J. Phys. Chem. 1994,98, 8101-8107
8101
Probing the Cybotactic Region of PRODAN in Tetramethyl Orthosilicate-Derived Sol-Gels Upvan Narang, Jeffrey D. Jordan, Frank V. Bright,' and Paras N. Prasad' Department of Chemistry and Photonics Research Laboratory, Acheson Hall, State University of New York at Buffalo, Buffalo, New York 14214 Received: February 14, 1994; In Final Form: May 31, 1995"
6-Propionyl-2-(dimethylamino)naphthalene(PRODAN) is an extremely solvent sensitive probe. Changes in its photophysical characteristics are used to probe the microenvironment of the sol-gel matrix as a function of aging time. These results clearly demonstrate that P R O D A N senses a distribution of microenvironments within the sol-gel matrix. The ensemble of domains are evident immediately after gelation up until the drying stage. The extent of this microheterogeneity varies as a function of aging time. Steady-state emission anisotropy and time-resolved emission data provide detailed information on how the various stages of the sol-gel process affect the photophysics of PRODAN. Introduction Sol-gel processed materials14 have been used in areas ranging from photonics5-8 to the development of chemical sensors.9-12 They have also shown promise in encapsulating and maintaining the functionality of bioactive species.l3-I6 Their utility as a platform for chemical and biosensing schemes has also been previously demonstrated.17J8 However, in spite of substantial effort, there is no detailed understanding of the influence of the sol-gel matrix on the function and dynamics of encapsulated solutes. Sol-gel processing involves the transformation of a solution phase into a gel, followed by the loss of solvent, ultimately resulting (after heating and densification) in a solid, optically transparent glass. The initial hydrolysis is generally acid- or base-catalyzed, and its rate is highly pH dependent. Temperature, pressure, molar ratio of H20,and alcohol also affect the polymerization process and thus the physicochemical properties of the final material.'-" The chemistry of the sol-gel processing has been probed using various techniques like 29Si NMR, Raman and electron spectroscopy, gas chromatography/mass spectrometry (GC/MS), and X-ray and neutron scattering.1~4 For characterization of the microstructure of sol-gels, transmission electron microscopy (TEM), small-angle neutron scattering (SANS), GC/MS,NMR spectrometry, Brunauer-Emmett-Teller (BET) nitrogen adsorption for surface area, helium pycnometry for skeletal density, and UV-vis spectrophotometry are some of the prime techniques u~ed.19~Fluorescence spectroscopy has also been used to study the sol-gel process on a molecular level.11.16J9-44 Kaufman et aZ.4 used pyranine (8-hydroxy- 1,3,6-pyrenetrisulfonic acid trisodium salt), a molecule which is sensitive to protontransfer phenomena, to study the rate of water consumption of a tetramethoxy orthosilicate (TMOS) derived sol-gel as a function of hydrolysis pH. They concluded that as the pH was increased, hydrolysis slowed and condensation increased. Pouxviel et also used pyranine to study aging phenomena in a aluminosilicatederived sol-gel. These authors found a different trend with pH and concluded that the polymerization mechanism is specific to a particular sol-gel system. McKiernan et a1.35 used bipyridyltricarbonylchlororhenium(I), ReCl(CO)3bipy, to study tetraethoxysilane (TEOS) and mixed aluminosilicate-derivedsol-gels throughout the aging and drying process. The emission maximum of ReCl(C0)3bipy changes as a function of the rigidity of the surrounding e n ~ i r o n m e n t .In~ ~ ethanol, the emission maximum at room temperature is 610 nm
* Authors to whom all correspondence should be sent.
Abstract published in Advance ACS Abstracts, July 15, 1994.
0022-3654/94/2098-8 101%04.50/0
and it blue shifts to 530 nm in frozen solution (100 K), On the basis of the magnitude of blue shift in the emission maxima of the probe in frozen ethanol and TEOS, these authors concluded that there is a complete rigidification of the local environment about ReCl(CO)3bipy during the room temperature aging cycle. The rate of "rigidification" was different for the two systems, demonstrating that the mechanism of entrapment is specific to the individual system. Dunn and Zink29 suggested that the steady-state fluorescence anisotropy of an encapsulated fluorophore could be used to probe themicroviscosity of the sol-gel matrix. In this work, they studied the changes in fluorescence anisotropy of 1-(4-nitrophenyl)-6phenylhexa- 1,3,5-triene as a function of aging time. However, actual microviscosities were not recovered from the available fluorescence anisotropy data. Espbance and Chroni~ter~~." used time-resolved fluorescence decays of anisotropy of quinizarin (1,4-dihydroxy-9,1O-anthraquinone) to study the spatial distribution of chromophore^^^ and the orientational dynamics of such in aluminosilicate and silicate sol-gel materials. These results suggested that the form of the sol-gel matrix affected the mobility of the solute and that there may indeed be some residual rotational reorientation of the dopant within a xerogel. Unfortunately, the time resolution of the instrumentation used did not allow the authors to recover any reorientational process that was faster than about 2 ns. The data were also not analyzed in terms of an actual local microviscosity. More recently, we reported the first experiments on the picosecond rotational reorientation dynamics of rhodamine 6G (R6G) entrapped in a TMOS-derived ~ o l - g e l . ~R6G ~ was chosen because it is an isotropic rotor and its excited-state fluorescence lifetime is insensitive to its local environment. These properties make the system moderately simple, and the time-resolved anisotropy studies yield detailed information on the cybotactic region about R6G throughout the sol-gel aging cycle. This study showed that two distinct R6G microenvironments existed throughout the aging process. The microviscosity of one domain remains constant throughout the entire sol-gel aging cycle. The second microdomain becomes more viscous and dominates at longer aging times. In order to develop sol-gel-based chemical sensors and biosensors, photonics devices, and nonlinearly active materials, it becomes important to understand the effects of the surrounding sol-gel matrix on the photophysics and dynamics of the entrapped dopant. Toward this end, we report now on the steady-state and time-resolved fluorescence of a model solute, PRODAN,45-S1 in a TMOS-derived sol-gel as a function of aging time and hydrolysis pH. TMOS was chosen because it is one of the most widely studied sol-gel materials. PRODAN45-5I was used because 0 1994 American Chemical Society
8102 The Journal of Physical Chemistry, Vol. 98, No. 33, 1994
(unlike R6G)42 (1) it is a probe that is highly sensitive to the physicochemical properties of its local envirdnment (emission maximum ranges from 401 nm in cyclohexane to 530 nm in ~ a t e r ) , (2) ~ 5 its excited-state decay kinetics are a strong function of its local environment, and (3) it has recently been demonstrated to have a large nonlinear polarizability, 8, which in a two-level model is a consequence of a change in dipole moment on excitation.52 The current work focuses on improving our understanding of how the aging of a sol-gel matrix effects the photophysics and cybotactic region surrounding the dopant PRODAN. Three main experiments are carried out: (1) static emission spectroscopy, (2) time-resolved fluorescence spectroscopy, and (3) static fluorescence anisotropy. Together these techniques provide detailed information on the cybotactic region about the solute in the sol-gel environment.
Theory Multifrequency Phase and Modulation Fluorescence. In the frequency domain, the sample under study is excited with highfrequency (MHz-GHz) sinusoidally modulated light and one measures the frequency-dependent phase shift (e(@)) and modulation (M(w)) of the resulting fluorescence. From these data one can extract the corresponding excited-state decay kinetics.53-55 The experimental measurables (@(a))and M ( w ) ) are related to an assumed decay law ( Z ( t ) )
Narang et al. parameters, (2) smallest relative x2 value, (3) random residual terms, (4) consistency with the separate experimental information, and ( 5 ) physical significance of the chosen model. More recently, several research groups have demonstrated that more accurate models may involve a continuous distribution of decay times.5u3 Here, one goes beyond the simplistic view of a fluorophore located in a finite number of discrete environments (eq 1) to the case where single or multiple fluorescent centers are distributed among or are converting between an array of environments. In this case one would expect the observed timeresolved intensity decay to be described by an ensemble of decay times associated with each microdomain. Therefore, the corresponding decay kinetics can be modeled by a continuous distribution of the form
(7) where simple functions are used to describe the underlying distrib~tion:5~-63 uniform
Gaussian
Lorentzian
through the sine and cosine Fourier transforms:
here o is the angular modulation frequency (w = 2z-J f = linear modulation frequency) and Z ( t ) is given by eq 1. S(w) and C ( o ) are related to the experimental measurables by53-55
M ( o ) = [S(w)2= c ( w ) 2 ] ' / 2
(4)
@(LO)= arctan[S(o)/C(o)]
(5)
In these expressions ( T ) is the central or mean lifetime value, u is the standard deviation of the Gaussian distribution, Wis the full-width a t half-maximum (FWHM) for the Lorentzian and uniform distributions, and T is the lifetime. To determine if the experimental data are best described by a discrete (eq 1) or distributed (eq 7) model, one again selects a simple test model, minimizes the residuals, and compares x2 (vide supra). One can also discriminate between the distributions if multiple sets of frequency-domain data are analyzed simultaneously using global analysis protocols.6M7 Steady-StateAnisotropy. If a sample is excited by vertically polarized light, its anisotropy ( r ) is given by68969
and the decay terms (ai and T i ) are recoverd by minimization of the x2 function:
D where D is the number of degrees of freedom and 68 and 6M are theuncertaintiesin measured phase and modulation, respectively. (Subscript c denotes the calculated frequency-dependent phase and modulation values based on a particular set of ai and ~i). In practice, one acquires a frequency-dependent set of phase and modulation data and picks a general form of a test model (eq 1). One then tests the experimental data against a model via nonlinear least squares (obtaining the best sets of a's and T'S for the model and the data) and compares the goodness of the fit between the data and the model by the x2function. In the idealized case, the best model simultaneously meets the following criteria: (1) simplest model having the minimum number of total floating
where Zl and ZI are the parallel and perpendicular components of the total fluorescence, respectively. Rotational mobility of the fluorophore70 can be obtained from the Perrin equation:6*-69
where I$ is the average rotational correlation (reorientation) time, T is the excited-state fluorescence lifetime, and ro is the limiting anisotropy. The DebyeStokes-Einstein expression provides a link between the experimental variable I$ and the average microviscosity (7) encountered by the fluorophore.68.69
Probing the Cybotactic Region of PRODAN
The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 8103
In this expression Vis the volume of the rotating fluorophore, R is the gas constant, and T i s the Kelvin temperature. One potential outcomeofthecontinuousdistributionof lifetimes (eq 7) is a corresponding “distribution of local microviscosity”. In principle, such a microviscosity distribution can be computed from eqs 12 and 13,using the (7)and a(7)values recovered from various distribution models (eqs 8,9, or 10). For example, if the decay kinetics are described by a Gaussian distribution of lifetimes, one can replace 7 in eq 12 by (~(7) (eq 9) and compute $(T), which can be substituted into eq 13 to yield ~ ( 7 (Le., ) ya distribution of microviscosities”).
.-E-_
Experimental Section Materials. The following chemicals were used: tetramethyl orthosilicate (TMOS) (Aldrich); HCl, Na2HP04, NaH2P04-2H20, and glycerol (Fisher); ethanol (200proof; Quantum); 6-propionyl-2-(dimethylamino)naphthalene (PRODAN; Molecular Probes, Laser Photonics Technology, Inc.). All reagents were used as received, and aqueous solutions were prepared in double-distilled-deionized water. Sample Preparation. A TMOS stock solution was prepared by mixing TMOS, water, ethanol, and HCl in the molar ratio 1:2:2:1.6X 10-5. To 1 mL of the stock solution, in a quartz cuvette, was added 27.3 pL of a 0.55 mM PRODAN stock solution, followed by the addition of 0.5 mL of 0.01 M phosphate buffer (either pH 4.0 or 8.0). Similar protocols have been used previously.l3J6 Additional experiments were carried out with 1 00-fold less PRODAN, and there was no discernible difference in results. The quartz cuvettes were capped immediately after the addition of buffer. The cuvettes were maintained at room temperature (22-26 “C)for aging and drying. The cap was removed after 48 h to increase the aging rate. If the cuvettes were not capped initially, the monoliths often cracked upon aging and drying. Viscosity Scale. Solutions of known viscosity71 were prepared by mixing glycerol and water in different ratios. PRODAN (3 pM) was then added to each solution and the steady-state anisotropy measured ( T = 20 “C). From this data we extracted a volume of 140 cm3/mol for PRODAN, in excellent agreement with previously reported values.72 This volume and limiting anisotropy (ro) of 0.336 f 0.002 (hx= 363 nm) were used to determine the microviscosity surrounding the PRODAN molecule in the sol-gel systems (vide infra). Fluorescence Measurements. All steady-state fluorescence measurements were performed with a SLM 48000 M H F spectrofluorometer using a Xenon arc lamp as the excitation source. Emission spectra were background-subtracted and corrected for detector and monochromator transmission nonlinearities. The bulk of the instrumentation used for this work has been described in detail el~ewhere.73-~6All fluoresence lifetime measurements were performed in the frequency domain with the use of a modified version of the instrument described by Bright.75 An argon ion laser (Coherent, Innova 90-6)was used as the excitation source. The 363 nm line was used for the time-resolved experiments, and the fluorescence was collected through a 435 nm longpass filter (Oriel). Magic angle polarization was used for all sample lifetime measurement^.^^,^^ MeZPOPOP in ethanol ( T = 1.45 ns) was used as the reference flu0rophore.~9 For all experiments, the Pockel’s cell was operated at a repetition rate of 5 MHz. Typically, data were acquired for 60 s from 5 to 150 MHz (30 frequencies). The frequency-domain data were analyzed using software from Globals Unlimited (Urbana, IL). Results and Discussion Steady-State Emission. Changes in emission characteristics of PRODAN are a measure of immediate microenvironment about the probe (Le., the cybotactic region). Figure 1 summarizes the
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Aging Time ( m i n ) Figure 1. Summary of steady-state emission from PRODAN-doped TMOS-derived sol-gels as a function of aging time and pH [8 (0);4 (V)]. (Panel A) Emission maxima. (Panel B) Full-width at halfmaximum (FWHM in cm-I) for the emission spectra.
changes in PRODAN emission as a function of the sol-gel aging time. Several features merit additional discussion. First, the sensitivity of PRODAN to its local environment is evident from the large red shift of the emission maximum (Figure 1A) Second, the gelation time for the pH 4 and 8 processed sol-gels are 10 and 60 min, respectively, and are denoted by the dashed vertical lines. Third, the emission maxima remain constant well beyond the gelation point, despite significant changes in the physical state (liquid to solid) of the bulk sol-gel matrix. This suggests that the microenvironment about the PRODAN molecule remains moreor lessconstant throughout this period despitevisualchanges in the physical state of the sol-gel much after the gelation time. Fourth, the red shift following gelation (after ca. 95 h) indicates that PRODAN senses a more polar environment. We attribute this red shift to the initial expulsion of ethanol from the sol-gel matrix, leaving a more waterlike environment. It is reasonable to expect ethanol to be removed before water, due to its higher vapor pressure. Fifth, after about 520 h of aging, a blue shift is observed which we attribute to the expulsion of water from the sol-gel network, thus making the local environment appear less polar. Sixth, one could argue that these spectral shifts may simply arise from the emergence of a new spectral feature. However, there is only a slight change (A50 cm-1) in the full-width at half-maximum (FWHM) (Figure 1 B) throughout the sol-gel aging cycle. This result is consistent with the original band simply shifting. Finally, there is a larger red shift following gelation for the sol-gel processed a t pH 4,and the subsequent blue shift is smaller. These results indicate a more hydrophilic environment for the pH 4 processed sol-gel. This can be attributed to the higher Si-OH content, due to faster hydrolysis of the alkoxide at lower pH.23 A more substantial blue shift for the pH 8 I
8104 The Journal of Physical Chemistry, Vol. 98, No. 33, 1994
Narang et al. 70
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processed sol-gel also indicates that the extent of water consumption is greater at higher pH values. These results are consistent with previous reports.3’ They also demonstrate one of the advantages of PRODAN over R6G in that the R6G spectra do not change appreciably42throughout the sol-gel aging cycle. Steady-State Fluorescence Anisotropy. The changes in the average PRODAN fluorescence anisotropy as a function of the sol-gel aging time (pH 4 or 8) are shown in Figure 2. These results are in agreement with those obtained previously using R6G as the fluorescent probe; one observes four distinct stages of the sol-gel aging process.42 Because the final anisotropy is lower than the limiting anisotropy (ro), one can conclude that there is clearly nanosecond rotational freedom of PRODAN even after it is dried under ambient conditions. (Additional experiments using 100-fold less PRODAN eliminate the possibility of some form of energy transfer artifically lowering the measured anisotropy values.) The issue of the average microviscosity sensed by PRODAN will be discussed in conjunction with the timeresolved fluorescence results. Time-ResolvedFluorescence. Multifrequency phase and modulation fluorescence is a powerful technique for determining the excited-state decay kinetics as a fluorescent ~enter.53-51~9-63 Figure 3 shows a typical multifrequency data set for a PRODAN-doped TMOS sol-gel, aged for 573 h (average anisotropy reaches saturation at this time).80 In panel A, we present theexperimental data (points) along the best fits to single-exponential (dotted line) and unimodal Gaussian distribution (solid line) decay models. Figure 3B illustrates the corresponding residual phase angle for a single exponential (v)and unimodal Gaussian distribution (a). A unimodal Gaussian distribution is clearly the superior model describing the experimental data. Table 1 summarizes the fits of the data shown in Figure 3 to various decay models and compares the associated fit quality ( ~ 2 ) . A unimodal Gaussian distribution (based on x2 values), indicative of the existence of an ensemble of (static or interconverting) microenvironments, best describes the intensity decay. Specifically, the unimodel Gaussian distribution is statistically superiors1 to the single-, double-, and triple-exponential decay laws at greater than the 80% confidence level and better than the unimodal Lorentzian distribution above the 70%confidence level. At other aging times we sometimes found that the doubleexponential decay law yielded a xz comparable to the unimodal Gaussian distribution. However, the unimodal Gaussian model has one less floating parameter and is thus statistically superior. Moreover, the results of the unimodal Gaussian distribution are in concordance with the steady-state results and best describe the system physically. Thus, on the basis of the available experimental data, we conclude that a unimodal Gaussian distribution best
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doped sol-gel monolith. (Panel A) Phase angle (points) and fits (lines) to various decay models. The x2 term is a measure of the goodness of fit. (Panel B) Residual phase angles for fits to single-exponential (0) and unimodal Gaussian distributions (V),respectively.
TABLE 1: Recovered Excited-State Decay Parameters for PRODAN in a TMOS Sol-Gel Monolith’ recovered Darameters decay model ( r ) b rlC ~2~ nd ale WTf y2 single 2.31 1.00 44.7 double 3.18 1.00 0.58 1.26 triple 4.16 2.14 2.00 0.39 0.57 1.31 uniform 2.94 2.12 1.92 Lorentzian 2.69 1.46 1.22 Gaussian 2.86 1.20 1.15 The sample has been aged for 573 h. Central or mean value for the continuous distribution. Discrete lifetime (ns). Discrete lifetime (ps). e Preexponentialfactor (E:-, ai = 1). f Width term. “u” for the Gaussian and “W‘ for the uniform and Lorentzian distribution. describes the experimental intensity decay for a PRODAN-doped TMOS-derived sol-gel throughout the aging cycle. These results are to be contrasted to studies on TMOS-derived sol-gels droped with R6G in which we found42that a single excitedstate lifetime described the intensity decay. This again demonstrates that PRODAN is spectrally and temporally a far more sensitive measure of the cybotactic region compared to R6G. Figure 4 presents the recovered lifetime distribution (solid curve) of a PRODAN-doped sol-gel aged for 573 h (data of Figure 3). The corresponding fluorescence lifetime for PRODAN in water, ethanol, dimethylformamide, and cyclohexane (benchmarks) are indicated by the solid vertical lines. Prior to a discussion of these time-resolved results, it is important toquestion first the imprecision in the recovered decay parameters. To address this issue, we carried out a series of rigorous confidence interval analyses.s2 In this scheme, one fixes a particular decay
The Journal of Physical Chemistry, Vol. 98, No. 33, 1994 8105
Probing the Cybotactic Region of PRODAN 1.2
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Figure 4. Recovered unimodal Gaussian lifetimedistribution(solid curve) for a PRODAN-doped sol-gel monolith aged for 573 h under ambient conditions. The dashed traces illustrate the 67% confidence interval (Figure 4). Theverticallinesrepresentthediscretelifetimesfor PRODAN in the neat liquid solvents indicated.
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parameter (e.g., (7))to a predetermined value and refits the entire data set, adjusting all floating parameters, and calculates a new x2. This process is repeated over a range of (7)values, and the x2 vs ( T ) yields a convenient measure of the imprecision in ( 7 ) . 8 2 Further, because one investigates each parameter separately, any correlation between terms is compensated.82 Figure 5 illustrates the confidence interval results (x2surfaces) for the data shown in Figure 3. The dashed horizontal line represents the rigorous 67% confidence interval, and its position indicates the imprecision (measured from the points where the curves intersect the dashed line) in the parameters. Figure 5A,B shows the uncertainty in (7)and the width of the unimodal Gaussian distribution, respectively. It is clear from Figure 5 that therecoveredlifetimedistributionand the widthofthedistribution are moderately precise. Figure 4 illustrates (dashed curves) how the uncertainties in thedecay and width terms (at 67% confidence interval) affect the uncertainty in the recovered Gaussian distribution. Figure 6 summarizes the effects of pH and aging time on the mean excited-state lifetime (panels A and C) and the associated width of the Gaussian distribution (panels B and D) for PRODAN-doped TMOS-derived sol-gels. Several aspects of
these results merit further discussion. For example, the excitedstate lifetimes of PRODAN in water and in ethanol are 2.1 and 3.7 ns, respectively. The recovered mean lifetime of PRODAN in the sol-gel matrix soon after gelation is about 2.6 ns,suggesting an average environment intermediate between that of water and ethanol. We observed that the excited-state lifetime distribution remains constant much beyond the gelation point. The average lifetime then decreases to a value of 2.1 ns, indicating a more waterlike environment, followed by a subsequent increase to an average value of 2.6 ns, suggesting the transition to a more nonpolar environment. This is in direct correlation with the steady-state results (Figure l ) , and the changes all occur on the same time scale. Thus, it appears that the initial decrease in the average excited-state lifetime results from the expulsion of ethanol from the matrix, followed by the removal of water. Our previous work with R6G42 was unable to detect this subtle difference between ethanol and water expulsion which is now very clear from the PRODAN results. The width of the Gaussian distribution is a qualitative measure of the heterogeneity of the microenvironment in the system.83 Because one observes a distribution of lifetimes, it suggests the existenceof a heterogeneous microenvironment soon after gelation. The width of this distribution increases concurrently with the removal of ethanol. This increase in “heterogeneity” can be attributed to random removal of ethanol from the more accessible domains, with varying amounts of water remaining. These domains, containing different amounts of water and ethanol, account for the increased matrix heterogeneity and, as a result, the increase in the width of the Gaussian distribution. Upon the
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The Journal of Physical Chemistry, Vol. 98, No. 33, 1994
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region) remains constant well beyond the gelation point. The fluorescence emission characteristics of PRODAN indicate that the expulsion of solvent is a stepwise process, in which the removal of ethanol is followed by that of water. Our previous work42 using R6G did not observe this phenomenon because R6G is not nearly as environmentally sensitive compared to PRODAN. There is significant rotational freedom of the entrapped probe even after the sol-gel is dried under ambient conditions. Time-resolved fluorescence results are in agreement with steady-state measurements.83 The excited-state intensity decay of PRODAN is best described by a unimodal continuous Gaussian distribution throughout the entire sol-gel process. Earlier work with R6G did not reveal such a distribution mainly because R6G’s decay kinetics are not easily affected by the physicochemical properties of its local environment. The expulsion of solvent changes the PRODAN lifetime distribution, finally resulting in a microenvironment that is intermediate between those of water and ethanol. The microenvironment of the sol-gels is heterogeneous immediately after the onset of gelation. This heterogeneity increases upon expulsion of ethanol but decreases as water is removed from the sol-gel matrix. One possible interpretation of the recovered intensity decay data is a corresponding distribution of microviscosities. We have shown that the apparent microviscosity change is minimal until the removal of solvent. The recovered final mean microviscosity sensed by PRODAN is significantly lower than that previously reported for the bulk viscositys4 of sol-gels but agrees well with our previous R6G re~ults.~2 This result again demonstrates that there is a certain degree of mobility of some dopants within the sol-gel network. The nonlinear optical properties of PRODAN doped in a solgel matrix are currently under investigation. This system shows great promise for second-order optical n0nlinearity.5~
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Microviscosity (centipoise) Figure 7. Recovered microviscosity distribution for a PRODAN-doped TMOS sol-gel (pH 8) after aging for 4 (O), 95 (v),141 (V), 165 ( O ) , 245 (m), and 573 h (A). removal of water, the width of the distribution decreases as the system becomes more homogeneous. Sol-gels catalyzed at pH 4 and 8 follow similar trends for average lifetime and width of the Gaussian distribution. Once again, our earlier with R6G was unable to detect these subtle differences in microheterogeneity . By combining the excited-state lifetime (Figure 6) and anisotropy results (Figure 2) for the PRODAN-doped sol-gels, one can describe the local PRODAN microenvironment in terms of a “continuous distribution of microviscosities” (eqs 9, 12, and 13). The recovered distribution of microviscosities sensed by PRODAN in a PRODAN-doped TMOS-derived sol-gel (processed at pH 8) is illustrated in Figure 7 as a function of aging time. The total width of the distribution represents the extent of heterogeneity encountered by the PRODAN probe. Interestingly, as we observed with R6G,42the initial average microviscosity is near 2 cP. This result is somewhat surprising and suggests that PRODAN (a neutral probe) also experiences domains similar to those of R6G (a cationic probe) within the fresh TMOS sol-gel matrix. We also note that the apparent microviscosity is a strong function of aging time and follows the same trend as that reported for the steady-state anisotropy. That is, the microviscosity changes little throughout the initial aging cycle but increases dramatically upon removal of solvent. In Figure 7,. (4 h) and v (95 h) bracket the gelation region. Over this same time increment, the change in the microviscosity is minimal (note: microviscosity is plotted on a logarithmic scale for clarity). Upon the onset of ethanol removal, the width of each microviscosity distribution, represented by v (141 h), 0 (1 65 h), and (245 h of aging time), increases significantly. This suggests that the overall heterogeneity sensed by PRODAN increases as does the microviscosity sensed by PRODAN. Upon the onset of water expulsion, A (573 h), the width of the distribution begins to decrease, illustrating a trend toward a more homogeneous microenvironment about PRODAN. Finally, the final average microviscosity is significantly lower than that of the bulk viscosity84 and is comparable to the value recovered using R6G as the pr0be.~2
Conclusions We report steady-state and time-resolved fluorescence results for PRODAN-doped TMOS-derived sol-gels as a function of aging time. The steady-state results show that the microenvironment about the dopant with these sol-gels (Le., the cybotactic
Acknowledgment. This work was supported in part by the National Science Foundation Surface and Analytical Chemistry Program (Grant CHE-9300694). The work of the Photonics Research Laboratory was supported in part by the Office of Innovative Science and Technology of the Strategic Defense Initiative Organization and the Air Force Office of Scientific Research, Directorate of Chemistry and Materials Science, through Contract No. F49620-90-C-0053 and in part by the National Science Foundation Solid State Chemistry Program (Grant DMR-9213907). We also thank Jennifer Holt for her help in the data acquisition. References and Notes (1) Chemical Processing of Advanced Materials; Hench, L. L., West, J. K., Eds.; Wiley: New York, 1992. (2) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33. (3) Paul, A. Chemistry of Glasses, 2nd Ed.; Chapman and Hall: New York, 1990; p 51. (4) Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: New York, 1989. (5) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (6) Wung, C. J.; Pang, Y.; Prasad, P. N.; Karasz, F. E. Polymer 1991, 32, 605. (7) Zhang, Y.; Prasad, P. N.; Burzynski, R. In Chemical Processingof Wiley: New York, Advanced Materials; Hench, L. L., West, J. K.,Us.; 1992; p 825. (8) Zhana. Y.: Prasad. P. N.: Burzvnski. R. Chem. Mater. 1992.4. . . 851. (9) Kuselkan; I.; Lev; 0. Talanta 1993, 40, 749. (10) Wolfbeis, 0. S.; Rodriguez, N. V.; Werner, T. Mikrochim. Acta 1992, 108, 133. (11) Narang, U.; Dunbar, R. A.; Bright, F. V.; Prasad, P. N. Appl. Spectrosc. 1993, 47, 1700. (12) MacCraith, B. D.;Ruddy, V.;Potter, C.; OKelly, B.; McGilp, J. F. Electron Lett. 1991, 27, 1247. (13) Ellerby, L.; Nishida, C.; Nishida, F.; Yamanaka, S.A.; Dum, B.; Valentine, J. S.; Zink, J. I. Science 1992, 25, 1113. (14) Shtelzer, S.;Rappoport, S.;Avnir, D.; Ottolenghi, M.; Braun, S. Biotech. Appl. Biochem. 1992, 15, 227. (15) Braun, S.;Rappoport, S.:Zusman, R.; Avnir, D.; Ottolenghi, M. Mater. Lett. 1990, 10, 1.
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