Langmuir 1993,9, 3320-3323
3320
Characteristics of the Fluorescence Spectra of Pyrene Molecules during the Sol to Gel to Xerogel Transitions of Silica-Titania Binary Oxide Systems N. Negishi? T. Fujii? and M. AnPo”+
392 nm
c
392 nm
Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 593, Japan, and Department of Chemistry and Material Engineering, Faculty of Engineering, Shinshu University, Wakasato, Nagano 380, Japan Xerogel
Received December 21, 1992. In Final Form: July 6, 1993
Introduction Studies of the photophysical and photochemical behavior of organic moleculesdoped stably into the glasslike silica matrices by room temperature polymerization of alkoxysilane,*i.e., prepared by the so-called “sol-gel method”,have recently been given a great deal of attention due to various promising applications of this method in designing photosensitive materials such as optical nonlinearity materials,l as well as for photochromism2and photoenergy conversion3and as photo catalyst^.^ In addition, the importanceof the sol-gelmethod is its potential capability to incorporate guest organic molecules into the stable inorganic host matrices with no risk of thermal decomp~sition.~ Although pure silica gel is a poor catalyst for acidcatalyzed reactions because the surface SiOH groups have a weak acidic strength,6 silica-titania is one of the important acid-base catalysts, and its surface structure has been discussed from the standpoint of its surface acidityand its role in catalysis. Silica-titania binary oxides are also known as effective photo catalyst^.^ However, the detailed chemicalnature of the surface is still not clarified. On the other hand, it is well known that the fluorescent probe moleculessuch as pyrene easily reflect the chemical nature of their environmentsin their fluorescencespectra. However,most of the studies using pyrene as a fluorescent probe were carried out in the pure systems in/on SiO2, Si-A1 binary system, or A1203.8 Therefore, it is of special interest to study not only the surface characteristics of the silica-titania prepared by the sol-gel method but also the deference in the surface nature of these silica-titania binary oxides and pure silica by measuring the characteristics of the fluorescence of pyrene doped in the pure Si02 and Si-Ti binary oxide systems. In addition, it is also of special interest to make an effort to clarify + University of Osaka Prefecture.
Shinshu University. (1)(a) Nakamura, M.; Nasu, H.; Kamiya, K. J.Non-Cryst. Solids 1991, 135,l. (b) Devlin, K.; O’Kelly, B.; Tang, Z. R.; McDonagh, J. F. J . NonCryst. Solids 1991,135,8. (c) Kojima, K.; Taguchi, H.; Matsuda, J. J . Phys. Chem. 1991,95,7595. (2)McKiernan, J.; Pouxviel, J. C.; Dunn, B.; Zink, J. I. J . Phys. Chem. 1989,93,2129. (3)Slama-Schwok, A.; Avinr, D.; Ottolenghi, M. J. Am. Chem. SOC. 1991,113,3984. (4)(a) Negishi, N.; Matsuoka, M.; Yamashita, H.; Anpo, M. J . Phy? Chem. 1993,97,5211.(b) Matsui, K.; Sasaki, K.; Takahashi, N. Langmurr 1991,7, 2866. (5)Fujii, T.; Kawauchi, 0.; Kurihara, Y.; Ishii, A.; Negishi, N.; Anpo, M. Chem. Express 1990,5,917. (6) (a) Kaufman, V. R.; Avnir, D. Langmuir 1986, 2, 717. (b) Pelmenschikov, A. G.; Morosi, G.; Gamba, A. J. Phys. Chem. 1992,96, 7422. (7)(a) Walther, K. L.; Wokaun, A.; Handy, B. E.; Baiker, A. J. NonCryst. Solids 1991, 134, 47. (b) Anpo, M.; Nakaya, H.; Kodama, s.; Kubokawa, Y.; Domen, K.; Onishi, T. J . Phys. Chem. 1986,90,1633.(c) Shibata, K.; Kiyoura, T.; Kitagawa, J.; Sumiyoshi, T.; Tanabe, K. Bull. Chem. SOC.Jpn. 1973,46,2985.
Wavelength (nm)
‘
Figure 1. Fluorescence spectra of pyrene in the TEOS during t h e sol to gel to xerogel transition (excitation wavelength 372 nm, an arrow shows t h e Iv band).
chemically the gelation point in the sol-gel process by measuring the changes in the fluorescence and its excitation spectra of pyrene doped in the systems,because, in spite of the many studies done on the sol-gel systems, it has not been possible to define the gelation point chemically until In the present work, we have measured the characteristic changes in the fluorescence and fluorescence excitation spectra of pyrene and the ESR spectra of the pyrenedoped Si-Ti binary oxide systems by comparing the corresponding changes in pure Si02 systems.
Experimental Section Pyrene was purified by recrystallization from ethanol. Ethanol (spectroscopic grade), tetraethyl orthosilicate (TEOS, Si(OC&&,)4) and tetrabutyl orthotitanate (TBOT, Ti(OC4Hd4) (Tokyo-Kasei Co.) were used without further purification. TEOS and TBOT were mixed as follows: a mixture of TEOS and TBOT (concentration range of Si:Ti = 1000 to 1090) in ethanol was mixed with a pyrene-ethanolsolution with the concentration of pyrene at 1 X M. In this paper, we report the results obtained mainly with the systems of the concentration ratios of Si:Ti = 1oO:O and 95:5. T h e fluorescence and excitation spectra were recorded on a Shimadzu RF-5000 spectrofluorophotometer using the surface emission method at room temperature. The ESR spectra were observed by a JEOL RE-2X spectrometer (X-band) at 77 K. Infrared experiments were carried out at room temperature using a Shimadzu IR-460 spectrometer with the KBr pellet method. T h e gelation times of these samples were determined by losing fluidity in the cell, being 4 and 5 days for t h e SkTi = 1oO:O and 95:5 systems, respectively.
Results and Discussion Figure 1 shows a series of the fluorescence spectra of pyrene during the sol to gel to xerogel transitions of the (8) (a) Stiihlberg, J.; Almgren, M. Anal. Chem. 1985, 57, 817. (b) Lochmiiller, C. H.; Colborn, A. S.; Hunnicutt, M. L.; Harris, J. M. J. Am. Chem. SOC.1984, 106, 4077. (c) Avnir, D.; Busse, R.; Ottolenghi, M.; Wellner,E.; Zachariasse,K. A. Phys. Chem. 1985,89,3521.(d) SWberg, J.; Almgren, M.; Alsins, J. Anal. Chem. 1988,60,2487. (e) Bogar, R.G.; Thomas, J. C.; Callis, J. B. Anal. Chem. 1984,56, 1080. (0Fujii, T.; Murayama, K.; Negishi, N.; Anpo, M.; Winder, E. J.; Neu, D. R.; Ellis, A. B. Bull. Chem.SOC.Jpn. 1993,66,739.(g) Fujii, T.;Shimizu,E. Chem. Phys. Lett. 1987,137,448.(h) Fujii, T.; Shimizu,E.; Suzuki, S. J . Chem. SOC.,Faraday Trans. 1 1988,84,4387. (9)(a) Mackenzie,J. D. J . Non-Cryst. Solids 1988,100,162.(b) Chen, K. C.; Tsuchiya, T.; Mackenzie, J. D. Ibid. 1986,81,227. (c) Boonstra, A. H.; Baken, J. M. E. Ibid. 1990,122,171. (d) Brunet, F.; Cabane, B.; Dubois, M.; Perly, B. J . Phys. Chem. 1991,95,945.
0743-7463/93/ 2409-3320$04.0O/O 0 1993 American Chemical Society
Notes
Langmuir, Vol. 9,No. 11,1993 3321 392 nm
- Xerogel
ExcitationWavelength (nm) Magnitude of the 0-0 band shift (nm)
[AI
PI
Figure 2. (A) Fluorescence excitation spectra of pyrene in the TEOS during the sol to gel to xerogel transitions (emission band). (B) wavelength 470 nm, an arrow shows the (0) Magnitude of the (0) band shift in this system during the sol to gel to xerogel transitions.
pyrene-doped TEOS sample as a function of time. In Figure 1,it is clear that with the original TEOS sol having a pyrene (concentration of pyrene 1 X M/ethanol) both the pyrene excimer (peak position at around 470 nm) and the characteristicpyrene monomer fluorescence(peak position at 392 nm, Iv band of pyrene monomer) are also observed with high efficiencies.1° However, as shown in Figure 1, the fluorescence due to the pyrene excimer disappears and is unobservablejust after gelation begins, and the only fluorescencethat can be observed, at around 392 nm, is due to the pyrene monomer at the xerogel stage in the pure Si02 systems. Figure 2A shows the changes of the excitation spectra of pyrene during the sol to gel to xerogel transitions of the pyrene-doped TEOS samples as a function of the gelation time. In Figure 2B, the location of the (0-0)band of the excitation spectra of pyrene is plotted as a function of the gelation time. The excitation spectra of pyrene molecules in pure Si02 at 372 nm can be assigned to monomer absorption.8J0 As shown in Figure 2B, the position of the (0-0) band of the excitation spectra of pyrene monomer scarcely changes during the sol to gel to xerogel transitions and is almost constant at 372 nm. These results suggest that the excimer species are formed by collision of an excited-state pyrene with a ground-state pyrene within its lifetime in the sol. On the other hand, at the xerogel stage,the (0-0)band of the excitationspectra of the pyrene showsthe same monomericpyrene band at 372 nm. These results clearly indicate that, as mentioned above, at the xerogel stages in the pure Si02 system, pyrene molecules are separated by the repulsive forces between molecules and locate only as the monomeric pyrene on the surface of different pores.8 As shown in Figure 1,it is clear that in the TEOS systems the intensity of the fluorescence spectrum due to the pyrene excimer becomes weaker just after gelation begins and then disappears during the completion of the gelation. This phenomenon is well linked to the dispersion of the pyrene moleculesinto distorted (or fractal geometry)and (10) (a) Matsui, K.; Nakazawa, T.; Morisaki, H. J . Phys. Chem. 1991, 95,976. (b) Matsui, K.; Nakazawa, T. J . Phys. Chem. 1991,63,11. (c) Matsui, K. Langmuir 1992,8,673. (d) Avnir, D.; Kaufman, V.;Reisfeld, R. J . Non-Cryst. Solids 1985, 74,395.
350
400
450
500
550
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650
Wavelength (nm)
Figure 3. Fluorescencespectra of pyrene in the TEOSand TBOT mixed solutions during the sol to gel to xerogel transitions (excitation wavelength 372 nm, Si:Ti = 95:5).
smaller pores which are formed during the shrinkage of the systems. Avnir et al. and Brusilovsky et al. said that the repulsive forces cause local dispersion of the pyrene molecules,and since the pores become so smallthat pyrene separation inside one pore becomes impossible, the molecules migrate to adjacent pores, which continue to close and shrink.6aJ1 The observation of the excimer emission of pyrene in the concentrated solutionis explained in terms of the intermolecular reaction of a excited pyrene with a ground-state pyrene: Py + Py* (PyPy)*. With deepening of the shrinkage of the system, the number of pores increases with decreasingdiameter of each pore, resulting in the dopant molecules that had been trapped in the same pores being separated. This is the reason why the excimer emission is not observable at the xerogel stages of the pyrene-doped Si02 systems. Figure 3 shows a series of the fluorescence spectra of pyrene during the sol to gel to xerogel transitions of the pyrene-doped TEOS-TBOT mixture system as a function of the gelation time. As shown in Figure 1,it is clear that with the original mixture of TEOS-TBOT sol having a pyrene (concentration of pyrene 1X M/ethanol) both the characteristic fluorescencespectrum due to the pyrene monomer (peak position at 392 nm) and the fluorescence due to the pyrene excimer-likeemission (peak position at around 470 nm) are observedwith high efficiencies. Figure 3 also shows that not only the fluorescence due to the pyrene excimer-like emission but also the characteristic fluorescence spectrum due to the pyrene monomer is observable in the sol to gel transition and even at the complete gelation and xerogel stages. Figure 4A shows the changes of the fluorescence excitation spectra of pyrene during the sol to gel to xerogel transitions of the pyrene-doped TEOS-TBOT mixture system as a function of the gelation time. In Figure 4B, the location of the (0-0)band of the excitation spectra of pyrene is plotted as a function of the gelation time. It is seen that in the Si-Ti binary system the (0-0)band shifts from 372 and 376 nm, and this change occurs just on the gelation. The band located at 376 nm can be assigned to
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(11) (a) Brusilovsky, D.; Reisfeld, R. Chem. Phys. Lett. 1987,141,119. (b) Matsui, K.; Usuki, N. Bull. Chem. SOC.Jpn. 1990,63,3516.
Notes
3322 Langmuir, Vol. 9,No. 11, 1993
E l .
50G
Excitation Wavelength (nm)
Magnitude of the 0-0 band shift (nm)
14
[Bl
Figure 5. ESR spectrum of Ti3+observed after the gelation of the Si-Ti binary systems (ESRwas measured at the xerogel stage of the sample and observed at 77 K).
Figure 4. (A) Fluorescence excitation spectra of pyrene in the
TEOS and TBOT mixed solutions during the sol to gel to xerogel transitions (emissionwavelength 470 nm). (B)Magnitude of the (0-0) band shift in the system during the sol to gel to xerogel transitions.
the excitation spectrum of pyrene which originates the ground-state bimolecular association of pyrene.12J3 Comparingthe results obtained in the Si-Ti binary oxide systems with those obtained in the pure Si02 systems clearly shows that in the Si-Ti binary oxide systems not mly the characteristic fluorescence spectrum due to the pyrene monomer but also the fluorescence spectrum due to the pyrene excimer-like emission is observable in the sol to gel transition and even at the xerogel stages. Thus, the result clearly shows that the formation of the pyrene dimer begins just on the gelation and the excimer-like emission of pyrene is observed with a high efficiency even in the xerogel stages in the Si-Ti binary oxide systems. Being in agreementwith the results obtained by Y a m d et and Fujii et al.,12cthe position of the (0-0)band of the fluorescence spectrum of the pyrene dimer is found to be at around 376 nm (Figure 1B)while that of the pyrene monomer is at around 372 nm. These results clearly indicate that the formation mechanism of the pyrene excimer (or excimer-like emission),which is observed even after the gelation, is completely different from that in the liquid-sol system where the diffusion mechanism is predominant. Figure 5 shows the ESR spectrum observed in the Si-Ti binary systems during the gelation of the system. The signal w i t h g l = 1.9632 andgll= 1.9365 are assigned to the presence of the Ti3+ The shape and g values of this Ti3+ion being in good accordance with those of the Ti3+ anchored on the silica glass,14it is concluded that the Ti3+ ions in the Si-Ti binary oxides locate in the tetrahedral coordination atmosphere. Although the concentration of the Ti3+ ions in the sample is not determined yet, the direct observationof the Ti3+ions located in the tetrahedral coordination after the gelation begins suggests at least that the low-temperaturepolymerizationreaction proceeds to produce -Si-O-Ti- linkages (during the gelation) where some parts of the titanium ions are located in the al.12apb
(12)(a) Yamazaki,T.;Tamai,N.;Yamazaki,I. Chem.Phys.Lett. 1986, Chem. 1987, 124,326. (b)Yamazaki, I.;Tamai,N.;Yamazaki,T.J.Phys. 91,3572. (c) Fujii, T.;Ishii, A.; Suzuki,S.;Anpo, M. Chem. Express 1989, 8, 471. (13)(a) Bauer, R. K.; Mayo, P. D.; Ware, W. R.; Wu, K. C. J. Phys. Chem. 1982, 86, 3781. (b) Lochmcdler, C. H.; Wenzel, T. J. J. Phys. Chem. 1990,94,4230. (14)Anpo, M.;Shima, T.; Fujii, T.; Suzuki,S.;Che, M. Chem. Lett. 1987,1997.
1200
1000
800
600
Wavenumber/cm-l
Figure 6. Infrared absorption spectrum of the Si-Ti binary system after the gelation of the system.
unsaturated tetrahedral coordination. The ESR signal with 811 = 2.0061 is probably attributed to structural defects and/or carbon radicals, being formed during the sol-gel reactions,16 because the signals were not observed with the starting sample solution but observed only on the xerogel stage. The IR absorption spectrum observed in the Si-Ti binary systemsjust after the gelation of the system begins exhibited the characteristic IR band at 950 cm-I (Figure 61, which is often ascribed to the formation of the -Si+Ti- bond involving the tetrahedrally coordinated Ti ions. In addition, the IR absorption spectrum also exhibited the fundamental absorption bands at around 1070 and 780 cm-l which are assigned to the S i 4 and Si-0-Si stretching vibrations, respectively.l6 It was found that the changes in both the ESR spectrum due to the Ti3+ ions and the IR spectrum due to the formation of the Si-+Ti bond in the systems involving pyrene molecules were foundto be small as compared with those in the absence of pyrene molecules. These results clearly suggest that the adsorption interaction of pyrene molecules with the adsorption sites is not strong but weak, as expected from the fact that there is no actual chemical bonding between pyrene molecules and adsorption sites. As shown in Figure 4B, the drastic band shift in the (0-0)band of pyrene fluorescence spectra is well linked with the starting point of the formation of the Ti3+ adsorption sites which play a significant role as adsorption sites in the Si-Ti binary systems, and an astringency in the (0-0) band shifts to 376 nm seems to be related to the completion of the adsorption of pyrene molecules onto Ti3+ ions. However, as shown in Figure 3, the main emission band is attributed to the pyrene monomer, and the relative intensity of the excimer-like emission is not (15)Ueda,H.Nihon Kagakukai-shi (J.Chem.SOC.Jpn.) 1988,8,1194. (16)(a) Bittar, A.; Adnot, A.; Sayari, A.; Kaliaguine, S. Res. Chem. Zntermed. 1992,18,49. (b)Kamiya, K.; Sakka, S. Nihon Kagakukai-shi (J.Chem. SOC.Jpn.) 1981,10,1571.
Notes strong as compared with that of the pyrene monomer. As mentioned above, the number of Ti3+adsorption sites is far from abundant, and pyrene molecules tend to adsorb first selectivity on the Ti3+sites during the first stage, i.e., just after the gelation begins, though it is not clear at present whether pyrene molecules adsorbed on the Ti ions having a direct interaction between the Ti ion and pyrene or an indirect interaction having a HzO between the Ti ion and pyrene.l' When the adsorption of pyrene molecules onto the Ti3+ sites is completed, the remaining pyrene molecules are adsorbed on the SiOH sites as a pyrene monomer. This may be the reason why the relative intensity of the excimer is not strong as compared with that of the pyrene monomer. Thus, these results clearly suggest that in the Si-Ti binary systemsa couple of pyrene molecules are adsorbed on the same specific Ti sites and this situation may be closely associated with the observation of the pyrene dimer emission even after the complete gelation of the systems. This is not the case in the pure Si02 systems. The parallel changes in the wavelengths of the (0-0)band of the excitation spectra of the pyrene dimer with the extent of the gelation of the systems suggest a possibility of the (17) Chandrasekaran,K.;Thomas, J. K. J.Am. Chem. SOC.1983,105, 6383.
Langmuir, Vol. 9,No. 11,1993 3323
chemical definition of the gelation point by observation of the (0-0) band location of the fluorescence spectra of the probe dopant molecules in the systems. Conclusions The excimer-like emission of pyrene was observed even after the gelation of the systems began in contrast to that in the Si02 system where the pyrene excimer weakened and disappeared after gelation began. ESR and IR studies of the Si-Ti binary systems have revealed that after the gelation of the systems begins the -Si-O-Ti- linkages are formed and Ti ions form unsaturated sites in the tetrahedral coordination. Theseresults suggest that two pyrene molecules are weakly coordinated onto the Ti ions in the tetrahedral coordination having a direct and/or indirect interaction between the Ti ion and pyrene. This situation in the Si-Ti binary systems allows the formation of the pyrene dimer even after gelation begins to be retained without the separation of pyrene molecules by the shrinkage of the systems, though such a separation of the dopant molecules is well known in pure Si02 systems.
Acknowledgment. This work was supported in part by Grants-in-Aid (No. 02805099 and No. 03650659) from the Ministry of Education, Science and Culture of Japan.
