Effects of N-Substitution on Phosphorescence Efficiency and Color

Article pubs.acs.org/JPCC

Effects of N‑Substitution on Phosphorescence Efficiency and Color Tuning of a Series of Ir(III) Complexes with a Phosphite Tripod Ligand: A DFT/TDDFT Study Yuqi Liu,† Godefroid Gahungu,‡ Xiaobo Sun,§ Xiaochun Qu,† and Zhijian Wu*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China ‡ Université du Burundi, Faculté des Sciences, Département de Chimie, Unité de Chimie Théorique et Modélisation Moléculaire, BP. 2700 Bujumbura, Burundi § Department of Applied Chemistry, Qingdao Agricultural University, Qingdao 266109, P. R. China S Supporting Information *

ABSTRACT: A DFT/TDDFT investigation was applied to understand the unusual properties of the recently synthesized blue-emitting Ir(III) complexes [Ir(PMe2Ph)(dppit)(py2pz)] (1) [PMe2Ph = dimethylphenylphosphine; dppit = diphenyl phenylphosphonite; py2pz = 3,5-di(2-pyridyl)pyrazole] and [Ir(PMe2Ph)(dppit)(bptz)] (2) [bptz =3-tert-butyl-5-(2pyridyl)triazolate], which are successfully used as emitters in organic light-emitting diodes (OLEDs). The influence of Nsubstitution on optical and electronic properties of Ir(III) complexes was also explored by introducing a N atom on the pyridine moiety of N∧N ligands for 1 and 2. The calculated results reveal that introduction of N substitution leads to a blue shift for 1a, 1b, 1c, and 1d (a, b, c, d indicate different positions for N substitution) and slightly red shift for 2a−2d in absorption spectra compared with that of 1 and 2, respectively. The N substitution at different positions on N∧N ligands may also be an efficient approach of tuning emitting color for 1 and 2. The 1-position substituent (1a and 2a) leads to an obvious blue shift of emission spectra compared with 1 and 2, while a significant red shift is observed for the 3-substituted derivatives 1c and 2c. It is believed that the larger 3MLCT−3MC energy gap and higher μS1 value, as well as the smaller ΔES1−T1 for 1a/2a, are good indications for the higher quantum efficiency compared with that of experimental structures 1/2. These new structure−property relationships can provide improved design and optimization of OLED devices based on blue-emitting phosphorescent Ir(III) complexes.

1. INTRODUCTION Phosphorescent iridium(III) complexes have attracted considerable attention because of their intriguing photophysical properties and potential applications in the fabrication of organic light-emitting diodes (OLEDs).1−5 Due to the heavyatom-induced spin−orbit coupling effects, which can partially remove the spin-forbidden nature of the T1→S0 radiative relaxation, these Ir(III) complexes can harvest both singlet and triplet excitons as light, leading to a theoretical level of unity for internal quantum efficiency in phosphorescent OLEDs.4,6 Therefore, the Ir(III)-based phosphorescence complexes can reach an efficiency four times higher than fluorescent materials.6−8 Additionally, these Ir(III) complexes would display bright phosphorescent emission spanning the entire visible spectra, making it possible to realize the full-color displays. Within the past decade, considerable efforts have been devoted to search for highly efficient three primary color (red, green, and blue) emitters, which are essential components for full-color displays. However, tuning of phosphorescence over © 2012 American Chemical Society

the entire visible spectrum still remains a challenge. With respect to red and green emitters, which have been successfully used in OLEDs with high efficiency and stability,9−11 designing stable and efficient blue-emitting materials encounter more obstacles12,13 owing to the wide energy gap required between the excited triplet state and the ground state. Thus, improvements to the efficiency and lifetime of blue phosphorescence materials are still vital to the success of OLEDs. Representative blue Ir(III) phosphors include FIrpic,14 FIr6,15 FIrpytz,1 and FIrtaz,16 which consist of two cyclometalated difluorophenylpyridine-based ligands. Although the devices fabricated with these emitters show high efficiency, their inferior chromaticity limited their application in full-color displays. Prompted by these requirements, considerable research has been conducted in developing new blue phosphors with good color purity. According to the recent reports, materials with Received: July 18, 2012 Revised: November 6, 2012 Published: November 29, 2012 26496

dx.doi.org/10.1021/jp3071019 | J. Phys. Chem. C 2012, 116, 26496−26506

The Journal of Physical Chemistry C

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fluorine substituents are known to hamper the longevity of OLEDs.17 Thus, very recently, Lin et al. reported a new class of blue phosphors [Ir(PMe2Ph)(dppit)(py2pz)] (1) and [Ir(PMe2Ph)(dppit)(bptz)] (2) [PMe2Ph = dimethylphenylphosphine; dppit = diphenyl phenylphosphonite; py2pz = 3,5-di(2pyridyl)pyrazole; bptz = 3-tert-butyl-5-(2-pyridyl)triazolate] (Scheme 1) without fluorine substituents.18 They incorporated

OLEDs based on these complexes, such as the ionization potentials (IPs), the electron affinities (EAs), and the reorganization energies which are used to evaluate the device performances, have also been studied. We hope this work can provide valuable information for the design and synthesis of new Ir(III) phosphors in OLEDs with high phosphorescence quantum efficiency and good electroluminescence performance.

Scheme 1. Schematic Structures of the Investigated Complexesa

2. COMPUTATIONAL METHODOLOGY All calculations on electronic singlet and triplet states of the studied complexes were carried out using the density functional theory (DFT)25 with Becke’s three-parameter hybrid method26 combining with the Lee−Yang−Parr correlation functional27 (denoted as B3LYP). Restricted and unrestricted28 formalisms were adopted in the singlet and triplet geometry optimization, respectively. Vibrational frequencies were calculated at the same theoretical level to confirm that each configuration was a minimum on the potential energy surface. The electronic configurations of 3MC d−d states were also optimized following the methodology illustrated in Persson’s work.29,30 On the basis of the ground- and lowest excited-state equilibrium geometries, the time-dependent DFT (TDDFT)31 approach associated with the polarized continuum model (PCM)32 in dichloromethane (CH2Cl2) media was applied to investigate the absorption and emission spectral properties. Recent calculations with the TDDFT method for transition-metal complexes have proved its reliability and gave good agreement with experimental spectra.33−36 A ″double-ξ″ quality basis set consisting of Hay and Wadt’s effective core potentials (LANL2DZ)37 was employed for the Ir atom and a 6-31G* basis set38 for H, C, N, O, and P atoms. The relativistic effective core potential (ECP) replaced the inner core electrons of the Ir(III) metal atom, leaving the outer core (5s25p6) and the 5d6 as valence electrons. All calculations were performed with the B.01 revision of the Gaussian 09 program package,39 the Gausssum 2.5 program40 being used for the distribution of the total density-of-state analysis as well as UV/vis spectral analysis and the Gabedit 2.3.9 user interface41 for structures and orbitals manipulations.

n-N (n = 1, 2, 3, 4) represents the “CH” group at the n-position substituted by the N atom. a

a dicyclometalated phosphite (or phosphonite) tripod19 serving as ancillary ligands, coupled with a monodentate phosphorus donor and 2-pyridyltriazolate acting as a blue chromophore.20 These Ir(III) complexes showed good quantum efficiency in both solution and the solid state. Meanwhile, the electrophosphorescent devices were fabricated by doping these complexes as emitters and achieved better peak luminescence of 4084 and 6027 cd A−1.18 Besides, Lin et al. also performed a theoretical study on the energy gap between the lowest triplet metal-to-ligand charge transfer (3MLCT/π−π*) excited state and metal-centered (3MC d−d) state to get insight about the reason for different quantum yields observed on these complexes experimentally. To the best of our knowledge, phosphorescent quantum yields are greatly affected by the accessibility of metal-centered d−d excited states, which can open efficiently the channels of nonradiative decay.21−23 Therefore, a higher-lying 3MC d−d state and a large energy gap between 3MLCT/π−π* and 3MC d−d states is required for high quantum yield. In the literature, the relatively large energy gaps between 3MLCT/π−π* and 3MC d−d states of 1 and 2 account for their high efficiency.18 On the basis of the above results, we performed quantum chemical calculations on 1 and 2 to get a detailed understanding of structure−property relationships. Preliminary calculations on 1 and 2 have shown that the electron density of the highest occupied molecular orbital (HOMO) is found located mainly on the phenoxide sides and Ir(III) center, while the lowest unoccupied molecular orbitals (LUMOs) are mainly on the pyridine moiety of N∧N ligands. As we know, the efficient ligand-based phosphorescence is common in Ir(III) complexes,24 which makes the design of new phosphors straightforward by modification of the ligand structure. Therefore, we designed a series of derivatives by a systematic substitution of the “CH” group by the electron-withdrawing nitrogen (N) atom at 1-, 2-, 3-, and 4-positions on the pyridine moiety of py2pz and bptz ligands in 1 and 2, respectively (Scheme 1), to investigate the influence of substitution on photophysical and spectra properties of these Ir(III) complexes. Meanwhile, the potential electroluminescent (EL) properties of

3. RESULTS AND DISCUSSION 3.1. Geometries in the Ground and the Lowest-Lying Triplet Excited State. To investigate the solvent effect, the ground-state geometry optimization of 1 and 2 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 Information) illustrates the parameters of Ir−ligand bond lengths and bond angles in the gas phase and CH2Cl2 media. It showed that the maximum difference of bond distances is 0.007 Å, while changes in angles do not exceed 1.0°. This means that solvent effect has a minor influence on the optimized geometries for the studied complexes. The schematic structures of the investigated complexes are depicted in Scheme 1, and the fully optimized ground-state geometrical structures of 1 and 2 are presented in Figure 1, along with the numbering of some key atoms. Selected bond distances and angles between Ir metal and coordinating atoms are summarized in Table 1. It can be seen that all complexes studied here adopt a pseudo-octahedral coordination geometry, 26497



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weakest metal−ligand interaction. In addition, the changes of the calculated coordination bond angle are minor (less than 1.5°) from 1/2 to 1a−1d/2a−2d (Table 1) (1a−1d indicates 1a, 1b, 1c, and 1d, the same hereafter). To demonstrate the changes of geometry structures upon excitation, the geometry parameters of all complexes in the lowest-lying triplet states were also calculated and listed in Table 1. In addition, changes of the Ir(III)-related coordination bond lengths between the ground (S0) and excited state (T1) for the studied complexes are shown in Figure 2. No obvious variations between S0 and T1 states are found for the Ir−dppit ligand bond distances (Ir−C1, Ir−C2, and Ir−P1) with nearly negligible differences within 0.02 Å. An exceptional case is the Ir−C2 bond lengths in 1a and 2a which are reduced by 0.046 and 0.047 Å in T1 states, respectively (Table 1). For Ir−N1 and Ir−N2 bond lengths, they are shortened significantly on the excited state especially in complexes 2−2d (more than 0.022 Å, see Figure 2), resulting in the strengthened interaction between metal and N∧N ligands in T1 states. The Ir−P2 bond lengths are significantly stretched by 0.025−0.087 Å upon S0→T1 excitation for the studied complexes, which would suggest that the PMe2Ph ligand is less involved in the excited states. Generally, Figure 2 shows that 1a and 2a have the largest changes of bond distances between S0 and T1 states, which can increase the interaction of metal−N∧N ligands and might consequently result in the decrease of metal-centered (MC) nonradiative emission and enhancement of radiative deactivation compared with other complexes. Furthermore, when introducing the N atom at 1-, 2-, 3-, and 4-positions on the pyridine moiety of N∧N ligands in 1 and 2, it caused similar influences on the change of geometry parameters for 1a−1d and 2a−2d, respectively.

Figure 1. Optimized geometry structures of 1 and 2 at the B3LYP/ LANL2DZ level.

similar to most reported Ir(III) complexes owing to the d6 configuration of the Ir(III) center. In general, the systematic substitution of the “CH” group by the N atom on the pyridine moiety of N∧N ligands in 1 and 2 does not cause obvious changes (0.50 eV), which would increase the difficulty of electron transition to LUMO+1 or higher virtual MOs in the low-energy 26499



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Figure 4. Simulated absorption spectra of 1−1d and 2−2d, respectively, in CH2Cl2 media.

bptz)] with metal to ligand charge transfer (MLCT) and ligand to ligand charge transfer (LLCT) characters. For 1 and 2, the calculated absorption peaks with the largest oscillator strength are localized at 299 and 277 nm, respectively, corresponding well to the experimental values of 297 and 272 nm.18 The transition configuration of HOMO-1 → LUMO+2 contributes to the 299 nm absorption for 1 described as the [π(py2pz + dppit) → π*(py2pz)] transition, while the HOMO-5 → LUMO and HOMO-2 → LUMO+2 excitations are the main configuration for 277 nm absorption of 2 with the mixed transition characters of LLCT/ILCT (intraligand charge transfer). For 1a−1d, the transitions with the largest oscillator strengths are located at around 277 nm, blue-shifted about 20 nm compared with that of 1, and have the transition characters of LLCT (Table S12, Supporting Information). With respect to 2a−2d, the peak absorptions range from 244 nm (2d) to 308 nm (2a) with mixed transition characters. These different transition characters and absorption peak positions demonstrate the different intramolecule energy transfer process In addition, in combination with Figure 4 and Tables S12− S13 (Supporting Information), we also noticed that a nonnegligible absorption cross section of singlet → triplet transitions with 3MLCT and 3π → π* characters is expected around 400 nm owing to the strong spin−orbit coupling effects induced by the Ir(III) metal center. The calculated lowest vertical triplet absorptions of 1−1d are at 423, 455, 439, 474, and 430 nm (Table S12, Supporting Information), respectively, corresponding to excitation of an electron from the HOMO with the significant mixed character of d(Ir)-orbital and dppitbased π orbital to the LUMO with py2pz-based π* orbitals. Thus, these transitions can be described as mixed characters of MLCT/LLCT except for 1b, in which the HOMO-1 → LUMO transition is responsible for the lowest vertical triplet absorption with LLCT [π(dppit) → π*(py2pz)] character. It shows more complicated behavior for 2−2d compared with that for 1−1d (see Table S13 (Supporting Information) for details). From the above calculations, we have the following conclusions: (a) the N substitution on the pyridine moiety in N∧N ligands does not cause a significant influence on the shape of the absorption curve for 1−1d and 2−2d; (b) the intensities of the relative absorption bands for 1−1d are stronger than those of 2−2d; (c) incorporation of the N atom leads to the blue-shift of peak absorption for 1a−1d compared with that of 1, while for 2a−2d the peak absorptions are slightly red-shifted

region. This implies that the HOMO−LUMO transition may be the dominant configuration for the low-energy excitation of these complexes. Therefore, the investigation on the HOMO− LUMO gap will be helpful to understand the variation trend of absorption and emission spectra. Furthermore, the energies of HOMO and LUMO are related to the hole- and electron-injection abilities of these complexes. For 1a/2a and 1c/2c, the significantly lower LUMO energies will ensure the efficient electron-injection abilities, while the comparable HOMO energies for all the complexes will result in the similar hole-injection abilities. The comparable holeinjection abilities and different electron-injection abilities will inevitably cause an influence on hole- and electron-injection balance and consequently affect the device performance. We will discuss this aspect in detail in Section 3.6. 3.3. Absorption Spectra. On the basis of the optimized ground-state geometry, the TDDFT/B3LYP method with PCM in CH2Cl2 media was used to calculate the absorption properties of the studied complexes. The most leading excited states (with larger CI coefficients) and the first vertical triplet state associated with their oscillator strengths, dominant orbital excitations, and their assignments are listed in Tables S12−S13 (Supporting Information), along with the corresponding experimental data. The simulated absorption curves with the calculated absorption data for 1−1d and 2−2d are shown in Figure 4. In experiment, absorption spectra for 1 and 2 show intense features below 300 nm and less intense features in the range of 300−400 nm, with the tail extending into the visible region. Obviously, the calculated absorption spectra (Figure 4) can reproduce well the experimental features in terms of band positions, intensities, and separations, especially taking into account the rather limited dimensions of the basis set and neglecting the spin−orbit coupling of the excitation energies, which can demonstrate the accuracy and reliability of this calculation method.35 Seen from Tables S12 and S13 (Supporting Information), consistent with the variation rules of HOMO−LUMO energy gaps, the calculated lowest-lying absorption bands exhibit redshifting in the following order: 1/2 (362/366 nm) → 1d/2d (378/386 nm) → 1b/2b (397/407 nm) → 1a/2a (414/417 nm) → 1c/2c (418/423 nm). As expected, the S1 states are predominantly contributed by the HOMO → LUMO transition with more than 94% in composition. From the above analysis on FMOs, it is reasonable to assign the lowestlying absorptions to mainly [d(Ir) + π(dppit) → π*(py2pz/ 26500



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B3LYP, B3P86, B3PW91, and PBE0 functionals, while the M05-2X gives satisfactory results for 1 (only overestimated by 12 nm) and largely deviates from the experimental data by 31 nm for 2. The best results are obtained with the M06-2X functional with smaller discrepancy of 4 and 12 nm compared with the experimental data for 1 and 2, respectively. Therefore, we have employed the M06-2X functional for further emission spectra calculations. On the basis of the lowest triplet excited-state geometries, the emission spectra of all the studied complexes were calculated by the TD-DFT method with the M06-2X functional in CH2Cl2 media. Similar to vertical triplet absorption, the oscillator strengths for phosphorescent emissions are also zero since there are no spin−orbit coupling (SOC) effects included in TDDFT calculation. The emission wavelengths, emission energies, and transition nature are listed in Table 2, along with the experimental values. To conveniently discuss the transition property of emission, we list the partial compositions of FMOs related to emission in Table S14−S15 (Supporting Information). The calculated lowest-energy emissions are at 468 nm (2.65 eV), 455 nm (2.72 eV), 456 nm (2.72 eV), 516 nm (2.40 eV), and 459 nm (2.70 eV) for 1−1d and 489 nm (2.53 eV), 449 nm (2.76 eV), 488 nm (2.54 eV), 531 nm (2.34 eV), and 489 nm (2.53 eV) for 2−2d. The 468 and 489 nm emissions of 1 and 2 correspond well to the experimental values of 472 and 477 nm, respectively. A pronounced red-shift for 1c and 2c has been observed compared to 1 and 2, while a relatively smaller and larger blue-shift is detected for 1a/1b and 2a, respectively. Table 2 shows that the lowest-energy emissions of 1−1d originate mainly from LUMO → HOMO transition assigned to [π*(py2pz) → d(Ir) + π(dppit)] with the mixed characters of 3 MLCT/3LLCT except for 1a. Due to the significantly reduced composition of the d(Ir) orbital in the HOMO of 1a, the transition characters of 455 nm can be described as ILCT [π*(py2pz) → π(py2pz)] with HOMO and LUMO both mainly localized on the py2pz ligand (Table S14, Supporting Information). The emission of 2 at 489 nm is also mainly contributed by LUMO → HOMO transition with the configuration coefficient of 0.77. For 2a−2d with the N atom

compared with 2. The remarkable MLCT participation in the lower-energy region ensures their efficient intersystem crossing (ISC), and the dipole-allowed singlet−triplet transitions are beneficial for the high luminescence efficiency of Ir(III) complexes. 3.4. Phosphorescence Spectra. The above results indicate that the theoretical approaches used in our present work are reasonable for the absorption properties. However, with regard to predicting the color of the light, emission is the most important property to be calculated. Our previous work indicates that the B3LYP functional seems to overestimate the emission wavelength of Ir(III) complexes.35,36,42 Thus, to find a method that would allow for reliable prediction of emission properties, six different density functionals (B3LYP, M06-2X,43 M05-2X,44 B3P86,45 B3PW91,46 and PBE047) were performed to calculate the emission spectra for 1 and 2. The results are depicted in Figure 5 together with the available experimental

Figure 5. Emission wavelengths for 1 and 2 at B3LYP, M06-2X, M052X, B3P86, B3PW91, and PBE0 levels, respectively, together with the experimental values.

values for 1 and 2. As can be seen from Figure 5, the emission wavelengths are significantly overestimated in the case of

Table 2. Calculated Phosphorescent Emission of the Studied Complexes in CH2Cl2 Media at the TDDFT/M06-2X and TDDFT/B3LYP Levels, Respectively, Together with the Experimental Values M06-2X λ (nm)/E (eV)

a

configuration

B3LYP assignment

λ/E

configuration

assignment

expa (nm)

L→H (66%) L→H (88%) L→H (49%) L→H-1 (41%) L→H (64%) L→H (57%) L→H (51%) L→H (92%)

MLCT/LLCT IL MLCT/LLCT LLCT MLCT/LLCT MLCT/LLCT MLCT/LLCT MLCT/LLCT

472

548/2.26

L→H (35%) L→H-2 (54%)

MLCT/LLCT LLCT

608/2.04

L→H (45%) L→H-1 (30%) L→H-2 (36%) L→H (39%)

MLCT/LLCT LLCT LLCT MLCT/LLCT

1 1a 1b

468/2.65 455/2.72 456/2.72

L→H (75%) L→H (43%) L→H (56%)

MLCT/LLCT IL MLCT/LLCT

522/2.37 599/2.07 523/2.37

1c 1d 2 2a

516/2.40 459/2.70 489/2.53 449/2.76 488/2.54

2c

531/2.34

2d

489/2.53

MLCT/LLCT MLCT/LLCT MLCT/LLCT MLCT/LLCT LLCT LLCT MLCT/LLCT LLCT MLCT/LLCT LLCT LLCT MLCT/LLCT

593/2.09 520/2.38 546/2.27 618/2.00

2b

L→H (69%) L→H (64%) L→H (77%) L→H (37%) L→H-2 (22%) L→H-1 (44%) L→H (23%) L→H-2 (23%) L→H (53%) L→H-1 (36%) L→H-1 (44%) L→H (43%)

550/2.25

477

Ref 18. 26501



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Table 3. Selected Bond Distances (in Å) Calculated for the Studied Complexes in the Metal-Centered (3MC) Triplet Excited States Ir−N1 Ir−N2 Ir−C1 Ir−C2 Ir−P1 Ir−P2

1

1a

1b

1c

1d

2

2a

2b

2c

2d

2.292 2.287 1.988 2.035 2.414 2.518

2.253 2.314 1.987 2.037 2.414 2.515

2.290 2.313 1.987 2.036 2.417 2.519

2.292 2.299 1.986 2.037 2.412 2.518

2.298 2.295 1.985 2.036 2.410 2.519

2.293 2.302 1.990 2.033 2.416 2.520

2.261 2.334 1.987 2.036 2.417 2.511

2.292 2.330 1.989 2.034 2.418 2.521

2.296 2.313 1.988 2.035 2.414 2.521

2.295 2.293 1.988 2.035 2.414 2.520

incorporated in bptz ligands, mixed transitions of LUMO → HOMO, LUMO → HOMO-1, and LUMO → HOMO-2 are responsible for these emissions, which are attributed to MLCT/ LLCT mixed characters (Table 2). These results are consistent with the analysis of the geometry structure of the triplet excited states in the geometry structure section. The strengthened interactions between the metal and py2pz/bptz and dppit ligands caused by the shortened Ir−ligand bond distances (Ir− N and Ir−C) (Figure 2) result in the most participation of py2pz/bptz and dppit ligands in the FMOs in the excited states for the studied complexes, while the significantly elongated Ir− P2 bond length undoubtedly leads to the absence of the PMe2Ph ligand in the excited states (Tables S14−S15, Supporting Information). The remarkable involvement of the 3 MLCT transition in emission for the studied complexes is beneficial for a high quantum yield. A large Ir d orbital contribution in HOMOs is good for high 3 MLCT composition and hence increases the transition probability. A careful analysis of the character of the FMOs in Tables S14−S15 (Supporting Information) demonstrates a large contribution from Ir d orbitals to HOMOs (26−29%) for the studied complexes except for 1a and 2a, resulting in substantial metal−ligand mixing with the π-orbitals of the ligands. For 1a and 2a, the drastic decreased contribution from Ir d orbitals to HOMOs (9% and 16%, respectively) might lead to the relatively weaker metal−ligand mixing in the transitions, which might be rationalized by the largest changes of bond distances in T1 states compared with the S0 states for 1a and 2a (Figure 2). The above results indicate that the N substitution may be an efficient strategy to tune the emitting color of 1 and 2. Compared with both 1 and 2, a blue shift is predicted for the 1substituted derivatives, while an obvious red shift was observed for the 3-substituted ones. Therefore, 1a and 2a might be potential candidates for blue-emitting phosphorescent materials. These results provide a useful guide for designing and synthesizing new excellent phosphorescent Ir(III) complexes with tunable optical and electronic properties. 3.5. Efficiency Comparison. 3.5.1. Calculated ExcitedState Properties (3MLCT and 3MC States). Since one of the most important deactivation pathways of the phosphorescent emission from T1 in transition-metal complexes is the population of the metal-centered (3MC d−d) triplet excited states,48 additional calculations were carried out to obtain the lowest 3MLCT/π−π* and 3MC d−d states of all the studied complexes using unrestricted triplet optimizations, aiming to get an insight about the different quantum yield observed for the studied Ir(III) complexes. The 3MLCT/π−π* excited states were obtained by performing an unrestricted triplet optimization starting from the optimized ground-state geometry (see Computational Methodology section). The electronic configurations of 3MC d−d states were calculated following the

literature methodology,29,30 in which optimization is starting with a distorted molecular geometry by largely elongating the metal−ligand bonds. The calculated metal−ligand bond distances in 3MC d−d states are listed in Table 3. For the studied complexes, the distortion of the geometry from S0 to the 3MLCT/π−π* states is relatively small. The geometric discrepancy from S0 to the 3MC d−d states, on the other hand, is significant, and the Ir−C1/Ir−C2 and Ir−N2/Ir−P1 bonds are elongated significantly by 0.06−0.09 and 0.16−0.20 Å, respectively, in 3MC d−d states compared to their ground states (Tables 1 and 3). The more distorted structures of 3MC d−d states will increase the nonradiative probability due to the very weak chelating interaction between metal and ligands caused by elongation of bond distances. This will undoubtedly result in the decrease of the quantum yield which is controlled by the competition between the radiative (kr) and the nonradiative (knr) rate. In addition, the lowest triplet metal-to-ligand charge transfer (3MLCT/π → π*) excited state can be rapidly converted, by thermal activation, to a short-lived triplet metal-centered (3MC d−d) state from which no photochemistry occurs,49,50 and the conversion is often irreversible, due to the very fast nonradiative decay back to the ground state from the metal-centered state.30 Since the activation barrier for 3MLCT → 3MC conversion is believed to be closely related to the energy gap between 3 MLCT/π → π* and 3MC d−d states,18,20b we examined the relative energy difference between 3MLCT/π → π* and 3MC d−d excited states. The calculated results are shown in Figure 6 with the normalized S0 levels. Theoretically, the large separation between 3MLCT/π → π* and 3MC d−d states is believed to play a key role for maintaining the phosphorescence

Figure 6. Energy level diagram of the studied complexes in 3MLCT and 3MC excited states, respectively, along with the normalized S0 levels. 26502



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quantum yield. Although being qualitative, the results (Figure 6) show a correlation between the N-substituted positions on the pyridine moiety in N∧N ligands and 3MLCT/π → π* versus 3MC d−d state energy gap. Note that the N substitution stabilizes the 3MLCT/π → π* states for 1a−1d and 2a−2d compared with 1 and 2, with the most decrease found in 1a/2a and 1c/2c. Meanwhile, the 3MC d−d states are hardly affected except for 1a/2a which are lying at a relatively higher energy level compared with 1 and 2. Therefore, 1a/2a and 1c/2c give a larger energy gap between 3MLCT/π → π* and 3MC d−d states. We also note that the relative energy of the 3MC d−d state is higher than that of the lowest-lying 3MLCT/π → π* excited state for all the studied complexes. In other words, upon excitation of 1a/2a and 1c/2c, the 3MLCT/π → π* → 3MC → S0 radiationless pathway is expected to be less efficient, which would result in a high emission quantum yield. Moreover, the same separation in energy between 3MLCT/π → π* and 3MC d−d states for 1 and 2 (Figure 6) also accounts for their similar nonradiative (knr) rate of 1.40 × 105 (1) and 2.68 × 105 (2) observed in experiment.18 The above calculated results indicate that the tuning of the energy gap between 3MLCT/π−π* and 3MC d−d states is largely dependent on the positions of the N substituent on the pyridine moiety in N∧N ligands. The N substituent at the 1and 3-position leads to a significantly larger energy gap between 3 MLCT/π → π* and 3MC d−d states, which means a decrease of nonradiative pathway and a positive effect on improving the emission efficiency for these complexes. The 2- and 4-Nsubstitutions result in a slight increase of the energy gap. Thus, we can obtain a rough estimate of the different quantum yield for the studied complexes from the above analysis. 3.5.2. Comparison of Radiative Rate (kr). As mentioned above, the quantum yield (ΦPL) is linked to the radiative (kr) and nonradiative (knr) rate, which can be expressed by the following equation, where τem is the emission decay time ΦPL = k rτem

kr = k r + k nr

Table 4. Computed Singlet−Triplet Splitting (ΔES1−T1), Metal-Based Charge Transfer Character (3MLCT) (%), and Transition Electric Dipole Moment (μS1) for the Studied Complexes complex 1 1a 1b 1c 1d 2 2a 2b 2c 2d a

Φ 0.18

ΔES1−T1 (eV) a

0.24a

MLCT (%)

μS1 (Debye)

11.60 17.01 9.48 11.84 12.44 11.35 20.40 13.65 13.29 14.07

0.32 0.48 0.24 0.36 0.28 0.21 0.48 0.18 0.33 0.24

3

0.60 0.22 0.30 0.46 0.47 0.51 0.28 0.22 0.41 0.39

Ref 18, measured in CH2Cl2.

(0.41) and 0.24 (≈1.0), respectively, for 1 and 2 in degassed CH2Cl2 media (in solid state), while the nearly same MLCT contribution was calculated for 1 and 2 (11.60 and 11.35%, respectively, Table 4), implying that the MLCT % does not contribute significantly to the different quantum yields for 1 and 2. The second aspect elucidating the SOC effects is the singlet− triplet splitting energy (ΔES1−T1).53 The S1 → T1 intersystem crossing 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.54 A minimal ΔES1−T1 is favorable for enhancing 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 expressed below55,56 kr = γ

(1)

⟨ΨS1|HSO|ΨT1⟩2 ·μS12 (ΔES1 − T1)2

, with γ =

3 16π 3106n3Eem 3hε0

(2)

where μS1 is the transition electric dipole moment in S0 → S1 transition; Eem represents the emission energy in cm−1; while n, h, and ε0 are the refractive index, Planck’s constant, and the permittivity in vacuum, respectively. According to the equation, we can provide a theoretical analysis of kr for the studied complexes. The singlet−triplet splitting energy (ΔES1−T1) was calculated considering the fixed triplet molecular geometry using TD-B3LYP. It shows that the decreasing ΔES1−T1 can lead to a higher kr, while a decreasing μS1 leads to a lower kr according to eq 2. From Table 4, ΔES1−T1 decreases in the following order: 1 < 1d < 1c < 1b < 1a (2 < 2c < 2d < 2a < 2b), with the largest value being calculated for complexes 1 (0.60 eV) and 2 (0.51 eV) and relatively smaller ones for 1a/1b (0.22/0.30 eV) and 2a/2b (0.28/0.22 eV). Thus, in comparison with the experimental structures of 1 and 2, the designed complexes 1a/1b and 2a/2b have a favorable ISC rate which would lead to a higher kr for them. This feature is even supported by the computed ΔES1−T1 value (Table 4) for 2 which favors well its higher kr (8.36 × 105 S−1)18 in comparison with that for 1 (3.02 × 104 S−1).18 As for μS1, the largest values were observed for 1a/2a (0.48/0.48 D), implying their higher kr again. Overall, the ΔES1−T1 and μS1 are the parameters mainly influencing kr and thus ΦPL, which were also found to be closely related to the positions of N position on the pyridine moiety in N∧N ligands

Obviously, a large kr and a small knr are required for a high quantum yield. Since the knr has been qualitatively discussed above, herein we will investigate the ΦPL in detail with a special emphasis on radiative rate (kr). The heavy atom participation, such as iridium, is believed to increase spin−orbit coupling (SOC) effects and intersystem crossing (ISC), provided that its orbitals make a significant contribution in the excited states involved. The SOC effects are elucidated mainly from the following two aspects. One is the contribution of MLCT in the T1 state.51 The direct involvement of the Ir(III) d orbital can enhance the firstorder SOC in the T1 → S0 transition, which would result in a drastic decrease of the radiative lifetime and avoid the nonradiative process.52 Therefore, a large MLCT contribution will increase the quantum yield (ΦPL). In Table 4, we list the MLCT contributions for the studied complexes calculated following Chou’s work.18 We noted that the MLCT % is found to be affected by the different positions of N atom for these complexes. To be specific, the introduction of N substituent on the pyridine moiety in N∧N ligands increases MLCT % for 1a− 1d and 2a−2d except for 1b, with the smallest contribution of 9.48% among these complexes. The highest MLCT is found in 1a/2a, indicating a chance for high ΦPL. Nonetheless, experimental work by Lin et al.18 has shown a ΦPL of 0.18 26503



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studied molecules. According to the semiclassical Marcus theory,58 the rate of intermolecular charge (hole and electron) transfer (Ket) can be estimated by the following formula

of these complexes. Thus, the radiative rates (kr) of 1a/2a can be qualitatively anticipated to be higher than that of 1 and 2. To sum up, from the above discussion, we can safely draw a conclusion that, due to the relatively larger kr and smaller knr (see Section 3.5.1), 1a/2a with the 1-position N substituent are considered to have relatively higher ΦPL compared with the experimentally synthesized complexes 1 and 2. Thus, 1a/2a could become the potential blue-emitting materials with high quantum efficiency. Of course, we should remember that besides the factors discussed above other factors may also affect radiative (kr) and nonradiative (knr) rates which are not taken into account in our discussion. 3.6. Comparison of Performance in OLEDs. A good device performance of OLEDs is attributed to the excellent electrical carrier-injection and -transport abilities as well as the balance between the hole and electron transport. To evaluate the energy barrier for the injection of holes and electrons, it is necessary to investigate the ionization potential (IP) and electron affinity (EA), which are also closely related to the HOMO and LUMO, respectively.57 For photoluminescent materials, a smaller IP value means easier injection of holes, while a larger EA value will facilitate electron injection into the emitting materials from the transporting layer. The calculated vertical IP (IPv), adiabatic IP (IPa), vertical EA (EAv), and adiabatic EA (EAa), together with hole extraction potential (HEP) and electron extraction potential (EEP), are listed in Table 5. The details for these definitions can be obtained from

Ket = A exp( −λ /4KBT )

where λ is the reorganization energy; A is a prefactor related to the electronic coupling between adjacent molecules; and T and KB are the temperature and Boltzmann constant, respectively. According to a previous report, the mobility of charges has been demonstrated to be largely related to the reorganization energy λ for OLED materials, owing to the limited intermolecular charge transfer range in the solid state.58a,c The reorganization energy λ is obtained by ignoring any environmental relaxation and changes. From eq 3, a low λ value is necessary for an efficient charge transport process. Avoiding the details, the reorganization energy for hole/ electron transfer can be simply defined as follows59

1 1a 1b 1c 1d 2 2a 2b 2c 2d

IP(a)

HEP

EA(v)

EA(a)

EEP

λh

λe

6.30 6.31 6.45 6.46 6.39 6.49 6.48 6.62 6.65 6.59

6.14 6.12 6.25 6.29 6.23 6.29 6.27 6.41 6.45 6.39

5.94 5.89 6.04 6.09 6.04 6.08 6.03 6.18 6.23 6.18

0.17 0.54 0.44 0.57 0.31 0.24 0.59 0.51 0.64 0.41

0.30 0.82 0.61 0.76 0.46 0.37 0.85 0.69 0.83 0.55

0.44 1.12 0.79 0.97 0.63 0.52 1.13 0.89 1.04 0.71

0.36 0.42 0.41 0.37 0.36 0.41 0.45 0.43 0.42 0.42

0.27 0.58 0.35 0.40 0.32 0.28 0.54 0.37 0.40 0.30

λh = IPv − HEP

(4)

λe = EEP − EA v

(5)

As shown in Table 5, for most complexes (except 1a/2a and 1c), the reorganization energies for hole transport (λh) are slightly larger than those for electron transport (λe), which reveals that the electron transport performance of these complexes is slightly better than hole transport ability. Table 5 also shows that the introduction of the N substituent on the pyridine moiety in N∧N ligands for 1a−1d and 2a−2d leads to their relatively higher λh and λe in comparison with those of 1 and 2, meaning that they have poor charge-transporting properties. The N substitution does not cause significant change of λh, while the λe values are more influenced, especially for 1a and 2a, in which the λe is 0.31 and 0.26 eV larger than those for 1 and 2, respectively (Table 5), resulting in the reduced electron-transporting rate for them. Moreover, we also note that the N substitution leads to a decreased difference between λh and λe, demonstrating the improvement of the transfer balance between hole and electron except for 1a, which has the most unbalanced hole and electron transfer process among these complexes. The differences between λh and λe of the assumed complexes 1c/2c and 1d are relatively smaller (0.02−0.03 eV), indicating that hole and electron transfer balance could be achieved more easily in the emitting layer, which is the key factor for materials used in OLEDs. Therefore, they are more suitable emitters in OLED applications.

Table 5. Ionization Potentials, Electron Affinities, Extraction Potentials, and Internal Reorganization Energies (in eV) for the Studied Complexes IP(v)

(3)

our previous work.42 It shows that 1/2 and 1a/2a have smaller IP values, which is consistent with their relatively higher HOMO energy levels, and thus their hole injection is easier compared with other complexes. It also indicates that the N substituent at the 1-position on the pyridine moiety causes a minor improvement of hole-injection ability for 1a/2a compared with that of 1/2 due to their similar IP values (Table 5). The N substitution on 2-, 3-, and 4-positions leads to the poor hole-injection ability. Corresponding to the lower LUMO energy levels, the assumed complexes 1a/2a and 1c/2c have larger EA values and enhanced electron injection ability compared with the experimental structures 1 and 2, which have the smallest EA values and worst electron injection ability. It is obvious that 1a/2a exhibits the relatively lower IPv and higher EAv values, indicating that the introduction of the N substituent at the 1-position on the pyridine moiety of N∧N ligands can effectively improve the carrier-injection ability Additionally, to evaluate the charge transfer rate and balance properties, reorganization energy (λ) was calculated for the

4. CONCLUSIONS In this paper, we have employed DFT and TDDFT methods to investigate the geometrical and electronic structures, absorption and emission spectra, charge-injection and -transport abilities, and phosphorescence efficiency of recently synthesized blueemitting Ir(III) complexes 1 and 2 bearing the structure of [Ir(PMe2Ph)(dppit)(N∧N)] without fluorine substituents. Focused on the effect of the N-substituted positions, a series of cyclometalated Ir(III) complexes on the basis of the structures of 1 and 2 have been designed by a systematic substitution of the “CH” group with the N atom at 1-, 2-, 3-, and 4- positions on the pyridine moiety in N∧N ligands of 1 and 2, respectively. According to the results, we found that the introduction of the N substituent on the pyridine moiety can stabilize the energy levels of LUMOs more than HOMOs and induce variations in the energy gap between HOMO and LUMO. What’s more, the N substituent at the 1-position 26504



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induces the blue-shift in absorption and emission spectra and improves the electron injection performances for 1a and 2a, while the 3-position of the N-substituent (1c/2c) leads to the red-shift in optical spectra and better balance properties between the hole- and electron-transporting abilities. The detailed analysis on quantum efficiency showed that the assumed 1a/2a are considered to be potential candidates of blue-emitting materials with high quantum efficiency, due to their relatively larger radiative (kr) rate and smaller nonradiative (knr) rate. We hope that our work can help to better understand the structure−property relationship of phosphorescent Ir(III) complexes and provide constructive information for designing novel and highly efficient OLED materials in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Tables: (S1) Selected optimized ground-state parameters of 1 and 2 in the gas phase and CH2Cl2 media at the B3LYP/ LANL2DZ leve. (S2−S11) Frontier molecular orbital energies (eV) and compositions (%) of different fragments in the ground state for complex 1−1d and 2−2d, respectively. (S12− S13) Selected calculated wavelength (nm)/energies (eV), oscillator strength ( f), major contribution, transition characters, and the experimental wavelength (nm) for 1−1d and 2−2d, respectively, in CH2Cl2 media at the TD-B3LYP level. (S14− S15) Partial molecular orbital composition (%) of 1−1d and 2−2d in the excited states, respectively. (S16−S17) The xyz coordinates for the optimized structures for 1 and 2 in the S0 and T1 states are also provided. Finally, the complete author lists of ref 11, 30, and 39 are given. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China for financial support (Grant Nos. 20921002, 21273219).

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REFERENCES

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Effects of N-Substitution on Phosphorescence Efficiency and Color

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