Investigation of the structures and electronic spectra of two coumarins with heterocyclic substituents through TD-DFT calculations

A theoretical study on various properties of the ground and first excited states of two 7-aminocoumarin dyes with heterocyclic substituents at the 3-position is presented. The ground-state and excited-state geometries were optimized at the Hartree-Fork and configuration interaction singles levels of theory, respectively. The geometric relaxation between the first excited state and the ground state was examined and explained in terms of the nodal patterns of the highest occupied and lowest unoccupied molecular orbitals. The most striking geometrical relaxation is the twisting motion between the parent coumarin and heterocyclic substituent at the 3-position. The absorption and emission wavelengths were calculated using the time-dependent density functional theory, and the solvent effect on geometries and spectra has been taken into account using the polarized continuum model. Our calculated results are in good agreement with the experimental measurements, and the influences of solvents and heterocyclic substituents on spectra are also discussed.