Iridium chromophore

Let us first consider the situation when porphyrin chromophores are excited. In this case, only electron transfers are possible within the triads, and also within the model dyads (not represented) used as reference compounds. Upon excitation of the iridium chromophore in the UV, an ET process to both porphyrins occurs. In these systems, it is interesting to note that the nature of the solvent used for the photophysical studies has a strong impact on the photoinduced processes and on the lifetimes of the CS states generated. These data are summarized in Figure 13.61 in the form of energy diagrams. 

This is in contrast to the results obtained following selective excitation of the PH2 unit discussed above, and yielding a multi-step electron transfer leading to charge separation. The different outcome can be discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semi-empirical calculations [48]. Remarkably, the zinc porphyrin based array PZn-Ir-PAu, 254+, displays an efficient electron transfer with the formation of a CS state with unitary yield also upon excitation of the iridium complex. This happens because the selective excitation of the zinc porphyrin chromophore discussed above, and the deactivation of the excited state PZn-3Ir- PAu, follow the same paths as those reported in Scheme 8. 

These approaches have been adopted more recently to incorporate phosphorescent chromophores into PF in order to make use of the fact that a large proportion (up to 75%) of all excitons formed in LEDs are triplet states, whose energy can only be harvested by using phosphorescent units. The first fluorene copolymers with phosphorescent units 34-35 were made by Holmes and coworkers who added monobrominated red- or green-emitting iridium complexes to an AA-BB Suzuki polycondensation [57]. With short fluorene chains, only emission from the iridium complexes are observed, but with longer fluorene chains some blue emission is also seen. Other groups have since incorporated different phosphorescent units such as platinum [58] or zinc salen [59] units or porphyrins [60,61 ]. 

The electroluminescence (EL) efficiency and the emission energy of iridium(III) complex based devices are greatly influenced by the organic ligand chromophores (Tang VanSlyke 1987 Tang VanSlyke 1989 Baldo et al. 1989). In the way to improve and tune the emission colors, we synthesized and reported the iridium(III) complexes using 2-... 

Neve, F Crispini, A. Serroni, S. Loiseau, F. Campagna, S. (2001). Novel dinuclear luminescent compounds based on iridium(III) cyclometalated chromophores and containing bridging ligands with ester-linked chelating sites. Inorganic Chemistry, Vol. 40, pp. 1093-1101. 

Polystyrenes have also been used to support chromophores useful in organic light-emitting diodes (OLEDs). Week and coworkers have attached tris(2-phenylpyridine) iridium complexes to aminomethylated polystyrene using a Schiff base reaction, 4 [21]. There was no major diminution of the desirable luminescence properties of the iridium complexes (high emission quantum yields of 0.23 and lifetimes of about a microsecond). Similar results have been reported for aluminum and boron 8-hydroxy quinoline complexes tethered to polystyrene using Schiff base condensation [22]. 

Flamigni, L., G. Marconi, I.M. Dixon, J.P Collin, and J.P Sauvage (2002). Switching of electron- to energy-transfer by selective excitation of different chromophores in arrays based on porphyrins and a polypyridyl iridium complex. J. Phys. Chem. B 106, 6663-6671. 

Scheme 29.7 Hybrid excited states including excitation, charge transfer, and triplet emission from Pt to Ir chromophores in conjugated Pt-Ir polymer containing poly[traw5 -[(5,5 -ethynyl-2,2 -bipyridine) bis(phenyl-2-yl-pyridine)-iridium(III)].

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