Experimental and DFT/TD-DFT computational investigations of the solvent effect on the spectral properties of nitro substituted pyridino[3,4-c]coumarins

The UV/Vis absorption characteristics of newly synthesized pyridino[3,4-c] coumarins in different neat solvents were investigated experimentally and computationally. It is noted that the nitro-substituted pyridocoumarins can exhibit spectral features with characteristic band in the visible region, which in turn is solvent's polarity dependent with negative solvatochromic behavior. Using the Kamet-Taft approach, the obtained results (R = 0.991) revealed that this negative solvatochromic behavior is dominantly influenced by the solvents' polarizability and hydrogen bonding capability. The experimental results of the spectral properties in solution were interpreted at the molecular level with aid of the DFT and TD-DFT/CAM-B3LYP/6-31+G(d) computational methods with IEFPCM implicit solvation approach. Per benchmarking the TD-DFT simulated spectra with the experimental one, it is demonstrated that the nitro group can induce an intramolecular charge transfer to afford two resonance structures of distinctive spectral features. Such difference in spectral features is interpreted in terms of molecular orbitals, where the two potential resonance structures exhibit different natures for the frontier molecular orbitals, namely the HOMO and LUMO with a significant difference in the corresponding energy gaps. Moreover, the electrostatic potential surfaces of both structures indicate relatively different accessibility toward intermolecular hydrogen bonding with the solvent molecules. The achieved results would provide valuable insights concerning the noteworthy influence of the substituents of pyridocoumarins on their spectral properties and correspondingly their solvatochromic behaviors at the molecular level in different media