Design, Synthesis and Photophysical Properties of 5-Aminobiphenyl Substituted [1,2,4]triazolo[4,3-c]- and [1,2,4]triazolo[1,5-c]quinazolines

Two series of novel [1,2,4]triazolo[4,3-c]- and [1,2,4]triazolo[1,5-c]quinazoline fluorophores with 4’-amino[1,1′]-biphenyl residue at position 5 have been prepared via Pd-catalyzed cross-coupling Suzuki-Miyaura reactions. The treatment of 2-(4-bromophenyl)-4-hydrazinoquinazoline with orthoesters in solvent-free conditions or in absolute ethanol leads to the formation of [4,3-c]-annulated triazoloquinazolines, whereas [1,5-c] isomers are formed in acidic media as a result of Dimroth rearrangement. 1D-NMR and 2D-NMR spectroscopy as well as single‐crystal X‐ray diffraction analysis unambiguously confirmed the annelation type and determined the molecular structure of p-bromophenyl intermediates and target products. Photophysical properties of the target compounds were investigated in two solvents and solid state and compared with those of related 3-aryl-substituted [1,2,4]triazolo[4,3-c]quinazolines. The excluding of aryl fragment from triazole ring has been revealed to improve fluorescence quantum yield in solution. Most of the synthesized structures show moderate to high quantum yields in solution. Additionally, the effect of solvent polarity on the absorption and emission spectra of fluorophores has been studied and considerable fluorosolvatochromism was stated. Moreover, electrochemical investigation and DFT calculations have been performed, their results are consistent with the experimental observation.