Inorganic Hybrid Gels

Only APTS gels obtained by carboxylic acid solvolysis are luminescent. In the case of ... In both cases gels emit bright photoluminescence7,8 in a bro...
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Chem. Mater. 2003, 15, 1855-1859

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Photoluminescence from Sol-Gel Organic/Inorganic Hybrid Gels Obtained through Carboxylic Acid Solvolysis Tatyana Brankova, Vlasoula Bekiari, and Panagiotis Lianos* Engineering Science Department, University of Patras, 26500 Patras, Greece Received May 21, 2002. Revised Manuscript Received January 31, 2003

Hybrid organic/inorganic sol-gel materials have been synthesized by carboxylic acid solvolysis of (aminopropyl)triethoxysilane or ureasil precursors. The hybrid nature of these gels is expanded with the introduction of silica esters. A main feature of the ensuing materials is the formation of hybrid organic/inorganic nanoclusters, which are founded on silica backbone but they are facilitated and assisted also by forces arising from hydrophilic/ hydrophobic balance and hydrogen-bonding interactions. The silicious domains of these clusters with attached functional groups, for example, amino groups, are luminescence centers. Some precursor compounds bearing amino groups without silicious groups but with a tendency to form clusters in solution also emit luminescence. Luminescence is the result of electron-hole recombination on delocalized states so that emission wavelength depends on excitation wavelength. Luminescence can thus be emitted in almost the entire visible spectrum and it can be tuned by choosing excitation wavelength. Amino-group-containing gels give the highest photoluminescence yield. It is, generally, expected that the presence of chemical groups with electron-donating capacity within the light-generating nanoclusters will be a favorable factor for making an efficient photoluminescent gel.

Introduction A large share in the impressive development of new high-technology materials during the recent years has been made by materials used for new light sources. Organic light-emitting diodes make the best example of this case. Silicon-based materials have also attracted an increasing interest, particularly, since the discovery that porous silicon can emit visible light.1,2 Less interest has been paid to silicon compounds since, in most cases, they are considered inert toward light absorption or emission, particularly SiO2, which has a high-energy band gap (5.4 eV). It is nevertheless known that various impurities introduced into the silica network, such as C, Sn, or excess O, can decrease the band gap and make interaction with visible light possible.3-5 Recently, a novel interest has been demonstrated for silicon-based compounds, following the finding that organic/inorganic hybrids obtained through the sol-gel route can emit broad-band photoluminescence with relatively high quantum yield.6-13 Even though some controversy has * To whom correspondence should be addressed. Tel.: 30-2610997587. Fax: 30-2610-997803. E-mail: [email protected]. (1) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (2) Lehmann, V.; Gosele, U. Appl. Phys. Lett. 1991, 58, 856. (3) Sendova-Vasileva, M.; Tzenov, N.; Dimova-Malinovska, D.; Marinova Ts.; Krastev, V. Thin Solid Films 1996, 276, 318. (4) Chiodini, N.; Meinardi, F.; Morazzoni, F.; Paleari, A.; Scotti, R.; Di Martino, D. J. Non-Cryst. Solids 2000, 261, 1. (5) Sakurai, Y.; Nagasawa, K. J. Non-Cryst. Solids 2000, 261, 21. (6) Green, W. H.; Le, K. P.; Grey, J.; Au, T. T.; Sailor M. J. Science 1997, 276, 1826. (7) Bekiari, V.; Lianos, P. Langmuir 1998, 14, 3459. (8) Bekiari, V.; Lianos, P. Chem. Mater. 1998, 10, 3777. (9) Ribeiro, S. I. L.; Dahmouche, K.; Ribeiro, C. A.; Santili, C. V.; Pulcinelli, S. H. J. Sol-Gel Sci. Technol. 1998, 13, 427.

arisen as to the origin of this light emission,13 subsequent publications have helped make the problem more and more clear so that prediction of the photoluminescence capacity of a silicious organic/inorganic hybrid can be made with satisfactory efficiency. The present work makes a short review of the intrinsic photoluminescence of a few chosen organic/ inorganic hybrid sol-gel materials and presents some new data which show that light emission from these materials is a property of organic/inorganic hybrid nanoclusters and that light is generated by transitions between delocalized energy states. The term “intrinsic photoluminescence” is used to denote materials which contain no dyes, semiconductors, or other luminescent dopants and that photoluminescence exclusively originates from the hybrid material itself. The most efficient light-emitting gels of the kind are those containing amino groups (quantum yield up to 35%).6-13 The functionality of the nitrogen-containing chemical group is recognized by all works in that field.6-13 However, it seems that it is nanocluster formation in some aminorich (or amide-rich) compounds that offers the possibility (10) Bekiari, V.; Lianos, P.; Judenstein, P. Chem. Phys. Lett. 1999, 307, 310. (11) (a)Carlos, L. D.; de Zea Bermudez, V.; Sa Ferreira, R. A.; Marques, L.; Assuncao, M. Chem. Mater. 1999, 11, 581. (b) Carlos, L. D.; de Zea Bermudez, V.; Duarte, M.; Silva, M. M.; Silva, C. J.; Smith, M. J.; Assuncao, L.; Alcacer, L. Phys. Chem. Lumin. Mater. VI, Electrochem. Soc. Proc. 1998, 97, 352. (c) de Zea Bermudez, V.; Carlos, L. D.; Duarte, M. C.; Silva, M. M.; Silva, C. J.; Smith, M. J.; Assuncao, L.; Alcacer, L. J. Alloys Compd. 1998, 275-277, 21. (12) Bekiari, V.; Lianos, P.; Lavrencic-Stangar, U. L.; Orel, B.; Judenstein, P. Chem. Mater. 2000, 12, 3095. (13) Carlos, L. D.; Sa Ferreira, R. A.; de Zea Bermudez, V.; Ribeiro, S. J. L. Adv. Funct. Mater. 2000, 11, 111.

10.1021/cm0212055 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/09/2003

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of light emission. This phenomenon is studied in the present work by employing two typical precursors: (1) APTS, that is, (aminopropyl)triethoxysilane, bearing an amino group, and (2) ureasils, that is, compounds made of a poly(alkylene oxide) chain, end-capped with triethoxysilane groups, covalently linked by urea bridges, that is, bearing amide groups.10-13 Both these precursors give photoluminescent gels through the sol-gel method by alkoxide solvolysis and inorganic (-O-SiO-) polymerization. We then believe that further clarification of the conditions for luminescence emission by these materials will lead to the search for other materials with similar or even greater efficiency in visible light emission. Experimental Section All materials used in the present work were purchased from Aldrich or Fluka and were used as received. Synthesis of Ureasil Gels. Ureasil hybrid precursors were synthesized as in previous publications.12,14 Poly(propylene glycol)-bis(2-aminopropyl ether) of different chain lengths was mixed with 3-isocyanatopropyltriethoxysilane (molar ratio 1:2) in tetrahydrofuran (THF) under reflux (64 °C) for 6 h. The isocyanate group of 3-isocyanatopropyltriethoxysilane reacts with the amino groups of poly(propylene glycol)-bis(2-aminopropyl ether) (acylation reaction), producing urea connecting groups between the polymer units and the inorganic part. After evaporation of THF under vacuum, a viscous precursor was obtained, which is stable at room temperature for several months. Four similar hybrid precursors have been prepared by modification by the poly(oxypropylene) chain length [abbreviated PP-230, PP-400, PP-2000, and PP-4000], where the number indicates the average chain size. Inorganic polymerization was realized by acetic acid solvolysis in the absence of water. To avoid any influence by atmospheric humidity, the procedure was run in closed containers by making the following steps. First, 0.5 g of the hybrid ureasil precursor was dissolved in 3 mL of ethanol in a glass flask designed to be put under vacuum. Second, the solution was frozen at liquid nitrogen temperature and it was submitted to two or three freeze-pump-thaw cycles to clear it of oxygen or any other volatile impurity. Finally, glacial acetic acid was injected into the solution under stirring at a molar ratio of ureasil/acetic acid ) 1:10. The mixture was kept in the airproof flask for a week and then it was poured in 1-cm square PMMA cuvettes and was exposed to ambient conditions for 1 week more until gelation was obtained. Synthesis of APTS Gels. Commercial APTS ((aminopropyl)triethoxysilane) precursors were used to synthesize APTS gels, which were made in a way similar to those in previous publications7,8 or to that in the above case of ureasils. Three milliliters of APTS (which is a liquid) was introduced into the vacuum flask and subjected to two or three freeze-pumpthaw cycles as above. Finally, formic or acetic or valeric acid was injected at a molar ratio of APTS/carboxylic acid ) 1:3, under stirring. The rest of the procedure was the same as that for the ureasil gels. Steady-state fluorescence measurements were made with a spectrofluorometer that consists of ORIEL components in a standard configuration: 150-W xenon lamp, excitation monochromator (25 cm), thermostated sample holder, emission monochromator (25 cm) at a right angle, and a computerdriven detection system. Spectra were corrected for both lamp and photomultiplier spectral response profiles. Measurements were performed at 20 °C. Fluorescence quantum yield mea(14) (a) de Zea Bermudez, V.; Baril, D.; Sanchez, J.-Y.; Armand, M.; Poinsignon, C. J. Opt. Mater. Technol. Energy Efficiency Sol. Convers. XI: Chromogenics Smart Windows, SPIE Proc. 1992, 1728, 180. (b) Dahmouche, K.; Atik, M.; Mello, N. C.; Bonagamba, T. J.; Panepucci, H.; Aegerter, M.A.; Judeinstein, P. J. Sol-Gel Sci. Technol. 1997, 8, 711.

Brankova et al. surements were made by the standard procedures used for fluid samples. This is permitted by the fact that gels take the orthogonal shape of the cuvettes where the sol is put in. Of course, dimension contraction was taken into account. Quinine sulfate was employed as a standard.

Results and Discussion Why Solvolysis with Organic Acid and Why in Airproof Flasks? As explained in the previous section, ureasil and APTS gels were made by interaction with acetic acid in vacuum flasks, while in the case of APTS other carboxylic acids such as formic and valeric acid have also been used. The choice was made first of all for practical reasons. All gels made in this way are more luminescent than gels made by typical hydrolysis in an HCl environment, such as in a standard sol-gel procedure. As a matter of fact, APTS gels made by HCl hydrolysis are not luminescent at all. Only APTS gels obtained by carboxylic acid solvolysis are luminescent. In the case of ureasils, fluorescence quantum yield measurements have shown that gels made by acetic acid solvolysis are 30% more luminescent than corresponding gels made by HCl hydrolysis. For both materials, carboxylic acid solvolysis is then preferable. Solvolysis with acetic acid was studied in previous works7,15by FTIR spectroscopy. In particular, in ref 15, it was found that acetic acid solvolysis proceeds by a two-step reaction, confirming a mechanism previously proposed by Pope and Mackenzie.16 First, a silica ester (CH3COOSi-) is formed while in the second step, by reaction with ethanol, SiOH is formed. From the latter, by inorganic polymerization a -O-Si-O- network is created, thus providing the gelling agent. A simplified reaction scheme coming out from FTIR data is represented by the following reactions:

-SiΟC2H5 + CH3COOH f -SiCH3COO + C2H5 OH f -SiOH + CH3COOC2H5 solvolysis-ester formation -SiOH f SiΟ2

inorganic polymerization

One main spectroscopic characteristic of this process is the rapid consumption of ethanol,15 which produces gels with practically no alcohol content. This is very important for the luminescence efficiency of the gels since the presence of alcohol destroys clusters, particularly, in the case of APTS. As it will be discussed below, the existence of nanoclusters is a prerequisite for luminescence emission. A second spectroscopic characteristic of the process is an important increasing contribution from a Si-O-C bond, which is attributed to silica ester. Even after a long ripening period, the silica ester content is still high. In other words, the hybrid organic/inorganic nature of these gels is enlarged by carboxylic acid solvolysis, in the sense that this process introduces additional organic groups chemically bound to the silica network. Such additional organic groups are expected to affect the dynamics of nanocluster formation and thus influence the capacity of the material to emit light. It is preferable (15) Stathatos, E.; Lianos, P.; Lavrencic-Stangar, U. L.; Orel, B Adv. Mater. 2002, 14, 354. (16) Pope, E. J. A.; Mackenzie, J. D. J. Non-Cryst. Solids 1986, 87, 185.

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Figure 2. Photoluminescence spectra of APTS gels obtained through carboxylic acid solvolysis: (1) formic acid, (2) acetic acid, and (3) valeric acid. Curves 1a, 2a, and 3a were obtained by excitation at 360 nm and curves 1b, 2b, and 3b by excitation at 467 nm. Insert: maximum photoluminescence curves obtained by excitation at 360, 420, and 467 nm for curves 1, 2, and 3, respectively.

Figure 1. (a) Photoluminescence spectra of fresh APTS gels, obtained by acetic acid solvolysis, at various excitation wavelengths: (1) 360 nm, (2) 380 nm, (3) 420 nm, and (4) 440 nm. (b) Photoluminescence spectra of mature APTS gels at various excitation wavelengths: (1) 360 nm, (2) 400 nm, (3) 460 nm, and (4) 500 nm.

to carry out this process in airproof containers to avoid environmental humidity as well as to be free of oxygen and other volatile impurities, which could affect the photoluminescence emission properties of the gels. Photoluminescence Spectral Characteristics of Gels Obtained with APTS or Ureasils through Carboxylic Acid Solvolysis. Figures 1 and 2 show the photoluminescence spectra of gels obtained from APTS. Figure 1a corresponds to a fresh sample, that is, right after completion of preparation, and Figure 1b corresponds to a mature sample. Maturing has been done by putting the gel in an oven at 50 °C, for about 30 days. In both cases gels emit bright photoluminescence7,8 in a broad band. The luminescence shifts to longer wavelengths with the excitation wavelength. Practically, the whole visible spectrum can be scanned by choosing the appropriate excitation wavelength. Mature samples emit at relatively longer wavelengths than fresh samples. The short wavelength excitation reveals the existence of two species, distinguished by two weakly separated peaks. The shorter wavelength species is detected only by short wavelength excitation. Emission in these gels originates from delocalized energy states by electronhole recombination and this is, first of all, concluded from the dependence of the emission wavelength on the excitation wavelength. Emission is red-shifted when the excitation is done at longer wavelength because of

nanocluster size polydispersity in these materials. Smaller clusters tend to absorb and emit at shorter wavelengths due to size effects, and this is a common place in several nanostructured materials. A typical case is a bright phosphor made by dispersion of CdS nanocrystallites in poly(ethylene glycol) oligomers,17 where the whole visible spectrum can also be scanned by changing excitation wavelength.17 A similar idea, but from a different point of view, has been previously supported by Carlos et al.11a These authors suggest that the dependence of emission wavelength on excitation energy is due to the existence of discrete energy states within the energy gap. Transition between such states will depend on excitation energy.11a We believe that the energy levels of such discrete energy states are related to nanocluster size so that higher energies are met in nanoclusters of smaller size. Naturally, mature gels emit at relatively longer wavelengths since they consist of larger nanoclusters. In these terms, nanocluster size is larger when solvolysis is done with a longer chain carboxylic acid. This conclusion is drawn by inspection of the data of Figure 2. This figure shows photoluminescence of APTS gels made by formic, acetic, or valeric acid solvolysis. Excitation at a relatively short wavelength (360 nm) makes all three species emit blue luminescence, the more intense being the one emitted by the formic acid-catalyzed species and the less intense being the one catalyzed by valeric acid. With excitation at a relatively long wavelength (467 nm), the order is reversed. All three species emitted green luminescence, which was very intense for the valeric acid-catalyzed species and scarcely detectable for the formic acidcatalyzed species. The case of acetic acid was always intermediate. The insert of Figure 2 also shows the position of maximum emission for the three species. These data then indicate that the obtained luminescent species tend to emit at longer wavelengths when the carboxylic acid chain length is longer. It is concluded that longer acids tend to form bigger clusters, obviously, due to the different hydrophilic-hydrophobic balance (17) Bekiari, V.; Lianos, P. Langmuir 2000, 16, 3561.

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Figure 4. Photoluminescence intensity of mixtures of ethanol with poly(ethylene glycol)-800-bis(2-aminopropyl ether) (b-b-b) poly(propylene glycol)-2000-bis(2-aminopropyl ether) ([-[-[). Figure 3. Chemical structure of PP-2000 and photoluminescence spectra of the ensuing acetic acid-catalyzed gel at various excitation wavelengths: (1) 340 nm, (2) 370 nm, (3) 386 nm, and (4) 420 nm.

on each molecule and the ensuing different aggregation tendencies. This trend continues with butyric acid and longer chain homologues, even though practical difficulties forbid accurate recording of spectra. The next figure, Figure 3, shows the photoluminescence spectra of one of the studied ureasils, PP-2000, which is the one that is derived from the precursor that has a poly(propylene oxide)-2000 component. The chemical structure of PP2000 is shown also in Figure 3. A behavior similar to the one demonstrated by APTS gels is also demonstrated by the ureasil gels. Shorter wavelength excitation reveals the existence of two emitting species, while a shoulder formed in the long-wavelength-excitation spectrum reveals that the existence of three distinct species is not excluded. Similar results were also registered with the rest of the species in the homologous series, that is, with PP-230, PP-400, and PP-4000. In going from smaller to larger poly(propylene oxide) chains, the spectral structure remained the same but luminescence intensity decreased, apparently due to a dilution effect, that is, an increase of the polyether volume fraction, which decreases the concentration of silica and urea groups. This means that luminescent centers are associated with silica and urea groups and that ether groups do not take part in the luminescence process, in agreement with previous findings.11-13 This conclusion is further supported by the fact that the spectral structure did not change with polyether chain length, as already said. Apparently, the organic subphase affects silica nanoclusters, but it is only the latter that are capable of emitting light. The appearance of discrete luminescence peaks, particularly in the highenergy domain, is most probably due to a tendency of small silica clusters to attain discrete sizes, as pointed out by Carlos et al. in ref 11a, based on observations made by SAXS. Role of the Nitrogen-Containing Chemical Group. Photoluminescent phosphors can be obtained by solvolysis of silicon alkoxides not containing amino or amide groups, for example, solvolysis of tetraethoxysilane by acetic acid, as it has been previously found6

and also verified by us. However, the highest photoluminescent efficiency obtained so far is with compounds possessing an amino group functionality, as is the case of APTS. The amino group by itself cannot emit luminescence. Thus, pure APTS is not luminescent. Cluster formation through the sol-gel process leads to APTS gels that emit light. In contrast, poly(propylene glycol)bis(2-aminopropyl ether) and the ensuing ureasil precursor after interaction with 3-isocyanatopropyltriethoxysilane are both photoluminescent. All ureasil precursors based on poly(propylene oxide) chains gave photoluminescence behavior similar to that of dry gels,12 that is, they emit a broad-band photoluminescence peaking at an excitation-dependent wavelength, similarly to the case of Figure 3 (cf. ref 12). However, both poly(propylene glycol)-bis(2-aminopropyl ether) and the corresponding ureasil precursors can be easily dissolved in ethanol where very rapidly they lose their photoluminescence capacity.12 This can be seen in Figure 4, where the luminescence intensity of poly(alkylene glycol)bis(2-aminopropyl ether) is plotted against its volume fraction in alcohol mixtures for two characteristic samples, one corresponding to poly(propylene oxide)2000 and one to poly(ethylene oxide)-800. We note a fast decrease of luminescence intensity with the increase of the percentage of ethanol, a decrease which is better marked in the case of the smaller-ether-chain homologue, poly(ethylene oxide)-800, which not only has a smaller molecular weight but also carries smaller aliphatic repeat units. In addition, we have previously found by light-scattering observations12 that the ureasil precursors form colloidal particles when dissolved in ethanol with a hydrodynamic radius ranging up to 200 Å, decreasing with an increasing percentage of ethanol. We have verified that the observed decrease of photoluminescence is not due to an artifact. First, the presence of impurities was excluded. Luminescence of impurities does not depend on excitation wavelength, as is the present case. A decrease of photoluminescence in alcohol due to self-absorption phenomena cannot be excluded but it is of small importance as it will be seen below. If self-absorption in the present bulk material played an important role, then one would expect an increase and not a decrease of photoluminescence intensity with increasing alcohol percentage. Interest-

Sol-Gel Organic/Inorganic Hybrid Gels

Figure 5. Comparison of photoluminescence spectra of: (1E) pure poly(propylene glycol)-2000-bis(2-aminopropyl ether) and (2E) diluted in 60% ethanol. Excitation wavelenth, 340 nm. The spectra are normalized to the shorter wavelength peak. The corresponding absorption spectra are given by curves 1A and 2A. Insert: Variation of luminescence intensity of poly(propylene glycol)-2000-bis(2-aminopropyl ether) by dilution with ethanol as recorded by excitation at different wavelengths: (a) 340 nm, (b) 370 nm, and (c) 420 nm.

ingly, the spectral structure of the photoluminescence of the diluted species varied with alcohol content in favor of the short-wavelength emission; that is, diluted species tend to emit at shorter wavelength. This can be seen by the spectra of Figure 5. Especially from the insert of Figure 5, it is seen that dilution effects are dramatically different when observation is made at different emission wavelengths. Thus, a small increase of luminescence intensity was observed in diluted species at short wavelength. This last result may be partly due to self-absorption since in that area absorption and emission overlap. However, as can be seen from the absorption spectra, also presented in Figure 5, the absorbance is not very large in the emission spectral region so that self-absorption does not play a crucial role. These data are compatible with the formation of clusters, with decreasing sizes in the presence of alcohol. It is obvious from these results that the emission properties of the above amino- or amide-rich compounds are connected to the existence of nanoclusters and they are demonstrated only in the presence of nanoclusters. There are three possible mechanisms of cluster formation in precursor compounds: (1) hydrophobic-hydrophilic balance forces causing folding-unfolding of the polymer chain and aggregation of several monomers; (2) aggregation by hydrogen bonding; and (3) valid only for silicon alkoxide precursors, partial hydrolysis and condensation due to ambient humidity. Care was, of course, taken to avoid this third possibility. When the clusters are in ethanol solutions, the forces sustaining them are eliminated and the clusters progressively disappear together with their photoluminescence capacity. We can then safely conclude that an amino or amide group has an important part in the luminescence process but the physical conditions for light generation is the formation

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of nanoclusters. Nanocluster formation in ureasil gels was previously studied by fluorescence probing.18,19 It was then concluded that both the silica groups with the surrounding urea groups as well as the polyether groups tend to aggregate so that one of the main features of these gels is the formation of nanoclusters. It is within the silica-based nanocluster that a certain level of conjugation between adjacent groups is obtained so that emissive states by electron-hole recombination are created within the visible. This conjugation and subsequent state delocalization is not as efficient as in π-conjugation but is efficient enough to give detectable photoluminescence. It seems that the large electrondonating capacity of the nitrogen-containing group is responsible for the efficiency of the conjugation. We have tried to produce light by making gels using mercaptopropyltrimethoxysilane as a precursor. The mercapto group has a lower electron-donating capacity than the amino group. It was not surprising that no light was generated by using such gels. In contrast, as already said, gels made by pure silicon alkoxides through acetic acid solvolysis are photoluminescent. We believe that the electron-donating capacity of the groups introduced through the formation of silica ester (CH3COOSi-) are responsible for light generation. These groups are preserved to an important extent in the gel, as already said above. The most concrete model, so far, to explain ureasil photoluminescence, which applies equally well also to APTS photoluminescence, was proposed by Carlos et al. in some recent publications.11a,13 The authors suggest that, in these hybrid organic/inorganic materials, the hierarchy in the silica backbone dimension, which is applied by the presence of the organic groups at increasing proportions, defines the emission wavelength.11a This is in accordance with the above findings since larger clusters emit at longer wavelength. When time-resolved luminescence spectroscopy was employed, Carlos et al. found that the broad-band photoluminescence emitted by the above materials results from a convolution of a longer lived emission originating in the amide groups and a shorter lived emission originating from electron-hole recombinations on nanometer-sized siliceous domains.13 From any point of view that one sees it, previous data and the above data underline the importance of the amine functionality for the photoluminescence of these systems. This is obvious from the fact that luminescence is produced even in the absence of silicon-based compounds, nanocluster formation being a prerequisite (cf. Figure 4). We further believe that all chemical groups with electron-donating capacity play a role similar to the amino group and we propose carboxylic acid solvolysis as a favorable process for making efficient photoluminescent gels. CM0212055 (18) Stathatos, E.; Lianos, P.; Lavrencic-Stangar, U. L.; Orel, B.; Judenstein, P. Langmuir 2000, 16, 8672. (19) Keeling-Tucker, T.; Brennan, J. D. Chem. Mater. 2001, 13, 3331.