Tunable Emission\nand Structural Insights of 6‑Arylvinyl-2,4-bis(2′-hydroxyphenyl)pyrimidines\nand Their O<sup>∧</sup>N<sup>∧</sup>O-Chelated Boron\nComplexes

In this study, we present the synthesis and photophysical\ncharacteristics\nof a novel series of 6-arylvinyl-2,4-bis(2′-hydroxyphenyl)pyrimidines.\nThese compounds exhibit nonemissive properties attributed to the potential\noccurrence of a excited-state intramolecular proton transfer process\nfrom the OH groups to the nitrogen atoms of the pyrimidine ring. The\nintroduction of an acid for protonation of the pyrimidine ring results\nin a significant enhancement of the fluorescence response, easily\nperceptible to the naked eye. Notably, these molecules serve as intriguing\nrigid O<sup>∧</sup>N<sup>∧</sup>O ligands for boron\nchelation. The incorporation of boron atoms promotes structural planarity,\nincreases rigidity, and successfully restores fluorescence in both\nsolution and the solid state. Moreover, the photoluminescence was\nfound to be strongly influenced by the nature of the end groups on\nthe arylvinylene fragment, allowing for the modulation of the emission\ncolor and covering the optical spectrum from blue to red. Strong emission\nsolvatochromism was observed in various solvents, a finding that supports\nthe formation of intramolecular charge-separated emitting states,\nparticularly when terminal electron-donating groups are present in\nthe structure. X-ray diffraction analysis enables the determination\nof inter- and intramolecular interactions, as well as molecular packing\nstructures, aiding in the rationalization of distinct luminescent\nbehaviors in the solid state. All experimental findings are elucidated\nthrough extensive density functional theory (DFT) and time-dependent\nDFT calculations.