Energy transfer antenna effect

The antenna effect is illustrated in Fig. 6. The necessary conditions for its efficient implementation are the high molar absorbance of antenna dyes, efficient energy transfer to acceptor dye, and high quantum yield of emission of the latter. At the same time, while evaluating the highly increased apparent brightness of the... 

Fig. 6 Illustration of antenna effect. Great number of donors (D) absorb light and transfer the excitation energy to a small number of acceptors (A). Excitation of acceptors via FRET increases their emission intensity and, often, lifetime in comparison with direct excitation. Homo-FRET between the donors is not shown...

Nowadays, most reaction center models carry suitable antenna pigments and acceptor groups and in effect are photosystem models. A typical example for a state-of-the-art system that incorporates many aspects of a photosystem consisted of a boron dipyrrin covalently linked to a zinc(II) porphyrin, which carried a suitably modified C60 derivative as axial ligand. Selective excitation of the boron dipyrrin as antenna pigment resulted in energy transfer to a zinc(II) porphyrin followed by electron transfer to the acceptor109. 

An important measure of the luminescence is the quantum yield. In effect, this is the probability that a photon will be emitted by the lanthanide given that one photon has been absorbed by the antenna ligand. Since measurement of absolute quantum yields is particularly difficult, the overall quantum yield ( ) is normally measured with reference to certain standards (26) these are routinely [Ru(bpy)3]2+ in water or SulfoRhodamine 101 in methanol for Eu3 +, and quinoline sulfate in 0.1 M HC1 or fluorescein in 1 N NaOH for Tb3+ (27,28). A method has been developed that measures energy transfer from the lanthanide complex to an acceptor of known quantum yield (28). 

As the energy of the excited states and the redox levels of each metal-polypyridine unit depend on metal and ligands in a predictable way, the simultaneous presence of different metals in a dendritic structures gives rise to intramolecular energy transfer processes as well to different redox patterns with multielectron processes. In particular, the tetranuclear [Os(2,3-dpp)3 (2,3-dpp)Ru(bpy)2 3]8+ (OsRu3) shown in Fig. 5.3 has been designed to achieve an efficient antenna effect. This species can also be considered a first-generation mixed-metal dendrimers.31... 

Compound 99 was made more hydrophobic by butylation of the 2-OH and 3-OH groups [88], The prepared CD derivative (100) solubilized 9-anthrylmethyl pivalate (AP) much more effectively than 99 or (3-CD. In the presence of AP, the naphthyl emission (Xex = 314 nm) of 100 was efficiently quenched and intense AP emission was observed. Because essentially all of the light is absorbed by the naphthyl chromophores, the strong AP fluorescence demonstrates that energy transfer occurs efficiently. The naphthyl antenna chromophores of 100 were shown to sensitize a selective photoreaction of AP included in the CD cavity (Scheme 2). The photoirradiation of AP in methanol gives 9-neopentylanthracene... 

Another water-soluble (3-CD (105) bearing seven naphthoyl chromophores forms very stable 1 1 complexes with a merocyanine laser dye DCM-OH [90], The energy transfer from the naphthoyl antenna chromophores to the included dye is shown to occur with 100% efficiency (antenna effect) (Scheme 3). 

The antenna effect of 105 was also used for reaction of nitrone as a guest [91]. In this case, ismerization of nitrone to the product was promoted by energy transfer from the antenna (Scheme 4). 

Note that in the case where the Forster mechanism concerns an organic species transferring its energy to the lanthanide center to which it is bound, this is often referred to as the antenna effect and the ligand is sometimes called a sensitiser. Such an effect becomes efficient provided that the organic chromophore possesses a triplet excited state close to but at least 1700 cm-1 above that of the lanthanide emissive state (Parker and Williams, 1996). 

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