). In addition, the nature of the counter ion can... 

The first cage lanthanide complexes studied for their photophysics were the simple 2.2.1 cryptands. The lack of a strongly absorbing chromophore, and easy approach of solvent molecules meant that their luminescence properties were disappointing in comparison to many recently studied complexes. The Lehn cryptand (L53) (Scheme 6... 

Recently, two cryptands and their lanthanide complexes have been synthesized which include either a bipyridyl (L56) or pyridyl (L57) chromophore (89). These have proved effective at populating the lanthanide excited states. Aqueous luminescence lifetimes of up to... 

It is interesting to note that the same type of interpretation (i.e., the role of OH oscillators in the fcraad term) has been proposed for the Eu2+ lifetime changes induced by complexation with various cryptands and crown-ethers in methanol (for a review of this question, see Jiang et al., 1998). The experimental data are presented in sect. 5, which is devoted to spectroscopic changes induced by complexation. In the case of Eu2+ luminescence in solution, data are scarce because Eu(II) is easily oxidised into Eu(III). Thus, the quantitative interpretation of the data in terms of an equation such as eq. (12b) has not been performed. However, in view of the interesting discussion that has been initiated by Jiang and co-workers, these results will be discussed below in sect. 3.6.4. 

The three Schiff-base bibracchial lariat ethers H2l7a-c derivatives have been designed for their ability to form a cryptand-like cavity upon reaction with Lnm ions, the size of which can be tuned by varying the number of -CH2-CH2-O- units in the macrocycle. The two 2-salicylaldiminobenzyl pendant arms fold in such a way that jt-jt interactions between aromatic rings result in the formation of a cryptand-like cavity, as demonstrated by the X-ray structure of the Cem complex with 17b depicted on fig. 25 (Gonzales-Lorenzo et al., 2003). The triplet state of the di-anionic receptor is located at 18750 cm-1 (0-phonon component measured on the Gd111 complex) so that sensitization of the NIR luminescence is not optimum nevertheless, Ndm emission could be detected. 

Following this study, Korovin and coworkers tested cryptands 23b-f which are used in time-resolved luminescent immunoassays (Mathis, 1993) for the sensitization of Ybm luminescence (Korovin et al., 2002b). From the lifetimes determined in both water and deuterated water, one calculates that the hydration number varies from 2 (23b), to 1.5 (23a, 23e), and finally to 1 (23c, 23d, 23f). Quantum yields were not determined, but luminescence intensities relative to the cryptate with 23a (in water, at room temperature) point to cryptands 23c and 23d being the best sensitizers of the Yb111 luminescence with a seven-fold enhancement, while cryptates with 23e and 23b are only 1.5- to 1.8-times more luminescent. 

Ligand 131 contains both cryptate and crown ether binding units. It reacts with neodymium triflate to give a 1 1 cryptate [Nd c 131]3+ exhibiting NIR luminescence upon excitation at 355 nm. The lifetime of the Nd(4F3/2) level is 347 and 711 ns in methanol and methanol-, respectively, which translates to MeOH 0 using eq. (10b). When barium ions are added to a solution of this complex in acetonitrile, the intensity of the 1.06 pm emission band is reduced substantially, while the lifetime of the Nd(4F3/2) level remains unchanged at about 470 ns. The Ndm ion is therefore not displaced from the cryptand cavity, while its luminescence is modulated by the presence of the Ba2+ ions bound by the crown ether (Coldwell et al., 2006). 

Protons are relatively simple targets for sensor molecules and do not require engineered receptors, however, achievement of selective interactions with other chemical species requires much more elaborate receptors. In the most cases cations are bound via electrostatic or coordinative interactions within the receptors alkali metal cations, which are rather poor central ions and form only very weak coordination bonds, are usually bound within crown ethers, azacrown macrocycles, cryptands, podands, and related types of receptor moieties with oxygen and nitrogen donor atoms [8], Most of the common cation sensors are based on the photoinduced electron transfer (PET) mechanism, so the receptor moiety must have its redox potential (HOMO energy) adjusted to quench luminescence of the fluorophore (Figure 16.3). 

A derivative of the (bpy.bpy.bpy) cryptand, obtained by modifying one of the chains, Lbpy, forms a di-protonated cryptate with EuCb in water at acidic pH, [EuCl3(H2Lbpy)]2+ in which the metal ion is coordinated to the four bipyridyl and two bridgehead nitrogen atoms, and to the three chlorine ions (Fig. 4.25). The polyamine chain is not involved in the metal ion coordination, due to the binding of the two acidic protons within this triamine subunit. In solution, when chlorides are replaced by perchlorate ions, two water molecules coordinate onto the Eu(III) ion at low pH and one at neutral pH, a pH at which de-protonation of the amine chain occurs, allowing it to coordinate to the metal ion. As a result, the intensity of the luminescence emitted by Eu(III) is pH dependent since water molecules deactivate the metal ion in a non-radiative way. Henceforth, this system can be used as a pH sensor. Several other europium cryptates have been developed as luminescent labels for microscopy. 

J. Jiang, N. Higashiyama, K.I. Machida, G.Y. Adachi, The luminescent properties of divalent europium complexes of crown ethers and cryptands, Coord. Chem. Rev. 170, 1-29, 1998. 

KIg. 10.12. Schemaiic represeniation of luminescence immunoassay with a rare-earih cryplate. a cryptand b luminescent rare earth ion c connection to antibody d antibody e antigene f biomoicculc... 

However, nonradiative losses also occur here, because the cryptand does not shield the rare earth ion completely. Even for Tb " " the quantum efficiency is low. This is due to the presence of a charge-transfer state between Tb and the cryptand which leads to nonradiative return to the ground state (Sect. 4.5). Note again the analogy with YVO4 Tb , which also does not luminesce efficiently due to quenching via a charge (or electron) transfer state (Sect. 4.5). 

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