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.
The direct energy transfer interaction between excited singlet aromatic molecules and peroxide lattice defect sites in silica gel is demonstrated. The nonexponential fluorescence relaxation of excited naphthalene and 2-methoxynaphthalene adsorbed to the pore surface of silica gel 60 is shown to be described by a mechanism involving dipole-dipole coupling between the molecular excited singlet states and the intrinsic defect sites of silica. The spectral overlap between the defect absorption and the adsorbate excited singlet state emission is measured. Values for the critical energy-transfer radius, &, are determined to be approximately 10 A for the probe molecules studied. The density of defects in the silica gel 60 is calculated to be 1020 cm: 3.
IntroductionThe behavior of molecules adsorbed to porous substrates has interested investigators for many y e a r~. I -~~ Aromatic molecules have been adsorbed to various silica supports, including silica gel, to determine the effects of interactions with surface groups on the photophysical properties of the adsorbates. The fluorescence relaxation of aromatic probes adsorbed to such surfaces have been found in many cases to be nonexponential.610 Several investigators have suggested that this behavior arises from site heterogeneity of the silica surface which may affect the excited-state lifetime of the adsorbed molecule. These authors have attempted to describe the relaxation dynamics using models which account for a distribution of lifetimes arising from a distribution of inquivalent adsorption These models are not, however, supported by hole-burning measurements determining the inhomogeneous line broadening of the f l u o r e s~e n c e .~~~~~In this paper, we show that the nonexponential relaxation of naphthalene and 2-methoxynaphthalene adsorbed to silica gel arises from energy transfer from the excited adsorbate molecules to defect sites in the silica gel. We have previously shown that such interactions exist for these molecules adsorbed to a silica zeolite, silicalite.20 We demonstrate that the nonexponenetial decay of the adsorbate molecules may be described by a mechanism in which the excitation energy is transfered via a dipole-dipole mechanism to randomly distributed defect acceptors. We further show that the criterion for dipledpole coupling, spectral overlap between the defect absorption and adsorbate emission, is met.The model for dipoledipole energy transfer has been described in detail p r e v i o~s l y .~' -~~ Briefly, the reaction iswhere D is the donor species, A is the acceptor, and W(r) describes the hierarchical onestep energy-transfer rate that depends on the distance, r, between D* and A. The general form of the survival probability of D* may be ~r i t t e n~l -~~ as *D(t,ro) = exp(-pJdr p o~( 1 -e x p [ -~-r o ) t~) ) (2)where the position of the donor, ro, is excluded and p is the fraction of the total acceptor sites that is occupied. po(r) is the site density function which describes the spatial arrangement of acceptors around the donor. For a di...
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