Article pubs.acs.org/JPCA

Shedding Light on the Photophysical Properties of Iridium(III) Complexes with N‑Heterocyclic Carbene Ligands from a Theoretical Viewpoint Li Wang, Yong Wu, Yun Geng,* Jie Wu, Dong-Xia Zhu, and Zhong-Min Su* Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China S Supporting Information *

ABSTRACT: The phosphorescent efficiencies of the Ir(III) carbene complexes 1−3 with wide-range color tuning were focused on in this work. A DFT/TDDFT (density functional theory/time-dependent density functional theory) investigation on the geometries in the ground and lowest triplet excited states, the frontier molecular orbitals, the absorption spectra, and dorbital splittings of 1−3 were provided to get a better understanding of structure−property relationships. Importantly, to shed light on the difference in phosphorescent quantum yields for 1−3, radiative decay constants as well as zero-field-splitting parameters were calculated based on the estimation of spin−orbit coupling (SOC) matrix elements denoted as ⟨Tα1 |HSOC|Sn⟩. The results show that, for any complex, the radiative decay rates in the three substates (namely, Tx, Ty, and Tz) are not equal, and the largest radiative rates of 1−3 are all located in x substates with values of 1.0764 × 104, 0.8231 × 104, and 1.9596 × 104 s−1, respectively. Moreover, for 3 with the highest quantum efficiency, we make efforts to modify it through varying substituents and substituent positions not only to achieve blue shift in the emission but also to obtain improved triplet energy.

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INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted extensive attention in recent decades due to their advantages of flexibility in flat-panel displays and sunlight simulation in white light.1−7 Meanwhile, OLED devices with heavy metal complexes as the dopant in the emission layer can enhance the luminescent efficiency greatly through utilizing both the singlet and triplet excitons recombined in the electroluminescent process.8,9 Additionally, the better stability compared with pure small organic systems is another advantage for transition metal complexes in organic devices, which makes the transition metal complexes to be the focus of researchers’ attention. Hence, the study and development of these heavy metal complexes become important subjects for highly efficient phosphorescent OLED devices. In order to meet the requirements of full-color displays and solid-state lighting, enormous efforts have been devoted to tuning the colors of organometallic electrophosphors. Previous studies confirmed that the converting in isomers,10 the introducing of electron-withdrawing11−14 and electron-donating moieties15 to aryl rings or pyridine fragments, the altering in substitution positions,16 and the using of new ancillary ligands17 are efficient methods in adjusting the luminescent color. For instance, the phosphorescent emission of bis[4,6-difluorophenylpyridinato-N,C2′]iridium(acetylacetonate) [FIr(acac)]14 experiences a blue shift of about 40 nm compared with bis(2phenylpyridinato-N,C2′)iridium(acetylacetonate) [(ppy)2Ir© 2014 American Chemical Society

(acac)] due to the introduction of F atoms on the phenyl ring. Tsuzuki and co-workers13 found that Ir(III) complexes with substituents at the para position rather than those at the meta position of the phenyl ring in the phenylpyridine ligand have larger wavelengths in the emission spectra. Accompanied by these abundant experimental advancements, many theoretical investigations focusing on molecular structure and phosphorescent properties as well as the pursuing of deep blue phosphors have also been carried out. Avilov and coworkers18 found that fluorine and trifluoromethyl, which have different inductive and mesomeric effects when attached to heteroleptic phosphorescent Ir(III) complexes, could modulate the emission energy and also change the nature of the lowest excited states through a density functional theory (DFT) calculation. Wu et al.19 designed a potential blue-emitting material in the form of coordination systems based on a theoretical study of chemical structure as well as photophysical properties. Apart from emitting color, the quantum efficiency of the guest material is another important factor to achieving high working efficiency of an OLED device. Even though previous studies have qualitatively analyzed the influence of metal character20 and d-orbital splittings19 on quantum efficiency, the details about how these parameters work are still vague. Received: October 7, 2013 Revised: May 17, 2014 Published: May 20, 2014 5058

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Figure 1. Chemical structures of complexes (fpmi)2Ir(dmpypz) (1), (mpmi)2Ir(dmpypz) (2), and (mpmi)2Ir(pybi)(3).

Recently, Tong et al. 21 carried out a study on the phosphorescent quantum efficiency through calculations on the spin−orbit coupling (SOC) matrix for Pt(II) and Au(I) complexes. However, such investigations for Ir(III) complexes, a kind of widely used material in OLED application, are scarce. Recently, Lu and co-workers22 synthesized a series of Ir(III) carbene complexes which achieve wide-range color tuning from blue to green and to red. Among these complexes, 1−3 (see Figure 1) with N-heterocyclics (NHCs) as cyclometalating ligands arouse our attention because of their interesting experimental phenomena (1, 2, and 3 refer to (fpmi)2Ir(dmpypz), (mpmi)2Ir(dmpypz), and (mpmi)2Ir(pybi), respectively, where H2fpmiI = 1-(4-fluorophenyl)-3-methyl-imidazolium iodide, Hdmpypz = 3,5-dimethyl-2-(1H-pyrazol-5-yl)pyridine, H2mpmiI = 1-(4-tolyl)-3-methyl-imidazolium iodide, and Hpybi = 2-(pyridine-2-yl)-1H-benzo[d]imidazole). For example, on one hand, the changing in the emission wavelengths for 2 is only 11 nm while a bathochromic shift of about 70 nm has taken place on 3 when compared with 1, and great differences in the obtained efficiencies have also been observed for the three complexes. On the other hand, the structural framework for the three complexes is identical except for tiny differences; namely, the cyclometalating ligands for 1 and 2 are tailored by different substituents at the phenyl rings and the ancillary ligand for 3 is enhanced in the conjugation. Thus, to shed light on the natures of the above interesting experimental phenomena and provide a service for new material design, we carried out corresponding theoretical calculations. In this work, research on energy levels and frontier molecular orbitals together with the absorption and emission spectra were performed through density functional theory/time-dependent density functional theory (DFT/ TDDFT) method to inquire reasons for tuning colors. Mainly, zero-field-splitting (ZFS) values and radiative decay rates for the three Ir(III) complexes were estimated through the calculations on the SOC matrix elements to explore the reason for the differences in the phosphorescent quantum efficiency. Finally, we designed complexes 4−9 to obtain a blue shift in the emission as well as improved vertical triplet energy compared with complex 3. As a result, the relationships between geometry structures and photophysical properties are explained, which could provide significant guidance for further design of new efficient materials.

respectively. Thus, according to the Kasha rule, it is rational to only consider T1 in the following investigations when estimating the radiative decay rate. For Ir(III) ions, it is a d6 configuration with Oh symmetry and the d-orbitals can split into t2g and eg23 manifolds. However, when heterogametic ligands are coordinated to the metal center, the quasi-Oh will not be maintained under the influence of the ligand field. As a result, the t2g-manifold orbitals will be no longer degenerated and will split into dxy, dyz, and dxz.23 Due to spin-forbidden restriction, triplet excitons by direct excitation are very few. Thus, mixing the singlet states into the lowest triplet state through SOC is necessary23 for getting efficient phosphorescent emitters. Under the Born−Oppenheimer approximation and the firstorder perturbation theory, the radiative decay rate kr of the phosphorescence can be expressed in the following form:21,24,25 k rα(T1 → S0) =

3 3 16 × 106π 3E(T) 1 η |M T|2 3hε0

=

3 3 16 × 106π 3E(T) 1 η 3hε0

∑ n

⟨T1α|HSOC|Sn⟩ ⟨S0|M |Sn⟩ E(Sn) − E(T) 1

2

(1)

Here, E(T1) and E(Sn) are the vertical excited energies of the related triplet and singlet excited states in cm−1, respectively. h, η, and ε0 represent Planck’s constant, the refractive index of the medium, and the vacumm permittivity, respectively. The medium used in the current system is CH2Cl2, and its refractive index is 1.424. ⟨Tα1 |HSOC|Sn⟩ is the SOC matrix element between T1 and Sn. ⟨S0|M|Sn⟩ is the transition dipole moment from the ground state to the singlet excited state. As provided in ref 13, the SOC effect between 3MLCT(dπ*) (metal to ligand charge transfer) and 1MLCT(dπ*) states, which originate from the same d orbital, is very weak. A similar situation is found in the coupling between 3(ππ*) and 1(dπ*) which are related to two transition centers, as well as the coupling between 3(ππ*) and 1(ππ*) which involve a nonmetal center. All these coupling processes can be neglected in the estimation of SOC matrix elements. Thus, the only remaining effective coupling involves the 3(dπ*) state and the 1(d′π*) state. Here, the participating 3(dπ*) and 1(d′π*) states have different d orbitals and identical π* antibonding orbitals to produce a strong SOC effect23 as analyzed above. Additionally, the so-called indirect SOC referred to by Yersin,23 which involves the configuration interactions (CI) between T1 substates with LC (ligand center) character and higher lying

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THEORY OF COMPUTATION Generally, radiative and nonradiative decay rates determine the phosphorescent efficiency of a certain transition metal complex, so discussions on these two parameters are worthwhile for understanding the relationships between structures and quantum efficiencies. Meanwhile, singlet and higher triplet excited states can arrive at the lowest triplet excited states through intersystem crossing (ISC) and internal conversion, 5059

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Table 1. Optimized Bond Lengths (Å), Angles (deg), and HOMO Energy Levels (eV) of Complex 1 Obtained by Different Functionals Together with the Experimental Values Ir−C1 Ir−C2 Ir−N3 Ir−N4 Ir−C5 Ir−C6 C1−Ir−C2 N3−Ir−N4 C5−Ir−C6 HOMO

PBE0

B3LYP

mPW1PW91

BHandHLYP

M062X

TPSSh

expt

2.027 2.037 2.102 2.165 2.027 2.037 79.0 75.8 79.0 −5.43

2.045 2.062 2.133 2.209 2.051 2.057 78.9 75.1 78.9 −5.26

2.029 2.042 2.106 2.170 2.031 2.039 79.0 75.7 79.0 −5.45

2.037 2.060 2.115 2.193 2.048 2.052 78.9 75.2 78.8 −6.31

2.014 2.038 2.129 2.213 2.027 2.028 79.3 75.1 79.2 −6.62

2.040 2.052 2.116 2.176 2.040 2.049 78.9 75.6 78.9 −4.83

2.019 2.009 2.077 2.133 2.038 2.020 79.3 75.7 79.1 −5.2

Another important parameter for weighing the phosphorescent properties of transition metal complexes is ZFS values. Due to the effect of SOC, triplet states can split into three sublevels (denoted as x, y, z) even without a magnetic field. As mentioned above, SOC consists of direct and indirect coupling. The latter process includes two SOC and CI procedures, and here, SOC involves the higher triplet and singlet MLCT states while CI involves the LC T1 substate together with higher lying MLCT and MC triplets.23 Simultaneously, it is noteworthy that CI is equally efficient for the three substates of a triplet state, suggesting that the introduction of CI as perturbation leads to neither different stabilized energy of the substates nor any zerofield splittings and changes of the individual rates when acting alone. Based on this theory, the energy shift of the three individual triplet substates can be represented in the following equation.23

triplet states with MLCT or MC (metal centered) character, also makes great contributions to the final radiative decay constants. Assuming that the excited wave function is a linear combination of one-electron transition configurations such as the occupied p orbital to the unoccupied q molecular orbital (denoted as Φ(p → q)), and molecular orbitals can be expanded by natural atomic orbitals in molecular orbital (MO) theory, thus, the SOC between the singlet and triplet excited states can be represented as26 occ occ

⟨3Ψ m|HSOC|1Ψ n⟩ =

∑ ∑ ∑ apqmarqn⟨3Φ(p → q)|HSOC|1Φ q = occ + 1 p = 1 r = 1 occ occ

(r → q)⟩ =

∑ ∑ ∑ apqmarqn⟨∑ 3(Ckpdkpπ *)|HSOC| q = occ + 1 p = 1 r = 1

k

∑ 1(Clrdlrπ *)⟩ l

(2)

ΔE(Tmα ) =

Here, a represents the orthonormal coefficient and its square can be approximately considered as the contribution of a certain transition configuration to the excited wave function. Additionally, HSOC represents the N-electron Hamiltonian operator and hso represents the one-electron Hamiltonian operator. Regarding a particular matrix element, for example, the onecenter SOC element between 3(dyzπ*) and 1(dxzπ*) can be evaluated by23,24,27,28

n

(5)

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=

COMPUTATIONAL METHODOLOGY All calculations were carried out with the Gaussian 09 program.30 To find a suitable functional for the investigated systems, we performed a functional test on the optimization of S0 geometry for 1 in comparison with the experimental structural parameters, and the results are collected in Table 1. Taking a comprehensive consideration of angles and bond lengths, the results from B3LYP31 are the worst among all functionals, especially the bond lengths of Ir−N3 and Ir−N4 which are overestimated by 0.056 and 0.076 Å, respectively. Comparatively speaking, TPSSh, BHandHLYP, 31 and mPW1PW9132 have moderate accuracy just as presented in Table 1. Among the employed functionals, M062X33 and PBE0 give the most accurate structural parameters compared with the experimental data. To further explore which functional is suited for our systems studied here, we also calculated the corresponding highest occupied molecular orbital (HOMO) energy levels for complex 1 and the results are listed in Table 1.

(3)

The calculated process of other matrix elements is similar to eq 3, and the relevant results are listed in the Supporting Information. It should be noted that eqs 2 and 3 are under the assumption of the one-electron approximation. Additionally, the value of the one-electron spin−orbit coupling constant ξIr 5d used here is 4430 cm−1. Based on the energy gap law, knr of the T1 → S0 transition is usually given in the form as eq 4:29 k nr(T1 → S0) ∝ exp{−βE(T)} 1

|⟨Tmα|HSOC|Sn⟩|2 E(Tm) − E(Sn)

Here, α represents the substates of T1 perturbed by singlet excited states. It is clear that ΔE(ZFS) is large when SOC between the involving states is strong. However, ΔE(ZFS) values sometimes may be small even though the contribution of the metal center in the complexes is large enough. This can be ascribed to the near-equal energy shifts of all three substates. Additionally, it is worth noting that the calculations of ΔE(ZFS) can only reflect the quantum efficiency to some extent, but not absolutely.

⟨ 3(Cyzdyzπ *)|hso|1(Cxzdxzπ *)⟩ 1 α α (⟨Cxzdxz )|hso|Cyzdyz ⟩ − ⟨Cxzdxzβ )|hso|Cyzdyzβ ⟩) 2 i = ξIr 5dCxzCyz 2

∑

(4)

β is related to the geometry distortion usually; hence, we can evaluate the nonradiative decay rate through a comparison on the geometry distortions and the lowest triplet excited energies among different complexes studied here. 5060

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Table 2. Calculated Geometry Parameters in S0 and T1 States and Their Difference (ΔT1−S0 in Parentheses) for Complexes 1, 2, and 3 (in Å) 1 S0 Ir−C1 Ir−C2 Ir−N3 Ir−N4 Ir−C5 Ir−C6

2.027 2.037 2.102 2.165 2.027 2.037

2 T1 (ΔT1−S0) 2.032 2.038 2.093 2.135 2.031 2.036

(0.005) (0.001) (−0.009) (−0.030) (0.004) (−0.001)

S0 2.031 2.034 2.106 2.169 2.025 2.040

3 T1 (ΔT1−S0) 2.035 2.036 2.096 2.137 2.029 2.038

(0.004) (0.002) (−0.010) (−0.032) (0.004) (−0.002)

S0 2.027 2.036 2.140 2.179 2.034 2.033

T1 (ΔT1−S0) 2.037 2.040 2.048 2.162 2.036 2.054

(0.010) (0.004) (−0.092) (−0.017) (0.002) (0.021)

C1 will be shortened. In contrast, the well-conjugated pyrazole is supposed to be a good electron acceptor, which could decrease the electron delocalization between the Ir ion and the phenyl ring; hence, the Ir−C6 bond is elongated. Similar situations are also found in other investigated complexes. Moreover, the F atom usually has a conjugated and inductive effect which may draw more electron density to the phenyl and Ir ion, thus, the interaction between the phenyl and the Ir metal center is strengthened. As a result, the bond lengths between the Ir atom and the phenyl ring are shortened while those between methyl-imidazole and the Ir center are stretched. Furthermore, with methyl groups taking the place of F atoms, the inductive effects of the substituents are weakened, and thus, the degree of bond stretching effect is reduced, which would lead to a longer Ir−C1 bond for 2 compared with 1. Simultaneously, due to the trans effect induced by the better conjugated benzoimidozole compared with pyrazole, the bond lengths of Ir−C1 and Ir−C6 are shorter for 3 than those for 2. It can be seen from Table 2 that some changes have taken place on the selected geometrical parameters at T1 states compared with those at S0 states. The changes in the coordinated bond lengths between the cyclometalating ligands and the Ir ion are notably smaller than those between the ancillary ligand and the Ir ion. Additionally, the bond lengths of Ir−N3 and Ir−N4 become shorter from S0 to T1, especially for complex 3, whereas those of Ir−C1 and Ir−C2 are stretched when the complex is excited via absorbing energy. Thus, it is anticipated that the lowest unoccupied molecular orbitals (LUMOs) at Ir−N3 and Ir−N4 regions as well as the HOMOs at Ir−C1 and Ir−C2 regions possess bonding characters, which is well consistent with the electron density distributions as depicted below. Taking all the selected parameters into account, it can be concluded that the three investigated complexes have similar geometry distortions from S0 to T1 states. Frontier Molecular Orbital Properties. Usually, transitions from HOMO to LUMO play an important role in the emission process, so it is reasonable to take the HOMO and LUMO energy gap as an original parameter to estimate the emitting color in designing and synthesizing new materials. What is more, for those occupied molecular orbitals with obvious contributions from d orbitals at T1 optimized structures, the smaller the energy difference between the two highest occupied orbitals is, the stronger the SOC matrix element will be. Thus, the frontier molecular orbitals as well as the related energy levels are focused on in this section to shed light on the optical and chemical properties of the three complexes. The obtained calculated results are shown in Figures 2 and 3, whereas d-orbital splittings together with the SOC matrix will be discussed in subsequent sections.

It shows that the HOMO energy levels from PBE0, B3LYP, and mPW1PW91 are much closer to the experimental result compared with the other three functionals. Thus, combining with the structural and energetic factors, PBE034 may be a reasonable and credible choice for geometrical optimization of S0 at the gas phase. Additionally, the T1 state was also optimized under the unrestricted-PBE0 functional in the gas phase for simplification. To explore the absorption and emission properties of the excited states for the three complexes, TDDFT method35 with the same functional level as the optimization process was applied and the well-known polarizable continuum model (PCM)36,37 with dichloromethane was chosen to consider the solvent effect. Meanwhile, LANL2DZ38 and 6-31+G(d,p)39 basis sets were used for Ir and the other atoms in all calculations, respectively, which has been verified to be reasonable by Cao and co-workers.25

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RESULTS AND DISCUSSION Geometries in Ground and Excited States. To get a better understanding of the structural features, the selected geometry parameters for complexes 1−3 at optimized S0 and T1 structures are summarized in Table 2. Also, the optimized molecular structures of the ground states are shown in Figure S1 in the Supporting Information. As displayed in Figure S1 (Supporting Information), all investigated complexes adopt distorted octahedral geometry with two cyclometalating NHC ligands and one ancillary ligand coordinated to the Ir metal center. Furthermore, it is found that the methyl-imidazole segments of the cyclometalating ligands situate at the mutual trans orientation while the phenyl rings substituted by an F atom or a methyl group locate at the cis disposition, which is consistent with the configurations of many other reported heteroleptic complexes.25 According to Table 2, the calculated Ir−C bond lengths are significantly shorter than the corresponding Ir−N bond lengths at S0 states; for example, the lengths of four Ir−C bonds for 1 are 2.027, 2.037, 2.027, and 2.037 Å, respectively, much shorter than those of the two Ir−N bonds for 1 (about 2.102 and 2.165 Å, respectively). This indicates that the interaction between the NHC ligand and the metal center is stronger than the interaction between the ancillary ligand and the metal center.25 Meanwhile, it is noteworthy that the length of the Ir−C1 bond (about 2.027 Å) is shorter than that of the Ir−C6 bond (about 2.037 Å) for 1, and this can be probably ascribed to the trans effect induced by the dmpypz ligand. Since the pyridyl and pyrazole in the dmpypz ligand locate at the trans disposition of the two phenyl rings substituted by F atoms, and the indirect inductive effects via two methyl groups make the electron delocalization transfer through the Ir ion and spread to the trans-oriented phenyl ring, as a result, the bond length of Ir− 5061

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would withdraw electron density from the ligands and reduce the repulsive Coulomb interaction between the electrons of the σ-system and the electrons occupied on the ligand-localized πMOs,18 thus resulting in the inductive effect and decreasing the HOMO and LUMO energy levels. In addition, it is notable that the energy gap of 1 is different from that of 2, which is probably caused by the different degree of decrease in the HOMO and LUMO energy levels. Thus, making comparisons between 1 and 2, it can be suspected that the introduction of the F atom may make the emission band shift in the blue direction. Besides, the energy gap of 3 is calculated to be 3.84 eV, which is about 0.13 eV lower than that of 2. Simultaneously, 3 has a deeper LUMO energy level compared with 2 as depicted in Figure 3. These could be attributed to the fact that the wellconjugated pybi ligand in complex 3 may withdraw the electron density, and lead to better electron delocalization over the whole molecule due to the molecular orbital interactions. As a result, the MLCT states of 3 are stabilized and a bathochromic shift may occur in the emission band compared with 2. Electronic Absorption Spectra. In the OLED device, the energy transfer between the host and guest materials usually has two forms: Förster energy transfer and Dexter energy transfer. As the emission spectrum of the host and the absorption spectrum of the guest is important for studying and evaluating those energy transfer rates, we investigate the absorption spectra of the three guest complexes 1−3 in CH2Cl2 solvent through TDDFT methods here, and the corresponding results are sketched in Figure 4. Additionally, to get details about the

Figure 2. Calculated HOMO (bottom) and LUMO (top) diagrams of complexes 1, 2, and 3 at their optimized S0 geometries.

Figure 3. Calculated energy levels of complexes 1, 2, and 3 at their respective optimized ground states.

It can be seen from Figure 2 that, for complexes 1 and 2, the HOMOs are both mainly localized on the π orbitals of the cyclometalated fpmi ligands, and the LUMOs both mainly reside on the ancillary ligands. Thus, it is considered that the introduction of the electron-withdrawing groups and the electron-donating groups has similar effects on the electron density distributions. In contrast, the conjugation of the ancillary ligand has great influence on the electron density distributions. For example, the HOMO of complex 3 has been partly localized on the ancillary ligand, which is quite different from that of complexes 1 and 2. Additionally, for the three complexes, Ir d orbitals contribute much to the HOMOs while contributing little to those unoccupied molecular orbitals, probably suggesting remarkable MLCT character for the excited states, especially for those with transition configurations mainly from the HOMO to the unoccupied molecular orbitals. Moreover, the LUMO of complex 3 mostly locates on the π* orbital of the pybi ligand. Combining with the electron density distribution of the HOMO for complex 3, it is anticipated that 3 probably has obvious LC charge transfer character, especially for those excited states mainly originating from the HOMO to LUMO transition. Importantly, the case that the substates with LC character would be mixed into those states with MLCT character is favorable for the radiative decay rate; thus, complex 3 probably has a higher radiative decay rate compared with 1 and 2.40 As shown in Figure 3, 1 has lower HOMO and LUMO energy levels compared with 2, which may be ascribed to the inductive effect caused by the electron-withdrawing F atoms. Compared with the methyl substituents in 2, F atoms in 1

Figure 4. Absorption spectra of the investigated complexes calculated at their optimized S0 geometry by TD-PBE0/6-31+G(d,p) in CH2Cl2 solvent.

specific transition process, the related wavelengths, oscillator strengths, compositions, and transition natures of the main peaks for 1−3 are listed in Table 3. For 1, the strongest absorption with its oscillator strength of about 0.1688 is calculated at 270 nm which is very close to the experimental value 275 nm, and the HOMO − 2 to LUMO + 1 configuration contributes 61% to this absorption. Following the strongest absorption, there is a shoulder located at about 301 nm which is mainly assigned to the HOMO − 3 → LUMO transition and the corresponding oscillator strength is 0.1241. The relatively large oscillator strengths for these two absorptions could provide high intensity for the triplet excited states through the ISC procedure. As referred to the electron density distributions of the corresponding orbitals involved in the absorption process of 1 shown in Figure S1 (Supporting 5062

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become thermally inaccessible. Thus, the emission quenching will be effectively suppressed and the corresponding nonradiative decay rate will be reduced.41 For these reasons, it is necessary to focus on Δddocc (the energy difference between the two highest occupied molecular orbitals which have a character of Ir 5d orbitals) and Δdd* in evaluating the phosphorescent efficiency of the transition metal complexes.41−43 The calculated values of Δddocc and Δdd* for complexes 1−3 are listed in Table 4.

Table 3. Absorption Wavelengths (λ), Oscillator Strengths (f), Composition, and Transition Nature in Terms of Molecular Orbital Contributions for the Dominant S0 → Sn Transitions Calculated by PBE0 TDDFT Methoda complex

λ/nm

f

1

270 301 272

0.1688 0.1241 0.1651

301

0.1030

294 330

0.0923 0.2692

2

3

composition H H H H H H H H H

− 2 → L + 1 (61%) − 3 → L (73%) − 5 → L (37%) − 2 → L + 1 (27%) − 3 → L (36%) → L + 2 (26%) → L + 1 (23%) − 5 → L (57%) − 3 → L (72%)

nature MLCT/LLCT/ILCT MLCT/LLCT/ILCT MLCT/LLCT MLCT/LLCT/ILCT MLCT/LLCT/ILCT MLCT/LLCT MLCT/LLCT MLCT/LLCT MLCT/LLCT

Table 4. Gas Phase Values for the Δddocc and Δdd* (eV) of Complexes 1, 2, and 3 at Their Respective Optimized S0 and T1 Geometries S0

Here, “H” represents HOMO and “L” represents LUMO. LLCT is ligand-to-ligand charge transfer, and ILCT is intraligand charge transfer.

a

Information), both HOMO − 2 and HOMO − 3 mainly originate from the fpmi ligands and Ir ion with a little contribution from the pyrazole ligand. Meanwhile, the LUMO and LUMO + 1 of 1 are mainly localized on the pyridine fragment of the ancillary ligand. Thus, both the strongest absorption peak and the shoulder of 1 have mixed MLCT/ LLCT/ILCT character. Moreover, it should be noted that the calculated absorption wavelengths of about 270 and 301 nm for 1 are consistent with the experimental 275 and 301 nm, demonstrating the reliability of our theoretical calculations. Furthermore, though the shape of the absorption curve for complex 2 is similar to that of 1, the absorption strength of 2 is weaker under 295 nm and becomes stronger after 295 nm compared with that of 1. As for complex 3, there are two peaks in the absorption spectra, one located at 294 nm and the other at 330 nm, which demonstrates remarkable red shift compared with 1 and 2 due to the good electron delocalization over the whole molecule. Besides, the absorption spectra of 3 discussed above is much broader than those of 1 and 2, which benefits the energy transfer between the host and guest materials. In addition, as the triplet states need to borrow their intensity from the singlet excited states via SOC, the larger oscillator strength of the strongest absorption for 3 compared with the other two complexes is conductive to obtaining high phosphorescent efficiency. Phosphorescent Quantum Efficiency. There are many parameters that can be used to evaluate the phosphorescent quantum efficiency. In this work, we mainly focus on the dorbital splitting values, ZFS values, and radiative decay rates in discussing the phosphorescent quantum efficiencies of the studied complexes. The latter two parameters mentioned above are estimated through the calculations on the SOC matrix elements here. d-Orbital Splitting. To obtain efficient SOC, the related 3 MLCT and 1MLCT states must involve different d orbitals. Meanwhile, the smaller the energy difference between the coupling states is, the larger the SOC matrix elements will be. Furthermore, the MC d−d* excitation (d represents the occupied molecular orbital which has great contributions from 5d orbitals of Ir ion, and d* represents the unoccupied molecular orbital which has some contributions from 5d orbitals of Ir ion) could induce emission quenching,19 and with the splitting between d and d* (denoted as Δdd* in the following discussion) increasing, the MC excited states will

T1

complex

Δddocc

Δdd*

Δddocc

Δdd*

1 2 3

0.24 0.30 0.12

4.10 3.97 5.37

0.21 0.25 0.26

4.04 3.92 5.51

It can be seen from Table 4 that all three complexes have small Δddocc and large Δdd* values at their respective optimized S0 and T1 geometries, implying relatively high efficiency for the complexes studied here. Compared with 2, 1 has smaller Δddocc and larger Δdd* values, suggesting that the SOC effect of 1 is stronger and the MC excited state is less thermally accessible for the emission quenching. Hence, the efficiency of complex 1 may be a little higher than that of 2. With respect to complex 3, the value of Δddocc at T1 geometry is comparable with those of 1 and 2. The value of Δddocc for 3 at its optimized S0 geometry is about 0.12 eV, which is the smallest of the three complexes, probably indicating a larger radiative decay rate. Moreover, the values of Δdd* of 3 are 5.37 and 5.51 eV at its respective optimized S0 and T1 geometries, which are much larger than those of 1 and 2, showing that the MC excited states for emission quenching is less thermally accessible for 3. Combining with the values of Δddocc and Δdd* for 3, it can be suspected that 3 has the largest phosphorescent efficiency of the three complexes. Zero-Field Splitting and Radiative Decay Constants of the Lowest Triplet Excited States. To gain a deep insight into the photophysical properties of 1−3, the excitation energy, ZFS parameters, and radiative decay rate constants for the three complexes are calculated and presented in Table 5, and the SOC matrix elements and other corresponding calculated details are collected in the Supporting Information. As the singlet excited state Sn (n > 5) has larger vertical excited energy and smaller oscillator strength than the lower singlet excited states, we only calculate the coupling between the five lower singlet excited states (namely S1, S2, S3, S4, S5) and T1 here. As for S1 (H → L) from which the T1 state could borrow its coupling intensity, the oscillator strength of 1 is larger than that of 2 at the optimized T1 geometry. A similar situation is found in the S3 (H − 2 → L) excited state for 1 and 2. In contrast, as for S4 and S5 excited states, the oscillator strengths of 2 are larger than those of 1. Meanwhile, the oscillator strengths of the S2 (H − 1 → L) excited state for 1 and 2 are almost the same around 0.0920. Hence, in summary, 1 and 2 are comparable from the perspective of oscillator strengths. In addition, the calculated overall SOC matrix element of 2 is larger than those of 1; thus, the calculated radiative decay rate of 2 is larger than that of 1. With respect to complex 3, the good electron delocalization caused by the well-conjugated benzolimidazole 5063

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Table 5. Radiative Decay Rate Constants kr (s−1), ZFS Parameters (cm−1), and Energies (cm−1) of Complexes 1−3 at Their Respective Optimized T1 Geometry Obtained from DFT/TDDFT Calculations in CH2Cl2 Solution 1 2 3

energy (cm−1)

krx (s−1)

kry (s−1)

18 543 18 559 16 259

1.0764 × 10 0.8231 × 104 1.9596 × 104 4

krz (s−1)

0.2761 × 10 0.6547 × 104 0.1263 × 104 4

kr (s−1)

0.0516 × 10 0.2006 × 104 0.0026 × 104 4

ΔE(ZFS) (cm−1)

0.4682 × 10 0.5595 × 104 0.6962 × 104 4

6.60 12.20 44.80

distortions for the three complexes are comparable, while the T1 excited energies are different, just like the results represented in Table 5. Thus, the T1 excited energy would be the only remaining factor to determine the nonradiative decay rate. Meanwhile, it can be inferred from eq 4 that the larger the lowest triplet energy is, the smaller the nonradiative decay rate will be. Hence, as shown in Table 5, 3 has the smallest T1 excited energy compared with 1 and 2, indicating that the nonradiative decay rate of 3 is properly the largest among the three complexes. When taking both the radiative and nonradiative decay rates into account, the efficiency of 3 is possibly the largest among the three complexes as the radiative decay rate of 3 is much larger than those of 1 and 2. Moreover, the T1 excited energy of 2 is larger than that of 1, suggesting that the nonradiative decay rate of 2 is larger than that of 1. Thus, even though the radiative decay rate of 2 is larger than that of 1, the quantum efficiency of 2 is comparable with that of 1. Furthermore, according to Lee and co-workers,44 high triplet energy and short excited state lifetime are important factors for deep-blue dopant materials, whereas in our studied systems, 3 has the shortest triplet exciton lifetime as its radiative decay rate is the largest of the three complexes and, simultaneously, the triplet energy of 3 is the smallest among the three investigated complexes. Therefore, we make an effort to introduce substituents on 3 to improve its performance in OLED device as discussed under Effects of Substituents for Color Tuning. Effects of Substituents for Color Tuning. As discussed above, 3 has the highest quantum yield among the three studied complexes, and this is probably caused by the fact that the contribution of Ir d orbitals to the occupied molecular orbitals for 3 is larger than those for the other two complexes. Meanwhile, due to the introduction of electron-withdrawing benzo[d]imidazole ligand, the character of LC charge transfer takes part in the emission transition of 3. Thus, the degree of SOC for 3 will be improved and the corresponding ISC procedure will be much easier to take place than in 1 and 2. However, even though 3 possesses so many advantages in efficiency, its emission wavelength locates in the green region. Hence, to make the emitting color of 3 blue shift, we adopt some strategies to tailor complex 3 with the structural framework unchanged, and the corresponding modified structures are shown in Figure 6. To simplify the calculation, the frontier molecular orbital energy levels of complexes 4−9 are estimated as a preliminary prediction for the emission wavelengths. The calculated frontier molecular orbital (FMO) energy levels for 4−9 are shown in Figure 7, and the corresponding contour plots are collected in Figure S3 in the Supporting Information. Meanwhile, the corresponding properties of complex 3 are also given as a reference to display the results of modification. It is clear that the introduction of electron-withdrawing trifluoromethyl groups at the R1 site which has contributions to the HOMO can improve the degree of delocalization for complex 3; thus, both the HOMO and LUMO energy levels can be stabilized. However, when the electron-donating methyl

leads to a lower energy difference between the triplet and singlet excited states compared with the other two complexes. Furthermore, the d-orbital coefficients of 3 are also much larger than those of 1 and 2. Hence, even though the oscillator strengths of the studied singlet excited states are small, 3 has the largest radiative decay rate among the three complexes. T1 excited states of 1 and 2 have three configurations, and the transitions from HOMO to LUMO make 50 and 29% contributions, respectively, while that of complex 3 has two configurations and the contribution from HOMO to LUMO is 82%. Simultaneously, analysis on the electron distributions show that complex 3 has the largest d contributions of Ir to the occupied orbitals at T1 geometry, which indicates that 3 probably has higher MLCT character compared with the other complexes. When carrying out a horizontal comparison, it is very interesting to find that the component of radiative decay rates for complexes 1 and 3 is much larger in x substates when compared with y and z substates (the corresponding Cartesian coordinates are displayed in Figure 5). Such a result suggests

Figure 5. Cartesian coordinates displayed in molecular systems.

that complexes 1 and 3 experience larger admixtures between singlet and triplet excited states on x substates while there are smaller admixtures on the other two substates, especially z. Additionally, the similar distributions of the radiative decay rates for 1 and 3 could be probably ascribed to the introduction of the electron-withdrawing groups or the well-conjugated ancillary ligands. Meanwhile, the differences in radiative decay rates between the three substates for 2 are not so obvious as those for 1 and 3. As a result, it can be concluded that the orientation and strength of the ligand field may change with the introduction of the electron-withdrawing moiety or ligands. Furthermore, the calculated ZFS values for the three complexes are all larger than 1 cm−1, which exhibits that the systems we discussed here have significant MLCT characters,40 especially complex 3 whose ZFS value is about 44.80 cm−1. In addition, due to the defect of the TDDFT method in long-range CT states and artificial ignorance in calculating SOC matrix elements for simplicity, the calculated values are different from the experimental ones, but the tendency of the quantum yields is consistent with the experimental observations. Both kr and knr have influences on the phosphorescent quantum yields. Additionally, not only the geometry distortion but also the lowest triplet excited energy can reflect the nonradiative rate.21 As discussed above, the geometry 5064

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from 2.036 to2.032 Å. Additionally, for 8, the N−C bond discussed above consists two bonding types: one is similar to that for 3 while the other one has main contributions from the N atom in the carbene ring. The corresponding results of NBO45 analysis are displayed in Table S12 in the Supporting Information. Thus, when the methyl is substituted by trifluoromethyl, the interaction between the substituents and the carbene ring as well as the interaction between Ir and the carbene ring becomes stronger. Thus, both the HOMO and LUMO energy levels of 8 become destabilized, especially the LUMO; as a result, the HOMO−LUMO energy gap for 8 is widened. Additionally, we also calculate the HOMO and LUMO energy levels for complexes with increased delocalization on the cyclometalating ligands as well as other complexes with F and Cl as the substituents at R1 sites (the corresponding result for the complex with F as the substituent is shown in the Supporting Information). From the results, we can see that, although the three cases mentioned above can stabilize the HOMO/LUMO energy levels and widen the energy gap when compared with 3, the electronegativity scale has little influence on the HOMO/ LUMO energy levels and the HOMO−LUMO energy gap, and the increased delocalization in the cyclometalating ligands could not influence the energy gap remarkably. Analysis of the electron density distribution shows that the means employed above may make the overall electron distribution more localized, and when the excited states mainly arise from the HOMO to LUMO transition, MLCT/LC characters become more significant while LLCT character becomes smaller or even disappears. In general, we suspect that the emission spectra of complexes 4−8 will successfully blue shift while that of complex 9 will red shift with respect to 3. Additionally, the energy gap of complex 1 is 3.97 eV and its emitting color is blue. In the modified complexes, there are two complexes whose energy gaps are close to that of complex 1; thus, they had the potential to emit in the blue region. To test whether this prediction can become fact, we underwent a further investigation through the TDDFT method and the related results are listed in Table 6.

Figure 6. Chemical structures of complexes 4−9.

Figure 7. Calculated energy levels of complexes 4−9 at their respective optimized ground states.

moiety is introduced to the R2 site which contributes to the LUMO, both the HOMO and LUMO are destabilized. Moreover, it is interesting to note that the introduction of the electron-withdrawing groups at regions where the HOMO is distributed will have a greater impact on the HOMO, while the introduction of electron-donating groups at regions where the LUMO is distributed will have a greater impact on the LUMO. A similar observation has also been examined in ref 18. Thus, the methods mentioned above can widen the HOMO− LUMO energy gap, and such effects will be enhanced when mixing the two methods together as with complex 6. As a result, the modified complexes will be potentially blue shifted compared with 3. Usually, each C atom on the benzene ring adopts sp2 hybridization, and the six single electrons in the remaining nonhybridized p orbitals in the six C atoms delocalize above the plane of the benzene ring; thus, a single electron rather than lone pairs for each C atom in the benzene ring takes part in the delocalization. Unlike the above case, lone pairs in the N atom on the carbene ring delocalize with the single electron in the nonhybridized p orbital of the middle C atom. (Here, the middle C atom on the carbene ring is the one coordinated to the Ir metal center.) Meanwhile, when the methyl group attached to the N atom in the carbene ring is replaced by one trifluoromethyl moiety, the above delocalization may be destroyed due to the strong electron-withdrawing ability of the trifluoromethyl group. This can be confirmed by the fact that, comparing 8 with 3, the N−C bond length between the carbene ring and the trifluoromethyl group is shortened from 1.448 to 1.426 Å, and the coordinated bond length between the Ir ion and the carbene ring is also shortened

Table 6. Phosphorescent Emissions of T1 for Complexes 1, 3, 6, and 8 Calculated with the TDDFT Method, Together with the Available Experimental Values of Complexes 1 and 3 complex

calc (nm)

expt (nm)

ETa (eV)

1 3 6 8

539 615 590 592

455 530

2.30 2.01 2.10 2.09

main config (contrib) H H H H

→ → → →

L L L L

(50%), H − 1 → L (25%) (0.82%) (91%) (90%)

a

ET represents the vertical triplet energy of T1 excited state at optimized T1 geometry using TDDFT method with CH2Cl2 as the solvent.

The calculated results indicate that the emissions of complexes 6 and 8 are blue shifted indeed compared with 3; however, their emitting wavelengths have not been in the blue region relative to the blue phosphorescent emitter 1. The reason may result from the one-electron excitation configurations of the lowest triplet excited states. Usually, in TDDFT calculation, the excited wave functions can be approximately expressed as a linear combination of one-electron excitation 5065

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configurations.46 As a result, the eigenvalue of the excited wave function is not the one of the one-electron excitation configuration. Thus, even though the transition from the HOMO to the LUMO makes a great contribution to the excited states, the HOMO−LUMO energy gap as a judgment on emitting colors has its limitations. For the complexes we study here, the T1 state of 1 is mainly composed of two configurations: H → L and H − 1 → L. The contribution from the former is 50% while the contribution from the latter is 25%, whereas the T1 states of 6 and 8 have about 90% contributions from the HOMO to LUMO transition. Thus, there is no doubt that even though the energy gaps between the HOMO and the LUMO for complexes 1, 6, and 8 are similar around 3.97 eV, their emitting colors are quite different: the emission spectra of 6 and 8 may be in the blue-green region while that of 1 is in the blue region. Simultaneously, the result proves that 6 and 8 are successful in promoting the 3 blue shift though they not reach the blue region like 1. Additionally, the simplified configuration for 6 and 8 is favorable for the high quantum efficiency, and the remarkably improved triplet energy benefits the deep-blue emission for the OLED device.

withdrawing groups as well as the increase of the delocalization for the coordinated ligands is favorable for electron trapping, and thus, the energy loss will be restrained in the OLED device. The related deep investigations will be carried out in our following work.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S3; Tables S1−S15; results for calculations of other matrix elements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel.: +86-0431-85099108. Fax: +86-431-85684009. Notes

The authors declare no competing financial interest.

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■

ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from NSFC (20903020 and 21131001), 973 Program (2009CB623605), SRF for ROCS, and the Science and Technology Development Planning of Jilin Province (201201071).

CONCLUSIONS In the present work, we performed DFT/TDDFT calculations on molecular geometries, electronic structures, absorption spectra, d-orbital splitting, radiative decay rates, and ZFS values to understand the difference in the phosphorescent efficiencies for complexes 1−3. According to the discussions mentioned above, we can draw the following conclusions: (i) Compared with the electron-donating methyl group attached to the phenyl ring of the phenylpyridine ligand in 2, the electron-withdrawing F atom in 1 could widen the HOMO−LUMO energy gap, increase the d-orbital splittings, restrain the MC quenching, and lead to blue shift in the emission, although it may have little influence on the electron density distributions. In contrast, the increase of the conjugation on the ancillary ligand for 3 could greatly influence the electron density distributions, make the LC character participate in the emission transition, and lead to a larger Δdd* value. Thus, the MC emission quenching state is less thermally accessible for 3. Additionally, for complex 3, even though the oscillator strengths of the singlet excited states are smaller, the radiative rate and ZFS value are the largest of the three complexes due to the relatively smaller Sn−T1 energy difference and much larger d-orbital coefficients, indicating that all three parameters can reflect the final radiative decay rate. (ii) The calculated radiative decay rates are 0.4682 × 104, 0.5595 × 104, and 0.6962 × 104 s−1, and the obtained ZFS values are 6.60, 12.20, and 44.80 cm−1 for the three complexes, respectively, demonstrating that the introduction of the electron-withdrawing moiety and ancillary ligand could influence the ligand field. Such conclusions can provide directions for further designing highly efficient phosphors. (iii) Our modification on complex 3 achieves success in obtaining blue-shift emission and improving the triplet energy. Meanwhile, as the T1 states for our studied complexes have different transition configurations, it is not enough to only take the HOMO−LUMO energy gap as a criterion when judging the emitting color. Additionally, for different complexes, the introduction of substituents at regions where the FMO is distributed has a more significant influence on the related FMO energy level. Furthermore, the introduction of the electron-

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