TD-DFT Study on the Electronic Structures and Optoelectronic

May 26, 2010 - Efficient Blue-Emitting Ir(III) Complexes with Phosphine .... with 2-pyridyl triazolate chromophores and fabrication of sky blue- and w...
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J. Phys. Chem. A 2010, 114, 6559–6564

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DFT/TD-DFT Study on the Electronic Structures and Optoelectronic Properties of Several Blue-Emitting Iridium(III) Complexes Lili Shi, Bo Hong, Wei Guan, Zhijian Wu,* and Zhongmin Su* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, College of Resource and EnVironmental Science, Jilin Agricultural UniVersity, Changchun 130118, People’s Republic of China, and Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal UniVersity, Changchun 130024, People’s Republic of China ReceiVed: February 3, 2010; ReVised Manuscript ReceiVed: May 10, 2010

The electronic structures and optoelectronic properties of several blue-emitting phosphors (dfppy)2Ir(pyN2), (dfppy)Ir(pyN2)2, and (fpmb)2Ir(pyN3) [dfppyH: 2-(2,4-difluorophenyl)pyridine; pyN2H: 5-(2-pyridyl)-3trifluoromethylpyrazole; Hfpmb: 1-(4-fluorophenyl)-2,3-dihydro-3-methyl-1H-benzo[d]imidazole; and pyN3H: 2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine] are investigated with density functional theory. The injection abilities of holes and electrons are estimated by evaluating the ionization potentials and electron affinities. It is found that the properties of the ligands have great influence on the photophysical properties, such as energy gap, absorption spectra, emission spectra, etc. The assumed complex (dfppy)2Ir(pyN2) is found to be a good candidate for blue-emitting material. We suggest that the luminescent properties of this class of materials can be tuned by modifications of the corresponding ligands. 1. Introduction Phosphorescent iridium(III) complexes have received intensive attention due to their potential applications in highly efficient organic light-emitting diodes (OLED).1-4 The phosphors can utilize both singlet and triplet excitons, resulting in a theoretical level of unity for the internal quantum efficiency in phosphorescent OLEDs.5,6 Moreover, the short radiative lifetimes of Ir(III) emitters could reduce the probabilities of triplet-triplet annihilation (at high current densities), which may lead to a significant decrease in photoluminescence (PL) quantum efficiencies. Developing highly efficient phosphors that can emit all three primary colors is the key factor to achieve full-color displays. Highly efficient green- and red-light-emitting OLEDs based on cyclometalated iridium complexes have been reported in the literatures.3,4,7-9 However, achieving room temperature blue phosphorescence with high quantum efficiency remains a challenge. The approaches to obtain highly efficient blue phosphorescent complexes might be achieved either by adopting ligands with high triplet energy levels or by using electronwithdrawing ancillary ligands. Therefore, the selection of suitable chelate ligands with large ligand-centered transition energies and/or metal-to-ligand charge transfer (MLCT) energies is the key factor. One of the well-known ligands for providing short emission wavelengths is 2-(2,4-difluorophenyl)pyridine (dfppy). Most of these blue-emitting phosphors have the form (dfppy)2IrL, where L is a monoanionic ancillary ligand, such as pic (picolinate),10,11 tetrakis(1-pyrazolyl)borate,12,13 pyN4 and pyN311 [pyN4H: 2-(1Htetrazol-5-yl)pyridine; and pyN3H: 2-(5-(trifluoromethyl)-2H1,2,4-triazol-3-yl)pyridine]. These ancillary ligands are the decisive structural factor for blue-emitting properties. However, the emissive color of the above-mentioned phosphors is not * To whom correspondence should be addressed. E-mail: [email protected] (Z.W.) and [email protected] (Z.S.).

saturated and close to cyan. Chi et al. reported a saturated deepblue-emitting device fabricated with (dfppy)Ir(pyN2)2 [pyN2H: 5-(2-pyridyl)-3-trifluoromethylpyrazole] as the emitting material, with the electroluminescent (EL) spectra centered at 450 nm.14 Recently, another important class of potential blue-emitting phosphors is Ir(III) complexes with N-heterocyclic carbene (NHC) ligands, such as fac-Ir(pmi)3, fac-Ir(pmb)3,15,16 and (fpmb)2Ir(pyN3)17 [pmi: 1-phenyl-3-methylimidazolin-2-ylidene; pmb: 1-phenyl-3-methylbenzimidazolin-2-ylidene; and fpmb: 1-(4-fluorophenyl)-2,3-dihydro-3-methyl-1H-benzo[d]imidazole]. Usually, Ir(III) carbene complexes are relatively rare saturated blue or even near-UV phosphorescent materials, in which the strong field of NHC ligand plays an important role in governing the blue-emitting properties. These investigations show that subtle changes in the ligands could make the emissive color shift from cyan to deep blue. Thus, it is worthwhile to study the effect of different ligands on the optoelectronic properties. Relative to the extensive experimental work, there are few quantum chemistry studies on blue-emitting complexes.18-20 With the development of computing facilities, theoretical study becomes more and more important in elucidating experimental phenomena and predicting new functional materials. Gu et al. studied the effects of ancillary ligands pic and acac (acetoylacetonate) on the spectral properties of blue-emitting phosphors (dfppy)2Ir(pic) and (dfppy)2Ir(acac).18 Moreover, Liu et al. investigated a series of fluoro-substituent derivatives of (ppy)2Ir(acac) and it was found that the photophysical properties of blue-emitting Ir(III) complexes can be finely tuned by systematic control of the number and site of the substituents on the phenyl ring.19 Recently, we have studied the optoelectronic properties for a series of blue-emitting Ir(III) complexes Ir(dfppy)2(pta-X/pyN4/pyN3) [pta ) pyridine-1,2,4-trazole; X ) phenyl, p-tolyl, and 2,6-difluororophenyl], and the structureproperty relationships were established.21 In this paper, on the basis of our previous work, we carried out density functional

10.1021/jp1010617  2010 American Chemical Society Published on Web 05/26/2010

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theory (DFT) calculations on the electronic structures, charge injection and transport, and spectral properties of several blueemitting phosphors (dfppy)2Ir(pyN2) (which has not been synthesized yet), (dfppy)Ir(pyN2)2, and (fpmb)2Ir(pyN3), aimed at exploring the effects of the electron-withdrawing abilities of the ancillary ligands on the electronic structures and optoelectronic properties. The differences of the electronic structures and optoelectronic properties between Ir(III) complexes coordinated with NHC and dfppy ligands are also discussed. 2. Computation Methods The ground state for each molecule was optimized by using the density functional theory. All open-shell calculations were performed with unrestricted methods, and spin contamination in the radical species can be neglected (〈S2〉 is in the range 0.7576-0.7643 and equals 0.75 before and after annihilation, respectively). Becke’s three-parameter hybrid method22 combined with the Lee-Yang-Parr correlation functional23 (denoted as B3LYP) was adopted here. The geometry optimizations of the lowest triplet states (T1) were performed by the configuration interaction with single excitations (CIS)24 approach. Vibrational frequencies were calculated at the same theoretical level to confirm that each configuration was a minimum on the potential energy surface. On the basis of ground- and excited-state optimization, the TD-DFT approach associated with the polarized continuum model (PCM) in the dichloromethane (CH2Cl2) media was applied to investigate the excited-state electronic properties. Due to large numbers of electrons, the LANL2DZ basis set25,26 was employed on the Ir atom. A relativistic effective core potential (ECP) on Ir replaces the inner core electrons, leaving the outer core 5s25p6 and 5d6 as the valence electrons of Ir(III). The 6-31G(d) basis set was used on nonmetal atoms in the gradient optimizations. To describe electronic affinities (EA) more accurately, the 6-31+G(d) basis set was further employed on the optimization of the neutral and the ionic species. These computational methods and basis sets have been proved reliable for cyclometalated Ir(III) complexes.21 All calculations were performed with the Gaussian 03 software package.27 3. Results and Discussion 3.1. Molecular Geometries in the Ground State. The schematic structures and optimized geometry parameters of (dfppy)2Ir(pyN2), (dfppy)Ir(pyN2)2, and (fpmb)2Ir(pyN3) in the ground state together with the available X-ray crystal diffraction data14,17 are shown in Figure 1. For comparison, the experimentally found blue-emitting complex (dfppy)2Ir(pyN3) (which has been calculated in our previous work as well21) is also selected. Ir(III) has d6 configuration, and adopts a pseudooctahedral coordination geometry. The most stable isomers of these complexes adopt the configurations shown in Figure 1. For example, in (fpmb)2Ir(pyN3), the two NHC fragments reside at the trans location, and the two phenyl fragments are at the cis location. The optimized bond distances for (dfppy)Ir(pyN2)2 and (fpmb)2Ir(pyN3) agree well with the corresponding X-ray results,14,17 except for the Ir-Npyridyl bond in (fpmb)2Ir(pyN3), in which our calculated bond distance 2.258 Å is 0.104 Å larger than the experimental value 2.154 Å. The discrepancy is due to the fact that the crystallographic data correspond to the complex (fpmb)2Ir(bptz) [bptz: 4-tert-butyl-2-(5-(trifluoromethyl)-2H-1,2,4-triazol-3-yl)pyridine], and the bulky butyl group on the pyridyl of (fpmb)2Ir(pyN3) makes significant solid state effects on the Ir-Npyridyl bond distance. Figure 1 also shows that bonds sharing the similar electronic environment have nearly the same bond distance. For example,

Figure 1. The schematic structures and the calculated bond parameters (in Å) for the studied complexes as well as the experimental data (in parentheses, ref 14 for (dfppy)Ir(pyN2)2 and ref 17 for (fpmb)2Ir(pyN3)). For comparison, (dfppy)2Ir(pyN3) from our previous study (ref 21) is also presented.

in (dfppy)2Ir(pyN2) and (dfppy)Ir(pyN2)2, the bond distances of Ir-Npyridyl trans to the pyridyl ring are 2.074, 2.072, 2.071, and 2.092 Å, which are nearly the same. The bond distances of Ir-Npyridyl trans to phenyl fragment are in the range 2.231-2.258 Å. Due to the significant trans influence of the phenyl group, the bonds trans to it are the longest. It is also interesting to note that the Ir-N bond distances trans to the difluorophenyl moiety follow the order of Ir-Npyridyl (2.231-2.258 Å) > Ir-NN3 (2.155 Å) > Ir-NN2 (2.147 Å), while the Ir-C bond distances at the trans location have the opposite trend (2.012-2.016 < 2.025 < 2.028 Å). From these results, three points can be obtained: (a) The shortened Ir-N bond distances result from improving the cooperative effect, in which a donation from a σ orbital of the ligand toward an empty dσ orbital of the metal and a back-donation from a filled dπ orbital to a π* antibonding orbital of the ligand occur simultaneously, and two processes strengthen each other. More electrons back-donating from a 5d orbital into an empty π*(N2) orbital weakens the Ir-C bond trans to the N2 group.28 Thus, the shorter the Ir-N bond, the longer the Ir-C bond at the trans location. (b) The slightly lengthened Ir-C bond distances show that the order of trans influence is N2 > N3 > pyridyl. (c) Among the Ir-N bonds, the strongest Ir-NN2 bond will further destabilize the d-d energy gap and prevent the nonradiative decay from the d-d excited state. 3.2. Electronegativity of the Ligands. The electronwithdrawing abilities of the ligands could be measured by the electronegativity (χ), which can be estimated from the average value of the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energies: χ ) -(EHOMO + ELUMO)/2.29 By reoptimizing the ligands, the calculated χ values are pyN3H (4.43 eV) > pyN2H (4.07 eV) > dfppyH (3.76 eV) > Hfpmb (3.18 eV), indicating that the electron-withdrawing abilities of the ligands decrease along this order. 3.3. Frontier Molecular Orbitals (FMOs). Since the optoelectronic properties, such as ionization potential (IP), electron affinity (EA), luminescent color, and charge transportation, are

DFT/TD-DFT Study of Blue-Emitting Ir(III) Complexes

J. Phys. Chem. A, Vol. 114, No. 24, 2010 6561 TABLE 1: Ionization Potentials, Electron Affinities, Extraction Potentials, and Inner Reorganization Energies for Electron/Hole (λie/λih)a IPv

IPa

HEP EAv EAa EEP

λie

λih

(dfppy)2Ir(pyN2) 7.10 6.99 6.87 0.99 1.07 1.14 0.15 0.23 (dfppy)Ir(pyN2)2 7.30 7.15 6.98 1.11 1.19 1.28 0.17 0.32 (fpmb)2Ir(pyN3) 6.75 6.62 6.48 0.78 0.95 1.16 0.38 0.27 a

Figure 2. Molecular orbital diagrams and HOMO and LUMO energies for (dfppy)2Ir(pyN2), (dfppy)Ir(pyN2)2, and (fpmb)2Ir(pyN3).

closely related to the FMOs, especially HOMO and LUMO, they have been studied in this work. In this subsection, we focus on the HOMO and LUMO as shown in Figure 2. The detailed information for other FMOs of (dfppy)2Ir(pyN2), (dfppy)Ir(pyN2)2, and (fpmb)2Ir(pyN3) is given in Tables S1-S3 of the Supporting Information. For (dfppy)2Ir(pyN2), the HOMO is mainly localized on 5d(Ir) and phenyl moieties of dfppy ligands, while the LUMO is primarily contributed by pyridyl moieties of one dfppy and the ancillary pyN2 ligands. This is similar to the situation in (dfppy)2Ir(pyN3).21 In addition, the HOMO and LUMO energies are affected significantly by the electron-withdrawing abilities of ancillary ligands. The HOMO and LUMO energies increase with decreased χ values of ancillary ligands: (dfppy)2Ir(pyN3) (-5.69 and -1.84 eV) < (dfppy)2Ir(pyN2) (-5.57 and -1.72 eV). Thus, the electronegativity change of the ancillary ligand could modulate the energy level of HOMO and LUMO. The HOMO-LUMO gaps are the same (3.85 eV) for (dfppy)2Ir(pyN2) and (dfppy)2Ir(pyN3).21 This indicates that similar to (dfppy)2Ir(pyN3), the assumed complex (dfppy)2Ir(pyN2) may be a promising candidate for blue-emitting material. In comparison with (dfppy)2Ir(pyN2), the HOMO of (dfppy)Ir(pyN2)2 is composed of 5d(Ir) and π(dfppy/pyN2), while LUMO is contributed by π*(pyN2), showing that the influence of pyN2 is significant. Moreover, the energy levels of HOMO (-5.74 eV) and LUMO (-1.87 eV) are lowered compared with the corresponding values -5.57 and -1.72 eV in (dfppy)2Ir(pyN2) due to the larger electronegativity of pyN2 ligand than dfppy ligand. For (fpmb)2Ir(pyN3), the HOMO is mainly localized on 5d(Ir) and phenyl fragments of fpmb ligands, while the LUMO is primarily contributed by pyridyl moieties of the ancillary pyN3 ligand. The strong field of the NHC ligand makes the energy level of HOMO (-5.28 eV) and LUMO (-1.69 eV) enhance significantly compared with the corresponding values -5.59 and -1.84 eV in (dfppy)2Ir(pyN3),21 as well as (dfppy)2Ir(pyN2) and (dfppy)Ir(pyN2)2 (Figure 2), especially for HOMO. In addition, its energy gap is the smallest (3.59 eV) among the studied complexes. Moreover, it is interesting to note that the HOMOs of the studied complexes in this work are contributed by the ligand with the weak electron-withdrawing ability, on the contrary, the LUMOs are mainly localized on the ligand with the strong electron-withdrawing ability.

Units: eV.

3.4. IP, EA, and Inner Reorganization Energy. To obtain devices with better performance, it is necessary to know the charge injection and transfer properties of luminescent materials. The electron (hole) injection abilities could be estimated by EAs (IPs).21,30 A larger EA (smaller IP) suggests that it is easier to inject electrons (holes) into the emitting materials from the electron (hole) transporting layer. Thus, the turn-on voltage would be low and the performance of devices would be enhanced. The calculated vertical IP (IPv), adiabatic IP (IPa), vertical EA (EAv), and adiabatic EA (EAa) are listed in Table 1. It shows that (dfppy)Ir(pyN2)2 has the largest IP and EA values among the studied complexes. Therefore, the electron injection would be the easiest and hole injection would be the hardest. In contrast, (fpmb)2Ir(pyN3) has the smallest IP and EA values, leading to the easiest hole injection and hardest electron injection. As widely used host materials for blue phosphorescent OLEDs, CzSi (9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole) and UGH2 (p-bis(triphenylsilyl)benzene) have HOMO energies -5.36 and -6.52 eV, respectively, from our calculation. From Figure 2 and Table 1, it can be found that the order of calculated -EHOMO (-ELUMO) is consistent with that of calculated IPs (EAs). Thus, except for (fpmb)2Ir(pyN3), the hole injection from CzSi to (dfppy)2Ir(pyN2) and (dfppy)Ir(pyN2)2 is an endothermic process and the majority holes will be accumulated on the HOMO of CzSi, because the HOMO energies of the latter are lower than that of CzSi. This means that CzSi is not a suitable host material for (dfppy)2Ir(pyN2) and (dfppy)Ir(pyN2)2, but suitable for (fpmb)2Ir(pyN3). However, for UGH2, the holes can be injected directly into the HOMO of the studied complexes, because their HOMO energies are higher than that of UGH2. At the microscopic level, the charge transport mechanism can be described as a self-exchange transfer process, in which an electron (or a hole) transfers from a charged molecule to an adjacent neutral molecule.31-33 The rate of intermolecular charge transfer (Ket) can be estimated by using the semiclassical Marcus theory34 described as follows:

Ket ) A exp(-λ/4KBT)

(1)

where λ is the reorganization energy, A is a prefactor related to the electronic coupling between adjacent molecules, T is the temperature, and kB is the Boltzmann constant. It has been demonstrated that due to the limited intermolecular charge transfer range in the solid state, the mobility of charges dominantly relates to the reorganization energy λ for OLEDs materials.35,36 Therefore, at constant temperature, the minimal reorganization energy is necessary for an efficient charge transport process. Generally, λ is determined by the fast change of the molecular geometry when a charge is added or removed from a molecule (the inner reorganization energy λi) and slow variations in the surrounding medium due to the polarization effects (the external contribution λe). A previous report,30 however, has indicated

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Figure 3. Schematic description of the inner reorganization energy.

that λe is very small and λi is dominant in λ. Thus, we focus on the inner reorganization energy λi, which is caused by the change of the internal nuclear coordinates from the reactant A to the product B and vice versa (Figure 3). It can be evaluated as the sum of two relaxation energies according to the following formula:

λi ) λ0 + λ1 ) (EBA - EA) + (EAB - EB)

(2)

where EA and EAB are the energies of A and B at the optimized geometry of A, respectively, and EBA and EB are the energies of A and B at the optimized geometry of B, respectively. Figure 3 shows that the reorganization energy for hole transport λih ) IPv - HEP. HEP represents hole extraction potential, which is the energy difference between M (neutral molecule) and M+ (cationic), using M+ geometry. Similarly, the reorganization energy for electron transport λie ) EEP - EAv. EEP represents the electron extraction potential, which is the energy difference between M and M- (anionic), using M- geometry. The λi values of the investigated complexes are given in Table 1. For Ir(III) complexes coordinated with dfppy ligand(s), λie is smaller than λih. This reveals that their electron-transporting performance is better than the hole-transporting performance. In contrast, Ir(III) carbene complex (fpmb)2Ir(pyN3) has a smaller λih and thus better hole-transporting performance. On the other hand, besides eq 2, λ can also be evaluated from the summations running over all the vibrational normal modes, which provides the partition of the total relaxation energy into the contributions from each vibrational mode:37

λ)

∑ λj ) ∑ pωjSj

(3)

ωj is the vibrational frequency, and Sj denotes the Huang-Rhys factor (electron-vibration coupling constant). Equation 3 indicates that the vibrational frequencies play an important role in determining the λ values. Therefore, since λ in (dfppy)2Ir(pyN2) and (fpmb)2Ir(pyN3) (Table 1) is larger than that of (dfppy)2Ir(pyN3) (λie and λih of (dfppy)2Ir(pyN3) are 0.08 and 0.12 eV,21 respectively), it can be assumed that the vibrational modes of -CH in (dfppy)2Ir(pyN2) and -CH3 in (fpmb)2Ir(pyN3) contribute to the λ significantly. Thus, to design optoelectronic materials with smaller λ values, a reasonable way would be to use the relatively heavy atoms such as F instead of H to suppress the vibrational frequencies of -CH and -CH3. 3.5. Photoexcitation in CH2Cl2 Media. Simulated Gaussiantype absorption curves in CH2Cl2 media for the studied

Figure 4. Simulated absorption spectra in CH2Cl2 media for complexes (dfppy)2Ir(pyN2), (dfppy)Ir(pyN2)2, and (fpmb)2Ir(pyN3).

complexes are shown in Figure 4. The detailed information, such as excitation energies, oscillator strengths (f), dominant configurations (with larger CI coefficients), transition nature, and experimental values of these complexes, is listed in Tables S4-S6 of the Supporting Information. Because the absorption spectra of the studied complexes are very complicated, only the excited states with f > 0.07 are selected for discussion. The character of each excited state is assigned on the basis of the compositions of the occupied and virtual MOs of the dominant configuration(s). The excited states originating from transitions between orbitals located on different moieties are classified as charge transfer (CT) excited states such as MLCT, and ligandto-ligand charge transfer (LLCT). The excited states from π-occupied to π-virtual orbitals located on the same ligand are described as intraligand (IL). The simulated absorption curves of (dfppy)Ir(pyN2)2 and (fpmb)2Ir(pyN3) well reproduce the experimental spectra in terms of band locations, intensities, and separations.14,17 Comparing (dfppy)2Ir(pyN2) with (dfppy)2Ir(pyN3),21 the change of the ancillary ligand has little influence on the absorption spectra. Their S0 f S1 excited states mainly correspond to the transition from HOMO to LUMO, resulting in almost identical absorptions at 384 (f ) 0.039)21 and 383 (f ) 0.041) nm for (dfppy)2Ir(pyN3) and (dfppy)2Ir(pyN2), respectively, and both of them are characterized as MLCT/ ILdfppy/LLCT. On the other hand, the absorption spectra of (dfppy)Ir(pyN2)2 are similar to that of (dfppy)2Ir(pyN2) and can be divided into three wave bands. The calculated maximum absorption band at 368 nm corresponds to HOMO f LUMO transition and is blueshifted about 15 nm compared with (dfppy)2Ir(pyN2). The characters of the lowest-lying absorptions for (dfppy)Ir(pyN2)2 and (dfppy)2Ir(pyN2) are predominated by the pyN2 and dfppy ligands, respectively, and assigned as MLpyN2CT/ππ*pyN2/ LdfppyLpyN2CT and MLdfppyCT/ππ*dfppy/LdfppyLdfppy′CT, respectively. A moderate absorption band is produced by the overlap of the absorptions ranging from 278 to 297 nm and assigned as MLCT/IL/LLCT character [d(Ir) + π(dfppy/pyN2) f π*(dfppy/ pyN2)]. The intensive absorption band in the region of 268-256 nm corresponds to MLCT/LLCT/IL character [d(Ir) + π(dfppy/ pyN2) f π*(pyN2)]. The calculated absorptions at 368, 329, 312, 297, and 268 nm agree well with the experimental values at 356, 326, 311, 299, and 264 nm.14 In experiment,14 the lowlying absorption band is assigned to a mixed MLCT and ππ* transition, which is consistent with our calculated results. In

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TABLE 2: Calculated Emission Energies of T1 and Their Transition Nature for Ir(III) Complexes λ (nm)/E (eV)

configuration

nature

(dfppy)2Ir(pyN2)

503/2.46

(dfppy)Ir(pyN2)2

497/2.49

(fpmb)2Ir(pyN3)

484/2.56

H-2 f L(0.51) H f L (0.51) H f L (0.63) H-1 f L(0.29) H-2 f L(0.53) H-5 f L(0.35)

IL/MLCT [d(Ir) + π(dfppy) f π*(dfppy)] IL/MLCT/LLCT [d(Ir) + π(dfppy) f π*(dfppy)] IL/MLCT [d(Ir) + π(dfppy) f π*(dfppy)] IL/MLCT/LLCT [d(Ir) + π(dfppy/pyN2) f π*(dfppy)] IL/MLCT [d(Ir) + π(pyN3) f π*(pyN3)] IL/MLCT/LLCT [d(Ir) + π(pyN3/fpmb) f π*(pyN3)]

the high-energy region (268 nm), the intensities of (dfppy)2Ir(pyN2) are stronger than that of (dfppy)Ir(pyN2)2, which would increase the probability of intersystem crossing (ISC) from singlet to triplet states and hence the possibility of increasing phosphorescent quantum yield. The differences of absorption intensities are especially obvious in the region of 268-300 nm, as a consequence of the change of ligands. The reason for the stronger intensity of (dfppy)2Ir(pyN2) is that the absorptions in this region are mainly ππ* transition on the two dfppy ligands (see the excited state with the largest oscillator strength, S18 in Table S4, Supporting Information). Moreover, the absorptions of (dfppy)Ir(pyN2)2 around 262 nm are stronger than that of (dfppy)2Ir(pyN2), because its excited states S31 and S32 at 262 nm are mainly ππ* transition on the two pyN2 ligands (see Table S5, Supporting Information). The absorption spectra of Ir(III) carbene complex (fpmb)2Ir(pyN3) are different from that of Ir(III) complexes coordinated with dfppy ligand(s). Ligand fpmb has significant influence on the spectral characters (see Table S6, Supporting Information). The maximal allowed transition featuring MLpyN3CT/ LfpmbLpyN3CT is localized at 348 nm (f ) 0.036) and produces a large blue-shift of 36 nm compared with (dfppy)2Ir(pyN3).21 The bands ranging from 289 to 264 nm are characterized as MLCT/IL/LLCT [d(Ir) + π(fpmb) f π*(fpmb)]. The measured absorptions17 at 356 and 298 nm are consistent with the calculated absorptions at 348 and 289 nm. 3.6. Phosphorescence in CH2Cl2 Media. TD-DFT was used to calculate the phosphorescent spectra in CH2Cl2 media on the basis of the lowest triplet state (T1) geometries. The calculated emission energies, dominant configurations (with larger CI coefficients), transition nature, and the available experimental values are listed in Table 2. Meanwhile, a graphical display for the electron density distribution changes upon T1 f S0 excitation is shown in Figure 5. Similar to the HOMO-LUMO energy gaps, the emission energies of (dfppy)2Ir(pyN2) and (dfppy)2Ir(pyN3)21 are the same (2.46 eV). This demonstrates that the assumed phosphor (dfppy)2Ir(pyN2) can serve as a blue-emitting material. Table

Figure 5. Change of electron density distribution upon the T1 f S0 transition for (dfppy)2Ir(pyN2), (dfppy)Ir(pyN2)2, and (fpmb)2Ir(pyN3). Cyan and violet colors represent the decrease and increase of electron density, respectively.

exptl (nm)

47914 46117

2 and Figure 5 show that the phosphorescent character of (dfppy)2Ir(pyN2) is mainly assigned as ππ*dfppy/MLdfppyCT. Generally, although TD-DFT systematically underestimates the transition energies, it reproduces the general trend. Herein, the calculated emission spectrum of (dfppy)Ir(pyN2)2 at 497 nm is blue-shifted about 7 nm compared with (dfppy)2Ir(pyN3) (504 nm21), which is consistent with the experimental value 10 nm (489 nm for (dfppy)2Ir(pyN3) vs 479 nm for (dfppy)Ir(pyN2)2).11,14 Figure 5 shows that its phosphorescent character is still dominated by the dfppy ligand and mainly assigned as ππ*dfppy/MLdfppyCT. The calculated emission wavelength of (fpmb)2Ir(pyN3) at 484 nm is the shortest and blue-shifted by 20 nm compared with (dfppy)2Ir(pyN3),21 in good agreement with the 28 nm observed by the experiments.11,17 It is interesting to note that different from its lowest-lying absorption, the pyN3 moiety, rather than the fpmb ligand, predominates the phosphorescent nature, which has ππ*pyN3/MLpyN3CT character. Because the transition characters of (dfppy)Ir(pyN2)2 and (fpmb)2Ir(pyN3) mainly depend on the unique dfppy and pyN3 ligand, respectively, the luminescent properties of such materials can be tuned by modifications of dfppy and pyN3 ligand, providing a versatile and cheap approach to control molecular phosphorescent properties. 4. Conclusions We have investigated the electronic structures and optoelectronic properties of two classes of blue-emitting iridium(III) complexes: Ir(III) complexes coordinated with dfppy and NHC ligands. It is found that the properties of the ligand have great influence on the photophysical properties, such as energy gap, absorption spectra, emission spectra, etc. (dfppy)Ir(pyN2)2 has the lowest HOMO and LUMO energies among the considered complexes, followed by (dfppy)2Ir(pyN2) and (fpmb)2Ir(pyN3). This indicates that the injection abilities increase for hole injection in that order, and decrease for electron injection. This is also demonstrated by the calculated ionization potentials and electron affinities. The much higher HOMO and LUMO energy levels in (fpmb)2Ir(pyN3) are related to the strong ligand field of NHC. The calculated energy gaps decrease from 3.87 eV in (dfppy)Ir(pyN2)2, to 3.85 eV in (dfppy)2Ir(pyN2), to 3.59 eV in (fpmb)2Ir(pyN3). The energy gap, maximal absorption, and emission spectra of (dfppy)Ir(pyN2)2 have a blue-shift compared with (dfppy)2Ir(pyN2). The emission energy of (dfppy)2Ir(pyN2) is the same as the experimentally found blue-emitting complex (dfppy)2Ir(pyN3), suggesting that the assumed complex (dfppy)2Ir(pyN2) could be a promising candidate for efficient blue-emitting material. The absorption spectra of (fpmb)2Ir(pyN3) are predominated by fpmb ligand, while its emission spectra depend on the pyN3 ligand. Ir(III) complexes coordinated with dfppy ligand(s) have better electron-transporting performance, while Ir(III) carbene complex (fpmb)2Ir(pyN3) has better hole-transporting performance. Acknowledgment. The authors thank the National Natural Science Foundation of China for financial support (Grant Nos.

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90922015, 20921002, and 20831004), National Basic Research Program of China (973 Program;2009CB623605), Chang Jiang Scholars Program (2006), and Program for Changjiang Scholars and Innovative Research Team in University (IRT0714). Supporting Information Available: Frontier molecular orbital compositions and energies in the ground state for (dfppy)2Ir(pyN2), (dfppy)Ir(pyN2)2, and (fpmb)2Ir(pyN3); selected calculated wavelength/energies, oscillator strength, major contribution, transition characters, and the corresponding experimental data for absorption spectra of (dfppy)2Ir(pyN2), (dfppy)Ir(pyN2)2, and (fpmb)2Ir(pyN3). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K. Nature 2009, 459, 234. (2) Chiu, Y. C.; Hung, J. Y.; Chi, Y.; Chen, C. C.; Chang, C. H.; Wu, C. C.; Cheng, Y. M.; Yu, Y. C.; Lee, G. H.; Chou, P. T. AdV. Mater. 2009, 21, 2221. (3) Ikai, M.; Tokito, S.; Sakamoto, Y.; Suzuki, T.; Taga, Y. Appl. Phys. Lett. 2001, 79, 156. (4) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048. (5) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (6) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Lee, H. E.; Adachi, C.; Burrows, P. E.; Forrest, S. R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (7) Duan, J. P.; Sun, P. P.; Cheng, C. H. AdV. Mater. 2003, 15, 224. (8) Su, Y. J.; Huang, H. L.; Li, C. L.; Chien, C. H.; Tao, Y. T.; Chou, P. T.; Datta, S.; Liu, R. S. AdV. Mater. 2003, 15, 884. (9) Tsuboyama, A.; Iwawaki, H.; Furugori, M.; Mukaide, T.; Kamatani, J.; Igawa, S.; Moriyama, T.; Miura, S.; Takiguchi, T.; Okada, S.; Hoshino, M.; Ueno, K. J. Am. Chem. Soc. 2003, 125, 12971. (10) Holmes, R. J.; Forrest, S. R.; Tung, Y. J.; Kwong, R. C.; Brown, J. J.; Garon, S.; Thompson, M. E. Appl. Phys. Lett. 2003, 82, 2422. (11) Yeh, S. J.; Wu, M. F.; Chen, C. T.; Song, Y. H.; Chi, Y.; Ho, M. H.; Hsu, S. F.; Chen, C. H. AdV. Mater. 2005, 17, 285. (12) Holmes, R. J.; D’Andrade, B. W.; Forrest, S. R.; Ren, X.; Li, J.; Thompson, M. E. Appl. Phys. Lett. 2003, 83, 3818. (13) Ren, X. F.; Li, J.; Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. Chem. Mater. 2004, 16, 4743. (14) Yang, C. H.; Cheng, Y. M.; Chi, Y.; Hsu, C. J.; Fang, F. C.; Wong, K. T.; Chou, P. T.; Chang, C. H.; Tsai, M. H.; Wu, C. C. Angew. Chem., Int. Ed. 2007, 46, 2418. (15) Sajoto, T.; Djurovich, P. I.; Tamayo, A.; Yousufuddin, M.; Bau, R.; Thompson, M. E.; Holmes, R. J.; Forrest, S. R. Inorg. Chem. 2005, 44, 7992.

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