Substituent Effect on the Luminescent Properties of a Series of Deep Blue Emitting Mixed Ligand Ir(III) Complexes Yi-Yeol Lyu, †,‡ Younghun Byun, † Ohyun Kwon, † Eunsil Han, † Woo Sung Jeon, † Rupasree Ragini Das,* ,† and Kookheon Char* ,‡ Samsung AdVanced Institute of Technology, P.O. Box 111, Suwon 440-600, Korea, and School of Chemical and Biological Engineering and NANO System Institute-National Core Research Center, Seoul National UniVersity, Seoul 151-744, Korea ReceiVed: December 22, 2005 The syntheses of the bright deep blue emitting mixed ligand Ir(III) complexes comprising two cyclometalating, one phosphine and one cyano, ligands are reported. In this study, a firm connection between the nature of the excited states and the physicochemical behavior of the complexes with different ligand systems is elucidated by correlating the observed crystal structures, spectroscopic properties, and electrochemical properties with the theoretical results obtained by the density functional theory (DFT) methods. The cyclometalating ligands used here are the anions of 2-(4′,6′-difluorophenyl)-pyridine (F 2 ppy), 2-(4′,6′-difluorophenyl)-4-methyl pyridine (F 2 ppyM), and 4-amino-2-(4′,6′-difluorophenyl)-pyridine (DMAF 2 ppy). The phosphine ligands are PhP(O- (CH 2 CH 2 O) 3 -CH 3 ) 2 and Ph 2 P(O-(CH 2 CH 2 O) n -CH 3 ), where Ph ) phenyl and n ) 1(P1), 3 (P3), or 8 (P350). The thermal stabilities of the complexes were enhanced upon increasing the “n” value. The crystal structures of the complexes, [(DMAF 2 ppy) 2 Ir(P1)CN], (P1)DMA, and [(F 2 ppyM) 2 Ir(P3)CN], (P3)F2M, show the cyano and phosphine groups being in a cis configuration to each other and in a trans configuration to the coordinating C ring atoms. The long Ir-C ring bond lengths are ascribed to the trans effect of the strong phosphine and cyano ligands. DFT calculations indicate that the highest occupied molecular orbital (HOMO) is mainly contributed from the d-orbitals of the iridium atom and the π-orbitals of cyclometalating and cyano ligands, whereas the lowest unoccupied molecular orbital (LUMO) spreads over only one of the cyclometalating ligands, with no contribution from phosphine ligands to both frontier orbitals. Dimethylamino substitution increases the energy of the emitting state that has more metal-to-ligand-charge-transfer (MLCT) character evidenced by the smaller vibronic progressions, smaller difference in the 1 MLCT and 3 MLCT absorption wavelengths, and higher extinction coefficients (ǫ) than the F 2 ppy and F 2 ppyM complexes. However, the increase in the basicity of the dimethylamino group in the DMAF 2 ppy complexes in the excited states leads to distortions and consequent nonradiative depopulation of the excited states, decreasing their lower photoluminescence (PL) efficiency. The effect of the substituents in the phosphine ligand is more pronounced in the electroluminescence (EL) than in the PL properties. Multilayer organic light emitting devices (OLEDs) are fabricated by doping the Ir(III) complexes in a blend of mCP (m-bis(N-carbazolyl benzene)) and polystyrene, and their device characteristics are studied. The (P3)F2M complex shows a maximum external quantum efficiency (η ex ) of 2%, a maximum luminance efficiency (η L ) of 4.13 cd/A at 0.04 mA/cm 2 , and a maximum brightness of 7200 cd/m 2 with a shift of the Commission Internationale de L’Eclairage (CIE) coordinates from (0.14, 0.15) in film PL to (0.19, 0.34) in EL. Introduction The photophysics and electroluminescence of cyclometalated heavy transition metal complexes have drawn much attention since the utilization of the green emitting phosphorescent, tris- cyclometalated Ir(III) complex, Ir(ppy) 3 , 1 as a dopant in organic light emitting devices (OLEDs). 2 Due to the strong spin-orbit coupling constant of these heavy transition metals, the singlet and triplet metal-to-ligand-charge-transfer (MLCT) states are mixed together, and the triplet MLCT state ( 3 MLCT) can emit effectively by burrowing the intensity from the singlet MLCT state ( 1 MLCT). 3 These complexes are able to provide 100% internal efficiency, as they can utilize both the singlet and triplet excitons on the host material through the Fo ¨rster and Dexter energy transfer. 4 For the development of full color phospho- rescent light emitting devices, the realization of all three green, 5 red, 6 and blue 7 (RGB) colors is necessary. The colors of phosphorescent complexes used in such devices are tuned by the variation of both cyclometalating and ancillary ligands. 8,9 Until now, several small molecular and polymeric blue emitting phosphorescent complexes 7,10,11 have been re- ported, but the realization of highly efficient deep blue emission is still a challenge to achieve. Both phosphine and cyano groups are known as very strong field ligands, and their inclusion in the coordination sphere can increase the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular or- bital (LUMO) gap to achieve the hypsochromic shift in the emission color. 12 Ir(III) complexes of N-phenyl pyrazole and * To whom correspondence should be addressed. E-mail: rd.rupasree@ samsung.com (R.R.D.); khchar@plaza.snu.ac.kr (K.C.). † Samsung Advanced Institute of Technology. ‡ Seoul National University. 10303 J. Phys. Chem. B 2006, 110, 10303-10314 10.1021/jp057446s CCC: $33.50 © 2006 American Chemical Society Published on Web 05/10/2006