J. Phys. Chem. 1995,99, 16999-17009
16999
Origins of Bound-Probe Fluorescence Decay Heterogeneity in the Distribution of Binding Sites on Silica Surfaces Haibo Wang and Joel M. Harris* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received: May 30, 1995@
Inhomogeneous fluorescence decay kinetics have been observed from 3-( 1-pyrenyl)propyldimethylsilane(3PPS) covalently bound to a silica surface. The excited-state decay kinetics were found to follow a discrete biexponential model rather than a continuous lifetime distribution model and were attributed to the specific site heterogeneity on the silica surface. The existence of two distinguishable silanol sites on silica surface was further supported by the difference in fluorescence decays when a chlorosilane versus a hydroxysilane reagent is used to bind pyrene to the surface. It was also found, for chlorosilane-bound pyrene, that the decay kinetics became more heterogeneous with increasing reaction time or upon exposure of the derivatized sample to water. These results can be explained by a selective reaction between the hydroxysilane and a small population of active silanols, while chlorosilane reacts indiscriminately with the surface under kinetic control. Fluorescence quenching studies of surface-immobilized pyrene by iodine were also conducted; enhanced quenching rates for the shorter lived excited-state population were observed and attributed to the adsorption of quencher molecules to active silanol sites, which also caused a faster decay rate of the surfacebound pyrene probe.
Introduction Chemically modified silicas are important materials, having a variety of applications in chemical separation, catalysis, and immobilized reagents. The silica surface is known to be both geometrically and energetically heterogeneous;'V2 geometric heterogeneities are related to the irregular surface topography, while the surface energetic properties are controlled by the chemistry and distribution of surface silanols. The silanols on the silica surface are of several kinds: geminal, isolated, and vicinal or hydrogen-bonded;'%2 their distribution on the surface is not homogeneou~.~ Fluorescence techniques have been used successfully to characterize silica surfaces. Probe molecules, which are adsorbed or covalently bound to the surface through interaction or reaction with surface silanol groups, can reveal information about their local chemical environment at the surface through changes in their photophysical properties such as fluorescence decay kinetics of excitation and emission ~ p e c t r a . ~Fluores-~ cence quenching measurements on surface-bound probes can yield information about molecular transport as well as the geometry of the substrate.6-'o Fluorescence decay kinetics of aromatic molecules adsorbed or covalently bound to silica or zeolite surfaces are often inhomogeneous"-'6 and have been modeled as multiexponential decays (discrete fluorescence lifetimes) and continuous distributions of fluorescence lifet i m e ~ . " - ' ~ The inhomogeneous fluorescence decay kinetics have been explained in terms of surface heterogeneity, where probe molecules occupy different surface sites. In addition, inhomogeneous decay may arise from energy transfer to electronic defects in the substrate.20 In the present work, inhomogeneous fluorescence decay kinetics of 3-( 1-pyrenyl)propyldimethylsilane (3PPS) covalently bound to a silica surface was observed. The inhomogeneous fluorescencedecay followed a discrete population model, which was confirmed by a Laplace inversion analysis. The decay kinetics were also found to depend on the wavelength at which @
Abstract published in Advance ACS Abstracts, October 15, 1995.
fluorescence was collected and the polarity of the solvent in which the sample was suspended. These results indicate that the inhomogeneous fluorescence decay is due to interactions with surface silanols having different activities. The influence of different sites on the silica surface was also apparent in the behavior of pyrene probes having different reactive functionalities to bind the probe to the surface. When a chlorosilane was used to bind pyrene to the silica surface, the fluorescence decay was homogeneous; when a hydroxysilane was used, however, the fluorescence decay was heterogeneous immediately after the derivatization reaction. Fluorescence quenching studies of surface-bound pyrene probes by iodine were also carried out to explore the influence of the surface near the probe molecules on interfacial transport. The quenching behaviors of the two populations of excitedstate molecules were distinct. The longest lived component produced linear Stem-Volmer plots with smaller quenching rate constants dominated by transport directly from solution. The quenching rate constants for the shorter lived component were larger than those for the longest lived component, which could be attributed to adsorption of quenchers to active silanols near the pyrene probe, indicating clustering of active silanol sites. Experimental Section Materials. Porous microparticulatesilica was obtained from Whatman (Partisil IO). The silica gel is 10 pm diameter irregular particles having mean pore diameters of 90 A and a nitrogen BET surface area of 320 m2/g. The synthesis of 3-( 1pyreny1)propyldimethylchlorosilane (3PPS-C1) and its subsequent reaction with the silica have been described in detail el~ewhere.~ 3-( 1-Pyrenyl)propyldimethylhydroxysilane(3PPSOH) was prepared by hydrolysis of 3PPS-Cl; a toluene solution of 3PPS-Cl was mixed and shaken with 1 M sodium hydroxide aqueous solution in a separatory funnel. The hydrolyzed product was isolated, and its structure was confirmed by 29SiNMR, in which only one peak was detected at 7.4 ppm, a chemical shift that agrees with that for hydroxysilane.*' Conditions of surface derivatization reaction using 3PPS-OH were the same as those
0022-365419512099-16999$09.00/0 0 1995 American Chemical Society
17000 J. Phys. Chem., Vol. 99, No. 46, 1995
of 3PPS-Cl except that the reaction time was 24 h instead of 2 h. Although a much longer reaction time was used for 3PPSOH, the yield of surface derivatization reaction of 3PPS-OH was only lo%, much lower than the 90% yield observed for 3PPS-C1. Spectral grade chloroform or toluene, dried over calcium chloride or molecular sieves, was used in derivatizing the silica gel. Spectral grade solvents, chloroform or toluene, tetrahydrofuran, acetonitrile, methanol, and acetone were used without further purification to rinse the chemically modified silica. Methanol, hexane (EM Science, Omnisolv), and iodine (Mallinckrodt, resublimed crystals 99.91%) were used without further purification in fluorescence decay and quenching experiments. The 3PPS-labeled silica gels were subjected to elemental carbon analysis, performed by M-H-W Laboratories (P.O. Box 15149, Phoenix, AZ 85018), to determine surface concentrations of the immobilized ligand. Silica samples used in this study have 3PPS coverages between 0.05 and 0.10 pmol/m2, which are less than 3% of a monolayer and are sufficiently low to avoid probe-probe interactions. At high surface coverage, interaction between neighboring 3PPS species can be inferred from the observation of excimer emission from silica in a ~ o l v e n t . To ~ test for interactions between probe molecules, fluorescence emission spectra from the 3PPS-derivatized silica were recorded. Weak excimer emission at 480 nm from the 3PPS-labeled silica gels was not detected in the tail of the monomer band for chlorosilane-derivatized samples (amplitude less than 3% of the peak monomer emission intensity); a small fraction of excimer (less than 6%) could be detected in the hydroxy silane-derivatized sample. Detectable excimer emission for the hydroxysilane-derivatizedsample agrees with the finding that hydroxysilane reacts preferentially with an active silanol population which is apparently clustered on the surface, as discussed below. This result confims the small fraction surface coverage by the probe and its dispersed bonding over the silica surface. In an experiment to hydrolyze surface-bound 3PPS from the surface, 0.03 g of 3PPS-labeled silica was shaken with 1.5 mL of 1 M sodium hydroxide aqueous solution for 5 min; the mixture was then extracted with spectral grade hexane. The supernatant was filtered with a 0.2 pm filter and freeze-pumpthawed twice to remove oxygen before steady-state and timeresolved fluorescence emission were measured. The degree of hydrolysis of surface-bound 3PPS was substantial even though no effort was made to determine it quantitatively. Chromatographic Measurements. The bare silica column used in chromatographic measurements was packed in-house with Partisil 10 from a ethanollsilica gel slurry using a Shandon column packer. The C1 column from ES Industries (25 cm x 2.51 mm) contains C1-modified 10 pm silica particles having a surface area of 180 m2/g. Chromatographic measurements to determine retention time were made with a HPLC system consisting of a Beckman Model 210 injector, an Isco Model 2350 isocratic pump, a HPLC column, a Beckman Model 153 detector or a Isco Model 229 UVlvis detector operated at 254 nm, and a chart recorder. Retention values on bare and C1 silica were determined relative to pentane (Spectral grade from EM Science) and deuterium dioxide (99.9% from Cambridge Isotope Laboratories), which have negligible affinity for bare and alkylated silica surfaces,20,2'respectively. From the retention times of these unretained species (to), the retention time for quencher on the column (tr)can be related to the fraction of the quencher population on the silica surface; the capacity factor, k' = (tr - to)/to, is equal to the mole ratio of quencher on the surface relative to quencher in solution.22
Wang and Harris Adsorption isotherms were determined using a frontal elution method.23 A Rheodyne Model 7010 six-port sampling valve was used to switch between the previously described Isco pump and a Waters 6000A HPLC pump; with each switch of the valve a successive increase in iodine concentration in the mobile phase was introduced into the column. Fluorescence Decay and Fluorescence Quenching Measurements. Fluorescence decay curves were obtained from slurries of 1% (by weight) 3PPS-labeled silica suspended in methanol, hexane, or their corresponding solutions containing quencher. Samples were transferred to a vacuum-tight spectroscopy Oxygen was removed by two freeze-pumpthaw cycles, after which the sample was allowed to equilibrate at room temperature for 1-2 h prior to the collection of fluorescence decay data. For the water vapor experiment, pyrene-labeled silica gel was allowed to equilibrate with water vapor in a closed container at room temperature for 20 days; the sample was then transferred into the spectroscopy cell and dispersed in methanol or hexane, followed by freeze-pumpthaw cycles to degas. To study the slow evolution of fluorescence decay kinetics in water and methanovwater mixtures, no degassing was used to avoid the change in fluorescence lifetime due to the change of oxygen concentration during the 2 month experiment. In fluorescence quenching experiments, fluorescence decay curves were collected at 375 and 405 nm for hexane and methanol solution, respectively; this change in wavelength was used to assure that both decay components exhibited sufficient amplitude to detect. Fluorescence decay curves were acquired on a nanosecond laser fluorometer using time-correlated single-photon counting. A mode-locked argon ion laser (Spectra Physics, Model 2000) synchronously pumps a rhodamine 6G dye laser (Spectra Physics, Model 375). The 600 nm laser pulse from the dye laser is extracted at 400 kHz by a cavity dumper (Spectra Physics, Model 454) and then frequency-doubled using a KDP crystal (Quantum Technology); the resulting 300 nm laser pulse has a duration less than 50 ps. Fluorescence emission was collected at a 90" geometry and isolated by a 0.25 m monochromator with a 1 nm band-pass; four long-pass filters (WG335 from Schott) were used to eliminate scattered excitation radiation. Data Analysis. Fluorescence decay curves were fit to a multiexponential model:
where t i is the fluorescence lifetime and Ai is the amplitude of the ith component. The analysis was performed using Metlab (Mathworks, Inc.). Best fits were found by minimizing the x2 statistic using a SIMPLEX algorithm to search for fluorescence lifetimes with a linear least-squares step to determine the bestfit a m p l i t ~ d e sthis ; ~ ~approach is much more efficient than using a nonlinear least-squares search for the component amplitudes. The experimental data were also analyzed according to a continuous distribution model of fluorescence decay rates by numerical Laplace inversion, which was performed with a FORTRAN program, CONTIN.25 Synthesized data sets with 2000 points and equal data intervals of 0.2 ns were also constructed according to a multiexponential model and analyzed. Gaussian noise with a variance equal to the mean, which is a good approximation to the Poisson-distributed noise of singlephoton detection at high counts, was created with a normal pseudorandom number generator on the basis of the Box, Muller, and Marsaglia algorithm26and added to synthetic data prior to analysis.
Origins of Bound-Probe Fluorescence Decay Heterogeneity
1
t
91
I
0
100
200
I
300
Time (ns) Figure 1. Fluorescence decay curves of 3PPS bound to silica surface suspended in methanol, monitored at 375 nm (top) and 405 nm (bottom).
Results and Discussion Inhomogeneous Fluorescence Decay Kinetics. Timeresolved fluorescence decays from pyrene probe (3PPS),covalently bound to a silica surface at low coverages, were found to be inhomogeneous. Furthermore, the decay kinetics strongly depend on the wavelength at which the time-resolved fluorescence emission is observed: the fluorescence decay is more homogeneous on short-wavelength sides of the peaks in the fluorescence vibronic bands, while it is very heterogeneous on long-wavelength sides of the emission peaks. Figure 1 shows typical fluorescence decay curves at two representative wavelengths: 375 and 405 nm,which are on the shorter wavelength side of the first emission peak and the longer wavelength side of the second emission peak, respectively. These semilog plots of the fluorescence decays clearly show that the decay kinetics are inhomogeneous and the relative contribution from a rapidly decaying population changes with wavelength. A possible mechanism for generating a short-lived component in the pyrene excited-state decay is through formation of excimer which can influence the decay kinetics of pyrene bound to ~ i l i c a . This ~ mechanism requires a high surface coverage so that a significant fraction of surface-bound pyrene probes are in close proximity on the surface. In our case, surface coverage of 3PPS is very low, less than 2% of a monolayer; emission in the pyrene excimer wavelength range (480-500 nm) does not rise above the tail of the monomer band. Furthermore, no characteristic rise time associated with dynamic formation of excimer was detected in the time-resolved emission measured in the excimer range. The inhomogeneous decay kinetics observed in this study are, therefore, not due to excimer formation. Silica surfaces are known to be both geometrically and energetically heterogeneous. On silica surfaces there are different silanol groups, including geminal, isolated, and hydrogen-bonded or vicinal silanols; the acidity of these silanol groups varies.'V2 The fluorescence decay kinetics of pyrene and its derivatives are sensitive to their local environment; interactions with polar surroundings break the symmetry of the electronic states of ~ y r e n e ?which ~ shortens the fluorescence lifetimeI2and also shifts the fluorescence emission to the longer wavelength^.^^*^^ Upon covalently binding to a silica surface, 3PPS molecules are immobilized and sample only a small area of the surface. 3PPS bound at different surface sites can have very different fluorescence lifetimes; therefore, the observed fluorescence emission is from a collection of 3PPS molecules at various surface sites, and it is not surprising to observe the inhomogeneity in the fluorescence decay kinetics. The wavelength dependence of the fluorescence decay kinetics is caused
J. Phys. Chem., Vol. 99, No. 46, 1995 17001 by slight shift of the emission spectra from 3PPS at more polar surface sites; due to this red ~ h i f t ,the ~ ~fluorescence ,~~ decays from 3PPS in more polar sites contribute more intensity on the long-wavelength sides of the emission peaks. To exclude the possibility of a surface photochemical reaction generating a short-lived decay component, immobilized 3PPS, which produced inhomogeneous fluorescence decay kinetics on the silica surface, was removed from the surface by hydrolysis with 1 M sodium hydroxide in aqueous solution and then extracted into hexane. The resulting hexane solution containing hydrolyzed 3PPS produced fluorescence characteristics of pyrene substituted in the 1-position on the ring; its time-resolved fluorescence decay kinetics were single-exponential, with a lifetime of 226 f 1 ns, which agrees with that of 1-methylpyrene in hexane.I4 This experiment shows that pyrene probes remain chemically intact on the surface. There is additional evidence to support the conclusion that heterogeneous surface environments cause the inhomogeneous fluorescence decay kinetics; this evidence includes the observation that the decay kinetics depends on the type of silane reagent used to bind pyrene to the silica surface (see below). While the evidence suggests that the inhomogeneous fluorescence from 3PPS is due to heterogeneity of the silica surface, quantitative modeling of inhomogeneous or nonexponential decay kinetics is not ~ i m p l e . ~ O The - ~ ~ choice between fitting inhomogeneous decay data to a sum of exponentials or treating the data as the Laplace transform of an underlying distribution of first-order decay rates is difficult to evaluate. Ware et al.32 used synthetic data to demonstrate the difficulty of this question; they generated decay data according to continuous lifetime distributions with significant widths and a diversity of shapes, but found the data transients could be fit well to a twoexponential decay model and yield satisfactory fit statistics and random residuals. To evaluate the choice between continuous distribution and multiexponential decay models, we analyzed our decay curves using a numerical Laplace i n v e r s i ~ n .Laplace ~~ inversion can be done analytically, by fitting the decay curve to some chosen analytic function and solving for its Laplace transform pair, or numerically without a prior choice of the mathematical form of the decay kinetics. Numerical inversion is preferable in circumstances where a prior choice of a model cannot be made; some control can be exercised during the inversion process by applying reasonable physical constraints to the solution. By applying the technique of constrained regularization, the solution is limited to values of the rate constants that are nonnegative and defined over an experimentally accessible range. With the additional application of the principle of parsimony, wherein the least detailed solution consistent with the data is chosen, numerical inversion can provide stable, physically reasonable results at the cost of larger variance in the resulting distribution and possible blurring of real detail in the d i s t r i b u t i ~ n . ~ ~ Results of a series of numerical studies have shown that the ability to distinguish between discrete and continuous decay models is strongly dependent on the noise level in the data.30 To avoid the problems caused by noise associated with photoncounting statistics, integration times of up to 2 h were used to acquire data with a signal-to-noise ratio of 2500. A typical result from the CONTIN analysis of surface-immobilizedpyrene fuorescence decay is shown in Figure 2, which has four resolved peaks; since these peaks have finite width, it is not clear whether the fluorescence decay kinetics should be four-exponential (four discrete decay rates) or four continuous distributions centered around different mean decay rates. To determine the contribution to the observed peak widths from the noise, synthetic data
17002 J. Phys. Chem., Vol. 99,No. 46, 1995 2000
Wang and Harris TABLE 2: Lifetime and Amplitude of the Data Analyzed According to the Four-Exponential ModeFb
A
375 nm 1 so0
solvent methanol
1000
4
hexane
so0
Decay Rate (ns') Figure 2. CONTIN analysis results of fluorescence of surface-bound 3PPS shown in Figure 1, bottom (solid line) and synthesized fourexponential data with added noise (dotted line). TABLE 1: Comparison of Fluorescence Decay Results Analyzed by the Continuous Lifetime Distribution Model (CONTIN Program) and the Discrete Exponential ModeP
experimental data
data synthesized according to a
four-exponential model silica background
t,(ns)
A,IAT,, (5%)
t r(ns)
A,/AT,, (%)
176 46 8 1 128 38 9
74 11 9 6 22 12 26 40
169 36 IO 3 125 32 8 2
18 27 22 33 13 15 35 31
L
0
169zt5 3613 10f3 311 169 f 5 3613 10f3 3fl 23 1 3 513 1&1 10f3 211
silica background fitted to a two-exponential model The amplitude uncertainty is ~ 5 % .
18 27 22 33 18 26 23 33 8 36 55 27 73
142
31 9 3 138 30 9 3 23 5 2
22 27 20 32 22 26 20 31 6 36 59
were constructed according to a discrete four-exponential model (eq l), where the decay rates and amplitudes were acquired from a discrete exponential fit of experimental data; noise was added at a level equivalent to the shot noise in the experimental data. The result of CONTIN analysis of synthetic data is shown in Figure 2 for comparison; interestingly, the two results have nearly identical shapes and widths. This agreement indicates that peak width in the CONTIN analysis of the experimental data is derived from noise in the data and that the results cannot be distinguished from a discrete four-exponential decay model. Table 1 compares fitting results from a continuous distribution analysis versus a least-squares fit to a four-exponential decay model. The mean lifetimes and relative amplitudes from these analyses are nearly identical. As shown in Table 1, the lifetimes of these decay components are widely separated; the smallest separation, which is between the second and third component, is a factor of 3. According to a numerical study of decay rate distribution analysis,30a wide separation of discrete decay rates makes distinguishing between discrete and continuous distribution models more reliable. Having adopted a discrete exponential model as the simplest description of the inhomogeneous fluorescence decay kinetics that is consistent with the data, we quantitatively analyze our decay curves using this model, and the results are listed in Table 2. Since the fluorescence lifetime of pyrene depends on its environment, it is possible to assign the origin of the fluorescence decay components from their lifetime values and their dependence on overlaying solvents. To better understand the relationship between fluorescence lifetime of 3PPS and its
405 nm
In data analysis, each decay transient was fit to our model separately at two wavelengths. The uncertainty of lifetimes is similar to that in Table 1; the amplitude uncertainty is ~ 5 % . environment, fluorescence lifetimes of 1-methylpyrene ( 1MP) in solution are first considered. 1MP fluorescence lifetimes were found to be 102,202, and 220 ns in water, methanol, and hexane solution, respe~tively;~,'~ as expected, 1MP fluorescence lifetime decreases with the increase in the polarity of its environment. Because it has an alkyl substitution on the same position of the aromatic ring system as 3PPS, 1MP is expected to exhibit similar fluorescence behavior as 3PPS. From Table 2, it is noticed that the lifetime of the longest lived component of bound 3PPS changes from 170 ns in methanol to 125 ns in hexane. The lifetime of the longest lived component in hexane is similar to that of 1MP in aqueous solution; this indicates that this population of surface-bound 3PPS is in contact with a polar (surface water andor silanol) environment. Methanol, on the other hand, is capable of forming hydrogen bonds with surface silanol groups; it can displace surface water33and compete with the surface for interactions with 3PPS. As a result, methanol gives the longest lived 3PPS population in a methanol-solvated surface environment. Assigning the origin of this longest lived component is relatively straightforward; this appears to be a surface-bound 3PPS population that is perturbed weakly by its surface environment and readily displaced from interactions with the surface by a suitable solvent. The sources of the other three, shorter lived components, however, are not as simple to identify. The two shortest lived components exhibit fluorescence lifetimes that are shorter by more than an order of magnitude compared to 1-methylpyrene in water. Furthermore, the lifetimes of these two shortest components are comparable to those observed for the background fluorescence from the silica substrate. A summary of a CONTIN analysis of the decay kinetics of the silica background fluorescence at 405 nm is given in Table 1. Three discrete components are found with mean lifetimes and relative amplitudes that agree well with a discrete exponential analysis (see Table 1). Two of these components account for 94% of the silica background fluorescence; a two-exponential fit of the background data gives lifetimes and relative amplitudes that closely match the two shortest decay components in the 3PPS-labeled silica fluorescence (see Tables 1 and 2). We therefore assign the two shortest lived decay components to the silica background fluorescence. Further support of this assignment is found in the fluorescence quenching studies (see below) where the two longer lived decay components produce linear Stem-Volmer behavior with encounter rate constants consistent with quenching of surface-bound 3PPS. The decay rates of the two shortest lived components show no change with quencher concentration within the uncertainty of determining their rates; this result is consistent with a luminescent impurity or defect site associated with the amorphous silica matrix. The second longest lived decay component exhibits a fluorescence lifetime longer than those of the silica background,
Origins of Bound-Probe Fluorescence Decay Heterogeneity and its relative amplitude at 405 nm rises well above the background upon binding of 3PPS to the silica surface; it is thus assigned to a population of surface-bound 3PPS. The shorter fluorescence lifetime of this component would be characteristic of a population of 3PPS molecules in a strongly perturbing polar environment. This environment could be due to silanols and/or surface water at uniquely active surface sites. The interaction between 3PPS and these sites must be strong since solvation of the surface by methanol does not eliminate the amplitude of this population to yield a uniform methanolsolvated environment. While solvation by methanol does not eliminate the shorter lived population, its amplitude relative to the silica background decreases in methanol as compared to the sample in hexane, as shown in Table 2. Methanol appears to be sufficiently polar to partially compete with active silanols and reduce the fraction of 3PPS that is subject to strong surface perturbations. As the amplitude of the short-lived component decreases in a displacing solvent, a new population of decaying species is not observed. This result indicates that the fraction of the short-lived population that was displaced from the surface is now part of the 170 ns population; this population appears to be solvated by methanol and dominates the intensity at 375 nm, as shown in Table 2. Inhomogeneous fluorescence decay kinetics of molecules adsorbed or bound to silica surfaces have been previously reported. W. R. Ware et al. have studied photophysics of aromatic molecules adsorbed on silica the fluorescence decay kinetics from these molecules, iqcluding pyrene, were found to be inhomogeneous. Decay kinetics were affected by heat treatment of the silica, added coadsorbates, and silylation and were attributed to the heterogeneity of surface sites. Fluorescence decay kinetics from 3PPS bound to a fumed silica have been found to be inhomogeneous, exhibiting stretchedexponential behavior. I 4 Inverse Laplace transform analysis of the region (k = (0.1-3.0) x lo7 s-I revealed a continuous distribution of decay rates centered very near the long-lived component of Figure 2. The finite width of the rate distribution peak was attributed to the interaction with surface silanols having a dispersion of surface energies. The distribution of decay rates is different from the discrete decay kinetics that we observe for precipitated porous silica. This difference is perhaps not surprising since the surface chemistry of these two silicas is different. Fumed silica is made by oxidizing silicon halide compounds in a hydrogen and oxygen flame' and has a much lower surface silanol density compared to the precipitated silica used in this study; this could lead to a dispersion of surface interactions due to local variations in silanol den~ities.'~ Heterogeneous fluorescence decay kinetics has also been reported for pyrene adsorbed onto zeolites.I6 Data analysis based on a two-exponential model revealed that there were two populations of pyrene molecules in the zeolite occupying two classes of surface sites: one producing a lifetime in the range 25-60 ns and the other 85-160 ns. Lifetime distribution analysis yielded two overlapped, broad Gaussian distributions of lifetime when the zeolite is dry. Interestingly, with the addition of water, it was found that the width of the two distributions decreased significantly and that the lifetimes increased. These results are remarkably similar to the behavior we observe for bound 3PPS on silica; the fluorescence lifetime values and the effects of solvation are very similar. Leheny et aL20 recently observed inhomogeneous fluorescence decay of naphthalene adsorbed onto a silica surface and proposed that it was due to energy transfer to a peroxide defect on the silica surface. Evidence of a defect was found in an electronic absorption band at 290 nm detected by a sensitive
J. Phys. Chem., Vol. 99, No. 46, 1995 17003
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I
0
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200
300
Time (ns) Figure 3. Fluorescence decay curves of surface-bound 3PPS. Samples were suspended in methanol. (A) Surface binding of 3PPS to silica using chlorosilane (top) versus hydroxysilane (bottom) reagent. (B) Two chlorosilane-derivatized samples stored under different conditions: kept dry in a desiccator (top, same as A above) versus exposed to water vapor for 20 days (bottom). photochemical beam-deflection measurement. It is unlikely that the inhomogeneous decay kinetics observed in the present study originate with this defect. The wavelength dependence is different from what would be expected for an energy transfer mechanism: at 375 nm, where overlap between pyrene emission and the tail of the reported silica defect absorption is significant, the fluorescence decay is much more homogeneous than at longer wavelengths, where the defect absorption is negligible. To further test this conclusion, we checked our silica material for absorbance in the ultraviolet region; chloroform was used to match the refractive index of silica at 300 nm, near the peak in the reported defect band. No band was observed to rise above the small base line of residual scattering, which established an upper bound for any defect absorption by our silica, A < 0.02 mm-I. From the extinction coefficient estimated from energy transfer parameters,*O the concentration of defects in our silica is less than 3 x IO'%m3 which is too small to influence the decay kinetics of pyrene excited states. Binding Site Origins of Silica Surface Heterogeneity. The inhomogeneous fluorescence decay kinetics of bound 3PPS suggest the existence of a population of uniquely active sites on the silica surface. The fluorescence decay results, however, indicate little about their chemical nature. Using different silane reagents to bind pyrene to the surface, the chemical origins of the observed surface heterogeneity can be probed. From their fluorescence decay kinetics, it can be inferred that pyrenechlorosilane (3PPS-C1) and pyrene-hydroxysilane (3PPS -OH) bind pyrene ligands to different surface sites. As shown in Figure 3A, fluorescence decay kinetics of surface-bound 3PPS from reaction of the silica surface with 3PPS-C1 are quite homogeneous, in contrast with the inhomogeneous decay kinetics observed from pyrene immobilized using the 3PPSOH reagent. The decay curves of Figure 3 were analyzed according to a discrete exponential model, with results shown
17004 J. Phys. Chem., Vol. 99, No. 46, 1995
TABLE 3: Fluorescence Lifetimes and Amplitudes under Different Condition& 375 nm 405 nm T,(ns) A J A T (%) ~ ~ A,IAI+z(%I t,(ns) AJAT,~(%) A A + 2 (%I Sample Kept Dry following Derivatization with Chlorosilane 90 94 168 82 93 168 6 36 6 7 36 6 7 2 7 10 0.2 1 0.2 3 Sample Kept Dry following Hydroxysilane Derivatization 175 72 90 175 45 85 45 8 10 35 8 15 7 9 6 21 2 12 2 26 Chlorosilane-Derivatized Sample Exposed to Water Vapor for 20 Days 164 54 87 161 39 81 36 8 13 31 9 19 5 16 6 13 1 22 1 39 In the data analysis, each decay transient was fit to our model separately at two wavelengths. The only exception was for the sample kept dry following derivatization using chlorosilane at 375 nm, in which case lifetimes from the same sample at 405 nm were used to calculate the amplitudes. The uncertainty of lifetimes is similar to that in Table 1; the amplitude uncertainty is % 5%. in Table 3. The lifetimes and the wavelength dependence of the amplitudes for the 3PPS-OH produced are similar to the results for 3PPS-Cland are consistent with the hydroxysilane reagent binding to two different surface sites. From the much greater amplitude of the shorter lived component (which is especially apparent in the 405 nm emission), it is clear that the hydroxysilane reagent binds a larger fraction of pyrene in the region of more active polar silanol sites on the silica surface. The fact that chlorosilane and hydroxysilane reagents bind 3PPS preferentially to different sites on the silica surface is probably related to differences in reactivity of these reagents. Unger et al. conducted a systematic study to compare reactivities of silanes with different reactive functional groups on silica surfaces34 and found a much lower yield for hydroxysilanes compared to chlorosilane reagents. The different reactivities of these two silanes are probably related to different enthalpies of reaction; from bond energies,35 one can estimate that the reaction between a surface silanol and chlorosilane is accompanied by a large change in enthalpy, about - 15 kcal/mol, while the enthalpy of reaction between a silanol and hydroxysilane is much smaller,
