Theoretical study on a series of iridium complexes with low efficiency roll-off property

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 406–412

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Theoretical study on a series of iridium complexes with low efficiency roll-off property Ming-Xing Song a, Guo-Feng Wang a, Jin Wang a, Yu-Hai Wang a, Fu-Quan Bai b,⇑, Zheng-Kun Qin a,⇑ a b

College of Information Technology, Jilin Normal University, Siping 136000, People’s Republic of China Institute of Theoretical Chemistry, State Key Laboratory of Theoretical and Computational Chemistry, Jilin University, Changchun 130023, People’s Republic of China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A series of Ir (III) complexes were

investigated by DFT and TD-DFT method.  The Ir complexes appeared here have an advantage of low efficiency roll-off property.  The materials appeared here are practical for OLED industrialization.

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 31 March 2014 Received in revised form 26 May 2014 Accepted 1 June 2014 Available online 26 June 2014

A series of heteroleptic cyclometalated Ir (III) complexes for OLEDs application have been investigated theoretically to explore their electronic structures and spectroscopic properties. The geometries, electronic structures, and the lowest-lying singlet absorptions and triplet emissions of (piq)2Ir(acac) (labeled 1) and theoretically designed models (piq)2Ir(dpis) (labeled 2), (4Fpiq)2Ir(dpis) (labeled 3), (4F5M-piq)2Ir(dpis) (labeled 4), (4,5-2F-piq)2Ir(dpis) (labeled 5) and (5-F-piq)2Ir(dpis) (labeled 6) were investigated with density functional theory (DFT)-based approaches, where, piq = 1-phenylisoquinolato, acac = acetylacetonate and dpis = diphenylimidodisilicate. Their structures in the ground and excited states have been optimized at the DFT/B3LYP/LANL2DZ and TDDFT/B3LYP/LANL2DZ levels, and the lowest absorptions and emissions were evaluated at B3LYP and M062X level of theory, respectively. Furthermore, the energy-transfer mechanism of these complexes also be analyzed here, and the result shown that the complexes 1–6 are having the low efficiency roll-off property. Except that, the oscillator strength analyze shown that the complexes 2–6, which were designed by theory, are suitable for OLED since their high oscillator strength property. Ó 2014 Elsevier B.V. All rights reserved.

Keywords: OLEDs Ir(III)-complex TD-DFT Excited state Efficiency roll-off

Introduction Luminescent transition metal complexes are employed in a diverse range of applications, notably as phosphorescent emitters ⇑ Corresponding authors. Tel./fax: +86 434 3292050 (Z.-K. Qin). E-mail addresses: (Z.-K. Qin).

[email protected]

(F.-Q.

Bai),

http://dx.doi.org/10.1016/j.saa.2014.06.088 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

[email protected]

for organic light-emitting diodes (OLEDs) and for solid-state lighting in the future [1]. In this regard, cyclometalated iridium (III) complexes have received special attention as dopants for harvesting the otherwise nonemissive triplet states formed in OLEDs [2]. The complexes are charge neutral and generally have good chemical and photochemical properties, such as high thermal stability, strong spin–orbit coupling effect of heavy metal, which can, to a large extent, partially remove the spin-forbidden nature of the

M.-X. Song et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 134 (2015) 406–412

T1 ? S0 radiative relaxation. Among them, iridium (III) complexes are regarded as the most effective materials in OLEDs, because of which, they would display bright phosphorescent emission spanning the whole visible spectra, making them suitable to serve as ideal phosphors for OLEDs applications. Nowadays, many interesting Ir complexes emitting in green, red, blue, and other color regions have been developed with good device performances [3]. Unfortunately, most of them cannot satisfy to be used for applications in displays, solid-state lighting, and so on, since that a fast reduction in efficiency known as rolloff, however, occurs when the drive current increases, This leads to a much lower luminance and more power consumption [4]. In 2011, Zheng et al. reported the first sky blue phosphorescent material ((dfppy)2Ir(tpip)) with low efficiency roll-off [3k], and in order to perfect the luminous mechanism of it, we also gave a lecture to analysis it by theory [5]. so, in this article, we theoretical design another ligand dpis like tpip. In order to verification the methods we used are suitable for Ir complexes, we theoretically investigated the molecular structures, the absorption and phosphorescence properties of iridium (III) complexes (piq)2Ir(acac) (labeled 1) in this paper, and the results we got are similar to the experimental datas. Furthermore, based on the designing idea of (dfppy)2Ir(tpip), the diphenylimidodisilicate (dpis) ligand is used on designing a series of red Ir complexes here: (piq)2Ir(dpis) (labeled 2), (4Fpiq)2Ir(dpis) (labeled 3), (4F5M-piq)2Ir(dpis) (labeled 4), (4,5-2F-piq)2Ir(dpis) (labeled 5) and (5-F-piq)2Ir(dpis) (labeled 6). And we also investigated the electronic structures, a part of the frontier molecular orbital composited by fragments (FMOfs) and the frontier molecular orbital composited by atom orbitals (FMOas), absorption and phosphorescence properties of iridium (III) complexes 1–6 by DFT and TDDFT method. It is worth noting that we discussed the mechanism of charge transfer in the process of emitting in the final of this paper, and the computational result shown that the Ir complexes with dpis ligand not only have a lower efficiency roll-off, who are similar to (dfppy)2Ir(tpip) (our previous work) [5], but also cause the electrons actively, so we hope that the materials we design can be synthesis in future.

Methodology The geometries of ground-state and the lowest-lying triplet excited-state have been investigated by the DFT and the timedependent DFT (TD-DFT) methods with Becke’s LYP (B3LYP) exchange–correlation functional with the double-f quality basis set: 6-31G* [6] and LanL2DZ [5,7] respectively, in addition, one f-type polarization function (af = 0.938) [8] was augmented to the Ir atom [9]. And there were no symmetry constraints on these complexes. A relativistic effective core potential (ECP) for Ir atom replaces the inner core electrons leaving the outer layer[(5s2)(5p6)] electrons and the (5d6) valence electrons [7b]. The basis sets were depicted as Ir (8s6p3d/3s3p2d), C, N, O, F (10s4p1d/3s2p1d), Si(16s10p1d/4s3p1d) and H (4s/2s). This combination of basis set is adequate to describe the ground and excited state geometries of the Ir complexes, and it has been verified and discussed elsewhere [10]. The respective optimized geometries of ground and excited states and the spectral data were associated with the polarized continuum model (PCM) in CH2Cl2 medias, since the experimental spectral data are obtained in CH2Cl2 solution, with the default parameters embedded in Gaussian09 to obtain a valid approximation of chemical environment, which have been shown to provide accurate interpretation and predication for the transition metal complexes in numerous applications in our previous work [5,11]. The M062x functional together with the same basis set mentioned

407

above were adopted to evaluate the emission nature [12]. Furthermore, the stable configurations of these complexes can be confirmed by frequency analysis, in which no imaginary frequency was found for all configurations at the energy minimal. All calculations have been performed with Gaussian09 suite of program with a tight self-consistent field convergence threshold for both gradient and wave function convergence [13]. Results and discussion Geometries in the ground state S0 and the lowest-lying triplet state T1 The ground-state geometries were optimized by the DFT method with Becke’s LYP (B3LYP) exchange–correlation function, and the optimized geometries of the lowest-lying triplet excitedstate were obtained by TD-DFT/B3LYP approach. There were no symmetry constraints on these complexes. The sketch maps of the complexes are shown in Fig. A.1. And the optimized ground-state geometrical structures for the complexes are shown in Fig. A.2 ((piq)2Ir(acac) (labeled 1)) and Fig. B.1, along with the numbering of some key atoms. The main geometry structural parameters of the ground states (S0) of the complexes are summarized in Table A.1 together with the BP86/TZ2P/TZP, B3LYP/TZP/DZP computational structure datas and experimental datas of the complex 1 [14], from which, we can find that our calculated structural parameters in the complex 1 are in good agreement with BP86/TZ2P/TZP, B3LYP/TZP/DZP and experimental values. All complexes maintain the quasi-octahedral geometry around the metal centers as being observed in other typical six-coordinated Ir (III) complexes. But, it should be noted that there are subtle distortion among the structural parameters of them. As the ground states (S0), compere to complex 1, the bond lengths Ir–O of complexes 2–6 are increasing 0.44–0.59 Å, and the bond angles N1–Ir–O1 are decreasing 0.2–0.5 degree, this should be attributed to the phenyl (abbreviated as Ph), which cause the steric hindrance of the molecules increased, connecting to the atoms Si. In addition, the atom F, whose atomic radius is larger and electron-withdrawing ability is stronger than atom H, cause the electron cloud come on closer to the main ligands of complexes, so that, the bond lengths Ir–O of complexes 3–6 are shorter than 2. The structure parameters of lowest-lying triplet states (T1) are also listed in Table A.1. Compared with the ground states, the

Fig. A.1. Sketch structures of the complexes.

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Fig. A.2. Optimized structures of complexes (piq)2Ir(acac) in the ground states at DFT/B3LYP/LANL2DZ level.

variation of structure parameters in T1 is more severely. Fortunately, from these variations we can obtain a lot of useful information below: First, there have been large changes in bond lengths Ir–N (0.006–0.037 Å) while they are very slightly in Ir–C (0–0.015 Å), it means that the ligand-to-metal charge transfer transition (3LMCT) is mainly appears between isoquinoline (abbreviated as Iql) and metal atom iridium in phosphorescence emissions T1 ? S0; Second, there are small changes in bond lengths Ir–O (0.003–0.01 Å), it is indicate that the ligand-to-ligand charge transfer transition (3LLCT) plays a bit role in all the emission progress. In summary, we can roughly conclude that in complexes 1–6, the 3LMCT is the main charge transfer transition, and the influence of 3LLCT is very slightly. Frontier molecular orbital properties It is known that the observed differences in optical and chemical properties of these complexes depend mainly on the changes of the ground-state electronic structure. The concept of emission color turning by grafting various substituents and use the electron-withdrawing substituents at the cyclometalating ring relies on the fact that the lowest excited state is relatively well described

as a HOMO to LUMO transition in a given ligand [15]. Therefore, we will discuss in detail the ground-state electronic structure with the special emphasis on the HOMO and LUMO distribution, energy levels, and energy gaps. The frontier molecular orbital composited by fragments (FMOfs) and the frontier molecular orbital composited by atom orbitals (FMOas) of the complexes are listed in Tables B.1–B.12 (Supporting Information). The HOMO and LUMO distribution, energy levels, and energy gaps are plotted in Fig. A.3, and the serial number of the atoms is shown in Fig. B.2. In complexes 1–6, the HOMO maily resides on the metal Ir (3D and 4D) and the Ph of main ligands (Ir: 52%, 54%, 52%, 49%, 49% and 54%; Ph: 44%, 42%, 42%, 46%, 46% and 42%), and the metal Ir d orbital (3D and 4D) is an anti-bonding combination with the Ph moieties as p-orbital which is contributed by atoms C5 (2P), C7 (2P), C19 (2P) and C21 (2P); while the LUMO distributes on the Iql of main ligands (96%, 96%, 96%, 96%, 96% and 94% respectively), and the contributing by atoms is not same: for complexes 1, 3 and 4, the electron cloud located on C8 (2P and 3P), C22 (2P and 3P), C15 (2P and 3P), C29 (2P and 3P), C10 (3P), C24 (3P), N1(2P and 3P) and N2 (2P and 3P); while for complex 2, either atom Ir (3D) has the same contribution with the atoms mentioned in 1, 3 and 4; and there is no contribution by atoms C10 and C24 for the other two complexes. This means that in absorption S0 ? T1 (HOMO ? LUMO), metal to ligand charge transfer transition (1MLCT) (d[Ir] ? p*[piq/Iql] character) is appearing between metal atom Ir (III) and Iql (part of main ligand), with intraligand charge transfer transition (1ILCT) (p[piq/Ph] ? p*[piq/Iql] character) is appearing in main ligands (between Iql and Ph), and there is no ligand to ligand charge transfer transition (1LLCT) and metal central (1MC)(d[Ir] ? d[Ir] character) in these complexes, so, we speculate that the complexes in this article are also have low efficiency rolloff, since they have a similar property to complex (dfppy)2Ir(tpip). The electron-withdrawing group F is connected to the Ph of main ligand, where the HOMO orbital located, in complexes 3 and 5, so the HOMOs of them are lower than 1 and 2 is reasonably (0.08–0.16 eV), while the energy levels of 4 is similar to 1 and 2, that is because the methyl, which is an electron donating group, is existing with F, and for complex 6, F is connecting to the end of Iql (The influence is slimly), thus the energy levels of it nearly have no changes to complexes 1 and 2. (Shown in Fig. A.3.) Absorption and emission in CH2Cl2 media The calculated absorption spectra in CH2Cl2 Solution associated with their oscillator strengths, assignment and excitation energies

Table A.1 Main optimized geometry structural parameters of the complexes in the ground and the Lowest lying triplet states at the B3LYP level, respectively, together with the computational values and X-ray diffraction from the reference for complexes (piq)2Ir(acac). 1 Bond

a,b c

S0(Comptl/Ref.ma,b,c)

2 T1(Comptl/Ref.mb)

Selected bond distances (Å) Ir–C1 1.999/1.995a/2.001b/1.987c Ir–C2 1.999/1.995a/2.001b/1.993c Ir–N1 2.064/2.043a/2.061b/2.045c Ir–N2 2.064/2.043a/2.061b/2.053c Ir–O1 2.209/2.175a/1.172b/2.169c Ir–O2 2.209/2.175a/1.172b/2.134c

1.987/1.977a/1.977b 1.987/1.977a/1.977b 2.034/2.046a/2.052b 2.034/2.048a/2.052b 2.207/2.175a/2.167b 2.207/2.176a/2.168b

Bond angles N1–Ir–C1 N1–Ir–C2 N1–Ir–O1 N1–Ir–O2 N1–Ir–N2

81.1 97.3 94.6 87.7 176.1

(deg) 79.7 96.7 95.0 88.9 174.8

Computational parameters from Ref. [14]. Experimental parameters from Ref. [14].

3

S0

T1 1.993 1.993 2.061 2.061 2.268 2.268

79.8 96.7 94.6 89.0 174.9

4

S0 1.992 1.979 2.077 2.031 2.259 2.258

79.7 96.8 94.8 87.9 176.4

T1 1.992 1.992 2.062 2.062 2.260 2.260

79.8 96.6 94.7 88.9 174.8

5

S0 1.992 1.986 2.078 2.025 2.254 2.256

79.7 96.1 94.4 88.8 176.0

T1 1.991 1.991 2.063 2.063 2.259 2.259

79.7 96.7 94.8 88.9 174.8

6

S0 1.980 1.991 2.030 2.077 2.255 2.256

81.1 97.5 94.0 88.2 176.1

T1 1.992 1.992 2.062 2.062 2.253 2.253

79.7 96.8 94.8 88.8 174.9

S0 1.977 1.992 2.034 2.076 2.248 2.250

81.1 97.5 94.0 88.1 176.3

T1 1.993 1.993 2.060 2.060 2.265 2.265

79.8 96.8 94.5 89.0 175.0

1.980 1.992 2.031 2.075 2.257 2.258 81.2 97.7 93.9 88.2 176.6

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409

Fig. A.3. Presentation of the energy levels, energy gaps and orbital composition distribution of the HOMO and the LUMO for the complexes.

are listed in Table A.2 together with the experiment details, and the fitted Gaussian type absorption curve is depicted in Fig. A.4. The phosphorescent emissions of the complexes in CH2Cl2 solution under the TDDFT/M062X/LanL2DZ;6-31G* level calculations together with major contribution and charge transfer transition

characters are listed in Table A.3, and the frontier molecular orbital composited by fragments (FMOfs) and the frontier molecular orbital composited by atom orbitals (FMOas) properties are listed in Table B.1-S12 (Supporting information). The calculated lowestlying absorption and emission of the complex 1 are comparable

Table A.2 Calculated absorption of the complexes in CH2Cl2 media at TD-B3LYP level, together with experimental values. Complex

States

Energy (nm/eV)

Oscillator

Main configurations

Assign

1

S1 S5 S13 S25 S54

519(477a)/2.39 393(377a)/3.15 336(336a)/3.69 292(286a)/4.25 245(226a)/5.05

0.088 0.138 0.201 0.228 0.117

HOMO ? LUMO(70%) HOMO  2 ? LUMO(66%) HOMO  4 ? LUMO(65%) HOMO ? LUMO + 5(40%) HOMO  6 ? LUMO + 3(35%) HOMO  11 ? LUMO(35%)

Ir/piq-1/piq-2 ? piq-1/piq-2 (1MLCT/1ILCT) Ir ? piq-1/piq-2 (1MLCT) Ir/piq-1/piq-2 ? piq-1/piq-2 (1MLCT/1ILCT) Ir/piq-1/piq-2 ? piq-1/piq-2 (1MLCT/1ILCT) piq-1/piq-2/acac ? piq-1/piq-2 (1LLCT/1ILCT) piq-1/piq-2/acac ? piq-1/piq-2 (1LLCT/1ILCT)

2

S1 S6

530/2.34 400/3.01

0.074 0.177

S11 S21 S59

349/3.55 301/4.11 258/4.80

0.238 0.511 0.288

HOMO ? LUMO(70%) HOMO  2 ? LUMO(52%) HOMO  1 ? LUMO + 1(43%) HOMO  3 ? LUMO + 1(65%) HOMO  1 ? LUMO + 4(63%) HOMO  7 ? LUMO + 2(53%)

Ir/piq-1/piq-2 ? piq-1/piq-2 (1MLCT/1ILCT) Ir/piq-1/piq-2 ? piq-1/piq-2 (1MLCT/1ILCT) Ir/piq-1/piq-2 ? piq-1/piq-2 (1MLCT/1ILCT) Ir/piq-1/piq-2 ? piq-1/piq-2 (1MLCT/1ILCT) Ir/piq-1/piq-2 ? piq-1/piq-2 (1MLCT/1ILCT) piq-1/piq-2/dpis ? dpis (1LLCT/1ILCT)

S1 S6

502/2.47 389/3.18

0.074 0.158

S21 S57

299/4.15 259/4.79

0.384 0.201

HOMO ? LUMO(70%) HOMO  2 ? LUMO(58%) HOMO  1 ? LUMO + 1(34%) HOMO  1 ? LUMO + 4(61%) HOMO  8 ? LUMO + 2(45%) HOMO  7 ? LUMO + 2(32%)

Ir/4Fpiq-1/4Fpiq-2 ? 4Fpiq-1/4Fpiq-2 (1MLCT/1ILCT) Ir/4Fpiq-1/4Fpiq-2 ? 4Fpiq-1/4Fpiq-2 (1MLCT/1ILCT) Ir/4Fpiq-1/4Fpiq-2 ? 4Fpiq-1/4Fpiq-2 (1MLCT/1ILCT) Ir/4Fpiq-1/4Fpiq-2 ? 4Fpiq-1/4Fpiq-2 (1MLCT/1ILCT) 4Fpiq-1/4Fpiq-2/dpis ? dpis (1LLCT/1ILCT) 4Fpiq-1/4Fpiq-2/dpis ? dpis (1LLCT/1ILCT)

S1 S6

516/2.40 391/3.17

0.083 0.152

S10

354/3.50

0.247

S21 S57

301/4.12 258/4.79

0.431 0.124

HOMO ? LUMO(70%) HOMO  2 ? LUMO(59%) HOMO  1 ? LUMO + 1(32%) HOMO  3 ? LUMO + 1(56%) HOMO ? LUMO + 3(32%) HOMO  1 ? LUMO + 4(64%) HOMO  11 ? LUMO + 1(44%) HOMO  8 ? LUMO + 2(40%)

Ir/4F5Mpiq-1/4F5Mpiq-2 ? 4F5Mpiq-1/4F5Mpiq-2 (1MLCT/1ILCT) Ir/4F5Mpiq-1/4F5Mpiq-2 ? 4F5Mpiq-1/4F5Mpiq-2 (1MLCT/1ILCT) Ir/4F5Mpiq-1/4F5Mpiq-2 ? 4F5Mpiq-1/4F5Mpiq-2 (1MLCT/1ILCT) Ir/4F5Mpiq-1/4F5Mpiq-2 ? 4F5Mpiq-1/4F5Mpiq-2 (1MLCT/1ILCT) Ir/4F5Mpiq-1/4F5Mpiq-2 ? 4F5Mpiq-1/4F5Mpiq-2/dpis (1MLCT/1ILCT) Ir/4F5Mpiq-1/4F5Mpiq-2 ? 4F5Mpiq-1/4F5Mpiq-2/dpis (1MLCT/1ILCT) dpis ? 4F5Mpiq-1/4F5Mpiq-2 (LLCT) dpis ? 4F5Mpiq-1/4F5Mpiq-2 (LLCT)

S1 S6

520/2.38 392/3.16

0.086 0.159

S10

352/3.52

0.216

S21 S59

298/4.17 259/4.81

0.407 0.266

HOMO ? LUMO(70%) HOMO  2 ? LUMO(59%) HOMO  1 ? LUMO + 1(33%) HOMO ? LUMO + 3(48%) HOMO  3 ? LUMO + 1(47%) HOMO  1 ? LUMO + 4(63%) HOMO  6 ? LUMO + 2(59%)

Ir/4,5-2F-piq-1/4,5-2F-piq-2 ? 4,5-2F-piq-1/4,5-2F-piq-2 (1MLCT/1ILCT) Ir/4,5-2F-piq-1/4,5-2F-piq-2 ? 4,5-2F-piq-1/4,5-2F-piq-2 (1MLCT/1ILCT) Ir/4,5-2F-piq-1/4,5-2F-piq-2 ? 4,5-2F-piq-1/4,5-2F-piq-2 (1MLCT/1ILCT) Ir/4,5-2F-piq-1/4,5-2F-piq-2 ? 4,5-2F-piq-1/4,5-2F-piq-2/dpis (1MLCT/1ILCT) Ir/4,5-2F-piq-1/4,5-2F-piq-2 ? 4,5-2F-piq-1/4,5-2F-piq-2 (1MLCT/1ILCT) Ir/4,5-2F-piq-1/4,5-2F-piq-2 ? 4,5-2F-piq-1/4,5-2F-piq-2 (1MLCT/1ILCT) dpis ? dpis (1ILCT)

S1 S6

537/2.31 402/3.09

0.071 0.176

S11 S20 S60

353/3.51 308/4.03 258/4.80

0.214 0.524 0.352

HOMO ? LUMO(70%) HOMO  2 ? LUMO(52%) HOMO  1 ? LUMO + 1(42%) HOMO  3 ? LUMO + 1(65%) HOMO  1 ? LUMO + 4(62%) HOMO  8 ? LUMO + 2(57%)

Ir/5-F-piq-1/5-F-piq-2 ? 5-F-piq-1/5-F-piq-2 (1MLCT/1ILCT) Ir/5-F-piq-1/5-F-piq-2 ? 5-F-piq-1/5-F-piq-2 (1MLCT/1ILCT) Ir/5-F-piq-1/5-F-piq-2 ? 5-F-piq-1/5-F-piq-2 (1MLCT/1ILCT) Ir/5-F-piq-1/5-F-piq-2 ? 5-F-piq-1/5-F-piq-2 (1MLCT/1ILCT) Ir/5-F-piq-1/5-F-piq-2 ? 5-F-piq-1/5-F-piq-2 (1MLCT/1ILCT) 5-F-piq-1/5-F-piq-2/dpis ? dpis (1LLCT/1ILCT)

3

4

5

6

a

The absorption of experimental data from Ref. [16].

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where me is the mass of an electron and ⁄ is the reduced Planck constant. The quantum states jnmn i; n = 1, 2, . . ., are assumed to have several degenerate sub-states, which are labeled by mn. ‘‘Degenerate’’ means that they all have the same energy En. The operator Ra is the sum of the x-coordinates ri,a of all N electrons in the system, etc.:

Ra ¼

N X ri;a

ðA:2Þ

i¼1

Fig. A.4. Simulated absorption spectra of the complexes in CH2Cl2 media with the calculated data under the TD-B3LYP/LANL2DZ level.

to the experimental value in Tables A.2 and A.3 [16]. As shown in Table A.2 and Fig. A.4, the lowest absorption bands are 519, 530, 502, 516, 520 and 537 nm for complex 1–6, respectively, and the HOMO–LUMO (70%) is the predominant transition. Generally, upon substitution on the main ligands with strongly electron-withdrawing groups, like F, an extreme variability in the absorption properties is manifested, and a large bathochromic shift or hypochromic shift of whole absorption spectrum is observed [17]. However, it is very interesting that for these complexes, the inserted F group on the Ph moiety results the spectrum shift weakly. That is attributed to both the HOMO and LUMO energy levels are localized on main ligands, thus, it is slightly about the influence of F group on energy gap. On another hand, all the important transitions with the greatest oscillation strengths are not attributed by HOMO–LUMO transition. By taking complex 1 for example, the absorption at 292 nm with the highest oscillation strength is attributed to HOMO ? LUMO + 5 (40%). The HOMO is mostly localized on the atom Ir and Ph (C5, C7, C19 and C21) of main ligand, and the Iql (C4, C7, C10, C13, C18, C21, C24, and C27) of main ligand are predominantly responsible for the distribution of LUMO + 5, the excitation HOMO ? LUMO + 5 can be assigned to (p[piq/ph] ? p*[piq/Iql]) transition with the 3ILCT transition character. Similarly, all the intense absorption bands of the other complexes have been summarized in Table A.2. Notably, compare to 1, the complexes 2–6 have the higher oscillation strengths (nearly 2–3 times), so according to Eq. (A.1) [18], we think the ligand dpis are suitably for these complexes. The oscillator strength f12 of a transition from a lower state j1m1 i to an upper state j2m2 i may be defined by:

f12 ¼

XX 2 me ðE2  E1 Þ jh1m1 jRa j2m2 ij2 3 h2 m2 a¼x;y;z

ðA:1Þ

The oscillator strength is the same for each sub-state j1m1 i. According to our previous work [5], the calculated emission by M062X is a little better to B3LYP compered to experiment for Ir complexes. So in this paper, the emission calculations under the TDDFT/M062X/LanL2DZ;6-31G* level for complexes 1–6, and the result are 653, 639, 657, 651, 652 and 652 nm respectively. For complex 1, the computational data present a little red shift compere to experimental value 632 nm [6], that because in theory calculation, the structure calculation of excited states cause looseness of structure since the process of optimize is under unimolecular environment but it is crystal structure in experiment, thus the computational value within the margin of error. According to our calculation, as Table B.1 (Supporting information) shown, the observed emission can be assigned as 3LMCT (between main ligands and metal atom, p*[piq/Iql] ? d[Ir]) and 3ILCT (in main ligands, p*[piq/Iql] ? p[piq/Ph]). The emission data for the other compounds also listed in Table A.3 and the FMOfs and FMOas properties listed in Tables B.1–B.12 (Supporting information), and we will not give much more discussion here. Furthermore, compere to the absorption, the emission of the complexes 1–6 present red shift 134, 109, 155, 135 and 132 nm respectively, that because the energy gap between HOMO and LUMO for the triplet excited states. Namely, the HOMO energy levels are decreased but the LUMO energy levels are increased for the triplet states. And the energy levels of the triplet states about complexes 1–6 are list in Table B.20. Energy-transfer mechanism In general, the Ir(III) trication is a 5d6 center and the electronic properties of its polyimine complexes share several features with those of other well-known octahedral complexes of Fe(II), Ru(II), Os(II), and Re(I) [19], whose metal centers are 3d6, 4d6, 5d6, and 5d6, respectively. And the degenerate d orbitals of the Ir (III) cations are destabilized and split in an octahedral ligand field, which because of the different spatial extension of the d orbitals and the field strength exerted by the ligands [8]. It should be noted that the absorption of Ir (III) complex is associated with electronic transitions from the ground state to the singlet levels of various nature and electronic localization, 1ILCT, 1LLCT, 1MLCT and 1MC transitions. Furthermore, since the 5d6 orbital, the ligand field splitting is very strong and the MC levels are pushed so high in energy, so generally they do not affect the emission properties. Thus in this section, we will discuss the affection of 1MLCT, 1ILCT

Table A.3 Phosphorescent emissions of the complexes in CH2Cl2 solution under the TD-M062X calculations, together with the experimental values.

a

Complex

Emissions (nm/eV)

Major contribution

Character

1 2 3 4 5 6

653(632a)/1.90 639/1.95 657/1.89 651/1.90 652/1.90 652/1.90

L ? H(56%); L ? H(55%); L ? H(60%); L ? H(57%); L ? H(58%); L ? H(55%);

3

The emission of experimental data from Ref. [16].

L ? H  1(35%) L ? H  1(39%) L ? H  1(31%) L ? H  1(35%) L ? H  1(34%) L ? H  1(39%)

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

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also is described in molecule of complex 1, shown in Fig. A.6 (we shown an electrons transition passageway in it). In this figure, we find that the effective electron transitions are mainly appearing among atoms Ir, N1, N2, C7, C8, C10, C15, C21, C22, C24 and C29, which belong to MLCT and ILCT, so we are sure that the ILCT are related to the low efficiency roll-off, but LLCT cause a longer charge transport distance and a longer phosphorescence lifetime, and of cause it will take a higher efficiency roll-off property to complexes. The other complexes in this article are similar to 1, we will not give more discussion for them.

Conclusion

Fig. A.5. The electron transition process of the complexes in this article.

and the 1LLCT transition characters on absorptions of these complexes. In order to make the discussion clearly, we take the complex 1 as an example here. As Fig. A.4 and Table A.2 and Table B.13 shown, there are five peaks in the absorption curve of complex 1, at 519, 393, 336, 292 and 245 nm respectively. For 519 nm, the main configuration as HOMO ? LUMO, assigned 1MLCT (d[Ir] ? p*[piq/Iql]) and 1ILCT (p[piq/Ph] ? p*[piq/Iql]), and the contributing atoms are: HOMO: Ir, C5, C7, C19, C21; LUMO: N1, N2, C8, C10, C15, C22, C24, C29, the information of the other wavelengths also be listed in Table B.13. And from Table A.3, the emission can be assigned as 3 LMCT (between main ligands and metal atom, p*[piq/Iql] ? d [Ir]) and 3ILCT (in main ligands, p*[piq/Iql] ? p[piq/Ph]) of complex 1. Therefore, the electron transition process can be divided into three parts (shown in Fig. A.5): First: Absorption, the electrons are excited to the LUMOs (LUMO, LUMO + 3 and LUMO + 5) from the HOMOs (HOMO, HOMO  2, HOMO  4, HOMO  6 and HOMO  11) orbital; Second: Vibrational Relaxation (VR) and Intersystem Crossing (ISC), the excited electrons go to LUMO from the higher orbitals (LUMO + 3 and LUMO + 5), this is a nonradiative process; Third: Emission, the electrons jump to the lower energy levels (HOMO and HOMO  1) from LUMO orbital in the form of photons. In order to discuss it clearly, the electron transfer process

In order to understand the phosphorescence mechanism of Ir complex, we have analyzed the charge transition path of the iridium (III) complexes (piq)2Ir(acac) (S0 ? S1, S1 ? T1 and T1 ? S0), and base on DFT/B3LYP/LANL2DZ:6-31G*, TDDFT/B3LYP/ LANL2DZ:6-31G* and TDDFT/M062X/LANL2DZ:6-31G* level, the geometrical structures, absorptions and emissions of complex (piq)2Ir(acac), (piq)2Ir(dpis), (4Fpiq)2Ir(dpis), (4F5M-piq)2Ir(dpis), (4,5-2F-piq)2Ir(dpis) and (5-F-piq)2Ir(dpis) were calculated, and the conclusions are as follows: (1) The computational optimized geometry parameters of (piq)2Ir(acac) are similar to the details of literature, and the calculated absorptions and emissions agree well with the experimental values. Thus the methods we used in this article are available; (2) The frontier molecular orbital properties of the complexes in this article are similar to (dfppy)2Ir(tpip) (our previous work), 1MLCT between metal and main ligands and 1ILCT in main ligands, and for this, we can concluded that the iridium (III) complexes appeared in this article are having the low efficiency roll-off property; (3) As having the same main ligand piq, the Ir complexes we designed with a dpis auxiliary ligand have stronger oscillator strength; it is means that the ligand dpis, which cause the electrons actively, is suitable for Ir complex. Namely, the Ir complex with a dpis ligand will have a high luminous efficiency.

Acknowledgments The authors are grateful to the financial aid from the National Natural Science Foundation for Creative Research Group (Grant No. 21003057), China Postdoctoral Science Foundation (Grant No. 2013M541286), and the Science and Technology Development of Jilin Province of China (Grant Nos. 201201078, 20101512, 20110320 and 20140520109JH).

Appendix A. Supplementary material

Fig. A.6. The electrons transition passageway of the complexes in this article.

The frontier molecular orbital composited by fragments in the ground state for the complexes at the DFT/B3LYP level are listed in Tables B.1–B.6, and the frontier molecular orbital composited by atom orbitals are listed in Tables B.7–B.12, and the relevant orbitals components and assignment of absorption spectra is listed in Table B.13, The Cartesian coordinates for complex 1–6 are list in Tables B.14–B.19. And the HOMO and LUMO energy level of the triplet states about complex 1–6 are list in Table B.20. The optimized ground-state geometrical structures for the complex 1–6 are shown in Fig. B.1, the sketch structures of the complexes 1–6 together with the numbers is shown in Fig. B.2. Supplementary

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