Carbazole dendrimers

A. Kimoto, J. Cho, M. Higuchi, and K. Yamamoto, Novel carbazole dendrimers having a metal coordination site as a unique hole-transport material, Macromolecular Symposia 209 (Organo-metallic and Coordination Clusters and Polymers) pp. 51-65 (2004). 

For some examples, see e.g. (a) Hogan CF, Harris AR, Bond AM et al (2006) Electrochemical studies of porphyrin-appended dendrimers. PhysChemChemPhys 8 2058-2065 (b) Jang W-D, Nishiyama N, Zhang G-D et al (2005) Supramolecular nanocarrier of anionic dendrimer porphyrins with cationic block copolymers modified with polyethylene glycol to enhance intracellular photodynamic efficacy. Angew Chem Int Ed 44 419 -23 (c) Loiseau F, Campagna S, Hameurlaine A et al (2005) Dendrimers made of porphyrin cores and carbazole chromophores as peripheral units. Absorption spectra, luminescence properties, and oxidation... 

More examples include the use of Ir(III) complexes in dendrimers [136] and as pendants in oligo- or polymeric lluorene (or carbazole), in part already discussed in Sect. 3.4 [76,137,138]. Both cases illustrate viable methods to solve the problem of phase separation in electroluminescent polymers doped with Ir(III) complexes and are not treated here in detail. 

Dehaen and coworkers utiUzed carbazole dendrons attached to 1,10-phenanthroline ligands (phen) to construct a dendrimer around a Ru(ll) core (12, Scheme 6) (12]. Absorbance transitions assigned to carbazole, phen, and carbazole-to-phen CT transitions aU gave rise to emission from the Ru(ll) core, providing yet another hght-harvesting dendrimer, which illustrates the antennae effect. 

Two similar systems with carbazole donor chromophores and porphyrin core acceptors were reported by the groups of Dehaen [78] and Lu [79]. With varying linker architecture and branching methods, Dehaen s system included dendrimers with 4, 8, and 12 carbazole chromophores per porphyrin core (Scheme 27). Lu s system also included materials with 4, 8, and 12 carbazole chromophores per porphyrin core, but with a different branching architecture (Scheme 27). The absorption spectra of the dendrimers in both of these studies are additive, with the visible transitions of the porphyrin cores. 

Scheme 27 Porphyrin core dendrimers with carbazole dendrons by Dehaen [78] (top) and Lu [79] (bottom)... 

Tian and co-workers have prepared and investigated the properties of flexible dendrons and dendrimers that have NI or PDI cores with carbazole (CZ) or oxadiazole (OXZ) peripheral units (Scheme 38) [103,104]. Excitation of the peripheral chromophores result in emission of the NI of PDI cores. Interestingly, in the case of the NI core dendrons, excitation of the peripheral oxadiazole residues of 75c results in a 3.9 times enhancement of core liuni-nescence, while excitation of the peripheral carbazole residues of 76c results in only a 20% emission intensity, suggesting a second pathway for donor quenching in the CZ systems. This pathway is probably photo-induced electron transfer (PET) from the CZ units to the NI core [104], The OXZ units, however, have a relatively higher electron affinity and no PET can take place. 

FIGURE 5.7 Second-generation irido-dendrimers with carbazole dendrons used for the formation of highly efficient green light- (left) and red light (ng/ Remitting diodes. 

J. Lu, P. F. Xia, P. K. Lo, Y. Tao, M. S. Wong, Synthesis and properties of multi-tiiarylamine-suhstituted carbazole-hased dendrimers with an oUgothiophene core for potential appUcations in organic solar cells and light-emitting diodes, Chem. Mater., 18, 6194-6203 (2006). 

PAR 11a] Park J.Y., Kim D.-E., Ponnapati R. et al, Electroluminescent properties of an electrochemically cross-linkable carbazole peripheral poly(benzyl ether)dendrimer , ChemPhysChem, vol. 12, no. 5, pp. 1010-1015, 2011. 

Other important feature of triarylamine-zinc porphyrin dendrimers is the intramolecular energy transfer from the triarylamine moieties to the zinc porphyrin unit. Interestingly, zinc porphyrin fluorescence emission takes place with higher quantum yield upon excitation in the triarylamine moieties UV absorption bands than upon direct excitation of the ZnTPP unit. Similar antenna effect has been previously reported for a series of carbazole-porphyrin dendrimers [132] encompassing up to 21 redox centers. Their electrochemical pattern is strongly dependent on the way the carbazole are structured along the dendrimer branches and connected to the porphyrin core. 

Loiseau F, Campagna S, Hameurlaine A, Dehaen W (2005) Dendrimers made of porph)nin cores and carbazole chromophores as peripheral units. Absorption spectra, luminescence properties, and oxidation behavior. J Am Chem Soc 127(32) 11352-11363... 

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