Synthesis, Photophysical Properties, and Ab Initio Calculations of 2-(2-Hydroxy/methoxyphenyl)pyrimidines Substituted by Sterically Hindered Electron-Donating Groups

A series of 2-(2-hydroxyphenyl)pyrimidines bearing sterically hindered electron-donating groups (DMAC, PXZ, PTZ, and NPh2), together with their methoxy precursors, were synthesized and investigated by photophysical studies and ab initio calculations. While the hydroxy derivatives were essentially nonemissive due to efficient nonradiative deactivation associated with excited-state intramolecular proton transfer (ESIPT) processes, the methoxy analogues displayed clear fluorescence and positive solvatochromism, consistent with an intramolecular charge transfer (ICT) character. The emission properties strongly depended on the donor nature and substitution pattern, with NPh2 derivatives showing the highest fluorescence quantum yields. Theoretical calculations revealed that steric effects and twisted donor–acceptor geometries govern the balance between ICT, ESIPT, and radiative pathways. These results provide valuable insights for the design of pyrimidine-based color-tunable fluorophores.