J. MATER. CHEM., 1991, 1(6), 903-913 903 FEATURE ARTICLE Optical Properties of Sol-Gel Glasses doped with Organic Molecules Bruce Dunna and Jeffrey 1. Zinkb a Department of Materials Science and Engineering and Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA 90024, USA The sol-gel process enables one to prepare oxide glasses at room temperature with little or no heating. By using this method, it is possible to encapsulate a wide variety of organic and organometallic molecules in the inorganic matrix. Studies of this new type of organic/inorganic composite have evolved towards two different objectives: the use of luminescent molecules as probes of the sol-gel process and the deliberate doping of organics to produce materials with specific optical properties.This review emphasizes the ability of encapsulated luminescent molecules to provide unique insights regarding local chemistry and structure during the sol-gel- xerogel transition. The second part of this review gives several examples of how organic-doped sol-gel materials are emerging as an important means of producing photonic materials. Keywords: Sol-gel processing ; Organic molecule ; pH sensor; Chemical sensor ; Feature article The sol-gel process is a chemical synthesis technique for preparing gels, glasses and ceramic powders. The method has received considerable attention because it possesses a number of desirable characteristics. It enables one to prepare glasses at far lower temperatures than is possible by using conven- tional melting.Compositions which are difficult to obtain by conventional means because of volatilization, high melting temperatures or crystallization problems can be produced. In addition, the sol-gel method is a high-purity process which leads to excellent homogeneity. Finally, the sol-gel approach is adaptable to producing bulk pieces as well as films and fibres. During the past 5 years it has been widely recognized that the sol-gel process may be used to encapsulate organic and organometallic molecules in an inorganic medium. It is now evident that the synthesis of organic-doped gel-glasses is not limited to just a few organics. The flexible solution chemistry and the ability to prepare an essentially inorganic matrix with little or no heating means that the sol-gel approach is compatible with a wide variety of organic molecules.The list of organic-doped sol-gels is constantly expanding. Prior to the sol-gel work, the incorporation of organic molecules in solids generally was restricted to the use of frozen solvents or organic polymer matrices. The present approach represents a totally new type of organic/inorganic composite material because the oxide matrix not only offers a signifi- cantly more ionic environment but also it is thermally, chemi- cally and dimensionally more stable. Thus, studies of organic- doped sol-gels have begun to develop substantial breadth; from investigations of doped sol-gels for spectroscopic and matrix-isolation photochemistry, to the effect of solvent chem- istry on luminescence properties, to the development of lasers and non-linear optical materials.The present paper will review the research in the two most developed areas for organic-doped sol-gel glasses; the use of luminescent molecules as probes of the sol-gel process and the development of specific optical properties based on the properties of the organic dopants. The emphasis of the paper will be on the ability of luminescent molecules to probe local chemical and structural changes which occur as the matrix evolves from a sol through the sol-gel transition, the ageing process and the drying stage. Optical spectroscopic techniques are used to detect the changes and to interpret the chemical and structural environment responsible for them.The second part of the review will consider the aspect of inducing specific optical properties based upon the deliberately chosen dopant. This work has led to the development of optical materials that exhibit laser action, optical gain, chemical sensing and non-linear optical effects. Preparation of Inorganic Glasses by the Sol-Gel Method The synthesis of materials by the sol-gel process generally involves the use of metal alkoxides which undergo hydrolysis and condensation polymerization reactions to give gels. The synthesis of oxides by this approach has been the subject of several books and reviews and the reader is referred to this literature for more in-depth disc~ssion.'-~ In this section, an overview of the sol-gel approach is given that places an emphasis on those aspects which are particularly relevant for this paper.The sol-gel process can ordinarily be divided into the following steps: forming a solution, gelation, drying and den~ification.~In the preparation of a silica glass, one starts with an appropriate alkoxide [e.g. Si(OC2H5)4, tetraethyl- orthosilicate or TEOS] which is mixed with water and a mutual solvent, such as ethanol (EtOH), to form a solution. Hydrolysis leads to the formation of silanol groups Si-OH. These species are only intermediates as they react further to form siloxane Si-0-Si groups. An example of the formation of a silica gel by the organometallic route is as follows: Si( OC2H5)4 +4H20-+ +4C2 H 50H nSi(OH),+nSiO2 +2nH20 (2) In general, the processes of hydrolysis and condensation polymerization are difficult to separate.Numerous studies have been performed in this area, and there is good under- standing of the chemistry involved and how the reactions relate to gelation and structural evol~tion.~-~ There are several parameters that influence the hydrolysis and conden- sation polymerization reactions including the temperature, solution pH, the particular alkoxide precursor and solvent, J. MATER. CHEM., 1991, VOL. 1 and the relative concentrations of the alkoxide precursor, water and solvent. Another consideration is that catalysts (usually acid, but basic ones have been used as well) are frequently added to help control the rate and the extent of hydrolysis.The conditions under which the hydrolysis and conden- sation occur have a profound effect on gel growth and m~rphology.~?~It is well appreciated that one can produce linear polymers, branched clusters or colloids in the solution depending upon various factors including the H20:Si ratio and pH. Silicate gels prepared at low pH (<3) and low water content (< 4mol water per mol of alkoxide) produce primarily linear polymers with low crosslink density. Additional crosslinks form during gelation and the polymer chains become increasingly entangled. Silicate gels prepared under more basic conditions (pH 5-7) and/or higher water contents produce more highly branched clusters which behave as discrete species. Gelation occurs by linking clusters together.At still higher pH and excess water content, colloidal silica is formed. As the hydrolysis and condensation polymerization reac- tions continue, viscosity increases until the solution ceases to flow. The time to gelation is an important characteristic which is sensitive to the chemistry of the solution and nature of the polymeric species. This sol-gel transition is irreversible and there is little if any change in volume. At this stage the one- phase liquid is transformed to a two-phase ~ystem.~ The gels formed in alcohol are termed alcogels, while those formed in aqueous solutions are termed hydrogels. The gel consists of amorphous particles of variable size (5-10 nm or smaller) with an interstitial liquid phase.After gelation, gels are generally subjected to an ageing process during which the gels are kept sealed and very little solvent loss or shrinkage occurs. Condensation reactions continue, increasing the degree of crosslinking in the network and producing an increase in gel viscosity. Syneresis is frequently observed at this stage. ' T~ The drying process involves the removal of the liquid phase. Low-temperature evaporation is frequently employed and there is considerable weight loss and shrinkage. As evapor- ation occurs, drying stresses arise because of the differential strains associated with the pressure gradient in the liquid phase. A series of papers by Scherer have been directed at understanding the fundamental mechanisms involved in dry- ing.*-12 The drying stage is critical as the ability to dry without cracking serves to limit the sizes of monolithic pieces.The final stage of the sol-gel process is that of densification. It is at this point that the gel-to-glass conversion occurs and the gel achieves the properties of the glass. As the temperature increases, several processes occur including elimination of residual water and organics, relaxation of the gel structure, and ultimately densification, usually by viscous sintering. The temperatures involved depend upon the specific system. In the case of SOz materials, temperatures corresponding to a viscosity of 1013 P (ca. 1200 "C)are generally involved in the densification stage.The high temperatures required for densification generally will destroy any incorporated organic molecules. Probes of Chemistry In this section we review the use of spectroscopic probes to follow the gelation process through its various stages. The most important probes monitor solution water content, pH effects and thermal effects. For each of these types of probes we consider the nature of the measurement, the changes which are observed, and the interpretation of the spectroscopic Changes in Water/Alcohol Content The chemical changes that occur during the polymerization reactions are an important aspect of the gelation process which has been studied by using molecular probes. A particu- lar change which was used to follow the rate of gelation was the change in the water:alcohol ratio.As the hydrolysis reaction proceeds, alcohol is continuously being produced and the ratio of the amount of water to the amount of alcohol continuously decreases. It is desirable to probe these changes on the molecular level. Pyranine The fluorescence spectrum of pyranine, 8-hydroxy- 1,3,6-trisul- phonated pyrene, is sensitive to proton-transfer phenomena.' The molecule can be encapsulated in gel-glasses and retain its fluorescence properties in the materials. As discussed below, the fluorescence spectrum is sensitive to the proton acceptor ability of the medium surrounding the molecule and can be used to monitor changes in the water: alcohol ratio.The spectroscopy of pyranine was studied in control solu- tions consisting of water :isopropyl alcohol mixt~res.'~ The protonated form of the molecule emits in the blue region of the spectrum at 430 nm whereas the deprotonated form emits in the green at 515 nm. The relative intensities of the blue and green emission bands depend on the surrounding medium. In water-rich environments the green emission band domi- nates because water acts as a proton acceptor. In propanol- rich environments, the blue band is dominant because the pyranine remains protonated. The ratio of the intensities, called F in our work, is defined as the intensity of the blue peak divided by that of the green peak.l4 The changes in the ratio, F, that accompany ageing and drying of the pyranine- doped aluminosilicate gels are shown in Fig.1. Consider first the initial part of the figure up to a processing time of ca. 150 h. F increases smoothly during the gelation process from t=O to an intermediate value of Fx0.45 at t= 15 h at the gel point. The ratio continues to evolve in closed gels over a period of time equal to ca. eight times the gelation time, eventually reaching a maximum value of Fx0.9 after 200 h. These results indicate that changes in the gel chemistry continue well beyond the gelation period. The macroscopic rigidity observed at the gel point does not mean that the reactive entities are immobile at the microscopic level. The rates of the reactions involved in the aging of the gel appear to be slower than those in the liquid state, but the extent of these changes is as great in the gel state as in the liquid state.after drying at 70 "C ->*# I drying 0*5Y0.0 : I I I I' I 0 100 200 300 400 tlh results in terms of the chemical changes during gelation, Fig. 1 Evolution of the emission intensity ratio (F) during gelation, ageing and drying. ageing and drying processes l4 J. MATER. CHEM., 1991, VOL. 1 Drying experiments were carried out after the luminescence of the sealed gel reached a stable value.14 F decreased slowly from 0.9 to 0.7 during the first 60% of weight loss. In contrast, the curve became irregular when the weight loss reached 70%. At this stage F increased sharply to a maximum of 1.6.This value is comparable to that observed for a gel that has been heated for 10 h under vacuum at 70 "C and is well dried. The spectral changes that occur are significant despite the fact that aluminosilicate sols do not undergo significant compositional changes. These results suggest that the pyranine molecules are probing a very selected aspect of the gel polymerization chemistry which is strongly influenced by the protonation/deprotonation characteristics of the solvation shell. The initial value of F just after hydrolysis is substantially different from that in a water-propanol mixture with the same water concentration. In the early moments of the gelation process, pyranine is not homogeneously distributed in the solution. Rather, it appears that the probe is already part of the small particles which result from partial conden- sation of the Al-OH groups.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. The continuous changes in F during gelation and ageing indicate that significant chemical modifications occur both in the adsorbed layers and consequently in the solvation sphere of the pyranine molecules. The increase in F from 0 just after the solution preparation to 0.45 at the gel point and eventually to 0.9 at the end of ageing indicates that the proton-accepting ability of the adsorbed layers on the polymer surface decreases substantially.Ultimately an equilibrium is established that reflects the initial composition of the sol. The changes in emission during the drying of the gels can be explained on a similar basis. Propanol is the most abundant solvent present in the pore system of the gel and has a higher vapour 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. F decreases accordingly. Later as drying continues, the free water content approaches zero and F increases sharply. It must be emphasized that the above results are specific to aluminosilicate gels. The changes in the fluorescence spectra of pyranine were used to study the effect of pH on water consumption in the early stages of gelation of silicate glass prepared from tetramethyl orthosilicate (TMOS)." The most important result of this study was the demonstration that there is a slower decrease in water content at higher pH values.This conclusion was explained in terms of gradual changes in the rates of the opposing processes of water consumption and water release with pH. For example, at pH 2.5, the probe shows that the rate of increase in water content was significantly slower than the initial rate of con- sumption. However, as the pH increased, hydrolysis slowed down and condensation accelerated. It was noted that in the pH range 2.5-7.6, all polymerization reached a similar water content, estimated at 30-35%, after ca.6 h. Thus, it was concluded that the rate of water release by condensation is less affected by pH than the hydrolysis rate. Europium(@ and Terbium(@ The luminescence of europium(II1) ions was used to follow the sol-gel tran~ition.'~?'~ The total luminescence from the 'Do state exhibited a gradual increase in intensity as a function of both time and temperature of dehydration for TMOS gels. In addition the specific 5DO-+7F1 transitions showed a relative increase in intensity. This behaviour was explained in terms of formation of a chemical bond between the Eu"' and the oxygens of silicate groups. It was proposed that the metal ion acquires a symmetry similar to that in glasses obtained by melting silica. The formation of glasses prepared from TMOS and TEOS was compared by using the Eu"' probe.In these studies the peaks arising from the transitions 5Do-7F1 and 5Do-7F2 were compared. It was concluded that glasses prepared from TEOS are more dense and that the probe ion is surrounded sooner by the silica groups which replace water than in glasses prepared from TMOS. Effects of pH Coumarin dyes have been used to probe the effects of pH on sol-gel processing. Many coumarin dyes are luminescent when incorporated in silicate and aluminosilicate matrices. The dyes that have been studied and the positions of their emission maxima are given in Table 1.'* In all cases small shifts in the positions of the maxima from those found in solution are observed.Of all of the dyes studied, only coumarin 338 lost its luminescence properties in the silicate xerogel (Table 1). In aluminosilicate xerogels all of the coumarin dyes retain their luminescence properties. Significant shifts of the emission and the excitation spectra as well as the presence of new luminescence bands are observed for these dried gels. For example, coumarin 540 exhibits a new peak at 540nm in the dried gel whereas only one peak at 504 nm is observed in the aged gel. In order to examine the effects of the matrix and pH on the luminescence spectra of coumarin dyes, the luminescence of coumarin 4 in aged and dried silica gels was examined in detail. Coumarin 4(7-hydroxy-4-methylcoumarin) belongs to the family of hydroxycoumarins whose fluorescence properties depend strongly on pH." The neutral form of coumarin 4 shown in Fig.2 is the dominant species in aqueous alcoholic solutions for a wide range of pH and water contents. The various reactions leading to the species responsible for the fluorescence are shown in Fig. 2.20*21In neutral and alkaline regimes (pH>6) the fluorescence is mainly composed of a strong blue emission at 440-450nm from the anionic (A*) form. As the acidity is increased, there are two posssible sites of protonation. Protonation at the phenolic oxygen leads to the neutral molecule (N*) with emission at 390 nm while Table 1 Luminescence band maxima of coumarin-doped gels ( mol dm-3) and xerogels band maxima/nm coumarin silicate gel silicate xerogel aluminosilicate gel aluminosilicate xerogel 4 395,430, 486 421, 486 394, 420,488 399,420 338 494 - 482 509 440 426 419 426 428 460 450 439,476 443, 469 445,469 523 495 496 504 493 540 508 525 504 512, 540 540A 530 512 GROUND STATE y3 HOmo neutral form EXCITED STATE anionicform neutral form y-43 y 3 +H HO HO OH zwitterionicform cationic form Fig.2 Molecular structure of different forms of coumarin 4 and the corresponding acid-base equilibriaI8 protonation at the carbonyl oxygen leads to the zwitterionic form (Z*) which emits at 480nm. Another form of the zwitterion which emits at 530 nm has also been reported.22 In very strong acid, the second protonation will lead to the cationic form (C*) which has an emission peak at 412 nm.The luminescence spectra in the gels are different from those observed in solutions. The differences suggest that the dye is not simply dissolved in solvent-filled pores in the gel. Previous work with pyranine in aluminosilicate gels indicated that the organic molecule tended to be adsorbed on the gel particles during polymerization.14 In this regard it is import- ant to recognize that the variation of pH will change the composition of the gel surface. Colloidal silica has an iso- electric point at pH 2.2,23and only relatively small deviations from this value would be expected for silicon oxide based polymer structures. At low pH (<2.2) the net charge of the silica network is expected to be positive owing to the presence of Si-OH; groups on the surface of the gel particles.In contrast, at pH >2.2, the presence of Si-0- anionic groups is expected to produce a negative charge. The changes of the surface composition lead to changes in the interactions between coumarin 4 and the gel matrix and subsequently cause the luminescence of aged and dried gels to differ substantially from the luminescence in solution. For gels synthesized at pH <2, the protonated gel surface leads to a more acidic environment than that of the reference solvent (ie. with 20% water). At pH 0.2, the cationic form is observed in gels [Fig. 3(a)],but not in the reference solution, while at pH 1 [Fig. 3(b)],the spectrum for the aged gels is comparable to the more acidic pH 0.2 emission for the 20% water-ethanol solution." When the gel is prepared at a pH >2.2, the emission spectrum resembles that of the reference solution at higher pH.It should be recognized, however, that in aged gels the solvent is still present in the matrix and that it is likely that the dye molecules will be solvated. The luminescence of xerogels differs from that of aged gels in two important respects. First, the luminescence intensity from xerogels is greater because gel shrinkage produces an increase in dopant concentration. Both the absorbance and J. MATER. CHEM., 1991, VOL. 1 the luminescence intensity increase. The second important spectral difference is that the relative intensities of specific emission bands show substantial changes as drying occurs.These changes cannot be explained by considering the compo- sition of the interstitial liquid phase of the gel. Rather, the changes in relative peak heights are due to enhanced dye/ matrix interactions as compared to aged gels. The role of the dried gel matrix is to control effectively the environment of the coumarin molecule. The surface character- istics of the gel produced by the pH during gel synthesis become even more important in establishing the luminescence characteristics. At pH <2, the peak heights for C*in the dried gel increase substantially from that of the corresponding aged gel [Fig. 3(a) and (b)].The gel prepared near the isoelectric point at pH 2.5 shows a marked growth during drying of the luminescence peak for the neutral species, N* [Fig.3(c)].In the most negatively charged gel (pH 3.8), peaks from both the neutral and the basic forms increase in intensity. The reasons for the enhanced interaction in the several gels prepared under different pH conditions arise from the chemi- cal and structural changes which occur during drying. First, the solvent loss during drying decreases the concentration of molecules which are likely to solvate the dye and the active sites on the gel surface. Secondly, the gel shrinks and the pore system collapses. The net result of these two processes is that the dye molecules are forced into increased contact with the gel surface and the screening effect of the solvent is diminished. Thus, drying the gels synthesized under the most acidic conditions leads to an increase in the intensity of the band from the protonic form of coumarin 4.Drying the gels synthesized under less acidic conditions causes an increase in the intensity of the bands from the neutral and the anionic forms. Chemical Changes during Heating and Drying The electronic absorption spectra of cobalt@) complexes have been used to follow the transformation of gels into dried glasses by heating.24 The cobalt(I1) ion forms complexes with both octahedral and tetrahedral geometries. Less regular co- ordination geometries are also known. Because the absorption spectra of the complexes with the different geometries are drastically different, the spectra are a sensitive means of following changes in the co-ordination as the environment changes. The geometry of the cobalt(I1) complexes in the sol precursor is primarily octahedral.The absorption spectra show intensit- ies higher than that of Co(H20)Ef. When the gel is dried at 60 "C, the geometry remains octahedral. However, at tempera- tures between 200 and 600 "C, the metal ion adopts a lower co-ordination number and the spectrum shows three strong absorption bands. Analysis of the spectrum showed that it does not consist merely of a mixture of octahedral and tetrahedral species. It was suggested that the species at high temperatures might be a distorted five-co-ordinate complex. The detailed and intricate spectroscopic changes that occur were not simple to interpret, and the assignment of the bands was not definite.The studies provide an interesting method of following the gel-glass process through relatively high temperature regimes. They also indicate that the analysis of optical spectra of metal complexes in the inorganic matrix will be a fascinating area of research. Probes of Matrix Rigidity Rigidochromism as a Probe of Gelation, Ageing and Drying2' The rigidochromic molecule bipyridyltriscarbonylchloro-rhenium(I), ReCl(CO),bipy, is a probe of the changes in J. MATER. CHEM., 1991, VOL. 1 = 340 400 500 600 340 400 500 340 400 500 600 340 400 500 600 I/nm I/nm Fig. 3 Luminescence spectra of silicate gels synthesized at different pH doped with coumarin 4 mol dm-3).'8 The spectra are shown for aged (a) and dried (d) gels.pH: (a)0.2, (b) 1, (c)2.5, (d) 3.8 rigidity of sols and gels prepared from TEOS and diisobutoxy- aluminotriethoxysilane (OBu),AlOSi(OEt), (or ASE). Rigi- dochromism, defined as the changes in the energy of the 600 luminescence maximum as a function of the rigidity of the surrounding medium,26 was measured in fluid and solid (frozen) reference solvents to calibrate the magnitude of the 1580 .:-2.09):effect. Above the freezing point of the solution, the emission maximum is 605nm. As the temperature is decreased, the I--g 560 .-emission maximum is virtually unchanged down to a tempera- ture of ca. 180 K.A sudden blue shift is observed in the region of the freezing point of the mixture. The band maximum in tn .-5 540 -1 .o the frozen solution is ca. 530 nm. The changes were then used to probe the rigidity of the two systems during gelation and drying. These studies revealed large differences between the two systems. Probing the Aluminosilicate Sol and Gel The band maximum of the emission of ReCl(CO),bipy in ASE exhibits large shifts at various stages of the gelation and drying processes. The shifts of the position of the band maximum and changes in the weight of the sample as a function of time are shown in Fig.4. For convenience, the processing time is plotted on a logarithmic scale. The emission lo-' 1oo 10' 1o2 1o3 tl h Fig. 4 Wavelength of the emission maxima of ReCl(CO),bipy in the aluminosilicate system as a function of processing time; 0, the concomitant weight changes2' them related to a specific sequential step of the sol-gel transformation: gelation, ageing and drying.First, during the gelation stage, the most drastic chemical and physical changes occur. Interestingly, the ReCl(CO),bipy probe molecule is band in the liquid state just after the mixing of the components /, is broad and centred around 600 nm (v = 16 660 cm- ',Avl insensitive to these changes and the emission peak maximum = remains constant at 590 nm. This observation indicates that although the system is macroscopically solid, the rigido- 2500 cm-'). No change is observed at the gelation point, although the solution has turned to a solid amorphous material which does not flow when the container is tilted.After a long period of ageing in a closed container, the maximum emission is blue-shifted to 560 nm (v = 17 860 cm- ', Avl/,=365Ocm-'). At the end of the drying, the emission peak has shifted to 523 nm (v= 19 120 cm-') and has a width of ca. 3580 cm-'. The small change in the FWHM indicates that the position of the band maximum has uniformly shifted and that the effect is not caused by the growth of a new band with a concomitant decline of the original band. The evolution of the ReCl(CO),bipy luminescence shown in Fig.4 can be separated into three distinct parts, each of chromic molecules are unconstrained in a solvent-like environment. The second step of the process, the ageing period, takes place in a sealed vessel.Although the gel is kept in a closed container and no evaporation of the organic molecules occurs, the luminescence maximum continuously shifts to the blue. At the end of the ageing period, the emission maximum stabilizes at 560 nm, intermediate between the value observed in a completely fluid medium, 600 nm, and that in a completely solid medium, 525 nm. The blue shift during this period is caused by partial rigidification of the gel on the molecular level. The rigidochromic probe is not simply surrounded by the interstitial liquid phase as it was during the gelation stage, but now it becomes entrapped in the oxide polymer network.The third stage of the process, which is related to the final changes of the emission of ReCl(CO),bipy, is the drying of the gel. When the solvents are removed, the gel structure collapses and the gel shrinks continuously. The final volume is about one eighth of that of the aged gel. As indicated by the blue shift of the emission of ReCl(CO),bipy, this step is accompanied by a progressive and eventually complete rigid- ification of the matrix. The final emission wavelength is the same as that in frozen ethanol solution. Both the increase in surface energy due to modification of the interface between the oxide polymer and the liquid and the capillary forces of the solvents provide the driving force for this partial densification.New condensation reactions occur because some reactive groups are in closer proximity. As a result, the oxide network becomes more compact. Its flexibility drastically decreases with the departure of the solvating species. The ReCl(CO), bipy molecules, which were previously entrapped in the network, are now completely immobilized by this collapsing structure. Probing the TEOS Sol and Gel The same procedure is followed for the preparation of silica sonogels containing the rigidochromic probe. However, the TEOS system doped with ReCl(CO),bipy exhibits a very different behaviour. From the initial liquid state to almost the end of drying, the emission maximum remains fixed at ca. 600nm. A blue shift occurs only with the final removal of the alcohol.The contrasting behaviour of the silica gel indicates that there are significant structural differences between the two kinds of investigated systems and that ReCl(CO),bipy is a sensitive probe of this difference. In the silica gels, the con- stancy of the emission at 590 nm indicates that the probe molecules are in a non-rigid environment during gelation, ageing and a major period of the drying. The fact that this behaviour is different from that of aluminosilicate gels is not surprising. Indeed, the chemistry of the precursors, the mech- anisms of polymerization and, therefore, the structure of the gels are different.'Y2 Moreover, the characteristics of the pore walls including polarity and electric charge are expected to be substantially different.This may change the interactions of the probe with the oxide network and consequently its position inside the gel structure. The adsorption of the probe on the surface of a pore once the water and ethanol are almost completely evaporated may account for the evolution of the fluorescence towards the characteristic blue shift of the rigidified state. Fluorescence Depolarization Fluorescence depolarization of the probe molecules DPH (1,6- diphenyl-hexa-l,3,5triene)and DPHPC (2,3-diphenylhexa- trienepropanoyl-3-palmitoyl-~-or-phosphatidylcholine)was used to study the rotational diffusion of the large probes during gelati~n.~~ The method is based on the idea that rotational motion of the probe during its emission lifetime changes the direction of the intrinsic polarization. If the molecule is held rigidly, the emission of these probes is polarized. If the molecule rapidly tumbles, emission from many orientations is observed and the polarization is scrambled.The depolarization provides a relative measure of the microscopic viscosity if the probe interacts with its environment. Several parameters define the degree of polarization of the emitted light.28 The most relevant parameter is the polariz- J. MATER. CHEM., 1991, VOL. 1 ation anisotropy, r The anisotropy can have maximum and minimum values of 0.4 and 0.0 for immobile and mobile chromophores, respect- ively. The mobility of a chromophore in isotropic media affects the polarization anisotropy as given by the relationship ro/r= 1+6Rz, where r is the experimentally determined ani- sotropy of the chromophore in the medium, ro is the exper- imentally determined limiting anisotropy of the chromophore in a rigid matrix, z is the emission lifetime of the chromophore in the medium and R is the rotational relaxation rate.The latter term is a measure of the rate at which the chromophore reorients between the time it absorbs and emits the photon. The reorientation rate is related to the viscosity of the medium. Ideally (4) where kB is Boltzmann's constant, T is the absolute tempera- ture, V is the molar volume of the chromophore and q is the viscosity of the medium. A given luminescent probe can be calibrated and thus used to probe the viscosity of a gel.Several factors may prevent the exact numerical values of the local viscosity from being obtained, but relative changes in the viscosity can be readily determined. The results of the study indicated that the fluorescence polarization signal of DPH is highly depolarized and that the amount is invariant during gelation and ageing. In contrast, the much larger DPHPC molecule detects the onset of gelation by an increase in polarization. It was concluded that DPH does not detect the growth of the polymer chains resulting in gelation because residual solvent-filled pores are sufficiently large to permit rapid tumbling and complete depolarization. On the other hand, the larger DPHPC molecule detects polymer growth but still experiences substantial rotational motion after gelation.We have carried out studies with MNPH [1-(4-nitrophenyl)-6-phenylhexa- 1,3,5-triene] doped in silica gels.29 No changes in polarization anisotropy (r) were observed while the gel samples, in sealed containers, proceeded through the stages of gelation and ageing. However, once solvent removal was initiated, progressive changes in r were observed as shown in Fig. 5. It is interesting to note that during the time that the viscosity increased by 10-fold, sample shrinkage was ca. 15%. These results clearly demonstrate the feasibility of using fluorescence polarization techniques to detect microviscosity in the gel. 0 200 400 600 tlh Fig. 5 Changes in polarization anisotropy of MNPH during sol-gel processing.The sample was opened for drying after CQ. 250 h J. MATER. CHEM., 1991, VOL. 1 Photochromic Gels and Glasses Photochromism is defined as the reversible light-induced colour change caused by absorption of a photon.30 The photochromic molecules which have been doped into sol-gel glasses undergo large structural changes after absorption of a photon. The rates of both the light-driven structural changes and of the back reactions are sensitive to the environment in the matrix. Aluminosilicate Gels Three derivatives of the photochromic 1',3',3'-trimethylspiro-[2H-1-benzopyran-2,2'-indoline]molecule,abbreviated BIPS, were studied in our lab~ratories.~' All three showed reversible photochromism in aluminosilicate gels.Of these, PNM-BIPS, l'-phenyl-6-nitro-8-methoxyBIPS was the most stable in the gels and gel-glasses and was studied in the most detail. Freshly prepared gels containing PNM-BIPS were clear and highly photochromic. Ageing of the gels did not significantly change the photochromic properties. Photo-chromic transparent solids of dimensions of 1 cm x 1 cm x 1 cm were easily prepared and studied. Transient absorption spectra of gels containing PNM-BIPS were measured. The visible spectrum is dominated by a band growing in at 590 nm when the sample is irradiated. Upon blocking the irradiation the band disappears over several seconds to leave the net zero transient absorption seen before the pump beam was unblocked. The total disappearance of the transient absorption band shows that the photochromism is reversible in the gel.The reversibility exists in gels 3 months old. The rate at which the transient absorbing species is formed and subsequently reverts back to the starting material was monitored near the maximum of the transient absorption spectrum at 632.8 nm by using an He-Ne laser. Upon exci- tation with a mercury lamp at wavelengths shorter than 4000A, the absorbance at 632.8 nm rapidly rises and then levels off as the irradiation continues. When the excitation ceases, the absorption gradually decreases. A plot of the transmittance at 632.8 nm of a PNM-BIPS gel is shown in Fig. 6. The plot shows the baseline transmittance before irradiation (A), the increase in the absorbance when the irradiation begins (A-B), and the decrease in the absorbance after the irradiation is stopped.(B-C). The figure shows the results for a 7 day old gel. A plot of the natural log of the absorbance as a function of time after the excitation ceased (point C) is linear. During the initial gelation of the aluminosilicate, no change in the rates of the rise or decay of the 16 950 cm-' absorption 0.6 0.5 8 0.4 f 0.3 v)a 0.2 0.1 0.0 0 20 40 60 80 100 120 tls Fig.6 Absorbance at 632.8 nm for an ASE gel containing PNM- BIPS. The excitation of the sample began at point A and continued through part B to part C at which time the excitation ceased. The decay of the absorption of the transient follows point C3' caused by the photointermediate is observed.As the gels age over a period of ca. 100 days, the lifetimes of the photointerme- diates slightly increase but do not undergo major changes. The solid gels are fully photochromic, i.e. the ring-opening reaction is not a sensitive probe of the polymerization and ageing processes. In contrast, a rigidochromic probe of the microscopic rigidity of the medium clearly indicated that the oxide skeleton becomes increasingly rigid during ageing.25 During drying, no change in rate constant is evident until some 70% weight loss has occurred at which point the rate increases. One explanation for the increase is that the BIPS molecule interacts with the wall of the pore. This interaction could then inhibit the ring opening. Another possibility is that the effective viscosity of the solvent increases as the fraction of solvent phase in the pores becomes small and just a thin film of liquid covers the pore wall.As previous studies have indicated, a viscosity increase can lead to an increase in the rate ~onstant.~' The increase in the lifetime of the inter- mediate during drying provides a sensitive probe of the changes occurring during the drying process. The rigido- chromic probe is also sensitive to the internal changes occur- ring during drying.25 It is interesting to note that drying to a weight loss of ca. 75% causes both the photochromism to cease and the rigidochromic emission to level out at its maximum energy.Silicon Dioxide and Modijied Silicon Dioxides Photochromic BIPS molecules have been incorporated in Si02 and various chemically modified Si02 matrices.32 The properties of the unmodified Si02 xerogels were similar to those of the aluminosilicate xerogels: the photochromism stopped and these materials cannot be used for practical applications. However, BIPS molecules retained their activity even at the dry stage when incorporated in modified silica xerogels prepared from Si(OC2H,)3CH2CH3 and from the copolymerization of TMOS with polydimethylsiloxanes under mildly basic conditions. In the case of the former xerogel, normal photochromism, i.e. a change from colourless to coloured upon irradiation, was observed. In the latter case, materials with reversed photochromism (coloured to colour- less) were obtained.The difference between the normal and the reversed photochromism was explained in terms of the differences in the properties of the pores in which the BIPS molecule is trapped. In the case of the normal photochromism, the pore has an apolar surface composed of Si-CH2CH3 groups which do not stabilize the coloured form, whereas in the case of the reversed photochromism, the pore surface consists of Si-OH groups and is similar to that of SO2 glasses. Photochromic glasses have several important uses depending on the rates of the transformations. For example, if the transformations are fast, the photochromic glasses can be used as optical switches. Light of one colour causes the change in the absorption spectrum of the glass which, in turn, transmits or blocks light of a second colour.If the transform- ations are very slow (on the order of years), then the photo- chromic glasses can be used as optical data storage media. Light of one colour is used to 'write' on the glass and light of a second colour is used to 'read' the stored information. In addition to these applications, photochromic glass can be used to form energy-conserving coatings, eye-protection glasses and privacy shields. The ability to prepare doped sol- gels in thin film form suggests that these materials may play an important role in the emergence of photochromic appli- cations. Excimers of Pyrene Chemical and physical changes that occur during the sol- gel-xerogel transitions have been studied by using pyrene as a photophysical pr~be.~~,~~ The luminescence spectrum of this molecule exhibits vibronic structure.The ratio of the intensities of the first to fifth peaks in the spectrum (Z1/Z5) is sensitive to the polarity of the environment. The ratio increases with increase in polarity. In addition, pyrene forms excimers, i.e. complexes between one pyrene in its lowest excited state and a second pyrene in its ground state. The intensity of the excimer emission increases with increase in the concentration of the molecule and with trapping in small pores. Both of these changes facilitate the intermolecular complexation. Changes in the emission spectra of pyrene and its excimer have been used to probe the sol-gel-xerogel transitions in TEOS and TMOS systems.In the case of TMOS gels, two general patterns of behaviour were observed depending on the water to silane ratio. At low ratios, geometric irregularity and porosity build up gradually along the whole process, resulting first in an increase in excimer emission intensity as aggregation becomes more pronounced, followed by a decrease in excimer intensity (but an increase in the monomer band) as pyrene molecules become isolated. At high water to silane ratios, a smooth surface was formed. These results suggest that polymerization-gelation occurs at low water to silane ratios whereas formation of a colloid followed by its gelation occurs at high water to silane ratios.Ethoxy groups slow down the polymerization in comparison with methoxy groups, but the reaction rates coincide once all of the alkoxy groups are hydrolysed. Thus, the gel-xerogel transitions in both TEOS and TMOS systems proceed at the same rate. The pyrene excimer disappears at the final xerogel state, proving that the sol-gel process is an effective method for isolating organic molecules. The changes in the polarity of the pyrene environment along the sol-gel process, as reflected by the Zl/Z5 ratio are due to pyrene aggregation and not due to the support itself. The fluorescence of pyrene probe molecules has also been used to follow the formation of micelles in sol-gel glasses.35 The 11/Z3ratio was different in the sol, gel and xerogel stages.When the concentration of sodium dodecylsulphate, the micelle former, was high, the ratio had values similar to those of solutions of micelles. This result suggested that micelles are trapped in the sol-gel glasses. Optical Properties of Doped Sol-Gel Materials Another significant feature of the sol-gel process is that it is possible to induce new optical properties based upon the deliberately chosen dopant. For example, the addition of laser dyes leads to a new solid-state gain medium while the addition of soluble organic polymers and molecules produces non-linear optical effects. In the following section we review progress in areas where the optical properties of sol-gel are already giving evidence of applications.Sol-Gel Lasers There has long been an interest in fabricating solid-state gain media containing organic laser dyes. Organic laser dyes possess several advantages over their inorganic counterparts including much larger cross-sections for absorption and emis- sion and lower threshold powers for laser action. Their gain characteristics and tunability in the visible spectrum are also quite attractive for device applications. Despite the advantages offered by solid-stage gain media, attempts to incorporate organic dye materials in polymeric hosts have met with limited success. In general, polymeric hosts have not exhibited the necessary thermal and mechanical properties, photostability or refractive index uniformity to become successful laser hosts. In contrast, inorganic glasses offer superior optical, thermal J.MATER. CHEM., 1991, VOL. 1 and chemical properties. The problem here is that the organic dyes cannot withstand the thermal conditions posed by the processing temperatures of conventional glasses. The sol-gel technique offers a low-temperature method for synthesizing amorphous materials which are essentially inor- ganic. Both the alkoxide precursors and the common laser dye families (coumarin, xanthenes, oxazines) are soluble in the same solvents, ensuring that the dyes could be incorpor- ated in the matrix without undesirable aggregation effects. A particularly attractive feature of these materials is that by providing the dye with an inorganic environment, increased photostability is possible.Research activities during the past 3 years have clearly established that tunable solid-state lasers based on sol-gel glasses can be achieved. A variety of laser dyes doped in different sol-gel matrices have demonstrated laser action. To date, different rhodamine, coumarin and perylene dyes have lased in a variety of matrices including silica, alumina, aluminosilicate, organic-modified silicates (ORMOSILs) and sol-gel composites.'8~36-4' A summary of solid-state sol-gel lasers, their dyes and wavelengths over which lasing occurs is provided in Table 2. The majority of the sol-gel laser studies have emphasized the demonstration of laser action. Specific laser characteristics have been explored in relatively few systems and only initial results have been reported.Tunability has been demonstrated in gels doped with sulphorhodamine B (SRB)39 and ~erylene.~~ The threshold for lasing has been reported for these systems and is in the range of 1-10 pJ, although one must be aware of the pump source for each of these measurements. Slope efficiencies show substantial variation among the systems. In the case of rhodamine-doped sol-gel materials, it is evident that slope efficiencies are substantially dependent upon the matrix. Values range from 2% for R6G in alumina films37 to ca. 25% for R6G in aluminosilicate xerogels41 to 30% in R6G/ORMOSIL.43 These values for R6G are still less than the 40% reported for aged silica gels.44 In the case of SRB, the slope efficiency for SRB/ORMOSIL was 39%,43 which is nearly twice as large as that reported for SRB in dried siica ge1.39 Photostability represents one of the most important issues for sol-gel laser materials.Photodegradation under laser action provides a rigorous assessment of this behaviour. R6G in alumina films exhibited a 50% output reduction in laser intensity within 600 pulses when pumped at 1 MW cm-2.37 R6G in ASE monoliths pumped at 1 mJ per pulse reached the 50% level in 1500 pulses (I Hz repetition rate) and continued to exhibit laser action after 40000 pulses.41 No specific results were reported for SRB/silica; however, there was an indication that photostability was relatively poor as thermal effects at low pumping rates (10 Hz) led to decreased output power.39 The photostability of dye-doped ORMOSILs have been studied in greater detai1.43*45 Recent work has shown that these materials possess substantially longer life- times and operate at much higher repetition rates than the other sol-gel systems or polymer hosts.Rhodamine 6G and SRB doped ORMOSILs reach the 50% laser output level after 1 1 000 and 14 500 pulses, respectively, when pumped at 95 pJ (8 kW)at 30 Hz. The future prospects for tunable lasers based on dye-doped sol-gel materials are extremely promising. The number of potential dyes and the versatility of the sol-gel process to achieve a favourable chemical environment for the dyes suggests that solid-state tunable lasers can be fabricated throughout the visible spectrum and into the near infrared.The recent photostability results with ORMOSILs have greatly surpassed the laser lifetimes obtained with polymer hosts and these characteri'stics are likely to improve by using J. MATER. CHEM., 1991, VOL. 1 91 1 Table 2 Summary of sol-gel laser systems oscillating range (peak)/ matrix coumarin 1 coumarin 102 coumarin 153 rhodamine B sulphorhodamine 640 rhodamine 6G coumarin 153 fluorescein BASF 241 (perylene derivative) rhodamine 6G rhodamine 6G coumarin 153 rhodamine 6G sulphorhodamine B rhodamine B nile blue ~~~~ ~ ~~~~~~ SiO," SiO," SiO," SiO, SiO, ASE ASE ASE MMA-impregnated SiO, ORMOSIL (EP/MA) ORMOSIL (EP)ORMOSIL (MA)ORMOSIL (MA)ORMOSIL (MA)ORMOSIL (MA)ORMOSIL (MA) nm 433-457 (444) 487-495 (491) 545-572 (558) 612-620 (618) 605-647 (621) 561-580 (569) 545-565 (555) 545-559 (552) 568-583 (575) 557-598 (568) 559-587 (571) 498-574 (525) 562-590 (570) 600-625 (609) 593-607 (598) 680-746 (696) pump source (Ajnm) ref.XeCl excimer (308 nm) 18 XeCl excimer (308 nm) 18 XeCl excimer (308 nm) 18 bXeCl excimer (308 nm)Dbl Nd :YAG (532 nm) 39 dye laser (540 nm) 41 dye laser (460 nm) 47 dye laser (5 1 1 nm) 47 Dbl Nd: YAG (532 nm) 42 dye laser (540 nm) 36 dye laser (540 nm) 36 dye laser (460 nm) 36 Dbl Nd :YAG (532 nm) 43 Dbl Nd: YAG (532 nm) 45 Dbl Nd:YAG (532 nm) 45 Dbl Nd: YAG (532 nm) 45 " Aged Gels.ASE =aluminosilicate. MMA =methyl methacrylate. MA =methacrylate. EP =glycidoxy. E. T. Knobbe, B. Dunn, P. Fuqua, F. Nishida and J. I. Zink, in Ultrastructure Processing of Ceramics, ed. D. R. Uhlmann, M. Weinberg and S. Risbud, Wiley, New York, in the press. dyes with enhanced photostability and optimizing both the synthesis techniques and matrix composition. Third-Order Non-linear Optical Materials There is a vital need for new materials whose x(2)and f3) (second- and third-order non-linear susceptibilities) can be used to produce the various non-linear functions necessary for such operations as optical information processing and optical data storage. In general, the desired non-linear material must not only possess large x(2)and f3) but also exhibit good chemical and mechanical stability, low optical loss, a high optical damage threshold and be fabricable into useful forms such as fibres and films.Since the non-linear optical properties of organics are frequently the highest known, the application of the sol-gel method to synthesize non-linear optical (NLO) materials is quite promising. In initial reports, the chemical flexibility of the sol-gel approach has enabled researchers to incorporate specific constituents (conjugated polymers, organic molecules) which provide third-order non-linear susceptibility within an essen- tially inorganic matrix designed to provide superior stability, optical properties and fabrication capability. The high degree of 7c conjugation found in conducting polymers makes them attractive candidates for third-order NLO materials because of the extended network of delocalized .n electrons.Although conjugated polymers have demonstrated large f3 ) coefficients, they generally exhibit poor processing characteristics and lack environmental stability. Two different methods have been used to incorporate conducting polymers into sol-gel matrices. Prasad and co-workers have successfully synthesized silica gel/conjugated polymer composites containing up to 50% (by weight) They used a water-soluble precursor of the conjugated polymer, poly(pphenyleneviny1- ene) (PPV), mixing it with the silica gel precursor and a common solvent. When it was heated above 140 "C the PPV precursor was converted to form the conjugated polymeric structure.High optical quality as-cast films were synthesized and a number of optical waveguide studies have been per- formed. At 1.06 pm, f3)z3 x lo-'' esu was measured by the degenerate four-wave mixing (DFWM) technique. Not only is this value comparable to those of conducting polymers, but the non-linear response time is in the subpicosecond range. Prasad also reported the fabrication of two-dimensional grat- ings using the silica/PPV films and demonstrated the feasibil- ity of using these films for optical recording.46 Another sol-gel approach has been used to synthesize polyaniline-doped silica gels.47 Polyaniline (or PANi) was selected because it can be made highly soluble in a variety of solvents.The PANi guest was incorporated into a silica sol host by dissolving the polymer into a mutually compatible solvent, N-methylpyrrolidone (NMP). This characteristic was used to prepare silica gel monoliths containing PANi or 2-ethyl PANi.48 The resulting material contained substantially less of the conjugated polymer (ca. mol dmP3) than the films obtained by polymerizing PPV. Despite the low concen- tration, DFWM measurements indicated that f3) for the material at 1.06 pm was ca. 30% of the value of a CS2 reference sample (i.e. x(3) x5 x 10-l3 esu). Since nanosecond pulses were used in these DFWM experiments, it is not yet established whether the f3) response is electronic or if there is a thermal contribution. Another method for producing NLO sol-gel materials has been to add organic molecules that exhibit large non-linear effects. The properties of the saturable absorber dyes fluor- escein, acridine orange and acridine yellow doped in sol-gel silica matrices were in~estigated.~~.~' The short luminescence lifetimes measured with these materials, however, led to sub- stantially larger saturation intensities than those of the dyes contained in low melting glasses.It was proposed that the presence of interconnected pores in the sol-gel matrix enabled oxygen quenching of the triplet state to occur. The use of 'composite' sol-gels, where the porous matrix is impregnated by an organic polymer thus eliminating the porosity, seems quite promising as long lifetimes were measured and low saturation intensities were calculated.Optically Based Chemical Sensors The overall requirements for a selective ion or molecular sensor are an indicator molecule that reversibly interacts with a target molecule with a high degree of specificity and that has some detectable property that changes when the target molecule has interacted. One of the most familiar types of chemical sensors are acid-base pH indicators that change colour when the target molecule (hydronium or hydroxyl ions) interacts with the indicator. Optically based sensors are especially convenient and accurate because changes in the absorbance or luminescence of the medium containing the sensor can be readily measured or even visually detected.Sol-gel glasses are almost ideal host matrices for optical chemical sensors. They have the obvious advantages of chemi-cal inertness, mechanical stability and optical transparency. More importantly, they are able to encapsulate the indicator molecule physically in pores in the glass such that these molecules are immobilized and cannot be leached out, while at the same time they are porous enough to allow transport of metal ions, solvent and other small molecules into the interior of the glass. The glasses can be fabricated into desired shapes and sizes such as monolithic blocks or thin films for specific applications. Several recent observations revealed the feasibility of mak- ing sol-gel glass based optical sensors. During our studies of pyranine as a probe of chemical changes occurring during gelation and drying, we discovered that luminescence of the pyranine probe molecule was sensitive to the surrounding en~ir0nment.l~Gas and liquid diffusion in the xerogel was very rapid owing to the interconnecting pores which represent approximately half of the gel volume.Avnir and co-workers found that the luminescence of molecular dopants in sol-gel glasses could be quenched by molecules and gases surrounding the glass.” Reisfeld” reported that oxazine- 170 doped in sol- gel glass films exhibited significantly different absorption and emission spectra depending upon whether the films were immersed in water or aqueous ammonia. The exciting out- growth of these observations is the feasibility of making glasses having optical properties which change in the presence of target molecules, i.e.of making sol-gel glass chemical sensors. The simplest chemical sensors which have been demon- strated to date are pH sensors because the response to the target ion is an easily recognisable colour change. Thymoph- thaleine,52 phen~lphthaleine’~ and phenol reds3 have been doped into TEOS-based gel-glasses. When the glass is immersed in appropriate acid-base solutions, the glasses change colour at the same pH values as do the indicators in solutions of the same pH. Response times are typically fast, on the order of 1 s for the phthaleines. Films having thick- nesses of the order of several thousand 8, have been coated on glass substrate^.'^ The films doped with acid-base indi-cators reversibly respond to pH in <1 s.Gel-glasses have also been made which respond to the presence of metal ions. All of the sensors reported to date have been studied in a very preliminary fashion, and details about the reversibility and response times are generally lack- ing. In some cases, colorimetric reagents such as dimethyl- glyoxime (for Ni2 +) and o-phenanthroline (for Fe2 +) have been incorporated in silica glasses and have been shown to produce the characteristic colour when exposed to the metal (red for both Ni2+ and Fe2+).52 In other cases, a metal ion (e.g. iron) has been trapped and the glass has been exposed to solutions containing hexacyanoferrate to produce intensely coloured Prussian blue.53 Luminescence of Molecules Luminescence has played a major role in the development of sol-gel optical materials.When researchers recognised the possibilities of incorporating organic molecules in sol-gel matrices, luminescent dyes were the first organics to demon- strate that the synthesis of these organic-doped inorganic materials was feasible. Dye-doped sol-gels represent the most widely studied systems. This paper has given several examples of how luminescent dyes are used as probes of both the sol-gel transformation and the accompanying chemical and matrix rigidity changes. The luminescence of dyes has J. MATER. CHEM., 1991, VOL. 1 also been highlighted in the development of specific optical properties including laser action, optical gain, chemical sensing and non-linear optical effects.It should be noted that many other organic molecules have been incorporated in sol-gel matrices and that sol-gel optical materials exhibit lumi-nescence throughout the visible regi~n.”.’~ One interesting aspect of the dye-doped sol-gel materials is the opportunity to use this approach for matrix-isolation purposes and to investigate the fundamental spectroscopy of the molecule in the sol-gel environment. Several of these studies have appeared and subsequently been used in the development of laser and non-linear optical proper tie^.'^*^^,'^ A molecule of interest is the tris(2,2’-bipyridyl)ruthenium(rr) cation. Ru(bpy)i+ has been widely studied because of its application in photochemical conversion for solar energy.The spectroscopic properties of Ru(bpy)g + in sol-gel were com- pared to those of the molecule in low-temperature glasses and in water.” Although the absorption spectra are similar for all the solids, the emission characteristics are quite different. The molecules in sol-gel materials exhibit lower lifetimes and quantum yields than the other glasses, but higher values than those in water. The interpretation is that the porous nature of the sol-gel matrix (50~01%) permits much more oxygen quenching and freedom of motion of the Ru(bpy)g + complex than occurs in the dense glasses. As discussed previously, the saturable absorber dyes were affected by similar microstruc- ture considerations. Another important luminescence property which is likely to generate more interest in the future is energy transfer.The sol-gel approach provides an opportunity to investigate dye- dye energy transfer in solid matrices. In recent work energy transfer from R6G to rhodamine B in alumina films was detected and enhanced laser emission from rhodamine B (N2 pumped) was observed.” Another type of energy transfer that has been demonstrated involved that between a dye and an inorganic ion. Genet et al. synthesized ThPO, gels using coumarin 460 as sensitizer and Eu3+ and Tb3+ ions as activators.60*61Excitation spectroscopy was used to verify the existence of energy transfer. In the case of the coumarin/Eu3 + samples, the maximum Eu3 + emission was observed using 0.1% of coumarin.Summary and Conclusions The ability to use the sol-gel approach to produce organic- doped gels and glasses is now well established. By using appropriate precursors and solvents, researchers have demon- strated that a wide variety of organic and organometallic molecules can be incorporated into sol-gel matrices. The work to date has, in general, evolved towards two different objectives. One direction has been to use the organic dopant as a luminescent probe of the sol-gel process. In this role, the probe molecules furnish substantial insight regarding local sol-gel chemistry and structure. The information obtained by the technique is in a dimensional regime between the molecu- lar-level resolution offered by NMR and the macroscopic- level results obtained by thermal and gravimetric methods.Moreover, the ability to use solution chemistry as a reference point for interpreting the spectroscopic response of the probe molecules in sols and gels provides a sound scientific basis for this work. The second direction of research, that of the deliberate doping of organics in sol-gels, is clearly emerging as an important means of developing new photonic materials. It is now evident that dye molecules, photochromic molecules and even conducting polymers are able to retain their specific optical properties in sol-gel matrices. Within a relatively short period of time several significant results have been achieved J. MATER.CHEM., 1991, VOL. 1 913 in the areas of tunable solid-state lasers, third-order suscepti- bility and photochromic behaviour. A new generation of optically based chemical sensors based on the tailoring of gel porosity is likely to be the source of the next dramatic results in this area. 28 29 30 31 M. Shinitzky and Y. Barenholz, Biochem. Biophys. Acta, 1978, 515, 367. M. Cox, B. Dunn, J. I. Zink, manuscript in preparation. R. C. Bertelson, in Techniques of Chemistry, Vol.ZZZ: Photochro-mism, ed. G. H. Brown, Wiley-Interscience, New York, 1971 and ch. VII, p. 100. D. Preston, T. Novinson, W. C. Kaska, B. Dunn and J. I. Zink, The authors are grateful for the support of this work by the National Science Foundation (DMR 9003080). Additional support has been furnished by Lockheed Corp.under the UC MICRO program. We greatly appreciate the many contri- butions of our students and colleagues: J. Altman, P. Fuqua, R. Kaner, W. Kaska, E. Knobbe, J. McKiernan, W. Nie, F. Nishida, S. Parveneh, J. C. Pouxviel, D. Preston, 32 33 34 35 36 J. Phys. Chem., 1990,94, 4167. D. Levy and D. Avnir, J. Phys. Chem., 1988,92, 4737; D. Levy, S. Einhorn and D. Avnir, J. Non-Cryst. Solids, 1989, 113, 137. D. Brusilovsky and R. Reisfeld, Chem. Phys. Lett., 1987, 14, 119. V. R. Kaufman and D. Avnir, Langmuir, 1986, 2, 717. K. Matsui, T. Nakazawa and H. Morisaki, J. Phys. Chem., 1991, 95, 976. E. T. Knobbe, B. Dunn, P. D. Fuqua and F. Nishida, Appl. Op., 1990,29,2729. 0.Stafsudd and S. Yamanaka. 37 Y. Kobayashi, Y. Kurokawa and Y. Imai, J.Non-Cryst. Solids, 1988, 105, 198. 38 B. Dunn, E. Knobbe, J. McKiernan, J. C. Pouxviel and J. I. Zink, References 39 Mater. Res. SOC. Symp. Proc., 1988, 121, 331. F. Salin, G. LeSaux, P. Georges, A. Brun, C. Bagnall and 1 2 3 4 L. L. Hench and J. K. West, Chem. Rev., 1990, 90, 33. C. J. Brinker and G. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, San Diego, 1989. Sol-Gel Technology, ed. L. C. Klein, Noyes Publications, Park Ridge, NJ, 1988. L. C. Klein, Annu. Rev. Mater. Sci., 1985, 15, 227. 40 41 42 43 J. Zarzycki, Opt. Lett., 1989, 14, 785. R. Reisfeld, D. Brusilovsky, M. Eyal, E. Miron, Z. Burstein and J. Ivri, Chem. Phys. Lett., 1989, 160, 43. J. M. McKiernan, S. A. Yamanaka, B. Dunn and J.I. Zink, J. Phys. Chem., 1990, 94, 5652. R. Reisfeld, D. Brusilovsky, M. Eyal, E. Miron, Z. Burstein and J. Ivri, Proc. SPZE, 1989, 1182, 230. J. C. Altman, R. E. Stone, B. Dunn and F. Nishida, Photonics 5 6 7 W. G. Klemperer and S. D. Ramamurthi, in Mater. Res. SOC. Symp., 1988, 121. C. J. Brinker, J. Non-Cryst. Solids, 1988, 100, 31. C. J. Brinker and G. W. Scherer, J. Non-Cryst. Solids, 1985, 70, 301. 44 45 Tech. Lett., 1991, 3, 189. G. B. Altshuler, V. A. Bakhanov, E. G. Dulneva, A. V. Erofeev, O.V. Mazurin, G.P. Roskova and T.S. Tsekhomskaya, Opt. Spectrosc., 1988, 62, 709. B. Dunn, F. Nishida, J. C. Altman and R. E. Stone, in Ultrastruc- 8 9 10 11 12 13 14 15 16 17 18 G. W. Scherer, J. Non-Cryst. Solids, 1986, 87, 199. G. W. Scherer, J. Non-Cryst.Solids, 1987, 92, 122. G. W. Scherer, J. Non-Cryst. Solids, 1988,99, 324; G. W. Scherer, J. Non-Cryst. Solids, 1988, 100, 77. G. W. Scherer, J. Non-Cryst. Solids, 1989, 107, 135. G. W. Scherer, J. Non-Cryst. Solids, 1987, 89, 217. N. R. Clement and M. Gould, Biochemistry, 1981, 20, 1534; K. Kano and J. H. Fendler, Biochim. Biophys. Acta, 1978, 509, 289. J. C. Pouxviel, B. Dunn and J. 1. Zink, J. Phys. Chem., 1989, 93, 2 134. V. R. Kaufman, D. Avnir, D. Pines-Rojanski and D. Huppert, J. Non-Cryst. Solids, 1988, 99, 379. R. Reisfeld, J. Non-Cryst. Solids, 1990, 121, 254. D. Levy, R. Resifeld and D. Avnir, Chem. Phys. Lett., 1984, 109, 593. J. M. McKiernan, S. A. Yamanaka, E. T. Knobbe, J. C. Pouxviel, S. Parveneh, B. Dunn and J. I. Zink, J. Znorg. Organomet. Polym., 1991, 1, 87.46 47 48 49 50 51 52 53 54 ture Processing of Ceramics, ed. L. L. Hench and J. K. West, Wiley, New York, in the press. P. N. Prasad, Mater. Res. SOC. Symp. Proc., 1990, 180, 741. J. 1. Zink, B. Dunn, R. B. Kaner, E. T. Knobbe and J. McKiernan, in Materials for Nonlinear Optics-Chemical Perspectives, ed. S. R. Marder, J. E. Sohn and G. D. Stucky, American Chemical Society, Washington, D.C., 1991, ch. 36, pp. 541-552. F. Nishida, B. Dunn, E. T. Knobbe, P. D. Fuqua, R. B. Kaner and B. R. Mattes, Mater. Res. SOC. Symp. Proc., 1990, 180, 747. R. Reisfeld and C. K. Jorgensen, in Struct. Bonding (‘Berlin), 1991, in the press. R. Reisfeld, J. Non-Cryst. Solids, 1990, 121, 254. A. Slama-Schwok, D. Avnir and M. Ottolenghi, J. Phys. Chem., 1989,93, 7544. R. Zusman, C. Rottman, M. Ottolenghi and D. Avnir, J. Non- Cryst. Solids, 1990, 122, 107. J. I. Zink, B. Dunn and S. Yamanaka, unpublished results. D. Avnir, D. Levy and R. Reisfeld, J. Phys. Chem., 1984, 88, 5956. 19 20 K. H. Drexhage, Topics in Applied Physics, Vol. I Dye Laser, ed. F. P. Schaefer, Springer, Berlin, 1975. G. J. Yakatan, R. J. Juneau and S. G. Schulman, Anal. Chem., 1972, 44, 1044. 55 56 D. Avnir, V. Kaufman and R. Reisfeld, J. Non-Cryst. Solids, 1985, 74, 395. Y. Kobayashi, Y. Imai and Y. Kurokawa, J. Mater. Sci. Lett., 1988, 7, 1148. 21 S. G. Schulman and L. S. Rosenberg, J. Phys. Chem., 1979, 83, 4. 57 R. Reisfeld, M. Eyal and R. Gvishi, Chem. Phys. Lett., 1987, 138, 377. 22 23 24 A. Bergman and J. Jortner, J. Lumin., 1973, 6, 390. R. K. Iler, The Chemistry of Silica, Wiley, New York, 1979. R. Reisfeld, V. Chernyak, M. Eyal, C. K. Jorgensen, Chem. Phys. Lett., 1989, 164, 307. 58 59 R. Reisfeld, R. Zusman, Y. Cohenand and M. Eyal, Chem. Phys. Lett., 1988, 147, 142. H. Sasaki, Y. Kobayashi, S. Muto and Y.Kurokawa, J. Am. Ceram. SOC., 1990, 73, 453. 25 26 27 J. McKiernan, J. C. Pouxviel, B. Dunn and J. I. Zink, J. Phys. Chem., 1989, 93, 2129. M. S. Wrighton and D. L. Morse, J. Am. Chem. SOC., 1974, 96, 998; P. J. Giordano and M. S. Wrighton, J. Am. Chem. SOC., 1979, 101, 2888. R. Winter, D. W. Hua, X. Song, W. Mantulin and J. Jonas, J. 60 61 M. Genet, V. Brandel, M-P. Lahalle and E. Simoni, C.R. Acad. Sci. Paris, 1990, 311, 1321. M. Genet, V. Brandel, M. D. Lahalle and E. Simoni, in Proc. SPZE Con5 on Sol-Gel Optics, ed. J. D. Mackenzie and D. R. Ulrich, SPIE, Bellingham, WA, 1990, vol. 1328, pp. 194-200. Phys. Chem., 1990,94, 2706. Paper 1/03546K; Received 12th July, 1991