Correlating Experimental Photophysical Properties of Iridium(III

Article pubs.acs.org/JPCC

Correlating Experimental Photophysical Properties of Iridium(III) Complexes to Spin−Orbit Coupled TDDFT Predictions Jarod M. Younker* and Kerwin D. Dobbs Central Research and Development, E. I. DuPont de Nemours and Co. Inc., RT 141 Henry Clay, Wilmington, Delaware 19880, United States S Supporting Information *

ABSTRACT: Ir(III) complexes are efficient phosphorescent emitters. The transition dipole moment between the triplet and singlet manifolds is formally spin forbidden. However, spin−orbit coupling (SOC), induced by the high angular momentum orbitals in Ir, efficiently mixes the triplet manifold with higher-energy singlets, increasing the transition probability. Spin−orbit coupled time-dependent density functional (TDDFT) calculations within the zero-order relativistic approximation (ZORA) are used to study nine complexes that have a range of emissions from 450 to 630 nm and quantum efficiencies of 0.1−0.9. We find that using the singlet groundstate geometry to calculate radiative rates produces the best correlation with experiment. We also show that the equal thermal population of the three sublevels in the triplet manifold is sufficient to understand rates at 300 K. We find that emission energies and radiative rates are best reproduced at the TD-B3LYP/TZP/DZP//BP86/TZ2P/TZP level of theory, where the larger basis set is supplied for Ir.

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INTRODUCTION

A growing number of reports in the literature are using timedependent density functional theory (TDDFT) to predict emission energies and lifetimes (using S0 → T1 vertical excitation energies), with the goal to understand what controls quantum efficiency and the color of phosphorescence at the molecular level.6−27 In order to accurately predict the emission energy and lifetime of an iridium complex, the geometry employed is critical. Both the optimized excited-state triplet and ground-state singlet geometries have been advocated by different groups. It is given that exciton recombination populates the T1 manifold, from which phosphorescent emission is observed. A review of the literature found a handful of reports where emissive properties were predicted using an optimized triplet geometry.8,9,16,18,19,24 Conversely, others have reported that emission properties are better correlated with experiment using the optimized singlet geometry.7,10,12,22 In fact, two reports that looked at emission properties from both geometries found better experimental correlation when vertical excitation energies were calculated at the singlet geometry.20,22 Phosphorescence is a long-lived process (on the order of microseconds), so it is anticipated that the triplet will have time to reorganize to a lower-energy geometry. Jansson et al. looked at the potential energy surface cross section of Ir(ppy)3, along the vibrational mode corresponding to the geometric change between the singlet and triplet geometries.22 They found that the triplet occupied a shallow anharmonic potential such that

A key property in organic light-emitting diodes (OLEDs), sensors, probes, imaging agents, and photosensitizers is molecular luminescence. Exciton recombination provides the required energy for these applications. Unfortunately, some of the materials used suffer from low electroluminescence quantum yield due to spin statistics. During exciton recombination, two non-geminate electrons couple to give a singlet and triplet spin manifold. Spin statistics limit the fluorescent quantum efficiency to a maximum of 25% for purely organic molecules. The remaining energy is lost from the triplet manifold in the form of heat. However, efficiency can be increased to nearly 100% in transition metal complexes featuring π-conjugated ligands. The presence of the metal center performs two important roles. First, spin−orbit coupling (SOC) increases intersystem crossing from the excited singlet manifolds (Sn, n > 0) to the triplet manifold (T1). This process has been referred to as triplet harvesting and is well documented in the literature.1,2 Second, the formally spin forbidden T1 → S0 transition is activated via SOC.1−3 The matrix element between the ground state (S0) and T1 π−π* states in purely organic systems is negligible due to low orbital angular momentum. Transition metals have high orbital angular momentum that couples efficiently to the electron spin, activating the previously forbidden transitions. The emitter tris(2-phenylpyridine) iridium or Ir(ppy)3 is prototypical of the process of triplet harvesting.4,5 Ir(ppy)3 emits a green color with ∼90% efficiency and has been used as a springboard for similar phosphorescent emitters. © 2013 American Chemical Society

Received: October 25, 2013 Revised: November 15, 2013 Published: November 18, 2013 25714

dx.doi.org/10.1021/jp410576a | J. Phys. Chem. C 2013, 117, 25714−25723

The Journal of Physical Chemistry C

Article

Figure 1. The cyclometalated iridium(III) complexes studied and their abbreviations.

the maximum of the vibrational wave function lies between the S0 and T1 equilibrium positions. The goal of this investigation is to develop an efficient computational protocol for reliably predicting phosphorescent emission energies and lifetimes for organoiridium complexes. This protocol was validated against the published experimental results for nine Ir complexes with reported phosphorescent wavelengths from 450 to 630 nm and quantum efficiencies of 0.1−0.9 (Figure 1 and Table 1). In developing the protocol, both ground-state singlet and first excited-state triplet

structures were optimized, using standard functionals and basis sets. In addition, the emission energies and lifetimes were explored with respect to changes in the basis sets.

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COMPUTATIONAL MODELS The excited state(s) leading to phosphorescence are formed by electron−hole recombination. Two distance-dependent mechanisms, Dexter and Förster, have been proposed to describe the excitation where we refer the interested reader to the literature.1,32−37 Alternatively, recombination can occur on the organoiridium complex with hole trapping occurring first.1 Following excitation of the emitter, there are two limiting cases which may be considered and which are a function of the environment (Figure 2). If the triplet state is long-lived and the environment reorganization energy is small, the excited emitter will relax on the triplet potential energy surface. However, if phosphorescence is faster than reorganization of the triplet manifold, the excited emitter would remain near the groundstate singlet geometry. If the former case is true, emission can be described by vertical transitions from the ground-state singlet at the triplet geometry into the triplet manifold. In the latter, vertical transitions from the ground-state geometry would be adequate. High-resolution results from low-temperature spectroscopies probably fall into the former category. Most likely, the emitting complex occupies many local minima on the potential energy surface and macroscopic properties are better described by an ensemble of geometric and electronic

Table 1. Experimental Emission Energies (λmax), Photoluminescence Quantum Yields (ΦPL), and Phosphorescence Lifetimes (τ) for the Complexes Studied complex

λmax (nm)

ΦPL

τ (μs)

a

449 516 519 548 597 624 627 631 632

0.66 0.34 0.9 0.27 0.1 0.45 0.37 0.4 0.2

1.08 1.6 1.6 4.5 2 1.25 1.39 1.38 1.1

Ir(ptz)3 Ir(ppy)2(acac)b Ir(ppy)3c Ir(bzq)2(acac)b Ir(pq)2(acac)b Ir(piq)3b Ir(piq)2(ppy)b Ir(piq)(ppy)2b Ir(piq)2(acac)c a

In toluene.28 bIn 2-MeTHF.29,30 cIn CH2Cl2.5,31 25715



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kir = k r(S0 , T1i) =

4α0 3 ΔES0 − T1i 3 3t0

∑

|M ji|2 (1)

j ∈ {x , y , z}

where α0 is the fine-structure constant, t0 = (4πε0) ℏ /mee4, ΔES0−T1i is the transition energy, and Mij is the spin−orbit coupled S0 → T1 transition moment. Mij is the only nonvanishing component of the first-order corrected wave functions: 2 3

∞

M ji =

∑

⟨S0|μĵ |Sn⟩⟨Sn|Ĥ SO|T1i⟩ E(Sn) − E(T1)

n=0 ∞

+

∑ m=1

⟨S0|Ĥ SO|Tm⟩⟨Tm|μĵ |T1i⟩ E(Tm) − E(S0)

(2)

where the electric dipole transition moment from the ith sublevel of T1 and S0 is evaluated according to linear response theory and the Cartesian components j ∈ {x, y, z} are used to represent the spin eigenfunctions. The operators μ̂j and Ĥ SO are the electric dipole and spin−orbit Hamiltonian, respectively. The two terms in eq 2 have an inverse dependence on the energy differences between the spin manifolds; the relative importance of each term can be qualitatively deduced (see illustration in Figure 3). Since the energy differences between

Figure 2. Limiting case vertical excitations from the optimized singlet geometry (green) and optimized triplet geometry (red). Inset: Relativistic effects split the T1 manifold into three sublevels that are separated in energy in the absence of an applied field. This splitting is referred to as the zero-field splitting (ZFS).

states. We decided to study the minimum-energy S0 and T1 geometry extremes.22 The experimental electronic properties are dependent upon the environment. The experimental numbers we reference in this paper were obtained from measurements made in toluene, CH2Cl2, and 2-MeTHF. To minimize the number of response variables, we decided first to focus on gas-phase calculations, with the hope that this work could serve as a benchmark to broadly predict emission properties. Second, we included environmental effects by implicitly modeling solvent according to the conductor-like screening model (COSMO). At the non-relativistic limit, the solution to the Schrödinger equation is a complex scalar field. However, SOC is a relativistic property and plays a role in energetics of transition metals, such as Ir. For electrons treated relativistically, the solution to the Dirac equation is a four-spinor. In the Dirac wave function, two spinors represent antimatter and for our purposes can be neglected. Subsequent application of the Born−Oppenheimer approximation significantly simplifies the equation while keeping the most important relativistic corrections. Analytic expansion of the resulting equation yields the zeroth-order regular approximation (ZORA) to the Dirac equation, whose two-component solution is determined self-consistently.38 Several excellent discussions of relativistic TDDFT calculations with SOC exist in the literature.7,8,11,22,39−41 Within the Dirac−Kohn−Sham equations for closed-shell systems, spin dependencies appear only at second order, allowing them to be treated as a perturbation of the scalar relativistic or one-component wave functions. We, as well as others, have found that the use of an effective single-electron approximation significantly decreases computational cost via elimination of the two-electron spin−orbit integrals.8,41 Treating SOC as a perturbation, following the determination of singlet−singlet and singlet−triplet vertical excitation energies and transition moments, is as accurate as the self-consistent two-component calculation. SOC splits the T1 manifold into three sublevels that are separated in energy in the absence of an applied field. This splitting is referred to as the zero-field splitting (ZFS, see inset of Figure 2). The radiative rate from each triplet sublevel is given by the following equation:

Figure 3. Relative strength and deconvolution of terms from eq 2 as predicted by the denominator (e.g., E(Sn) − E(T1)). The wider the solid line, the more important the contribution (red/magenta > blue > green).

the singlet ground state (S0) and the excited triplet states (Tm) are much greater than the energy differences between the first excited triplet state (T1) and the first few excited singlet states (Sn), the second term in eq 2 is much less than the first term (as is the n = 0 element of the first term). As such, Mij is proportional to the product of the singlet transition dipole moments for n > 0 (⟨S0|μ̂|Sn⟩) and the spin−orbit coupling element between the emitting triplet state and the first few excited singlet states (⟨Sn|Ĥ SO|T1⟩). To further conceptualize this equation to the chemical level, we note that the magnitude of the singlet transition dipole moment is proportional to the oscillator strength divided by the energy of the respective transition, or in other words the “allowedness” of the singlet− singlet transitions between the excited-state singlets and the 25716



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Table 2. Optimized Gas-Phase Singlet (S0) and Triplet (T1) Ir−C/N/O Bond Distances (Å) at BP86 and B3LYPa BP86 S0 Ir−C1 Ir−C2 Ir−C3 Ir−N1 Ir−N2 Ir−N3

2.030 2.029 2.029 2.173 2.173 2.174

Ir−C1 Ir−C2 Ir−C3 Ir−N1 Ir−N2 Ir−N3

2.026 2.027 2.024 2.152 2.152 2.152

Ir−C1 Ir−C2 Ir−C3 Ir−N1 Ir−N2 Ir−N3

2.024 2.024 2.023 2.153 2.154 2.153

Ir−C1* Ir−C2 Ir−C3 Ir−N1* Ir−N2 Ir−N3

2.029 2.022 2.022 2.156 2.154 2.147

Ir−C1* Ir−C2* Ir−C3 Ir−N1* Ir−N2* Ir−N3

2.027 2.028 2.020 2.153 2.155 2.147

B3LYP T1

Ir(ptz)3 2.034 1.994 2.040 2.193 2.148 2.168 Ir(ppy)3 2.016 2.019 2.014 2.153 2.152 2.150 Ir(piq)3 2.014 2.014 2.013 2.148 2.147 2.148 Ir(piq)2(ppy) 2.024 2.031 1.999 2.173 2.152 2.115 Ir(piq)(ppy)2 2.018 2.038 2.009 2.164 2.164 2.105

BP86

S0

T1

S0

2.030 2.034 2.034 2.199 2.188 2.196

2.037 1.995 2.048 2.227 2.145 2.197

Ir−C1 Ir−C2 Ir−N1 Ir−N2 Ir−O1 Ir−O2

2.003 2.002 2.041 2.040 2.171 2.172

2.024 2.026 2.025 2.169 2.166 2.166

2.020 2.041 1.985 2.192 2.176 2.128


2.010 2.009 2.049 2.050 2.165 2.167

2.022 2.023 2.022 2.163 2.163 2.162

2.015 2.017 2.013 2.153 2.153 2.149


1.989 1.989 2.080 2.081 2.178 2.179

2.028 2.021 2.020 2.171 2.163 2.157

2.023 2.034 2.008 2.183 2.167 2.102


1.995 1.995 2.043 2.043 2.175 2.175

2.026 2.026 2.019 2.167 2.169 2.158

2.021 2.041 2.008 2.178 2.177 2.100

B3LYP T1

Ir(ppy)2(acac) 1.988 1.987 2.045 2.045 2.162 2.163 Ir(bzq)2(acac) 1.989 1.988 2.060 2.060 2.152 2.154 Ir(pq)2(acac) 1.979 1.978 2.083 2.083 2.166 2.166 Ir(piq)2(acac) 1.977 1.977 2.046 2.048 2.175 2.176

S0

T1

2.006 2.006 2.054 2.054 2.170 2.171

1.982 1.982 2.055 2.055 2.172 2.171

2.015 2.014 2.061 2.062 2.163 2.163

1.989 1.989 2.072 2.073 2.146 2.146

1.992 1.992 2.101 2.100 2.188 2.187

1.980 1.979 2.095 2.096 2.168 2.166

2.001 2.001 2.061 2.061 2.172 2.172

1.977 1.977 2.052 2.052 2.167 2.168

a Atoms are defined in Figure 4. For heteroleptic complexes, the C/N atoms from ppy are designated with an asterisk (*). Changes in bond distances, upon going from the singlet to the triplet geometry, are shown in italic (>0.02 Å) and bold-faced ( Ir(piq)3 > Ir(piq)(ppy)2 > Ir(piq) 2(ppy) > Ir(ppy)2(acac) > Ir(piq) 2 (acac) > Ir(bzq)2(acac) > Ir(pq)2(acac). The proposed protocol predicts

Figure 7. Correlation of calculated (TD-B3LYP) and experimental radiative rates (average) for the BP86- and B3LYP-optimized singlet (S0) geometries as a function of basis set: TZP/DZ (open circles), TZP/DZP (solid squares), TZP/TZP (open squares), and TZ2P/ TZP (solid circles). Experimental errors are shown via the vertical error bars. Linear fits can be found in the Supporting Information. 25720



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Table 4. Experimental and Theoretical Radiative Rates kr (s−1) for the Complexes Studieda

emission energies and phosphorescent lifetimes with experimental values for nine complexes (in which we have a high degree of confidence of the characterization) having a variety of different ligands, emission energies, and radiative rates. We find that vertical excitation properties calculated at the BP86 S0 geometry correlate the best with experiment. We acknowledge that the long-lived triplet state should have time to relax and that phosphorescence from such a geometry is physical. However, correlations using the triplet geometry are poor (both with BP86 and B3LYP). An extensive study of Ir(ppy)3 by Jansson et al. concluded that the true geometry of the emitting species is intermediate between the singlet ground and excited triplet states, as the triplet state exists in a shallow anharmonic potential energy well.41 Although the “true” geometry of the emission state was not determined in this work, time-dependent calculations from the S0 geometry are sufficient to produce results in agreement with experiment, and thus satisfying the stated intent of our research: to develop a protocol which is sufficient to reproduce and understand experimental trends. The use of the singlet geometry is also advocated by Minaev et al., where they employed SOCquadratic response theory to predict photophysical properties of several iridium complexes.25−27 We have also presented results on the basis set dependence of the time-dependent calculation of the emission energies and radiative rates. The former can be adequately calculated without including polarization functions on the main group elements, whereas, for the latter, such higher angular momentum functions are necessary. We propose that photophysical properties for cyclometalated Ir complexes be calculated at the TD-B3LYP/TZP/DZP level of theory, using the S0 geometry optimized at BP86/TZ2P/TZP, where the larger basis set is used for Ir. We have also analyzed the response of an implicit solvent (both on the vertical excitations and the geometry) and have found that the predicted radiative rates differ from experiment. We also show that, at 300 K, the radiative rate can be described by a simple average of the rates from the three T1 sublevels. Nevertheless, a slightly better correlation is achieved when accounting for the thermal population of the sublevels according to Boltzmann statistics. This unequal distribution of states is needed for complexes which have a large calculated ZFS. The lowest T1 sublevel has a radiative rate an order of magnitude or more smaller than the other two sublevels. By using a simple average of rates, the predicted values may be somewhat faster. This work represents the first step toward a general protocol suitable for the study of photophysical properties of cyclometalated iridium. The universality of our approach, and subsequent conclusions, will be validated by the growing size of the literature, wherein SOC-TDDFT is used to study similar systems.

computational complex

experimental

Ir(ptz)3

(6.1 ± 0.8) × 105 b

Ir(ppy)2(acac) Ir(ppy)3

(2.1 ± 0.5) × 105 c (5.6 ± 0.4) × 105 d

Ir(bzq)2(acac) Ir(pq)2(acac) Ir(piq)3

(0.6 ± 0.1) × 105 c (0.5 ± 0.1) × 105 c (3.6 ± 0.4) × 105 c

Ir(piq)2(ppy)

(2.7 ± 0.3) × 105 c

Ir(piq)(ppy)2

(2.9 ± 0.3) × 105 c

Ir(piq)2(acac)

(1.8 ± 0.4) × 105 d

average 5.9 4.3 1.8 4.0 4.8 6.1 1.2 1.8 2.9 3.5 2.7 2.2 2.7 2.7 1.1

× × × × × × × × × × × × × × ×

105 f,g 105 f,h 105 f,g 105 f,g 105 f,i 105 e 105 f,g 105 f,g 105 f,g 105 e 105 f,g 105 e 105 f,g 105 e 105 f,g

kBT-weighted 4.7 × 105 f,g 1.4 × 105 f,g 3.3 × 105 f,g

1.0 × 105 f,g 1.0 × 105 f,g 2.4 × 105 f,g 2.2 × 105 f,g 1.6 × 105 f,g 1.1 × 105 f,g

a

Theoretical rates were calculated using eqs 4 (average) and 3 (kBTweighted). bIn toluene.28 cIn 2-MeTHF.29,30 dIn CH2Cl2.5,31 eIn THF.19 fGas phase. gThis work, TD-B3LYP/TZP/DZP//BP86/ TZ2P/TZP S0 geometry. hFrom ref 9. iFrom ref 12.

the general trend of radiative rates, based on the average of the three triplet sublevels, with one exception: Ir(ptz)3 > Ir(ppy)3 > Ir(piq)3 > Ir(piq)(ppy)2 ∼ Ir(piq)2(ppy) > Ir(ppy)2(acac) ∼ Ir(pq)2(acac) > Ir(piq)2(acac) > Ir(bzq)2(acac). The one exception is Ir(pq)2(acac), which is predicted to have a higher rate than observed experimentally. As discussed earlier, the average predicted rate for Ir(pq)2(acac) is high due to the large ZFS value. Taking into an account the thermal populations of the triplet sublevels lowers the rate for Ir(pq)2(acac) to be more inline with experiment. Finally, we explored the effect of solvent on the emission energies and radiative rates. Solvent (toluene, THF, or CH2Cl2) was implicitly included via the conductor-like screening model (COSMO) within ADF. First, the response of the implicit dielectric was included within the TDDFT calculation (TDB3LYP/TZP/DZP) using the gas-phase-optimized geometries. Second, the organoiridium complexes were optimized within the implicit solvent (BP86/TZ2P/TZP), with subsequent excitation properties calculated at the level given in the prior sentence. The resulting data can be found in the Supporting Information (Figure S5 and Table S5). In summary, correlations to emission energies were excellent (R2 = 0.96− 0.97), with predicted emissions blue-shifted in relation to the gas-phase calculations. However, correlation between the predicted and experimental radiative rates was poor (R2 = 0.40−0.59). A large increase in radiative rates was predicted upon including the solvent response. We also compared electronic properties predicted when including the solvent response in the TDDFT calculation using either the gas-phaseor solvent-optimized geometries. Emission energies and radiative rates predicted with both geometries were nearly identical, signifying that the gas-phase geometry is adequate to predict electronic properties.

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ASSOCIATED CONTENT

S Supporting Information *

Tables of emission energies and radiative rates of individual sublevels and linear fits. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS In order to develop an efficient and reliable computational protocol to predict phosphorescent properties for cyclometalled iridium complexes, we have correlated theoretical

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+1) 302695-7376. Fax: (+1) 302-695-9873. 25721



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Notes

(17) De Angelis, F.; Santoro, F.; Nazeruddin, M. K.; Barone, V. Ab Initio Prediction of the Emission Color in Phosphorescent Iridium(III) Complexes for OLEDs. J. Phys. Chem. B 2008, 112, 13181− 13183. (18) De Angelis, F.; Belpassi, L.; Fantacci, S. Spectroscopic Properties of Cyclometallated Iridium Complexes by TDDFT. J. Mol. Struct.: THEOCHEM 2009, 914, 74−86. (19) Liu, Y.; Gahungu, G.; Sun, X.; Qu, X.; Wu, Z. Effects of NSubstitution on Phosphorescence Efficiency and Color Tuning of a Series of Ir(III) Complexes with a Phosphite Tripod Ligand: A DFT/ TDDFT Study. J. Phys. Chem. C 2012, 116, 26496−26506. (20) De Angelis, F.; Fantacci, S.; Evans, N.; Klein, C.; Zakeeruddin, S. M.; Moser, J.-E.; Kalyanasundaram, K.; Bolink, H. J.; Grätzel, M.; Nazeeruddin, M. K. Controlling Phosphorescence Color and Quantum Yields in Cationic Iridium Complexes: A Combined Experimental and Theoretical Study. Inorg. Chem. 2007, 46, 5989− 6001. (21) Abedin-Siddique, Z.; Ohno, T.; Nozaki, K.; Tsubomura, T. Intense Fluorescence of Metal-to-Ligand Charge Transfer in [Pt(0)(binap) 2] [binap = 2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl]. Inorg. Chem. 2004, 43, 663−673. (22) Jansson, E.; Minaev, B.; Schrader, S.; Ågren, H. TimeDependent Density Functional Calculations of Phosphorescence Parameters for fac-tris(2-Phenylpyridine) Iridium. Chem. Phys. 2007, 333, 157−167. (23) Liu, Y.; Gahungu, G.; Sun, X.; Su, J.; Qu, X.; Wu, Z. Theoretical Study on the Influence of Ancillary and Cyclometalated Ligands on the Electronic Structures and Optoelectronic Properties of Heteroleptic Iridium(III)) Complexes. Dalton Trans. 2012, 41, 7595−7603. (24) Qu, X.; Liu, Y.; Godefroid, G.; Si, Y.; Shang, X.; Wu, X.; Wu, Z. Theoretical Study on Cationic Iridium(III) Complexes with a Diphosphane Ligand - Geometry, Electronic Properties, and Application for Light-Emitting Electrochemical Cells. Eur. J. Inorg. Chem. 2013, 2013, 3370−3383. (25) Minaev, B.; Ågren, H.; De Angelis, F. Theoretical Design of Phosphorescence Parameters for Organic Electro-Luminescence Devices Based on Iridium Complexes. Chem. Phys. 2009, 358, 245− 257. (26) Minaev, B.; Minaeva, V.; Ågren, H. Theoretical Study of the Cyclometalated Iridium(III) Complexes Used as Chromophores for Organic Light-Emitting Diodes. J. Phys. Chem. A 2009, 113, 726−735. (27) Li, X.; Zhang, Q.; Tu, Y.; Ågren, H.; Tian, H. Modulation of Iridium(III) Phosphorescence via Photochromic Ligands a Density Functional Study. Phys. Chem. Chem. Phys. 2010, 12, 13730−13736. (28) Lo, S.-C.; Shipley, C. P.; Bera, R. N.; Harding, R. E.; Cowley, A. R.; Burn, P. L.; Samuel, I. D. W. Blue Phosphorescence from Iridium(III) Complexes at Room Temperature. Chem. Mater. 2006, 18, 5119−5129. (29) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Synthesis and Characterization of Phosphorescent Cyclometalated Iridium Complexes. Inorg. Chem. 2001, 40, 1704−1711. (30) Deaton, J. C.; Young, R. H.; Lenhard, J. R.; Rajeswaran, M.; Huo, S. Photophysical Properties of the Series fac-and mer-(1̂ ̂ Phenylisoquinolinato-NC2′)x(2-Phenylpyridinato-NC2′)3−x Iridium(III) (x= 1−3). Inorg. Chem. 2010, 49, 9151−9161. (31) Su, Y. J.; Huang, H. L.; Li, C. L.; Chien, C. H.; Tao, Y. T.; Chou, P. T.; Datta, S.; Liu, R. S. Highly Efficient Red Electrophosphorescent Devices Based on Iridium Isoquinoline Complexes: Remarkable External Quantum Efficiency Over a Wide Range of Current. Adv. Mater. 2003, 15, 884−888. (32) Andrews, D. L. A Unified Theory of Radiative and Radiationless Molecular Energy Transfer. Chem. Phys. 1989, 135, 195−201. (33) Wu, C.; Djurovich, P. I.; Thompson, M. E. Study of Energy Transfer and Triplet Exciton Diffusion in Hole-Transporting Host Materials. Adv. Funct. Mater. 2009, 19, 3157−3164. (34) Jeon, W. S.; Park, T. J.; Kim, S. Y.; Pode, R.; Jang, J.; Kwon, J. H. Ideal Host and Guest System in Phosphorescent OLEDs. Org. Electron. 2009, 10, 240−246.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge support from the developers at SCM (Erik van Lethe and Stan van Gisbergen), as well as support from the high-performance computing group at DuPont (Alistair J. McGhie and Douglas Cloud).

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Correlating Experimental Photophysical Properties of Iridium(III

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