Direct energy transfer from excited organic adsorbates to intrinsic defect sites in silicalite

Abstract

The decay of the excited singlet state of an aromatic molecule such as naphthalene adsorbed to the surface of silica is nonexponential. This nonexponential decay is shown to result from a multipolar interaction between the excited adsorbate and the intrinsic defects of silica. We present experimental evidence for direct energy transfer between the donor molecules, naphthalene and 2-methoxynaphthalene, and acceptor defects sites of a crystalline silica zeloite (silicalite). The principal condition for direct energy transfer, spectral overlap between the optical absorption of the defects and the fluorescence emission of the adsorbate is demonstrated for this system, and the nonexponential relaxation of the donor is shown to be described by a Forster-type stretched exponential. Based on the spectral overlap and energy transfer rate determined, the intrinsic defect density of silicalite is calculated to be 3 &2x 102' cme3.

Defect Centers in Microporous Aluminum Silicate Materials

“…The high concentration of the defect sites with two adjacent silanol groups (disilanols) was also demonstrated in Na and proton exchange and in NMR experiments on TPA-templated zeolite ZSM-5 . A direct energy transfer from the excited adsorbates to the structural defects was reported in optical experiments on silicalites (thermal deflection spectroscopy), which allowed the authors to estimate the defect concentration as one defect per unit cell; the optical absorption of the defect sites was thus identified at 293 and 283 nm, which in comparison with previously reported data suggested that the defect sites are either peroxy bridges or peroxy radicals. The role of dioxygen species formed at or next to Brønsted sites was also discussed in the context of Lewis and redox activity of zeolites based on the EPR data .…”

Computational Investigation into the Origins of Lewis Acidity in Zeolites

“…[66] A direct energy transfer from the excited adsorbates to the structural defects was reported in optical experiments on silicalites (thermal deflection spectroscopy). [81] The authors estimated the defect concentration at one species per unit cell. The optical absorption of the defect sites was identified at 293 nm and 283 nm, which by comparison with previously reported data suggested that the defect sites are either peroxy bridges or peroxy radicals.…”

Photoinduced Reactions in Porous Systems: Reactions at the Solid−Liquid Interface1

“…A general finding is that the kinetics of the reactions are more complex than those in bulk solvent. This is not unexpected due to the heterogeneous nature of the surfaces, and possibly due to fractal effects. 2jk, For the most part, the observed chemistry can be described by reasonable models involving expected properties of the oxide surfaces. The photochemistry in these systems occurred at solid−vacuum or solid−gas interfaces.…”

“…Photoinduced reactions of molecules on silica and alumina surfaces 2fg,3 have been the focus of numerous studies. A general finding is that the kinetics of the reactions are more complex than those in bulk solvent.…”