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Updated: May 28, 2026

Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
Published on: July 19, 2019
A Computational Investigation into the Substituent-dependent Modulation of ESIPT Behavior and Fluorescent Properties
Yanni Wang1, Zishan Lin2, Sisi Yu2
1Pharmacy Department, The Third Affiliated Hospital of Wenzhou Medical University, Wenzhou, Zhejiang, 325200, China.
None:
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.
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