Computational analysis of coumarin-pyridine hybrid derivatives: Molecular structure, electrostatic potential surfaces, Mulliken charge distribution, and electronic excitation spectra

Abstract

This study computationally investigates the structural, electronic, and photophysical properties of a parent 3-acetylcoumarin core and seven coumarin-pyridine hybrid derivatives using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods at the CAM-B3LYP/6-311G(2d,p) level. Geometrical analysis reveals a stable, twisted 3D propeller-like conformation driven by steric hindrance between the central pyridine and peripheral phenyl rings. Molecular electrostatic potential and Mulliken charge distributions successfully identify the carbonyl and cyano groups as strong nucleophilic sites, while the amine framework acts as an electrophilic hydrogen-bond donor. Frontier molecular orbital analysis classifies the hybrids into highly reactive, chemically soft systems with narrow energy gaps ΔE ⁓ 0.10 a.u., for example, 3-NO2, 3-Br, and chemically hard, rigid structures with wider gaps ΔE ⁓ 0.20 a.u., for example, 4-CN, 4-CF3. TD-DFT absorption spectra show significant bathochromic shifts for heavy halogens (3-Br- at 423.09 nm; 4-Cl at 424.57 nm) through localized highest occupied molecular orbitals → lowest unoccupied molecular orbitals transitions, contrasted by hypochromic shifts for fluorinated and electron-withdrawing groups. These results provide a clear structure-property relationship, and the reactivity profiles and optical band gaps of these hybrids can be tuned by peripheral substitution in medicinal and biomolecular applications.. KEYWORDS :Coumarin, Pyridine, Density functional theory, Mulliken Charge, ESP, HOMO, LUMO.