Tuning the electronic and phosphorescence properties of blue-emitting iridium(III) complexes through different cyclometalated ligand substituents: a theoretical investigation.

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Cite this: DOI: 10.1039/c5dt00048c

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Tuning the electronic and phosphorescence properties of blue-emitting iridium(III) complexes through different cyclometalated ligand substituents: a theoretical investigation† Jieqiong Li,a Qing Zhang,a Hongqing He,b Li Wang*a and Jinglai Zhang*a The geometric and electronic structures, phosphorescence properties and the organic light-emitting diode (OLED) performance of a series of Ir(III) complexes based on bis[(4,6-di-fluorophenyl)-pyridinatoN,C2’]picolinate (FIrpic) were investigated by using density functional theory/time-dependent density functional theory (DFT/TD-DFT), including Ir(III)bis[2-(2,4-difluorophenyl)-4-(tert-butyl)pyridinato-N,C2’] picolinate (1a), Ir(III)bis[2-(2,4-difluorophenyl)-4-(n-heptyl)pyridinato-N,C2’]picolinate (2a), Ir(III)bis[2-(2,4difluorophenyl)-4-(3-ethylheptyl)pyridinato-N,C2’]picolinate

(3a),

Ir(III)bis[2-(2,4-difluorophenyl)-4-

(2,4,6-trimethylphenyl)pyridinato-N,C2’]picolinate (5a), and Ir(III)bis[2-(2,4-difluoro-3-(2,4,6-trimethylphenyl)phenyl)-pyridinato-N,C2’] picolinate (5b). To explore the influence of the substituted positions on the optical and electronic properties of the Ir(III) complexes, seven other new complexes were designed by introducing the substituted groups on the difluorophenyl rings or pyridine rings. After introducing the phenyl substituted groups on the pyridine or difluorophenyl rings of cyclometalated ligands, the HOMO– LUMO energy gap is decreased. Thus, the absorption spectra of 4a and 4b undergo a red-shifting, especially for 4a, and have a stronger absorption strength that will be beneficial to improving their quantum yields, which is proved by the further evaluation of the radiative (kr) and non-radiative (knr) rate Received 6th January 2015, Accepted 18th February 2015

constants. The combinations of a larger 3MLCT-3MC energy gap, smaller ΔES1–T1, and higher contribution

DOI: 10.1039/c5dt00048c

of 3MLCT in the emission process result in the higher quantum yields for complexes 4a and 4b. Besides, the designed complexes 4a and 4b are considered to be potential candidates as blue-emitting materials

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with better equilibrium between the hole transport (λhole) and electron transport (λelectron).

1.

Introduction

Phosphorescent organic light-emitting diodes (OLEDs) based on heavy transition-metal complexes have been attracting widespread attention in recent decades because they display bright phosphorescence emission spanning the whole visible spectra.1–3 Due to the strong spin–orbit coupling (SOC) introduced by the central transition metal atom, the spin-forbidden nature of the T1→S0 radiative relaxation can be removed to a large extent. As a result, the electrophosphorescence can achieve a theoretical level of unity for the internal quantum

a Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, P.R. China. E-mail: [email protected], [email protected] b Wuhan Center for Magnetic Resonance, State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5dt00048c

This journal is © The Royal Society of Chemistry 2015

efficiency by harvesting both the singlet and triplet excitons, which is three times higher than fluorescent materials.4 The short lifetime and low quantum efficiency of phosphorescent OLEDs are two important issues that impede the development of this promising technology. Considerable attention has been paid to searching for highly efficient red-green-blue emitters, which are the essential components for emitting various colors. Within the past decade, highly efficient red- or greenemitting materials have been developed well and been in the commercialization phase. However, due to the required wide energy gap between the excited triplet state and the ground state, designing blue phosphorescence emitters with high quantum yield and purity sufficient for commercial applications has encountered more obstacles.5,6 During the past few decades, the bidentate cyclometalated iridium complexes have been established as potential blue phosphorescence emitters due to their excellent emission wavelength tunability.7,8 The FIrpic9 is one of the representative blue emitters for phosphorescent OLEDs, however, its significant lowering of the quantum yield and poor equilibrium

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between the hole and electron transportation have hampered its further application. The modulation in the ligand structure is one of the efficient pathways to tune emission color and phosphorescence quantum efficiency.10,11 Inspired by this strategy, considerable effort has been devoted to the modification of the cyclometalating ligand. For instance, Zhao et al.12 proposed two artificial complexes on the basis of FIrpic by altering the conjugation length of the cyclometalating ligand from 2-(2,4-difluorophenyl)pyridine to 2-(2,4-difluorophenyl)isoquinoline and 2-(2,4-difluorophenyl)-3-(2,6-dimethylphenoxy)pyridine, respectively. An alternative method is to alter the auxiliary ligand. For example, Park et al.13 have successfully synthesized a series of phosphorescent Ir(III) complexes FIrmpic, FIrpia, and FIrprza by changing the pic auxiliary ligand in the complex FIrpic into mpic, pca, and prza ligands, respectively. Although these new synthesized Ir(III) complexes have a better quantum yield or a smaller deviation between the hole and electron transportation as compared with FIrpic, they exhibited a much-increased green-shifting with respect to the maximum emission of FIrpic, which is not beneficial to be the efficient true-blue phosphors. Recently, Bryce et al.14 synthesized a series of Ir(III) complexes, 1a, 2a, 3a, 5a, and 5b by introducing tert-butyl (R1), n-heptyl (R2), 3-ethylheptyl (R3), and 2,4,6-trimethylphenyl (R5) substituted groups into FIrpic, respectively, aiming to gain high-efficiency blue-emitting PhOLEDs and to improve their solubility. To elucidate the structure–property relation-

Scheme 1

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ship, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed to gain an in-depth understanding of the geometries, electronic structures, quantum yields, and electroluminescence (EL) properties. If the same substituted group is introduced into the pyridine ring or the difluorophenyl ring, what is the difference between them? And which position is more beneficial to improve efficiency? It is widely accepted that a blue shift of the emission will be induced by introducing electron-withdrawing substituents such as fluorine. However, the position of the substituents is also an important factor that should be considered. Baranoff et al.15 have stated that electron-donating groups can effectively replace electron-withdrawing substituents on the orthometallated phenyl to induce a blue shift of the emission in a series of phenylpyridine emitters. Both the nature of the substituted group and the substituted position should be considered to result in the blue shift of the emission. Inspired by previous work, we designed seven Ir(III) complexes 4a, 6a, 1b–4b, 6b (Scheme 1) by changing the substituted positions. Moreover, the properties of new designed complexes were also studied by the same theoretical methods. It is expected that the predicted structure–property relationships may give more insights into the core issues regarding the design and synthesis of new Ir(III) phosphors suitable for OLEDs with high phosphorescence quantum efficiency and good electroluminescence performance.

Sketch structures for all the investigated Ir(III) complexes.


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2. Computational details The neutral ground-state, radical-cation, radical-anion, and the lowest-lying triplet excited-state geometries for each complex were optimized by using density functional theory (DFT) with Becke’s three-parameter hybrid exchange functional16 combined with the Lee–Yang–Parr correlation functional (B3LYP)17 and unrestricted B3LYP. Vibrational frequencies were calculated to characterize the nature of the optimized structures. ‘None’ imaginary frequency on the optimized geometries indicated that each configuration was a minimum on the potential energy surface. To investigate the absorption and emission spectral properties, time dependent DFT (TDDFT) calculations18,19 associated with the polarized continuum model (PCM)20,21 in toluene media were performed on the basis of the respective ground state (S0) and lowest triplet excited state (T1) equilibrium geometries. In order to choose a more reliable method, four different functionals, the hybridtype Perdew–Burke–Ernzerhof exchange correlation functional (PBE0),22 B3LYP,16,17 CAM-B3LYP,23 and M06-2X,24 were performed on the absorption and emission spectra calculations for 1a–3a, 5a, and 5b, respectively. Furthermore, one more functional, LC-BLYP25,26 was also employed to simulate the absorption spectra with a range-separation parameter of 0.18 since it has been successfully applied to other Ir(III) complexes.27,28 The fitted Gaussian type absorption curves obtained from the different methods are displayed in Fig. S2† and Fig. 4 with full width at half maximum (FWHM) of 0.4 eV. As compared with the experimental results, the PBE0 and M06-2X are more appropriate in the absorption and emission calculations, respectively (the detailed discussions are presented in the Results and discussion section). The lowest 50 singlet–singlet and 20 singlet–triplet excitations on the basis of the S0 and T1 optimized geometries were calculated respectively to gain insight into the nature of the absorption and emitting states. In terms of the basis set, the “double-ζ” quality basis set consisting of Hay and Wadt’s effective core potentials (LANL2DZ)29,30 was employed for the Ir atom and 6-31G(d)31 for all the non-metal atoms. A relativistic effective core potential (ECP) on Ir(III) replaced the inner core electrons, leaving the outer-core [(5s25p6)] electrons and the [(5d6)] valence electrons. All the calculations were performed without symmetry restrictions using the Gaussian 09 software package.32

3. Results and discussion 3.1. Geometries in the ground state and the lowest-lying triplet excited state The schematic structures of the investigated complexes FIrpic, 1a–6a (1a–6a indicate 1a, 2a, 3a, 4a, 5a, and 6a, the same hereafter), and 1b–6b are depicted in Scheme 1 along with the numbering of some key atoms. All Ir(III) complexes are coordinated with three ligands, two primary ligands (dfppy) and one ancillary ligand ( pic). According to the different positions


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of pyridine and difluorophenyl rings in the two primary ligands, there are four possible stereoisomers for each complex. They can be classified into four types with the N1–N2 (A), N1–C2 (B), C1–N2 (C), and C1–C2 (D) from two dfppy ligands in the trans-direction. Here, the “trans” notation is used more liberally, meaning the angles of N1–Ir–N2, N1–Ir– C2, C1–Ir–N2, and C1–Ir–C2 are in a range of 175°–179°. The four configurations of FIrpic were calculated as an example. At the B3LYP/6-31G(d)-LANL2DZ level, isomer A is the most energetically favorable followed by configuration B with 1.34 kcal mol−1 higher in energy. The energy of isomer C is 3.89 kcal mol−1 higher than that of isomer B. Isomer D is the most unstable one with a deviation of 14.95 kcal mol−1 as compared with isomer A. Owing to the ligands of all studied complexes having the same skeleton, the substitutions in the family will not have a substantial impact. So the isomers of other complexes will present the same sequence, which has been testified by Brédas et al.33 Both the energy sequence and the energy difference among four isomers are similar to the results for other FIrpic-like Ir(III) complexes.33 Thus, only the most energetically favorable configuration A is considered for other complexes in the following sections. As compared with the experimental geometry34 for complex 1a listed in Table S1 of ESI,† the geometric parameters are overestimated by the B3LYP method, which is also observed for other Ir(III) complexes based on the FIrpic derivatives.33,35 In this contribution, a larger basis set Def2-SVP36,37 was employed to perform the optimization for 1a. The geometric parameters are even further overestimated. To ensure a high ratio of performance/cost, the B3LYP/6-31G(d)(E)ULANL2DZ(Ir) is finally chosen in this work to optimize the geometries for all complexes. The main optimized geometrical parameters in ground-state S0 are summarized in Table 1. Moreover, the corresponding ball–stick models of fully optimized groundstate geometrical structures are presented in Fig. S1.† All these Ir(III) complexes with d6 configuration adopt a pseudo-octahedral coordination around the metal center with two N atoms (N1 and N2) from dfppy ligands residing at the trans location and C1 and C2 atoms located at the cis location to form the cis-C–C and trans-N–N chelate dispositions. Moreover, both the pyridine rings of dfppy ligands are almost perpendicular to the pic ligand with the bond angles of N1–Ir–O1 and N2–Ir–O1 being 93°–94° and 88°–89°, respectively. According to the calculated results, the introduction of different substitutions into the primary ligands does not cause a great variation on the key bond distances around the Ir(III) atom. In addition, for all the thirteen complexes, the bond lengths of Ir–N1/N2 are about 0.05–0.06 Å longer than that of Ir–C1/C2, which should be attributed to the different features of the N and C elements. A lone-pair of electrons for the hybrid N atom cause the Ir–N coordination bond to be formed through a pair of electrons from N entering into the unoccupied orbit of the Ir(III) ion, whereas the Ir–C bond is formed by sharing electrons because of the absence of the lone-pair of electrons for the hybrid C atom.38,39 The shorter distances indicate a stronger interaction between the difluorophenyl rings and the metal center.

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Table 1 Main optimized geometric parameters of all investigated Ir(III) complexes in the S0 and T1 states determined at the B3LYP/6-31G(d)(E)ULANL2DZ(Ir) level of theory

FIrpic

2a

3a

4a

5a

6a

T1

S0

T1

S0

T1

S0

T1

S0

T1

S0

T1

S0

T1

Bond length/Å Ir–C1 2.011 Ir–C2 2.013 Ir–N1 2.062 Ir–N2 2.072 Ir–N3 2.211 Ir–O1 2.180

1.982 2.002 2.044 2.089 2.257 2.167

2.010 2.013 2.060 2.072 2.208 2.182

1.981 2.003 2.045 2.086 2.253 2.170

2.010 2.013 2.061 2.074 2.206 2.184

1.981 2.003 2.043 2.088 2.250 2.172

2.010 2.013 2.061 2.074 2.206 2.183

1.982 2.004 2.043 2.089 2.247 2.172

2.010 2.013 2.060 2.072 2.206 2.182

1.981 2.000 2.040 2.090 2.259 2.162

2.010 2.013 2.061 2.073 2.208 2.182

1.981 2.002 2.042 2.088 2.251 2.169

2.010 2.014 2.061 2.071 2.209 2.182

1.983 2.004 2.038 2.089 2.247 2.170

Bond angle/° C1–Ir–O1 171.56 C1–Ir–C2 91.03

168.83 94.07

171.55 90.89

168.63 94.06

171.42 91.09

168.70 94.25

171.41 91.05

168.84 93.62

171.48 90.75

168.16 94.24

171.64 90.84

168.79 93.94

171.33 91.41

169.20 93.64

S0


1a

1b

2b

3b

4b

5b

6b

T1

S0

T1

S0

T1

S0

T1

S0

T1

S0

T1

Bond length/Å Ir–C1 2.007 Ir–C2 2.010 Ir–N1 2.061 Ir–N2 2.074 Ir–N3 2.207 Ir–O1 2.183

1.976 1.997 2.045 2.090 2.257 2.169

2.010 2.013 2.061 2.074 2.208 2.184

1.977 1.999 2.047 2.089 2.260 2.170

2.010 2.012 2.062 2.074 2.209 2.181

1.977 1.999 2.046 2.089 2.260 2.170

2.008 2.011 2.061 2.074 2.207 2.182

1.976 1.997 2.047 2.089 2.259 2.170

2.009 2.011 2.061 2.073 2.208 2.183

1.978 1.998 2.046 2.089 2.258 2.170

2.009 2.010 2.061 2.073 2.210 2.182

1.979 2.000 2.045 2.088 2.253 2.173

Bond angle/° C1–Ir–O1 171.54 C1–Ir–C2 91.15

168.43 94.56

171.63 90.83

168.79 94.80

171.39 91.40

168.37 94.84

171.43 91.01

168.27 94.83

171.30 91.10

168.56 94.60

171.37 92.21

169.16 95.18

S0

Another common point for all complexes is that the bond lengths of Ir–N3 and Ir–O1 are longer than those of Ir–N1/N2 and Ir–C1/C2, which can be rationalized by the trans effect within these Ir(III) complexes.40 The ancillary ligand is located at the trans-position of the difluorophenyl rings of two dfppy ligands, while the two pyridine rings of two ppy ligands are in the trans-direction. The trans influence of the difluorophenyl groups is larger than that of the pyridine groups or other groups, resulting in the longer Ir–N3 and Ir–O1 distances.39 To observe the changes of geometry structures upon excitation, the geometry parameters of all complexes in the lowestlying triplet states (T1) are also listed in Table 1. Simultaneously, the variations of the Ir(III)-related coordination bond lengths between the ground (S0) and excited (T1) states for the investigated complexes are plotted in Fig. 1. All six metal– ligand bond lengths in the T1 state are contracted in a range of 0.01–0.03 Å as compared with those in the S0 state, except for the Ir–N2 and Ir–N3 bonds. In contrast, the Ir–N2 and Ir–N3 bonds are elongated, which suggests the larger involvement from one primary ligand in the T1 state rather than from the other one and the secondary ligand. Moreover, the shortened distances are helpful to decrease the metal-centered (MC) nonradiative decay that accounts for the higher efficiency of some complexes. 3.2.

Electronic structure

To explore the influence of different substitutions on the electronic structures especially for the FMOs (Frontier Molecular

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Orbitals), the contour plots of the FMOs and energy levels for all complexes are shown in Fig. 2. The HOMOs of all complexes present similar features which are principally localized on the Ir(III) d-orbital and two phenyl rings of the dfppy ligands. In addition, the LUMOs primarily distribute over the pyridine ring of one of the dfppy ligands and the pic ligand rather than two pyridine rings, which is aroused by the different positions of two pyridine rings. There is no electron density distribution over the one located at the trans-position of the pyridine ring of the pic ligand because of the inductive effect caused by the electron-withdrawing pyridine group of the pic ligand. Except for the above common properties, the LUMO of complex 4a extends over one substituted phenyl ring attached on the pyridine ring of the dfppy ligand because of the mesomeric effect caused by the substituted phenyl group (R4). The influence of the substituents is usually described in terms of the mesomeric and inductive effects.41 Except for the R-phenyl group, the inductive effects of other substituted groups R1–R3, R5, and R6 are stronger than their mesomeric effects, and because they are the electron-donating groups, no electron density is extended over these substituted groups. Simultaneously, the influence of different substitutions on the energy level is also investigated. Taking the energy level of FIrpic as a criterion, the energy levels of both HOMO and LUMO for all other complexes are destabilized with an exception of complexes 4a and 4b whose energy levels of LUMO are stabilized. As a result, the energy gaps of complexes 4a and 4b are the lowest one among their respective groups. In addition,


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Fig. 1 Calculated bond length variations between the lowest singlet state and the lowest triplet state for all investigated Ir(III) complexes (the negative values represent the bond distances contracted in the T1 state, while the positive ones indicate an elongation for the bonds in the T1 state).

the energy levels of HOMO and LUMO are increased by different degrees because the substituted groups are grafted at the positions where the HOMO and LUMO have different electronic distributions. The energy level will be more affected when the substituted group is introduced into the position where the distribution of electron density is obvious. Neither HOMOs nor LUMOs of complexes 1a–6a have obvious electron density over the pyridine rings. When the different substituted groups are grafted on the two pyridine rings of dfppy ligands, the energy levels of HOMOs and LUMOs are increased to a similar extent with an exception of 4a. Since the electron density of LUMO for complex 4a is mainly extended over one pyridine ring of the dfppy ligand, the energy level of LUMO rather than that of HOMO is more affected. The LUMOs of complexes 1b–6b are less destabilized than their HOMOs by the introduction of various substitutions on two difluorophenyl rings because the HOMOs have obvious electron density on the phenyl moieties of dfpppy ligands while the LUMOs have few distributions over them. 3.3.

Comparison of performance in OLEDs

The charge mobilities and a comparable balance between the hole and electron transportation are two key factors to evaluate the performance of OLED devices. The ionization potential (IP) together with the electron affinity (EA) are usually employed to qualitatively evaluate the hole- and electron-injection properties, respectively.42,43 For photoluminescent materials, a smaller IP value means easier hole injection ability, while a larger EA value will facilitate electron injection. In addition, in the doped OLED device, the reorganization energy (λ) can be utilized to roughly weigh the mobility of charges due to the randomized molecular pack in the deposition process.44 The corresponding results of the IP, EA, and λ as well as the hole extraction potential (HEP), electron extraction potential (EEP), and “small-polaron” stabilization energy (SPE) for all the studied complexes are demonstrated in Table 2. The IP and EA can represent either vertical excitations


or adiabatic excitations, i.e., IP(v/a) and EA(v/a), while the symbol v indicates that the excitation energies are calculated based on the geometry of the neutral molecule and the symbol a indicates that the excitation energies are calculated based on the respective optimized structure for both the neutral and charged molecules. In addition, HEP is the energy difference between M (neutral molecule) and M+ (cationic), using the M+ geometric structure in the calculation, and EEP is the energy difference between M and M− (anionic), using the M− geometric structure in the calculation. Similarly, SPE is the difference between EA(a) and EA(v) (for electron) and IP(v) and IP(a) (for hole). As compared with FIrpic, all complexes 1a–6a/1b–6b have smaller IP values, which is consistent with their higher HOMO energy levels, and thus, their hole injection is easier than that of FIrpic. It indicates that the introduction of the alkyl/aryl groups into FIrpic can cause improvement of the hole injection capability. However, the substituted alkyl and aryl groups result in different situations for electron injection ability. Complexes 4a–6a/4b–6b have larger EA values and enhanced electron injection ability with respect to FIrpic, especially for 4a and 4b with the largest EA values. In contrast, the alkyl substitutions will lead to worse or comparable electron injection ability for complexes 1a–3a/1b–3b. On the basis of the above analysis, complexes 4a/4b have relatively smaller IP values and larger EA values, indicating that the introduction of phenyl substituted group will effectively refine the carrier-injection ability. It has to be mentioned that the trends of IP and EA are not strictly consistent with the orders of HOMO and LUMO energy levels, respectively. One possible reason is that the B3LYP functional cannot predict orbital energies accurately. Moreover, for all commonly used exchange–correlation functionals, the energy of HOMO is not a good approximation to the negative of experimental IP in contrast to the results of Hartree–Fock (HF) theory for which Koopmans’ theorem is valid.45 However, it is not an obstacle to predicting the most suitable complex with the best IP and EA values from the

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Fig. 2

Calculated energy-level diagram and contour plots of the HOMO and LUMO for all investigated complexes.

calculated energies of HOMO and LUMO since the estimated relative sequence is more reliable with the increase of the difference. Moreover, a generally applicable linear correlation exists between the calculated HOMO/LUMO energy and the corresponding experimental IP/EA value.46 Furthermore, it is possible to accurately reproduce experimental IPs with corrections.47 In this work, no corrections have been added to evalu-

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ate IP and EA, since the aim of this paper is to compare the relative trend of similar complexes rather than predict the exact IP and EA values. Although no corrections are considered for all complexes, the qualitative analysis of them is still reliable and can be compared with the experimental results. To evaluate the charge transfer rate and balance properties, the reorganization energy (λ) is estimated for all the investi-


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Table 2 Ionization potentials (IP, eV), electron affinities (–EA, eV), extraction potentials (HEP and EEP, eV), stabilization energy (SPE, eV), and internal reorganization energies (λ, eV) for all investigated Ir(III) complexes determined at the B3LYP/6-31G(d)(E)ULANL2DZ(Ir) level of theory

Species

IPv

IPa

HEP

SPE(h)

–EAv

–EAa

EEP

SPE(e)

λhole

λelectron

ΔSPE

Δλ

FIrpic 1a 2a 3a 4a 5a 6a 1b 2b 3b 4b 5b 6b

6.69 6.50 6.51 6.49 6.51 6.53 6.48 6.47 6.41 6.40 6.38 6.40 6.44

6.55 6.37 6.37 6.36 6.37 6.40 6.35 6.35 6.29 6.27 6.28 6.29 6.32

6.39 6.20 6.21 6.19 6.20 6.22 6.19 6.20 6.15 6.13 6.16 6.16 6.19

0.13 0.13 0.13 0.13 0.14 0.14 0.13 0.13 0.12 0.12 0.11 0.11 0.11

0.50 0.41 0.43 0.43 0.76 0.54 0.62 0.51 0.48 0.48 0.60 0.58 0.63

0.59 0.49 0.51 0.52 0.88 0.65 0.72 0.59 0.56 0.57 0.68 0.66 0.71

0.67 0.58 0.60 0.60 1.00 0.76 0.82 0.67 0.64 0.65 0.76 0.74 0.80

0.08 0.08 0.08 0.09 0.11 0.10 0.10 0.08 0.08 0.09 0.08 0.08 0.08

0.30 0.30 0.30 0.30 0.31 0.32 0.29 0.28 0.27 0.26 0.22 0.24 0.24

0.16 0.17 0.17 0.17 0.23 0.21 0.20 0.17 0.16 0.17 0.17 0.17 0.16

0.05 0.05 0.05 0.05 0.03 0.04 0.03 0.05 0.04 0.03 0.03 0.03 0.03

0.14 0.13 0.13 0.13 0.08 0.11 0.09 0.11 0.11 0.09 0.05 0.07 0.08

gated species. According to the Marcus/Hush model,48,49 the charge (hole or electron) transfer rate k can be expressed by the following formula:50 k¼

π λkB T

1=2

V2 λ exp 4kB T ℏ

ð1Þ

transfers can be simply defined by the following respective expressions: λhole ¼ ½Eþ ðMÞ EðMÞ ½Eþ ðMþ Þ EðMþ Þ ¼ IPv HEP ð2Þ λelectron ¼ ½EðM Þ E ðM Þ ½EðMÞ E ðMÞ ¼ EEP EAv ð3Þ +

where kB is the Boltzmann constant, T is the temperature, λ is the reorganization energy, and V is the coupling matrix element between the ions and molecules which is dictated by the overlap of orbitals. According to eqn (1), the intermolecular charge transfer rate k is determined by two factors, λ and V. Due to the limited intermolecular charge transfer range in the solid state, the mobility of charges has been demonstrated to be primarily associated with the reorganization energy λ for OLED materials.48 A small λ value is essential for an efficient charge transport process. Generally, the λ for hole and electron

as illustrated in Fig. 3, where E, E , and E represent the energies of the neutral, cation, and anion species, respectively, and M, M+, and M− denote the optimized geometries of neutral, cation, and anion, respectively. For all complexes, the reorganization energies for hole transport (λhole) are slightly larger than those for electron transport (λelectron), which reveals that the electron transport performance of these complexes is slightly better than hole transport ability. Moreover, when the substituted group is the same, the imbalances between λhole and λelectron for 1b–6b are much smaller than those for respective 1a–6a, demonstrating that the performance of the OLED devices is more affected by the substituted position than the substituted group. In addition, the Δλ of complexes 4a (0.08 eV) and 4b (0.05 eV) are the smallest ones among complexes 1a–6a and 1b–6b, respectively, revealing that 4a and 4b are the more potential emitters for OLED applications. 3.4.

Fig. 3 Schematic description of internal reorganization energy for the hole transfer.


−

Absorption spectra

The absorption properties were performed by TDDFT calculations with PCM in toluene media on the basis of the optimized ground-state geometry. Since the position and shape of absorption spectra greatly rely on the choice of the theoretical functional, five different DFT functionals with the same basis set 6-31G(d)(E)ULANL2DZ(Ir), i.e., TD-B3LYP, TD-PBE0, TD-CAM-B3LYP, TD-M06-2X, and TD-LC-BLYP, were employed to perform the calculations for 1a–3a, 5a, and 5b whose corresponding experimental values are available. The simulated Gaussian-type absorption curves are plotted in Fig. S2.† For all the five complexes, the experimental absorption spectra show intense absorption bands around 230–350 nm and an obscure moderate absorption band at 350–400 nm with long tails extending into the visible region about 450–475 nm. Neither

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the band shape nor the position obtained at the TD-B3LYP level agrees well with the experimental results. The band shapes simulated by other four TDDFT methods show good consistency with the experimental ones. However, the intense absorption band positions simulated at TD-CAM-B3LYP and TD-M06-2X levels are less than 250 nm, which deviates by ∼60 nm as compared with the experimental value of ∼280 nm. The TD-PBE0 and TD-LC-BLYP could be a valuable compromise for the excited-state as they give both reasonable singlet– singlet transition energies and band shapes. However, the intense absorption peaks obtained at the TD-PBE0 level of theory look less flat as compared with the experimental results. As a result, the electronic transition properties in the absorption process for other complexes are determined by the TD-LC-BLYP functional. The detailed information, such as excitation energies, oscillator strengths, dominant configurations, assignments, is listed in Table 3 and the simulated absorption curves of complexes 1a–6a/1b–6b are presented in Fig. 4. All complexes have one strong absorption peak, one moderate shoulder peak, and one weak absorption peak. Except for the common features, there are two distinct points for complexes 4a and 4b. One is that the absorption peaks of complex 4a present an obvious red-shifted phenomenon as compared with other complexes in group I (1a–6a), including both the maximum absorption and the strongest absorption. The absorption peaks of 4a located at 400 and 286 nm are mainly contributed by the HOMO → LUMO/HOMO−1 → LUMO+4 transitions, which are assigned as [d(Ir) + π( ph1 + ph2)→π*(R-ph + py1)]/[d(Ir) + p(a)→π*( py1 + py2)] with a ILCT (intraligand charge transfer)/LLCT (ligand-toligand charge transfer)/MLCT (metal-to-ligand charge transfer) character. Because the LUMO of complex 4a not only extends

over the pyridine ring of one dfppy ligand but also delocalizes on the substituted phenyl group, then, an additional ligand center (LC) charge transfer character is involved in the absorption process. Normally, LC charge transfer is always accompanied by a lower energy gap and corresponds to the red-shifted absorption phenomenon. The other point is that the oscillator strength of complex 4b is the largest in all complexes, which is beneficial for the SOC process. The strong SOC can remove the partial spin-forbidden nature of the T1→S0 radiative relaxation and promote the triplet to singlet radiative transition. 3.5.

Phosphorescence spectrum

On the basis of the optimized lowest triplet excited-state geometries, the phosphorescence properties of the investigated complexes were calculated in toluene solution provided by PCM. Similar to absorption spectra, four density functionals (B3LYP, M06-2X, CAM-B3LYP, and PBE0) were adopted to calculate the emission spectra for FIrpic, 1a–3a, 5a, and 5b. The results are plotted in Fig. 5 together with the available experimental values. Clearly, the emission energies are dramatically underestimated when performed using B3LYP, CAM-B3LYP, and PBE0 functionals, while the M06-2X gives more favorable results for all complexes and deviates from the experimental data by a trifling amount within 0–0.06 eV. Hence, the TD-M06-2X functional was employed to predict the emission spectra for other complexes. The detailed information, such as the calculated emission wavelengths, dominant configurations, the transition nature as well as the available experimental values, is listed in Table 4. To conveniently discuss the transition properties of emission spectra, the contributions of single orbital exci-

Table 3 Selected absorptions for Ir(III) complexes in toluene solution determined at the TD-LC-BLYP/6-31G(d)(E)ULANL2DZ(Ir)//B3LYP/6-31G(d)(E)ULANL2DZ(Ir) level of theory

Species

State

λcal (nm)

F

Main configuration

Assignment

1a

S1 S24 S1 S24 S1 S24 S1 S17 S1 S18 S1 S18 S1 S24 S1 S25 S1 S25 S1 S23 S1 S23 S1 S26

385 263 387 263 387 263 400 286 392 279 395 280 398 265 402 263 401 263 399 274 398 267 395 261

0.0787 0.3127 0.0807 0.2652 0.0830 0.2855 0.0871 0.5100 0.0890 0.3524 0.0994 0.2665 0.0599 0.1668 0.0630 0.1868 0.0639 0.1893 0.0696 0.6094 0.0637 0.4486 0.0631 0.3346

H → L+1 (60%) H−2 → L+4 (49%) H → L+1 (44%) H−2 → L+4 (53%) H → L+1 (45%) H−2 → L+4 (55%) H → L (83%) H−1 → L+4 (60%) H → L (67%) H−1 → L+5 (25%) H → L (79%) H−1 → L+5 (46%) H → L (61%) H → L+6 (29%) H → L (59%) H−3 → L+4 (29%) H → L (57%) H−3 → L+4 (29%) H → L (69%) H−1 → L+5 (24%) H → L (65%) H → L+6 (34%) H → L (65%) H−3 → L+4 (29%)

d(Ir) + π(ph1 + ph2)→π*(ph1 + py1 + py3)/LL/MLCT d(Ir) + π(ph1 + py1)→π*(py1 + py2)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1 + py3)/LL/MLCT d(Ir) + π(ph1 + py1)→π*(py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1 + py3)/IL/LL/MLCT d(Ir) + π(ph1 + py1)→π*(py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(py1 + R-ph)/IL/LL/MLCT d(Ir) + p(a)→π*(py1 + py2)/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1 + py3)/IL/LL/MLCT d(Ir) + p(a)→π*(py2 + py3)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1 + py3)/IL/LL/MLCT d(Ir) + p(a)→π*(py2 + py3)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(py1 + py3)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + ph2)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(py1 + py3)/IL/LL/MLCT d(Ir) + π(ph1)→π*(py1 + py2)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(py1 + py3)/IL/LL/MLCT d(Ir) + π(ph1)→π*(py1 + py2)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1 + py3)/IL/LL/MLCT d(Ir) + p(a)→π*(py2 + py3)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1 + py3)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + R-ph)/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(py1 + py3)/IL/LL/MLCT d(Ir) + π(ph1)→π*(py1 + py2)/IL/LL/MLCT

2a 3a 4a 5a 6a 1b 2b 3b 4b 5b 6b

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tations to the S0→T1 excitation are depicted in Fig. S3 and S4.† The emission wavelengths are in a range of 458–476 nm, indicating that all the studied complexes are potential candidates for blue-emitting materials. All the calculated lowest-energy emissions are dominantly originated from the SOMO → L transition designated as [d(Ir) + π( ph1 + ph2)→π*( ph1 + py1)] with 3 ILCT/3LLCT/3MLCT characters except for 4a. While the transition character of 458 nm emission for 4a is designated as 3 MLph1+py1+R-ph/3Lph1+ph2Lph1+py1+R-ph/3ILph1+py1CT. The remarkable involvement of the 3MLCT transition in emission for the studied complexes is beneficial for a high quantum yield. It is interesting to note that the phosphorescence nature is dominated by the cyclometalating ligands rather than the ancillary ligand, which is consistent with the contracted distances between Ir and primary ligands. The conclusion has been demonstrated in several literature studies that for Ir(III) compounds the contracted distance of the T1 state will facilitate the MLCT transition in emission.51,52 Therefore, the modification of the cyclometalating ligands is an efficient approach to control molecular phosphorescence properties. 3.6.

Phosphorescence quantum efficiency

The phosphorescence quantum yield (Φem) is affected by the competition between the radiative (kr) and non-radiative (knr) rate constants, which can be expressed by the following equation, where τem is the emission decay time: Φem ¼ kr τem ¼ kr =ðkr þ knr Þ

Fig. 4 Simulated absorption spectra of the investigated Ir(III) complexes in toluene solution determined at the TD-LC-BLYP/6-31G(d)(E)ULANL2DZ(Ir)//B3LYP/6-31G(d)(E)ULANL2DZ(Ir) level of theory.

Fig. 5 Emission energies (eV) for FIrpic, 1a–3a, 5a and 5b at B3LYP, M06-2X, PBE0, and CAM-B3LYP levels, respectively, together with the experimental values.14


ð4Þ

To ensure a high quantum yield, a large kr and a small knr are required. In this section, we investigate the Φem in detail through a respective discussion on the qualitatively variation of kr and knr. 3.6.1. Comparison of radiative rate constants (kr). According to the Einstein theory of spontaneous emission, kr is proportional to (ET1)3|MT–S|2,53,54 where ET1 is the emitting energy and |MT–S| is the transition dipole moment from the triplet state. The emitting energies of complexes 5a and 5b are comparable, with a small deviation. However, the kr of complex 5a is three times that of 5b. Thus, the other related factor |MT–S| plays a very important role in the emission process. Since the participation of the heavy atom center is the key to breaking the spin-forbidden nature of singlets and triplets and results in the phosphorescence of these transition metal complexes, the kr is related to the spin–orbit coupling (SOC) effects in the emission process. The SOC effects are evaluated by the following two aspects. One is the percentage of metal character (3Mc, %) in the lowest singlet–triplet transition.55 An increase of metal center character can enhance SOC and hence result in a larger kr, which is beneficial to decreasing the radiative lifetime and avoiding the non-radiative process.56,57 We summarized the calculated metallic character (3Mc, %) for the corresponding excitations in the second column of Table 5. The metallic contribution is increased provided that the substituted group is grafted on the pyridine ring. In contrast, it will be decreased if

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Table 4 Selected phosphorescence emissions of Ir(III) complexes in toluene solution determined at the TD-M06-2X/6-31G(d)(E)ULANL2DZ(Ir)// B3LYP/6-31G(d)(E)ULANL2DZ(Ir) level of theory together with the experimental values14

Species

State

λcal (nm)

Main configuration

Assignment

λExpt. (nm)

1a 2a 3a 4a 5a 6a 1b 2b 3b 4b 5b 6b

T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1 T1

458 465 466 458 465 474 465 471 470 465 466 476

SOMO → L (60%) SOMO → L (60%) SOMO → L (59%) SOMO → L (50%) SOMO → L (59%) SOMO → L (59%) SOMO → L (57%) SOMO → L (61%) SOMO → L (61%) SOMO → L (58%) SOMO → L (57%) SOMO → L (62%)

d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1 + R-ph)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT d(Ir) + π(ph1 + ph2)→π*(ph1 + py1)/IL/LL/MLCT

467 465 464

Table 5 Computed emitting energy (ET1), singlet–triplet splitting energy (ΔES1–T1), and metal-based charge transfer character (3Mc) (%) for the investigated complexes, together with the radiative (kr) and nonradiative (knr) rate constants for 5a and 5b and the experimental quantum efficiency (Φem, %)

Species

ET1

3

FIrpic 1a 2a 3a 4a 5a 6a 1b 2b 3b 4b 5b 6b

2.30 2.34 2.31 2.31 2.28 2.31 2.27 2.28 2.25 2.25 2.26 2.27 2.26

33.13 32.77 33.14 33.35 33.55 33.16 33.44 32.64 32.15 32.06 25.50 27.19 30.48

Mc (%)

ΔES1–T1 (eV) 0.424 0.434 0.452 0.447 0.310 0.415 0.431 0.376 0.393 0.391 0.376 0.391 0.452

kr a (×106 s−1)

knr a (×106 s−1)

Φem a 0.54 0.68 0.66 0.71

1.07

0.09

0.92

0.39

0.32

0.55

a

Obtained from the experimental14 quantum yield (Φem) and lifetime (τ), kr = Φem/τ and knr = (1 − Φem)/τ.

the same substituted group is introduced into the difluorophenyl position. Correspondingly, the calculated metallic character of 5a is higher than that of 5b, which accounts for the experimental result that the kr of 5a (kr = 1.07 × 106 s−1) is larger than that of 5b (kr = 0.39 × 106 s−1). Another aspect is the singlet–triplet energy gap (ΔES1–T1).58 Since the ISC rate decreases exponentially as the singlet–triplet splitting energy increases,59 a minimal difference between the S1–T1 splitting energy is favorable for enhancing the ISC rate, leading to an increased kr. Therefore, it would be informative to obtain further insights into the evolution of kr by concentrating on the singlet–triplet ISC rate for these complexes. It is important to point out that it is not only S1 that can contribute to the intensity borrowing by T1. According to eqn (5),60 3

kr / ð E1 Þ 3

( X

) 1

½, ϕ1 jH SO j ϕi > =ð Ei E1 Þ 3

1

3

1

f i =1 Ei

ð5Þ

i

the radiative rate constant (in some approximations) indeed contains a sum over all the singlet states, where 1ϕi and 3ϕ1

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473

475

and 1Ei and 3E1 are the wavefunctions and energies, respectively, of the singlet manifold of excited states and of the lowest triplet excited state, HSO is the spin–orbit coupling operator and 1fi is the oscillator strength of the singlet–singlet excitations, which is calculated together with the transition energies by TDDFT. Moreover, to ensure the non-vanishing of the spin–orbit coupling element in the specific case of MLCT transitions, two restrictions should be followed: (i) the coupled triplet and singlet excited states need to involve the same π* orbital and (ii) they need to involve metal t2g orbitals with different symmetries.61,62 To gain insight into the nature of SOC, the lowest singlet–singlet (S0–S1 and S0–S2) and singlet– triplet (S0–T1) excitation energies at the optimized geometries of the ground state and lowest triplet state are calculated and listed in Table S2.† On the basis of the considerations, coupling between S0–T1 and S0–S1 should be mainly responsible for the emission because of the substantial oscillator. Since the lowest triplet and singlet excited states involve Ir t2g orbitals with the same symmetry, their coupling is ensured as far as the triplet and singlet excited states involve the same π* orbital. As a result, only the contribution from S1 is considered to estimate the transition dipole moment (|MT–S|). Actually, the value of kr is underestimated when the contributions from other singlet–singlet states are omitted. However, it will not affect the relative sequence for these complexes with the same skeleton. Since the S0–S2 transitions of all complexes have close oscillator strengths, excited energies, and the same orbital symmetry, it is reasonable to expect similar contributions from them to kr. Consequently, the qualitative judgment for the sequence of kr will not be affected. Two obvious points are derived: (1) the ΔES1–T1 values for complexes 4a, 1b, and 4b are the three lowest ones in all complexes, which is helpful to increase their kr ; (2) the ΔES1–T1 value of complex 5a is larger than that of 5b, which is inverse with the sequence of kr. Thus, the effect of non-radiative rate constants (knr) is not negligible. 3.6.2. Comparison of non-radiative rate constants (knr). Next, let us turn our attention to the non-radiative rate constants knr that is the other key factor in controlling the quantum yield (Φem). The population of the metal-centered (3MC d–d) triplet excited states is considered to be one of the


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Ir–C1 Ir–C2 Ir–N1 Ir–N2 Ir–N3 Ir–O1

Paper

Selected bond distances (in Å) calculated for the studied complexes in the metal-centered (3MC) triplet excited states

FIrpic

1a

2a

3a

4a

5a

6a

1b

2b

3b

4b

5b

6b

2.035 2.029 2.481 2.258 2.216 2.244

2.036 2.029 2.475 2.248 2.216 2.250

2.036 2.030 2.475 2.245 2.219 2.252

2.036 2.029 2.478 2.245 2.217 2.253

2.035 2.029 2.475 2.245 2.216 2.250

2.034 2.029 2.481 2.255 2.214 2.240

2.035 2.031 2.485 2.256 2.217 2.240

2.033 2.026 2.474 2.254 2.219 2.246

2.034 2.028 2.477 2.260 2.218 2.244

2.034 2.028 2.480 2.261 2.218 2.242

2.033 2.027 2.473 2.252 2.220 2.249

2.033 2.028 2.476 2.252 2.219 2.251

2.031 2.027 2.483 2.259 2.216 2.237

most important deactivation pathways of the phosphorescence emission from T1 states in transition-metal complexes.63 Additional calculations were carried out to obtain the lowest 3 MLCT/π–π* and 3MC d–d states for all the investigated complexes to gain insight into their different quantum yields. The 3 MLCT/π–π* excited states were obtained by performing an unrestricted triplet optimization on the basis of the optimized ground-state geometries, while the electronic configurations of 3 MC d–d states were calculated starting from the distorted molecular geometries by largely elongating the metal–ligand bond length following the literature.64 In Table 6, we list the calculated metal–ligand bond distances in 3MC d–d states. It shows that the geometric discrepancy from S0 to 3MLCT/π–π* states is relatively small. In contrast, the distortion of the geometry from S0 to 3MC d–d states is significant, especially for the Ir–N1 and Ir–N2 bonds. They are elongated by 0.42 and 0.18 Å in 3MC d–d states, respectively, as compared to their ground states (Tables 1 and 6). The significant extent of distortion in the 3MC d–d state will increase the non-radiative probability due to the very weak chelating interaction between the metal and ligands induced by the elongated bond distances. This will ultimately decrease the quantum yield that is inversely proportional to the knr. Besides, the lowest triplet metal-to-ligand charge transfer (3MLCT/π–π*) excited state can be rapidly converted thermally to the short-lived triplet 3MC d–d state, from which no photochemistry occurs.65 Moreover, the conversion is often irreversible, due to the very fast non-radiative decay back to the ground state from the 3MC d–d state.64 Therefore, a far energy separation between the 3MC d–d state and the corresponding 3 MLCT/π–π* emissive state is regarded as an efficient way to reduce non-radiative transition. The calculated relative energy differences between the 3MC d–d state and the corresponding 3 MLCT/π–π* emissive state are shown in Fig. 6 with the normalized S0 levels. For all the investigated complexes, the relative energy of the 3MC d–d state is higher than that of the lowest-lying 3MLCT/π–π* excited state. It means upon excitation, the 3MLCT/π–π* → 3MC d–d → S0 radiationless pathway has a negligible possibility of happening, which would result in a relatively low knr. From the viewpoint of substituted position, grafting the substituted groups on the difluorophenyl rings will stabilize the 3MLCT/π–π* state and lead to a larger energy gap between 3 MLCT/π–π* and 3MC d–d states (except for 2a and 2b) as compared with those on pyridine groups. Furthermore, complexes 4a, 3b, and 4b have larger energy gaps, indicating that the


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

phenyl group and 3-ethylheptyl group have a positive influence on decreasing the knr. Considering both the kr and knr, complexes 4a and 4b will have the better quantum yields with a relative large kr and low knr. As a result, introducing the phenyl group looks like a more favorable route to increasing the quantum yield, especially on the pyridine ring.

4.

Conclusion

In this article, we have carried out DFT and TDDFT investigations on the structures, spectral properties and phosphorescence efficiency of recently synthesized blue-emitting Ir(III) complexes FIrpic, 1a–3a, 5a, and 5b and newly designed complexes 4a, 6a, and 1b–6b. According to the detailed discussion above, the following conclusions can be drawn: both the substituted group and position have a slight effect on the geometric parameters. Except for the Ir–N2 and Ir–N3 bonds, other metal–ligands bond distances in the T1 state are greatly contracted, which is beneficial to decrease the possibility of non-radiative. The smaller HOMO–LUMO energy gap is gained by stabilizing the LUMO energy level for complexes 4a and 4b. So their absorption spectra present the red-shifting and strongest oscillator strength situations. However, the emission spectra are slightly influenced by switching the substituted group and position. In addition, the larger 3MLCT-3MC energy gap, smaller ΔES1–T1, and higher contribution of MLCT in the emission process result in the higher quantum yields for

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complexes 4a and 4b. Moreover, the quantum yields for complexes 1a–6a in group I are all higher than the corresponding complexes 1b–6b in group II, which proves that the quantum yields are greatly affected by the substituted position rather than the substituted group. Simultaneously, the phosphorescence EL efficiency is also more related to the substituted position. Grafting a substituted group on the difluorophenyl ring will have a smaller value for Δλ than that on the pyridine ring. Combining all aspects discussed in this work, the designed complexes 4a and 4b might be the promising blueemitting materials with higher quantum yields and smaller differences between λhole and λelectron as compared with 5a and 5b, which are the potential candidates recommended by experiments. So the strategy of introducing the conjugated substituted group in FIrpic may provide valuable clues for further experiments.

Acknowledgements We thank the Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences for providing computational resources. This work was supported by the National Natural Science Foundation of China (21376063, 21476061), the Natural Science Foundation of He’nan Province of China (134300510008, 144300510032, 142300410120), the Science Foundation of Henan Province (14A150034), and the Foundation for University Key Teachers from the He’nan Educational Committee.

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Dalton Trans.

trans isomerism.

Cyclometalated iridium(III) complexes with deoxyribose substituents.

Tuning the electronic coupling in cyclometalated diruthenium complexes through substituent effects: a correlation between the experimental and calculated results.

Tuning reactivity and electronic properties through ligand reorganization within a cerium heterobimetallic framework.

Tuning the electronic properties in ruthenium-quinone complexes through metal coordination and substitution at the bridge.

Tuning the luminescent properties of Pt(II) acetylide complexes through varying the electronic properties of N-heterocyclic carbene ligands.

Heteroleptic cyclometalated iridium(III) complexes supported by triarylborylpicolinate ligand: ratiometric turn-on phosphorescence response upon fluoride binding.

Phosphorescent, Cyclometalated Cinchophen-Derived Platinum Complexes: Syntheses, Structures, and Electronic Properties.

Exploring Second-Order Nonlinear Optical Properties and Switching Ability of a Series of Dithienylethene-Containing, Cyclometalated Platinum Complexes: A Theoretical Investigation.

Manipulation of phosphorescence efficiency of cyclometalated iridium complexes by substituted o-carboranes.

Cyclometalated iridium(III) complexes for phosphorescence sensing of biological metal ions.

Deep red phosphorescence of cyclometalated iridium complexes by o-carborane substitution.

Experimental and theoretical investigation of the spectroscopic and electronic properties of pyrazolyl ligands.

Dinuclear palladium complexes with two ligand-centered radicals and a single bridging ligand: subtle tuning of magnetic properties.

Thiocyanate-free cyclometalated ruthenium(II) sensitizers for DSSC: a combined experimental and theoretical investigation.

Tuning the optical properties of 2-thienylpyridyl iridium complexes through carboranes and anions.

Tuning the emission lifetime in bis-cyclometalated iridium(III) complexes bearing iminopyrene ligands.

Fine-tuning of single-molecule conductance by tweaking both electronic structure and conformation of side substituents.

Investigation of VO-salophen complexes electronic structure.

Influence of Electron-Withdrawing Substituents on the Electronic Structure of Oxidized Ni and Cu Salen Complexes.

Rhodium, iridium and nickel complexes with a 1,3,5-triphenylbenzene tris-MIC ligand. Study of the electronic properties and catalytic activities.

Cyclometalated iridium(III) bipyridyl-phenylenediamine complexes with multicolor phosphorescence: synthesis, electrochemistry, photophysics, and intracellular nitric oxide sensing.

Tuning thiophene with phosphorus: synthesis and electronic properties of benzobisthiaphospholes.

Tuning the Electronic Structure of Anatase Through Fluorination.