Fluorescent Probe as Reporter on the Local Structure and Dynamics

Jun 26, 2007 - To get some information on the aggregation behaviors of the products derived from different organotrialkoxysilanes, the ...
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J. Phys. Chem. B 2007, 111, 7963-7971

7963

Fluorescent Probe as Reporter on the Local Structure and Dynamics in Hydrolysis-Condensation Process of Organotrialkoxysilanes Shu K. Shen and Dao D. Hu* Key Laboratory for Macromolecular Science of Shaanxi ProVince, School of Chemistry and Materials Science, Shaanxi Normal UniVersity, Xi’an 710062, People’s Republic of China ReceiVed: January 21, 2007; In Final Form: March 31, 2007

To get some information on the aggregation behaviors of the products derived from different organotrialkoxysilanes, the hydrolysis-condensation processes of some organotrialkoxysilanes have been examined by means of pyrene as fluorescent probe. The organotrialkoxysilanes used in the research were n-octadecyltrimethoxysilane (ODTMS), n-octyltrimethoxysilane (OTMS), 3-glycidoxypropyltrimethoxysilane (GTMS), 3-methacryloxypropyltrimethoxysilane (MAPTMS), and propyltrimethoxy-silane (PTMS). The results show that pyrene as fluorescence probe can respond sensitively not only to the organization state of the hydrolysates but also to the change in the organization state during the condensation process. The organization states during the hydrolysis and condensation can be explained in terms of structures of the products. In the initial stage, the silanols with long organic chains are amphiphilic molecules, and such nature of the silanols can be compared to that of a surfactant. Therefore, the excimer emission of pyrene is extremely obvious because of such silanols being prone to form aggregates. In the case of silanols having short alkyl groups or epoxy groups, these silanols homogenously disperse in solution, which results in the appearance of an only monomer emission of pyrene. In the late stage, the fluorescence behavior of pyrene is also sensitive to structural evolution of the silicates. The fluorescence spectra of pyrene during the condensation of the silanols with short alkyl groups or epoxy groups are almost in silence, indicating that the condensation products, with a low condensation degree, homogeneously disperse in solution. For the silanols with long hydrophobic substituents in different lengths, the changes in fluorescence spectra of pyrene during the condensation are varied. Commonly, the excimer emission is noticeable, implying that the condensation products with high condensation degree inhomogenously disperse in solution. However, the relative excimer/monomer fluorescence intensity is alkyl chain-length dependent. The longer alkyl chains in the condensation products result in the appearance of the obvious excimer emission. These phenomena imply that the condensation degree of the products increases with the length of the alkyl chains. Additionally, the distorted spectrum of pyrene appears in the case of the organotrialkoxysilanes with side chain substituent, illustrating that the steric hindrance between the substituents can be monitored by fluorescence of pyrene. All these results are verified by the fluorescence-quenching measurements. The approach in the present study gives new insights into the local structure and dynamics in hydrolysis-condensation process of organotrialkoxysilanes and emphasizes the influence of the self-assembling behavior.

Introduction Organotrialkoxysilanes as precursors have commanded interest in preparation of hybrid organic-inorganic materials through sol-gel process. Since hybrid materials offer a large range of applications, including multifunctional coatings and films, electrical and optical materials, biomedical and chemical sensors, catalyst and porous supports, and so forth, different routes to prepare hybrid materials as powders, fibers, thin films, or bulk materials of virtually any shape and size were developed.1-3 Monitoring the route from the precursor to the final xerogel (dry gel) is required since the performances of the final material are intimately linked to its structure. Therefore, much effort has been executed to investigate the sol-gel process of siloxane.4-11 A wide variety of analytical techniques, such as X-ray and neutron diffraction and Fourier transform infrared (FTIR), Raman, or NMR spectroscopies, have been commonly used in * To whom correspondence should be addressed. Tel: +86 29 85307534. Fax: +86 29 85307534-8221. E-mail: [email protected].

this field. Fluorescence technique is more attractive not only to follow the process of heterogeneous systems but also to offer important information especially in microenvironments at the molecular scale. Employing the fluorescence technique to monitor the structural changes at the molecular scale during the sol-gel process was reported for the first time by Avnir and Levy.12 Pyrene as a fluorescence probe was added to the system to monitor the sol-gel process of Si(OMe)4. Some information, such as the sol-gel chemistry or structural development during polymerization, gelation, and drying of gels, was obtained. After that, Rhodamine 6G,13 pyrene,14 pyranine,15 and europium cation16 have been widely used as probes to monitor the sol-gel process. Because probe mobility, solvation, and reorientation processes have a dominant effect in their electronic excited states, the fluorescence behaviors for a given probe is variable in various solid matrixes or sol-gel processes.17 Most of the previous papers focused on the sol-gel process of precursor with lower molecular weight, such as tetramethyl orthosilicate (TMOS) and tetraethyl orthosilicate

10.1021/jp0705121 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/26/2007

7964 J. Phys. Chem. B, Vol. 111, No. 28, 2007 CHART 1: Chemical Structures of Organotrimethoxysilanes

Shen and Hu its high sensitivity to the local environment, and its excimer emission reflects the variation of its local concentration, and the ratio in intensity of peak 1 to peak 3 (I1/I3) in its monomer emission spectrum is sensitive to environmental polarity.18-21 In our research, both parameters were considered. The I1/I3 ratio was taken as a measure of the polarity of the environment, while the monomer/excimer emission ratio was taken as an indication of the confinement of the probe in the hydrophobic aggregates formed during the reaction. The obtained results provided detailed information on the sol conformation as hydrolysis and condensation reactions developed. They gave new insights into the sol-gel process of organotrialkoxysilanes and emphasized the influence of a long hydrophobic chain. Experimental Section

(TEOS). Additionally, the research on the sol-gel process generally concentrated on a variation of the system with time in large scale so that some detailed information, such as the self-assembling behaviors of hydrolysates, is seldom obtained. Such information is especially important for the siloxane-derived sol-gel process. Being different from tetramethoxysilane (TMOS) or tetraethoxy-silane (TEOS), organotrialkoxysilanes have nonhydrolyzed organic substituents. The effects of these substituents on the sol-gel chemistry of organotrialkoxysilanes, therefore, could not be ignored. In fact, the substituent has significant influences not only on the reaction rate of hydrolysis-condensation but also on the microstructure and performance of final materials. The effects of electronic and steric factors of various substituents on rates of both hydrolysis and condensation of ethoxysilanes have been investigated.5 Loy et al.6 proved that propensity of trialkoxysilanes to form gels, insoluble precipitates, soluble polymers, or crystalline polyhedral oligosilsesquioxanes depended on the organic groups. The effects of the organic substituents on cyclization and self-organization in polymerization of trialkoxysilanes have been also systematically studied.7 Some organotrialkoxysilane behaviors at air-water interface have been considerably noticed.8-10 However, the sol-gel process of organotrialkoxysilanes is seldom monitored in minute scale. As our previous research,11 the sol-gel process of methacryloxypropyltrimethoxysilane (MAPTMS) has been followed in minute scale by fluorescence spectroscopy with 2-naphthol as a probe. Spectroscopic studies revealed fluctuations of the maximum emission intensity and wavelength as a function of time. The results reflect some detailed information in aggregation behavior of the hydrolysates. Especially, fluorescence spectroscopy emphasized the reversibility of monomeric silanol aggregates and changes in hydroxy group number of the silica network during the sol maturation. These behaviors were mainly attributed to the amphiphilic behavior of the hydrolysates. To systematically study the evolution of local structure of organotrialkoxysilanes, we have examined the acid hydrolysis of a variety of organotrimethoxysilanes with different organic substituents including n-octadecyltrimethoxysilane (ODTMS), n-octyltrimethoxysilane (OTMS), (3-glycidoxypropyl) trimethoxysilane (GTMS), 3-methacryloxypropyltrimethoxysilane (MAPTMS), and propyltrimethoxysilane (PTMS). Chemical structures of the organotrialkoxysilanes are shown in Chart 1. In this research, pyrene as photophysical probe is used to investigate the structural development. The major advantages of pyrene as probe are its relatively long lifetime in the excited singlet state that leads to a well-resolved vibronic spectrum and

Materials. The following organotrialkoxysilanes RSi(OMe)3 were used as obtained from Aldrich: n-octadecyltrimethoxysilane (ODTMS), n-octyltrimethoxysilane (OTMS), 3-glycidoxypropyltrimethoxysilane (GTMS), 3-methacryloxypropyltrimethoxysilane (MAPTMS), and propyltrimethoxysilane (PTMS). Pyrene (Aldrich) was reagent-grade. Nitromethane, copper acetate, copper nitrate, and hydrochloric acid were of analytical grade. Water was deionized. Sample Preparation. Solutions for spectroscopic measurements were prepared by the following method. First, a stock solution of pyrene was prepared in ethanol (1 × 10-3 M). A 100 µL of the above stock solution was introduced in a volumetric flask and was spread over the flask, and the solvent was evaporated under room temperature. Then, 10 mL of acidified water (0.1 M HCl) was added to the flask, and the solution was sonicated for about 30 min. After 12 h aging, a fixed quantity of precursor was added to the flask, and the mixture solution was shaken until the solution became clear. The resulting clear solution was then directly added into the spectrophotometer cuvette. The concentration of pyrene in solutions was less than 10-5 M. The concentrations of all precursors in solutions were 10-6∼10-5 M (more details on concentrations of probe and precursors used in our research are found in Supporting Information I). Steady-State Fluorescence Measurement. Fluorimetric measurements were done on a Perkin-Elmer LS50-B fluorescence spectrometer. Emission spectra were collected with an excitation wavelength of 340 nm at 25 °C. In-situ fluorescence measurements were performed at 5 min intervals. The fluorescent cell was closed for all measurements. Fluorescence Quenching. In view of the fact that the solutions studied are violently reactive at early stage, fluorescencequenching measurement is not suitable for this stage. So, the solutions aged for 10 h were used as the samples for fluorescencequenching measurement. The aqueous solution of quencher (nitromethane, copper acetate, copper nitrate) was dropped to the aged solutions. The fluorescence intensity of pyrene was measured with excitation wavelength of 340 nm. A ratio of I to I0 as a parameter indicates the efficiency of fluorescence quenching, where I0 and I are the fluorescence intensity in the absence and the presence of quencher, respectively. Results and Discussion Fluorescence Spectroscopy of Pyrene in Different Organotrimethoxysilane-Derived Systems. Steady-State Fluorescence Measurement at Early Stage. Figure 1 depicts the normalized steady-state fluorescence emission spectra of pyrene in different organotrimethoxysilane-derived systems during the sol-gel process at the early stage. The spectra reveal that the emission spectra are mainly composed of two parts. The first

Fluorescent Probe in Organotrialkoxysilanes

Figure 1. Flourescence spectra of pyrene in different organotrialkoxysilane-derived systems at the early stage: a, HCl; b, OTMS; c, ODTMS; d, PTMS; e, MAPTMS; f, GPTMS.

part is characterized by a few sharp bands (360-440 nm), which has been assigned to the monomer emission of pyrene.22-23 The second part refers to the wavelength from 440 nm to the far end of emission spectrum with simple and structureless band, which is attributed to the perfect excimer emission.24-25 To make a comparison between these spectra, there are obvious distinctions in spectra profiles (see Figure 1). On the basis of the feature of spectral profiles, the trimethoxysilane-derived systems are generally divided into three types. One is the spectra with an obvious excimer emission centering around 470 nm for OTMS and ODTMS (see Figure 1b and 1c), the second one is the spectra without the obvious excimer emission for PTMS and GPTMS (see Figure 1d and f), and the last one refers to the spectrum which has the indistinct shoulder peak around 470 nm and the small shoulder peak around 420 nm for MAPTMS (see Figure 1e). It is known that the excimer emission is due to the interaction of an excited pyrene species with a pyrene molecule in ground state.25,26 This interaction depends on both orientation and distance of pyrene molecules. The emission centering around 470 nm is related to the presence of a perfect “sandwich-like” structure organized between pyrene molecules.19,27-28 The required interplanar distance for a perfect sandwich-like structure is about 0.35 nm.29 So, the spectra for OTMS and ODTMS systems imply that the local concentration of pyrene in the systems is higher, and pyrene molecules are located in hydrophobic domains.30 These results reveal that the hydrolysates in OTMS and ODTMS systems inhomogeneously disperse in the solutions. Generally, an inhomogeneous dispersion reflects the presence of micelle in solution. With the proceeding of hydrolysis reaction of OTMS or ODTMS, the silanols are produced. These silanols can be compared to amphiphilic molecule because of the silanols with the polar heads and the long apolar hydrocarbon tails. As a result, these silanols may form O/W micellelike aggregates in a manner analogous to typical surfactant in bulk water, and pyrene as a hydrophobic molecule is incorporated into the hydrophobic core of the aggregates. On the basis of the model of the explanation mentioned above, the observation of the obvious excimer emission can be understood because of extraction of pyrene molecules into the hydrophobic microdomain. Additionally, it is not difficult to imagine that the hydrophobic core formed by the longer alkyl groups of the silanols makes the motion and

J. Phys. Chem. B, Vol. 111, No. 28, 2007 7965 reorientation of pyrene relatively easy and thereby makes the opportunity for formation of perfect excimer. The more obvious perfect excimer emission for ODTMS system than that for OTMS system may be rationalized by the same reason. The fluorescence spectra of pyrene for PTMS and GPTMS systems are similar to that for free precursor solution (cf. Figure 1a, d, and f), which are characterized by the higher I1/I3 (I1 (λ ) 373 nm) and I3 (λ ) 384 nm)) value, and the perfect excimer emission is nearly negligible. According to the explanation for OTMS and ODTMS systems, the spectra regarding PTMS and GPTMS systems imply that pyrene molecules locate in homogeneous polar environment, and local concentration of pyrene is lower. For the PTMS system, the propyl in silanol is not long enough to drive the corresponding silanols to form aggregates. For GPTMS system, the epoxy group in GPTMS could be protonated in the acidic condition,31-32 which leads to the silanols produced by GPTMS that do not easily form aggregates because of the presence of two polar ends in the silanols. Consequently, the probe molecules are homogeneously dissolved in PTMS or GPTMS system, the excimer emission of pyrene is absent, and the ratio I1/I3 is higher. For MAPTMS systems, there is a small shoulder peak around 470 nm, and the I1/I3 is lower. These results may be understood by considering the structure feature of MAPTMS. The silanols produced by MAPTMS are amphiphilic molecules owing to the presence of hydrophilic head (Si-OH) and hydrophobic tail (methacryloxypropyl group).11,33 Accordingly, the aggregates formed from the silanols lead to the appearance of excimer emission around 470 nm and the I1/I3 with lower volume. Differing from the OTMS and ODTMS systems, there is the weak excimer emission rather than the obvious one at 470 nm for MAPTMS system. Focusing our attention on the spectrum of pyrene for MAPTMS system, it is found that the spectrum is notably distorted and that the contribution of the shoulder peak around 420 nm to the total emission is obvious. The distorted excimer emission of pyrene has been extensively reported by several groups.34-38 The distorted excimer emission around 420 nm is related to the partially overlapped pyrene excimer. On the basis of this consideration, the anomalous fluorescence for MAPTMS system is attributed to a twisted conformation of two rings of pyrene.39 By contrast, for OTMS and ODTMS systems, the two pyrene rings assume a completely parallel conformation. This twisted conformation of the two pyrene rings decreases the overlap of the π surfaces, resulting in the shift of the excimer fluorescence band to the shorter wavelength region. The two featureless emissions are believed to originate from the loosely and tightly coupled molecular pairs of pyrene prior to excitation.30 It has been reported that trans1,8-bis(1-pyrenyl)naphthalene, in which the two pyrene residues assume an imperfectly stacked conformation, emits fluorescence at 425 nm.40 The similar phenomenon was recently observed by Suzuki et al.41 So, for MAPTMS system, the excimer emission at about 420 nm implies that pyrenes in the system are not only concentrated but also are constrained. On the basis of these statements, the fluorescence behaviors for MAPTMS system might be a result of the fact that pyrenes located in a microdomain composed of the bulky methacryloxypropyl groups make pyrene difficult to reorient.42 Such a conclusion was also confirmed by other experiments that will be discussed in detail. Figure 2 shows the optical micrographs of different organotrialkoxysilanes systems at early stage. It can be seen in Figure 2 that PTMS and GPTMS systems are homogeneous and that OTMS, ODTMS, and MAPTMS systems, however, are het-

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Figure 2. Optical micrographs of different organotrialkoxysilane systems at early stage: a, PTMS and GPTMS; b, OTMS; c, MAPTMS; d, ODTMS.

Figure 3. Changes in the I1/I3 ratio and IE/I1 as a function of reaction time for PTMS, GPTMS, OTMS, and ODTMS systems.

erogeneous. These results are in accord with that obtained from the fluorescence research. In-Situ Steady-State Fluorescence Measurement. The results from the steady-state fluorescence measurement at early stage indicate that the organic substituent in precursor plays a major role in the self-organization of hydrolysates. Naturally, the selforganization of hydrolysates certainly affects evolution of condensation products. It is well-known that the sol-gel process is very complicated because of the simultaneous presence of varied species and time-dependent change in species.1-3,43-44 Although the systems we studied are complicated, on the basis of the fluorescence technique being especially suitable to explore the aggregation in a molecular scale, it is possible to get some information on the aggregation derived from the evolution of the condensation products. The in-situ steady-state fluorescence measurement used here is expectative to probe the difference on evolutions of condensation products from the different organotrialkoxysilanes systems. Figure 3 shows the changes in the I1/I3 ratio and IE/I1 as a function of reaction time for PTMS, GPTMS, OTMS, and ODTMS systems. PTMS and GPTMS Systems. The changes in the I1/I3 ratio as a function of reaction time for PTMS and GPTMS systems are shown in Figure 3, and the corresponding fluorescence spectra are shown in Figure 4. For PTMS system, the value of I1/I3 continuously stayed at a higher level during measurement, and the changes in ratio of I1/I3 and the spectral profile were slight. As the previous explanation for PTMS system, the results imply that the polarity of species-involved condensation products did not significantly change along the polymerization. Similar results for GPTMS system were also obtained (refer to Supporting Information II). These conclusions agree with that from others. Mateˇjka et al.7 reported that the structure evolution in the sol-gel polymerization of organotrialkoxysilanes RSi(OR′)3 is dependent on the type of the substituent R. Trialkoxysilanes with small substituents undergo faster condensation and intermolecular branching because of lower steric hindrance. As a result, the growth of a high molecular weight polymer with

Figure 4. Fluorescence spectra of pyrene in PTMS system (from 0 to 75 min). (Inset) Fluorescence spectra of pyrene in PTMS system (from 75 to 600 min).

dangling organic groups is preferred. On the basis of their experiments, they inferred that no micelles were created. Their inferences on absence of micelles in solution are verified by our experiments. Evidence for the evolution features of condensation products in PTMS system was also seen in mass spectrometry data of the polymerized n-propyl silsesquioxane.45 The result suggested that the local environment was the same with regard to degree of intramolecular condensation at any place in the molecule and was independent of the size of the molecule. For GTMS system, Mateˇjka et al. have found that the intermolecular condensation is preferred, and a loose structure of linear and high molecular weight branched polymer is formed under acid catalysis. The main products in unstrained structure and the minor products in strained Si3O3 rings were also found in GTMS system,46 however, basic and neutral catalysis results in pronounced cyclization and self-organization.7 They explained these observations as follows. Under acid catalysis, an increasing H-bond interaction with oxygen atom in the organic chain leads to a better compatibility, and microphase separation is restricted. On the basis of the explanation from Mateˇjka et al., the structure of condensation products under basic and neutral catalysis is difficult to understand. From the in-situ steady-state fluorescence measurement of GPTMS system, the difference in the structures of condensation products formed under neutral and acidic catalysis can be understood. In our opinion, the better compatibility of the condensation products is related to the protonated expoxyl group of GPTMS in acid condition. Owing to the protonated expoxyl group of GPTMS, the silanols produced from GPTMS do not easily form aggregates because of the presence of two polar ends in the silanols. As a result, the loose structure of linear and high molecular weight branched polymer is preferable to form. It is not difficult to image that the polarity of the expoxyl group under basic or neutral conditions results

Fluorescent Probe in Organotrialkoxysilanes

Figure 5. Fluorescence spectra of pyrene in OTMS system (selected from 0 to ∼35 min).

in the formation of amphiphilic silanol. The silanols, produced under basic or neutral catalysis, inhomogeneously disperse in solution, which results in pronounced cyclization and selforganization. OTMS System. For OTMS system, there is a continuous and pronounced decrease in the I1/I3 before 200 min, and then it trends toward stability (Figure 3). The ratio IE(λ)460 nm)/I1(λ)373 nm) as a function of time is also shown in Figure 3. The IE/I1 presents an initial decrease before 35 min followed immediately by an increase during 35-200 min and then changes slightly after 200 min. The I1/I3 is taken as a measure of the polarity of the microenvironment sensed by the probe, and the monomer/ excimer emission ratio is taken as an indication of the confinement of the probe in the hydrophobic aggregates.30 Therefore, these observations are attributed to local changes in polarity and pyrene concentration forced by the species in system. As the previous exposition for OTMS system, a rapid hydrolysis reaction is dominant in the early stage, resulting in the formation of silanols as surfactant-like molecules during this period. With the hydrolysis proceeding, more and more silanols participate in the formation of the oil-in-water emulsions. As a result, surfactant-like silanols greatly ameliorate the interface energy of the emulsion, leading to a transition from a macroemulsion to a microemulsion, in a manner analogous to an increase of surfactant driving emulsion system into microemulsion. Consequently, pyrene molecules are redistributed from the macroemulsions to the microemulsions. Such effect could be characterized by fluorescence behavior in two aspects: (1) this effect causes a decrease in local concentration of pyrene characterized by a sharp decrease in IE/I1 and (2) the polarity of the domain in which the pyrene molecules are located changes slightly, resulting in a slight change in I1/I3. The corresponding fluorescence spectra during this period shown in Figure 5 directly reflect this transition. The fluorescence spectra selected from 0 to 35 min obviously indicate the presence of an isoemissive point. More detailed spectra are shown in Supporting Information II. The isoemissive point demonstrates the transition between monomer emission and excimer emission.11,43,47-48 A reasonable explanation for the isoemissive point observation is that a transformation of fewer macroemulsions to more microemulsions occurs in the system, resulting in a migration of pyrene from the macroemulsions to the microemulsions. It is known that the relative intensity of the monomer to excimer emulsion in pyrene fluorescence spectrum is concentrationdependent. The increase in the monomer emission and the

J. Phys. Chem. B, Vol. 111, No. 28, 2007 7967 decrease in excimer emission imply that the local concentration of pyrene decreases because the concentration of pyrene in the macroemulsions is higher than that in microemulsions as a given amount of pyrene transfers from fewer macroemulsions into more microemulsions. On the basis of this explanation, the slight decrease in I1/I3 and the obvious increase in IE/I1 during 0∼35 min could be understood. In the next stage, the formation of Si-O-Si silicate oligomers is dominant because the monomer is quickly depleted under acidic catalysis. So, the micelles formed by the silanols are gradually destroyed, Instead, the oligomers with small polymerization degree gradually transform to higher molecular weight products. For organotrialkoxysilanes RSi(OR′)3, increasing the length of R results in slowing down the polymerization. The narrow distribution of polyhedrons, with emanating organic chains as octopus-like molecules, is formed, especially, in dilute systems. Because of incompatibility of the polymer framework and pendant organic chains, microphase separation takes place and self-organization into micelles occurs without any surfactant present.7 According to these findings, another type of aggregate composed of polyhedrons might form because of the hydrophobic interaction of the emanating organic chains in the polyhedrons. This change makes pyrene molecules transfer from the dispersed microemulsions to the compact hydrophobic domains of the aggregates. As a result, pyrene molecules are concentrated, resulting in an increase in the IE/I1 and a decrease in I1/I3 during 35-200 min (Figure 3). Regarding a fluctuation in IE/I1 during 200-800 min, it can be explainable in terms of phase separation. In the system, a fast formation of small polyhedrons and their interconnection to form higher molecular weight polyhedral structure is most probable. As the size increases of a polyhedral cluster, the solubility of products decreases, which results in separation of the products trapped in pyrene from the bulk phase. In this situation, it is possible to cause the decrease in IE/I1. After that, the residual small polyhedrons in solution might form polyhedral clusters, resulting in the increase in IE/I1 again. Although the change in condensation products drives the fluctuation in IE/I1, the I1/I3 is comparatively stable owing to less change in local polarity. ODTMS System. For ODTMS system, the changes in the I1/I3 and IE/I1 with time are shown in Figure 3. Before 100 min, the I1/I3 obviously increases from 1.0 to 1.3, while the IE/I1 significantly decreases. After 100 min, the increase in I1/I3 and the decrease in IE/I1 retard. Similarly to OTMS system, the obvious excimer emission appears for ODTMS system (cf. Figure 1). However, fluorescence behaviors for the ODTMS system, that is, the relative intensity of excimer emission, the variant trends in I1/I3 and IE/I1, are different from that for OTMS system. These differences in fluorescence behaviors could be interpreted in terms of the effect of the substituent’s length. As previously mentioned, the silanols derived from OTMS and ODTMS could form aggregates because of the silanols being amphiphilic molecules, and the tendency to form the aggregates depends on the length of substituent. Compared with the silanols formed by OTMS, the silanols formed by ODTMS more easily form aggregates because the silanols formed by ODTMS have a longer alkyl group. Evidence on this conclusion can be seen in Figure 2. There are more well-distributed microemulsions with smaller size in ODTMS system than in OTMS system. Additionally, the longer substituent results in a fast formation of small polyhedrons.7 As a result, for ODTMS system, more pyrene molecules concentrated in the microemulsions lead to the more significant excimer emission of pyrene, and the observation about the isoemissive point as found in OTMS

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Figure 7. Photograph for indicating hydrophobicity of the product separated from ODTMS system.

Figure 6. Fluorescence spectra of pyrene in ODTMS system (selected from 0 to 80 min after measurement).

system did not appear. In view of the effect of the substituent’s length on the sol-gel chemistry of organotrialkoxysilanes reported in the literature, in the case of ODTMS with acidic hydrolysis, a rapid formation of a white gelatinous precipitate composed of small polyhedrons was found because of a strongly hydrophobic interaction.8,49-50 This observation indicates that the condensation products are prone to precipitate from bulk phase. Accordingly, pyrene molecules are also gradually separated from the solution through the hydrophobic incorporation into the precipitate. As a result, the decrease in content of pyrene in solution leads to a gradual decrease not only in total fluorescence intensity but also in the relative intensity of excimer emission (Figure 6 and Figure 3). Moreover, the aggregates of the polyhedrons that pyrene is located in are so small that the partial pyrene molecules are easily exposed to the outer aggregates, resulting in the increase in I1/I3. Our fluorescence observations for ODTMS system also coincide with the structural evolution of the condensation products detected by time-resolved small-angle X-ray scattering (SAXS) and 29Si NMR experiments. Mateˇjka et al. explained the structural evolution of the condensation products as follows. Increasing the length of R in RSi(OR′)3 results in slowing down the polymerization, sterically hindering the intermolecular reaction, and advancing the cyclization. Judging by our fluorescence observations, the effect of the aggregation behavior on the structural evolution of the condensation products should be noticed. Namely, the self-association driven by the hydrophobic interactions directly affects the arrangements of molecules, which is a dominant factor to influence the structure of the product. According to this explanation, the difference between OTMS and ODTMS systems on structural evolutions of the condensation products could be understood. Compared with silanols from OTMS, the silanols from ODTMS easily form the tight aggregates. Accordingly, the silanols closely confined prefer the intermolecularly full condensation within the aggregates rather than without the aggregates. Therefore, the silsesquioxane polyhedrons with more narrow distribution are the main reaction products. A fluorescence-quenching experiment described in the next section verifies this conclusion. In view of the structural feature of the condensation product, the precipitation from bulk solution is strongly hydrophobic because of the presence of the emanating hydrophobic chains in the condensation product. Small-contact angle of the product to water clearly shows the fact that the product is hydrophobic (cf. Figure 7). The results from the fluorescence measurements of OTMS and ODTMS systems indicate that the structural

evolution of the condensation product could be sensitively probed by pyrene fluorescence. MAPTMS System. MAPTMS system, probed with pyrene, was made in the same fashion as other systems. However, the distorted emission spectrum of pyrene appeared as especially remarked in steady-state fluorescence measurement, indicating that pyrene molecules in MAPTMS system are not only concentrated but also constrained. On the basis of this research, the dramatic appearance of the fluorescence behavior of pyrene in MAPTMS system might be a result of pyrenes located in microdomains composed of the bulky methacryloxypropyl groups to make pyrene difficult to reorient. Although the variations in I1/I3 ratio and IE(λ)460 nm)/I1(λ)373 nm) in this case are not adaptive to reflect the changes in local polarity and local concentration of pyrene, it was found that the fluorescence spectroscopy of pyrene could provide detailed information about functional group mobility and excimer formation because the interaction between the excited pyrene species and the pyrene in its ground state is both orientation- and distance-dependent.51 All this information could be characterized by the change in intensities of I1 and IE. In the fluorescence spectrum of pyrene, a broad structureless band (E1) has a maximum at 420 nm, of pyrene excimer of overlap conformation, and the well-known band (E2) of pyrene excimer of sandwich conformation has a maximum at 470 nm.36 The distorted fluorescence spectrum and the diminished fluorescence intensity because of the concentration inner filter effect have been found.32,48 On the basis of a detailed spectroscopic analysis mentioned above, the variation in intensities of IM (I1(λ)373nm)) or IM (I5(λ)397nm)) and IE (IE(λ)420nm)) might generally reflect the change of pyrene concentration in the microdomains during the hydrolysis and condensation of MAPTMS. As a result, some information on evolution of the species in the system could be expectedly obtained. The fluorescence spectra of pyrene in MAPTMS system at different times are shown in Figure 8, and the variations of intensity in I1(λ)373nm), I5(λ)397nm), and IE(λ)420nm) as a function of time for MAPTMS system are shown in Figure 9. From start to 680 min, the spectra present obvious fluctuations in intensity and spectral profile (Figure 8). The variations of pyrene in I1(λ)373nm), I5(λ)397nm), and IE(λ)420nm) as a function of time in MAPTMS system directly illustrate the fluctuations in intensities (Figure 9). A possible reason for these observations is attributed to a reversible change in local concentration of pyrene. The decrease in total intensity is attributed to the increase in local concentration of pyrene because of self-quenching.36,38,52 Oppositely, the increase in total intensity is ascribed to the decrease in local concentration of pyrene because of inhabitation of selfquenching. In light of these conclusions, the fluctuation in intensity can be understood. From start to 200 min, the decrease in the intensity is attributed to pyrene solubilization into the mi-

Fluorescent Probe in Organotrialkoxysilanes

Figure 8. Fluorescence spectra of pyrene in MAPTMS system at different times: a, 0 min; b, 40 min; c, 300 min; d, 400 min; e, 540 min; f, 680 min.

Figure 9. Variations of pyrene in I1(λ)373nm), I5(λ)397nm), and IE(λ)420nm) as a function of time in MAPTMS system.

croemulsion formed by silanols, partial unreacted MAPTMS, and water, which results in increase in the local concentration of pyrene. From 200 to 400 min, it might be that as consumption of the unreacted MAPTMS and condensation between the silanols occurs, the originally formed microemulsions are disrupted, which makes the local concentration of pyrene decrease. Additionally, for MAPTMS system, the condensation products have a lower degree of intramolecular condensation,45 and major condensation species with -OH groups have significant concentrations at the time when the first cluster is formed. These fundamental building blocks are incompletely condensed polyhedra or ladder structures.40 The gradual formation of such products makes not only disruption of the microemulsions but also insulation of pyrene. Therefore, the decrease and increase in fluorescence intensity of pyrene from start to 400 min could be observed. From 400 to 600 min, a plausible explanation for the decrease in intensity during this time is given as follows. The condensed species in the first generation mainly consist of incompletely condensed polyhedra and ladder-type structures. These species with -OH groups give place to a second generation of condensation products with more condensed structures, and third and fourth generations of condensation products are also present. With the condensation proceeding, the condensation products may become more hydrophobic because of decrease in -OH groups and increase

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Figure 10. Variations of pyrene in I5(λ)397nm) and IE(λ)420nm)/I1(λ)373nm) as a function of time in MAPTMS system.

in hydrophobic methacryloxyproply groups in condensation products. These products hydrophobicly aggregate to make pyrene concentrated in hydrophobic domains.46 This effect leads to the gradual decrease in the fluorescence intensity of pyrene because of concentration quenching. Although the condensation is continual, the hydrophobicity of the microdomain pyrene does not change significantly. For this reason, the intensities keep lower levels after 600 min. The changes in the intensity at the three wavelengths are synchronous, and the variation tendencies in I5(λ)397nm) and IE(λ)420nm)/I1(λ)373nm) are complementary (cf. Figure 10). This information also indicates the change in local concentration of pyrene. A decrease in IE(λ)420nm)/I1(λ)373nm) reflects a relative decrease in excimer emission, implying that the local concentration of pyrene decreases. An increase in IE(λ)420nm)/I1(λ)373nm) certainly implies an increase in the local concentration of pyrene. These conclusions are completely identical to that mentioned above. Fluorescence-Quenching Measurements. The results from quenching measurements can provide helpful information about molecular behaviors and conformations of polymer or surfactants in solution.26 The aim in this section is to employ fluorescence quenching confirming the difference in organization state of the organotrialkoxysilanes in solution. An effective quenching occurs when the quencher is easily accessible to the fluorophore probe. For the desired purpose, quenchers including nitromethane, copper acetate, and copper nitrate were used in this research. Figure 11 shows fluorescence quenching of pyrene by copper nitrate in different organotrialkoxysilane-derived systems aged 10 h. It is clearly seen that the slopes of the quenching curves are higher for PTMS, GPTMS, and ODTMS systems than that for OTMS and MAPTMS systems when copper nitrate was used as quencher. These results indicate that pyrene molecules are easily accessible to copper nitrate in PTMS and GPTMS systems, implying that PTMS and GPTMS systems are homogeneous. As for ODTMS system, partial pyrene molecule is easily exposed to the outer aggregates (as mentioned in the section of In-Situ Steady-State Fluorescence Measurement, ODTMS System), and therefore, the quencher easily accesses pyrene, and the quenching efficiency is relatively higher. The levels of I0/I for both OTMS and MAPTMS systems almost remain constant. These results imply that the condensation products at late stage adopt more compact conformation so that pyrene molecules deeply incorporate into the hydrophobic

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Shen and Hu species in the hydrolysis and condensation of organotrialkoxysilane is a very important factor to influence final structure of materials. Although the structures of condensation products are not directly determined in the present study, the obtained results from the fluorescence measurements coincide with the structural evolution of the condensation products detected by others, and the approach in the present study gives new insights into the local structure and dynamics in hydrolysis-condensation process of organotrialkoxysilanes and emphasizes the influence of the self-assembling behavior. Acknowledgment. The authors would like to thank the NSF of China (20576068), SRF for ROCS, SEM (Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry), and the NSF of Shaanxi Province (2005B12) for financial support.

Figure 11. Quenching curves of pyrene by copper nitrate in different organotrimethoysilane-derived systems aged 10 h.

domain. As a result, the quencher is inaccessible to pyrene. The results agree with that from the in-situ steady-state fluorescence measurements. The fluorescence quenching of pyrene by hydrophobic quenchers (nitromethane and copper acetate) also verifies these conclusions (Supporting Information III). Conclusions To get some detailed information about the effect of different substituents on the structural evolution of the condensation products, the hydrolysis-condensation processes of some organotrialkoxysilanes have been examined by means of pyrene as fluorescent probe at minute time scale. For organotrialkoxysilanes with short alkyl group or epoxy group, the only monomer emission of pyrene in these precursor systems appears during the hydrolysis and condensation, indicating that the silanols and the condensation products homogenously disperse in solution. For organotrialkoxysilanes with long alkyl substituents, the changes in fluorescence spectra of pyrene during the hydrolysis and condensation are varied. Generally, the excimer emission is noticeable, implying that the silanols and condensation products inhomogenously disperse in solution. However, the relative excimer/monomer fluorescence intensity is alkyl chainlength dependent. The longer alkyl chains in the silanols and condensation products result in the appearance of the more obvious excimer emission of pyrene. These phenomena imply that the silanols and condensation products are easier to form aggregates with increase in the length of alkyl chain. Additionally, the distorted spectrum of pyrene appears when the organotrialkoxysilane with bulky substituent is monitored by fluorescence of pyrene, illustrating that the steric hindrance between the substituents can be monitored by fluorescence of pyrene. This paper emphasizes changes at a minute scale of the environment of a polarity-sensitive fluorescent molecule during a sol-gel process. The results show that pyrene as fluorescence probe could sensitively respond not only to the organization state of the hydrolytes but also to the change in the structural evolution of condensation products. Additionally, the organization state of silanols formed from the different precursors is related to the nature of substituent. Judging by our fluorescence observations, the effect of the aggregation behavior on the structural evolution of the condensation products should be noticed. Namely, the self-assembling behavior of intermediate

Supporting Information Available: The concentrations of probe and precursors used in our research depended on a series of experiments. For details, see Supporting Information I. More detailed fluorescence spectra for different organotrialkoxysilanes-derived systems are shown in Supporting Information II. Quenching curves of pyrene by copper acetate and nitromethane in different organotrimethoysilane-derived systems are shown in Supporting Information III. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wen, J. Y.; Wilkes, G. L. Chem. Mater. 1996, 8, 1667. (2) Yaho, S.; Iwata, K.; Kurita, K. Mater. Sci. Eng. C 1998, 6, 75. (3) Baney, R. H.; Iton, M.; Sakakibara, A.; Suzuki, T. Chem. ReV. 1995, 95, 1409. (4) Schottner, G. Chem. Mater. 2001, 13, 3422. (5) Hook, R. J. J. Non-Cryst. Solids, 1996, 195, 1. (6) Loy, D. A.; Baugher, B. M.; Baugher, C. R.; Schneider, D. A.; Rahimian, K. Chem. ReV. 1995, 95, 1431. (7) Mateˇjka, L.; Dukh, O.; Hlavata, D.; Meissner, B.; Brus, J. Macromolecules 2001, 34, 6904. (8) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Aurentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135. (9) Kojio, K.; Takahara, A.; Kajiyama, T. Langmuir 2000, 16, 9314. (10) Carino, S. R.; Duran, R. S.; Baney, R. H.; Gower, L. A.; He, L.; Sheth, P. K. J. Am. Chem. Soc. 2001, 123, 2103. (11) Hu, D. D.; Barghorn, C. C.; Feuillade, M.; Carre, C. J. Phys. Chem. B 2005, 109, 15214. (12) Avnir, D.; Levy, D. R. J. Phys. Chem. 1984, 88, 5956. (13) Innocenzi, P.; Kozuka, H.; Yoko, T. J. Non-Cryst. Solids 1996, 201, 26-36. (14) Chambers, R. C.; Haruvy, Y.; Fox, M. A. Chem. Mater. 1994, 6, 1351. (15) Levy, D. R.; Avnir, D. Chem. Phys. Lett. 1984, 109, 593. (16) Matsui, K.; Tomonaga, M.; Arai, Y.; Saton, H.; Kyoto, M. J. NonCryst. Solids 1994, 169, 295. (17) Pouxiel, J. C.; Dunn, B.; Zink, J. I. J. Phys. Chem. 1989, 93, 2134. (18) Tucker, T. K.; Brennan, J. D. Chem. Mater. 2001, 13, 3331. (19) Winnik, F. M. Chem. ReV. 1993, 93, 587. (20) Deng, Q.; Hu, Y.; Moore, R. B.; McCormick. C. L.; Mauritz. Chem. Mater. 1997, 9, 36. (21) Kaufman, V. R.; Avnir, D. Langmuir 1986, 2, 717. (22) Chu. D. Y.; Thomas, J. K. Macromolecules 1984, 17, 2142. (23) Olea, A. F.; Thomas, J. K. Macromolecules 1989, 22, 1165. (24) Ezzell, S. A.; Hole, C. H.; Creed, D.; McCormick, C. L. Macromolecules 1992, 25, 1887. (25) Char. K.; Gast, A. P.; Frank, C. W. Langmuir 1988, 4, 989. (26) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/Plemum Publishers: New York, 1999. (27) Van der Veen, N. J.; Flink, S.; Deij, M. A.; Egberink, R. J. M.;van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 2000, 122, 6112. (28) Matsui, J.; Mitsuishi, M.; Miyashita, T. J. Phys. Chem. B 2002, 106, 2468. (29) Kayanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (30) Jones, G.; Vullev, V. J. Phys. Chem. A 2001, 105, 6402-6406. (31) Oyama, T.; Miyake, H.; Kusunoki, M.; Nitta, Y. J. Biochem. 2003, 133, 467-474.

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