Sensitization of Lanthanide Luminescence

Depending on the electromagnetic nature of Ti , a double-electron exchange (Dexter) mechanism or an electrostatic multipolar (Forster) mechanism have been proposed and theoretically modeled. They are sketched on Fig. 8 for the simple S - T -Ln path. Their specific dependences on the distance d separating the donor D from the acceptor A, i.e., for double-electron exchange and for dipole-dipolar processes, respectively, often limit Dexter mechanism to operate at short distance (typically 30-50 pm) at which orbital overlap is significant, while Forster mechanism may extend over much longer distances (up to 1,000 pm). 

In addition, a i -dependent dipole-quadmpolar mechanism may also be quite effective at short to medium-range distances in fact, depending on Qda it may be as efficient as the dipole-dipole mechanism up to distances as long as 300 pm. 

For the molecular edifices discussed here, another definition of quantum yield ought to be made the overall quantum yield, that is the quantum yield of 

The three components in the middle term represent (1) the efficiency r p pWith which the feeding level ( T, ILCT, LMCT, MLCT, possibly a 4f5d state) is populated by the initially excited state (the corresponding rate constant is if S is excited and T is the donor level, see Fig. 7), (2) the efficiency of the energy transfer (r et) from the donor state to the accepting Ln ion, and (3) the intrinsic quantum yield. The overall sensitization efficiency, rj ens can be accessed experimentally if both the overall and intrinsic quantum yields are known or, alternatively, the overall quantum yield and the observed and radiative lifetimes  

The lifetime method is especially easy to implement for Eu compounds since the radiative lifetime is readily determined from the emission spectrum via (23). Some data are reported in Table 8. 

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Fig. 7. Simplified diagram showing the main energy flow paths during sensitization of lanthanide luminescence. From (Biinzli and Piguet, 2005), reproduced by permission of the Royal Society of Chemistry.

Xu, H.-B., Zhang, L.-Y, Chen, Z.-H., et al (2008) Sensitization of lanthanide luminescence by two different R Ln energy transfer pathways in RLns heterotetranuclear complexes with 5-ethynyl-2,2 -bipyridine. Dalton Transactions, 4664. 

Li, X.-L., Dai, F.-R., Zhang, L.-Y, et al (2007) Sensitization of lanthanide luminescence in heterotrinu-clear PtLn2 (Ln = Eu, Nd, Yb) complexes with terpyridyl-functionalized alkynyl by energy transfer from a platinum(II) alkynyl chromophore. Organometallics, 26, 4483. 

Pope, S.J.A., Coe, B.J., Faulkner, S., et al. (2004) Self-assembly of heterobimetalhc d-f hybrid complexes sensitization of lanthanide luminescence by d-block metal-to-Ugand charge-transfer excited states. Journal of the American Chemical Society, 126, 9490. 

Fig. 7. Simplified diagram showing the main energy flow paths during sensitization of lanthanide luminescence.

Lanthanide chelates also can be used in FRET applications with other fluorescent probes and labels (Figure 9.51). In this application, the time-resolved (TR) nature of lanthanide luminescent measurements can be combined with the ability to tune the emission characteristics through energy transfer to an organic fluor (Comley, 2006). TR-FRET, as it is called, is a powerful method to develop rapid assays with low background fluorescence and high sensitivity, which can equal the detection capability of enzyme assays (Selvin, 2000). 

Due to the presence of hard anionic oxygen atoms, phenolate and carboxylate groups are often employed as donors in lanthanide coordination chemistry. Ligand [L18]4- is reported as an excellent triplet sensitizer for lanthanide luminescence (61). Indeed aqueous lifetimes of 0.57 and 1.61 ms are reported for europium and terbium, respectively quantum yields of 0.20 and 0.95 respectively refer to the efficiency of the energy transfer process alone. 

Fig. 5. Schematic representation of the sensitization process of lanthanide luminescence via the surroundings of the...

The preparation, characterization, aqueous stability, and photophysical properties of NIR emitting lanthanide complexes with tetradentate chelating ligands 36 and 37 were described by Raymond and coworkers [61, 62]. In aqueous solution, the chelating ligand 36 or 37 forms stable complexes with Ln(III) ions, and sensitized NIR lanthanide luminescence was detected for the complexes with Pr(III), Nd(III), Ho(III), or Yb(III) ions. For [Ln(36)2] complexes, the luminescence decay curves were biexponential due to partial hydrolysis of the complexes or alternately the presence of a slowly exchanging equilibrium mixture with a hydrated form of the complexes. For [Ho(37)2] , the NIR band due to Fs -> I transition of the Ho(III)... 

Porphyrin is one of the most widely studied macrocyclic systems suitable for complexation with lanthanide(III) ions. Porphyrins can absorb strongly in the UV-vis region so as to serve as efficient photo-sensitizers, making lanthanide(III)porphyrinate complexes ideal candidates for luminescence imaging agents. Indirect excitation of porphyrin antenna chromopheres in close proximity to lanthanide ions can make the energy in the triplet state of the porphyrin ligand transfer efficiently to the excited state of the lanthanide ion so as to sensitize the lanthanide luminescence, particularly NIR emission. 

Mono- or dinuclear platinum(II) complex capped with one or two [Pt(Bu3tpy)]+ units can be incorporated with Ln(hfac)3 units to produced a series of PtLn (Ln = Nd 88, Yb 89) and Pt2Ln (Ln = Nd 90, Yb 91) heteronuclear complexes [131]. With excitation at 350 nm < L < 550 nm, sensitized NIR lanthanide luminescence was detected in these Pt-Ln heteronuclear complexes with microsecond ranges of lifetimes, whereas Pt-based luminescence from the MLCT and LLCT states was mostly quenched. This reveals that fairly effective Pt Ln energy transfer is operating from the platinum(II) terpyridyl alkynyl chromophores to the lanthanide(III) centers. 

Veggel and coworkers [137, 138] first reported the use of ruthenium(II) tris(2,2 -bipyridine) complexes ([Ru(bpy)3] +) and ferrocene as light-harvesting chromophores for sensitization of NIR luminescence from Nd(III) and Yb(III) ions. The Ru-Ln complexes (Ln = Nd 104, Yb 105) resulted from incorporating [Ru(bpy)3] + with m-terphenyl-based lanthanide complexes. Upon excitation of the Ru(bpy)s chromophore absorption with visible light up to 500 nm, both Ru-Nd and Ru-Yb complexes exhibited typical NIR luminescence because of effective Ru Ln energy transfer with the rates of 1.1 x 10 s for Ru-Nd complex and <1.0 X 10 s for Ru—Yb species. 

In addition, pyrazoyl-azaxanthone can also be used as a sensitizer of lanthanide complexes. An emissive terbium complex Tb-58 incorporating a pyrazoyl-l-aza-xanthonechromophore (Figure 13.25) exhibits cellular uptake and possesses a much lower sensitivity to excited state quenching. For example, Tb-58 was incubated for varying periods of time (from 1 to 12 h 50 or 100 xM complex) with CHO or NIH/3T3 cells. Examination of the loaded cells by luminescence microscopy revealed complex uptake, and localization within endosomes in the cytoplasm, presumably following receptor mediated endocytosis, but no tendency to nuclear localization [75]. 

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