Novel 5-Aryl-[1,2,4]triazoloquinazoline Fluorophores: Synthesis, Comparative Studies of the Optical Properties and ICT-Based Sensing Application

Novel tricyclic fluorophores were obtained from 2-aryl-[1,2,4]triazolo[1,5-c]quinazoline-5(6<i>H</i>)-ones through chlorodesoxygenation and subsequent Suzuki-Imamura cross-coupling reactions. Their π-extended analogues were synthesized from 5-(4-bromophenyl)-[1,2,4]triazoloquinazolines. The structure of target fluorophores was confirmed by X-ray single crystal diffraction. Photophysical properties in solutions and solid state were studied. 5-Aminoaryl-substituted [1,2,4]triazolo[1,5-<i>c</i>]quinazolines revealed bright blue fluorescence in toluene (Φ_F > 95%), which can be tuned by solvent polarity, the electronic nature and rigidity of the donor fragment, the π-spacer length, and the annelation pattern. Intramolecular charge transfer (ICT) behaviour was demonstrated using both experimental and theoretical data. Distinct acid-induced spectral and fluorescence changes upon protonation were observed for diethylamino-containing derivatives, indicating their potential applicability as dual-mode (polarity and pH) molecular sensors.