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 AlO 4 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.