Structure and electronic properties of quinizarin chemisorbed on alumina

The anthraquinone dye molecule quinizarin is known to allow for persistent spectral hole burning up to liquid nitrogen temperatures after chemisorption on alumina surfaces. The mechanism underlying these improved hole-burning properties is not known, though is has been speculated that it might be related to intrinsic surface effects on the electronic structure of the dye. We approach this problem theoretically using gradient corrected density functional theory. The chemisorbed compound system is modelled by a periodically replicated nine layer slab which represents the (0001) surface of alpha-Al2O3. The chemisorption geometry obtained by geometry optimization and confirmed by Car-Parrinello molecular dynamics runs at room temperature is shown to be a perpendicular arrangement of quinizarin on the surface, where a chelate-like bond is formed with one exposed surface aluminum atom. In order to get information about the electronic structure, the frontier orbitals that are relevant for the description of the electronic excitation to the first excited state are evaluated for the isolated molecule, the chemisorbed molecule, and a quinizarin-aluminum-water complex. The strong red shift of the absorption frequency found in experiment upon chemisorption is reproduced. However, the results show that the shape of the frontier orbitals and hence the properties of the electronic excitation remain essentially unchanged by chemisorption. Thus, the differences in the behavior of the isolated and the chemisorbed dye observed in persistent spectral hole-burning experiments cannot be explained by genuine surface induced effects on the molecular electronic structure. (C) 1996 American Institute of Physics.