Theoretical study on electronic structures and optical properties of blue phosphorescent Iridium(III) complexes with C∧N and N∧N ligands

Journal of Luminescence 143 (2013) 402–408

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Theoretical study on electronic structures and optical properties of blue phosphorescent Iridium(III) complexes with C∧N and N∧N ligands Xiaohong Shang a,b, Yuqi Liu a, Xiaochun Qu a, Zhijian Wu a,n a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China b College of Chemistry and Life Science, Changchun University of Technology, Changchun 130024, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 24 March 2013 Received in revised form 11 May 2013 Accepted 30 May 2013 Available online 10 June 2013

We report a quantum-chemical study on the electronic structures and optical properties of two series of heteroleptic iridium(III) complexes [(dfb-pz)2Ir(N∧N+sub)], [dfb-pz ¼2,4-difluorobenzyl-N-pyrazole, sub indicates substituent group, N∧N+sub ¼tphppz ¼4-tert-butyl-2-(5-phenyl-[1,2,4]triazol-3-yl)-pyridine (1a), tmppz ¼4-tert-butyl-2-(5-methyl-[1,2,4]triazol-3-yl)-pyridine (1b), fphppz ¼ 4-fluoro-phenyl-5-(2pyridyl)pyrazole (1c), and fmphppz ¼4-trifluoromehtyl-phenyl-5-(2-pyridyl)pyrazole (1d)]; with [(C∧N +sub)2Ir(fppz)], [C∧N ¼b-pz ¼benzyl-N-pyrazole, fppz ¼3-trifluoromethyl-5-(2-pyridyl)pyrazole, C∧N +sub ¼dfb-pz ¼2,4-difluorobenzyl-N-pyrazole (2a), tfmfb-pz ¼2-trifluoromethyl-5-fluorobenzyl-N-pyrazole (2b), phb-pz¼ 3-phenyl-benzyl-N-pyrazole (2c), and dfphb-pz ¼ 3-phenyl-2,4-difluorobenzyl-Npyrazole (2d)]. The calculated results shed light on the reasons of the remarkably manipulated excitedstate and electroluminescent properties through substitution effect. The phenyl ring on main ligands can enhance the π-conjugation of the main ligands moiety and increase the metal-ligand bond strength for 2c and 2d, then enhancing the transition strength. From 1c, 1d, 2c, and 2d, it can also be seen that substituents on the terminal phenyl ring have a slight effect on the excited energy because the distance between the substituents and the ancillary (or main) ligand is interrupted by the phenyl moiety. The calculated absorption and luminescence properties of the four complexes 1a, 1b, 2a, and 2b are compared with the available experimental data and a good agreement is obtained. Furthermore, the assumed complex 1c, 2c, and 2d possess better charge transfer abilities and more balanced charge transfer rates. The designed complexes 2c and 2d are potential candidates for blue phosphorescent materials. & 2013 Elsevier B.V. All rights reserved.

Keywords: Ir(III) complexes Absorption Emission Phosphorescent DFT TDDFT

1. Introduction Phosphorescent metal complexes have become the research core in the field of optoelectronic materials since the Pt-porphyrin complexes were successfully applied in organic light emitting diodes (OLEDs) in 1998 [1]. Although the second and third row transition metal complexes (such as Ru(II), Pd(II), Os(II), Re(I) and Ir (III)) have been most studied in this area, the high costs and environmental concerns result in inherent limitations [2–4]. Therefore, an attractive alternative is emerging, Ir(III) complexes, which are developed to solve the problems. Phosphorescent iridium(III) complexes have attracted considerable attention because of their intriguing photophysical properties and potential applications in

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0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.05.046

the fabrication of organic light-emitting diodes (OLEDs) [5–9]. Due to the heavyatom-induced spin-orbit coupling effects, which can partially remove the spin-forbidden nature of the T1-S0 radiative relaxation, these Ir(III) complexes can harvest both singlet and triplet excitons as light, leading to a theoretical level of unity for internal quantum efficiency in phosphorescent OLEDs [10]. Therefore, the Ir(III)-based phosphorescence complexes can reach an efficiency four times higher than fluorescent materials [11–13]. Additionally, these Ir(III) complexes would display bright phosphorescent emission spanning the entire visible spectra, making it possible to realize the full-color displays. Developing highly efficient phosphors that can emit all three primary colors is the key factor to achieve full-color displays. Highly efficient green- and red-lightemitting OLEDs based on cyclometalated Ir(III) complexes have been reported in the literatures [14–17]. However, achieving room temperature blue phosphorescence with high quantum efficiency remains a challenge. In order to design excellent Ir(III) complexes,

X. Shang et al. / Journal of Luminescence 143 (2013) 402–408

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better understanding their structure-property relationships is very important. Song and co-workers systematically synthesize a new series of phosphorescent Ir(III) complexes with nonconjugated Nbenzylpyrazole ligands that show the required saturated blue phosphorescent emission [18]. We are very interested in the possibility of fabricating OLEDs using these Ir(III) complexes due to their saturated blue phosphorescent emission. Therefore, to provide the experimental results a powerful theoretical support, we investigated four complexes from the experimental molecular structures (1a, 1b, 2a, and 2b in Fig. 1) and proposed four assumed structures (1c, 1d, 2c, and 2d in Fig. 1) using density functional theory (DFT) and time-dependent density functional theory (TDDFT).

2. Computational methods

Fig. 1. Schematic structures for the studied complexes (a) and optimized structure of 1a in the ground state (b).

The ground-state and the lowest-lying triplet excited-state geometries were optimized by the density functional theory (DFT) method [19–22] with Becke's three-parameter hybrid method [23] combining with the Lee–Yang–Parr correlation functional (B3LYP) [20] and the unrestricted B3LYP (UB3LYP) [20] approach, respectively. All geometrical structures were fully optimized without any symmetry constraints. Vibrational frequencies were also calculated at the same theoretical level to confirm that each configuration was a minimum on the potential energy surface. On the basis of the optimized ground- and excited-state geometry structures, the time-dependent DFT (TDDFT) [24–26] approach associated with the SCRF (self-consistent reaction field) theory using the polarized continuum model (PCM) [27] in dichloromethane (CH2Cl2) [18] media was applied to simulate the absorption and emission spectral properties from the experimental results by Song et al. [18]. The S1–T1 energy gap (ΔES1 −T 1 )

Table 1 Selected bond distances (Å), bond angles (1) and dihedral angles (1) of the optimized geometry for the studied complexes, together with the experimental values for 2b. 1a

Ir–N1 Ir–N2 Ir–N3 Ir–N4 Ir–C1 Ir–C2 C3–C4 N3–Ir–N4 N1-Ir-N2 C1–Ir–C2 N3–Ir–N4–N1 N3–C3–C4–N4

1b

S0 2.052 2.064 2.234 2.143 2.060 2.051 1.442 75.7 177.0 88.6 93.6 4.0

T1 2.058 2.061 2.202 2.097 2.065 2.060 1.390 76.6 177.8 88.1 92.8 1.6

2a

Ir–N1 Ir–N2 Ir–N3 Ir–N4 Ir–C1 Ir–C2 C3–C4 N3–Ir–N4 N1–Ir–N2 C1–Ir–C2 N3–Ir–N4–N1 N3–C3–C4–N4 a

1c

S0 2.052 2.063 2.235 2.142 2.061 2.051 1.442 75.8 176.7 88.4 93.6 3.2

T1 2.058 2.061 2.201 2.090 2.067 2.061 1.386 77.5 177.7 88.0 92.7 0.9

2b

1d

S0 2.054 2.063 2.238 2.144 2.062 2.051 1.441 75.8 177.0 88.4 93.7 3.3

T1 2.060 2.061 2.203 2.093 2.067 2.061 1.391 77.5 178.2 88.3 92.9 1.4

2c

2d

S0 2.055 2.064 2.239 2.146 2.061 2.052 1.442 75.8 177.0 88.3 93.7 3.3

T1 2.060 2.061 2.204 2.091 2.067 2.062 1.388 77.5 178.2 88.2 92.8 1.2 2b

S0

T1

S0

T1

S0

T1

S0

T1

Exp.a

2.055 2.065 2.245 2.147 2.059 2.052 1.443 75.7 176.8 88.2 93.6 3.3

2.059 2.063 2.196 2.100 2.063 2.061 1.382 77.5 177.2 87.8 92.5 1.0

2.050 2.060 2.234 2.132 2.071 2.063 1.442 75.8 176.7 88.4 94.3 4.3

2.056 2.057 2.195 2.076 2.078 2.074 1.382 77.6 176.8 88.0 92.9 1.6

2.052 2.061 2.256 2.160 2.058 2.051 1.443 75.4 177.0 88.5 93.9 3.7

2.055 2.060 2.181 2.122 2.056 2.036 1.404 76.8 176.4 90.3 90.7 0.1

2.054 2.063 2.245 2.147 2.057 2.049 1.443 75.7 177.1 88.3 93.6 3.3

2.057 2.063 2.179 2.112 2.059 2.052 1.387 77.4 177.1 88.1 91.6 0.8

2.042 2.027 2.175 2.089 2.064 2.045 1.449 76.5 177.2 93.3 85.4 2.3

Experimental values from Ref. [18].

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was calculated by considering the fixed triplet molecular geometry. The “double-ξ” quality basis set LANL2DZ [28,29] associated with the pseudopotential was employed on atom Ir. The 6–31 Gn basis set was employed on non-metal atoms in the gradient optimizations. The properties of the Ir(III) complexes, such as ionization potential (IP), electron affinity (EA), reorganization energy (λ), etc. were also calculated by the method mentioned above. All calculations were performed with Gaussian09 software package [30].

3. Results and discussion 3.1. Geometries in ground and lowest lying triplet excited states The schematic structures of the studied complexes 1a–2d (1a– 2d indicates complexes from 1a to 2d, the same hereafter) are shown in Fig. 1(a). The representative optimized structure of complex 1a in the ground state (S0) at the B3LYP level is given in Fig. 1(b), along with the numbering used in this study. The main optimized geometry parameters for 1a–2d in S0 and lowest lying triplet excited state (T1) are summarized in Table 1 together with the X-ray crystal structure data of 2b [18]. All complexes show a pseudo-octahedral coordination around the Ir metal centers. For 1a–2d, the two N atoms (N1 and N2) are at the trans position, while C1 and C2 are at the cis position. The optimized bond distances of 2b are in quite good agreement with available experimental data, and the deviation is within 1.3%. The largest discrepancies are for the Ir–N3 bond and C1–Ir–C2 angle, which deviate from the measured data by about 0.049 Å and 4.91, respectively. This is probably due to the fact not only that the theoretical results refer to the gas phase with respect to the closepacked crystal lattice in experiment, but it is well known that B3LYP overestimates the structural parameters (particularly bond lengths) in transition metal complexes. The Ir–C bond length in 2b is longer than those of other complexes, while the bond lengths of Ir–N1 and Ir–N2 in 2b are shorter, especially Ir–N1 bond. This can be attributed to a strong electron-accepting effect from the –CF3 substituent at the main ligands. The influence of −F and −CF3 are also found in the ancillary ligands. For example, the Ir–N3 and Ir–N4 bond distances in 1c and 1d are longer than those in 1a and 1b, due to the electronaccepting effect and the enhanced interannular π-conjugation effect of triazolate ring [31]. Therefore, the strong field ligand attached to the triazolate ring can weaken the metal-ligand bond strength. The Ir–C bonds for 2c and 2d are shorter than those of 1a-1d, 2a, and 2b. This may be attributed to the electron-donating effect of phenyl ring and the extended π-conjugation, which may increase the π-accepting ability of phenyl moiety on the main ligands, and therefore, lead to the strengthened metal–ligand interaction, then enhancing the transition strength. It is also noted that the Ir–N4 bond length is shorter than Ir–N3 ones for all the studied complexes. This is also reflected in the Ir–C1 and Ir–C2 bond lengths, in which Ir–C1 is longer than Ir–C2. The elongated Ir–C1 bond length is caused by the strengthened Ir–N4 bonding interaction, which eventually weakened the Ir–C1 bond at the trans disposition. From Table 1, it can be seen that the Ir–N2, Ir–N3, Ir–N4, and C3-C4 bonds for 1a–2d are contracted in T1 states compared with those in S0 states, while Ir–N1, Ir–C1, and Ir–C2 (except Ir–C1 and Ir–C2 for 2c) have the opposite trend. These changes indicate that the interaction between metal and the ancillary ligand will be strengthened in T1 states for 1a–2d compared with the interaction between metal and main ligands, resulting in the greater effect of ancillary ligand on frontier molecular orbitals (FMOs) in the excited states. Furthermore, this different strength between the

metal and the main/ancillary ligands will result in different electron transition characters. 3.2. Frontier molecular orbitals Since the properties of the excited states and electron transport of organic light-emitting materials are closely related to the characters of the frontier molecular orbitals (FMOs), especially highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), we calculated the FMOs and energy gaps between LUMO and HOMO of the studied complexes. The contour plots of HOMO and LUMO are shown in Fig. 2. The detailed descriptions of the molecular orbitals, in terms of energies, composition and the assignment of different fragments are collected in Tables S1–S8. As shown in Fig. 2, the electron density of HOMO is predominantly delocalized over the ppz(N∧N)+sub in 1a, 1c, and 1d, indicating that different substituent group on ppz(N∧N) ligand does not cause obvious difference in HOMO distribution for 1a, 1c, and 1d. For the HOMO of 1a lying at −5.51 eV, the extended πconjugation length decreases the Ir d-orbital composition (11%) and increases the proportion for tphppz moiety up to 86% (tph moiety (34%), Table S1). The HOMO of 1c is composed of Ir atom (10%) and fphppz moiety (88%) (fph moiety (39%), Table S3), while in 1d, the 17% metal 5d orbital and 75% fmphppz are responsible for the HOMO (Table S4). Thereby, for 1a, 1c, and 1d, it is not favorable for the charge transfer between the metal–ligand due to the lower contribution from metal–orbital. On the contrary, complexes 1b, 2a, and 2b possess a large contribution from the Ir dorbital in their HOMOs, resulting in substantial metal–ligand mixing with the π-orbitals of the ligands. For 2, the HOMO, lying at −5.66 eV, distributes over the Ir center (40%) and b-pz(C∧N) (53%) moieties with negligible contribution from ppz(N∧N) (Table S2). The HOMOs of 2a and 2b are contributed by 59% and 55% b-pz (C∧N) respectively, with the same composition of d(Ir) (39%). For 2c and 2d, the HOMOs are mainly localized on main ligands b-pz (C∧N) (54% and 50%, respectively), along with relatively large contribution from Ir d-orbital of 27% and 23% in proportion. We also found that (Fig. 2) the HOMO and LUMO energies of 2a are lower than those of 1a, which is probably due to the incorporation of the stronger electron-accepting group (−CF3) at the ancillary ligand. While the −CF3 group attached on 2a does not cause larger effect on changing the HOMO–LUMO energy gap with respect to 1a. This is because the –CF3 groups can significantly

Fig. 2. Energy levels, energy gaps (in eV), and contour plots of HOMO and LUMO for the studied complexes.

X. Shang et al. / Journal of Luminescence 143 (2013) 402–408

decrease both of HOMO and LUMO energies in similar degree. Although inductive and negative mesomeric effects caused by –F and –CF3 are largely local, both of them can strongly impact the energies of HOMO, LUMO and the electronic structure of these iridium complexes. These groups have significant contribution to HOMO, hence HOMO undergo a large extent perturbed in energy than LUMO in 2a and 2b compared with 1a and 1b. It is also noted that the LUMOs of 1a–2d are predominantly localized on the ancillary ligand πn(N∧N) with the contributions of about 95%. Thus, it is possible to apply some electron-donating or electronaccepting substituents at the specific position on the ligands to design promising OLED materials. The stabilized LUMO energies will benefit the electron injection, whereas the lowered HOMO energies will not facilitate the hole injection. This will have significant effect on hole and electron injection balance and the position of recombination zone, as well as device performance.

3.3. Absorption spectra in CH2Cl2 media Starting from the S0 geometries, the lowest singlet-singlet and singlet-triplet excited states were calculated by the TDDFT/B3LYP method with PCM in dichloromethane (CH2Cl2) media. The calculated absorption data are listed in Table S9, and the simulated absorption spectra according to the singlet transition results are presented in Fig. 3. The lowest energy absorption wavelengths follow the order: 2c (405 nm) 42d (377 nm) 41c (369 nm)42b (366 nm) ¼2a (366 nm)4 1a (362 nm) 41d (361 nm) 41b (350 nm), which is not consistent with the variation trend of HOMO and LUMO energies because the HOMO-LUMO transition configurations are not predominant contributions to S0-S1 transitions for all cases (Table S9). The lowest energy absorptions at 362, 350, 366, and 366 nm for 1a, 1b, 2a, and 2b, are in good agreement with the experimental values of 343, 340, 368, and 370 nm, respectively [18] (Table S9). The 362 nm absorption for 1a can be described as π (N∧N+tph)-πn(N∧N) transition with ILCT (intraligand charge transfer) character. The lowest energy absorptions of 2a and 2b are assigned to MLCT(metal to ligand charge transfer)/LLCT(ligand to ligand charge transfer) transition characters described as d(Ir) +π(C∧N)-πn(N∧N) transitions. The absorption at 369 nm for 1c can be described as π(N∧N+fph)-πn(N∧N) with ILCT character. The lowest lying absorptions of 1d can be characterized as ILCT [π(N∧N +fmph)-πn(N∧N)] character. For 2c and 2d, the d(Ir)+π(C∧N +ph)-πn(N∧N)/ d(Ir)+π(C∧N+dfph)-πn(N∧N) transition contributes to S1 excitation with the character of MLCT/LLCT. The observed strongest absorptions located at the higher energy regions in experiment are 276, 267, 261, and 261 nm for 1a, 1b, 2a, and 2b, respectively [18]. The calculated results in

405

CH2Cl2 solution are 273, 273, 267, and 266 nm, which agrees well with the measured absorptions in terms of the absolute values and the trends. The absorption at 273 nm for 1a is mainly contributed by the H-L+2 configuration with the largest oscillator strength of 0.3895 (Table S9), and attributed to the π(N∧N+tph)-πn(C∧N) transition with the LLCT character. For 1b, 2a and 2b, HOMO-5L[π(C∧N)-πn(N∧N)], H-3-L+1[d(Ir)+π(C∧N+N∧N)-πn(N∧N)] and H-L+5 [d(Ir)+π(C∧N)-πn(C∧N)] are mainly responsible for the 273, 267 and 266 nm absorptions, respectively. The 272 nm absorption of 1c with oscillator strength of 0.5026 is assigned to LLCT/ILCT [π(C∧N+fph)-πn(C∧N)] characters. For 1d, 2c, and 2d, the calculated 294, 297, and 273 nm absorptions are significantly red-shifted compared with those of 1a, 1b, 2a, and 2b (Fig. 3). It is also noted that the transition from ancillary ligands in the higher energy region has vanished in 2c and 2d, which is due to the weakened Ir-ancillary interaction. The Ir-ancillary bond lengths (Ir–N3 and Ir–N4) are longer relative to those of the Ir-b-pz (C∧N) ones. The calculated vertical triplet absorptions of these complexes are at 433, 420, 439, 426, 394, 395, 411, and 395 nm for 1a-2d (Table S9) with the transition characters of MLCT, LLCT and ILCT.

3.4. Phosphorescence in CH2Cl2 media The TDDFT method was used to calculate the phosphorescent spectra in CH2Cl2 media on the basis of the lowest triplet state (T1) geometries. The calculated emission energies, transition nature, and the available experimental values are listed in Table 2. Generally, although TDDFT systematically underestimates the transition energies, it reproduces the general trend. To check the computational method, two different density functionals (B3LYP and M062X [32]) were used. A better agreement with experimental data was obtained for M062X relative to an unsatisfactory result for B3LYP. The calculated emission energies for 1a, 1b, 2a, and 2b at B3LYP level are 533, 520, 513, and 494 nm, with significant deviations of 45, 40, 53, and 37 nm. vs. 493, 495, 463, and 463 nm deviating from measured values (488, 480, 460, and 457 nm for 1a, 1b, 2a, and 2d) [18] by 5, 15, 3, and 6 nm at the M062X level. Obviously, the M062X functional yield more satisfactory results. For 1a and 1b, the phosphorescent emissions were contributed mainly by the LUMO-HOMO transition (83% and 91% at the M062X level, respectively) and assigned to 3MLCT/3ILCT [πn(N∧N)-d(Ir)+π(N∧N)] and 3MLCT/3LLCT/3ILCT [πn(N∧N)-d(Ir) +π(C∧N+N∧N)] characters, respectively. In addition, the LUMOHOMO (76%) also predominantly contributes to the emissive transition for 2a, while for 2b, it is 71% from LUMO-HOMO and assigned to 3MLCT/3LLCT [πn(N∧N)-d(Ir)+π(C∧N)] characters (Table 2). For 1c, 1d, 2c, and 2d, the emission wavelengths are

Fig. 3. Simulated absorption spectra of (a) 1a–1d and (b) 2a–2d in CH2Cl2 media.

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X. Shang et al. / Journal of Luminescence 143 (2013) 402–408

Table 2 Comparison of the calculated emission wavelength (λ, in nm)/energies (E, in eV) in CH2Cl2 media at the TDDFT/M062X and TDDFT/B3LYP levels, respectively, along with the emission wavelength (nm) in experiment for 1a, 1b, 2a, and 2b. M062X λ/E

Exp.a

B3LYP Configuration

Nature

Configuration

Nature

L-H(89%) L-H(86%) L-H(89%) L-H(86%) L-H(52%) L-H-1(31%) L-H-2(58%) L-H-2(56%) L-H-2(65%)

3

1a 1b 1c 1d 2a

493/2.52 495/2.51 494/2.51 483/2.56 463/2.67

L-H(83%) L-H(91%) L-H(82%) L-H(83%) L-H(76%)

MLCT/ ILCT MLCT/3LLCT/3ILCT 3 MLCT/3ILCT 3 MLCT/3ILCT 3 MLCT/3LLCT

533/2.32 520/2.38 562/2.20 545/2.27 513/2.41

2b 2c 2d

463/2.67 455/2.72 460/2.69

L-H(71%) L-H-2(81%) L-H-2(75%)

3

494/2.50 537/2.31 5.07/2.44

a

3

λ/E

3 3

MLCT/3LLCT MLCT/3LLCT 3 MLCT/3LLCT 3

488 480

460 457

Experimental emission wavelength values from Ref. [18].

located at 494, 483, 455, and 460 nm, respectively, and the emission characters are predominantly ascribed to the LUMOHOMO (L-H-2 for 2c and 2d) transition and assigned to 3 MLCT/3ILCT characters for 1c and 1d (3MLCT/3LLCT for 2c and 2d). The emission wavelengths of 1a–1d are similar to each other, indicating that the emission properties are not sensitive to the selection of substituent group attached directly to the ancillary (N∧N) ligands. Whereas, compared with 1a–1d, the emission wavelengths of 2a–2d are blue-shifted by about 20 nm, owing to the substituent groups attached to the main ligands (C∧N). Thus, the emission spectra of 2a–2d are in the light-blue to blue region. Especially, the 2c and 2d with pure-blue-emitting wavelength are expected to be potential candidates for blue emitters in phosphorescent dopant emitters in OLEDs. We will further investigate the performance of the complexes in OLEDs in the following section.

Table 3 Computed metal-based charge transfer character (3MLCT) (%), the energy of lowest triplet excited state (ET1 , in eV), singlet-triplet splitting energy (ΔES1 −T1 , in eV) for the studied complexes, together with the radiative decay rate kr and nonradiative decay rate knr, the measured quantum yields Φ [%] for 1a, 1b, 2a, and 2b in CH2Cl2 media.

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

The emission quantum yield (Φ) can be determined by the competition between kr and knr, which can be generally formulated as: kr ðkr þ knr Þ

MLCT

E T1

ΔES1 −T1

Φa

(kr  106)a

(knr  106)a

8.0 32.7 7.1 12.9 13.3 29.7 24.1 17.4

2.52 2.51 2.51 2.56 2.67 2.67 2.72 2.69

0.81 0.91 0.79 0.59 0.69 0.66 0.30 0.55

45 20

0.2 0.2

0.2 0.7

10 4

1.0 0.6

9.0 14.0

Experimental values from Ref. [18].

By use of first-order spin-orbital theory, the jM T−S j can be calculated by the formula:

3.5. The quantum yield in CH2Cl2 media

Φ¼

MLCT/3ILCT MLCT/3LLCT/3ILCT 3 MLCT/3ILCT 3 MLCT/3ILCT 3 MLCT/3LLCT 3 MLCT/3LLCT/3ILCT 3 MLCT/3LLCT 3 MLCT/3LLCT 3 MLCT/3LLCT 3

ð1Þ

where kr and knr are the radiative and nonradiative rate constants, respectively. Thus, a large kr and a small knr are required to increase the quantum yield. From Table 3, it can be seen that the kr value for 2b is about three times larger than that of 1a and 1b. However, the knr of 2b is largest among these complexes, resulting in its smaller Φ. Besides, 2a has the largest kr and relatively larger knr value, leading to the lower Φ value. The rate constants depend strongly on the energy of lowest triplet excited state (ET1 ) for phosphorescence, expressed as: kr ¼ γðET1 Þ3 jM T−S j2

ð2Þ

knr ¼ αexpð−βET 1 Þ

ð3Þ

where α, β, and γ are constant; jM T−S j is the emission transition moment from the triplet state. Eq. (2) shows that kr increases with the increase of ET1 . In contrary, eq. (3) is well-known as “the energy gap law” [33], which indicates that knr decreases with the increase of ET1 . Thus, a large ET1 is essential for an efficient material. Table 3 indicates that 2c has the largest ET1 followed by 2d, 2a and 2b in CH2Cl2 media. Hence, the assumed complex 2c is a good candidate to be an efficient phosphorescent material followed by 2d.

^ SO jϕ 〉 〈ϕT1 jH Sm 〈ϕSm er ϕS0 〉 ESm −ET1 Sm

M T−S ¼ ∑

ð4Þ

^ SO is the spin-orbit coupling operator. ϕ is the wave where H function of the corresponding state. ESm is the energy of the mth , singlet excited state (Sm), and er is the electric dipole operator. ! 1=2 〈ϕSm je r jϕS0 〉 ¼ ½ðf Sm Þ=ðESm Þ , where f Sm is the oscillator strength of the transition S0-Sm in absorption spectra. The spin-orbit coupling (SOC) calculations have been performed and analyzed in many literatures [34–37]. It can be seen that SOC effects can be elucidated mainly from the following two aspects. One is the contribution of MLCT in the T1 state [38]. The direct involvement of the d(Ir) orbital enhances the first-order SOC in the T1-S0 transition, which would result in a drastic decrease of the radiative lifetime and avoid the nonradiative process [39]. Thus, a large MLCT contribution is beneficial to increase the quantum yield. In Table 3, we list the MLCT contributions of 8.0%, 32.7%, 13.3%, and 29.7% for 1a, 1b, 2a, and 2b along with the experimental quantum yields of 45%, 20%, 10%, and 4% [18], respectively. It's noted that the 32.7% of MLCT for 1b is significantly higher than that for 1a (8.0%), while the quantum yield of 1a (45%) is relatively larger than that of 1b (20%). The same case occurs between 2a and 2b. This revealed that the MLCT contribution may be not the most crucial factor for the quantum yield. The second aspect is the S1–T1 energy gap (ΔES1 −T1 ) [40]. According to eq. (4), transition moment may partially depend on the ΔES1 −T1 , the S1-T1 intersystem crossing (ISC) due to SOC interactions of the triplet state with singlet state plays an important role in phosphorescent process. The minimal

X. Shang et al. / Journal of Luminescence 143 (2013) 402–408

407

Table 4 Ionization potentials, electron affinities, extraction potentials and internal reorganization energies, along with the Δ¼ ∣λh−λe∣ for the studied complexes (values are given in eV).

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

IPv

IPa

HEP

EAv

EAa

EEP

λh

λe

Δ

6.46 6.65 6.59 6.78 7.02 7.06 6.26 6.60

6.35 6.54 6.46 6.66 6.88 6.92 6.13 6.48

6.22 6.37 6.31 6.52 6.70 6.76 6.99 6.35

0.24 0.14 0.31 0.42 0.38 0.45 0.28 0.47

0.43 0.34 0.51 0.63 0.60 0.68 0.48 0.68

0.62 0.53 0.71 0.82 0.81 0.89 0.69 0.89

0.26 0.28 0.28 0.25 0.32 0.30 0.25 0.25

0.38 0.39 0.30 0.40 0.43 0.44 0.27 0.24

0.12 0.11 0.02 0.15 0.11 0.14 0.02 0.01

ΔES1 −T1 is good for enhancing the transition moment and ISC rate, leading to the increased kr. The calculated ΔES1 −T1 values (Table 3) indicate that 2c has the smallest ΔES1 −T1 (0.30 eV) followed by 2d. A , large value of 〈ϕSm jer jϕS0 〉 will make jM T−S j and kr increase. Because the absorption band locations of 2c and 2d are similar, and the oscillator strength of 2c and 2d are stronger than others, the value , of 〈ϕSm jer jϕS0 〉 ¼ ½ðf Sm Þ=ðESm Þ1=2 for 2c and 2d could be larger than others. From the above discussion, we can conclude that the assumed complexes 2c and 2d are the most efficient blueemitting phosphorescent materials among the studied complexes. 3.6. Comparison of performance in OLEDs The device performances of OLEDs strongly depend on the charge injection, transfer, and balance, as well as the exciton confinement in a device [41]. The charge injection properties of luminescent materials can be evaluated by the ionization potential (IP) and electron affinity (EA), which are also closely related to the HOMO and LUMO, respectively [42]. For photoluminescent materials, a larger EA (smaller IP) suggests that it is easier to inject electrons (holes) into the emitting materials from the electron (hole) transporting layer. The calculated vertical IP (IPv), adiabatic IP (IPa), vertical EA (EAv), and adiabatic EA (EAa) are listed in Table 4. The results show that 1a has smaller IP value (6.46 eV) and larger hole injection abilities compared with 1b, 2a, and 2b, which is consistent with its higher HOMO energy level (Fig. 2). The designed complex 2c has the smallest IP value (6.26 eV) among the complexes, indicating its easiest hole injection. While, the EA values of 2c (0.28 eV) is significantly small, resulting in the poor ability of electron injection. Corresponding to the lowest LUMO energy level, 2b has the largest EA value (0.45 eV) and enhanced electron injection ability among the experimentally obtained complexes. The highest EA (0.47 eV) is found for 2d, suggesting its easiest electron injection, while 1b (EA¼ 0.14 eV) is found to be the most difficult for the electron injection. According to the semi-classical Marcus theory [43], the charge (hole or electron) transfer rate (Ket) can be expressed by the following formula:   −λ ð5Þ K et ¼ Aexp 4kB T where λ is the reorganization energy, A is a prefactor related to the electronic coupling between adjacent molecules, and T and kB are the temperature and the Boltzmann constant, respectively. As previously reported, due to the limited intermolecular charge transfer range in solid state, the mobility of charges dominantly relates to the reorganization energy λ for OLEDs materials [44,45]. Therefore, at constant temperature, the low λ value is necessary for an efficient charge transport process. Herein, any environmental influence is ignored, and we focus on the inner reorganization

Fig. 4. Schematic description of the inner reorganization energy.

energy λi, which is caused by the change of the internal nuclear coordinates from the reactant A to the product B and vice versa (Fig. 4). It can be evaluated as the sum of two relaxation energies according to the following formula: λi ¼ λ0 þ λ1 ¼ ðEAB −EA Þ þ ðEBA −EB Þ

ð6Þ

B

where EA and EA are the energies of A and B at the optimized geometry of A, respectively; EBA and EB are the energies of A and B at the optimized geometry of B, respectively. The hole extraction potential (HEP) is the energy difference between M (neutral molecule) and M+ (cationic), using M+ geometry. The electron extraction potential (EEP) is the energy difference between M and M− (anionic), using M− geometry. Fig. 4 shows that the reorganization energy for hole (λh) and electron transport (λe) can be evaluated as: λh ¼ IP v −HEP

ð7Þ

λe ¼ EEP−EAv

ð8Þ

The reorganization energy (λ) can be approximately used to estimate the charge transport rate and balance between holes and electrons. The results are also listed in Table 4. Complex 1a and 1b has the better hole transport ability with the smaller λh value (0.26 and 0.28 eV, respectively) compared with 2a and 2b (0.32 and 0.30 eV, respectively). The nearly identical λh values of 1d, 2c, and 2d indicate their comparable hole transport abilities. The energy differences between λh and λe for 1c (0.02 eV), 2c (0.02 eV), and 2d (0.01 eV) are significantly smaller than that of 1a (0.12 eV), 1b (0.11 eV), 1d (0.15 eV), 2a (0.11 eV), and 2b (0.14 eV), which suggests that the hole and electron transfer balance could be achieved more easily in the emitting layer for complexes 1c，2c, and 2d, which is the key factor for materials used in OLEDs.

4. Conclusions In this article, we have reported the detail investigation on geometrical structures, spectra, and electroluminescent properties for two series of iridium(III) complexes. The phosphorescence wavelength, quantum yields, and electroluminescent efficiency of these complexes are closely related to the π-conjugation length of the main ligand (C∧N), substitution positions, and substituent's inductive effect. The calculated results reveal that –CF3 groups can

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X. Shang et al. / Journal of Luminescence 143 (2013) 402–408

result in contracted sphere structure and make metal-ligand bond strengthened, which is the most important factor to increase the emission quantum yields. The substituents (–F, –CF3) on the terminal phenyl ring have a slight effect on the excited energy because the distance between the substituents and the ligand is interrupted by the phenyl moiety. Complexes 1c, 2c, and 2d are better phosphorescent OLED materials with the better charge transfer abilities and more balanced charge transfer rates. The designed complexes 2c and 2d are potential candidates for blue phosphorescent materials. Acknowledgments The authors thank the National Natural Science Foundation of China for financial support (Grant nos. 90922015, 20921002, and 21273219). Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jlumin.2013.05. 046.

References [1] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson S.R. Forrest, Nature 395 (1998) 151. [2] C. Adachi, M.A. Baldo, M.E. Thompson, S.R. Forrest, J. Appl. Phys. 90 (2001) 5048. [3] Y.L. Tung, S.W. Lee, Y. Chi, Y.T. Tao, C.H. Chien, Y.M. Cheng, P.T. Chou S.M. Pengc, C.S. Liud, J. Mater. Chem. 15 (2005) 460. [4] P.Y. Chen, T.J. Meyer, Chem. Rev. 98 (1998) 1439. [5] J.J. Lin, W.S. Liao, H.J. Huang, F.I. Wu, C.H. Cheng, Adv. Funct. Mater. 18 (2008) 485. [6] G.J. Zhou, C.L. Ho, W.Y. Wong, Q. Wang, D.G. Ma, L.X. Wang, Z.Y. Lin T.B. Marder, A. Beeby, Adv. Funct. Mater. 18 (2008) 499. [7] X.M. Yu, H.S. Kwok, W.Y. Wong, G.J. Zhou, Chem. Mater. 18 (2006) 5097. [8] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.E. Lee, C. Adachi, P. E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc. 123 (2001) 4304. [9] S. Lamansky, P. Djurovich, D. Murphy, F.A. Razzaq, R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau, M.E. Thompson, Inorg. Chem. 40 (2001) 1704. [10] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E. Thompson S.R. Forrest, Nature 395 (1998) 151. [11] C. Adachi, M.A. Baldo, S.R. Forrest, M.E. Thompson, Appl. Phys. Lett. 77 (2000) 904.

[12] W.Y. Wong, C.L. Ho, Coord. Chem. Rev. 253 (2009) 1709. [13] W.Y. Wong, C.L. Ho, J. Mater. Chem. 19 (2009) 4457. [14] M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl. Phys. Lett. 79 (2001) 156. [15] D.M. Han, G. Zhang, H.X. Cai, X.H. Zhang, L.H. Zhao, J. Lumin. 138 (2013) 223. [16] J.P. Duan, P.P. Sun, C.H. Cheng, Adv. Mater. 15 (2003) 224. [17] A. Tsuboyama, H. Iwawaki, M. Furugori, T. Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S. Miura, T. Takiguchi, S. Okada, M. Hoshino, K. Ueno, J. Am. Chem. Soc. 125 (2003) 12971. [18] Y.H. Song, Y.C. Chiu, Y. Chi, Y.M. Cheng, C.H. Lai, P.T. Chou, K.T. Wong, M.H. Tsai, C.C. Wu, Chem. Eur. J. 14 (2008) 5423. [19] P. Hohenberg, W. Kohn, Phys, Rev 136 (1964) B864. [20] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [21] T. Wei, W.H. Zhu, J. Zhang, H.M. Xiao, J. Hazard. Mater. 179 (2010) 581. [22] X.W. Fan, X.H. Ju, H.M. Xiao, J. Hazard. Mater. 156 (2008) 342. [23] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [24] R.E. Stratmann, G.E. Scuseria, M.J. Frisch, J. Chem. Phys. 109 (1998) 8218. [25] N.N. Matsuzawa, A. Ishitani, D.A. Dixon, T. Uda, J. Phys. Chem. A 105 (2001) 4953. [26] M.E. Casida, C. Jamorski, K.C. Casida, D.R. Salahub, J. Chem. Phys. 108 (1998) 4439. [27] E. Cancès, B. Mennucci, J. Tomasi, J. Chem. Phys. 107 (1997) 3032. [28] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270. [29] P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 299. [30] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09 (Revision B.01), Gaussian, Inc., Wallingford, CT, 2010. [31] X.N. Li, Z.J. Wu, Z.J. Si, H.J. Zhang, L. Zhou, X.J. Liu, Inorg. Chem. 48 (2009) 7740. [32] Y. Zhao, D.G. Truhlar, Theor. Chem. Acc. 120 (2008) 215. [33] J.S. Wilson, N. Chawdhury, M.R.A. Al-Mandhary, M. Younus, M.S. Khan P.R. Raithby, A. Kohler, R.H. Friend, J. Am. Chem. Soc. 123 (2001) 9412. [34] B.F. Minaev, E.M. Khomenko, L.B. Yashchuk, J. Appl. Spectrosc. 76 (2009) 772. [35] B. Minaev, V. Minaeva, H. Ågren, J. Phys. Chem. A 113 (2009) 726. [36] X. Li, B. Minaev, H. Ågren, H. Tian, J. Phys. Chem. C 115 (2011) 20724. [37] X. Li, B. Minaev, H. Ågren, H. Tian, Eur. J. Inorg. Chem. (2011) 2517. [38] J. Li, P.I. Djurovich, B.D. Alleyne, M. Yousufuddin, N.N. Ho, J.C. Thomas, J.C. Peters, R. Bau, M.E. Thompson, Inorg. Chem. 44 (2005) 1713. [39] C.H. Yang, Y.M. Cheng, Y. Chi, C.J. Hsu, F.C. Fang, K.T. Wong, P.T. Chou C.H. Chang, M.H. Tsai, C.C. Wu, Angew. Chem. Int. Ed. 46 (2007) 2418. [40] I. Avilov, P. Minoofar, J. Cornil, L.De Cola, J. Am. Chem. Soc. 129 (2009) 8247. [41] A.J. Epstein, W.P. Lee, V.N. Prigodin, Synth. Met. 117 (2001) 9. [42] L.L. Shi, Y. Geng, H.Z. Gao, Z.M. Su, Z.J. Wu, Dalton Trans. 39 (2010) 7733. [43] R.A. Marcus, Rev. Mod. Phys. 65 (1993) 599. [44] N.S. Hush, J. Chem. Phys. 28 (1958) 962. [45] R.A. Marcus, J. Chem. Phys. 24 (1956) 966.