Electronic structure and optical properties of chelating heteroatomic conjugated molecules: a SAC-CI study

The electronic structure and optical properties of 13 chelating heteroatomic conjugated molecules such as pyridine, benzoxazole, and benzothiazole derivatives, which are used as C–N ligands in organometallic compounds, have been investigated. The geometries of the ground and first excited states were obtained by the DFT and CIS methods, respectively, followed by the SAC-CI calculations of the transition energies for absorption and emission. For six compounds whose experimental data are available, the SAC-CI calculations reproduced the experimental values satisfactorily with deviations of less than 0.3 eV for absorption and 0.1 eV for emission except for benzoxazoles. For other molecules, the theoretical absorption and emission spectra were predicted. The lowest ππ* excited-state geometries was calculated to be planar for most of the molecules with two or three conjugated rings connected by single bond. The geometry change due to the ππ* excitation was qualitatively interpreted by electrostatic force theory based on SAC/SAC-CI electron density difference. The excitations are relatively localized in the central region and in the lowest ππ* excited state, the inter-ring single bond shows large change, with a contraction of 0.05–0.09 Å. The present calculations provide reliable information regarding the energy levels of these chelating heteroatomic conjugated compounds.