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 3 d(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 tech nique 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 chann els, and it is coordinated only to oxygen atoms of an AlO4 tetrahedron. The refore, the emission energies depend not only on the type of the Cu+ site b ut also on the position of the Al atom in the zeolite framework. The smalle st emission energies (corresponding to 520-540-nm peaks in observed emissio n spectra) are found for Al located at the intersection of two channels. Th e higher-emission energy peaks observed at 470-490 nm are due to the Cu+ si tes where copper is located inside one of the channels. Because in the trip let state the optimized structures are alike for all Cu+ sites considered, the emission energies are also relatively similar. In the singlet state, di fferent types of the Cu+ coordination exist, which cause large variations i n excitation energies. Our results indicate that photoluminescence excitati on scan should be preferably used for Cu+ site determination.