FULL PAPER DOI: 10.1002/ejic.201100084 Theoretical Study of Phosphorescence of Iridium Complexes with Fluorine-Substituted Phenylpyridine Ligands Xin Li, [a,b] Boris Minaev, [b,c] Hans Ågren,* [b] and He Tian* [a] Keywords: Iridium / Fluorine / Organic light-emitting diodes / Phosphorescence / Density functional calculations Time-dependent density functional theory (TD-DFT) with linear and quadratic response approaches was applied to cal- culate absorption and luminescence spectra of a number of facial and meridional iridium complexes with fluorine-substi- tuted phenylpyridine (F n ppy) ligands. The absorption and lu- minescence spectra were studied to simulate the photophysi- cal properties of electroluminescent devices fabricated on the basis of these iridium complexes used to increase spin–orbit coupling and the triplet-state blue emission of the corre- Introduction Heavy-transition-metal complexes, particularly irid- ium(III) and platinum(II) compounds, that contain large π- conjugated ligands, such as phenylpyridines (ppy), bipyr- idines (bpy), and porphyrins, have received tremendous interest because of their potential for full utilization of both the singlet and triplet excitons upon electron–hole annihila- tion in organic light-emitting diodes (OLEDs). [1–5] At the same time, an enhanced triplet generation following photo- induced charge transfer (CT) has recently been reported in organic donor–acceptor polymer blend films that are im- portant for use in photovoltaic devices. [6,7] In all these appli- cations, the singlet–triplet (S–T) transitions are strongly en- hanced relative to typical organic polymer films, like poly- phenylene vinylene (PPV), usually applied in molecular electronics. [2] Such electroluminescent devices typically consist of few layers of a luminescent organic polymer film, like PPV, sandwiched between two metal electrodes. Upon an applied bias, the electrons and holes are injected from the metal electrodes into the polymer. When these opposite-charged carriers start to migrate through the organic polymer, they [a] Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, People’s Republic of China E-mail: tianhe@ecust.edu.cn [b] Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, 106 91 Stockholm, Sweden E-mail: agren@theochem.kth.se [c] Bogdan Khmelnitskij National University, 18031 Cherkassy, Ukraine Eur. J. Inorg. Chem. 2011, 2517–2524 © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2517 sponding organic light-emitting diodes (OLEDs). By using the quadratic response technique, the phosphorescence radi- ative rate constant and lifetime of the studied iridium com- plexes were calculated through spin–orbit coupling pertur- bation and compared with the measured data in experi- ments. A satisfactory agreement between these data permits us to guide improvements in the design of phosphorescence- based OLEDs by predicting the structure–property relation- ships through quantum chemical calculations. can form nearest-neighbor excitons. In this case, the non- geminate pair of the oppositely charged polarons (produced independently at different electrodes) recombines and cap- tures each other, being excited by charge transfer between the singly occupied molecular orbital (SOMO) of the elec- tron carrier and the lowest unoccupied molecular orbital (LUMO) of the hole. Since the charge pairs are nongemi- nate, they have random spin orientation; thus the singlet and triplet colliding pairs are equally probable. [8] In general, the triplet state has three spin projections (M S = 0, 1) and the singlet spin state has only one microstate (M S = 0). The singlet excitons in PPV and in other organic conjugated polymers provide intense luminescence. On the other hand, the radiation from the triplet excitons is spin-forbidden and is therefore much less intense in organic species. [9–11] This triplet–singlet (T 1  S 0 ) transition from the lowest triplet excited state to the singlet ground state provides the phos- phorescence, which is about seven to eight orders of magni- tude weaker than the fluorescence (the S 1  S 0 transition) of organic polymers. [10,11] On this background it has been commonly assumed that the quantum yield of electrolumi- nescence in such OLEDs has an upper statistical limit of 25 %. [12] The triplet excitons in pure organic polymers are dark and spend 75% of the electric energy of the recombi- nation of charge carriers just to heat the polymer. The spin-forbidden phosphorescent T 1  S 0 transition can acquire dipole activity through spin–orbit coupling (SOC); this perturbation is very weak in organic π-conju- gated polymers because the orbital angular momentum ma- trix elements between the optically active singlet and triplet π–π* states of the conjugated systems are negligible, [9,10] and orbital magnetization is practically quenched in such