Computational unveiling of solvent-polarity-dependent ESDPT behavior in typical asymmetric PIQ fluorophores

Molecules exhibiting excited-state intramolecular proton transfer (ESIPT) hold significant promise for bioimaging applications. While symmetric ESIPT systems are well-studied, the mechanism of asymmetric excited-state double proton transfer (ESDPT) remains underexplored. This study deciphers the solvent-dependent ESDPT dynamics in the asymmetric fluorophore 3-(3-hydroxypyridin-2-yl)isoquinolin-4-ol (PIQ) using density functional theory (DFT) and time dependent density functional theory (TDDFT). Key evidence-including bond lengths, IR vibrational spectra, IRI, the CVB index, and <i>E</i>_HB-collectively confirms significant hydrogen bond strengthening upon S₀ → S₁ excitation, which promotes the ESDPT reaction. Additionally, employing the potential energy surface (PES), we conducted detailed investigations into solvent polarity effects on ESDPT pathways. The findings revealed a polarity-regulated ESDPT mechanism for PIQ fluorophores, presenting distinct excited-state behaviors in low-polarity <i>versus</i> high-polarity environments.