The Antenna Effect

The overall efficiency of sensitised emission is given by Equation 1.77. 

The efficiency of intersystem crossing and efficiency of energy transfer (p j combine to 

In the course of the pioneering work on sensitised luminescence, Yuster and Weissman smdied the promotion of intersystem crossing due to spin-orbit coupling with heavy atoms [69]. They found that dibenzoylmethanide coordinated to the non-emissive La, Lu and Gd displayed different efficiencies of intersystem crossing, as seen by different ratios of fluorescence versus phosphorescence intensity and changes in phosphorescence lifetimes, directly related to spin-orbit coupling and to the magnetic moment of the lanthanide ion. 

The lifetime t( 5) of the singlet excited state is given by Equation 1.78. 

The emission efficiencies for the fluorescence (f)pi and phosphorescence (j)ph can then be defined by Equations 1.80 and 1.81, respectively. 

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Remarkable concentration of the photon energy absorbed by aromatic dendritic scaffold toward the focal point (herein called antenna effects ) is one of the most distinct features of the dendritic macromolecules. Poly(benzyl ether) (PBE) dendron [1] has been studied most extensively as an efficient antenna dendron. Careful studies have revealed the crucial role of the symmetric dendritic structure for the antenna effect. 

The morphological dependence of the antenna effect was quantitatively studied by the comparison of the three focal isomers of benzene-cored 4th-genera-tion dendrimers [7]. The energy of the photon absorbed by the dendron subunits must be dissipated either through radiative (luminescence) or nonradia-... 

A supramolecular assembly of macromolecules bearing antenna dendron has been reported. Pyrazole-anchored PBE dendrons were synthesized to examine the coordination behavior to transition-metal cations (Cu, Au, Ag) [31]. Self-assembled metallacycles were found. The Cu-metallacycle further formed luminescent fibers about 1 pm in diameter. The luminescence (605 nm) occurred by the excitation of the dendron (280 nm) and the excitation spectrum was coincident with the absorption spectrum of the dendron, suggesting the antenna effect. Interestingly, the luminescence of the Cu-metallacycle fiber disappeared when the fiber was dissociated into the individual metallacycles in C2H2. 

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... 

The antenna effect as it is found in natural photosynthetic systems is an attractive tool for increasing light absorption of solar cells. Some of the work done on dye sensitization of polycrystalline titanium dioxide shows aspects of antenna behavior [76,83-87]. Most of the problems in the systems where an electron is injected into the semiconductor arise in the regeneration process of... 

The low absorption cross-sections of the trivalent ions means that an indirect method of populating the metal excited states is required to generate a significant emission. Such a mechanism is the Antenna Effect. 

The mechanism for sensitized luminescence using the antenna effect is shown schematically in Fig. 2. 

Areas of high microwave flux are checked with a Pelco 36140 microwave bulb array (Ted Pella). Specimens are not placed in areas indicated by illuminated bulbs. Vials containing the specimens are placed in a cold tap water bath (50 ml) that is preheated to the required temperature. The temperature is regulated by placing the microwave temperature probe into a vial of the same solution that is present in the specimen vial. The built-in temperature probe displays the specimen temperature on the oven front panel. The wire that attaches the probe to the oven is submerged in the water to decrease the antenna effect. An additional 400 ml of static water load is placed in the oven at an optimal position determined with the microwave bulb array. This water is changed between every step. 

The principle of the antenna effect is based on long-range interactions (see Section 5.11) between periphery and core. These make it possible to influence... 

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). 

When looking for R(III) determination, use can be made of the antenna effect already discussed in sect. 3.7, provided the lanthanide of interest is luminescent. The basic principle is to add a given organic ligand, which will ensure a high complexation constant with the lanthanide together with an efficient UV absorption and energy transfer to the R(III)... 

An antenna for light harvesting is an organized multicomponent system, in which an array of chromophoric molecules absorbs the incident light and channels the excitation energy to the common acceptor components [49-57], The antenna effect... 

FIGURE 6. The antenna effect in the luminescence of rare-earth complexes. Reproduced by permission from Reference 2, Copyright 2005 the Royal Society of Chemistry... 

Fg/i FIi3/2 and " F9/2 FIi5/2 transitions of Dy +, respectively. The antenna effect was adduced to discuss the energy transfer mechanism between the ligand and the central Dy + ion. ... 

Figure 6. A typical europium cryptate featuring the antenna effect [19].

The unique luminescent properties of rare earth metal clathrochelates have been used in the development of luminescent materials (luminophores and laser materials). The luminescence of these clathrochelates in solution makes their application as biological probes and concentrators of the luminescence (i.e., the antenna effect ) promising. These complexes can also serve as efficient molecular devices to convert UV light absorbed by the ligand to lanthanide ion luminescence in the visible region. Even in very dilute (10-5 mol l-i) solutions, the conversion of irradiated photons to luminescent ones has been observed to occur at a rate of approximately 1%. For rare earth metal aqua ions at the same concentration, the efficiency of conversion is equal to 4 x IQ- % [212, 390-392]. 

In addition to the surface plasmon resonance, other phenomena contribute to the field enhancement. The oscillating dipole of the molecular vibration induces an image field in the metal, sometimes called the antenna effect (1). These effects have also been treated theoretically, leading to a combined theory that predicts overall field enhancement. Detailed discussions of these considerations have been presented and should be consulted by the interested reader (6,7). 

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