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The influence of numbers and ligation positions of the triphenylamine unit on the photophysical and electroluminescent properties of homoleptic iridium(III) complexes: a theoretical perspective† Yuqi Liu,a Xiaobo Sun,b Ying Wang*a and Zhijian Wu*a A DFT/TDDFT investigation was carried out on a series of homoleptic triphenylamine-featured Ir(III) complexes 1a–1c [1a: ( fac-tris[2-phenyl-4-(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 1b: ( fac-tris[2phenyl-4-(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 1c: ( fac-tris[2-phenyl-4-(4-(N,N-diphenylamino)phenyl)pyridine]iridium(III))] with one triphenylamine unit in the 2-phenylpyridine ( ppy) ligand and 2a–2c [2a: ( fac-tris[2,4-bis(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 2b: ( fac-tris[2,4-bis(3(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 2c: ( fac-tris[2,4-bis(4-(N,N-diphenylamino)phenyl)pyridine] iridium(III))] with two triphenylamine units in the ppy ligand, respectively, aiming to gain insight into the influence of number and ligation position of triphenylamine units on the photophysical and electronic properties of the studied complexes. Complexes 2a–2c have been synthesized recently. For comparison, the parent complex Ir( ppy)3 was also investigated. The calculated results reveal that the introduction of the triphenylamine unit leads to enhanced charge-injection abilities and a balanced charge-transfer process compared with Ir( ppy)3. The different ligation positions of triphenylamine unit have an obvious
Received 9th April 2014, Accepted 16th May 2014
effect on the absorption intensities for these complexes. The emissions of 1a–1c and 2a–2c undergo significant red shift with the introduced triphenylamine unit in ppy ligands compared with that of Ir( ppy)3,
DOI: 10.1039/c4dt01049c
while the extent of red shift shows an apparent dependence on the number of triphenylamine units.
www.rsc.org/dalton
The factors that might affect the quantum yield have been discussed.
1.
Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) have attracted intense interest because phosphors with heavy metals can make use of both singlet and triplet excitons for
a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China. E-mail: [email protected], [email protected]; Tel: +86-431-85682801 b Department of Applied Chemistry, Qingdao Agricultural University, Qingdao 266109, People’s Republic of China † Electronic supplementary information (ESI) available: The method for calculating the MLCT contribution in the excited state is given first, followed by the tables and figures. Tables: (S1–S7) Frontier molecular orbital energies (eV) and compositions (%) of different fragments in the ground state for the studied complexes. (S8) Selected calculated wavelength (λ, in nm)/energies (E, in eV), oscillator strength (ƒ), major contribution and transition characters for the studied complexes in toluene, along with the experimental data for 2b and 2c. (S9–S11) The Cartesian coordinates for the optimized structures for 2a–2c in the S0 states. (Fig. S1) Spin-density contours of the studied complexes in the triplet electronic configuration. (Fig. S2) Contour plots of the HOMO and the LUMO for the studied complexes obtained from DFT calculations at their S0 optimized geometries. See DOI: 10.1039/c4dt01049c
This journal is © The Royal Society of Chemistry 2014
emission and consequently realize a theoretical internal quantum efficiency of 100%.1–6 Generally, the mixtures of three primary (red, green and blue) or complementary colors (blue and orange) are required to generate white emission.7 Hence, much effort has been spent on investigating third-row transition-metal complexes to develop highly efficient phosphors that can cover the entire visible light spectrum.8–13 The OLED device has multilayer architecture consisting of a hole-transporting, an electron-transporting, and a composite guest–host emissive layer. As guests, the phosphors are normally doped into a host matrix with low concentration to reduce the triplet–triplet (T–T) annihilation and concentration quenching.14–20 Among all the metallated phosphorescent emitters, 2-phenylpyridine ( ppy) based Ir(III) complexes are obviously in a prominent position for developing both fullcolor flat-panel displays and low-cost lighting sources because of their high phosphorescence quantum yield, relatively short triplet lifetime and tunable emission color.21–25 In recent work by Yang and coworkers,26–28 the triphenylamine fragment was included as a part of the 2-phenylpyridine ( ppy) ligand framework to synthesize the homoleptic Ir(III) complexes 2a ( fac-tris[2,4-bis(2-(N,N-diphenylamino)phenyl)-
Dalton Trans., 2014, 43, 11915–11924 | 11915
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dine)iridium(III)), which emits a green color with ∼90% efficiency and has been used as a springboard for similar phosphorescent emitters,3 is also investigated. Our goal is, through quantum chemical investigations on electronic structures and properties for all the studied homoleptic Ir(III) complexes, to rationalize the reported results of 2a, 2b and 2c, and to further evaluate the potential application of the designed 1a, 1b and 1c as phosphorescent emitters. On the basis of the conclusions, we hope to better understand the structure–property relationship of phosphorescent Ir(III) complexes and provide valuable information for designing novel and highly efficient OLED materials in the future.
2. Computational methods Scheme 1
Schematic structures of the investigated complexes.
pyridine]iridium(III)), 2b ( fac-tris[2,4-bis(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)) and 2c ( fac-tris[2,4-bis(4-(N,Ndiphenylamino)phenyl)pyridine]iridium(III)) (Scheme 1). Usually, the triphenylamine derivatives are used as host materials owing to their high triplet energy (ca. 2.9 eV) and good hole-transporting ability. On the other hand, it is demonstrated that incorporating the triphenylamine groups into the ligand frame raises the HOMO (highest occupied molecular orbital) levels, reducing the barrier height for hole injection and triplet–triplet annihilation. Finally, the EL (electroluminescence) performance of the resulting complexes has been improved.29–33 In addition to the enhanced hole-injection/ transporting properties endowed by the triphenylamine units, remarkable color tuning could be realized by simply altering the ligation positions of the triphenylamine units. The green (2a), orange (2c) and red (2b) emitters were obtained and the OLED devices fabricated with them showed excellent performance. These emitters have been demonstrated as active components for white polymer light-emitting diodes (WPLEDs). The high performances render these triphenylamine-featured Ir(III) phosphors very promising candidates for application in full-color display and solid-state lighting. Encouraged by the excellent performances of these dendritic phosphors, we performed quantum chemical calculations to explore how the photophysical and electronic properties are affected by altering the number and ligation positions of the triphenylamine units in the ppy ligand. Meanwhile, to gain further insight into the role of the triphenylamine unit in governing the photophysical and electronic properties of the experimental complexes, 1a ( fac-tris[2-phenyl-4-(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)), 1b ( fac-tris[2phenyl-4-(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)) and 1c ( fac-tris[2-phenyl-4-(4-(N,N-diphenylamino)phenyl)pyridine]iridium(III)), with one triphenylamine unit in the pyridyl moiety of the ppy ligand are also studied (Scheme 1). For comparison, the parent complex Ir( ppy)3 ( fac-tris(2-phenylpyri-
11916 | Dalton Trans., 2014, 43, 11915–11924
All the calculations for the studied complexes were performed using the Gaussian 09 program package.34 Density functional theory calculations (DFT)35 were carried out using the hybridtype Perdew–Burke–Ernzerhof exchange correlation functional (PBE0)36–38 together with the 6-31G(d) basis set39 for C, H, and N atoms and the “double-ζ” quality LANL2DZ basis set40,41 for the Ir element. In the LANL2DZ basis set, a relativistic effective core potential (ECP) replaces the inner core electrons of Ir, leaving the outer core [(5s)2(5p)6] electrons and the (5d)6 valence electrons of Ir(III). Calculated structures of transition metal complexes in the ground-state (S0) with the PBE0 hybrid functional often provide better agreement with crystal geometries versus other functionals.42–44 We have optimized the ground state (S0) and the first triplet excited state (T1) geometries without symmetry constraints. T1 optimized geometries were obtained via the Kohn–Sham approach using UPBE0 (unrestricted PBE0). The vibrational frequencies were calculated at the same theoretical level to confirm that each configuration was a minimum on the potential energy surface. Single-point calculations were performed based on S0 geometries for the molecular orbital population, while the lowlying S1 and T1 energy gap (ΔES1–T1) was calculated considering the fixed triplet molecular geometry. To obtain the absorption and emission spectral properties, time dependent DFT (TD-DFT)45–47 calculations associated with the polarized continuum model (PCM)48–50 in toluene were performed on the basis of the optimized ground- and lowest triplet excited-state equilibrium geometries. GaussSum 2.5 program51 was used for the distribution of the total density-of-state and the UV/Vis spectra analysis. The molecular orbitals were generated using Molekel 4.3.52,53 The method for calculating the metal-based charge transfer character (3MLCT%) in the excited states is based on the work of Chou and coworkers.54 For further details, see the ESI.†
3. Results and discussion 3.1. Molecular geometries in ground state S0 and triplet state T1 The schematic structures of the investigated complexes are presented in Scheme 1 and the fully optimized ground state
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lengths (∼0.013–0.022 Å) compared with 1a. The different variations in the Ir-related bond lengths caused by the ligation position of the diphenylamine unit would result in the different photophysical properties for 2a–2c. The three Ir–C (Ir–N) bond lengths are similar to each other in the S0 states, but drastically different upon excitation to the T1 state, which implies differing participation of the three ligands in the excited states. The changes in bond lengths can also be rationalized by looking at the unpaired-electron spin density distribution of these complexes presented in Fig. S1 (ESI†), which demonstrates a charge transfer process from the Ir 5d orbital to the ligand upon S0→T1 excitation. After relaxation in the T1 state, the electron distribution localizes on the ligands, then moves closer to the Ir(III) center. Further analysis on the unpaired-electron spin density distribution of these Ir(III) complexes could explain the variations in metal–ligand bond length upon S0→T1 excitation. Specifically, for Ir( ppy)3, the spin density resides on the Ir(III) center and the ppy2 ligand, with negligible contribution from the ppy1 and ppy3 ligands. Thus, the ppy2 ligand comes closer to the Ir(III) center and the ppy1 and ppy3 ligands seem to move away, which is consistent with the shortened Ir–ppy2 (Ir–C2 and Ir–N2) and elongated Ir–ppy1&3 (Ir–C1 Ir–N1 and Ir–N3) bond lengths in the T1 state (Table 1). For 1a–1c, the spin density is mainly localized on the (C^N)3 ligand and the Ir(III) center along with the contribution from the phenyl moiety in the (C^N)1 ligand (Fig. S1†). Compared with those in the S0 state, this leads to the shortened Ir–N3 (in ppy3 ligand) and Ir–C1 (in ppy1 ligand) bonds, and elongated Ir–N2 and Ir–C2 (in ppy2 ligand) bonds. The (C^N)3 ligands in 2a–2c get closer to the Ir(III) center due to the spin density of these complexes predominantly localized on the Ir(III) and (C^N)3 ligands, whereas the (C^N)1 and (C^N)2 ligands are pushed away upon excitation to the T1 state. These shortened bond distances in the T1 state would suggest the larger involvement of the ligands in the excited states.
Fig. 1 Optimized geometry structures of 1a in the ground state. C and N atoms are shown in gray and blue, respectively, while for clarity H atoms are not displayed.
structure of 1a is shown in Fig. 1, along with the numbering of some key atoms. It reveals that the Ir center coordinated by the three anionic C^N ligands adopts the fac-Ir(C^N)3 chelate disposition in a distorted octahedron. The main structural parameters of the studied complexes in the S0 and T1 states are summarized in Table 1. It is noted that the Ir–C (Ir–C1, Ir–C2 and Ir–C3) bond lengths show negligible changes (within ∼0.005 Å) from Ir( ppy)3 → 1a–1c → 2a–2c, indicating that the introduction of the triphenylamine/diphenylamine unit into the pyridyl/phenyl moiety of the ppy ligand causes minor effects on the geometric structures of these complexes. For Ir–N (Ir–N1, Ir–N2 and Ir–N3) bonds, the triphenylamine unit in the pyridyl moiety of the ppy ligand leads to a slight contraction (
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The influence of numbers and ligation positions of the triphenylamine unit on the photophysical and electroluminescent properties of homoleptic iridium(III) complexes: a theoretical perspective† Yuqi Liu,a Xiaobo Sun,b Ying Wang*a and Zhijian Wu*a A DFT/TDDFT investigation was carried out on a series of homoleptic triphenylamine-featured Ir(III) complexes 1a–1c [1a: ( fac-tris[2-phenyl-4-(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 1b: ( fac-tris[2phenyl-4-(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 1c: ( fac-tris[2-phenyl-4-(4-(N,N-diphenylamino)phenyl)pyridine]iridium(III))] with one triphenylamine unit in the 2-phenylpyridine ( ppy) ligand and 2a–2c [2a: ( fac-tris[2,4-bis(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 2b: ( fac-tris[2,4-bis(3(N,N-diphenylamino)phenyl)pyridine]iridium(III)); 2c: ( fac-tris[2,4-bis(4-(N,N-diphenylamino)phenyl)pyridine] iridium(III))] with two triphenylamine units in the ppy ligand, respectively, aiming to gain insight into the influence of number and ligation position of triphenylamine units on the photophysical and electronic properties of the studied complexes. Complexes 2a–2c have been synthesized recently. For comparison, the parent complex Ir( ppy)3 was also investigated. The calculated results reveal that the introduction of the triphenylamine unit leads to enhanced charge-injection abilities and a balanced charge-transfer process compared with Ir( ppy)3. The different ligation positions of triphenylamine unit have an obvious
Received 9th April 2014, Accepted 16th May 2014
effect on the absorption intensities for these complexes. The emissions of 1a–1c and 2a–2c undergo significant red shift with the introduced triphenylamine unit in ppy ligands compared with that of Ir( ppy)3,
DOI: 10.1039/c4dt01049c
while the extent of red shift shows an apparent dependence on the number of triphenylamine units.
www.rsc.org/dalton
The factors that might affect the quantum yield have been discussed.
1.
Introduction
Phosphorescent organic light-emitting diodes (PhOLEDs) have attracted intense interest because phosphors with heavy metals can make use of both singlet and triplet excitons for
a State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China. E-mail: [email protected], [email protected]; Tel: +86-431-85682801 b Department of Applied Chemistry, Qingdao Agricultural University, Qingdao 266109, People’s Republic of China † Electronic supplementary information (ESI) available: The method for calculating the MLCT contribution in the excited state is given first, followed by the tables and figures. Tables: (S1–S7) Frontier molecular orbital energies (eV) and compositions (%) of different fragments in the ground state for the studied complexes. (S8) Selected calculated wavelength (λ, in nm)/energies (E, in eV), oscillator strength (ƒ), major contribution and transition characters for the studied complexes in toluene, along with the experimental data for 2b and 2c. (S9–S11) The Cartesian coordinates for the optimized structures for 2a–2c in the S0 states. (Fig. S1) Spin-density contours of the studied complexes in the triplet electronic configuration. (Fig. S2) Contour plots of the HOMO and the LUMO for the studied complexes obtained from DFT calculations at their S0 optimized geometries. See DOI: 10.1039/c4dt01049c
This journal is © The Royal Society of Chemistry 2014
emission and consequently realize a theoretical internal quantum efficiency of 100%.1–6 Generally, the mixtures of three primary (red, green and blue) or complementary colors (blue and orange) are required to generate white emission.7 Hence, much effort has been spent on investigating third-row transition-metal complexes to develop highly efficient phosphors that can cover the entire visible light spectrum.8–13 The OLED device has multilayer architecture consisting of a hole-transporting, an electron-transporting, and a composite guest–host emissive layer. As guests, the phosphors are normally doped into a host matrix with low concentration to reduce the triplet–triplet (T–T) annihilation and concentration quenching.14–20 Among all the metallated phosphorescent emitters, 2-phenylpyridine ( ppy) based Ir(III) complexes are obviously in a prominent position for developing both fullcolor flat-panel displays and low-cost lighting sources because of their high phosphorescence quantum yield, relatively short triplet lifetime and tunable emission color.21–25 In recent work by Yang and coworkers,26–28 the triphenylamine fragment was included as a part of the 2-phenylpyridine ( ppy) ligand framework to synthesize the homoleptic Ir(III) complexes 2a ( fac-tris[2,4-bis(2-(N,N-diphenylamino)phenyl)-
Dalton Trans., 2014, 43, 11915–11924 | 11915
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Dalton Transactions
dine)iridium(III)), which emits a green color with ∼90% efficiency and has been used as a springboard for similar phosphorescent emitters,3 is also investigated. Our goal is, through quantum chemical investigations on electronic structures and properties for all the studied homoleptic Ir(III) complexes, to rationalize the reported results of 2a, 2b and 2c, and to further evaluate the potential application of the designed 1a, 1b and 1c as phosphorescent emitters. On the basis of the conclusions, we hope to better understand the structure–property relationship of phosphorescent Ir(III) complexes and provide valuable information for designing novel and highly efficient OLED materials in the future.
2. Computational methods Scheme 1
Schematic structures of the investigated complexes.
pyridine]iridium(III)), 2b ( fac-tris[2,4-bis(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)) and 2c ( fac-tris[2,4-bis(4-(N,Ndiphenylamino)phenyl)pyridine]iridium(III)) (Scheme 1). Usually, the triphenylamine derivatives are used as host materials owing to their high triplet energy (ca. 2.9 eV) and good hole-transporting ability. On the other hand, it is demonstrated that incorporating the triphenylamine groups into the ligand frame raises the HOMO (highest occupied molecular orbital) levels, reducing the barrier height for hole injection and triplet–triplet annihilation. Finally, the EL (electroluminescence) performance of the resulting complexes has been improved.29–33 In addition to the enhanced hole-injection/ transporting properties endowed by the triphenylamine units, remarkable color tuning could be realized by simply altering the ligation positions of the triphenylamine units. The green (2a), orange (2c) and red (2b) emitters were obtained and the OLED devices fabricated with them showed excellent performance. These emitters have been demonstrated as active components for white polymer light-emitting diodes (WPLEDs). The high performances render these triphenylamine-featured Ir(III) phosphors very promising candidates for application in full-color display and solid-state lighting. Encouraged by the excellent performances of these dendritic phosphors, we performed quantum chemical calculations to explore how the photophysical and electronic properties are affected by altering the number and ligation positions of the triphenylamine units in the ppy ligand. Meanwhile, to gain further insight into the role of the triphenylamine unit in governing the photophysical and electronic properties of the experimental complexes, 1a ( fac-tris[2-phenyl-4-(2-(N,N-diphenylamino)phenyl)pyridine]iridium(III)), 1b ( fac-tris[2phenyl-4-(3-(N,N-diphenylamino)phenyl)pyridine]iridium(III)) and 1c ( fac-tris[2-phenyl-4-(4-(N,N-diphenylamino)phenyl)pyridine]iridium(III)), with one triphenylamine unit in the pyridyl moiety of the ppy ligand are also studied (Scheme 1). For comparison, the parent complex Ir( ppy)3 ( fac-tris(2-phenylpyri-
11916 | Dalton Trans., 2014, 43, 11915–11924
All the calculations for the studied complexes were performed using the Gaussian 09 program package.34 Density functional theory calculations (DFT)35 were carried out using the hybridtype Perdew–Burke–Ernzerhof exchange correlation functional (PBE0)36–38 together with the 6-31G(d) basis set39 for C, H, and N atoms and the “double-ζ” quality LANL2DZ basis set40,41 for the Ir element. In the LANL2DZ basis set, a relativistic effective core potential (ECP) replaces the inner core electrons of Ir, leaving the outer core [(5s)2(5p)6] electrons and the (5d)6 valence electrons of Ir(III). Calculated structures of transition metal complexes in the ground-state (S0) with the PBE0 hybrid functional often provide better agreement with crystal geometries versus other functionals.42–44 We have optimized the ground state (S0) and the first triplet excited state (T1) geometries without symmetry constraints. T1 optimized geometries were obtained via the Kohn–Sham approach using UPBE0 (unrestricted PBE0). The vibrational frequencies were calculated at the same theoretical level to confirm that each configuration was a minimum on the potential energy surface. Single-point calculations were performed based on S0 geometries for the molecular orbital population, while the lowlying S1 and T1 energy gap (ΔES1–T1) was calculated considering the fixed triplet molecular geometry. To obtain the absorption and emission spectral properties, time dependent DFT (TD-DFT)45–47 calculations associated with the polarized continuum model (PCM)48–50 in toluene were performed on the basis of the optimized ground- and lowest triplet excited-state equilibrium geometries. GaussSum 2.5 program51 was used for the distribution of the total density-of-state and the UV/Vis spectra analysis. The molecular orbitals were generated using Molekel 4.3.52,53 The method for calculating the metal-based charge transfer character (3MLCT%) in the excited states is based on the work of Chou and coworkers.54 For further details, see the ESI.†
3. Results and discussion 3.1. Molecular geometries in ground state S0 and triplet state T1 The schematic structures of the investigated complexes are presented in Scheme 1 and the fully optimized ground state
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lengths (∼0.013–0.022 Å) compared with 1a. The different variations in the Ir-related bond lengths caused by the ligation position of the diphenylamine unit would result in the different photophysical properties for 2a–2c. The three Ir–C (Ir–N) bond lengths are similar to each other in the S0 states, but drastically different upon excitation to the T1 state, which implies differing participation of the three ligands in the excited states. The changes in bond lengths can also be rationalized by looking at the unpaired-electron spin density distribution of these complexes presented in Fig. S1 (ESI†), which demonstrates a charge transfer process from the Ir 5d orbital to the ligand upon S0→T1 excitation. After relaxation in the T1 state, the electron distribution localizes on the ligands, then moves closer to the Ir(III) center. Further analysis on the unpaired-electron spin density distribution of these Ir(III) complexes could explain the variations in metal–ligand bond length upon S0→T1 excitation. Specifically, for Ir( ppy)3, the spin density resides on the Ir(III) center and the ppy2 ligand, with negligible contribution from the ppy1 and ppy3 ligands. Thus, the ppy2 ligand comes closer to the Ir(III) center and the ppy1 and ppy3 ligands seem to move away, which is consistent with the shortened Ir–ppy2 (Ir–C2 and Ir–N2) and elongated Ir–ppy1&3 (Ir–C1 Ir–N1 and Ir–N3) bond lengths in the T1 state (Table 1). For 1a–1c, the spin density is mainly localized on the (C^N)3 ligand and the Ir(III) center along with the contribution from the phenyl moiety in the (C^N)1 ligand (Fig. S1†). Compared with those in the S0 state, this leads to the shortened Ir–N3 (in ppy3 ligand) and Ir–C1 (in ppy1 ligand) bonds, and elongated Ir–N2 and Ir–C2 (in ppy2 ligand) bonds. The (C^N)3 ligands in 2a–2c get closer to the Ir(III) center due to the spin density of these complexes predominantly localized on the Ir(III) and (C^N)3 ligands, whereas the (C^N)1 and (C^N)2 ligands are pushed away upon excitation to the T1 state. These shortened bond distances in the T1 state would suggest the larger involvement of the ligands in the excited states.
Fig. 1 Optimized geometry structures of 1a in the ground state. C and N atoms are shown in gray and blue, respectively, while for clarity H atoms are not displayed.
structure of 1a is shown in Fig. 1, along with the numbering of some key atoms. It reveals that the Ir center coordinated by the three anionic C^N ligands adopts the fac-Ir(C^N)3 chelate disposition in a distorted octahedron. The main structural parameters of the studied complexes in the S0 and T1 states are summarized in Table 1. It is noted that the Ir–C (Ir–C1, Ir–C2 and Ir–C3) bond lengths show negligible changes (within ∼0.005 Å) from Ir( ppy)3 → 1a–1c → 2a–2c, indicating that the introduction of the triphenylamine/diphenylamine unit into the pyridyl/phenyl moiety of the ppy ligand causes minor effects on the geometric structures of these complexes. For Ir–N (Ir–N1, Ir–N2 and Ir–N3) bonds, the triphenylamine unit in the pyridyl moiety of the ppy ligand leads to a slight contraction (
