Incorporation of Luminescent Species

Coming back to incorporated luminescent species, the text to follow is divided in terms of the active luminescent species, that is, lanthanide ions and dyes. Of course many other important luminescent species, among them transition metal ions and semiconductor nanoparticles (quantum dots (QDs)), can be incorporated in OIH giving rise to several applications, but these will not be treated here. 

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We have discussed how to make highly luminescent species, but we have left unaddressed the more difficult question of how to incorporate specific sensitivity into molecular probes. There are two basic problems. First, one must develop a moiety with the desired specific sensitivity. Second, in doing so, one must not violate the basic criteria established earlier and inadvertently turn off the luminescence or introduce unacceptable photochemical sensitivity. 

A dinuclear cyclometalated platinum(II) complex [Pt2L2 (dppm)] + (L = 4-(jo-diethylphosphonophenyl)-6-phenyl-2, 2 -bipyridine) has been employed as a luminescent probe for SDS micelles. Addition of SDS micelles to an aqueous solution of the complex led to an entission band at 530 mn, which was similar to the MLCT emission of mononuclear species. The observations were attributed to the absence of intramolecular and/or intermolecular Pt(II) Pt(II) interactions when the complex was incorporated in the SDS nticelles. Addition of NaCl solution into a micellar solution of the complex resulted in an additional MMLCT ((da )— (tt )) emission band at 650 nm. It was suggested that addition of sodium cations would force two cyclometalated platinum(II) units sufficiently close for intramolecular metal-metal and/or ligand-ligand interactions to occur, and thus the observation of MMLCT luminescence. 

In both cases, the luminescence of the species A is quenched, and in the case of energy transfer, it can be replaced by the luminescence of species B (sensitization process). In dendrimers, energy- and electron-transfer processes can also occur among nearby molecular components incorporated in the dendrimer structure. For example, in a dendrimer containing A and B component units, excitation of A may be followed by energy (Eq. 10) or electron (Eqs. 11 and 12) transfer to B (5) (Fig. 3b) ... 

Intermolecular reactions taking place between a unit incorporated in the den-drimer structure and an external partner. This is the most commonly observed situation. Often, well-known luminescent species like Ru -polypyridine complexes [30, 31] or metallo-porphyrin species [31] are used as dendrimer cores (Figure 2a). In this case, interesting issues are how the dendritic branches can influence the photophysical properties of the core, and to which extent quencher access to the core can be prevented. 

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