J. Phys. Chem. 1992, 96, 8498-8502
8498
Dipote-Dlpole Coupllng between Excited Aromatic Probe Molecules and Defect Sites in Silica Gel A. R. Leheny, N. J. Turro, Department of Chemistry, Columbia University, New York,New York 10027
and J. M. Drake* Exxon Research and Engineering Company, Annandale. New Jersey 08801 (Received: March 9, 1992; In Final Form: May I I , 1992)
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-methoxynaphthaleneadsorbed 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.
Introduction The behavior of molecules adsorbed to porous substrates has interested investigatorsfor 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.20We 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 is
D* + A-
W)
D
+ A*
(1) where 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 ~as l-~~ *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 dipole-dipole energy transfer, W(r)may be defined as21-23 (3) with T~ the lifetime of the isolated donor, K the anisotropicfactor 0022-3654I92 12096-8498SO3.00IO
containing the angular dependence of the dipolar interaction, and Ro the critical energy transfer radius. Ro is defined as21-24
where NA is Avagadro's number, n is the refractive index of the embedding medium, tA(v) is a molar extinction coefficient of the acceptor, and FD(v) is the fluorescence intensity, and QD the fluorescencequantum yield of the donor. In the limit of an infiite system with random acceptor distribution, po(r) = po, and we can write the survival probability as21-23
where d is the dimension of the embedding medium, r is the gamma function, and A. is the energy-transfer p r e f a c t ~ r ~ ' - * ~ A0 = ppoC~rRo~ (6) A. defines the number of acceptors within a sphere of radius Ro with C a constant that depends on d (e.g., for d = 3, C = 4 f 3 for a sphere and C = 2/3 for a hemisphere). An alternative mechanism for energy transfer, the exchange interaction, has also been considered. In exchange interaction, W(r)falls off exponentially with distance and the interaction is very short range. This leads to a markedly different behavior for @ ( t ) ,which we have attempted to fit to our data.*O The temporal dependence of the data is not, however, described by this model. We show that a spectral overlap between the emission of naphthalene and 2-methoxynaphthaleneand the optical absorption of silica defects exists. On the basis of the energy of the defect absorption, we suggest that the defects are peroxide radicals (Si-00') and peroxide bridges (Si-o-0-Si) in the lattice.25s26 Other defect sites such as oxygen vacancies and dangling hydroxyl groups have absorption energies that do not overlap with the adsorbates' emission and probably do not interact with these molecules. The survival probability of the fluorescence of the adsorbed aromatics is also measured and fit to eq 5 . From this, values for T~ and A. are determined. The value for Ro may be calculated from the overlap if a reasonable value for the quantity p p o is assumed. The presence of dipole-dipole energy-transfer interactions between the silica gel defects and excited aromatic probe molecules offers an appealing explanation for the nonexponential relaxation of the probe fluorescence. From the dipole+.de model, we are able to define a single lifetime, T ~for , the adsorbate excited state which we find depends on the density of dangling hydroxyl groups on the surface. The nonexponential relaxation arises from en@ 1992 American Chemical Societv
The Journal of Physical Chemistry, Vol. 96, No. 21, 1992 8499
Defect Sites in Silica Gel
1.2
1.0
-
o.8
GEL
I ON
In
8 0.6 N, -I < K L
0.4
0.2 0 2
0
4
6
8
1
-
Energy Density (mJ/cm2*pulse)
Figure 1. Linearity of the photothermal deflection signal as a function of the energy density of excitation.
ergy-transfer interactions with randomly distributed acceptor defect sites in silica gel.
' * '
ENERGY
'
' '
;; 00
Figure 2. Spectral overlap between silica gel 60 absorption measured using photothermal deflection spectroscopy and naphthalene emission measured using the SLM-Aminco 4800s. .. 1.2
Experimental Seetion
The silica gel used in these experiments was silica gel 60 obtained from E. Merck (Darmstadt, Germany). It has an average pore radius of 35 A and a BET surface area of 390 f 10 mZ/g.4sz7 Two sets of samples were heated in a tube furnace to either 165 or 600 O C . The samples heated to 165 OC (SG165) have 2.9 vicinal and 1.5 isolated surface hydroxyl groups per nmz while those heated to 600 'C (SG600) have 1.5 isolated group per nm2. These values were determined by heating the silica gel to 800 O C to obtain the total surface hydroxyl concentration by thermogravimetric analysis (TGA). The water loss at 165 and 600 O C was further estimated using TGA. At 165 OC only those water molecules physisorbed to the surface are desorbed while at higher temperatures the removal of hydroxyl groups by geminate dehydrogenation ~ c c u r s . ~After ~ * ~ heat treatment, the solids were stored under nitrogen until used. Naphthalene and 2-methoxynaphthalene were obtained from Aldrich and used without further treatment. These were adsorbed to the silica gel 60 from isooctane solutions. Adsorption was done in sealed air-tight vials where the samples were allowed to sit without agitation until equilibrium was reached (approximately 1 h). The supernatant was decanted from the samples and the wet powder was dried under vacuum at 50 O C for 30 min. The adsorption of the organic by the solids was calculated from the difference between the optical absorption of the starting solutions and that of the equilibrium Supernatant. The samples used in these studies contained between 5 and 20 pM/g silica gel which is a surface density of (0.7-3.1) X lo-' nm-'. Samples were stored in air-tight vials and 2-methoxynaphthalenesamples were stored in the dark to avoid photodegradation. We found that samples could be stored for months without any change in their photophysical behavior. The absorption spectrum of silica gel defects from 278 to 337 nm (35 970 to 29 670 cm-') was measured using photothermal deflection spectroscopy as described The silica gel samples were pressed into a
