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Cite this: DOI: 10.1039/c5cp00211g

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Theoretical research on the effect of regulated p-conjugation on the photophysical properties of Ir(III) complexes† Li-Ming Xie, Fu-Quan Bai,* Wei Li, Zhi-Xiang Zhang and Hong-Xing Zhang* In this work, the effect of regulated host and auxiliary ligand p-conjugation on the photophysical properties of a series of Ir(III) carbene complexes is examined by using the start-of-the-art theoretical methods. According to our results, all of the lowest-lying and strongest absorption peaks can be assigned as having a mixed ligand-to-ligand/metal-to-ligand charge transfer (LLCT/MLCT) character, but the different ways of introducing phenyl have a great effect on the absorption wavelength variation. In addition, the charge transfer characteristics of lowest-lying emission have some minute differences. In addition, when the extended p-conjugation is broken, the emission wavelength can be effectively retained due to the similar emission charge transfer related electronic density distribution of occupied molecular orbitals and unoccupied molecular orbitals. However, the larger p-conjugation can give rise to remarkably blue-shifted emission. This blue-shifted emission can be attributed to the alteration in the transition character due to intense interactions between nearly degenerate unoccupied molecular

Received 14th January 2015, Accepted 9th March 2015

orbitals. Through the evaluation of the spin–orbit coupling (SOC) effect, we can gain a deeper understanding of the radiative decay rate processes. These results reveal that the larger p-conjugation can

DOI: 10.1039/c5cp00211g

also lead to higher quantum efficiency due to the larger radiative decay and the smaller nonradiative

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and thus, can pave the way for the design of novel and efficient blue phosphorescent materials.

decay rate. Our theoretical studies highlight the role of p-conjugation of the host and auxiliary ligand,

1 Introduction During the past two decades, organic light-emitting diodes (OLEDs) have drawn much attention due to their advantages of flexibility in full-colour displays and low energy consumption in whitelight illumination.1–12 In the emission layer of OLED devices, organic cyclometalated complexes, particularly the Ir(III) and Pt(II) complexes, are commonly used as phosphorescent dopants because of their relatively short excited-state lifetimes, wide colour tuneability, and high emission quantum yields. And these properties result partly from the strong spin–orbit coupling (SOC) effect of heavy metal atoms, which can promote an effective intersystem crossing from the singlet to triplet excited state.13–17 Recently, the phosphorescence colour and quantum efficiency have been widely investigated by theoretical and experimental researchers, and highly efficient red and green phosphorescent materials have been successfully applied in OLEDs.18–20 However, the development

State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People’s Republic of China. E-mail: [email protected], [email protected] † Electronic supplementary information (ESI) available: Tables S1–S6. DOI: 10.1039/c5cp00211g

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of deep-blue phosphorescent emitter materials with high quantum efficiency is still challenging.21,22 Generally, with regard to the common Ir(III) and Pt(II) complexes bearing the C^N (2-phenylpyridine) ligand, the expansion of p-conjugation in the C^N ligand can lead to a red-shifted emission spectrum, which is not desirable for the application of blue phosphorescent OLEDs.23 Therefore, in order to prevent the red-shift in the emission spectrum, an effective approach is to break the p-conjugation.24 Monkman and co-workers investigated the steric effect of the mesityl groups of the bis[4,6(difluorophenyl)-pyridinato-N,C2]picolinate (FIrpic) complex by twisting the adjacent pyridyl rings to block the expansion of p-conjugation, and they showed that the introduction of the steric groups to auxiliary ligands is an effective way to maintain the blue emission.25 In contrast, Thompson et al. found that the larger extension of p-conjugation can give rise to the counterintuitively blue-shifted emission and a high radiative rate constant; it was ascribed to the electronic density redistribution that induced an effective spin–orbit coupling in the lowestlying triplet state.26 Furthermore, there are also some empirical strategies developed for the design of efficient blue phosphorescent complexes. On the one hand, the electron-withdrawing and electron-donating groups can be used to regulate the position

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gap law approximation, the nonradiative decay rate constants (knr) can be simplified as34–38 knr(Tm - S0) p exp{b[E(Tm)  E(S0)]}

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Fig. 1

Structures of complex A and designed complexes B–D.

of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), leading to a larger HOMO and LUMO gap; thus the blue emission occurs. On the other hand, using a moiety with a high ligand field strength can increase the energy level of the nonradiative triplet metal-centred (3MC) d–d states, reducing thermal quenching.27–31 These have been verified in the work of Hogan et al., where the blue-shift in the emission spectrum of the F-substituted Ir(C^N)2(C^C*) (C^C* = 2-phenylmethylimidazole) complex (here we denoted as A, as shown in Fig. 1) occurs due to the stabilization of the HOMO by the introduction of an electron-withdrawing group.32 However, it is worth noting that a large HOMO–LUMO gap might lead to a low quantum efficiency because nonradiative decay pathways become thermally accessible. Hence, in this work, based upon the experimental complex A, the effects of p-conjugation on the photophysical properties and quantum efficiency are elucidated by the introduction of phenyl into the host ligand C^N and the auxiliary ligand C^C*. The structures of complex A and the designed complexes (B, C, and D) are shown in Fig. 1. The electronic structures and the involved photophysical properties of the ground state and the excited state of the four complexes are determined using the density functional theory/time-dependent density functional theory (DFT/TDDFT) method. The intrinsic radiative decay rate and zero-field splitting parameter values are estimated by calculating SOC matrix elements. The nonradiative decay rate is also discussed via the energy gap law. The SOC processes relevant for the quantum efficiency are analysed in detail. Our results reveal the striking correlation between the p-conjugation alteration and the radiative/nonradiative decay rate, namely the larger p-conjugation on the host ligands, the faster radiative decay rate (due to the stronger intersystem crossing by SOC interaction) and the slower nonradiative decay rate (due to the larger energy gap between the excited triplet state and the ground state). We expect that this preliminary work would provide important guidelines for the design of highly efficient blue luminescent materials for applications in OLEDs.

2 Computational details 2.1

Theoretical background

In general, the phosphorescence quantum efficiency is determined by intersystem crossing (ISC), radiative decay, and nonradiative decay. The ISC process is related to the strength of SOC, that is, the stronger SOC causes the faster ISC process, improving the phosphorescence quantum yield.33 According to the energy

Phys. Chem. Chem. Phys.

where the parameter b is related to the structural distortion between the ground state and the involved potential energy surfaces (PESs) of the excited triplet states. The larger structural distortion of PESs leads to a faster nonradiative decay rate, which is adverse to the phosphorescence efficiency. In addition, as the energy gap between the excited triplet state and the ground state decreases, the nonradiative decay rate increases exponentially. Therefore, the nonradiative decay rate can be well evaluated bythe estimation of structural distortions in PESs and the energy gap between the excited triplet state and the ground state. When it comes to the radiative decay, within the regime of the Born–Oppenheimer approximation and perturbation theory, the excited triplet state is perturbed by various excited singlet states due to spin–orbit interactions. The radiative decay rate constants (kr) from the Tm state to S0 can be represented as39–43 8 92

Theoretical research on the effect of regulated π-conjugation on the photophysical properties of Ir(III) complexes.

In this work, the effect of regulated host and auxiliary ligand π-conjugation on the photophysical properties of a series of Ir(III) carbene complexes...
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