2134
J . Phys. Chem. 1989, 93, 2134-2139
Fluorescence Study of Aluminosilicate Sols and Gels Doped with Hydroxy Trfsulfonated iyrene J. C. Pouxviel,+B. Dunn,*?+and J. I. Zinkt Department of Materials Science and Engineering and Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90024 (Received: July 6, 1988)
Aluminosilicate sols and gels doped with pyranine (8-hydroxy trisulfonated pyrene) have been prepared by hydrolytic polycondensation of an organometallic precursor. We report upon the evolution of the dye luminescence during polymerization, aging, and drying of the gels. Under selected experimental conditions, we show that the ratio of the green and blue emission peaks, which reflect the relative degree of deprotonation of the excited pyranine molecules, is mainly controlled by the water content of the medium surrounding the dye. Kinetic and structural information concerning the gel and its interaction with the dye are discussed. From the early stages of polymer growth, the pyranine molecules are in intimate contact with the surface of the inorganic network. As polymerization proceeds, the water content in the local surroundings of the pyranine molecules decreases as a result of condensation reactions involving Al-OH groups. These reactions continue until well after the onset of gelation, although at a much slower rate. The drying of the gels is also investigated.
Introduction The sol-gel process is a recently developed technique used to prepare oxide powders or glasslike materials by hydrolysis and polycondensation of organometallic compounds. The low processing temperatures enable one to dope the gel with luminescent organic molecules.1d Such molecules have poor thermal stability and cannot be included in traditional inorganic hosts. The sol-gel method, however, affords an opportunity to incorporate organic molecules in an inorganic matrix. The number of potential applications of such materials is expected to be substantial. Possible applications include devices for use as nonlinear optical elements, luminescent solar concentrators, and chemical and pH sensors. The inorganic matrices produced from the sol-gel process are expected to possess several advantages over organic polymers such as poly(methy1 methacrylate) (PMMA). The photostability is enhanced because no radicals are generated by UV irradiation, and subsequent fluorescence quenching is significantly reduced. The thermal stability of the oxide is decidedly better and the index of refraction is not perturbed by the formation of temperature gradients. Avnir et al. were the first to report the preparation and the resulting optical properties of silica gels doped with rhodamine 6Gl and pyreneS2They have also shown that other dyes, including fluorescein, crystal violet iodine, malachite green oxalate, and oxanine-4 perchlorate can be successfully incorporated in the inorganic silica n e t ~ o r k .More ~ recently, they have used pyranine as a fluorescent probe to monitor the water content within silica sols obtained by hydrolysis of tetramethoxy~ilane.~ Although materials have been synthesized, numerous fundamental questions have yet to be considered. Little is known, for example, about the nature of the interaction between the guest dye molecules and the solid host matrix, or about the parameters that control the luminescence of molecules embedded in these inorganic systems. A given dye solution is likely to behave differently in a solid matrix because molecular mobility, solvation, and reorientation processes have a dominant effect in their electronic excited states. Another interesting aspect is that the sensitivity of dyes to chemical environment suggests the potential for using luminescent probes during polymerization, gelation, and drying of gels to investigate the sol gel chemistry and/or structural development. This paper considers the interaction of a guest dye molecule with an aluminosilicate sol-gel matrix. The emission and excitation spectroscopy of aluminosilicate sols and gels prepared by the hydrolytic condensation of a double alkoxide, (Bu0)2Al-O*Author to whom correspondence should be addressed. 'Department of Materials Science and Engineering. *Department of Chemistry and Biochemistry.
Si(OEt),, doped with 8-hydroxy 1,3,6-trisulfonated pyrene is presented. The luminescence of this dye is studied in mixtures of various solvents in order to determine the effects of pH, proton donor capacity, and water content. Changes in the luminescence of the dye molecule are monitored during the sol-gel process from its initial solution phase to the eventual dried aluminosilicate gel. The results are discussed in terms of the chemical changes that occur during the sol-gel process and provide insight regarding the structural aspects of the gel and the interactions between the dye and the sol-gel matrix.
Starting Materials and Methods 1 . Hydroxy Trisulfonated Pyrene or Pyranine. The chemical structure of the pyranine molecule and a representation of its optical response to chemical changes are shown in Figure 1. The molecule of 8-hydroxy 1,3,6-trisulfonated pyrene (also referred to as pyranine in this paper) is derived from the pyrene molecule which is composed of four benzene rings arranged in a planar configuration. Three sodium sulfonated groups and one OH group are substituted in positions 1, 3, 6 and 8 on the two rings sharing only two edges with the whole structure. Absorption and fluorescence spectra of pyranine are very sensitive to protontransfer phenomena. The basic photochemistry of pyranine has been ~haracterized'-~'and the molecule has been successfully used to detect pH variations in phospholipid vesicles by measuring the luminescence inten~ity,',~ and to probe the structure of reverse micelles as a function of water c ~ n t e n t . ~This * ' ~ molecule has been also employed as a proton generator by flash illumination (pH jump]'). In an ionic medium, the S03-Na+ groups are believed (1) Kaufman, V.; Avnir, D. Langmuir 1986, 2, 717. (2) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984, 88, 5956. (3) D. Avnir, D.; Kaufman, V. R.; Reisfeld, R. J . Non-Cryst. Solids 1985, 74. 395. (4) Kaufman, V. R.; Avnir, D.; Pines-Rojanski, D.; Huppert, D. J . NonCryst. Solids 1988, 99, 379. (5) Makishima, A,; Tani, T. J . Am. Ceram. SOC.1986, 69, 4. Tani, T.; Namikawa, H.; Arai, K.; Makishima, A. J. Appl. Phys. 1985,58,3559. Pope, E. J. A.; MacKenzie, J. D. M R S Bull. 1987, 12, 29. Newsham, M. D.;
.-
Cerrata, M. K.; Berglund, K.A.; Nocera, D. G. In Better Ceramics Through Chemistry III; MRS Symposium, Reno, April 1988; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds. (6) Dunn, B.; Knobbe, E.; MacKiernan, J. M.; Pouxviel, J. C.; Zink, J. In Better Ceramics Through Chemistry IIk MRS Symposium, Reno, April 1988: Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds. (7) Clement, N. R.; Gould, M . Biochemistry 1981, 20, 1534. (8) Kano, K.; Fendler, J. H. Biochim. Biophys. Acta 1978, 509, 289. (9) Kondo, H.; Miwa, I.; Sunamoto, J. J. Phys. Chem. 1982, 86, 4826. (10) Bardez, E.; Goguillon, B. T.; Keh, E.; Valeur, B. J . Phys. Chem. 1984, 88, 1909. (11) Pines, E.; Huppert, D. J. Phys. Chem. 1983, 87, 4471.
0022-3654/89/2093-2134$01.50/0 0 1989 American Chemical Society
Fluorescence Study of Aluminosilicate Sols
The Journal of Physical Chemistry. Vol. 93, No. 5, 1989 2135
pKa= 1
RO-.+H+
ROH'C---)
TABLE I: Composition of the Investigated Aluminosilicate Sols and Gels"
composition precursor concn, mol/L propanol/water voI fraction water/precursar molar ratio gel time, h
142
HI0
0.8
0.4
4
II
10
IO
H20 0.4 5 20
I5
43
17
aThe concentration of pyranine in the initial sols is I
Figure 1.
Chemical structure of pyranine and absorption-emission pro
CeSSeS.
> t In
z W
+
5 400
500
600
WAVELENGTH(nm) Figure 2. Emission spectra of pyranine dissolved in water/propanol mixtures; the percentage shown is the volume percent of water. The excitation wavelength is 345 nm. to be fully dissociated. In the ground state, the protonated molecule p y o H is reported to be a weak acid with a pK. of about 7.5. In the excited state, however, it is a more acidic molecule, PyOH*, with a pK. of about 1. The protonic ground-state form is stabilized in acidic conditions (pH < 7.5) while the deprotonated PyO- is present in hasic condition (pH > IO). The two forms present a maximum in absorption a t 400 and 450 nm, respectively.',8 After excitation by light, the competition between the kinetics of the protonation4eprotonation and the deactivation processes of the two forms controls the relative intensity of the green emission peak at 515 nm (due to the RO-* form) and of the blue emission at 430 nm (due to the R O W form). In water solution, the excited states have a fluorescence lifetime of approximately 5 ns and the deprotonation and protonation rate The relative constants are and 3 x IO-'ss', re~pectively.~ intensities of the blue and green emission are affected by the proton donor-acceptor ability of the surrounding medium or by large changes in pH from neutral to very acidic conditions. In aqueous solution the fluorescence intensity is pH dependent. For pH greater than 3, the emission is restricted to the green emission a t 515 nm, which is characteristic of PyO-*. The peak a t 430 nm arises only at much lower pH? As shown in Figure 2, the luminescent behavior in alcoholic solutions is quite different from that of aqueous solutions. 2. Aluminosilicate Sols and Gels. The sol-gel matrix is prepared by hydrolysis and subsequent condensation of a bifunctional silicon aluminum alkoxide." It is difficult to prepare transparent aluminosilicate gels possessing good chemical homogeneity from a mixture of AI and Si alkdxides because the hydrolysis rates ofthe individual alkoxides are too different. The double alkoxide allows one to prepare clear monolithic gels with the nominal composition AI,0,-2SiOz by a direct process. No base or acid is necessary to catalyze the gel formation. Depending on the water and precursor concentrations, gels can be obtained in a relatively short time, from 7 h to a few days. The preparation methods, gelation mechanisms, and the structure of the polymers have been studied by Pouxviel and Boilot
X
H30 0.4 3 30 8
IO" mol/L.
using '9Si and "AI N M R and by small-angle scattering techniques.'2-'5 Polymerization is initiated upon the addition of water, as organic groups attached to the aluminum atoms are rapidly hydrolyzed. The resulting AI-OH species have an amphoteric nature and thus buffer the pH of the solution, maintaining values close to 7.1. These species rapidly condense, forming AI-0-AI bonds and leading to the generation of nearly dense particles some 15-20 .& in diameter. This first stage of AI hydrolysis and condensation is very short ( I or 2 min) compared to the total gelation time. The second step consists of the aggregation of these initial particles by means of hydrolysis of some Si-OR groups, immediately followed by the condensation of silanol groups. The rate constant for this process is a direct function of the water content. Near the gel point, clusters having a gyration radius of about 100 8, are present in the sol. These clusters have been characterized to have a mass fractal structure with a dimension of 2. As the clusters become entangled and overlap, information from small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) is difficult to interpret. Thus, little is known concerning the structural changes which occur during the aging and drying steps for these aluminosilicate gels. The aluminum silicon ester precursor was supplied by Dynamit Nobel. The synthesis parameters employed are shown in Table I. The preparation conditions and sample identification numbers are identical with those used in previous works.'2-'5 Isopropyl alcohol is used as a mutual solvent for water and the aluminosilicate precursor. The volume of added water represents 2-5 times the amount necessary for total hydrolysis. Depending upon the compositions, gelation times ranged from 8 h for composition H30 to 40 h for composition H10. The gels were doped with pyranine during the sol preparation. The pyranine dye was dissolved in the necessary amount of distilled water used to hydrolyze the alkoxide. The pyranine concentration in the initial sols was 1 X M. The presence of the organic dyes was found to have no influence on the gelation kinetics. After gelation, the gel samples were kept in closed containers for time periods of at least I O times that required for gelation. Once the aging step was complete, a small hole was opened on the top of the container to permit drying at room temperature. 3. Optical Measurements. Excitation and emission spectra were recorded on a Spex Flurolog spectrophotometer, Model F1 IM. All fluorescence measurements were performed in a front face configuration. Liquid samples were kept in styrene cuvettes, while dry solid samples were measured in air. Although the initial concentration of the dye in the sol or in M, this concentration increased the solvent mixture was 1 X during drying. A solid, dried gel had a typical concentration of 8 X IO' M. Strong luminescence was observed for all samples and the intensity was slightly increased as the material progressed from the sol to the dried gel. The ratio, F, used in this paper is defined as the intensity of the blue peak (438 nm) divided by that of the green peak (515 nm). A more rigorous determination of F, which involves normalizing the two spectra and subtracting the initial pure green emission curve, did not lead to significant differences from that of the simple ratio, F. Small variations of Paurviel, I. C.: Boilot, J. P. J . Mater. Sei., in press. (13) Pouxviel, J. C.; Boilot, J. P.; h m t c . A,; Dauger. A. J. Phys. Paris 1987, 48, 921. (14) Pouxviel, J. C.: Boilot, J. P.: Dauger. A,; Wright, A. C. J. " ICryst. Solids, in press. (IS) Boilot, I. P.; Pouxviel, J. C.; Daugcr, A,; Wright, A. C. In Betlei Ceramics Through Chemistry IIC MRS Sympasium, Reno, April 1988; Brinker, C. J., Clark, D. E., Ulrich, D. R.,Eds. (12)
2136
The Journal of Physical Chemistry, Vol. 93, No. 5, 19619 2.0
00
1
Pouxviel et al. in,
1
\
1 0
20
40
60
80
100
I 111
Yo WATER
Figure 3. Evolution of the emission intensity ratio F of the pyranine emission in different water/solvent solutions as a function of the volume percent of water. The excitation wavelength is 345 nm.
0.0,
0
100
IO0
WAMLWTM mm,
A ,
I
50
,
I
,
100
I
150
,
200
TIME (hours)
the excitation wavelength do not produce measurable changes in this ratio.
Figure 4. Evolution of the ratio Fof the blue (438 nm) and green (515 nm) intensities during the gelation of the 142 composition. The gel point (B) is indicated with an arrow.
Results 1 . Pyranine Spectroscopy in Solution. Emission spectra of pyranine dissolved in isopropyl alcohol and water mixtures were determined by using an excitation wavelength of 345 nm (Figure 2). The pH of the solution was fixed at 7 by addition of a small amount of pH 7 buffer. The intensities of green and blue emission vary as a function of relative water content. At low water concentration (25% in volume), the green peak arising from the fluorescence of the excited deprotonated form, PyO-*, is very weak. This peak becomes dominant when the water volume fraction reaches 40%. The evolution of the blue and green peaks can be quantified by the change of the ratio, F, which decreases from 1.7 to almost zero as the water content varies from 25 to 75%. The variation of F was also investigated with other watermiscible solvents. Figure 3 compares the change of the ratio F for dimethyl sulfoxide, isopropyl alcohol, and formamide mixed with water. The same trend is observed for these three types of solutions. The increase of F at low water content is more abrupt in DMSO and isopropyl alcohol than in formamide. The three solvents are known to have different polarities and proton-acceptor abilities which can account for the different spectral responses. DMSO, despite its strong polarity, does not allow the deprotonation of pyranine when the amount of water is less than 40%. Formamide is known to be a weak proton acceptor. Thus, even when formamide represents more than 90% of the solution, the green peak of the deprotonated form continues to dominate the emission spectrum. The effect of the pH in the alcoholic solution, 50% propanol-50% water, was investigated by the addition of a few drops of HCI or NH40H. The ratio, F, was found to be invariant. The only changes observed were those of the total luminescence intensity. The behavior shown here for pyranine in solution is consistent with literature reports.’-I’ The spectra demonstrate that protonation/deprotonation kinetics of pyranine in water-organic solvent mixtures are controlled by the proton-acceptor character of the medium. In the reference solution used as a control, small changes in pH were found to affect the total emission intensity, but not the ratio, F, within the experimental uncertainty of the measurement. 1 . Luminescence Changes during Gelation and Aging. The emission spectra of the pyranine-doped aluminosilicate sols were recorded at regular intervals for the 142, H10, H20, and H30 compositions. In all cases, polymerization was accompanied by an increase of the blue peak. l a . Polymerization and Aging of the 142 Gel. Figure 4 shows the emission of the 142 composition just after preparation (A), a t the gel point (B), and after aging (C). The gel was kept in a closed container for all these measurements. The evolution of the individual blue and green peaks is shown in the inset. The ratio, F, increases almost linearly during the gelation process from 0 at the beginning of preparation ( t = 0 h) to an intermediate value of about 0.45 at t = 15 h at gel point. This ratio increases
because a prominent blue peak develops, while the green peak decreases slightly. Significantly, the ratio continues to evolve in closed gels over a period of time equal to about 8 times the gelation time, eventually reaching a maximum value of 0.9 after 200 h. These results clearly indicate that the dye molecule is sensitive to changes associated with gel polymerization and that changes in gel chemistry continue well beyond the gelation period. The macroscopic rigidity observed at the gel point does not mean that, at the microscopic level, all the reactive entities are immobile. Although the kinetics of the reactions involved in the aging of the gel appear to be slower than the ones in the liquid state, the extent of these changes, as indicated by the evolution of blue/green peaks intensities, is as great in the gel state as in the liquid state. Excitation spectra for the 430- and 515-nm peaks remain parallel during the entire gelation process. This behavior is not unexpected since prior work has showni0that luminescence in the pH 7 range is dependent upon the equilibrium established between the excited state (ROH* and RO-*) and not that of the ground state. l b . Effect of Water Content. Pyranine was added to aluminosilicate sols having a range of different water contents. The water content directly controls the polymerization rate of this aluminosilicate system.i2-i4~i5 SANS data have shown that the growth kinetics (gyration radius, total scattered intensity) are dependent on the water concentration. H10, H20, and H30 solutions have exactly the same precursor concentration but with a molar water/precursor ratio equal to 10, 20, and 30, respectively. The gelation times decrease accordingly with increasing water content (Table I). The change in F as a function of time for these three solutions is shown in Figure Sa. Immediately after sol preparation, the values of F exhibit slight differences but are still small. The ratio for H10 ( F = 0.26) is greater than the other solutions ( F = 0.13). In the initial stages of gelation (Le., over the first 10 h), F evolves almost linearly for all samples as was observed for the 142 composition. A surprising feature here is that, despite a great difference in the polymerization kinetics and gelation times for H10, H20, and H30, the rate of increase of F, dF/dt, is nearly the same for these three compositions. This result implies that the increase in viscosity which accompanied the polymer growth prior to the gel point does not influence the progressive changes occurring in the emission spectra. There is also no indication that the luminescence characteristics are altered by the abrupt viscosity increase near the gel point. Moreover, in light of the neutron-scattering data, these results also indicate that the rate, dF/dt, is not related to the growth rate of clusters and therefore to the types of reactions that are responsible for the aggregation of these clusters. Similar to the trend observed in the case of the 142 composition, the slope, dF/dt, begins to decrease after the gel point and only begins to approach zero after a period of time which is approximately 8 times the gelation period (Figure 5b). Although the kinetics may be comparable, the values of F at the end of aging are quite different: 1.3, 1 , and 0.8 for the HIO, H20, and H30
Fluorescence Study of Aluminosilicate Sols
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2137
i!2&
h
!n
r
. !n
I
-
I-
z 400
m P 0
v
U
500
600
WAVELENGTH(nm)
00
0
2
8
6
4
Figure 7. Comparison of the emission of a 142 aged gel doped with pyranine when the solvent has been replaced by water (1) or propanol (2).
10
TIME (hours) 15
-.
1
h v)
r
1.0
(0
U 0
I
v
0.5
I
0.0kl 0
300
200
100
400
TIME (hours)
Figure 5. Evolution of the ratio Fof the blue (438 nm) and green (515 nm) intensities during the gelation and aging of the closed gels H10,H20, and H30. The upper figure shows the F ratio during the first 10 h. The lower figure shows the entire gelation and aging process.
,
2.0
I
A F E R DRYING AT7CC
".", 0
.
, 100
.
I
200
.
, 300
.
-4 I0'
I
,
.
400
TIME (hours) Figure 6. Evolution of the emission intensity ratio, F, during the entire gelation, aging, and drying process. Weight loss during drying occurred at a constant rate of 8 . 7 mg/h.
gels, respectively. As discussed later, these equilibrium values are associated with the water content of the system. 1c. Drying. Drying experiments were performed after the luminescence of the closed gel had obtained a stable value. Wet samples of composition 142 (initial weight of 3 g) exhibited solvent loss at a constant rate of approximately 8.7 mg per hour. The total weight loss reached 85% in 2 weeks. The final volume of the dried samples was only 1/8 of the initial volume. The evolution of the emission intensity ratio, F, during the entire process is shown in Figure 6. The first 60% of the weight loss produces minor changes in emission. The ratio, F, decreases slowly from 0.9 to 0.7. In contrast, the curve becomes much more irregular when the weight loss reaches 70%: F increases sharply up to a maximum of 1.6. This value of F is comparable to that observed for a gel that has been heated for 10 h under vacuum at 70 O C and is well-dried. The precise mechanisms involved during the initial and latter stages of the drying are not completely clear at the present time. The drying of the gels consists of the removal of entrapped alcohol and water. As these constituents vaporize, the capillary forces, surface energy, and reactivity of pore walls induce a contraction and a subsequent modification of the gel structure. It is interesting
to note that, after drying, samples kept in the open atmosphere are able to readsorb moisture. The blue fluorescence decreases and F drops to 0.2-0.4. Thus, once the dried gels are at this stage, it is possible to control their emission properties by addition or removal of moisture. When the sample is fully desiccated (at 70 OC under vacuum), the ratio, F, approaches 1.7 and once the sample is in contact with room humidity, this ratio returns to low values. Immersing the sample in anhydrous propanol has the same effect as vacuum desiccation. 1d . Infiltration Tests. Subtle variations in the preparation procedures were performed in order to study the position of entrapped dye molecules within sols and gels. A 142 composition sol was initially prepared in the same manner as in the previous experiments and then divided into three parts, A, B, and C. Solutions A and B were doped during the mixing of water with the precursor, i.e., the usual method. Solution C was doped after 10 h of polymerization. In addition, after 10 h, water was added to solution B until complete precipitation of gel powder had occurred. This precipitate was washed several times at 50 O C with water and propanol and then dried at room temperature. The emission spectra for solutions A and C after 10.5 h are very different. In solution A, the ratio is near 0.4 while in solution C, its value is now 1.1. This spectral difference between the two compositions remains during the entire gelation and aging period. In this regard, the fluorescent emission is sensitive to the processing history of the gel. The results indicate that pyranine luminescence is not only sensitive to the average composition of the sols (since A and C are the same in regard to the nature of solvent and extent of polymerization) but also to the polymerization mechanism. The emission spectra suggest that the dye molecules interact with the polymers during the early stages of network growth. Consequently, the position of the dopant in the matrix and its interaction with the polymer system depend upon the processing history of the sample. This supposition is supported by the behavior of solution B. After precipitation of the oxide or hydroxide particles, the remaining liquid phase does not exhibit a detectable level of fluorescence activity. In contrast, it is the powder which maintains a high degree of fluorescence even after several washings. This demonstrates that, when the polymer sol is destabilized, the pyranine molecules are already enclosed and tied to the oxide network. The dye molecules cannot be removed by washing. Although the pyranine molecules are trapped in the oxide matrix, they are still sensitive to the solvent environment. This behavior was demonstrated in a series of experiments involving solvent exchange. In this case the aged but still wet gels were immersed in water or propanol and the solutions were periodically refreshed. Emission spectra indicate that the luminescence changes from having an exclusively green emission, when water is used for infiltration, to a predominantly blue one when propanol is diffused into the gels (Figure 7). This effect is reversible. Moreover, no leaching of the dyes occurs during the solvent exchange. Dried gels behave similarly to the aged gels; the organic molecules cannot be leached from the structure and the emission spectra are controlled by the medium to which the gel is exposed. Discussion The results of the spectroscopic study have established the main features regarding the luminescence of pyranine and the factors
2138
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989
affecting this luminescence in aluminosilicate sols and gels. In this section, questions concerning the position of the dye species within the gel structure, and the meaning of the spectral changes observed during gelation, aging, drying, and solvent exchange, are considered. I . Incorporation of the Dye within the Structure. The exclusive presence of the dye in the powder phase when precipitation is forced (solution B), and the inability to leach the dye from either the wet, aged, or dried gels, clearly demonstrate that the pyranine molecules are trapped within the polymeric structure. The nature and mechanism for such encapsulation are of considerable interest because they help to explain the observed optical properties. It is significant to note that pyranine is a triple anion (with SOY groups). Thus, there is the possibility that it will interact electrostatically with the inorganic network, which contains a large number of charged or polar species such as Al-OH groups possibly solvated by water molecules. However, the absence of a pronounced shift in either the absorption or emission spectra during the sol-to-gel transformation indicates it is unlikely that any significant bonding between pyranine and the polymer network occurs. The results obtained concerning the effect of processing history on luminescence underscore the notion that the luminescent properties of pyranine are sensitive to the polymerization process. In view of what is known about the polymerization of aluminosilicate gels, there is the likelihood that pyranine molecules are located on the surfaces of the initial sol particles. As a clustercluster aggregation process takes place, the molecules become increasingly entangled and ultimately trapped within the growing clusters. Despite entrapment, it is evident from Figure 7 that the pyranine emission is very sensitive to the presence and removal of solvents. Thus, even when the gel network has completely encapsulated the dye molecules, the ambient solvation conditions have a strong influence on the spectroscopy of pyranine. This behavior indicates that the molecule is connected via a pore network to the external surface of the gel. This model is in good agreement with what is known about the structure of aluminosilicate gels as determined by small angle scattering techniques. In that work, the polymers were reported to exist with a very open and ramified s t r ~ c t u r e . ' ~ J ~ 2. Spectral Changes during the Gelation Process. One of the principal results of the present study is that the ratio, F, changes during the gelation process. This behavior is shown in Figures 4 and 5. The reasons for this change must be related to the factors known to significantly affect pyranine luminescence, e.g., protonation/deprotonation events and rate kinetics. Thus, pH and the presence of proton acceptors or donors would be expected to influence the emitting species and its luminescence. In the experimental conditions investigated (excitation wavelength of 345 7), the ratio F clearly reflects the presence of a nm and pH proton acceptor in the medium surrounding the pyranine molecules. For this situation, small variations in the vicinity of pH 7 will simply modify the intensity of the fluorescence. No catalysts are added to the sols, and the precursor itself acts as a pH 7 buffer by means of the AI-OH amphoteric groups. Measurement of the sol pH using a glass electrode confirmed there were no significant pH changes during polymerization. Another significant experimental result is the luminescence similarities (Figure 5 ) for the three compositions (Le., H10,H20, H30) which possess very different gelation kinetics. The results with H10, H20, and H30 clearly establish that other physical parameters which are characteristic of polymerization, including the size of the polymer and the viscosity of the sols, are apparently not responsible for the evolution of the ratio F. If the spectral changes which occur (Le., Figure 4) are not directly due to either pH or viscosity, they must arise from other factors. It is important to realize that only relatively small changes in overall solvent composition (that is, the water:propanol ratio) are expected after the initial hydrolysis reaction which forms A1-OH groups and AI-0-A1 bonds. These reactions are quite rapid and take place only a few minutes following the mixing of reactant^.'^*'^ In contrast, silica-based gel systems are charac-
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Pouxviel et al. terized by a slow and continuous change in solvent composition (absorption of water by hydrolysis, followed later by release of water by condensation reactions). Despite the fact that aluminosilicate sols do not undergo significant composition changes, the spectroscopic variations during gelation are rather significant (Figure 4) and imply far more pronounced chemical changes than anticipated. These results suggest, therefore, that the pyranine molecules are probing a very selected aspect of the gel polymerization chemistry which is strongly influenced by protonation/deprotonation characteristics of the solvation shell. This shell will be, in return, influenced by the nature of the chemical species which compose the immediate surroundings of the dye. The initial value of F in the 142 sol just after hydrolysis (Le., point A in Figure 4, where F = 0) is substantially different from a water-propanol mixture with the same water concentration, where F > 1.5 (Figure 2). This enormous disparity confirms that, even in the early moments of the gelation process, the dye is not homogeneously distributed in the solution. Rather, it appears that the dye species are already part of the small particles (10-20 A) which result from partial condensation of Al-OH groups.13J4 These are essentially aluminum hydroxide particles with a high proportion of AI-OH groups and a surface which is expected to be very hydrophilic. These particles, which have both water and pyranine molecules adsorbed, constitute a medium with a substantially higher proton-donor ability than the average composition of the sol. From this explanation, it is apparent why the pyranine luminescence is representative of a much higher water content than would be expected if the pyranine was homogeneously distributed in the sol. The continuous changes in the ratio F (Figure 4) during gelation and aging indicate that significant chemical modifications occur both in the adsorbed layers of these particles and, consequently, in the solvating sphere of the pyranine molecules. The fact that F increases from 0 just after the solution preparation to 0.45 at the gel point and eventually to 0.9 at the end of aging indicates that the proton-acceptor ability of the adsorbed layers on the polymer surface decreases substantially. In terms of waterpropanol volume ratio (Figure 2), this evolution is equivalent to a change in water concentration from 100% at point A to 60% at gel point (B) and eventually to 40% after aging (C). These results indicate that the surfaces of the gel particles alter their chemistry continuously during gelation and aging. Ultimately, an equilibrium is established which reflects the initial composition of the sol. This equilibrium is evident in the emission spectra for H10, H20, and H30 (Figure 5b) at the end of aging. The respective values for these samples are consistent with the total water content added during sol preparation. The spectral changes shown in Figure 7 are in agreement with this explanation. When the solvent phase is replaced by pure water or pure propanol, the resulting luminescence can be either green or blue, depending upon the liquid phase substituted. This behavior demonstrates that the adsorbed layers on the gel particles which represent the surrounding medium for the pyranine molecules can be modified by the solvent composition. A totally alcoholic solvent will reduce the number of water molecules in these layers and create a local environment which is unable to accept a proton from an excited pyranine molecule. The blue emission shown in Figure 7 arises from these considerations. The changes in emission during the drying of the gels can be explained on a similar basis (Figure 6). Propanol is the more abundant solvent present in the pore system of the gel and has a higher vapor pressure than water. The first step of drying corresponds to the vaporization of propanol, which gives rise to an increase in the relative concentration of water in the remaining solvent. Therefore, the deprotonation mechanisms of excited pyranine molecules will be facilitated and the ratio F should decrease accordingly. This behavior is observed. Later, as drying continues, the free water content approaches zero and the ratio F increases sharply. 3. Implications for Aluminosilicate Gels. The luminescence results during gelation are strongly influenced by the adsorption of the pyranine molecules on the initial particles. These particles
Fluorescence Study of Aluminosilicate Sols rapidly form after the hydrolysis step and the changes in the luminescence spectra are related to changes in the local structure of the polymers and their chemistry, rather than the long-range structure responsible for gelation. This analysis is consistent with the fact that no correlation has been found between the rate of increase of F, dF/dt, and the kinetics of cluster growth and gelation time (Figure 5 ) . The decrease in proton-acceptor character is observed spectrally and has been interpreted as an evolution of the solvating layers of the particles toward a less hydrophilic state with less solvating water. The luminescence results are in good agreement with the results and models developed concerning the gelation of the aluminosilicate The small particles (10-20 A diameter) which form from the hydrolysis and nearly immediate condensation of Al-OH groups are not completely dense. In addition, it is known that alkoxy groups tend to reorient after partial hydrolysis to form a more stable structure.'* Thus, the particles are apt to undergo chemical and structural changes as gelation continues and aging processes occur. These changes are evident from both N M R and optical spectroscopy. During the aggregation of these initial particles, progressive changes observed in the 27AlN M R spectra have been associated with condensation of the A1-OH groups and/or changes in the coordination of the AleL3These modifications are consistent with the luminescence spectra where an increase in the F ratio indicates that there is a decrease in the water present in the solvation shell of pyranine. Condensation reactions leading to the formation of less polar (and less hydrophilic) AI-0-AI species would cause an increase in F. The water content is a parameter which affects both the overall polymerization process (formation of the elementary particles and aggregation kinetics) and the emission of pyranine. It is difficult, therefore, to precisely determine the correlation between the aggregation of particles and local reaction kinetics. Nonetheless, general information concerning the kinetics of different aspects of gel synthesis can be established. One of the most significant features is that gels kept in a closed environment continue to evolve structurally, long after gelation. As probed by the luminescence of pyranine (Figure 4),the evolution of the oxide network during aging is as important as during the gelation process itself, although the kinetics are much slower. The time necessary to truly stabilize the gel is about 8 times the gelation period. At the gel point, the solution is macroscopically rigid due to interconnection of large clusters. However, at a microscopic level, there is a likelihood for structural changes which could be promoted by some flexibility of the polymer and by possible condensation reactions. Water is also able to diffuse in the gel, thus promoting further hydrolytic reactions. These structural changes are expected to continue until the structure has reached a stable state. The changes occurring in matrix rigidity during aging have been investigated in the same gels by using a rigidochromic probe.I6 The fluorescence of dried gels is also well explained by considering the local environment of pyranine. Upon drying at 70 OC, all the solvating water molecules are removed from the gel. As a consequence, the pyranine molecules are confined in a medium which has a very poor proton-acceptor ability. This causes the observed increase in F. This behavior suggests that the internal surfaces of the gel, which form the pore walls after drying, are mainly organic in nature. The AI-OH groups and their solvation shells have probably been eliminated by condensation reactions during the aging or drying periods, while most of the unreactive Si-OR groups still remain in the gel. It is important to note that the fluorescence intensity increases during the drying phase as a consequence of the shrinkage of the gel and concomitant con(16) MacKiernan, J. M.; Pouxviel, J. C.; Dunn, B.; Zink, J. J . Phys. Chem., in press. (17) Pouxviel, J. C.; Parvaneh, S.; Knobbe, E. T.; Dum, B. Solid State Ionics, in press. (18) Mehrotra, R. C. In Better Ceramics Through Chemistry III; MRS Symposium, Reno, April 1988; Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.
The Journal of Physical Chemistry, Vol. 93, No. 5, 1989 2139 centration of the dye. In contrast to aluminosilicate gels, the luminescence of pyranine and other dyes in fully hydrolyzed silica sonogel is almost completely lost after drying and this indicates that the matrix has a strong influence on the luminescence properties of the organic dyes.17 Once the samples are dried, they can easily reabsorb water vapor from the air or infiltration can be used to fill the pore volume. Gas or liquid diffusion into the gel is very rapid, due to interconnecting pores which represent 50% of the gel volume. The emission characteristics of the trapped organic dye will then be determined by the ambient medium in which the gel is exposed.
Conclusion Aluminosilicate sols and gels have been prepared by hydrolysis and condensation of a double silicon aluminum alkoxide and doped with a fluorescent probe, pyranine. From emission and excitation spectroscopy during the sol-gel process, we have established that pyranine luminescence is sensitive to the internal solvent chemistry and to ambient acceptor/donor proton characteristics. The ratio F of the blue to green intensities of pyranine luminescence peaks does not appear to be sensitive to sol viscosity or to the pH of the surrounding medium. Changes in the emission spectra of pyranine have been used to probe the local sol-gel chemistry. The dye molecules are captured or embedded in the matrix long before gelation is complete by interaction with the primary particles formed soon after hydrolysis. The ionic character of the molecules keeps them in intimate contact with the surface of the polymeric oxide network. During polymerization and aging, the proton-donor ability of the chemical species on the surface of the gel particles decreases in a pronounced manner. Internal condensation reactions which occur between AI-OH groups are likely to lead to the observed spectral changes. These condensation reactions are not directly related to the aggregation process that composes the gel, but rather affect the local structure of the particles constituting the clusters. The reactions continue during an aging period which is equal to about 8 times the gelation time. Based on the emission spectra, the extent of the fluorescence change is as large in the gel state as during the aggregation state, although the kinetics of the structural evolution during aging are slower. During the drying of the gels, the fluorescence intensity of the dye is maintained at a high level. When the gels are fully dried, the spectroscopic results suggest that the dye molecules are trapped in the gel. The pores of the gel are believed to have solventlike characteristics provided by the relatively unreactive Si-OR groups. In this pore system, however, the pyranine molecules are still sensitive to the outside medium by means of the open pore network. Although changes in fluorescence characteristics can be imposed by gas or diffused solvents, the dyes can not be leached from the gels. It should be emphasized that the conclusions given here are specific to the investigated system, pyranine in aluminosilicate gels. The results contrast with those of Kaufman et al. who considered pyranine-doped silica s o h 4 Enormous differences between silica and aluminosilicate sols and gels are expected because of their different polarity, polymerization mechanisms, and oxide network structures. The study of this new class of materials, inorganic glasses doped with organic molecules, is at its inception and it is not clear at this time how optical properties are affected by the nature of the gel matrix. In fact, initial s t ~ d i e s ~ , indicate ' ~ * ' ~ that differences between matrices can be very large at the microscopic level depending upon the type of the dye employed as the dopant. Nevertheless, the use of luminescent properties of organic dyes appears to be a very powerful approach for supplying new insights regarding the chemistry and structure of sol-gel materials. Acknowledgment. This work was made possible by a grant from the National Science Foundation (NSF DMR 87-06010). Registry No. (BuO),Al-0-Si(OEt),, trisulfonated pyrene, 27928-00-3.
93672-20-9; 8-hydroxy-1,3,6-
