Planarity‐Controlled ESIPT Versus Cu(II) Coordination in a Hydroxyl‐Rich Schiff Base: Structural Modulation of Excited‐State and Emission Properties

ABSTRACT Strategic control of molecular conformation is key to tuning excited‐state processes in functional organic chromophores. Here, the (E)‐4‐(((3‐(hydroxymethyl)phenyl)imino)methyl)benzene‐1,2,3‐triol ( 1K HBAB) was synthesized and investigated via experimental and theoretical approaches. SCXRD confirms the stabilization of the keto tautomer in the solid state. TD‐DFT calculations reveal energetically accessible enol–keto interconversion, where planar keto forms exhibit ESIPT enabled by strong intramolecular O–H···N hydrogen bonding. FMO analysis shows narrow energy gaps and spatial orbital separation in the planar forms, promoting intramolecular charge transfer (ICT), whereas non‐planar conformers show larger energy gaps. Solvent‐dependent TD‐DFT calculations demonstrate bathochromic shifts in polar media due to enhanced ICT character. Finally, molecular docking reveals a binding affinity of −9.720 kcal mol −1 . The ground‐ and excited‐state intramolecular proton‐transfer (GSIPT/ESIPT) mechanisms of the HBAB were investigated using DFT calculations, revealing that the preorganized O–H···N hydrogen‐bonded six‐membered pseudo‐ring governs proton migration, with a barrierless GSIPT pathway and a low 1.31 kcal mol −1 ESIPT barrier, while electronic excitation further lowers the energetic requirements for subsequent structural rearrangement, thereby establishing ESIPT as a facile pathway for excited‐state relaxation and a key‐contributor to the photophysical behavior of HBAB. These findings establish a direct structure–planarity–ESIPT relationship, guiding the design multifunctional Schiff‐base chromophores with potential applications.