Balancing the Push-Pull Effect on the Synthesis and Fluorescent Properties of New ESIPT Dyes for Thin Film Applications

The development of functional materials exploiting excited-state intramolecular proton transfer (ESIPT) requires a deep understanding of the push-pull balance on their reactivity, emission mechanism, and response to the surrounding environment. In this study, we focused on the model 2-(2-hydroxyphenyl)benzothiazole (HBT) core, and we have developed a simplified synthetic pathway to obtain two new ESIPT dyes with amide groups at the para or meta positions of the phenolic ring. We studied the impact of the geometry of the push-pull system for the two HBT isomers, unraveling that it strongly influenced their reactivity patterns and fluorescence emission. The ESIPT emission of the new molecules in the solution was affected by the solvent polarity, transitioning from pure keto to pure enol emission as the solvent polarity was increased. In the spin-coated films, the intermolecular interactions generated by the push-pull balance and the molecular shape of the deposited materials were also found to be essential for defining their morphology and the related emissive properties. Furthermore, these features were tuned by applying thermal annealing or by changing the solvent used for thin film deposition. The combination of both strategies was employed to provide a more favorable molecular ordering and to increase the fluorescence quantum yield of the neat films by 150%.