Phosphorescence color tuning

EB Namdas, TD Anthopoulos, IDW Samuel, MJ Frampton, SC Lo, and PL Burn, Simple color tuning of phosphorescent light-emitting diodes, Appl. Phys. Lett., 86 161104-161106, 2005. 

Tuning of Phosphorescence Colors in Neutral Iridium Complexes. 151... 

Tuning of Phosphorescence Colors in Cationic Iridium Complexes. 154... 

Tuning of Phosphorescence Colors in Anionic Iridium Complexes. 158... 

In order to tune the phosphorescence color of [Ir(ppy)3 ], Watts et al. synthesized several substituted ppy-based neutral Ir complexes [96-98]. Table 1 shows the list of complexes that show strong phosphorescence from a3 MLCT excited state. The phosphorescence lifetime of these complexes is in the range of 2-3 ps in nitrogen-saturated acetonitrile at room temperature [97]. The photophysical and electrochemical data (see Table 1) demonstrate the influence of ligands bearing electron-withdrawing and the electron-donating... 

The tuning aspects of phosphorescence colors in anionic Ir complexes are illustrated using the following representative complexes The (C4H9)4N[Ir(2-phenylpyridine)2 (CN)2] (46), (C4H9)4N[Ir(2-phenyl)-4-dimethylaminopyr-idine)2(CN)2] (47), (C4H9)4N[Ir(2-(2,4-difluorophenyl)-pyridine)2(CN)2]... 

Phosphorescent Color Shift in Anionic Iridium Complexes by Tuning of HOMO Levels... 

In addition to the incorporation of fluorescent chromophores in PF, phosphorescent moieties have been widely used to copolymerize with fluorene or as pendent group in PFs for color tuning (via the selection of ligands chelated to heavy metals) and performance improvement. The ability to enhance device performance via phosphors is because strong spin-orbit coupling (resulting from the inclusion of heavy metal atoms in the phosphor structure) can efficiently utilize triplet excitons for electroluminescence and theoretically there are three times as many as triplet as singlet excitons. 

The first dye used in an electrophosphorescent LED was the terbium-complex Tb(acetyhlacetonate)3 (Tb(acac)3, see Fig. 11.2) [21], LEDs based on complexes with rare-earth central metals such as terbium or europium are very interesting for display applications, because they emit light with a very small spectral line width. These sharp emission lines are due to f-f transitions located on the central metal ion. Disadvantages of these complexes are, however, that color tuning via the chemical modification of the ligand is not possible and that the radiative lifetime of phosphorescence is rather long. 

Thompson, M. E., Li, J., Tamayo, A. et al. 2005. Color tuning dopants for elec-trophosphorescent devices Towards efficient blue phosphorescence from metal complexes. SID Inti. Symp. Dig. Tech. Papers 36 1058. 

Abstract Mononuclear Ir(III)-polyimine complexes show outstanding luminescence properties, i.e., high intensities, lifetimes in the is time range, and emission wavelengths that can be tuned so as to cover a full range of visible colors, from blue to red. We discuss the approaches for the use of ligands that afford control on luminescence features. Emphasis is placed on subfamilies of cyclometalated complexes, whose recent enormous expansion is motivated by their potential for applications, including that as phosphorescent dopants in OLEDs fabrication. The interplay of the different excited states associated... 

By altering the structure of the ligand attached to the iridium, the color of the emission can be tuned, and red, green, and blue phosphorescent devices have been prepared [39]. 

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. 

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