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

Abstract

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. A 1D-NMR and 2D-NMR spectroscopy, as well as a 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 in the solid state and compared with those of related 3-aryl-substituted [1,2,4]triazolo[4,3-c]quinazolines. The exclusion of the aryl fragment from the 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 has been stated. Moreover, electrochemical investigation and DFT calculations have been performed; their results are consistent with the experimental observation.

Keywords: [1,2,4]triazolo[1,5-c]quinazoline; [1,2,4]triazolo[4,3-c]quinazoline; cross-coupling; fluorescence; solvatochromism.

Figure 1

Previously reported 3,5-diaryl[1,2,4]triazolo[4,3-c]quinazolines (A), 5-(4′-amino[1,1′]-biphenyl)quinazolines (B), and target triazoloquinazolines (I,II).

Scheme 1

Synthesis of 5-(4′-amino[1,1′]-biphenyl)[1,2,4]triazolo[4,3-c]quinazolines 4a–f and 5-(4′-amino[1,1′]-biphenyl)[1,2,4]triazolo[1,5-c]quinazolines 5a–f.

Scheme 1

Synthesis of 5-(4′-amino[1,1′]-biphenyl)[1,2,4]triazolo[4,3-c]quinazolines 4a–f and 5-(4′-amino[1,1′]-biphenyl)[1,2,4]triazolo[1,5-c]quinazolines 5a–f.

Figure 2

(a) ¹H NMR spectra of 2a and 3a in DMSO-d₆. (b) ¹H NMR spectra of 2b and 3b in DMSO-d₆. Hydrogen chemical shifts (δ) in ppm.

Figure 3

Molecular structure and selected torsion angles of the compounds 4a, 4e, 5d, and 5e in the thermal ellipsoids of 50% probability.

Figure 3

Molecular structure and selected torsion angles of the compounds 4a, 4e, 5d, and 5e in the thermal ellipsoids of 50% probability.

Figure 4

Normalized absorption spectra of compounds 4a–f (left) and 5a–f (right) in MeCN.

Figure 5

Photographs of 4a–f and 5a–f taken in the dark upon irradiation with a hand-held UV lamp (λ_em = 366 nm).

Figure 6

Absorption (a) and emission (b) spectra of compounds 4a in different solvents: cyclohexane, toluene, THF, DCM, DMSO, MeCN and MeOH; (c) photograph of solutions of 4a taken in the dark upon irradiation with a hand-held UV lamp (λ_em = 366 nm).

Figure 7

Changes of selected dihedral angles of 4e and 5e in MeCN upon photoexcitation.