4′-N,N-二乙氨基-3-羟基苯并黄酮激发态分子内质子转移机制的溶剂极性效应——实验和理论研究

doi:10.3866/PKU.WHXB202309027

Abstract

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

Yanglin Jiang, Mingqing Chen, Min Liang, Yige Yao, Yan Zhang, Peng Wang, Jianping Zhang. Experimental and Theoretical Investigations of Solvent Polarity Effect on ESIPT Mechanism in 4′-N,N-diethylamino-3-hydroxybenzoflavone[J]. Acta Phys. -Chim. Sin. 2025, 41(2), 100012. doi: 10.3866/PKU.WHXB202309027

Figures/Tables 10

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References 26

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	λ_abs/nm	λ_em^N*/nm	λ_em^T*/nm	Stokes shift (N*)/cm⁻¹	Stokes shift (T*)/cm⁻¹	I_N/I_T	Φ_FL
in CYH	443	471	618	1343	6392	0.03	13.6%
in DEE	435	500	629	2988	7090	0.13	31.9%
in THF	440	533	638	3966	7053	0.71	60.7%

Solvent	λ_probe/nm	τ (Amplitude, %)
in CYH	N*: 470 nm	16.1 ps (96.47)
		3.47 ns (3.53)
	T*: 650 nm	15.5 ps (−100)
		3.43 ns (100)
in DEE	N*: 500 nm	39.4 ps (91.58)
		2.39 ns (8.42)
	T*: 650 nm	40.1 ps (−100)
		2.36 ns (100)
in THF	N*: 530 nm	95.2 ps (80.83)
		2.25 ns (19.17)
	T*: 650 nm	93.5 ps (−100)
		2.27 ns (100)

Solvent	k_pt^–1/ps	k_-pt^–1/ps	K_eq
in CYH	16.6	453.2	27.3
in DEE	43.0	467.9	10.9
in THF	117.8	496.8	4.2

Solvent	State	d_O―H/Å	d_HB/Å	δ_O―H···O/(°)	ν_―OH/cm⁻¹
in CYH	N	0.97673	2.01827	118.75498	3598
	N*	0.99038	1.87741	123.21769	3372
	T	0.99137	1.90213	122.13116	3360
	T*	0.98128	2.00152	118.26560	3530
in DEE	N	0.97693	2.02402	118.54029	3604
	N*	0.98873	1.89246	122.78375	3396
	T	0.98978	1.91565	121.72884	3384
	T*	0.98105	2.00259	118.25833	3533
in THF	N	0.97620	2.02724	118.42082	3606
	N*	0.98801	1.89952	122.56224	3404
	T	0.98910	1.92181	121.54373	3395
	T*	0.98093	2.00363	118.25851	3535

	in CYH	in DEE	in THF
µ_N/D	7.21	8.13	8.61
µ_N*/D	19.04	20.76	21.59
µ_T/D	7.47	8.47	8.99
µ_T*/D	7.86	9.27	10.02
∆µ_N*-N/D	11.83	12.63	12.98
∆µ_{T*- T}/D	0.39	0.80	1.03

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

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