English

Stabilization of the E/Z Configuration for Cyanostilbene-based Luminogens by Enhanced Charge Transfer Excited State

• DONG Yujie ,
• XU Chendong ,
• WANG Shizhao ,
• LI Weijun ,
• SONG Qingbao ^, ,
• ZHANG Cheng ^,

• State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, P. R. China

Corresponding author: SONG Qingbao, qbsong@zjut.edu.cn ZHANG Cheng, czhang@zjut.edu.cn

• Received Date: 04 July 2018
  Accepted Date: 01 August 2018
  Revised Date: 31 July 2018
  Available Online: 15 June 2019
• Fund Project:

  The project was supported by the China Postdoctoral Science Foundation (2018M632498), the Zhejiang Provincial Postdoctoral fellowship, China (Z71101009), the National Natural Science Foundation of China (51603185, 51673174), and the Zhejiang Provincial Natural Science Foundation, China (LY17E030001)

Abstract: The E/Z isomerization reaction is found extensively in most organic molecules containing double bond unit. This limits their practical application as luminescent materials partly, especially under photoirradiation. Therefore, it is important to obtain E/Z isomers with stable configuration in the excited state after photoirradiation. It is well known that cyanostilbene and its analogues play an important role in the development of organic opto/electronic materials. The substituted cyano group on C＝C double bonds has strong electron-withdrawing ability and large steric hindrance, which benefits the formation of donor-acceptor (D-A) structures and formation of intramolecular charge transfer. In our previous work, we reported a triphenylamine-cyanostilbene molecule (TPNCF) formed by modifying the cyanostilbene structure with triphenylamine, which maintained a stable E/Z configuration as a film and in high polar solvents. According to solvatochromism mechanisms and the results of theoretical calculations, we proposed that the charge transfer (CT) excited state between the triphenylamine donor and cyanostilbene acceptor groups induced the stable configuration of the E- and Z- isomers under photoirradiation. Under irradiation, the E/Z isomerization process occurring at a higher energy locally excited (LE) state was suppressed by a rapid internal conversion process from the LE to CT state. This work inspired us to provide a universal and effective molecular design strategy by modifying D-A substituents on double bonds that can successfully stabilize E/Z isomers. To further confirm that the CT excited state induced stable E- and Z- isomers in the cyanostilbene structure under photoirradiation, we designed and synthesized a donor-acceptor phenoxazine-cyanostilbene molecule (PZNCF) and successfully characterized its two E/Z isomers. In comparison with the reported TPNCF molecule, the in-situ NMR and UV spectra of E- and Z- isomers of PZNCF demonstrated that the E/Z isomerization rate became slower under photoirradiation, which indicated that the stronger electron-donating group of phenoxazine substituted in the cyanostilbene structure induced a more stable E/Z isomer configuration in its excited state. DFT calculations and photophysical results indicated that a stronger CT state was generated in both E- and Z- isomers of PZNCF. This further confirmed our hypothesized mechanism where the stable E/Z configuration can be obtained under photoirradiation by forming a suitable donor-acceptor structure to suppress the E/Z isomerization reaction in the LE state by a rapid internal crossing process from the LE to CT state. This molecular design strategy is of great significance to organic photochemistry and photoelectronics for molecules with double bond units.

Key words:
• E/Z configuration
•  /  Charge transfer excited state
•  /  Cyanostilbene
•  /  Donor-acceptor structure
•  /  Locally excited state

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