Synthesis, characterization and density functional theory investigations of the electronic, photophysical and charge transfer properties of donor–bridge–acceptor triaminopyrazolo[1,5-a]pyrimidine dyes

• Pyrazole-carboxamides have been synthesized and characterized.
• DFT and TDDFT were applied to study the optical and charge transfer properties.
• The π-conjugation lowers the energy level of the frontier molecular orbitals.
• The lowered LUMO energy level would be beneficent for the flow of electrons.
• The electron injection, LHE and electronic coupling constant have been computed.

We have synthesized multifunctional dyes 3-(4-methyl-phenylazo)-6-(4-nitro-phenylazo)-2,5,7-triaminopyrazolo[1,5-a]pyrimidine (4a) and 3-(4-methyl-phenylazo)-6-(4-acetyl-phenylazo)-2,5,7-triaminopyrazolo[1,5-a]pyrimidine (4b), then characterized by IR, 1H NMR and 13C NMR techniques. The ground state geometries have been computed by using density functional theory at B3LYP/6-31G∗ level of theory. The absorption spectra have been calculated by using time dependent density functional theory with and without solvent. The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are delocalized and localized on throughout the backbone, respectively. Solvent also play important role towards elevating the dipole moment. Significant red shift in absorption wavelengths have been observed in methanol compared to without solvent. We discussed the electron injection, electronic coupling constant and light harvesting efficiency.

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