Coordination change of Cu+ sites in ZSM-5 on excitation in the triplet state:: Understanding of the photoluminescence spectra

The excitation and emission energies of the transition between the lowest 3d(10) singlet and 3d(9)4s(1) triplet states of Cu+ ions in ZSM-5 have been studied by a combined quantum mechanics/interatomic potential function technique for different sites of Cu+. In the triplet state, the coordination of the Cu+ ion to the zeolite framework is significantly different from that in the singlet state. The Cu+ ion moves away from the wall of zeolite channels, and it is coordinated only to oxygen atoms of an AlO4 tetrahedron. Therefore, the emission energies depend not only on the type of the Cu+ site but also on the position of the Al atom in the zeolite framework. The smallest emission energies (corresponding to 520-540-nm peaks in observed emission spectra) are found for Al located at the intersection of two channels. The higher-emission energy peaks observed at 470-490 nm are due to the Cu+ sites where copper is located inside one of the channels. Because in the triplet state the optimized structures are alike for all Cu+ sites considered, the emission energies are also relatively similar. In the singlet state, different types of the Cu+ coordination exist, which cause large variations in excitation energies. Our results indicate that photoluminescence excitation scan should be preferably used for Cu+ site determination.