Characteristics of the Phosphorescence Spectra of Benzophenone

Characteristics of the Phosphorescence Spectra of. Benzophenone Adsorbed on Ti-Al Binary Oxides. H. Nishiguchi, J.-L. Zhang,† and M. Anpo*. Departme...
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Characteristics of the Phosphorescence Spectra of Benzophenone Adsorbed on Ti-Al Binary Oxides H. Nishiguchi, J.-L. Zhang,† and M. Anpo* Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan Received October 4, 2000. In Final Form: February 2, 2001 The excited states of benzophenone adsorbed on well-characterized Ti-Al binary oxides were investigated by means of conventional molecular spectroscopic techniques such as UV and IR absorption as well as photoluminescence spectra measurements. It was found that the photochemical and photophysical properties of benzophenone adsorbed on the oxide surfaces strongly depend on their surface properties, which are systematically changed by changing the composition of the oxides. Specifically, the phosphorescence spectra of benzophenone adsorbed on the oxides have been found to be composed of the hydrogen-bonded benzophenone species and the protonated benzophenone species.

Introduction The last two decades have seen a growing interest in the study of the photophysical and photochemical behavior of adsorbed molecules, which is observed to be greatly different from the behaviors of molecules in the gas phase and in solution. However, studies of the photophysics and photochemistry of adsorbed layers or in heterogeneous systems have still been one of the most unexplored fields as compared with those of homogeneous systems. Therefore, in recent years, many investigations on the photophysics of adsorbed molecules have focused on discovering what effect the surface has on the behavior of adsorbed molecules and on examining the nature of the surface itself.1-7 The photochemical and photophysical properties of adsorbed molecules are entirely dependent upon the host-guest locations and the host-guest interactions in the local molecular environment. On the other hand, the general characteristics of the photolysis of adsorbed molecules have also been of great interest, that is, how the reactivities of the excited states or the radical species themselves vary when they are formed on the solid surfaces.8-16 More recently, studies of the photophysics and photochemistry of molecules adsorbed on inert adsorbents such * To whom correspondence should be addressed. E-mail: [email protected]. †Permanent address: Institute of Fine Chemicals, East China University of Science and Technology, Shaghai 200237, P. R. China. (1) Anpo, M.; Yamashita, H. In Surface Photochemistry; Anpo, M., Ed.; John Wiley & Sons: Chichester, U.K., 1996; p 117. (2) Anpo, M.; Nishiguchi, H.; Fujii, T. Res. Chem. Intermed. 1990, 13, 73. (3) Zhang, J.; Yamashita, H.; Anpo, M. Chem. Lett. 1997, 1027. (4) Leermakers, P. A.; Thomas, H. T.; Weis, L. D.; James, F. C. J. Am. Chem. Soc. 1966, 88, 5075. (5) Zhang, J.; Matsuoka, M.; Yamashita, H.; Anpo, M. Langmuir 1999, 15, 77. (6) Mao, Y.; Thomas, J. K. J. Phys. Chem. 1995, 99, 2048. (7) Wilkinson, F.; Willsher, C. J. Tetrahedron 1987, 43, 1197. (8) Kubokawa, Y.; Anpo, M. J. Phys. Chem. 1974, 78, 2442. (9) Anpo, M.; Kubokawa, Y. J. Phys. Chem. 1974, 78, 2446. (10) Kubokawa, Y.; Anpo, M. J. Phys. Chem. 1975, 79, 2225. (11) Anpo, M.; Wada, T.; Kubokawa, Y. Bull. Chem. Soc. Jpn. 1975, 48, 2663. (12) Anpo, M.; Kubokawa, Y. Bull. Chem. Soc. Jpn. 1976, 49, 2623. (13) Anpo, M.; Wada, T.; Kubokawa, Y. Bull. Chem. Soc. Jpn. 1977, 50, 31. (14) Anpo, M. Chem. Lett. 1987, 1221. (15) Anpo, M.; Yamamoto, Y.; Suzuki, S. Chem. Lett. 1989. 1339. (16) de Mayo, P.; Nakamura, A.; Tsang, P. W. K.; Wong, S. K. J. Am. Chem. Soc. 1982, 104, 6824.

as SiO2 and zeolites received a great deal of attention from the standpoint of elucidating photochemical processes on solid surfaces, namely, “heterogeneous photochemistry”17-24 and/or “photochemistry in molecular assemblies”. Especially, the photochemistry of adsorbed molecules is one of the most attractive research fields for photochemists with regard to advances in the photochemical-vapor-deposition method to produce thin-layered electronic materials and new types of anchored catalysts.22 Developments in these fields are closely associated not only with the development of surface science but also with the development in photochemical techniques involving laser technologies and internal reflection spectroscopies.1,5,24 For example, it has been shown that diffuse reflectance laser flash photolysis is a powerful experimental technique27-29 for opaque and optically high scattering materials such as organic microcrystals,28 dyed fabrics,30 molecules adsorbed on silica gels31-34 or transparent substrates,35 and so forth, so that this method is being successfully applied to the detection of the excited species at the interfaces and in various crystalline systems. (17) Johnston, L. L.; Wong, S. K. J. Am. Chem. Soc. 1984, 106, 1999. (18) Kazanis, S.; Azarani, A.; Johnston, L. J. J. Phys. Chem. 1991, 95, 4430. (19) Vieira Ferreira, L. F.; Netto-Ferreira, J. C.; Khmelinskii, I. V.; Garcia, A. R.; Costa, S. M. B. Langmuir 1995, 11, 231. (20) Mao, Y.; Thomas, J. K. J. Phys. Chem. 1995, 99, 2048. (21) Weis, L. D.; Evance, T. R.; Leermakers, P. A. J. Am. Chem. Soc. 1968, 90, 6109. (22) Anpo, M.; Sunamoto, M.; Che, M. J. Phys. Chem. 1989, 93, 1187. (23) Okamoto, S.; Nishiguchi, H.; Anpo, M. Chem. Lett. 1992, 1009. (24) Masuhara, H.; Ikeda, N. Hyomen 1989, 27, 203. (25) Wilkinson, F.; Willsher, C. J. Tetrahedron 1987, 43, 1209. (26) Wilkinson, F.; Willsher, C. J. Appl. Spectrosc. 1984, 38, 897. (27) Wilkinson, F. J. Chem. Soc., Faraday Trans. 2 1986, 82, 2073. (28) Wilkinson, E.; Willsher, C. J. J. Chem. Soc., Chem. Commun. 1985, 142. (29) Wilkinson, F.; Willsher, C. J.; Casal, H. L.; Johnston, L. J.; Scaiano, J. C. Can. J. Chem. 1986, 64, 539. (30) Turro, N. J.; Zimmt, M. B.; Gould, I. R. J. Am. Chem. Soc. 1985, 107, 5826. (31) Turro, N. J.; Gould, I. R.; Zimmt, M. B.; Cheng, C. C. Chem. Phys. Lett. 1985 119, 484. (32) Drakr, J. M.; Levitz, P.; Turro, N. J.; Nitsche, K. S.; Cassidy, K. F. J. Phys. Chem. 1988, 92, 4680. (33) Kessler, R. W.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1981, 77, 309. (34) Ferin, V. A.; Shvets, V. A.; Kazanskii, V. B. Kinet. Katal. 1979, 19, 1041. (35) Lin, C. T.; Hsu, W. L.; Yang, C. L.; El-Sayed, M. A. J. Phys. Chem. 1987, 91, 4556.

10.1021/la001402n CCC: $20.00 © 2001 American Chemical Society Published on Web 05/30/2001

Benzophenone Adsorbed on Ti-Al Binary Oxides

On the other hand, it can be expected that absorption and photoluminescence spectra measurements (fluorescence and phosphorescence) on adsorbed molecules provide useful information on the physical and chemical nature of the adsorption sites or surfaces. Various host-guest systems offer interesting areas of research, and spectroscopic techniques have been successfully used to study such systems.36-39 It is known that various binary oxide catalysts often exhibit higher catalytic activity and selectivity than what can be predicted from the properties of their components.38,39 Therefore, it is of special interest to study the relationship between the structure of the binary oxides and the photophysical and photochemical behavior of the molecules adsorbed. In addition to this, with binary oxides, it is possible to obtain detailed information on these relationships by changing the composition of the binary oxides continuously and systematically from 0 to 100%.38,39 However, few studies have been performed on binary oxides from this point of view. In the present investigation, the characteristics of the excited states of benzophenone adsorbed on well-characterized Ti-Al binary oxides are investigated by means of dynamic photoluminescence spectroscopy along with conventional techniques such as UV absorption and IR absorption analyses, since benzophenone is a good candidate to act as “molecular probe” for the study of the acidic properties of solid surfaces.17,18,23,34 Experimental Section 1. Preparation of Ti-Al Binary Oxides. Ti-Al binary oxides with different atomic ratios of Ti to Al were prepared by coprecipitation of desired amounts of a mixed aqueous solution of TiCl4 and AlCl3 (0.5 M concentration) by addition of an aqueous solution of ammonia as the precipitation reagent. For precipitation, aqueous ammonia was added to the solution, which was kept cool in an ice bath. The final pH of the solution was 9.5. The precipitates were then filtered, washed, dried, and calcined at 773 K in air in order to convert to binary oxides before further treatments.40 The binary oxides were degassed at 290 K for 1 h, heated at 773 K under 20 Torr O2 for 1 h, and then degassed at about 773 K and finally cooled to 290 K. The ratio of the composition of TiO2 and Al2O3 of the binary oxides is defined as Ti/Al. 2. Adsorption of Benzophenone. Prior to the adsorption of benzophenone, the samples were evacuated to about 10-6 Torr at 473 K to purify the surfaces. Adsorption of benzophenone (BP) was carried out at 290 K from both the CCl4 solution of BP and the BP vapor. The amount of adsorbed BP was about 1 × 10-6 mol/g. The phosphorescence spectra of BP were recorded at 77 and 290 K with a Shimadzu RF-501 spectrofluorophotometer, and their lifetimes were recorded by a streak scope using Hamamatsu C-4334. The excitation source was a N2 laser. The FT-IR spectra of BP adsorbed on the binary oxides were recorded at 295 K with a Shimadzu FTIR-8500 spectrophotometer. The absorption spectra were recorded at 295 K with a Shimadzu UV-2200A UV-vis recording spectrophotometer.

Results and Discussion 1. Phosphorescence Properties of Benzophenone Adsorbed on SiO2. The UV absorption spectra of BP onto microcrystalline cellulose have been reported in (36) Itoh, T.; Baba, H.; Takemura, T. Bull. Chem. Soc. Jpn. 1978, 51, 2841. (37) Rusakowics, R.; Byers, G. W.; Leemarkers, P. A. J. Am. Chem. Soc. 1971, 93, 3263. (38) Kodama, S.; Nakaya, H.; Anpo, M.; Kubokawa., Y. Bull. Chem. Soc. Jpn. 1985, 58, 3645. (39) Anpo, M.; Nakaya, H.; Kodama, S.; Kubokawa, Y.; Domen, K.; Onishi, T. J. Phys. Chem. 1986, 90, 1633. (40) Anpo, M.; Kawamura, T.; Kodama, S.; Maruya, K.; Onishi, T. J. Phys. Chem. 1988, 92, 438.

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Figure 1. Phosphorescence spectra of BP in CCl4 solution and adsorbed on PVG (amount of adsorbed BP: 2 × 10-6 mol/g) at 77 K. (Excitation wavelength: 1, 280 nm; 2, 300 nm; 3, 340 nm; 4, 360 nm.) 5, in CCl4 solution (1 × 10-5 M).

previous literature.19 Their absorption peaks are located at around 250 and 350 nm. They can be assigned to the n-π* orbital transition of BP maximizing at ∼350 nm and also the π-π* orbital transition, which occurs at shorter wavelength and peaks at ∼250 nm. Upon excitation of BP in the (n, π*) band, the phosphorescence spectrum due to a radiative transition from the excited triplet state to the ground state is observed at around 400-500 nm.41-43 Figure 1 shows the phosphorescence spectra of BP in the CCl4 solution and BP adsorbed on porous Vycor glass at a low surface coverage (2 × 10-6 mol/g) at 77 K. In the nonpolar solution of CCl4 (10-2 M), the phosphorescence spectrum of BP at 77 K exhibits wellstructured vibrational fine structures due to the CdO groups of BP. (Figure 1 (5)). The peak positions did not change in the solution system (maximum peak positions, λmax ) 410, 445, and 475 nm) at different excitation wavelengths. The decay of the phosphorescence of BP in the CCl4 solution was analyzed by a single exponential, and the lifetime was determined to be 4.5 ms at 77 K. The lifetimes of BP in the CCl4 solution scarcely changed by changing with the BP concentration from 10-4 to 1.0 M. In the case of ethanol solution, the peak positions were observed at shorter wavelengths (λmax ) 405, 435, and 465 nm) as compared with those in nonpolar solvents. The lifetime of BP in the ethanol solution was determined to be 5.5 ms at 77 K. Figure 1 also shows the phosphorescence spectrum of BP adsorbed on porous Vycor glass at a low surface coverage (2 × 10-6 mol/g) at 77 K. The peak position of the phosphorescence spectrum of BP adsorbed on Vycor glass was observed at around 435 nm, being independent of the excitation wavelength. It was observed to shift to shorter wavelength regions as compared with those in nonpolar solvents. The maximum peak of the excitation spectrum was observed at around 330 nm. When BP molecules are adsorbed onto the surface of oxides without any strong acidic sites on their surfaces such as SiO2 and Vycor glass (PVG, major composition: SiO2 97% and B2O3 3%), they adsorb on the surface OH groups only through hydrogenbonding and in such systems their phosphorescence peaks (41) Nakayama, T.; Sakurai, K.; Hamanoue, K. J. Chem. Soc., Faraday Trans. 1991, 87, 1509. (42) Hsu, W. L.; Lin, C. T. J. Phys. Chem. 1990, 94, 3780. (43) de Mayo, P.; Johnston, L. J. Preparative Chemistry Using Supported Reagents; Academic Press: New York, 1987; p 61.

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Figure 2. IR absorption spectra of BP adsorbed on PVG: 1, 0 mol/g; 2, 5 × 10-5 mol/g; 3, 7.3 × 10-5 mol/g.

were found to be observed at around 430 nm with its excitation peak at around 330 nm. 2. Adsorbed States of Benzophenone on PVG. On the excitation of BP in the (n, π*) band in both the gas phase and various matrixes, the phosphorescence spectrum of BP is observed at around 450 nm. This is attributed to the radiative decay from the excited triplet state to the ground state. The orbital participating in a BP molecule has a (n, π*) character, leading to an appreciable dependence of the phosphorescence spectrum on the acidic properties of the media. It is well-known from the studies of the absorption spectra of BP in solutions that solvents such as ethanol and other polar protic solvents promote a blue shift in the (n, π*) absorption of the CdO groups of BP,17 due to the fact that the oxygen of the CdO is more strongly hydrogen bonded in the ground state than in the excited state. The energy of the ground state is, therefore, lowered more by the hydrogen-bonding interaction with solvent than in the excited state. From the results of the absorption and phosphorescence spectra of BP adsorbed on PVG, it is clear that BP exhibits a large blue shift in the (n, π*) absorption band due to the hydrogen bonding between the CdO group of BP and the surface OH groups as compared to those in the CCl4 solution, as shown in Figure 1. Figure 2 shows the IR absorption spectra of BP adsorbed on PVG. The appearance of a broad IR absorption spectrum observed at around 3400 cm-1 and the decrease in the absorption bands at 3750 and 3703 cm-1 due to Si-OH and B-OH groups, respectively,1,8,14 support the fact that the BP molecules adsorbed mainly on the surface OH groups by hydrogen bonding. Such a blue shift (in the maximum of the phosphorescence spectrum) in the absorption spectra of BP to shorter wavelengths could also be observed in polar solutions. These blue shifts were also reported for alkyl ketones such as acetone and 2-pentanone adsorbed on PVG through the hydrogen bonding between the CdO group and surface OH groups.1 When PVG was treated with H2O vapor after standard pretreatment, the phosphorescence spectra of BP adsorbed on the treated PVG scarcely exhibited any changes in the spectra and only the intensity of the phosphorescence increased markedly. These similarities also support the concept that BP molecules adsorb mainly on the surface OH groups of PVG through hydrogen bonding. The decay curve of the phosphorescence of BP adsorbed on PVG was found to be analyzed by biexponential factors with different lifetimes of τ1 ) 70 ms and τ2 ) 6 ms at 77 K. Identifying whether the decay of the phosphorescence spectrum of the BP adsorbed can be analyzed by a single

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Figure 3. Phosphorescence decay profile of BP adsorbed on Al2O3 oxide: excitation wavelength, 337 nm; curve a, observed decay; a-1 and a-2, deconvoluted decay curves. Table 1. Determined Lifetimes of BP Adsorbed on the Ti-Al Binary Oxides oxide TiO2(0)/Al2O3(100) TiO2(3)/Al2O3(97) TiO2(25)/Al2O3(75) TiO2(50)/Al2O3(50) TiO2(75)/Al2O3(25) TiO2(94)/Al2O3(6) TiO2(100)/Al2O3(0) PVG in CCl4

lifetimes (ms) at 77 K t1 ) 390 t1 ) 450 t1 ) 410 t1 ) 300 t1 ) 190

t2 ) 49 t2 ) 24 t2 ) 20

t1 ) 70 t1 ) 4.5

t2 ) 6

exponential or a multiexponential is not easy and quite complicated due to the heterogeneity of the adsorption sites.43 In the present case, however, these two components of the decay could well be attributable to the excited triplet BP adsorbed on the surface Si-OH and B-OH groups by hydrogen bonding, respectively. Table 1 also shows the phosphorescence lifetimes of the BP molecules adsorbed on the Ti-Al binary oxides together with the lifetime determined in the CCl4 solution. The phosphorescence lifetime of BP in CCl4 solution was analyzed by a single exponential, while that of BP adsorbed on the Ti-Al binary was analyzed by biexponential factors, as can be seen in Figure 3. The observed fluctuation in Table 1 is caused by differences in the chemical heterogeneity of the local molecular environment of the surfaces of each sample, which might be introduced by the variation in the sample preparation leading to the different concentrations of the surface OH groups. It is known that alkyl ketones adsorbed on PVG exhibit two different lifetimes, being attributed to the molecules adsorbed on the surface Si-OH groups and B-OH groups, respectively, with different adsorption strengths due to the different acidities of these surface hydroxyl groups.44 It can, therefore, be seen by the IR results that the excited triplet states of the adsorbed BP with two different lifetimes may be associated with the BP molecules adsorbed on the surface Si-OH and B-OH groups, respectively.45 3. Phosphorescence Properties of Benzophenone Adsorbed on TiO2, Al2O3, and Ti-Al Binary Oxides. Figure 4 shows a typical phosphorescence spectrum of BP adsorbed on Ti-Al binary oxides with different atomic ratios of Ti/Al and on PVG.46 As shown in Figure 4, the phosphorescence spectra of BP adsorbed on the Ti-Al (44) Sullivan, M. O.; Testa, A. C. J. Phys. Chem. 1970, 92, 258. (45) Van Duuren, B. L. Chem. Rev. 1963, 63, 325. (46) Nishiguchi, H.; Zhang, J.-L.; Anpo, M.; Masuhara, H. Submitted to J. Phys. Chem.

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Figure 4. Phosphorescence spectra of BP in various systems: 1, adsorbed on Ti-Al oxide (Ti/Al ) 3:97); 2, adsorbed on the Ti-Al oxide (Ti/Al ) 50:50); 3, adsorbed on PVG; 4, in CCl4 solution; excitation, 340 nm, recorded at 77 K.

Figure 6. Effect of the Ti-Al mole ratio on the phosphorescence intensity of BP adsorbed on Al2O3 and Ti-Al binary oxides (BP: 2 × 10-5 mol/g): 1, Al2O3; 2, Ti-Al 3:97; 3, Ti-Al 25:75; 4, Ti-Al 50:50; 5, Ti-Al 75:25; 6, Ti-Al 94:6.

Figure 5. Effect of the excitation wavelength on the phosphorescence spectra of BP adsorbed on the Ti-Al oxide (Ti/Al ) 3:97) at 77 K. (Excitation wavelength: 1, 280 nm; 2, 300 nm; 3, 340 nm; 4, 360 nm).

results obtained by XRD, UV reflectance, and XPS40 measurements clearly indicated that the crystallinity of the TiO2 species in the oxides decreases and the dispersion state of the TiO2 species is increased by the addition of alumina to TiO2. Especially, the results obtained by XPS measurements suggested that there is a significant enrichment of the Al3+ ions in the surface layer at lower percentage of alumina. Conversely, a steady enrichment of the surface Ti4+ ions with an increase in the percentage of alumina could also be seen in the higher content regions of alumina.34 The maximum in the yield of the phosphorescence spectra of BP adsorbed on Ti-Al binary oxides changed as the excitation wavelength varied from 280 to 360 nm, as shown in Figure 6. The vibrational fine structures in the phosphorescence spectrum due to the stretching of the CdO bond of BP cannot be observed in the phosphorescence spectra of BP adsorbed on Ti-Al binary oxides. A nearly complete reversal of the excited-state energy levels occurs in certain molecules which are very sensitive to moderate changes in the solvent polarity and the strength of hydrogen bonding. Van Duuren concluded that the lowest (n, π*) state of the BP molecules in benzene was replaced by a nearby (π, π*) state in water.45 The triplet energy level reversal has also been reported for many aromatic carbonyl compounds such as butyrophenone47 and acetophenone48 in polar solvents and/or acidic solvents. Hamanoue et al. have reported that the phosphorescence spectrum of BP at 77 K in a mixed solvent of trifluoroethanol (TFE) (which are good hydrogen donors) and water is a dual phosphorescence spectrum. They have attributed the short-lived phosphorescence to the free triplet BP of (n, π*) character and the long-lived phosphorescence to the complex of the triplet state of BP/TFE/ water with a mixed character of (n, π*) and (π, π*).49,50 On the other hand, Oelkung et al. have observed a broad phosphorescence spectrum of BP adsorbed on Al2O3 at 77 K with significantly long lifetimes of 150-350 and 400-

binary oxides shift to longer wavelength regions; that is, a red-shift different from those on PVG as well as in nonpolar solvents can be observed. Figure 5 shows the effect of the excitation energies upon the peak positions of the phosphorescence spectrum of BP adsorbed on the binary oxide with a Ti/Al ratio of 97:3 (amount of adsorption: 2 × 10-5 mol/g) at 77 K. The phosphorescence spectrum exhibits no vibrational fine structures due to the CdO group, and a red-shift (max. 445 to 475 nm) can be seen only when the sample is excited by the longer excitation wavelengths (280-360 nm). The maximum peak of the excitation spectrum of BP adsorbed on Al2O3 was observed at around 340 nm. Thus, the phosphorescence spectrum did not exhibit vibrational fine structures and only a red-shift. At high Ti composition, the extent of the shifts in the phosphorescence of BP adsorbed became smaller (453-463 nm at a Ti/Al ratio of 25:75). Moreover, the position of the excitation peak changed from around 340 to 370 nm when the Ti mole ratio became larger than 50%. Table 1 summarizes the lifetimes of BP adsorbed on PVG, Al2O3, and Ti-Al binary oxides. 4. Adsorbed States of Benzophenone on Ti-Al Binary Oxides. In the case of Ti-Al binary oxides, the

(47) Rauh, R. D.; Leermakers, P. A. J. Am. Chem. Soc. 1968, 90, 2246. (48) Lamola, A. A. J. Chem. Phys. 1967, 47, 4810. (49) Hamanoue, K.; Kajiwara, Y.; Miyake, T.; Nakayama, T. Chem. Phys. Lett. 1983, 94, 276. (50) Nakayama, T.; Sakurai, K.; Ushida, K.; Kawatrura, K.; Hamanoue, K. Chem. Phys. Lett. 1989, 164, 557.

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Figure 7. Phosphorescence spectrum of BP adsorbed on the Ti-Al binary oxide (3:97) at 77 K (amount of BP: 2 × 10-5 mol/g) and corresponding deconvoluted curves (A and B) (Excitation wavelength: 300 nm).

Figure 8. Phosphorescence spectrum of BP adsorbed on the Ti-Al binary oxide (3:97) at 77 K (amount of BP: 2 × 10-5 mol/g) and corresponding deconvoluted curves (A and B) (Excitation wavelength: 360 nm).

700 ms.51 Upon adsorption, the electronic state of BP is perturbed by the interaction of the nonbonding electrons of the CdO with the proton-donors or electron-acceptors on the surface of oxides, its extent being dependent on the strength of the surface acidity of oxides. These results indicate that the energy levels of the 1,3(n, π*) transitions of BP systematically increase and the 1,3(π, π*) transitions relatively decrease, while an increase in the acidity of the surface sites results in a crossover of the (n, π*) phosphorescence into the (π, π*) phosphorescence. Such an unusual perturbation occurs in the adsorption state of BP on Al2O3 and Ti-Al binary oxides, and as a result, an inversion in the energy levels results in a complete loss of the vibrational fine structure due to the CdO stretching in the phosphorescence spectra of adsorbed BP as well as a considerable increase in the lifetime of the phosphorescence. It is thus clear that the phosphorescence spectra of BP adsorbed on the Ti-Al binary oxides involve a (π, π*) transition character. Moreover, the phosphorescence spectrum from these transitions has long lifetimes and unresolved vibrational fine structures. Figure 7 shows the effect of the Ti/Al mole ratio upon the phosphorescence spectra of the BP adsorbed on the Ti-Al binary oxides. It is clear from Figure 7 that increasing the composition of Ti in the oxides results in a decrease of the yield of the phosphorescence of BP. The phosphorescence spectrum of BP could not be observed on pure TiO2 (TiO2: 100%) systems. This may be explained by the concept that the energy transfer (and/or electron transfer) occurs from the excited triplet state of BP to the conduction band of TiO2.52 Such energy (or electron) transfer from the excited state of molecules adsorbed on semiconductors to their conduction bands has been established from the standpoint of solar energy conversion into electrochemical and/or chemical energy by using semiconducting materials. The existence of such energy transfer processes can be directly observed by using a diffuse reflectance laser flash photolysis technique which was applied in the present systems.53 In fact, the time-resolved triplet-triplet transient absorption spectra of BP adsorbed on Al2O3 were

observed at around 540 nm at 298 K. However, such a triplet-triplet transient absorption spectrum of BP adsorbed on pure TiO2 was not observed even at the first stage of the delay time. This suggests that the excited triplet state of BP or BP ketyl radicals is not formed on pure TiO2. In other words, the energy transfer from the excited triplet state of BP into the TiO2 semiconductor may occur faster than the nanosecond time scale of the measurement.52 Figure 8 shows the phosphorescence spectrum of BP adsorbed on the binary oxide with a Ti/Al ratio of 3:97 (excitation at 300 nm) at 77 K and the corresponding deconvoluted curves (A and B). From these results, it is clear that the phosphorescence spectrum consists of two different bands; one observed at around 445 nm (A) and the other at around 480 nm (B). The A band observed at around 445 nm can be attributed to the phosphorescence from the excited triplet state of the hydrogen bonded BP adsorbed on the surface OH groups. BP adsorbed on PVG or dissolved in ethanol solution by hydrogen bonding exhibited a phosphorescence spectrum at around 445 nm. By taking these results into account, the B band observed at around 480 nm can be attributed to the phosphorescence from the excited triplet state of the protonated BP (BPH+) species. The phosphorescence spectra of BP in a 98% H2SO4 solution,37,54 in CH3Cl solution saturated with HCl,34 and in CH3CN-water systems have been observed at around 480 nm, being attributed to the phosphorescence from the protonated BP molecules. The protonated BP molecules can be produced by a proton transfer from the Bro¨nsted acid sites on the oxide surfaces to the BP molecule. In the case of the protonated BP species in an acidic solution, the phosphorescence spectrum was observed at around 490 nm and its excitation spectrum at 380 nm,34,39 respectively. However, in the case of Ti-Al binary oxides, such protonation of BP molecules occurs in the excited state of BP but not in its ground state, since the acidities of the surface Bro¨nsted acid sites are not high enough to donate a proton to the BP in its ground state. It seems possible only in the excited state of BP. On Ti-Al binary oxides, when the TiO2 mole ratio is larger than 50%, the protonation of BP molecules becomes possible in its ground

(51) Gunther, R.; Hubner, P.; Lege, R.; Oelkung, D. Abstracts of the XII IUPAC Symposium on Photochemistry, Bologna, Italy, 1988; p 814. (52) Nishiguchi, H.; Zhang, J. L.; Anpo, M.; Masuhara, H. J. Phys. Chem., accepted. (53) Gopidas, K. R.; Kamat, P. V. J. Phys. Chem. 1989, 93, 6428.

(54) Ireland, J. F.; Wyatt, P. A. H. J. Chem. Soc., Faraday Trans. 1 1973, 161.

Benzophenone Adsorbed on Ti-Al Binary Oxides

state, since these Ti-Al binary oxides have strong Bro¨nsted acid sites. Figure 8 shows the phosphorescence spectrum at 77 K of BP adsorbed on the Ti-Al binary oxide with a Ti/Al ratio of 3:97 (excitation at 360 nm) and the corresponding deconvoluted curves (A and B). It can also be seen that the ratio of the concentrations of the excited triplet state of BP (A) to the protonated state of BP (B) decreases; that is, the concentration of A decreases and the concentration of B increases with changes in the excitation wavelength from 340 to 360 nm. In the case of other Ti-Al binary oxides (excitation at 340 nm, phosphorescence spectra of BP adsorbed on Ti-Al biary oxide with Ti/Al ratios of 25:75 and 50:50), the ratio of the yield of the excited triplet state of BP to that of the protonated BP increases with the Ti content. The phosphorescence of the protonated BP species was clearly observed when the Ti-Al binary oxide with a Ti/ Al ratio of 50:50 was used as an adsorbent. Furthermore, in this system, the protonation of BP became possible even in its ground state, since the excitation spectrum of the phosphorescence exhibited a maximum at around 370 nm on this oxide with a Ti/Al ratio of 50:50, suggesting the protonation of BP in its ground state to form the PH+ species. Upon the excitation at long wavelengths (280360 nm), the phosphorescence of the protonated BP became predominant in the observed phosphorescence spectrum and, as a result, the total phosphorescence spectrum shifted to longer wavelength regions. The presence of the phosphorescence moiety of the protonated form of BP was also observed on the aluminosilicate, H-mordenite, and HY zeolite surfaces,34 which are all known as strong acidic

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catalysts. In fact, they have high acidities or proton donor centers with pka for BP (-5.6),54 where pka refers to Hammet’s function. Conclusions The photochemical and photophysical properties of benzophenone adsorbed on the oxide surfaces were found to be strongly dependent on their surface properties, that is, mainly on the surface acidic property and the properties of the surface OH groups. In the case of BP adsorbed on porous Vycor glass, the characteristics of the phosphorescence and decay of BP were attributed to the presence of two different species of BP adsorbed on two different types of surface OH groups, Si-OH and B-OH groups with different acidic properties. Ti-Al binary oxides prepared by a coprecipitation method exhibited different chemical natures when the Ti/Al compositions of the oxides were changed. When BP molecules were adsorbed on the Al-Ti binary oxides, the photophysical properties of BP were found to be remarkably different from those of BP adsorbed on PVG. Namely, the phosphorescence spectra of BP adsorbed on the Ti-Al binary oxides were found to be a dual emission which can be attributed to the phosphorescence from the hydrogen-bonded BP and the protonated BP molecules. Moreover, the contribution of this phosphorescence to the total emission was found to change with changes in the Ti/Al compositions of the oxides, reflecting the changes in the surface acidity of the oxides by changing the compositions of the oxides LA001402N