Theoretical Insights into the Phosphorescence Quantum Yields of

Article pubs.acs.org/JPCC

Theoretical Insights into the Phosphorescence Quantum Yields of Cyclometalated (C∧C*) Platinum(II) NHC Complexes: π‑Conjugation Controls the Radiative and Nonradiative Decay Processes Yafei Luo,† Yanyan Xu,† Wenting Zhang,† Wenqian Li,† Ming Li,† Rongxing He,† and Wei Shen*,† †

College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, China S Supporting Information *

ABSTRACT: In this article, the radiative and nonradiative decay processes of four cyclometalated (C∧C*) platinum(II) N-heterocyclic carbene (NHC) complexes were unveiled via density functional theory and time-dependent density functional theory. In order to explore the influence of π-conjugation on quantum yields of (NHC)Pt(acac) (NHCN-heterocyclic carbene, acac = acetylacetonate) complexes, the factors that determine the radiative process, including singlet−triplet splitting energies, transition dipole moments, and spin−orbit coupling (SOC) matrix elements between the lowest triplet states and singlet excited states were calculated. In addition, the SOC matrix elements between the lowest triplet state and the ground state as well as Huang−Rhys factors were also computed to describe the temperatureindependent nonradiative decay processes. Also, the triplet potential energy surfaces were investigated to elucidate the temperature-dependent nonradiative decay processes. The results indicate that complex Pt-1 has higher radiative decay rate than complexes Pt-2−4 due to the larger SOC matrix elements between the lowest triplet states and singlet excited states. However, complexes Pt-2−4 have smaller Huang−Rhys factors, smaller SOC matrix elements between the lowest triplet and the ground states, and higher active energy barriers than complex Pt-1, indicating that complexes Pt-2−4 have smaller nonradiative decay rate constants. According to these results, one may discern why complex Pt-2 has higher phosphorescence quantum efficiency than complex Pt-1; meanwhile, it can be inferred that the nonradiative decay process plays an important role in the whole photodeactivation process. In addition, on the basis of complex Pt-2, Pt-5 was designed to investigate the influence of substitution group on the photodeactivation process of rigid (NHC)Pt(acac) complex.

1. INTRODUCTION In recent years, as high-efficiency phosphorescent emitters, Nheterocyclic carbene (NHC)-based transition metal complexes have aroused extensive attention in the field of organic lightemitting diodes (OLEDs).1−4 Generally, in comparison with other coordinated ligands, NHC ligands exhibit distinctive advantages: they possess exceptionally strong σ-bonding and easily tunable steric and electronic properties. As is well-known, the strong ligand field of the carbene can raise the energy of metal-centered d−d excited states, thus leading to the increase of the energy gap with the emissive excited states, which is favorable for the improvement of phosphorescence quantum yield. Moreover, owing to the good stability of metal−carbene bonds, these materials are believed to have long operational lifetime when applied in organic electronic devices. Over the past decades, phosphorescent Ir(III) and Pt(II) complexes have already been widely doped into OLEDs as emitters because of the origination of their electroluminescence from both singlet and triplet excited states, and thus the achievement of utilization of nearly 100% electrogenerated excitons can be possible.5−8 Additionally, for both Ir(III) and Pt(II) complexes, a wide emission color tunability and luminescence quantum yield improvement can be achieved via wisely designing reasonable ligands, which has been proven in experimental and theoretical studies.9−14 Therefore, it is a sagacious strategy © 2016 American Chemical Society

to adopt NHC as cyclometalated ligand to coordinate with Pt for obtaining the promising phosphorescent emitters with high emission quantum yield. To improve the phosphorescence quantum efficiency, the molecular rigidity of transition metal complexes is an important parameter to which importance should be attached, since it has great influence on the value of nonradiative decay rate constant.15,16 Generally speaking, the more rigid the molecule, the smaller the degree of structural deformation between the emissive excited state and the ground state, which is unfavorable to the nonradiative decay process. Therefore, good structural rigidity is desirable in designing high-efficiency molecules.17−19 Recently, Thomas Strassner and co-workers have synthesized a series of bidentate (NHC)Pt(acac) complexes to explore the influence of enlarging the π system on phosphorescence quantum efficiency.20 According to their studies, enlarging the phenyl ring at the backbone of the ligand can result in the increase in the phosphorescence quantum yield owing to the sharp decrease in the nonradiative decay rate constants. Therein, the phosphorescence quantum efficiency of Pt-1 is 0.40, and that of Pt-2 is 0.58 (Figure 1). By their Received: December 14, 2015 Revised: January 23, 2016 Published: January 25, 2016 3462

DOI: 10.1021/acs.jpcc.5b12214 J. Phys. Chem. C 2016, 120, 3462−3471

Article

The Journal of Physical Chemistry C

ZFS. The radiative decay rate from each sublevel can be calculated according to the following equations κir =

4α0 3 1 i = κ r(S0 , Tem ΔES0 − Tiem 3 )= τi 3t0

∑

|M ji|2

j ∈ {x , y , z}

(1)

where the radiative lifetime from each sublevel i (i = 1, 2, 3) of triplet excited states to the ground state is denoted as τi and the fine structure constant is defined as α0 Moreover, t0 = (4πε0)2ℏ3/mee4, ΔES0−Tiem is the transition energy between the triplet excited states and the ground state, and Mij is on behalf of the spin−orbit coupled Tem → S0 transition dipole moment which can be showed as follows: ⟨S0|μĵ |Sn⟩⟨Sn|Ĥ SO|Tiem⟩

∞

M ji

=

∑

E(Sn) − E(Tem)

n=0 ∞

+

∑

⟨S0|Ĥ SO|Tm⟩⟨Tm|μĵ |Tiem⟩

m=1

Figure 1. Chemical structures of the investigated platinum(II) complexes.

E(Tm) − E(S0)

(2)

Based on the linear response theory, the transition dipole moment from the ith sublevel of triplet excited states and ground state can be evaluated, and the Cartesian components j ∈ {x, y, z} are used to represent spin eigenfunctions. The operators μ̂ j, Ĥ SO represent the electric dipole and spin−orbit Hamiltonian, respectively. According to a thermal population distribution determined by Boltzann statistics of the three sublevels, the total radiative decay rate constant can be computed by the following equation:

analysis, the main cause of this result is that complex Pt-2 has stronger rigidity than complex Pt-1. In order to further explore the influence of enlarging the phenyl ring at the backbone of the NHC ligand on nonradiative decay rate constant, complexes Pt-3 and Pt-4 are designed with two phenyl rings at NHC and phenyl ring moieties, respectively, which are shown in Figure 1. So far, although there has been abundant research focusing on the theoretical investigation of phosphorescent transition metal complexes, the computation and interpretation of their emissive properties are still difficult, especially the investigation of nonradiative decay pathways. Actually, nowadays more and more studies demonstrate that not only is the simplified energy gap law insufficient for analyzing the nonradiative decay rate, it is desirable to make a thorough and in-depth understanding of the complicated radiationless pathways. In this report, we make detailed analyzes on nonradiative decay of the studied complexes. Density functional theory (DFT) calculation is performed to obtain Huang−Rhys factors (Si) to elucidate the relationship between molecular rigidity and nonradiative decay rate constants. The thermally activated nonradiative photodeactivation processes, i.e., 3ES (emission state) → TS(3ES/3MC) → 3MC (metal-centered excited state) → minimum energy crossing point (MECP) conversion, are illustrated by DFT. In addition, the TD-DFT calculation including SOC is also applied. We hope this theoretical study on (NHC)Pt(acac) complexes would provide valuable guides for understanding and designing high-efficiency phosphorescent transition metal complexes.

κr =

κ1r + κ2r exp( −ZFS1,2 /κBT ) + κ3r exp( −ZFS1,3 /κBT ) 1 + exp( −ZFS1,2 /κBT ) + exp( −ZFS1,3 /κBT ) (3)

In general, for transition metal complexes, the values of ZFS1,2 and ZFS1,3 are typically