Origin of Rare and Highly Efficient Phosphorescent and

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Origin of Rare and Highly Efficient Phosphorescent and Electroluminescent Iridium(III) Complexes Based on C∧NdN Ligands, A Theoretical Explanation Xiao-Na Li,†,‡ Zhi-Jian Wu,† Xiao-Juan Liu,† and Hong-Jie Zhang*,† State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, Graduate School of the Chinese Academy of Sciences, Beijing, People’s Republic of China ReceiVed: May 12, 2010; ReVised Manuscript ReceiVed: July 13, 2010

The origin of the rare and highly strong phosphorescent (PL) and electroluminescent (EL) Ir(III) complexes with C∧NdN ligand, tris[3,6-bis(phenyl)pyridazinato-N1,C2]iridium (Ir(BPPya)3) (1), tris[1,4-bis(phenyl)phthalazine]iridium (Ir(BPPa)3) (2), and tris[1-(2,6-dimethylphenoxy)-4-(4-chlorophenyl)phthalazine]iridium ((Ir(MPCPPZ)3) (3) are investigated theoretically. By changing the conjugation length of the C∧NdN ligand from BPPya to BPPa, one can tune the emission color from green in 1 to saturated red in 2. The addition of sterically bulky phenolic substituents in 3 exhibits the highest external quantum efficiency of 20.2% ph el-1 and luminescence efficiency of 18.4 cd A-1. Density functional theory (DFT) and time-dependent DFT (TDDFT) methods are used to rationalize these properties. The more promising PL and EL properties of 3 result from the bulky phenolic group, which acts as a pendant at the periphery of the emitting core and protects the emitting core from the hazardous intermolecular interaction of emitters and reduces luminescence quenching. Introduction Since the organic light-emitting diodes (OLEDs) comprising an emissive layer containing green-emitting Ir(ppy)3 (fac-tris(2phenylpyridyl)iridium(III) blended with CBP (4,4′-bis(N-carbazolyl)-2,2′-biphenyl) are reported to have an external quantum efficiency of 19%,1 luminescence Ir(III) complexes have been investigated extensively as emitting layers in OLEDs because of their excellent emission properties, such as good color purity, short triplet excited lifetime, high quantum efficiency, and high stability.2 The strong spin-orbit coupling of a heavy metal can effectively promote singlet-to-triplet intersystem crossing and enhance the subsequent radiative transition, which can, to a large extent, partially remove the spin-forbidden nature from the triplet to the ground state. Among the three primary colors, however, the highly efficient pure-red-emitting complexes are still scarce3 because the low quantum yields result from a diminished HOMO-LUMO band gap, which facilitates the nonradiative process according to the energy gap rule.4 Modification of chelate ligand structure is an efficient way to improve the properties. Designing new cyclometalating ligands, auxiliary ligands, or attaching different substituents has received immense attention to tune the emission color or efficiency of the resulting complexes. Cyclometalated Ir(C∧N)3 and Ir(C∧N)2L (where C∧N ) 2-phenylpyridine and L ) ancillary ligand) complexes represent the well-known phosphorescent complexes in OLEDs. A fairly stable chelating interaction between Ir(III) cation and carbon, and the simultaneous formation of metal-nitrogen dative bonding makes these complexes having high thermal stability, high phosphorescence (PL), and electroluminescence (EL) efficiency.5 Theoretical investigations showed that the HOMO of these Ir(III) complexes consists of a mixture of * Corresponding author. Phone: +86 431 85262127. Fax: +86 43185698041. E-mail: [email protected]. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.

phenyl π- and Ir d-orbital, and the LUMO is localized largely on the pyridyl π*-orbital. Thus, a generally used strategy to tune emission color to red relies on the introduction of electrondonating groups to the phenyl moiety or extending the π-conjugation length of ppy, which can elevate the HOMO energy levels more than that of LUMO by increasing the dπ composition6 and therefore narrow the HOMO-LUMO gaps. Highly efficient phosphorescent dye doped PLED/OLED devices with the blending system may intrinsically suffer from efficiency and stability limitations because of positive energy transfer from host to low-lying triplet states, long PL lifetime, and triplet-triplet annihilation (TTA). They arise from the aggregation of dopants and potential phase separation, which result in the fast decay of efficiency with increasing current density. To suppress these negative effects, many strategies have been developed: introduction of ligand with stronger interaction with metal, improvement of the thermal stability, and consequently, extension of the lifetime of OLED devices. In addition, the strengthened metal-ligand interaction is a critical factor to increase the 3MLCT character and therefore shorten the phosphorescence lifetime, which can partially remove the possibility of fast decay and improve EL quantum efficiency at high doping concentration or at high current density due to the absence of TTA or excited-state saturation. Another effective strategy is the introduction of suitable dendritic structure at the periphery of the coordination environment7 or direct incorporation of the transition metal complexes into the polymer main chain,8 which can prevent the emission center from suffering from harmful aggregation or phase inhomogeneity. Recently, Mi and Wang’s groups reported the excellent works of Ir(III) complexes containing C∧NdN ligands, tris[3,6bis(phenyl)pyridazinato-N 1,C2]iridium (Ir(BPPya)3),9 tris[1,4bis(phenyl)phthalazine]iridium (Ir(BPPa)3),10 and tris[1-(2,6dimethylphenoxy)-4-(4-chlorophenyl)phthalazine]iridium ((Ir(MPCPPZ)3),11 which are more promising for optoelectronic applications compared with Ir(III) complexes with simple C∧N

10.1021/jp1043317  2010 American Chemical Society Published on Web 08/04/2010

Phosphorescent and Electroluminescent Ir(III) Complexes

J. Phys. Chem. A, Vol. 114, No. 34, 2010 9301

TABLE 1: Main Optimized Geometry Parameters for 1-3 at the B3LYP/LANL2DZ Level Together with the Experimental Data for 2a 1

a

2

3

S0

T1

S0

T1

S0

T1

exptl10

IrsC1 IrsC2 IrsC3 IrsN1 IrsN2 IrsN3 C4sC5 C6sC7

2.039/2.033/2.039 2.039/2.032/2.039 2.040/2.032/2.040 2.110/2.149/2.124 2.111/2.152/2.125 2.115/2.148/2.129 1.462/1.460/1.457 1.487/1.483/1.481

2.058 2.026 2.022 2.143 2.129 2.066 1.464 1.487

Bong Length (Å) 2.034 2.035 2.035 2.112 2.114 2.114 1.469 1.491

2.027 2.027 2.025 2.118 2.117 2.118 1.461 1.488

2.031 2.031 2.031 2.125 2.125 2.124 1.476

2.049 2.025 2.026 2.143 2.132 2.061 1.476

2.019(5)

C1sIrsN3 C1sIrsN2

171.9/172.1/171.7 79.6/79.1/79.2

170.9 78.8

Bong Angle (deg) 170.4 78.5

171.5 78.8

172.3 78.2

173.3 78.3

NdNsA NdNsB

0.7/0.7/1.2 18.3/17.5/20.9

1.2 13.5

7.6

11.0

Dihedral Angle (deg) 11.9 12.7 40.1 37.9

2.095(4)

The italic data are the geometrical parameters at the BS2 level, and the boldface are parameters at the BS3 level for 1.

CHART 1: Sketch Structures of 1-3 chelate ligands, due to their high thermal stability and high EL and PL efficiency. By changing the conjugation length of C∧NdN ligands from Ir(BPPya)3 to Ir(BPPa)3, one can tune the emission color from green to saturated red. The addition of sterically bulky phenolic substituents in Ir(MPCPPZ)3 exhibits the highest external quantum efficiency of 20.2% ph el-1 and luminescence efficiency of 18.4 cd A-1. In this paper, we carried out density functional theory (DFT) calculations on Ir(BPPya)3 (1), Ir(BPPa)3 (2), and Ir(MPCPPZ)3 (3), aiming at providing a theoretical understanding of the higher thermal stability and the high PL and EL efficiencies of these Ir(III) complexes. In addition, comparisons of electronic structure among 1-3 are made in an effort to rationalize the reasons of the red-shifted emissions in 2 and 3 with respect to 1, together with the rather higher EL efficiency of 3 due to the presence of bulky phenolic substituents. This work provides a theoretical insight for the structure-driven tuning of the excited-state properties, thus opening the way for future design and synthesis of new iridium(III) phosphors suitable for higher performance PLEDs. Computational Details The ground-state and the lowest-lying triplet excited-state geometries were optimized by DFT12 with Becke’s LYP (B3LYP) exchange-correlation functional13 and the unrestricted B3LYP (UB3LYP)14 approach, respectively. There were no symmetry constraints on these complexes. At the respective optimized geometries of ground and excited states, TDDFT/ B3LYP15 calculations associated with the conductor polarized continuum model (CPCM)16 in toluene (for 1) and dichloromethane (CH2Cl2) (for 2 and 3) media were carried out to obtain the vertical excitation energies of singlet (Sn) and triplet (Tn) states. Spin-orbit coupling (SOC) causes the mixture of singlet and triplet states, and the latter is allowed to participate in both absorption and emission; however, no SOC factor is included in current TDDFT calculation under G03 software. Su and his co-workers explored the SOC effect on the excited energy of Re complexes.17 The results revealed that SOC has a tiny effect on transition and it provides available spectral features for transition-metal Re complexes. In the calculation, the quasi-relativistic pseudopotentials of Ir atoms proposed by Hay and Wadt18 with 17 valence electrons

were employed, and a “double-ξ” quality basis set LANL2DZ was adopted. The selection of appropriate basis sets for a given system is very important in ensuring high-quality results. However, the trade-off between accuracy and computational costs has to be weighed, and this becomes increasingly important with large molecular systems. Therefore, as an example, we optimized the geometry structure of 1 in the ground state with three sets of basis sets: LANL2DZ on all the atoms was used as BS1, LANL2DZ on Ir and 6-31G(d) on nonmetal atoms was used as BS2, and LANL2DZ on Ir and 6-31G+(d) on other atoms was used as BS3. The calculated results are listed in Table 1. As shown, the more accurate results can be obtained from BS1. When BS1 and BS2 are compared, the differences of Ir-C and C-C bonds, as well as angles between two basis sets, are negligible, while the Ir-N bonds are greatly overestimated by BS2. In BS3, the addition of diffuse function improves the Ir-N bond lengths; however, it is still longer than that obtain from BS1. Therefore, we adopted BS1 for all the calculations. The frequency calculations were performed to verify the optimized structure was at an energy minimum. All calculations were performed with Gaussian 03 software package.19 Results and Discussion Geometries in the Ground and the Lowest Lying Triplet Excited States. The sketch map for 1-3 is presented in Chart 1 and the optimized ground-state structures are shown in Figure 1 along with the numbering of some key atoms for 1. The main structural parameters are summarized in Table 1 together with the X-ray crystal structures data of 2.10 All complexes show a pseudo-octahedral coordination around the metal centers with

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Figure 2. Mulliken atom charge distribution on center Ir(III), NdN, Ph(A), and Ph(B) moieties.

Figure 1. Optimized structures of 1-3 in the ground states at the DFT/ B3LYP level.

the nitrogen atom localized on the trans position to the carbon atom. Table 1 indicates that the optimized bond lengths of 2 are in good agreement with the experimental values.10 The calculated average IrsC bond length of 2.035 Å is ca. 0.8% longer than the experimental value of 2.019 Å, and the average deviations of IrsN bonds between the calculated and the experimental values are 0.9%. When compared with the case of the simple ppy ligand, the advantage of the C∧NdNH ligand is the easier proximity to the center metal due to the greatly reduced size of the NdN moiety. Therefore, the strengthened metal-ligand bond can be formed, which is significantly important to improve the phosphorescent quantum efficiency, and this may be the origin of the rare and high efficiency of these complexes. The discrepancy between the calculated and the measured values may result from the different environments and the rather limited dimensions of basis set. It is interesting to note that the average IrsC bonds decrease in the order 1 f 2 f 3; IrsN bonds in 3 are longer than in 1 and 2, and C4sC5 bonds increase from 1 to 3. This can be

attributed to the extended π-conjugation length of pyridazine (NdN) in 2 and 3 compared with that in 1, which causes significant repulsion between the NdN moiety and its adjacent phenyl rings. As shown in Chart 1 and Figure 1, the dihedral angles between NdN and phenyl ring A (Ph(A)) are 0.7, 11.9, and 7.6° for 1-3, respectively, and the dihedral angles between NdN and phenyl ring B (Ph(B)) for 1 and 2 are 18.3 and 40.1°. The increased dihedral angles in 2 and 3 distort the C∧NdN ligands and push the adjacent phenyl rings away from the NdN moiety and, consequently, lead to the elongated C4sC5 bonds. The longer IrsN bonds in 3 can be attributed to two reasons: The first is the steric bulky phenolic moiety, which prevents the NdN moiety from being close to the center Ir(III) metal and therefore decreases the coordination strength between the N atom and Ir(III). The second is the decreased static attraction between the NdN moiety and the center Ir(III) due to the electron-withdrawing effect of the phenolic substituent, which will be discussed in the next paragraph. To further investigate the bonding interactions, Mulliken population analysis is presented in Figure 2. For all cases, the Mulliken atom charges on the center Ir(III) are +1.148, +1.183, and +1.211 |e-|, which are much less ionic than the formal oxidation state of +3, indicating the considerable donation of electron density into the 5d orbital from the adjacent ligands. When 1 and 2 are compared, the increased positive charge on Ir(III) and the simultaneous increased negative charge on Ph(A) in 2 suggest the increased cooperative effect,20 which results from the repulsion of the NdN moiety and the stronger IrsC bonds in 2 because of static attraction. For 3, the inductive effect caused by the chlorine atom can further improve the backdonation from the dπ orbital and decrease the IrsC bonds in 3. The stronger inductive effect of phenol causes more positive charge on the metal ion and NdN moiety and more negative charge on the Ph(A) and Ph(B) moieties. In this case, the metal-ligand interaction is weakened and leads to the longest IrsN bonds among 1-3. In addition, the large positive-negative separation will result in a large dipole moment and consequently the high polarity of 3 compared with that for 1 and 2. On the basis of the optimized ground-state structures, the UB3LYP method is used to optimize the lowest-lying triplet excited states (T1), and the main geometry parameters are also presented in Table 1. It can be seen that the variation of geometrical parameters of 1 and 3 is different from that in 2. For 2, all IrsN bonds are slightly elongated and all IrsC bonds are contracted in the T1 state, which indicates that the metal-Ph(A) interaction is strengthened compared with metalsNdN. For 1 and 3, one ligand has the weakened IrsC and IrsN bonds, one has the same variation trend with that in 2, and the last has the strengthened IrsC and IrsN bonds. The



Figure 3. Presentation of energy levels, energy gaps, and orbital composition distribution of HOMO and LUMO for 1-3.

different situations of the three ligands in the T1 states will consequently result in uneven population densities on them, and the ligand with the strongest metal-ligand interaction will control the frontier molecular orbital in the T1 states. Frontier Molecular Orbital Properties. It is known that the observed differences in optical and chemical properties depend mainly on the changes of the ground-state electronic structure. The frontier molecular orbital (FMO) compositions for 1-3 are given in Tables S1-S3 (Supporting Information). The HOMO and LUMO distribution, energy levels, and energy gaps are plotted in Figure 3. For 1, Figure 3 and Table S1 show that the HOMO, at -5.05 eV, distributes over the d-orbital of Ir (49.3%) and the NdN (13.5%) and Ph(A) (35.1%) moieties, with negligible composition from Ph(B). The d-orbital is an antibonding combination with the C∧NdN ligand π-orbital. HOMO-1 and HOMO-2 are a couple of degenerate orbitals that have orbital distributions similar to that of HOMO and are separated by 0.19 eV from HOMO. HOMO-3 and HOMO-4 are another couple of degenerate orbitals localized mainly on Ph(A). 2 has an orbital composition very similar to that for 1, and the extended π-conjugation length slightly increases the contribution from metal d-orbital and the NdN moiety as shown in Table S2 (Supporting Information). In HOMO-3 and HOMO-4, the orbital composition is almost evenly distributed on the NdN and Ph(A) moieties with the disappeared composition from Ph(B). This can be attributed to the large torsion angle between NdN and Ph(B) because of steric hindrance, which breaks the π-conjugation. For 3, the electron density on the metal d-orbital in HOMO-HOMO-2 as well as the distribution on NdN in HOMO-3 and HOMO-4 decrease due to the electronwithdrawing effect of chlorine atom.

Figure 4. Density-of-states for 1-3.

For 1, LUMO and LUMO+1, LUMO+2 and LUMO+3 are two couples of degenerate orbitals, localized exclusively on NdN, Ph(A), and Ph(B), with the exception of LUMO+1, which is predominantly localized on the NdN moiety. The extended π-conjugation in 2 and 3 causes a significant difference in unoccupied molecular orbitals distribution compared with that in 1. For 2 and 3, the LUMO is mainly localized on the NdN moiety with a small composition on Ph(A). While LUMO+1 and LUMO+2 are a couple of degenerate orbitals with similar populations, and LUMO+1 is about 0.16 eV higher in energy than LUMO. LUMO+3 and LUMO+4 for 2 and 3 are predominantly localized on the NdN moiety with nearly zero contribution from the Ph(B) moiety. In addition, we noticed that the e*g-like orbitals lie at much higher energy, being at LUMO+18, LUMO+23, and LUMO+25 for 1-3, respectively, about 6.82, 6.62, and 6.67 eV higher than HOMO as shown in Figure 4, which shows the DOS spectra in the energy range from -6 to +2.5 eV. The emission color turning by grafting

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TABLE 2: Absorption of 1 in Toluene Solution According to TDDFT/B3LYP Calculations state

λ (nm)/E (eV)

oscillator

S1

486/2.55

0.0044

S13

419/2.96

0.0895

S18

386/3.21

0.1602

S34

314/3.95

0.1393

S36

311/3.99

0.8315

main configuration H f L+1 (76%) H f L (24%) H-2 f L+2 (26%) H-1 f L+3 (18%) H-1 f L+5 (29%) H-2 f L+4 (17%) H-4 f L +5 (29%) H-6 f L (17%) H-5 f L+1 (16%) H-5 f L (41%) H-6 f L (27%)

assign d(Ir) + π(NdN+ph(A)) f π*(NdN)/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph(A)) f π*(NdN+ph(A) + ph(B))/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph(A)) f π*(NdN+ph(A)+ph(B))/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph(A)) f π*(NdN+ph(A)+ph(B))/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph(A)) f π*(NdN)/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph(A)) f π*(NdN)/MLCT/LLCT/ILCT π(NdN+ph(A)+ph(B)) f π*(NdN)/LLCT/ILCT π(NdN+ph(A)) f π*(NdN+ph(A)+ph(B))/LLCT/ILCT π(NdN+ph(A)+ph(B)) f π*(NdN)/LLCT/ILCT π(NdN+ph(A)+ph(B)) f π*(NdN+ph(A)+ph(B))/LLCT/ILCT π(NdN+ph(A)) f π*(NdN+ph(A)+ph(B))/LLCT/ILCT

exptl9

400

284

TABLE 3: Absorption of 2 in Dichloromethane (CH2Cl2) Solution According to TDDFT/B3LYP Calculations state

λ (nm)/E (eV)

oscillator

S1 S4 S5 S21

562/2.21 499/2.49 498/2.49 357/3.47

0.0288 0.0994 0.0998 0.1049

S22

353/3.51

0.1307

S23

353/3.51

0.1303

S24

351/3.53

0.1651

S25

350/3.53

0.1654

S28

338/3.67

0.5200

main configuration H f L (100%) H-1 f L (96%) H-2 f L(96%) H-5 f L(60%) H-6 f L(33%) H-3 f L+2 (50%) H-4 f L+1 (50%) H-3 f L+1 (50%) H-4 f L+2(48%) H-4 f L+2(45%) H-3 f L+1(44%) H-4 f L+1 (40%) H-3 f L+2 (39%) H-6 f L (67%) H-5 f L (33%)

assign d(Ir) + π(NdN+ph) f π*(NdN+ph)/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph) f π*(NdN+ph)/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph) f π*(NdN+ph)/MLCT/LLCT/ILCT π(NdN+ph) f π*(NdN+ph)/LLCT/ILCT π(NdN+ph) f π*(NdN+ph)/LLCT/ILCT π(NdN+ph) f π*(NdN+ph+s-ph)/LLCT/ILCT π(NdN+ph) f π*(NdN)/LLCT/ILCT π(NdN+ph) f π*(NdN)/LLCT/ILCT π(NdN+ph) f π*(NdN+ph+s-ph)/LLCT/ILCT π(NdN+ph) f π*(NdN+ph+s-ph)/LLCT/ILCT π(NdN+ph) f π*(NdN)/LLCT/ILCT π(NdN+ph) f π*(NdN)/LLCT/ILCT π(NdN+ph) f π*(NdN+ph+s-ph)/LLCT/ILCT π(NdN+ph) f π*(NdN+ph)/LLCT/ILCT π(NdN+ph) f π*(NdN+ph)/LLCT/ILCT

exptl10

316

TABLE 4: Absorption of 3 in Dichloromethane (CH2Cl2) Solution According to TDDFT/B3LYP Calculations state

λ (nm)/E (eV)

oscillator

S1 S4 S5 S22 S23

515/2.41 440/2.82 440/2.82 336/3.69 336/3.69

0.0159 0.1119 0.1139 0.1242 0.1256

S24 S25

336/3.69 334/3.72

0.1160 0.1075

S28

325/3.82

0.2973

Main configuration H f L (100%) H-1 f L (86%) H-2 f L (86%) H-3 f L+1 (86%) H-4 f L+1 (50%) H-3 f L+2 (48%) H-4 f L+2 (80%) H-3 f L+2 (48%) H-4 f L+1 (47%) H-7 f L (74%)

various substituents relies on the fact that the lowest excited state is relatively well described as a transition from HOMO to LUMO in a given ligand. Thus, the investigation on the HOMO-LUMO (∆EH-L) difference will give some useful information on the variation trend of absorption and emission spectra. Figure 3 shows that the extended π-conjugation length in 2 can stabilize the LUMO energy by 0.31 eV and increase the HOMO energy by 0.19 eV, therefore resulting in the narrow HOMO-LUMO gap compared with the case for 1. For 3, inductive and negative mesomeric effects can decrease the dπorbital composition and lower the LUMO energy more than for the HOMO, which leads to the narrow HOMO-LUMO gap. There will be an electronic transition from the HOMO to the LUMO or LUMO+1 for 1 in the lowest singlet and triplet transitions, due to the degeneration of LUMO and LUMO+1. However, for 2 and 3, the HOMO f LUMO transition may be the dominant character. Considering the configuration of OLEDs, proper energy matching between host and dopants is an essential issue. Different HOMO and LUMO energies will have a significant effect on hole and electron injection balance and the position of the recombination zone, as well as device performance. Absorption Spectra. The calculated singlet absorption spectra data are listed in Tables 2-4 for 1-3, respectively. For clarity, only the most leading excited states (with larger CI

assign d(Ir) + π(NdN+ ph(A)) f π*(NdN+ph(A))/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph(A)) f π*(NdN+ph(A))/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph(A)) f π*(NdN+ph(A))/MLCT/LLCT/ILCT π(NdN+ph(A)) f π*(NdN+ph(A))/LLCT/ILCT π(NdN+ph(A)) f π*(NdN+ph(A))/LLCT/ILCT π(NdN+ph(A)) f π*(NdN+ph(A))/LLCT/ILCT π(NdN+ph(A)) f π*(NdN+ph(A))/LLCT/ILCT π(NdN+ph(A)) f π*(NdN+ph(A))/LLCT/ILCT π(NdN+ph(A)) f π*(NdN+ph(A))/LLCT/ILCT π(NdN+ph(A)+ph(B)) f π*(NdN+ph(A))/LLCT/ILCT

exptl11

296

coefficients) are listed. The triplet excited states for 1-3 are listed in Table S4 (Supporting Information). Tables 2-4 show that, consistent with the variation rules of energy gaps between HOMO and LUMO, the calculated lowestlying absorption bands exhibit red-shifting in the following order: 1 (486 nm) f 3 (515 nm) f 2 (562 nm). As expected, the S1 state comes from the HOMO f LUMO/LUMO+1 transition for 1, and the HOMO f LUMO transition contributes to the S0 f S1 state for 2 and 3. As a result of the negligible intensity for 1 (0.0044), the S0 f S1 transition is probably forbidden and would be practically absent in the absorption spectra. The calculated S0-S1 transition dipole moments for 1-3 are 0.2666, 0.7295, and 0.5199 D, respectively. The increased oscillator strengths for 2 and 3 originate from the increased transition dipole moment, which is defined as the transition density weighted by the distance of the atom from the center of gravity.21 In the presence of the extended π-conjugation of the NdN moiety in 2 and 3, the HOMO and LUMO distribution is more extensive than that in 1, which is most favorable for achieving large transition dipole moments. The smaller transition dipole moment for 3 than for 2 can be attributed to the steric effect between the phenol and NdN moieties, which further forces them out of coplanarity and results in the more distorted C∧NdN ligand. This distortion leads to the reduced decolalization and π-acceptor ability of the ligand. From the above



TABLE 5: Calculated Emission Energies and Dominant Orbital Emissions from TDDFT Results for 1-3 λ (nm)/E (eV) 1 2 3 a

T1 T1 T1

631/1.96 706/1.75 792/1.56

main configuration H-1 f H (84%) H-1 f H (91%) H-1 f H (76%)

assign d(Ir) + π(NdN+ph(A)+ph(B)) f π*(NdN+ph(A))/MLCT/LLCT/ILCT d(Ir) + π(NdN+ph(A)) f π*(NdN+ph(A))/MLCT/LLCT/ILCT d(Ir) + π(NdN+Ph(A)) f π*(NdN+Ph(A))/MLCT/LLCT/ILCT

exptl a,9

541 625b,10 590, 630(sh)b,11

Calculated in toluene solution. b Calculated in CH2Cl2 solution.

discussion on FMO, the calculated 486 nm absorption can be described as a {[d(Ir) + π(NdN+ph(A))] f [π*(NdN/NdN+ ph(A)+ph(B)]} transition with the character of MLCT and LLCT or ILCT. The 562 and 515 nm absorptions for 2 and 3 can be contributed by a {[d(Ir) + π(NdN+ph(A))] f [π*(NdN+ph(A))]} transition with the same character to 1. The lowest lying distinguishable absorption bands in the experiment, 471-475 nm for 1,9 395-456 and 557-564 nm for 2,10 and 450-500 and 550 nm for 3,11 do not appear in the singlet-to-singlet calculation, which indicates the singlet-totriplet absorption transition in nature. An experimentally used model of an excited state corresponds to excitation of an electron from an occupied to a virtual molecular orbital. As shown in Table S4, our calculated results demonstrate that excited-state electronic structures can best be described in terms of multiconfigurations, and a linear combination of several occupiedto-virtual molecular orbital excitations comprises a given transition. The 471-475 nm absorption for 1 is mainly contributed by T5-T7 states, which have large multiconfigurational character. Due to the absence of the SOC effect in current TDDFT calculations, the oscillator strengths for triplet excited states are zero. The involved orbitals in transition are mainly from dπ, NdN, and ph(A) based π-orbital to NdN based π*orbital. Therefore, the transition character can be described as MLNdNCT, LPh(A)LNdNCT, and ILNdNCT. Ph(B) is involved in the transitions at higher energy states, such as in T7. The initial orbital for 557-564 nm absorption arises from metal d-, NdN-, and Ph(A)-based π-orbitals. The acceptor orbitals are mainly localized on the NdN- and Ph(A)-based π*-orbitals with transition characters of MLCT, LLCT, and ILCT. The ligandcentered transition dominates the 395-456 nm absorption and mixes with some MLCT composition,as shown in Table S4 (Supporting Information) for 2. For 3, the calculated triplet states T2 and T3, T4-T9 correspond to the observed 550 and 450-500 nm bands, respectively, which can all be ascribed as MLCT, LLCT, and ILCT characters {[d(Ir) + π(NdN+ph(A))] f [π*(NdN+ph(A))]}. As shown in Tables 2-4, there are excited states with significant oscillator strength in the regions 306-309, 340, and 325 nm for 1-3, respectively, which correspond to the experimentally observed strong absorption bands at 284, 316, and 296 nm,9-11 with a deviation of 24, 25, and 29 nm. For 1, the HOMO-5/6 f LUMO/+2,+4 are responsible for the 284 nm absorption with the transition from [π(NdN+ph(A)+ ph(B))] to [π*(NdN+ph(A)+ph(B))] and the characters of LLCT and ILCT. For 2 and 3, the electronic transitions are mainly localized on the NdN and Ph(A) moieties, with negligible contribution from the Ph(B) moiety. This almost zero electronic transition from Ph(B) in 2 and 3 with respect to 1 implies that Ph(B) plays an important role in shielding the emission center against undesirable nonradiative interactions. According to knr ) (1 - Φ)/τ (where knr is nonradiative decay rate, Φ is quantum yields and τ is lifetime), the calculated knr for 1 and 3 are 1.18 × 105 and 2.35 × 104 s-1. The significantly decreased knr value for 3 can sufficiently demonstrate this conclusion. Phosphorescence. On the basis of the optimized triplet excited-state geometries, emission spectra were calculated using

Figure 5. Singlet electron emission of T1 states for 1-3 calculated at TDDFT/B3LYP level.

the TDDFT method, and the results are listed in Table 5. Partial compositions of FMOs related to emission are listed in Table S5 (Supporting Information). The plots of the molecular orbitals related to emissions of 1-3 are presented in Figure 5. Table 5 shows that the calculated lowest energy emissions are localized at 631, 706, and 792 nm for 1-3, respectively, and these calculated absolute emission energies deviate somewhat from the experimental results.9-11 Similar to triplet absorption, the oscillator strengths for phosphorescent emissions are also zero due to the without-SOC effect in TDDFT results. One possible reason is that the excited-state geometry we used here was optimized in the gas phase, and another reason is the limited dimensions of the basis set. Table 5 shows that all the calculated emissions originate mainly from the highest single occupied molecular orbital (HSOMO-1), which is mainly composed of Ir d-orbital antibonding combined with the π orbital of the NdN and Ph(A) moieties. The contribution from Ph(B) is not negligible in 1. However, from Figure 5, we note that the orbital distributions for 1 and 3 are only from a single ligand, while that of 2 is evenly distributed among the three ligands. This is consistent with the different metal-ligand interaction intensity among 1, 3, and 2 in the lowest triplet states. In 1 and 3, the ligand with the strongest interaction with metal may dominate this excited-state frontier molecular orbital. For 1, the accepting orbital HSOMO is distributed on the Ph(A) (68.7%) moiety of a single ligand, and the remaining orbital is largely localized on the NdN moiety (21.8%). Therefore, the 631 nm emission for 1 can be described as 3MLNdN/Ph(A)/Ph(B)CT, 3 LNdN/Ph(A)LPh(A)/NdNCT, and 3ILNdN/Ph(A)CT transition characters. The 706 and 792 nm emissions for 2 and 3 have transition characters 3MLNdN/Ph(A)CT, 3LNdN/Ph(A)LPh(A)/NdNCT, and 3ILNdN/ Ph(A)CT, and the participation of the NdN moiety is larger than that in 1. The large involvement of the 3MLCT transition in

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Li et al. TABLE 6: Ionization Potentials (IP) (eV), Electron Affinities (EA) (eV), Extraction Potentials (HEP and EEP) (eV), Stabilization Energy (SPE) (eV), and Internal Reorganization Energies (eV) for 1-3 1 2 3

1 2 3 Figure 6. Ground-state (S0) and the lowest triplet excited-state (T1) potential energy surface (PES).

emission agrees well with the experimental observations that the measured emission spectra is structureless, because emission bands from 3MLCT states are generally broad and featureless, while 3(π*fπ) states typically give highly structured emissions.22 Many factors, such as temperature,23 different polarities of solvent,24 different ligand bite angles,25 and vibrational decay,26 can influence the lifetime and quantum yield of transition-metal complexes to different extents. The peculiarity of these complexes is the novelty of the C∧NdNH ligand, which has stronger chelate interaction with the center metal. The general metalcentered (MC) excited state can be greatly destabilized and, consequently, reduce the deactivation through this process. To gain insight into the nature of the excited states involved in the phosphorescence emission process, the potential energy surface (PES) of the S0 and T1 states for 2 is represented in Figure 6 as an example by elongating and freezing the IrsN bond selectively. This is due to the fact that the IrsN distance shows elongation on going from S0 to T1 minima. The T1-PES was calculated by using the constrained geometry optimization method at the UB3LYP/BS1 level when the variations in bond lengths were in the range 1.90-2.46 Å. Then, for all the optimized geometries, the SCF energies of the T1 and S0 states were calculated at the UB3LYL/BS1 level, and the resultant data were interpolated using Origin 7.0 to sketch the smooth PESs. The vertical axis indicates the energy relative to the optimized ground-state geometry. As shown in Figure 6, both the S0 and T1 states become less stable in energy compared with the S0- or T1-equilibrium geometry in PES as IrsN distance becomes shorter or longer. In other words, the energy of the S0- or T1-equilibrium geometry is the global minima in both PESs. In addition, unlike iridium(III) complexes with fast d-d deactivation of excited states,27 the T1-PES for 2 is very deep, indicating the large activation energy barrier to its surrounding state and the T1-PES has only one minima corresponding to the phosphorescence state. Thus, there are no split peaks in the phosphorescent spectra for 2 as observed in the experiment.10 Furthermore, we noted that the T1-PES is very similar in shape to S0-PES, and there is no interaction between them in the studied Ir-N bond distances. This means no surface crossing and the radiationless transition from the 3metal-centered (3MC) state28 will not take place. This is consistent with the little geometrical structure changes from the S0 to T1 state, as shown in Table 1. Due to the limitation of computational resources, we assume that the high e*g-like orbital in 1 and 3 may also have PESs similar to those for 2 and do not facilitate fast deactivation of excited states from dd states.

IP(v)

IP(a)

HEP

SPE(h)

λhole

6.06 5.75 6.14

5.98 5.70 6.08

5.92 5.58 6.03

0.08 0.05 0.06

0.14 0.17 0.11

EA(v)

EA(a)

EEP

SPE(e)

λelectron

0.84 1.18 1.33

0.91 1.23 1.39

0.99 1.28 1.46

0.07 0.05 0.06

0.15 0.10 0.13

The main 3MLCT composition can be obtained from the following expression: 3MLCT% ) [M%(donor) - M%(accept)]CI, where M%(donor) is the metal composition in the donor orbital, M%(accept) is the metal composition in the acceptor orbital, and CI is the proportion of the main transition configuration in the whole lowest energy emission. The calculation shows that the 3MLCT transition compositions for 1-3 are 16.3%, 13.8%, and 17.1%, respectively. Chi el at.5,29 concluded that quantum efficiencies could be increased by large 3MLCT excited-state composition and the intersystem crossing could be enhanced by notable 3MLCT participation. Namely, the phosphorescence from the T1 f S0 radiative transition increased with increased 3MLCT: the 3π*-π ratio should increase the transition probability significantly, hence shortening the radiative lifetime. Maintaining high phosphorescent quantum yields requires a large radiative rate, which is directly proportional to the spin-orbit coupling (SOC) and siglet-triplet splitting (∆EST).30 Recently, it has been confirmed that MLCT character influences the ∆EST value, which in turn controls the radiative rate of the complexes.31 For 1 and 3, according to the obtained experimental lifetime (τ) and quantum (Φ) values, the calculated kr values are 4.73 × 105 and 4.77 × 105 s-1, respectively, which is consistent with the slightly larger 3MLCT for 3 than 1. The higher Φ value of 0.953 for 3 is arising from the knr (2.35 × 104 s-1) value being greatly smaller than that for 1 (1.18 × 105 s-1), and the protection of the bulky phenolic moiety can account for this decreased knr value. For 2, the lower quantum yield of only 0.20 may result from the rather lower radiative rate value according to 3MLCT composition. EL Efficiency. In this section, ionization potentials (IP), electron affinities (EA), and reorganization energy (λ) are calculated, together with hole extraction potential (HEP) and electron extraction potential (EEP). The details for the definitions can be obtained from our previous work.32 For photoluminescent materials, a smaller IP value means easier hole injection ability, while a larger EA value will facilitate electron injection. For IP and EA calculations, we did not include the solvent polarization of the surrounding medium. In the case of solidstate photoelectronic devices such as LEDs, this solvent factor can also be negligible. The obtained trend for charge injection abilities in IP and EA has been demonstrated to be reasonable from our previous study.33 It is assumed that the pure metal-centered oxidation process is centered on the metal, and the decreased dπ orbital composition increases the difficulty of the oxidation process. This indicates that the ability of losing an electron from the metal d-orbital becomes more difficult in 3 and easy in 2 with respect to 1. This can be demonstrated by the calculated IPs (Table 6), which decrease in the following order: 3 f 1 f 2. This is consistent with the measured oxidation potentials of 1.03 V for 1 and 0.91 V for 2.9,10 This also implies that the difficulties of



Figure 7. Schematic description of internal reorganization energy for hole transfer.

hole injection from the hole-transporting layer (HTL) to these complexes gradually decreases in the order 3 f 1 f 2. By analysis of the EA value, 3 more easily accepts an electron than 1 and 2, and this trend is also consistent with the order of LUMO energy levels and the measured reduction potentials. According to the Marcus/Hush model,34 the charge (hole or electron) transfer rate k can be expressed by the following formula:

κ)

( ) π λκbT

1/2 V2

p

(

exp -

)

(

λ λ ) A exp 4κbT 4κbT

)

(1)

where T is the temperature, kb is the Boltzmann constant, λ is the reorganization energy, and V is the coupling matrix element between the ions and molecules that is dictated by the overlap of orbitals. As shown in eq 1, two factors, λ and V, determine the k. Due to the limited intermolecular charge transfer range in the solid state, the mobility of charges has been demonstrated to be dominantly related to the reorganization energy λ for OLED materials.35 Generally, λ is determined by fast changes in molecular geometry (the internal reorganization energy λi) and by slow variations in solvent polarization of the surrounding medium (the external reorganization energy λe). In OLED devices, the contribution from λe can be neglected. Therefore, the internal reorganization energy λi is the determinant factor (Figure 7). λi for holes transfer can be expressed as follows:36

λhole ) λ0 + λ+ ) (E*0 - E0) + (E*+ - E+) ) IP(v) HEP (2) As illustrated in Figure 6, the definitions of E0, E+, E*0, and E*+ are similar to those reported previously.35 Emitting layer materials need to achieve hole and electron injection and transport balance, and a low reorganization energy is necessary for an efficient charge transport process. As shown in Table 6, for 1 and 3, the reorganization energies for hole transport (λhole) and electron transport (λelectron) are comparable, which indicates similar hole and electron transport abilities for 1 and 3. For 2, the electron transport ability is obviously better than hole. In comparison with the λhole ) 0.274 and λelectron ) 0.689 for TPD (a hole transport material),37 the Ir(III) complexes in this paper

have smaller λ values, and therefore they have better charge transport properties. More importantly, it is essential to elucidate the reasons of high EL efficiency of OLEDs with 3 as phosphorescent dopant emitter instead of 1 and 2. The lifetime and efficiency of the OLED device are greatly limited by the harmful intermolecular interactions of the dopant molecules, such as dimer or excimer formation, phase segregation, and luminescence quenching through TTA. The high EL efficiency and the reduced roll-off at high currents of 3 is closely related to the sterically bulky phenolic moiety at the periphery of the coordination environment. As discussed in the FMO section, the distribution of HOMO for 3 is with little contribution from the phenolic moiety, which only acts as a pendant at the periphery of the chromophore core. This is not surprising, since the phenolic moiety is not conjugated to the NdN moiety. In this case, the emitting core is protected by the bulky phenolic group and this encapsulated structure therefore meets the structural requirement of an efficient luminescent material, which can successfully control the hazardous intermolecular interaction of the phosphorescent emitters and reduce luminescence quenching in the blending system. In addition, the confinement of the recombination zone within the light-emitting layer has been claimed to play a major role in the long term stability of the phosphorescent device.38 In a doped system, unequal carrier trappings were expected if the corresponding trap depths are different.39 The self-trapping energies for the electron (SPE(e)) and hole (SPE(h)) are also calculated and listed in Table 6. The traps characterizing the electron transport in the material are as the states in which the injected electron is self-trapped in an individual molecule accompanied by structure relaxation. However, such a procedure provides an estimate of exciton trap energy rather than of the injected electrons.40 In Table 6, 2 and 3 have slightly smaller SPEs value than 1, which result from the higher dipole moment of 7.46 and 12.04 D than 5.80 D for 1. The high dipole moment can lead to a local electric field, which can induce charges, thus facilitating charge injection into the dopants.41 The smaller SPE values for 2 and 3 compared to that in 1 may be the reasons for the highest HOMO energy in 2 and lowest LUMO energy in 3 among the three complexes, because the higher HOMO energy or the lower LUMO energy can provide a more stable potential well for hole or electron trapping in 2 and 3, respectively. Conclusions Quantum chemistry methods were used to investigate the electronic structure, absorption, and phosphorescent and electroluminescent properties of three iridium(III) complexes with the C∧NdN ligand. The extended π-conjugation of the NdN moiety in 2 and 3 distorts the C∧NdN ligand and results in the negligible electron density distribution on Ph(B) and phenolic groups, especially in 3. In addition, this extended π-conjugation of the NdN moiety can increase the electronic S0-S1 transition dipole moment and therefore increase the lowest singlet absorption intensity in 2 and 3. The calculated phosphorescent potential energy surface (PES) of 2 indicates that the triplet excited state PES has a deep and similar profile to that of the ground state and the fast dd transition will not take place due to the absence of surface crossing. The bulky phenolic group plays a major role in the more promising PL and EL properties of 3. The large distortion between the phenolic and NdN moieties leads to negligible distribution on the Ph(B) moiety, which only acts as a pendant at the periphery of the chromophore core and protects the chromophore core from the hazardous intermolecular interaction of emitters and reduces luminescence

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quenching. Furthermore, the electron-withdrawing phenol increases the intramolecular charge separation and increases the dipole moment and therefore results in a smaller SPE value, which is important for OLEDs. Acknowledgment. We are grateful for the financial aid from the National Natural Science Foundation of China (Grant Nos. 20631040, 20771099) and the MOST of China (Grant Nos. 2006CB601103). Supporting Information Available: Frontier molecular orbital compositions for 1-3 in the S0 state are presented in Tables S1-S3, triplet absorptions for 1-3 are listed in Table S4, and frontier molecular orbital compositions (%) in the T1 state for 1-3 are listed in Table S5. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl. Phys. 2001, 90, 5048–5051. (b) Ikai, M.; Tokito, S.; Sakamoto, Y.; Suzuki, T.; Taga, Y. Appl. Phys. Lett. 2001, 79, 156–158. (2) (a) Lee, S. J.; Park, K. M.; Yang, K.; Kang, Y. Inorg. Chem. 2009, 48, 1030–1037. (b) Wong, W. Y.; Ho, C. L. J. Mater. Chem. 2009, 19, 4457–4482. (c) Yang, C. H.; Li, S. W.; Chi, Y.; Cheng, Y. M.; Yeh, Y. S.; Chou, P. T.; Lee, G. H.; Wang, C. H.; Shu, C. F. Inorg. Chem. 2005, 44, 7770–7780. (d) Justin, K. R.; Velusamy, M.; Lin, J. T.; Chien, C. H.; Tao, Y. T.; Wen, Y. S.; Hu, Y. H.; Chou, P. T. Inorg. Chem. 2005, 44, 5677– 5685. (3) (a) Ho, C. L.; Wong, W. Y.; Gao, Z. Q.; Chen, C. H.; Cheah, K. W.; Yao, B.; Xie, Z. Y.; Wang, Q.; Ma, D. G.; Wang, L. X.; Yu, X. M.; Kwok, H. S.; Lin, Z. Y. AdV. Funct. Mater. 2008, 18, 319–331. (b) Liu, Z. W.; Guan, M.; Bian, Z. Q.; Nie, D. B.; Gong, Z. L.; Li, Z. B.; Huang, C. H. AdV. Funct. Mater. 2006, 16, 1441–1448. (c) 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, 12971–12979. (d) Chen, X. W.; Liao, J. L.; Liang, Y. M.; Ahmed, M. O.; Tseng, H. E.; Chen, S. A. J. Am. Chem. Soc. 2003, 125, 636–637. (e) Sandee, A. J.; Williams, C. K.; Evans, N. R.; Davies, J. E.; Boothby, C. E.; Ko¨hler, A.; Friend, R. H.; Holmes, A. B. J. Am. Chem. Soc. 2004, 126, 7041–7048. (4) (a) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949–1960. (b) Wilson, J. S.; Chawdhury, N.; Al-Mandhary, M. R. A.; Younus, M.; Khan, M. S.; Raithby, P. R.; Ko¨hler, A.; Friend, R. H. J. Am. Chem. Soc. 2001, 123, 9412–9417. (5) Chou, P. T.; Chi, Y. Chem.sEur. J. 2007, 13, 380–395. (6) (a) Hay, P. J. J. Phys. Chem. A 2002, 106, 1634–1641. (b) Zhao, Q.; Liu, S. J.; Shi, M.; Wang, C. M.; Yu, M. X.; Li, L.; Li, F. Y.; Yi, T.; Huang, C. H. Inorg. Chem. 2006, 45, 6152–6160. (c) Lowry, M. S.; Hudson, W. R.; Pascal, Jr, R. A.; Bernhard, S. J. Am. Chem. Soc. 2004, 126, 14129– 14135. (7) Kim, J. J.; You, Y.; Park, Y. S.; Kim, J. J.; Park, S. Y. J. Mater. Chem. 2009, 19, 8347–8359. (8) (a) Park, M. J.; Lee, J.; Kwak, J.; Jung, I. H.; Park, J. H.; Kong, H.; Lee, C.; Hwang, D. H.; Shim, H. K. Macromolecules 2009, 42, 5551– 5557. (b) Lai, W. Y.; Levell, J. W.; Burn, P. L.; Lo, S. C.; Samuel, I. D. W. J. Mater. Chem. 2009, 19, 4952–4959. (9) Gao, Z. Q.; Mi, B. X.; Tam, H. L.; Cheah, K. W.; Chen, C. H.; Wong, M. S.; Lee, S. T.; Lee, C. S. AdV. Mater. 2008, 20, 774–778. (10) Mi, B. X.; Wang, P. F.; Gao, Z. Q.; Lee, C. S.; Lee, S. T.; Hong, H. L.; Chen, X. M.; Wong, M. S.; Xia, P. F.; Cheah, K. W.; Chen, C. H.; Huang, W. AdV. Mater. 2008, 20, 1–5. (11) Tong, B.; Mei, Q.; Wang, S.; Fang, Y.; Meng, Y.; Wang, B. J. Mater. Chem. 2008, 18, 1636–1639. (12) Runge, E.; Gross, E. K. U. Phys. ReV. Lett. 1984, 52, 997–1000. (13) Mayo, S. L.; Olafson, B. D.; Goddard, W. A., III. J. Phys. Chem. 1990, 94, 8897–8909. (14) Igor, A.; Payam, M.; Je´roˆme, C.; Luisa, C. D. J. Am. Chem. Soc. 2007, 129, 8247–8258. (15) (a) Autschbach, J.; Ziegler, T.; Gisbergen, S. J. A.; Baerends, E. J. J. Chem. Phys. 2002, 116, 6930–6940. (b) Helgaker, T.; Jørgensen, P. J. Chem. Phys. 1991, 95, 2595–2601. (c) Bak, K. L.; Jørgensen, P.; Helgaker, T.; Rund, K.; Jensen, H. J. A. J. Chem. Phys. 1993, 98, 8873–8887. (16) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151–5158. (17) Shi, L. L.; Liao, Y.; Zhao, L.; Su, Z. M.; Kan, Y. H.; Yang, G. C.; Yang, S. Y. J. Organomet. Chem. 2007, 692, 5368–5374.

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