Pyrazolo[4,3-a]quinindoline as a new highly fluorescent heterocyclic system: Design, synthesis, spectroscopic characterization and DFT calculations

• Pyrazolo[4,3-a]quinindolines are synthesized as new fluorescent heterocyclic systems.
• The spectral and photophysical properties of these compounds are investigated.
• Solvent effects on absorption and emission spectra of the fluorophore are studied.
• DFT and TD-DFT calculations of one structure are performed at the 6-311 + G(d,p) level.

After obtaining the desired precursors in several reactions, new N-alkyl-substituted heterocyclic system pyrazolo[4,3-a]quinindolines (pyrazolo[4,3-f]-indolo[2,3-b]quinolines) were synthesized by one-pot reaction of 1-alkyl-5-nitro-1H-indazole with 2-(1-alkyl-1H-3-indolyl)acetonitrile in MeOH/KOH solution via the nucleophilic substitution of hydrogen in excellent yields. Spectral (UV–Vis, FT–IR, NMR and fluorescence) and analytical data allowed the structures of the synthesized compounds to be established. The values of absorption and fluorescence maxima, extinction coefficients and fluorescence quantum yield of these new heterocyclic fluorophores were obtained and they show highlighting interesting photophysical properties. Density functional theory (DFT) calculations of one structure by using the B3LYP hybrid functional and the 6-311 + G(d,p) basis set were performed to provide the optimized geometry, relevant frontier orbitals and the prediction of 1H NMR chemical shifts. Calculated electronic absorption spectrum of one structure was also obtained by time-dependent density functional theory (TD-DFT) method. Solvatochromic properties of these dyes have been discussed and the results showed that the absorption and emission bands in polar solvents undergo a modest red shift.

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