English

Experimental and Theoretical Investigations of Solvent Polarity Effect on ESIPT Mechanism in 4′-N,N-diethylamino-3-hydroxybenzoflavone

• Yanglin Jiang ^1, ,
• Mingqing Chen ^1, ,
• Min Liang ^2, ,
• Yige Yao ^3, ,
• Yan Zhang ^{1, 4,} ,
• Peng Wang ^{1, ,} ,
• Jianping Zhang ^1,

• 1.

  Key Laboratory of Advanced Light Conversion Materials and Biophotonics, Department of Chemistry, Renmin University of China, Beijing 100872, China

• 2.

  Information Technology Center, Renmin University of China, Beijing 100872, China

• 3.

  State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China

• 4.

  State Key Laboratory of Elemento-organic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China

Corresponding author: Peng Wang, Email: wpeng_chem@ruc.edu.cn; Tel.: +86-10-62516604

• Received Date: 15 September 2023
  Accepted Date: 27 October 2023
  Revised Date: 26 October 2023
  Available Online: 15 February 2025
• Fund Project:

  the Fundamental Research Funds for the Central Universities 

  the Research Funds of Renmin University of China 

  the Natural Science Foundation of China 

Abstract: Excited-state intramolecular proton transfer (ESIPT) is a fundamental photoreaction of significant importance in both chemical and biological systems. This phenomenon typically occurs in chromophores featuring intramolecular hydrogen bonding. Among the molecules undergoing ESIPT, 3-hydroxyflavone derivatives (3-HFs) have garnered significant attention due to their natural origins and environmentally responsive fluorescence properties. A particular 3-HF compound, 4′-N,N-diethylamino-3-hydroxybenzoflavone (D-HBF), distinguished by its extended π-system and red-shifted electronic absorption, has recently been identified as a potent fluorescent probe highly sensitive to changes in environmental polarity. In this study, we systematically explored the ESIPT reaction mechanism of D-HBF in three aprotic solvents: cyclohexane, diethyl ether, and tetrahydrofuran, each possessing varying polarities. Our investigation involved a combination of spectroscopic and theoretical methods. In all three solvents, we observed the characteristic dual emission bands associated with ESIPT, with the intensity ratio of these bands being influenced by the solvent. As solvent polarity increased, we noted a decrease in the rates of both the forward and reverse proton transfer (PT) reactions based on our analysis of fluorescent kinetics. However, the reverse PT was favored. Through density functional theory (DFT) and time-dependent DFT (TDDFT) calculations of bond lengths and bond angles of the intramolecular hydrogen bond in these solvents, we confirmed that the ESIPT reaction in D-HBF is driven by the strengthening of the excited-state hydrogen bond. Notably, upon increasing solvent polarity, the intramolecular hydrogen bonds in the excited N* state weakened, as evidenced by the up-shifted IR absorption frequency of the O―H stretching mode in the S₁ state. Electron intensity analysis of frontier orbitals revealed characteristic intramolecular charge transfer (ICT) occurring in D-HBF upon photoexcitation, attributable to the introduction of a strong electron-donating group at the 4′ position (4′-N,N-diethylamino-). Calculations of potential energy curves for the S₀ and S₁ states confirmed that the PT process tends to occur in the S₁ state rather than the S₀ state, and a more polar solvent generates a more significant potential barrier, hindering the corresponding ESIPT reaction. An analysis of the Gibbs free energy of ESIPT further confirmed that increasing solvent polarity favors the equilibrium shifting toward the N* state. This research lays the foundation for potential future applications of D-HBF as a biological fluorescent probe sensitive to environmental polarity.

Key words:
• 4′-N,N-diethylamino-3-hydroxybenzoflavone
•  /  ESIPT
•  /  DFT/TD-DFT
•  /  Solvent polarity effect
•  /  Fluorescent dynamics

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