Theoretical investigation on electronic structure and photophysical properties of a series of mixed-carbene cyclometalated iridium(III) complexes with different ancillary ligand applied in phosphorescent organic light-emitting diodes

By using density functional theory (DFT) and time-dependent density functional theory (TDDFT), the geometrical structure, electronic structure and photophysical properties of a series of mixed-carbene cyclometalated iridium(III) complexes with different ancillary ligand have been explored. The frontier molecular orbital (FMO) components and energy levels for all studied complexes have been investigated. The lowest lying absorptions were calculated to be at 327, 322, 333, 332 and 332 nm for these complexes, which have the transition configuration of HOMO→LUMO. The lowest energy emissions for these complexes are localized at 413, 399, 498, 418 and 415 nm, respectively, simulated in CH2Cl2 medium at the M062X level. One complex designed could possess the largest radiative decay rate (kr) value and be a potential candidate for blue emitters in organic light-emitting diodes (OLEDs). The theoretical study can provide a useful guidance for design and synthesis of new iridium(III) complexes in phosphorescent materials.


Introduction
In recent two decades, organic light-emitting diodes (OLEDs) have been widely studied due to their applications in flat display and light sources [1][2][3][4][5]. Phosphorescent OLEDs can capture singlet excitons and triplet excitons at the same time due to the heavy atom effect, so theoretically 100% internal quantum yield can be achieved. Among them, phosphorescent iridium complexes have received the most attention [6][7][8][9][10]. Recently, the phosphorescence color and quantum efficiency have been widely investigated by theoretical and experimental researchers. Many green and red-emitting OLEDs devices with excellent performances have been reported. However, the development of blue-emitting phosphors still remains a challenging task to achieve high efficiency and stability for blue phosphorescence [11][12][13]. In order to obtain full-color display, it is necessary to obtain commercially available red, green, and blue light materials. Among them, the blue emitters require a wide energy gap between the excited triplet state and the ground state. The approaches to obtain efficient phosphorescent blue emitting materials are to either seek ligands with high triplet energy or use electron-withdrawing ancillary ligands to raise the emission energy by stabilizing/destabilizing the highest occupied/lowest unoccupied molecular orbital (HOMO/LUMO). N-heterocyclic carbenes (NHCs) are typical carbene ligands incorporated into blue-emitting iridium complexes [14][15][16]. The acyclic diaminocarbenes (ADCs) can be even stronger σ-donors than NHCs on account of the greater 2p character in their σ orbital [17,18], which could potentially destabilize metal-centered triplet excited state ( 3 MC) to an even greater extent than is possible with NHCs and improve the photostability of blue phosphorescent complexes. Recently, Hanah Na et al. have investigated a class of cyclometalated iridium complexes with general structure Ir(C^C: NHC )2(C^C: ADC ), where C^C: NHC is an N-heterocyclic carbene (NHC) derived cyclometalating ligand and C^C: ADC is a different type of cyclometalating ligand featuring an acyclic diaminocarbene (ADC) [19]. In this study, on the basis of complex 3b [19], that is, complex 1 in Fig. 1, four complexes [2, 3, 4 and 5 in Fig. 1(a)] have been designed. The electronic structures and photophysical properties of these complexes have been theoretically studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The ancillary ligand with different substituent group will have an effect on the electronic structure and photophysical properties of all the studied complexes.
To calculate electronic singlet and triplet states of all the studied complexes, we use the DFT (density functional theory) with PBE0 (hybrid-type Perdew-Burke-Ernzerhof exchange correlation functional) and UPBE0 (unrestricted PBE0) respectively [20][21][22]. The value of lowest-lying emission wavelength of complex 1 is calculated by M062X method which is in good agreement with the experimental wavelength value [19]. The quasi-relativistic pseudopotentials of Ir atom proposed by Hay and Wadt with 17 valence electrons were employed, and a ''double-ξ'' quality basis set LANL2DZ was adopted as the basis set [23].  [24][25][26] in dichloromethane (CH2Cl2) media was applied to simulate the absorption and emission spectral properties from the experimental results by Hanah Na et al. [19]. All calculations were performed by using the Gaussian 09 software package [27]. GaussSum 2.5 is used for UV/Vis absorption spectra analysis with a full width at half maximum (FWHM) of 3000 cm -1 based on the present TDDFT computational results [28].

Geometries in the ground state S0 and triplet excited state T1
The sketch map of Ir(III) complexes 1-5 has been presented in Fig. 1(a), and the partial atomic number of complex 1 as a representative has been shown in Fig. 1(b). In order to describe these complexes, the main ligand and ancillary ligand have been respectively named as NHC and ADC moieties. The main optimized geometry parameters of the ground state S0 and triplet excited state T1 are presented in Table 1.
The optimized bond distances of complex 1 are in quite good agreement with available experimental data [19], and the deviation is within 1.8%. The bond angles C1-Ir-C2, C3-Ir-C6 and C4-Ir-C5 are larger than 77°. The bond angles C1-Ir-C6, C2-Ir-C5 and C3-Ir-C4 are larger than 166°. This indicates that all studied Ir(III) complexes with d 6 configuration adopt a pseudo-octahedral coordination geometry. The dihedral angles C1-C3-C6-C4 in complexes 1-5 are less than 2°, which shows a nearly plane structure around the central Ir atom. Especially, the dihedral angle C1-C2-C6-C5 in complex 2 is less than those of other four complexes, which is probably due to the introduction of the strong electron-donating group -N(CH3)2. From the S0 to T1 states, the bond lengths Ir-C4 and Ir-C5 in complexes 1-5 slightly decrease and increase, respectively. The bond angles C3-Ir-C6 and C4-Ir-C5 in complexes 1-5 also slightly decrease and increase, respectively.

Absorption spectra
The vertical electronic excitation energies, oscillator strengths ( f ), dominant orbital excitations and their assignments of the singlet excited state are presented in Table S6 (Supplementary Information). Simulated absorption curves for complexes 1-5 in CH2Cl2 medium have been depicted in Fig. 3.
The absorption spectra shapes of complexes 1-5 are very similar. In addition, there is a small absorption peak at about 260 nm for complexes 4 and 5. It can be seen from Table S6 that the lowest lying singlet→singlet absorption of 1-5 is

Phosphorescence emission properties
In order to further explore the phosphorescent properties of the complexes, the TDDFT method was used to calculate the emission wavelength and transition properties based on the optimized T1 structure. In order to ensure the accuracy of the data, the density functionals B3LYP [29], PBE0 [30], CAM-B3LYP [31], M052X [32], M062X [33] and BP86 [34] were respectively used to calculate the complex 1. The calculated lowest energy emissions at these levels are localized at 2.356, 2.467, 2.808, 3.013, 3.001 and 1.908 eV, deviating from experimental value 2.952 eV [19] by 0.596, 0.485, 0.144, 0.061, 0.049 and 1.051 eV.
Obviously, a good agreement with measured datum was obtained for M062X. Therefore, we have employed the M062X method for emission property calculations of all studied complexes. The calculated emission wavelengths, emission energies and transition nature of complexes 1-5 in CH2Cl2 medium at the M062X level are listed in Table 2. The plots of the molecular orbitals related to emissions of complexes 1-5 have also been presented in Table 3.
In addition, partial frontier molecular orbital compositions (%) of complexes 1-5 in the triplet excited states are presented in Table S7 (Supplementary Information). Table 2 shows that the calculated lowest energy emissions of complexes 1-5 are located at 413, 399, 498, 418 and 415 nm, respectively. Complex 3 has the largest emission wavelength 498 nm among these studied complexes, which shows the introduction of phenyl ring to the ancillary ligand has an obvious effect on the phosphorescence emission properties.
From Table 2 and Table S7, it can be seen that the phosphorescence emission of all studied complexes mainly possesses the transition of LUMO→HOMO configuration. For example, complex 1 has the triplet metal-to-ligand charge transfer ( 3 MLCT)/triplet ligand-to-ligand charge transfer ( 3 LLCT)/triplet  intraligand  charge  transfer  ( 3 ILCT) [π*(NHC)→d*(Ir)+π*(NHC+ADC)] transition characters. Complex 2 has the smallest emission wavelength 399 nm among these studied complexes, which may be due to the introduction of -N(CH3)2 with strong electron-donating ability to the ancillary ligand. For example, the LUMO of complex 1 is distributed on the NHC ligand (91%). From Table 3 and Table S7, it can be seen that the LUMO of all studied complexes are mainly localized on the NHC ligand except complex 3. The HOMO of all studied complexes are mainly localized on the Ir atom and NHC ligand. For example, the HOMO of complex 1 is distributed on Ir atom (34%) and NHC ligand (55%).

Phosphorescence quantum yield
The phosphorescent quantum yield (ΦPL) is obtained by the following equation [35]: in which kr is the radiative decay rate and knr is the nonradiative decay rate. Therefore, it is necessary to increase kr and reduce knr to achieve high phosphorescent quantum efficiency.
The knr from the T1 to the S0 states is usually expressed in the form of the energy law equation (2), and the kr is given by equation (3) [36,37]: