Phosphorescence Color Tuning of Cyclometalated Iridium Complexes by <italic>o</italic>-Carborane Substitution

Heteroleptic (CN)2Ir(acac) (C ∧N = 4-CBppy (1); 5-CBppy (2), 4-fppy (4) CB = ortho-methylcarborane; ppy = 2-phenylpyridinato-C,N, 4-f ppy = 2-(4-fluorophenyl)pyridinato-C,N, acac = acetylacetonate) complexes were prepared and characterized. While 1 exhibits a phosphorescence band centered at 531 nm, which is red-shifted compared to that of unsubstituted (ppy)2Ir(acac) (3) (λem = 516 nm), the emission spectrum of 2 shows a blue-shifted band at 503 nm. Comparison with the emission band for the 4fluoro-substituted 4 (λem = 493 nm) indicates a substantial bathochromic shift in 1. Electrochemical and theoretical studies suggest that while carborane substitution on the 4position of the phenyl ring lowers the MLCT energy by a large contribution to lowest unoccupied molecular orbital (LUMO) delocalization, which in turn assigns the lowest triplet state of 1 as [dπ(Ir)→π*(C ∧N)] MLCT in character, the substitution on the 5-position raises the MLCT energy by the effective stabilization of the highest occupied molecular orbital (HOMO) level because of the strong inductive effect of carborane. An electroluminescent device incorporating 1 as an emitter displayed overall good performance in terms of external quantum efficiency (6.6%) and power efficiency (10.7 lm/W) with green phosphorescence. ■ INTRODUCTION Phosphorescent heavy metal complexes have attracted a great deal of interest as emitting materials in organic light-emitting diodes (OLEDs) because of their excellent properties such as good color purity, high quantum efficiency, relatively short phosphorescence lifetime, and high photoand thermal stability. In particular, modification of the cyclometalating ligand (C∧N ligand) has enabled control of emission color over the entire visible region that can be beneficial for realizing fullcolor and white-light displays. The tuning of phosphorescence color has usually been achieved by the variation of the substituent on the C∧N ligand, for example, ppy (2-phenylpyridinato-C,N), since the emissive lowest-energy excited states such as MLCT and π−π* (LC) are largely involved with the electronic structure of the ligand. Experimental and theoretical studies reported to date have established the basic principles for the emission color control as follows: (i) the highest occupied molecular orbital (HOMO) level of the complex is dominated by the π orbital of the phenyl ring of the ppy ligand together with the metal t2g manifold (dπ orbitals) and (ii) the π* orbital of the pyridyl ring of the ppy ligand governs the lowest unoccupied molecular orbital (LUMO) level. Consequently, the introduction of an electron-withdrawing group such as a fluorine atom into the phenyl ring of the ligand usually gives rise to an increase in the HOMO−LUMO band gap by lowering the HOMO level, while introduction into the pyridyl ring narrows the band gap by lowering the LUMO level. As a result, the energy level of the triplet state formed from the HOMO−LUMO transition can be fine-tuned in accordance with the change in the band gap. Moreover, the orbital analyses further suggest that the position of the substituent on each ring of ligand is also important for tuning the emission wavelength. For example, the electronwithdrawing effect of a fluorine atom attached on the 4and/or 6-carbon positions of the phenyl ring gives a large blue shift by the greater HOMO stabilization than LUMO, but substitution at the 5-position offsets the electron-withdrawing effect of the fluorine atom by weak π-donation from fluorine into the electron density of the HOMO at this position, thereby reducing the band gap. Although these approaches to phosphorescence color tuning by use of the electron-donating or -withdrawing substituents Received: July 18, 2012 Published: December 20, 2012 Article