DFT and TD-DFT Study on the Electronic Structures and

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DFT and TD-DFT Study on the Electronic Structures and Phosphorescent Properties of a Series of Heteroleptic Iridium(III) Complexes Xiaohong Shang,*,† Deming Han,‡ Qing Zhan,§ Gang Zhang,∥ and Dongfeng Li† †

College of Chemistry and Life Science, Changchun University of Technology, Changchun 130012, People’s Republic of China School of Life Science and Technology, Changchun University of Science and Technology, Changchun 130022, People’s Republic of China § Jilin Provincial Institute of Education, Changchun 130022, People’s Republic of China ∥ State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: The electronic structures and phosphorescent properties of a series of heteroleptic iridium(III) complexes (mpmi)2Ir(dmpypz) (1; mpmi = 1-(4-tolyl)-3-methylimidazolium, dmpypz = 3,5-dimethyl-2-(pyrazol-3-yl)pyridine), (bpmi)2Ir(dmpypz) (2; bpmi = 1-biphenyl-4-yl-3-methylimidazole), (dfmi)2Ir(dmpypz) (3; dfmi = 1-(2,6-difluorobiphenyl)-3-methylimidazole), (mtmi)2Ir(dmpypz) (4; mtmi = 1methyl-3-(4′-(trifluoromethyl)biphenyl-4-yl)imidazole), (fmmi)2Ir(dmpypz) (5; fmmi = 1-(fluoren-2-yl)-3-methylimidazole), and (mhmi)2Ir(dmpypz) (6; mhmi = 1-methyl-3phenanthren-2-ylimidazole) have been investigated by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The influence of different substituent groups and π-conjugation degrees on the optical and electronic properties of Ir(III) complexes was also explored by introducing phenyl, fluorophenyl, (trifluoromethyl)phenyl, and rigid construction on the phenylimidazole moiety of a cyclometalated ligand (C∧C) in complex 1. The calculated results show that the lowest energy absorption wavelengths of complexes 1−6 are 387, 380, 378, 375, 391, and 384 nm, respectively. The introduction of different substituent groups leads to different degrees of red shift for complexes 2−6 in emission spectra in comparison with that of complex 1. It is believed that the highest triplet metal to ligand charge transfer 3MLCT (%) contribution, smallest ΔES1−T1 and higher μS1 values, and larger 3 MC−3MLCT energy gap for 3 ensure its higher quantum yield in comparison with that of other complexes.

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INTRODUCTION

larger d-orbital splitting in comparison with the other metals in the series, (b) the closely lying π−π* orbitals and metal to ligand charge transfer (MLCT) state, and (c) the stronger heavy atom effect (namely, the larger SOC effect constant of the Ir atom) that enhances the SOC, giving a much allowed T1−S0 transition and reducing the probabilities of triplet− triplet annihilation, hence increasing the photoluminescent quantum yield.10 In order to obtain blue-emitting materials, several strategies have been adopted to broaden the gap of iridium complexes.11 A number of approaches to increase the emission energy of cyclometalated iridium(III) complexes have focused on methods to lower the HOMO energy while keeping the LUMO energy relatively unchanged.12 The addition of

In the past two decades, phosphorescent transition-metal− ligand complexes have attracted much attention for their potential application as highly efficient electroluminescent (EL) emitters in organic light emitting devices (OLEDs).1−4 The triplet state T1 is usually occupied by intersystem crossing (ISC) from the excited singlet state. Transitions between singlet and triplet states are formally forbidden, because they involve a change in the electron spin. However, due to the strong spin−orbit coupling (SOC), which is introduced by nuclei in the molecule, ISC and also phosphorescence become partially allowed. To obtain high phosphorescence efficiency, many complexes containing transition metals such as iridium(III), 5 platinum(II), 6 osmium(II), 7 rhenium(I), 8 and ruthenium(II)9 have been extensively investigated in experimental and theoretical studies. As reported, cyclometalated iridium(III) complexes are very promising because of (a) the © 2014 American Chemical Society

Received: December 11, 2013 Published: June 23, 2014 3300

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Figure 1. Schematic structures for the studied complexes.

complexes have been studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods.

electron-withdrawing groups to the phenyl ring has been used as one way to achieve this goal.13 The most commonly used electron-withdrawing group is fluoride, and the representative blue phosphors are a class of iridium(III) complexes possessing at least one cyclometalated 4,6-difluorophenylpyridine ((dfppy)H) ligand, known as FIrpic, FIr6, FIrtaz, and others.14 Our aim is to find a high luminous efficiency and a pure blue phosphorescent material. Thus, it is important to understand the relationship between structure and photophysical properties. Reports on iridium tris(carbene) complexes are known to have high triplet energy gaps and can be used as blue phosphorescent emitters.15 Kido et al. have reported a triscarbene iridium complex based device with a high external quantum efficiency of 18.6%.16 Recently, a new bis-carbene iridium complex that can efficiently emit blue light, (mpmi)2Ir(dmpypz) (1; mpmi = 1-(4-tolyl)-3-methylimidazolium, dmpypz = 3,5-dimethyl-2-(pyrazol-3-yl)pyridine) (Figure 1), has been reported and proven to be useful as a phosphorescent emitter for electroluminescent devices, giving extremely high device efficiencies.17 In our present work, on the basis of the recently synthesized blue-emitting Ir(III) complex (mpmi)2Ir(dmpypz) (1), we propose a series of designed structures (bpmi)2Ir(dmpypz) (2; bpmi = 1-biphenyl-4-yl-3-methylimidazole), (dfmi) 2 Ir(dmpypz) (3; dfmi = 1-(2,6-difluorobiphenyl)-3-methylimidazole), (mtmi)2Ir(dmpypz) (4; mtmi = 1-methyl-3-(4′(trifluoromethyl)biphenyl-4-yl)imidazole), (fmmi)2Ir(dmpypz) (5; fmmi = 1-(fluoren-2-yl)-3-methylimidazole), and (mhmi)2Ir(dmpypz) (6; mhmi = 1-methyl-3-phenanthren-2ylimidazole) with the 3,5-dimethyl-2-(pyrazol-3-yl)pyridine ligand considered to have great potential as a blue chromophore in comparison with the experimental molecule. The main goal is to theoretically provide a detailed understanding of physical properties, such as the electronic structure and charge injection, transport, and spectral properties. We also explore the influence of different π-conjugated extensions on the physical properties of these complexes. All of these

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COMPUTATIONAL METHODS

The ground state and the lowest lying triplet excited state geometries were calculated by using density functional theory (DFT)18 with Becke’s three-parameter hybrid method19 combined with the Lee− Yang−Parr correlation functional20 (B3LYP). Restricted and unrestricted14 formalisms were used in the singlet and triplet geometry optimizations, respectively. All geometrical structures were fully optimized without any symmetry constraints. The calculated vibrational frequencies with no imaginary frequencies based on the optimized geometries for complexes 1−6 verify that each of the optimized structures is a true minimum on the potential energy surface. In addition, the electronic configurations of the triplet metalcentered (3MC) d−d states were optimized according to the methodology presented in Thummel’s work. 21 Further, the TDDFT22 approach associated with the polarized continuum model (PCM)23 in dichloromethane (CH2Cl2) media was adopted to investigate the absorption and emission properties from the experimental results by Chen et al.17 The S1−T1 energy gap (ΔES1−T1) was calculated by considering the fixed triplet molecular geometry. In all calculations, the 6-31G* basis set24 was used for the ligands and the LANL2DZ25 basis set was adopted for the Ir atom. The relativistic effective core potential (ECP) replaced the inner core electrons of the Ir(III) metal atom, leaving the outer core (5s25p6) and 5d6 as valence electrons. All calculations were performed with the Gaussian 09 software package.26 The UV/vis absorption spectra were obtained by using the GaussSum 2.5 software.27

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RESULTS AND DISCUSSION Molecular Geometries in the Ground and Lowest Triplet States. To investigate the solvent effect, the groundstate geometry optimization of 1 was also performed within the self-consistent reaction field (SCRF) theory using the polarized continuum model (PCM) in dichloromethane (CH2Cl2) media to model the interaction with the solvent. Table S1 (Supporting 3301

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refer to the gas phase with respect to the close-packed crystal lattice in experiment but also to the fact that it is well-known that B3LYP overestimates the structural parameters (particularly bond lengths) in transition-metal complexes. When 1 is compared with 2, the substitution of the “−CH3” group by the phenyl group on the phenylimidazole moiety of cyclometalated ligand (C∧C) ligands does not cause obvious changes ( 1 (387 nm) > 6 (384 nm) > 2 (380 nm) > 3 (378 3303

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singlet to triplet states and hence the possibility of increasing the phosphorescent quantum yield. Phosphorescence in CH2Cl2 Media. It is known that a range-separated functional can possibly improve the agreement in phosphorescence calculations.28−30 Herein, two rangeseparated functionals (LC-BLYP and CAM-B3LYP) have been used to perform the phosphorescence calculations. However, there is a discouraging result that the lowest-energy emissions for complex 1 are at 645 nm (1.92 eV) at the LCBLYP level and 558 nm (2.22 eV) at the CAM-B3LYP level, respectively, which have large differences from the experimental value of 466 nm. Table 2 shows the calculated emission energies, transition natures, and available experimental values. It is known that the B3LYP functional usually overestimates the emission wavelength.31 Thus, in order to get a reliable method to predict the emission properties, five density functional methods (B3LYP, M062X,32 PBE0,33 BP86,34 and B3P8619) were adopted to calculate the emission spectra for complex 1. Figure 5 shows the results together with the available

Figure 4. Simulated absorption spectra of 1−6 in CH2Cl2 media.

nm) > 4 (375 nm), which is not closely related to the variation trend of HOMO−LUMO energy gaps because HOMO → LUMO transition configurations are not predominantly contributed by S0 → S1 transitions for all cases because of the quasidegenerate HOMO, LUMO, and LUMO+1 orbitals (for 6 only). From the discussion of FMOs, the lowest lying absorptions can be characterized as MLCT [d(Ir) → π*(N∧N)] and LLCT [π(C∧C) → π*(N∧N)] for 1−6, and [π(C∧C) → π*(C∧C)]/ILCT also contributes to this transition in 6. For 1, 5, and 6, the calculated maximum absorptions bands are located at 223, 310, and 275 nm described as d(Ir) + π(C∧C+N∧N) → π*(N∧N)/MLCT/LLCT/ILCT, π(N∧N) → π*(N∧N)/ILCT, and π(C∧C+N∧N) → π*(C∧C+N∧N)/ LLCT/ILCT transitions, respectively. For 2−4, their maximum absorptions are ascribed to d(Ir) + π(C∧C) → π*(C∧C) with MLCT/LLCT/ILCT character, and absorption bands are at 268, 277, and 287 nm contributed by the H-5 → L+1 (47%), H-4 → L+2 (61%), and H-4 → L+2 (64%) transitions, respectively. The calculated vertical triplet absorptions of these complexes are at 426, 426, 425, 425, 437, and 464 nm (Table S8, Supporting Information), respectively, having the transition characters of MLCT, LLCT, and ILCT. For the broad absorption band at 300−450 nm, we cannot exclude the participation of S0 → T1 transitions in the lower-energy regions, even though it is assumed that their contribution should decrease with decreasing wavelength. On the other hand, in the higher energy region around 250−300 nm, the absorption intensity of 6 is significantly stronger than that of others (Table S8 and Figure 4); the large absorption intensities would increase the probability of intersystem crossing (ISC) from

Figure 5. Emission wavelengths for 1 at the B3LYP, M062X, PBE0, BP86 and B3P86 levels, respectively, together with the experimental values.

experimental values for 1. It can be seen that the B3LYP, PBE0, BP86, and B3P86 functionals overestimated emission wavelengths in comparison with the experimental data, especially the B3LYP functional with a deviation of 69 nm, while the M062X gives a satisfactory result (479 nm) for 1 (only overestimated by 13 nm). Therefore, we have employed the M062X functional for further emission spectra calculations. On the basis of the optimized T1 structures, the emission wavelengths, emission energies, and transition natured were calculated by the TDDFT/M062X method (Table 2). To conveniently discuss the transition properties of emission, we give the partial compositions of FMOs related to emission in

Table 2. Calculated Phosphorescent Emission of the Studied Complexes in CH2Cl2 Media at the TDDFT/M062X and TDDFT/ B3LYP Levels, Respectively, and the Experimental Values M062X λ (nm)/E (eV) 1

479/2.58

2 3 4 5 6

487/2.54 489/2.53 487/2.54 486/2.55 522/2.37

confign L L L L L L L

→ → → → → → →

H (57%) H-1 (33%) H (73%) H (82%) H (84%) H (73%) H (83%)

B3LYP nature

λ (nm)/E (eV)

LMCT/LLCT/ILCT LMCT/LLCT/ILCT LMCT/LLCT/ILCT LMCT/ILCT LMCT/ILCT LMCT/LLCT/ILCT LMCT/LLCT/ILCT

535/2.31

L → H (73%)

519/2.38 545/2.27 495/2.49 513/2.41 560/2.21

L L L L L

3304

confign

→ → → → →

H (86%) H (89%) H (86%) H (52%) H-2 (58%)

nature

exptl17

LMCT/LLCT/ILCT

466

LMCT/LLCT/ILCT LMCT/ILCT LMCT/ILCT LMCT/LLCT/ILCT LMCT/LLCT/ILCT

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transition energies and/or the lowest triplet metal to ligand charge transfer 3MLCT energies is necessary to raise the energy of the d−d state. The 3MLCT/π → π* excited states were obtained by performing an unrestricted triplet optimization starting from the optimized ground-state geometry. The electronic configurations of 3MC d−d states have been calculated according to the literature methodology,21 in which optimization starts with a distorted molecular geometry by largely stretching the metal−ligand bonds. Table S9 (Supporting Information) gives the calculated metal−ligand bond distances in 3MC d−d states. In a comparison of the 3MC d−d states with their ground states (Tables S9 and S1), the Ir− N1, Ir−N2, and Ir−C2 bond distances are significantly elongated by 0.022−0.026, 0.157−0.233, and 0.025−0.034 Å, respectively. The elongation of bond distances due to the weak chelating interaction between the metal and the ligands results in a larger nonradiative probability. This will decrease the quantum yield, which is controlled by the competition between the radiative (kr) and nonradiative (knr) rates. In addition, by thermal activation, the 3MLCT/π → π* excited state can be rapidly converted to a short-lived 3MC d−d state from which no photochemistry occurs.37 Thus, the energy gap between 3 MLCT/π → π* and 3MC d−d states is the activation barrier for 3MLCT → 3MC conversion.38 The relative energies of 3 MLCT/π → π* and 3MC d−d excited states are shown in Figure 6 with the normalized S0 levels.

Table 3. The 479 nm emission of 1 corresponds well to the experimental value of 466 nm.17 The phosphorescent emissions Table 3. Partial Molecular Orbital Composition (%) in the Triplet Excited States (T1) at the TDDFT/M062X Level MO contribn (%) MO 1

2 3 4 5 6

LUMO HOMO HOMO-1 LUMO HOMO LUMO HOMO LUMO HOMO LUMO HOMO LUMO HOMO

energy/ eV −1.17 −4.64 −4.91 −1.03 −4.78 −0.95 −4.76 −1.22 −5.05 −1.24 −4.64 −1.10 −4.45

Ir 3 36 24 3 35 3 38 3 37 3 28 3 18

C∧C N∧N 2 51 19 1 50 1 6 1 7 2 56 96 81

95 13 57 96 15 96 56 96 56 95 16 2 1

assignt ∧

π*(N N) d(Ir) + π(C∧C+N∧N) d(Ir) + π(C∧C+N∧N) π*(N∧N) d(Ir) + π(C∧C+N∧N) π*(N∧N) d(Ir) + π(N∧N) π*(N∧N) d(Ir) + π(N∧N) π*(N∧N) d(Ir) + π(C∧C+N∧N) π*(C∧C) d(Ir) + π(C∧C)

for complex 1 were contributed mainly by the LUMO → HOMO (57%) and LUMO → HOMO−1(33%) transitions, and both have the same phosphorescent characters, which are assigned to an LMCT (ligand to metal charge transfer)/LLCT/ ILCT [π*(N∧N) → d(Ir) + π(C∧C+N∧N)] transition. On the other hand, the calculated lowest-energy emissions are at 487 nm (2.54 eV), 489 nm (2.53 eV), 487 nm (2.54 eV), 486 nm (2.55 eV), and 522 nm (2.37 eV) for complexes 2−6, respectively. In comparison with 1, a pronounced red shift for 6 has been observed, while a relatively smaller red shift is detected for 2−5. Table 2 shows that the lowest-energy emissions of 2−6 originate mainly from a LUMO → HOMO transition with the mixed characters of LMCT/LLCT/ILCT except for 3 and 4. Due to the significantly reduced composition of the C∧C moiety in the HOMO of 3 and 4, the transition characters of 489 and 487 nm can be described as LMCT/ILCT [π*(N∧N) → d(Ir) + π(N∧N)] with LUMO and HOMO both mainly localized on the N∧N ligand (Table 3). The emission of 6 at 522 nm can be described as a [π*(C∧C) → d(Ir) + π(C∧C)] transition. These results are also consistent with the analysis of the molecular structure of the T1 states above. For example, in complex 6, the strengthened interactions between the metal and C∧C ligands caused by the shortened Ir−ligand bond distances (Ir−C1 and Ir−C2) (Table 1) result in the greatest participation of the C∧C ligand in the FMOs in the excited states, while the significantly elongated Ir−N1 and Ir−N2 bond lengths undoubtedly lead to the absence of the N∧N ligand in the excited states (Tables 3). In comparison with that of 6 the emission wavelengths of 1−5 are even more blue-shifted, and the emission spectra of the complexes are in the blue region. We expect they might be potential candidates for blue emitters in phosphorescent dopant emitters in OLEDs. Efficiency Comparison. The significantly lower emission quantum yield hampers the fabrication of blue phosphorescent OLEDs.35 Accordingly, the population of the 3MC d−d excited states is one of the most important deactivation pathways of the phosphorescent emission from T1 in transition-metal complexes.36 In this regard, the selection of suitable stronger field ligands with sufficiently large ligand-centered (LC) π → π*

Figure 6. Energy level diagram of the studied complexes in T1 and 3 MC excited states, respectively, along with the normalized S0 levels.

To evaluate the phosphorescence quantum yield, it is necessary to investigate the separation between 3MLCT/π → π* and 3MC d−d states. The stability of 3MLCT/π → π* states for 2 is larger than that of 1. The 3MC d−d states of complexes 2, 3, 5, and 6, especially 3, are at relatively higher energy levels. From Figure 6 and Table 4, it can be seen that 2, 3, and 6 give a larger energy gap between 3MLCT/π → π* and 3MC d−d states. The ( 3 MLCT/π → π*) → ( 3 MC d−d) → S 0 radiationless pathway is expected to be less efficient for 2, 3, and 6, which would result in a high emission quantum yield. The quantum yield (ΦPL) can be expressed as eq 1, where kr, knr, and τem are the radiative rate, nonradiative rate, and ΦPL = k rτem =

3305

kr k r + k nr

(1)

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decreasing ΔES1−T1 can lead to a higher kr value , while a decreasing μS1 value leads to a lower kr value; thus, according to eq 2, large μS1 and small ΔES1−T1 values are also good for a high quantum yield. From Table 4, it can be seen that complex 3 has the smallest ΔES1−T1 value (0.4936 eV) and a large μS1 among these complexes, forecasting the largest kr value. From the discussion above, the lowest ΔES1−T1 value, the largest 3MLCT contribution, and a higher μS1 value for complex 3 may account for a larger kr according to eq 2. To sum up, from the above discussion concerning 3MC and 3MLCT states, we can safely draw the conclusion that, due to the relatively larger kr and smaller knr values, complex 3 is considered to have a relatively higher ΦPL. Thus, complex 3 could be a potential blue-emitting material with high quantum efficiency. However, we should understand that in addition to these factors discussed above other factors may also affect radiative (kr) and nonradiative (knr) rates. Comparison of Performance in OLEDs. The ionization potential (IP) and electron affinity (EA) have been investigated to evaluate the energy barriers for the injection of holes and electrons, which are important for the device performances of OLED materials. For complexes with similar structures, higher HOMO and lower LUMO energy levels will facilitate the hole transporting and electron transporting abilities.43 A larger EA (smaller IP) suggests that it is easier to inject electrons (holes) into the emitting materials from the electron (hole) transporting layer. The reorganization energy (λ) evaluates the charge transfer rate and balance. The calculated vertical IP (IPv), adiabatic IP (IPa), vertical EA (EAv), and adiabatic EA (EAa) values, together with hole extraction potential (HEP) and electron extraction potential (EEP), are given in Table 5. It can be seen that the calculated IP values decrease in the order 4 > 2 > 3 > 6 > 1 > 5, indicating the increased hole injection abilities from the hole-transporting layer to the HOMO of dopants. This order is consistent with the trend of HOMO energies, and thus hole injections for 1, 5, and 6 are easier in comparison with other complexes. The restricted rotation of the phenyl group on the cyclometalated ligands in 5 and 6 does not cause an obvious improvement of hole injection ability with respect to that of 1 due to their similar IP values (Table 5). In comparison with 1, the assumed complexes 4 and 6 have larger EA values and enhanced electron injection ability. It is also evident that 6 possesses relatively lower IP and higher EA values, indicating that the rigid skeletal structure can effectively improve the carrier injection ability. The reorganization energies for electron transport (λe) and reorganization energies for hole transport (λh) have been clarified in a previous article.44 From the calculated data in Table 5, it can be seen that the reorganization energies for electron transport (λe) of complexes 1, 2, and 5 are slightly

Table 4. Calculated Energy Difference (ΔE3MC−3MLCT, kcal/ mol) between the Metal-Centered (3MC) State and the Lowest Triplet State (3MLCT), Metal-Based Charge Transfer Character (3MLCT, %), Energy of Singlet−Triplet Splitting (ΔES1−T1, eV), and the Transition Dipole Moment in the S0 → S1 Transition (μS1, D) for the Studied Complexes in CH2Cl2 Medium ΔE3MC−3MLCT

3

3.5 5.8 8.1 1.8 4.9 7.1

1 2 3 4 5 6

MLCT

ΔES1−T1

μS1

24.2 23.4 28.7 28.5 13.1 20.8

0.5997 0.6628 0.4936 0.7083 0.5687 1.1024

0.012 0.025 0.047 0.049 0.006 0.001

emission decay time. It can be seen that a large kr value and a small knr value are required to increase the quantum yield. In addition, the metal participation in the triplet states and in the effective SOC is closely related to the photophysical properties of organometallic triplet emitters. The SOC effects are elucidated mainly from two aspects. One is the contribution of 3MLCT in the T1 state.39 The direct involvement of the Ir(III) d orbital enhances the first-order SOC in the T1 → S0 transition, which would result in a drastic decrease of the radiative lifetime and avoid the nonradiative process. Therefore, a large 3MLCT contribution will increase the quantum yield (ΦPL). As shown in Table 4, the introduction of −F and −CF3 substituents on the ending phenyl ring increases the percent 3 MLCT for 3 and 4, indicating a chance for a high ΦPL value. The second aspect elucidating the SOC effects is the singlet− triplet splitting energy (ΔES1−T1).40 The S1 → T1 intersystem crossing (ISC) induced by SOC plays an important role in the phosphorescent process. It is known that the ISC rate decreases exponentially as the singlet−triplet splitting energy increases. A minimal ΔES1−T1 is good for enhancing the transition moment and ISC rate, leading to an increased kr. Theoretically, kr is related to the mixing between S1 and T1, which is proportional to the SOC and inversely proportional to the energy difference between the two states according to eq 2, 41,42 where μS1 is the kr = γ

⟨ΨS1|HSO|ΨT1⟩2 μS2

1

(ΔES1‐ T1)2

γ=

3 16π 3106n3Eem 3hε0

(2)

transition electric dipole moment in the S0 → S1 transition, Eem represents the emission energy in cm−1, and n, h, and ε0 are the refractive index, Planck’s constant, and the permittivity in vacuum, respectively. According to eq 2, ΔES1−T1 was calculated considering a fixed triplet molecular geometry. It shows that the

Table 5. Ionization Potentials, Electron Affinities, Extraction Potentials, Internal Reorganization Energies, and Δ = |λh − λe| (in eV) for the Studied Complexes

1 2 3 4 5 6

IPv

IPa

HEP

EAv

EAa

EEP

λh

λe

Δ

5.72 5.87 5.86 6.13 5.70 5.74

5.61 5.76 5.73 6.01 5.60 5.62

5.57 5.65 5.60 5.89 5.49 5.49

0.11 0.22 0.14 0.43 0.21 0.24

0.11 0.15 0.12 0.24 0.13 0.21

0.03 0.04 0.02 0.32 0.03 0.13

0.26 0.22 0.25 0.23 0.21 0.22

0.40 0.29 0.19 0.22 0.24 0.00

0.14 0.07 0.06 0.01 0.03 0.22

3306

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Notes

larger than those for hole transport (λh), which reveals that the hole transport performances of these complexes are slightly better than their electron transport abilities. Table 5 also shows that a change in cyclometalated ligands for complexes 2−6 leads to relatively lower λh and λe values in comparison with those of complex 1, meaning that they have rich charge transporting properties. Complex 1 has poor hole and electron transfer ability with the largest reorganization energies for λh and λe values, whereas complex 6 has a strong chargetransporting ability. The nearly identical λh values of 2 and 4−6 indicate that they may have comparable hole transfer abilities. We also found that the energy differences between λh and λe for 4 and 5 (0.01 and 0.03 eV, respectively) are smaller than those for 1−3 and 6 (0.14, 0.07, 0.06, and 0.22 eV), especially for 4. This suggests that the hole and electron transfer balance could be achieved easily in the emitting layer for complexes 4 and 5, which is the key factor for materials used in OLEDs.

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CONCLUSIONS

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ASSOCIATED CONTENT

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jilin Province of China (Grant No. 20101548) and the Program of Science and Technology Development Plan of Jilin Province of China (Grant No. 20110333).

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The geometrical and electronic structures, absorption and emission spectra, charge injection and transport abilities, and phosphorescence efficiencies of a series of heteroleptic iridium(III) complexes (C∧C)2Ir(dmpypz) were investigated using DFT and TDDFT methods. It was found that complex 4 designed by the substitution of the “CH3” group with the a trifluoromethyl-phenyl group on the phenyl-imidazole moiety of 1 can stabilize the LUMO energy levels, improving the electron injection abilities. In addition, complexes 4 and 5 possess better charge transfer abilities and more balanced charge transfer rates. The rigid ligand C∧C in complex 6 leads to a significant red shift in optical spectra and rich hole and electron transfer ability with the smallest reorganization energies for λh and λe values. The theoretical analysis showed that complex 3 can be considered a potential candidate of blueemitting material with high quantum efficiency, due to its larger radiative (kr) rate and smaller nonradiative (knr) rate. These theoretical results can be anticipated to be useful for designing novel phosphorescent materials.

S Supporting Information *

Selected optimized ground-state parameters of 1 in the gas phase and CH2Cl2 media at the B3LYP/LANL2DZ level (Table S1), frontier molecular orbital energies (eV) and compositions (%) in the ground state for the studied complexes (Tables S2−S7), selected calculated wavelengths (nm)/ energies (eV), oscillator strength ( f), major contributions, transition characters, and experimental wavelengths (nm) for 1 in CH2Cl2 media (Table S8), selected bond distances (in Å) calculated for the studied complexes in the metal-centered (3MC) triplet excited states (Table S9), Cartesian coordinates from the optimized structures of S0 in CH2Cl2 media for 1 (Table S10), and Cartesian coordinates from the optimized structures of S0 and T1 in the gas phase for the studied complexes (Tables S11−S16). This material is available free of charge via the Internet at http://pubs.acs.org.

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