Fluorescence Emissive Fluorinated Pyrimido[5,4-<i>h</i>]quinazolines: Synthesis, Structure, and Photophysical and Halochromic Properties

Planar tricyclic aromatics and azaheteroaromatics are of significant interest to organic chemists in the field of materials science owing to their excellent photoconductivity, electroluminescence and fluorescence properties enabled by their rigid extended π-conjugation. Furthermore, tricyclic azaheteroaromatics are effective ligands for organometallic catalysis and a prominent drug discovery framework. Herein, we present the first one-step synthesis of a less-accessible tricyclic fluorophore comprising a 5,6-difluoro-2,9-diarylpyrimido[5,4-<i>h</i>]quinazoline core entailing the regiospecific tandem condensation of tetrafluoroterephthalonitrile and derivatives of benzamidine hydrochloride in the presence of potassium carbonate or cesium carbonate under mild conditions. Single-crystal X-ray diffractometry studies confirmed the structure of the 5,6-difluoro-2,9-diarylpyrimido[5,4-<i>h</i>]quinazolines, and various interactions such as π-π stacking. Spectroscopic measurements, including ultraviolet-visible (UV-vis) and fluorescence spectroscopy, of the synthesized pyrimido[5,4-<i>h</i>]quinazolines revealed that they have excellent fluorescence properties both in the solution and solid states, displaying red-shifted maximum fluorescence wavelengths in the solid state compared to those in the solution state. Density functional theory (DFT) calculations and electrochemical analyses revealed that the introduction of an electron-withdrawing group, such as a halogen, stabilized the energy levels. Thermogravimetric (TG) analysis indicated high decomposition temperatures for 5,6-difluoro-2,9-diarylpyrimido[5,4-<i>h</i>]quinazolines, confirming their favorable thermal stability. Finally, the halochromism of 5,6-difluoro-2,9-diarylpyrimido[5,4-<i>h</i>]quinazolines arising from the protonation of the amino group and/or pyrimidine backbone is described.