Phosphorescent dendrimers

TD Anthopoulos, MJ Frampton, EB Namdas, PL Burn, and IDW Samuel, Solution-processable red phosphorescent dendrimers for light-emitting device applications, Adv. Mater., 16 557-560, 2004. 

Charge-neutral metallic dendrimer complexes, (II), were prepared by Samuel et al. (2), which were luminescent in the solid state. In separate investigations phosphorescent dendrimers were prepared by Lo et al., (Ill), and Bum et al. 

Another very effective method of preventing triplet-triplet annihilation was reported by Burn and coworkers using dendrimer ligands to surround and protect the iridium core. The color of the emission can be tuned by altering the core of the molecule and the properties of the material controlled by variation of the dendritic structure [42]. For example, the iridium dendrimer 13 has an external quantum efficiency of 10.4% in a single layer device. These phosphorescent dendrimers also have the additional advantage that they are sufficiently soluble and massive enough to be spin-coated from solution. 

KO 10] Ko L.-C., Liu T.-Y., Chen C.-Y. et al, Multi-layer organic hght-emitting diodes processed from solution using phosphorescent dendrimers in a polymer host , Organic Electronics, vol. 11, no. 6, pp. 1005-1009, 2010. 

J.P. Markham, S.C. Lo, S.W. Magnnis, P.L. Burn, and I.D.W. Samuel, High-efficiency green phosphorescence from spin-coated single-layer dendrimer light emitting diodes, Appl. Phys. Lett., 80 2645-2647 (2002). 

Fig. 8 Energy level diagrams for the dansyl units of dendrimer 11 and the investigated lanthanide ions. The position of the triplet excited state of 11 is uncertain because no phosphorescence can he observed...

The photophysical effects and mechanisms described in general terms in the above sections will now be illustrated with the aid of findings and interpretations from the world of dendrimers. Since straightforward luminescence effects are also mentioned elsewhere in this book in the context of syntheses (Chapter 2) and the individual types of compounds (Chapter 4) and are, moreover, considered as part of more complex events in Section 5.2, just a few characteristic examples of fluorescence and phosphorescence of dendrimers will be presented The adjacency of many groups in a molecule may lead to the occurrence of complex processes, which sometimes make it difficult to draw precise conclusions, but may sometimes also be useful and amenable to optimisation for various applications. 

For phosphorescence, lanthanide metal ions can be used in a similar manner. Lanthanide ions have very interesting photophysical properties, but often exhibit weak absorption bands, and aggregate to form clusters, which limit their applications. Thus, a dendrimer that can provide a protective shell to isolate a cation and at the same time enhance the emission by transfer from the periphery to the lanthanide ion at the core could be of great interest. Self-assembled lanthanide-cored dendrimers have been prepared to prove such an assumption synthesis was carried out by mixing three equivalents of polyaryl ether dendrons bearing carboxylic acid entity at the focal point with Ln(III) cations [Er(III), Tb(III), and Eu(III)] (Fig. 5.4) [34]. The authors demonstrated that the enhancement of the lanthanide cation emission associated with the dendritic core shell was observed, and an antenna effect from the periphery to the core was shown to promote this process. 

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