Structural and electronic tuning of single-benzene fluorophores with simple methylations

Abstract

Recent advancements in the development of low molecular-weight fluorophores have led to the study of a single-benzene fluorophore (SBF) containing both electron-withdrawing and electron-donating groups within an aromatic ring, resulting in unique photophysical properties. In this study, we explored the modulation of emissions by adjusting the electron-donating properties via the simple methylation of the 2,5-diamino or 2,5-dihydroxy terephthalate structure. Introducing additional functional groups under both symmetrical and unsymmetrical conditions demonstrated changes in the photophysical properties, which was not only owing to the structural variations in the location and number of methyl groups but also due to alterations in the electron-donating properties. The observed effects appeared to be owing to the interactions between the amine and ester groups, as confirmed via single-crystal X-ray diffraction. Finally, hydrolysis of the ester to the corresponding carboxylic acid resulted in various emission wavelengths at different pH values via protonation and deprotonation. This study provides valuable insights for predicting the structural and electronic effects of additional functional groups in SBF.