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<p>This work applied DFT and TD-DFT to explore how substituent type (-NH<sub>2</sub> or -CN) and position (<i>ortho</i> vs. <i>meta</i> on the phenolic ring) regulate excited-state intramolecular proton transfer (ESIPT) and photophysical behavior in 2-(2′-hydroxyphenyl)benzothiazole (MEO) derivatives. Four model compounds were designed: MEO-<i>o</i>-NH<sub>2</sub>, MEO-<i>m</i>-NH<sub>2</sub>, MEO-<i>o</i>-CN, and MEO-<i>m</i>-CN. Geometry optimization, S<sub>0</sub>/S<sub>1</sub> IR analysis, QTAIM hydrogen-bond critical point evaluation, and IRI visualization show that the -CN group strengthens the intramolecular hydrogen bond (IHB) in the S<sub>1</sub> state and promotes ESIPT, whereas the -NH<sub>2</sub> group weakens the IHB and suppresses ESIPT. The S<sub>1</sub> IHB strength order is MEO-<i>o</i>-NH<sub>2</sub> &lt; MEO-<i>m</i>-NH<sub>2</sub> &lt; MEO-<i>o</i>-CN &lt; MEO &lt; MEO-<i>m</i>-CN. UV-Vis and fluorescence simulations reveal that stronger electron-withdrawing ability leads to larger Stokes shifts, <i>o</i>-CN induces the largest redshift (92&#xa0;nm), reflecting pronounced excited-state charge polarization. Fluorescence quenching in MEO-<i>o</i>-NH<sub>2</sub> and MEO-<i>m</i>-NH<sub>2</sub> arises from twisted intramolecular charge transfer (TICT) formation. S<sub>0</sub>/S<sub>1</sub> potential energy curves confirm that -CN lowers the ESIPT barrier (accelerating proton transfer), while -NH<sub>2</sub> raises it (inhibiting transfer). Crucially, <i>ortho</i>-substitution exerts stronger regulatory effects than <i>meta</i>-substitution, underscoring the decisive role of spatial proximity in tuning hydrogen-bond geometry and proton-transfer efficiency.</p>

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A Computational Investigation into the Substituent-dependent Modulation of ESIPT Behavior and Fluorescent Properties in MEO Derivatives

  • 期刊论文

  • 发表日期：

    2026年5月26日

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  • Yanni Wang,
  • Zishan Lin,
  • Sisi Yu,
  • Zhifeng Chen,
  • Xiandong Du,
  • Xiaoyun Xia,
  • Kun Wang,
  • Xiuwei Shen,
  • Chaojie Wang

摘要

This work applied DFT and TD-DFT to explore how substituent type (-NH2 or -CN) and position (ortho vs. meta on the phenolic ring) regulate excited-state intramolecular proton transfer (ESIPT) and photophysical behavior in 2-(2′-hydroxyphenyl)benzothiazole (MEO) derivatives. Four model compounds were designed: MEO-o-NH2, MEO-m-NH2, MEO-o-CN, and MEO-m-CN. Geometry optimization, S0/S1 IR analysis, QTAIM hydrogen-bond critical point evaluation, and IRI visualization show that the -CN group strengthens the intramolecular hydrogen bond (IHB) in the S1 state and promotes ESIPT, whereas the -NH2 group weakens the IHB and suppresses ESIPT. The S1 IHB strength order is MEO-o-NH2 < MEO-m-NH2 < MEO-o-CN < MEO < MEO-m-CN. UV-Vis and fluorescence simulations reveal that stronger electron-withdrawing ability leads to larger Stokes shifts, o-CN induces the largest redshift (92 nm), reflecting pronounced excited-state charge polarization. Fluorescence quenching in MEO-o-NH2 and MEO-m-NH2 arises from twisted intramolecular charge transfer (TICT) formation. S0/S1 potential energy curves confirm that -CN lowers the ESIPT barrier (accelerating proton transfer), while -NH2 raises it (inhibiting transfer). Crucially, ortho-substitution exerts stronger regulatory effects than meta-substitution, underscoring the decisive role of spatial proximity in tuning hydrogen-bond geometry and proton-transfer efficiency.