Insights into the Electronic Properties of Coumarins: A Comparative Study

The effect of substitution on some biologically active di- and tri-coumarins substituted 4-phenyl coumarins was investigated using absorption and fluorescence spectrum profiles. These studies helped in the estimation of excited-state dipole moments (μe) of coumarins by applying two renowned Solvatochromic shift methods Kawaski et al. and Ravi et al. Computational methodology was applied to find the ground state dipole moments (μg) and Onsager cavity of the coumarins from the optimized geometry of the molecules using DFT/B3LYP/6-31++ G(d,p) level of theory. The Mulliken charges, frontier molecular orbitals (FMO), and electrostatic potential surfaces (EPS) were theoretically generated utilizing their optimized geometries to validate the experimental findings. Time Dependent Density Functional Theory (TD-DFT) approach was applied to approximate the excitation energy, dispersion energy, cavitation energy, corresponding wavelengths, oscillator strength for different coumarins in gaseous phase in addition to benzene, ethanol and most polar aqueous medium. The large stoke’s shifts, and high μe values indicate that the intramolecular charge transfer (ICT) process occur in these coumarins which has coerced by an intense S0 → S1 electronic transition. The computational results were matched with the experimental analysis, and revealed characteristics electrical properties of coumarins, as well as confirming the presence of ICT in these coumarins.