Congratulations to Hao Chen for his article published in Chemistry of Materials-News-Huang Hui Research Group

Congratulations to Hao Chen for his article published in Chemistry of Materials

Congratulations to Chen Hao's article published in Chemistry of Materials!

Organic electroluminescent diodes (OLEDs) have developed rapidly in the field of light-emitting and display screens and gradually occupy the industry market due to their advantages of flexibility, low cost, light weight, and solution processing. In particular, white OLEDs play an important role in practical production and life applications. significance. Since its development in 1995, electro-white light composed of multiple luminescent species has achieved high luminous efficiency. However, the phase separation of multi-luminescent components due to kinetic instability during long-term device use, and single-molecule white electroluminescent materials can effectively solve this problem.

Different from the previous single-molecule electroluminescence white light complexed by excimers or exciplexes, the team of Professor Huang Hui of the University of Chinese Academy of Sciences, the team of Professor Zhao Zujin of South China University of Technology and the team of Professor Peng Qian of the Institute of Chemistry, Chinese Academy of Sciences, adjusted the heterocyclic atom , obtained room temperature photophosphorescence material, applied it to OLED, obtained a white light OLED device composed of single-molecule electrofluorescence and phosphorescence, and opened up a new way to obtain single-molecule electroluminescence white light.

The team synthesized three compounds as shown in Figure 1. After characterizing their basic physicochemical properties, they focused on their photophysical properties. The PhCz-S crystal state has only a short wavelength (450 nm) emission peak under the condition that 408 nm is the excitation wavelength, and exhibits fluorescence properties with a fluorescence lifetime of 1.1 ns. But PhCz-Se and PhCz-Te crystals exhibit fluorescence and phosphorescence double emission. The fluorescence lifetime of PhCz-Se crystal at 440 nm is 285.6 ps, and the fluorescence lifetime of PhCz-Te crystal at 490 nm is 59.9 ps. The phosphorescence lifetimes of the two crystals are 113.1 μs (@590 nm) and 12.2 μs (@650 nm), respectively.

Figure1. (A) Molecular structural formula; (B) Crystal emission spectrum,

In order to better understand the intermolecular interactions and the way of molecular packing, the single crystal structures of the small molecules of these three compounds were analyzed. The compound PhCz-Te has two single crystal structures, namely PhCz-Te-inner with tellurium atoms facing inward and PhCz-Te-outer with tellurium atoms facing outward. Each of the single crystal structures in Figure 2 contains various interaction forces, including C-H∙∙∙π (2.758 Å and 2.859 Å) in the compound PhCz-S; C-H∙∙∙π (2.736 Å) in PhCz-Se Å and 2.856 Å); while the inward-facing structure of the tellurium atoms in PhCz-Te has the interaction forces of C-H∙∙∙π (2.753 Å and 2.758 Å) and C-H∙∙∙O (2.500 Å and 2.658 Å), and the tellurium atoms are facing inwards. Exostructures exist as C-H∙∙∙π (2.738 Å, 2.753 Å and 2.874 Å), C-H∙∙∙O (2.610 Å and 2.658 Å) and C=O∙∙∙π (3.198 Å). In terms of the crystal packing of each compound, the compound PhCz-Te has more weak interactions than the other two compounds, and the compound PhCz-Te has a shorter heteroatom-heteroatom distance, which means it is more compact. The crystal packing of , which brings about stronger phosphorescence emission, which is also consistent with the crystal emission spectrum.

Figure 2. Interaction forces between crystals

Then, the photophysical properties of the three compounds were further verified from the computational point of view. It is not difficult to see that from S to Se to Te, as the atomic number increases, the molecular band gap gradually decreases, and the heavy atom effect becomes more obvious. According to the basic knowledge of photophysics, phosphorescent emission must go through the process of crossing from the singlet state to the triplet state. This spin-forbidden process is proportional to the spin-orbit coupling coefficient, and the heavy atom effect of Se and Te is greatly enhanced. The spin-orbit coupling coefficient, which explains that PhCz-S does not exhibit room temperature phosphorescence while PhCz-Se and PhCz-Te have room temperature phosphorescence properties.

Figure 3. Compound energy level, vibration intensity, and spin-orbit coupling coefficient

Based on the photo-induced double emission properties of PhCz-Se and PhCz-Te, the team introduced them into OLED devices, using PPF and CBP as host materials, respectively, to prepare devices. Figure 4 shows the corresponding electroluminescence spectra. The former obtains fluorescence and phosphorescence emission peaks consistent with the photoluminescence spectrum at low voltage, and forms a white light with CIE color coordinates (0.30, 0.29), which is the first white light OLED composed of fluorescence and phosphorescence of a single material. . In the electroluminescence spectrum of the latter, white light with CIE color coordinates (0.34, 0.33) is composed of the fluorescence and phosphorescence of the host materials CBP and PhCz-Te, and there is no triplet-triplet annihilation as the voltage increases. The reduction of the phosphorescence peak is due to the strong heavy atom effect brought by the tellurium element, which greatly accelerates the intersystem crossing process, making the generated triplet excitons rapidly radiatively transition back to the ground state. This work explains and elaborates the photophysical properties of compounds through the regulation of heteroatoms, and finally obtains a new strategy for electro-white light with single-molecule fluorescent phosphorescence and double emission.

Figure 4. (A,C) Device structure; (B,D) Electroluminescence spectra at different currents。

This result was recently published in Chemistry of Materials. Chen Hao, a doctoral student at the University of Chinese Academy of Sciences, is the first author of the article, and Huang Hui, Zhao Zujin, and Peng Qian are the corresponding authors.