Fluorescence of 2-Hydroxy Chalcone Analogs with Extended Conjugation: ESIPT vs. ICT Pathways

The chalcone derivatives with hydroxy group (<b>2</b>) have been examined using low-temperature fluorescence spectroscopy. The study aimed to freeze the intramolecular charge transfer (ICT) motion in order to reveal the potential hidden transition(s) that are difficult to observe at room temperature. Although chalcone <b>2</b> revealed one emission peak at ~667 nm at room temperature, it exhibited two emission peaks (λ_em = 580 and 636 nm) in EtOH at liquid N₂ temperatures (77 K). With the aid of model compound <b>3</b> with methoxy group and aluminum complex <b>2</b>-Al³⁺, attempts were made to assign these emission peaks. The results point towards the possibility of the coexistence of ICT and excited state intramolecular proton transfer (ESIPT) in the chalcone derivatives with extended conjugation.