Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Solid-state effect on luminescent properties of thermally activated delayed fluorescence molecule with aggregation induced emission: A theoretical perspective
Graphical abstract
Introduction
At present, organic materials are widely concerned and applied in various fields, especially for organic light emitting diodes (OLEDs) [[1], [2], [3]]. Discovered by Deng et al., OLED has gradually replaced traditional light-emitting materials after only a few decades of development. Now the latest models of mobile phones and TV monitors are starting to use OLED materials. Compared with traditional liquid crystal display (LCD), it has many advantages, such as self-emission, high contrast, flexibility, low consumption and so on. In particular, the characteristics of self-emission make wearable integrated systems possible [4]. In the future, OLED has unlimited development space. However, the material development of OLED light-emitting layer is not complete. Finding efficient and stable luminescent material has always been the goal of researchers. OLED materials are constantly being refined over the past few decades. From the first generation of fluorescent materials to the second generation of phosphorescent materials, the new thermally activated delayed fluorescence (TADF) is regarded as the third generation emitting materials [[5], [6], [7], [8], [9]]. According to spin statistics, the singlet and triplet excitons are generated with the ratio of 1:3. For fluorescent materials, only singlet excitons can be used and the highest exciton utilization is 25% [[10], [11], [12]]. As for phosphorescent materials, they can convert electrons originally in singlet states to triplet states through efficient intersystem crossing (ISC) process by the enhanced spin-orbit coupling (SOC) effect of heavy metals, this could help to reduce device energy consumption and extend device lifetime. However, its main disadvantage is that the heavy metals such as Ir and Pt are very scarce, costly and highly polluting, this hinders its further development. While for TADF materials, they can convert the triplet electrons to the singlet state through a rapid reverse intersystem crossing (RISC) process, which is associated with the energy gap (ΔEST) between the first singlet (S1) and triplet (T1) excited state following the equation kRISC ≈ exp (−ΔEST/kBT). Thus, 100% exciton utilization can be realized without the need of rare precious metals. The potential for TADF materials is very exciting [[13], [14], [15]]. However, a stubborn problem for TADF materials in practical application is that most TADF emitters suffer from aggregation-caused quenching (ACQ) in solid phase. TADF was widely used in devices when Tang et al. found the Aggregation-Induced Emission (AIE) phenomenon [16,17]. Most TADF molecules also have ACQ phenomenon, and it is usually necessary to doping the molecules in the host molecule as the subject to inhibit the π-π stacking interaction between molecules, so as to avoid the ACQ effect. However, considering from the perspective of technology, doping will increase the complexity of the process and increase the cost and difficulty of preparation. Therefore, the development of TADF molecule with AIE activity has far-reaching significance for the preparation of high-efficiency undoped OLED devices.
The advantages of TADF have been further amplified by the continuous research of scientists. Duan et al. came up with a new mechanism called TADF-sensitizing-fluorescence (TSF) [18]. It breaks through the contradiction between high mobility and high stability of electronic transmission materials. Tang and Zhao et al. made many excellent achievements in the research of Aggregation-Induced Delayed Fluorescence (AIDF) materials [[19], [20], [21]]. Recently, Zhao et al. reported a new multifunctional TADF molecule SBF-BP-DMAC (Fig. 1a), which shows enhanced photoluminescence and bipolar carrier transport ability with AIE properties [22]. To detect the inner mechanism from a theoretical perspective, the photophysical properties of SBF-BP-DMAC in Tetrahydrofuran (THF) solution is studied by the polarizable continuum model (PCM). Besides, in order to better simulate the molecular environment in the solid phase, the combined quantum mechanics and molecular mechanics (QM/MM) method is adopted [[23], [24], [25], [26]]. We qualitatively calculated the rates in the THF and solid phases respectively, and then the excited state energy consumption process was investigated. It was theoretically verified that SBF-BP-DMAC molecule had AIE-TADF property and the experimental measurements were reasonably elucidated. The AIE-TADF mechanisms of the molecule were further analyzed and elaborated from the aspects of geometry structures, energy gap, transition property, Huang-Rhys factor and reorganization energy.
Section snippets
Theoretical method
For TADF systems, the prompt fluorescence efficiency (ΦPF), intersystem crossing efficiency (ΦISC), reverse intersystem crossing efficiency (ΦRISC), thermally activated delayed fluorescence efficiency (ΦTADF) and the total fluorescence efficiency (ΦF) can be obtained by the following equations:
In above equations, kr is the radiative decay rate from S1 to ground state (S0), knr is the
Computational details
As abovementioned, the TADF properties are largely depended on their excited state energy gaps. Thus, the photophysical properties of excited states are studied by time-dependent density functional theory (TD-DFT). The solvent effect in THF is simulated by PCM. Moreover, in order to investigate the effect of the solid environment, we performed calculations by using QM/MM method with two-layer ONIOM approach. The ONIOM model is constructed from the X-ray crystal structure and is shown in Fig. 1
Excited state dynamics
In this section, the prompt fluorescence and delayed fluorescence efficiencies are evaluated, and the excited state properties of SBF-BP-DMAC are discussed. According to the Marcus equation, the ISC and RISC processes are related to the adiabatic energy gap between S1 and T1 and the SOC constants between them. Thus, the SOC constants (with the unit of cm−1) between S1 and T1 are calculated both in THF and solid phase by Dalton 2013 package. Corresponding data are collected in Table 2. Results
Conclusion
In this work, the excited state properties of SBF-BP-DMAC in solvent and solid state were theoretically studied, the AIDF mechanism is revealed. The calculated delayed fluorescence efficiency and total fluorescence efficiency are 0.4‰ and 3.0% in THF respectively, and corresponding values increases to 31.5% and 45.9% in solid phase, all these verify the properties of AIDF. For the inner mechanisms, the geometrical changes between S0 and S1 in solid phase are restricted with smaller RMSD value
CRediT authorship contribution statement
Yuchen Zhang: Writing - original draft. Yuying Ma: Investigation. Kai Zhang: Investigation. Yuzhi Song: Formal analysis.Lili Lin: Formal analysis. Chuan-Kui Wang: Formal analysis. Jianzhong Fan: Writing - review & editing.
Declaration of competing interest
There are no conflicts of interest to declare.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (Grant Nos. 11904210, 11874242, 21933002 and 11974216) and Shandong Provincial Natural Science Foundation, China (ZR2019BA020). Thanks to the supporting of Taishan Scholar Project of Shandong Province. Thanks to the supporting of the Project funded by China Postdoctoral Science Foundation (Grant No. 2018M642689) and the Open Fund of Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, Guangzhou
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