English

Aggregation-Induced Emission Property of a Novel Pt(Ⅱ) Metal Complex and Its Self-Sensitized Oxidation Reaction in Photo-Excitation State

• WANG Shizhao ,
• LI Weijun ^, ,
• YU Yue ,
• LIU Jin ,
• ZHANG Cheng ^,

• State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, College of Chemical Engineering, International Science & Technology Cooperation Base of Energy Materials and Application, Zhejiang University of Technology, Hangzhou 310014, P. R. China

Corresponding author: LI Weijun, liwj@zjut.edu.cn ZHANG Cheng, czhang@zjut.edu.cn

• Received Date: 19 February 2019
  Accepted Date: 21 March 2019
  Revised Date: 18 March 2019
  Available Online: 15 November 2019
• Fund Project:

  The project was supported by the National Natural Science Foundation of China (51603185, 51673174), Zhejiang Provincial Natural Science Foundation of China (LQ19E030016, LY19E030006, LZ17E030001), China Postdoctoral Science Foundation (2018M632498), and the Zhejiang Provincial Postdoctoral fellowship, China (Z71101009)

Abstract: The excited states of transition metal complexes with a wide range of photochemical and photophysical properties have attracted considerable attention recently. However, the luminescence property is affected by concentration quenching in practical applications. Aggregation-induced emission (AIE) is an effective strategy to solve this problem. In this work, a new imidazole-based N^C^N Pt(Ⅱ) metal complex, PtP2IM, with the AIE property was synthesized and characterized according to its single crystal structure. Under visible light, we found that the metal complex undergoes a photo-oxidation reaction with the generation of a new red-emitting, imidazole/benzoylimino-based N^C^N' Pt(Ⅱ) metal complex, PtPIMO, which was also confirmed by the crystal structure. Additional studies on the reaction process and conditions of this photo-oxidation reaction were conducted using different methods, such as NMR, UV-Vis spectroscopy, and so on. The experimental results showed that the change from PtP2IM to PtPIMO gradually occurred, and the new photochemical reaction was finally concluded as the C＝C double bond in either one of the two imidazole rings of the PtP2IM complex was attacked by oxygen to generate a new complex, PtPIMO, under photoirradiation in air. Electron paramagnetic resonance (EPR) measurements demonstrated the production of singlet oxygen, which is an excited state of oxygen with a high energy. Through the density functional theory (DFT) calculations, the electronic transition was determined to be a metal to ligand charge transfer (MLCT) in which more energy could transfer from the triplet excited state of PtP2IM to the ground-state oxygen to generate singlet oxygen (¹O₂) with a high intersystem crossing (ISC) efficiency due to the spin-orbit coupling of Pt heavy atoms. When large amounts of the singlet capture agent, triethylenediamine (TEDA) were added, the previously observed UV-Vis spectra change that corresponded to the photo oxidation reaction was not detected, which means that the photo-oxidation reaction observed in the case of PtP2IM was because of the oxidation by singlet oxygen. When oxygen was removed, excellent photostability and an obvious aggregation-induced emission (AIE) were observed for PtP2IM with the luminescent quantum efficiency of PtP2IM in solution and as a film at ~3% and ~20%, respectively. Based on the packing structure in the crystal, we observed that there were no strong intermolecular interactions, such as π-π or Pt-Pt interactions. Additionally, many intermolecular CH―π bonds between the two adjacent PtP2IM molecules were observed, which could effectively limit the rotation of the peripheral phenyl group linked to the imidazole ring. Thus, the AIE property of PtP2IM was attributed to the restricted intramolecular rotation (RIR) effect of the peripheral flexible phenyl group that was linked to the imidazole ring in the solid state in which the vibration of multiple peripheral benzene rings was effectively suppressed. This decreased the non-radiative transition rate and induced the high luminescent quantum efficiency. In the aggregation state, PtP2IM still demonstrated the photo-oxidation reaction by singlet oxygen. Thus, we report a new Pt(Ⅱ) metal complex, PtP2IM, with the AIE property that can undergo an uncommon photo-oxidation reaction in the photo-excitation state. This work aimed to elucidate the basic photochemical and photophysics of transition metal complexes with the AIE property.

Key words:
• Pt(Ⅱ) metal complex
•  /  Photo-oxidation reaction
•  /  Singlet oxygen
•  /  MLCT
•  /  AIE

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