4-(Aryl)-Benzo[4,5]imidazo[1,2- a]pyrimidine-3-Carbonitrile-Based Fluorophores: Povarov Reaction-Based Synthesis, Photophysical Studies, and DFT Calculations

Abstract

A series of novel 4-(aryl)-benzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles were obtained through the Povarov (aza-Diels-Alder) and oxidation reactions, starting from benzimidazole-2-arylimines. Based on the literature data and X-ray diffraction analysis, it was discovered that during the Povarov reaction, [1,3] sigmatropic rearrangement leading to dihydrobenzimidazo[1,2-a]pyrimidines took place. The structures of all the obtained compounds were confirmed based on the data from ¹H- and ¹³C-NMR spectroscopy, IR spectroscopy, and elemental analysis. For all the obtained compounds, their photophysical properties were studied. In all the cases, a positive emission solvatochromism with Stokes shifts from 120 to 180 nm was recorded. Aggregation-Induced Emission (AIE) has been illustrated for compound 6c using different water fractions (fw) in THF. The compounds 6c and 6f demonstrated changes in emission maxima or/and intensities after mechanical stimulation.

Keywords: Povarov reaction; aggregation-induced emission; aza-Diels–Alder reaction; benzimidazole; fluorescence; mechanochromic properties; oxidation; pyrimidine.

Scheme 1

Nucleophilic substitution of hydrogen (S_NH) in 2-phenyl-2H-benzo[4,5]imidazo[1,2-a][1,2,3]triazolo[4,5-e]pyrimidine [42].

Scheme 2

Scope of benzimidazole-2-arylimine 3a–f.

Scheme 3

Povarov reaction and rearrangement [49].

Scheme 4

Substrate scope of dihydropyrimidines 5a–f.

Scheme 5

Scope of the 4-(aryl)benzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles 6a–f.

Figure 1

Molecular structure of 6c.

Scheme 6

Plausible reaction mechanisms of dihydropyrimidines 5a–f formation and [1,3] sigmatropic rearrangement.

Figure 2

Absorption (a) and emission (b) spectra of fluorophores 6a–f in THF (c = 10⁻⁵ M).

Figure 3

Emission spectra of dyes 6a–f in powders (a) and photographs of the solid samples under daylight and 365 nm UV irradiation (b).

Figure 4

Emission spectra of dyes 6a–f in PVA films (a) and photographs of the samples under daylight and 365 nm UV irradiation (b).

Figure 5

Emission spectra of 6c in different ratios of THF–water (v/v) mixtures (a). Plot of I/I₀ versus water fraction (vol%), where I₀ is the fluorescence intensity in pure THF and emission images of the 6c in different water fraction mixtures under 365 nm UV illumination (λ_ex = 365 nm) with the concentration of 10⁻⁵ M (b).

Figure 6

(a) Emission spectra of 6c in solid states (λ_ex = 350 nm): as prepared (1), after grinding (2), and after treatment with CH₂Cl₂ (3). (b) Emission spectra of 6f in solid states (λ_ex = 350 nm): as prepared (1), after grinding (2), and after treatment with CH₂Cl₂ (3).

Figure 7

Photographs of 6c (a) and 6f (b) taken under 365 nm UV irradiation.

Figure 8

Energy gaps of fluorophores 6a–f in THF phase.

Figure 9

Donor-acceptor structure 4-aryl-substituted benzo[4,5]imidazo[1,2-a]pyrimidine chromophors 6a–f.

Figure 10

The compound 6c in the thermal ellipsoid at the 50% probability level.

Figure 11

π-π-contacts in the crystal of the compound 6c.