ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2009, Vol. 45, No. 6, pp. 685–692. © Pleiades Publishing, Ltd., 2009. Original Russian Text © K.E. Polunin, N.P. Sokolova, A.M. Gorbunov, R.A. Bulgakova, I.A. Polunina, 2009, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2009, Vol. 45, No. 6, pp. 603–610.
MOLECULAR AND SUPRAMOLECULAR STRUCTURES AT THE INTERFACES
Interaction of Stilbenes with TiO2 Studied by Fourier IR Spectroscopy K. E. Polunin, N. P. Sokolova, A. M. Gorbunov, R. A. Bulgakova, and I. A. Polunina Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31 Leninskii pr., Moscow, 119991, Russia e-mail:
[email protected] Received February 29, 2008
Abstract—Adsorption interactions of stilbene (1,2-diphenylethylene) and its hydroxy- and methoxyderivatives with nanodispersed TiO2 aerogel surface were studied by means of Fourier IR radiosity spectroscopy. Stilbene trans- and cis-isomers were shown to be weakly adsorbed on TiO2 surface forming hydrogen bonds. UV irradiation (wavelength 285–305 nm) of adsorbed compounds results in their partial destruction, yielding aldehydes, ketones, and carboxylates. The products of stilbene photolysis are strongly bound to TiO2 surface. Modification of TiO2 surface with trans-hydroxystilbenes is characterized by the formation of various hydrogen bonds and surface colored quinoid compounds. The interaction of TiO2 with trans-methoxystilbenes changes the state of surface hydroxyl groups and yields stable carboxylate compounds. UV irradiation results in the partial destruction of adsorbed stilbenoids, yielding aldehyde, ketone, and carboxylate surface compounds. PACS numbers: 68.43.-h DOI: 10.1134/S2070205109060082
INTRODUCTION Titanium, silicon, and aluminum oxides are often used as fillings and pigments in the manufacture of composite materials, paints, paper, polymeric materials, and pharmaceutical preparations, as well as products of electronic and radio engineering industry [1–2]. However, unlike other oxides,TiO2 has the own photochemical sensitivity to the sunlight and UV radiation [3]; this enables titanium dioxide to be used as an oxidizer to decontaminate industrial waste water [3, 4] because the photoreduction of Ti4+ ions into Ti3+ ions yields active atomic oxygen. The behavior of stilbene functional derivatives (stilbenoids) in the presence of Tié2 has not been studied in practice though filled materials often contain both of these components. Therefore, a study of stilbenoid adsorption, desorption, photolysis, and oxidation processes on the Tié2 surface is of both scientific and practical interest. Previously [5], using Fourier IR spectroscopy, we studied the composition of the adsorption layer of stilbene isomers and its hydroxy- and methoxyderivatives on aerosil. The equilibrium concentration of stilbenoids on metal oxides was studied in [6]. It was shown that, unlike the reversible adsorption of trans-stilbene functional derivatives on aerosol, the adsorption of stilbenoids on aluminum and titanium oxides is partially irreversible and described by more abrupt isotherms. The desorption of stilbenoids from the TiO2 surface was studied by means of thermal destruction mass spectrosmetry and chromatographic mass spectrometry in [7, 8]. The goal of this work was to study the adsorption mechanism stilbenoids on nanodispersed nonporous titanium
oxide and surface forms of stilbenoids on TiO2 by means of Fourier IR spectroscopy. EXPERIMENTAL Table 1 represents the structures of studied compounds, including synthetic trans- and cis-isomers of 1,2-diphenylethylene (stilbene), as well as hydroxy- and methoxyderivatives of trans-stilbene of 98–99% purity degree from Acrus (Russia) and Sigma (United States). The used adsorbent was nonporous TiO2 aerogel of P−25 mark from Degussa (80% anatase, 20% rutile, primary particle size 21 nm, specific surface area calculated by BET from low-temperature nitrogen adsorption Ssp = 55 m2/g). Prior to the adsorption measurements, titanium oxide was annealed in air for 6 h at 450°ë and slowly cooled to room temperature. The adsorption of stilbenoids was carried out from diethyl ether solutions (c = 1 mg/ml) with continuous stirring of TiO2 suspension for 3 h. Prior to the spectral studies, the modified adsorbent specimens were dried in air at 25°ë. The desorption of stilbenoids from the TiO2 surface was achieved by stirring the modified adsorbent suspension in pure solvent at 25°ë for 3 h followed by the decantation of solution. Diethyl ether, acetonitrile, and methanol of HPLC grade (Acrus) were preliminarily dried over sodium metal and purified by distillation in argon flow. IR spectra of dispersed specimens were recorded in the range of 400–4000 cm–1 with a Perkin-Elmer 2000 Fourier IR spectrometer. The radiosity method used a Harrick & Ko
685
686
POLUNIN et al.
Structure and properties of studied stilbenoids No.
Compound
UV absorption peak*, nm
Molecular structure
1. trans-stilbene (1,2-diphenylethylene) C14H12
294
2. cis-stilbene C14H12
276
3. trans-4-hydroxystilbene C14H12O
319 HO
4. trans-4-methoxystilbene C15H14O
317 MeO
5. trans-α,α’-diethyl-4,4’-dihydroxystilbene (diethylstilbestrol) C18H20O2
295 OH HO
6. trans-3,5,4’-trihydroxystilbene (resveratrol) C14H12O3
326
HO OH
HO 7. trans-1,6-diphenyl-1,3,5-hexatriene
355
Note: * Solutions of stilbenes in methanol.
device, which we modified to study the special specimens. In order to increase the signal-to-noise ratio, the spectra were recorded and averaged by 100 scans. The resolution was 4 cm–1. IR spectra of organic compounds and their solutions were recorded on a KRS–5 plate by the transmission method. The initial stilbenoids and TiO2 specimens modified by them were irradiated in air with a DRT 240–1 UV lamp (240 kW) for 10 h. The irradiance of specimens at wavelengths of 285–305 nm was 3 W/m2 at a distance of 0.5 m to the irradiated surface, which was continuously cooled with water.
RESULTS AND DISCUSSION Figure 1 shows the IR radiosity spectra of stilbene trans- and cis-isomers as well as trans-stilbene hydroxyand methoxyderivatives. According to [9, 10], low-frequency absorption at 600–520 cm–1 is attributed to the C=C deformation vibrations of olefine and arene, absorption at 980–965 cm–1 characterizes trans-configuration of stilbene, and bands at 860–770 cm–1 are attributed to deformation vibrations of arene linked to olefine (859 cm–1) and 1,4-disubstituted (897 cm–1) benzene ring. The absorption of aromatic fragments of stilbenoids is observed at 1600, 1512, and 1497 cm–1 and CO stretching vibrations are observed at 1335–1165 cm–1. The deformation vibrations
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES
Vol. 45
No. 6
2009
INTERACTION OF STILBENES WITH TiO2 STUDIED BY FOURIER IR SPECTROSCOPY
of OH groups in aromatic ring are characterized by absorption at 1390–1315 cm–1 and their stretching vibrations are observed at 3300 and 3243 cm–1. Figure 2 shows the IR spectrum of TiO2 dehydrated at 450°ë. This radiosity spectrum shows the absorption bands at 450, 900, 1627, 3409, and 3690 cm–1. The application of trans-stilbene changes the spectrum of TiO2 (Fig. 3). While initial TiO2 only shows the absorption band of free OH groups (3690 cm–1) and the band of linked OH groups (3409 cm–1) [1, 11], the interaction of stilbene broadens and shifts the band at 3409 cm–1 to 3270 cm–1; the range of free OH groups shows two absorption bands at 3682 and 3580 cm–1, which can be accounted for by different coordination of OH groups. On the background of TiO2 absorption, we see the absorption bands of stilbene: band at 960 cm–1, which is characteristic of trans-stilbene, and bands of its aromatic fragments at 1600, 1500, and 1450 cm–1. Somewhat broadened absorption in the range of 1600–1500 cm–1 can be accounted for by the existing absorption bands of stilbene and deformation vibrations of water, as well as its partial oxidation products. According to [11, p.162], the absorption of ethylene and other olefines on transition metal oxides, e.g., chromium oxide, can take place with the destruction of ë=ë bond and the formation of ëéé– fragments (1630–1640 and 1340–1480 cm–1). Oxidation is provided by the surface oxygen. Taking into account the existing cations with unsaturated coordination on the TiO2 surface, as well as properties of olefines as electron donors, we suggest the formation of surface π-complexes of olefins; these complexes were detected on aluminum, titanium, nickel, and other oxides in [11–12]. According to the data of [11], the weak distortion of C = C bonds (slight frequency reduction) in the adsorption complexes of olefins, as well as their low stability (destruction upon desorption at room temperature), are evidence of weak donor–acceptor interactions between titanium ions and olefines. The presence of some broadened bands in our spectra at 1600–1500 cm–1 and data on the low stability of π-complexes do not allow us to reach an unambiguous conclusion on the formation of stilbene π-complexes on TiO2. When TiO2 modified with stilbene was rinsed with solvent (diethyl ether), in addition to TiO2 absorption bands, the spectrum (Fig. 3, curve 2) retained very weak absorption in the range of oxidized structures and lost the band, which is characteristic of associates (3580 cm–1) with somewhat weakened hydrogen bonds (band shift from 3290 to 3370 cm–1). The spectrum of desorption products after the evaporation of solvent (curve 3) is closer to the spectrum of initial trans-stilbene (Fig. 1). The study of desorption products from TiO2 surface by means of chromatographic mass spectrometry confirmed the sole presence of trans-stilbene molecules [7]. Thermal desorption mass spectrometry detected that the process of trans-stilbene release from the modified surface is described by almost Gaussian curve within the range of 30–300°ë with the maximum at 150°ë [8]. Therefore, the
1600 1512
1260 1497 1300
1032
827
968
687 538
859 912
1623
4 780
A
3
1027 925
2 1 1700
1500
1300
1100
900
700
500 cm–1
Fig. 1. IR radiosity spectra of trans-stilbene (1), cis-stilbene (2), trans-4-hydroxystilbene (3), and trans-4-methoxystilbene (4).
901
1104
654
1182
A 1627
3409 3690
4000
3000
2000
1500
1000
400 cm–1
Fig. 2. IR radiosity spectra of íiO2 dehydrated at 450°ë.
909 961
2 971 1451
A
761 691
1495
3372
1071
3690
960
1
1598 1451
3269
3
1072
3579
1495
3683
4000
3000
2000
1500
1000
400 cm–1
Fig. 3. IR spectra of TiO2 modified with trans-stilbene (1); same specimen treated with ether (2); products of desorption from modified TiO2 surface (3).
majority of stilbene is weakly adsorbed on TiO2 and easily washed off with solvent. At the same time, the IR spectrum of TiO2 recovers the absorption of OH groups. The spectrum of desorbed trans-stilbene is almost the same as the initial spectrum (curve 3).
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES
Vol. 45
No. 6
2009
688
POLUNIN et al. 964
3
617
1410 1601 1450 1646
A
1716
1
3062 3313
1517
2 4000
3000
2000
1500
1000
400 cm–1
Fig. 4. IR spectra of TiO2 modified with trans-stilbene after UV irradiation (1), same specimen treated with ether (2); products of desorption from modified TiO2 surface after UV irradiation (3).
Similar changes that evidence the formation of hydrogen bonds with surface OH groups on TiO2 are observed also upon the adsorption of cis-stilbene on TiO2. The formation of bonds with surface oxygen ions can be hardly identified, since the spectral changes are minor and fall within the absorption range of OH stretching. The spectra of interaction products of trans-stilbene TiO2 upon UV irradiation with λ of 285–305 nm (Fig. 4) differ from the spectra before this irradiation (Fig. 3) because both stilbene and TiO2 shows the photochemical activity in this range of electromagnetic radiation [1, 13, 14]. UV irradiation significantly reduces the intensity of absorption bands at 960, 1450, 1500, and 1600 cm–1, which are characteristic of trans-stilbene. This fact evidences the partial destruction of this compound. The decreasing intensity of the absorption band of OH groups on TiO2 at 3690 cm–1 indicates to their involvement in formation of hydrogen bonds. UV irradiation results in new absorption bands in the spectrum of modified TiO2 in the range of 1400– 1750 cm−1, which may be related to the formation of aldehyde, ketone, formate, and caboxylate groups [15]. The absorption bands of carbonyl group in aldehydes and ketones are rather close to each other. In addition to the data of a carbonyl group, the identification of aldehydes also requires the frequency of stretching vibrations of C-H group in aldehydes (2900–2700 cm–1). Due to the strong effect of carbonyl oxygen, this frequency is almost independent of the structure of the whole molecule and, therefore, is rather characteristic [16]. In our case, no absorption in this range enables us to suggest that the band at 1716 cm−1 can be attributed to aromatic ketone (benzophenone). Moreover, we observe the general broadening in the absorption bands, which may be related to the dimerization of stilbene molecules on TiO2 surface after irradiation [14]. The IR spectra of the desorption products from a transstilbene-modified and irradiated TiO2 surface (Fig. 4)
showed the absorption bands of trans-stilbene, which excess is removed with ether from the oxide surface, as well as carbonyl compounds that are less strongly linked to the surface. These results agree well with the data obtained previously by means of chromatographic mass spectrometry on the presence of approximately the same quantities of trans-stilbene and benzophenone in the ether solution of the desorption products from irradiated TiO2 surface [7]. Unlike silica and alumina, the spectra of desorbed trans-stilbene photolysis products do not show any absorption bands of cis-stilbene as a characteristic product of photoisomerization [14]. This is possible in the case of changing the reaction route of photo excitation of trans-isomer on TiO2 surface or in the case of further transformation of cis-isomer into phenanthrene and cyclobutane structures [14]. Unfortunately, the absorption bands of phenanthrene (1600, 1550, 1500–900, and 900–650 cm–1) overlap with the absorption bands of other decomposition products of trans-stilbene, and identification of phenanthrene is complicated though its formation is not excluded. The dimerization of stilbene into a cyclobutane structure can be evaluated using by the observed broadening absorption bands in the spectra of stilbene interaction products with TiO2 upon irradiation. Upon rinsing irradiated TiO2- stilbene specimen with solvent, the IR spectrum (Fig. 4) retains absorption bands of photolysis products, which is proof of their strong adsorption on an oxide surface. The identification of these compounds by means of thermal desorption mass spectrometry [7, 8] enabled us to detect that the major product of thermal desorption from irradiated TiO2 specimen modified with stilbene is benzophenone, while its mass thermogram shows maxima at 120 and 290°ë, which indicates the existence of two surface forms of benzophenone with different surface bond energies. Therefore, our studies showed that adsorption of transstilbene on TiO2 changes the nature of hydrogen bonds involving OH groups of oxide, as well as that the partial oxidation of stilbene takes place with formation of formate and carboxylate structures. The effect of UV radiation causes the partial destruction and oxidation of adsorbed trans-stilbene yielding benzophenone, formate, and carboxylate structures, as well as dimers with cyclobutane structure. For comparison, Fig. 5 presents the spectra of several diphenylpolyenes adsorbed on TiO2. Regardless of the length of olefine fragment in diphenylpolyenes, almost the same spectral changes are observed upon their adsorption on TiO2. On the contrary, the size of molecule affects the structure of monolayer, as is clearly seen in Fig. 5, when TiO2 is modified with cis-stilbene (1,2-diphenylethylene) and 1,1diphenylethylene, the absorption band of free surface OH groups at 3680 cm–1 disappears almost completely (curves 3 and 4); however, the intensity of the band at 3600 cm–1 increases. The adsorption of trans-stilbene and trans-diphenylhexatriene (curves 2 and 5), as well as some surface OH of TiO2, are not involved in the formation of
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES
Vol. 45
No. 6
2009
INTERACTION OF STILBENES WITH TiO2 STUDIED BY FOURIER IR SPECTROSCOPY
hydrogen bonds. This may be caused by the lower density of the adsorption layers of trans-isomers compared to more compact cis-isomers. Stilbenes with hydroxysubstituents show properties of phenols, i.e., are proton- and electron-donor molecules. The presence of OH groups in para-position of two benzene rings in symmetric molecule of diethylstilbestrol and three OH groups in asymmetric molecule of resveratrol (Table 1) strengthens acid and electron-donor properties of these polyphenol compounds as compared to trans-4hydroxystilbene. The modification of a TiO2 surface with hydroxystilbenes (Fig. 6) results in the disappearance of the band of free OH groups of titanium dioxide at 3698 cm–1 in the case of 4-hydroxystilbene and diethylstilbestrol and its significant reduction in the case of resveratrol. The presence of OH groups in hydroxystilbenes changes the structure of the broad band in the low-frequency range, which is characteristic of hydroxyl groups forming hydrogen bonds. Trihydroxystilbene (reservatrol) shows numerous maxima, which are evidence of the variety of structures that form hydrogen bonds. Compared to trans-stilbene, the adsorption of hydroxystilbenes on the TiO2 surface (Fig. 5) changes the shape of the absorption band at 1600 cm–1, broadening it to higher frequencies, which is probably accounted for by the formation of new surface complexes. A comparison in a region of 1650–1550 cm–1 of the spectra of adsorbed hydroxystilbenes, n-benzoquinone, and hydroquinone (Fig. 7) showed that the observed changes upon the adsorption of hydroxystilbene and hydroquinone are very similar (band at 1609 cm–1). The absorption intensity in this range increases upon a transition from mono- to dihydroxystilbene (diethylstilbestrol). Visually, the adsorption interaction of hydroxystilbenes with TiO2 is accompanied by the changing color of oxide; upon the addition of hydroxystilbenes, white powder of TiO2 instantly turns brightly orange and, after several minutes, becomes dull brown. Upon the addition of a droplet of water to the orange powder, it turns white, while the dull brown powder does not change its color; in other words, the first color-changing stage is reversible and second is irreversible. We did not observe similar color reactions upon the adsorption of hydroxystilbenes on silica or alumina; therefore, the adsorption sites are probably titanium ions with unsaturated coordination rather than acid or basic OH groups on the adsorbent surface. As is known in phenol chemistry, in the presence of Lewis acids, phenols can enter electrophilic substitution reactions, which include two stages. The first (reversible) stage yields anion radicals of the quinoid type and the second (irreversible) stage is parasubstitution in the benzene ring [17]. In fact, phenol is oxidized into 1,4-dihydroquinones. The oxidation of spatially convenient polyphenols is a complex, multistage process. Its mechanism has not yet been studied in detail. For example, the oxidation of dihydric phenols to 1,4- or 1,2-quinones involves several stages, including the formation of anion radicals, followed by var-
689
5 911
4 3 2
809 1625
3408
A
3686
965
14431326 1610 1494 1654 1570 1406
3314
896
1
3591 3693
4000
3000
2000
1500
1000
430 cm
Fig. 5. IR spectra of TiO2 (1), TiO2 modified with trans-stilbene (2), cis-stilbene (3), 1,1-diphenylethylene (4), and trans-1,6-diphenyl-1,3,5-hexatriene (5).
3213
3315
3026 3566 3423 3696
3498
A
4
2970
3596
306
3411
2938 2877
3 2
2927 2857
3211
1
2938 3698
4000
3600
3200
2800
2500
Fig. 6. IR spectra of TiO2 (1), TiO2 modified with trans-4hydroxystilbene (2), diethylstilbestrol (3), and resveratrol (4).
1598 1593
3
1609 1625
A 1609
2 1606
1650
1630
1610
1 1590
1570
1550 cm–1
Fig. 7. IR spectra of TiO2 modified with 1,4-dihydroquinone (1), 1,4-dihydroxybenzene (2), and trans-4-hydroxystilbene.
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES
Vol. 45
No. 6
2009
690
POLUNIN et al.
930 3380
823 960
1447 1259
A
1594 3402
1171 1373
3011
3609
1750
2
1292
1506
1590 1510 1660
1
960 820
1248
3
684
4000
3000
2000
1500
1000
400 cm–1
Fig. 8. IR spectra of TiO2 modified with trans-4-stilbene after UV irradiation (1), same specimen treated with ether (2), products of desorption from modified TiO2 surface after UV irradiation (3).
2
3410
931
1252
1598 3579
A
1
1167
1258 1429 3417
4000
1657
3000
2000
1500
3 1000
400 cm–1
Fig. 9. IR spectra of TiO2 modified with trans-diethylstilbestrol after UV irradiation (1), same specimen treated with ether (2), products of desorption from modified TiO2 surface after UV irradiation (3).
A 3565
1178 1513 1255 1407 1606 1642 1447 1300 1572 964 813 1252 1178 688
3063 2841
3690 3299
1602 1510 1443 1296
2
1
536
3 4000
3000
2000
1500
1000
400 cm–1
Fig. 10. IR spectra of TiO2 modified with trans-4-methoxystilbene after UV irradiation (1), same specimen treated with ether (2), products of desorption from modified TiO2 surface after UV irradiation (3).
ious dimers resulting from new C-C and C-O bonds (quinolides, dihydroxybiphenols, quinol ethers, etc.); in turn, quinones can be also oxidized into more complex structures [18]. It is quite probable that this electrophilic substitution mechanism takes place during the interaction of hydroxystilbenes with titanium oxide, which has the properties of Lewis and Brönsted acid [1, 2]. In this case, adsorption first yields intermediate stilbene anion radicals with the characteristic orange color of quinine, which can interact with both surface Ti4+ cations and each other, yielding brownish polyconjugated oligomers grafted to the surface. The olefine polymerization reactions are well known [17]. Our studies of hydroxystilbene desorption from the TiO2 surface by means of IR spectroscopy, as well as chromatographic mass spectrometry [8], revealed that hexane, diethyl ether, and acetone cannot break the surface bonds of hydroxystilbenes. According to the data of thermal desorption mass spectrometry, the surface compounds of hydroxystilbenes have high hydro- and thermal stability and their destruction can only be observed at temperatures over 350°ë. The UV irradiation of trans-4-hydroxystilbene adsorbed on TiO2 surface changes the IR spectrum of the modified specimen in the range of 1750 to 1720 cm–1, evidencing the partial destruction and oxidation of adsorbed molecules (Fig. 8). The solvent (ether) washes off only an insignificant quantity of adsorbed hydroxystilbene (Fig. 8, curve 3). Approximately the same products of photolysis are also observed in the case of diethylstilbestrol adsorbed on TiO2 (Fig. 9). Following the modification of TiO2 with trans-4-methoxystilbene (Fig. 10, curve 3), the absorption band of free surface OH groups at 3692 cm–1 does not disappear, but rather weakens only slightly and the bands at 3400 and 3200 cm–1 broaden and almost become one. In the range of 1750–1400 cm–1, we observe absorption bands that can be attributed to antisymmetric (1572 cm–1) and symmetric (1406 cm–1) vibrations of carboxylate structures. These spectral changes are retained after rinsing with solvent, which enables us to reach a conclusion on the chemisorption of methoxystilbene molecules on TiO2. Excess adsorbed methoxystilbene is washed off with solvent (Fig. 10, curve 3). The same results were obtained in [8] by means of chromatographic mass spectrometry. Usin thermal desorption mass spectrometry, we detected that methoxystilbene molecules strongly linked to TiO2 can be removed from the surface by heating the specimen to over 200°ë [8]. As in the case of UV irradiation, the white color of TiO2 upon adsorption of trans-4-methoxystilbene was almost unchanged. Upon UV irradiation, the IR spectrum of methoxystilbene adsorbed on TiO2 showed a new absorption band at 1712 cm–1, which is characteristic of νëé vibrations in aromatic aldehydes or ketones (Fig. 11). The same changes are also retained in the spectrum after rinsing the specimen with solvent (Fig. 11, curve 2), which is evidence of the
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES
Vol. 45
No. 6
2009
INTERACTION OF STILBENES WITH TiO2 STUDIED BY FOURIER IR SPECTROSCOPY
946
3579 3683
4000
2
1174 1259 1606 1418 1510 1303
2923
A
691
1
1602 1395 1447 1639 1712 1255
3063 2841
1030 968 813
3 688
3000
2000
1500
1000
400 cm–1
Fig. 11. IR spectra of TiO2 modified with trans-4-methoxystilbene after UV irradiation (1), same specimen treated with ether (2), products of desorption from modified TiO2 surface after UV irradiation (3).
O
CH3
H3C O
O hν, O2, TiO2
+
O Fig. 12. Photolysis scheme of trans-4-methoxystilbene adsorbed on TiO2 surface.
strong adsorption of the methoxystilbene photolysis products on the surface. The intensity of the absorption band of methoxystilbene washed off the prepared specimen (Fig. 11, curve 3) is much lower than that before UV irradiation (Fig. 10). This is probably also related to the destruction and transformations of the substance on TiO2 surface. Using chromatographic mass spectrometry, we identified para-methoxystillene (M/z 136) as the major desorption product after the irradiation of trans-4-methoxystilbene on a TiO2 surface. Its formation indicates the cleavage of the ethylene bond in methoxystilbene due to photolysis and the oxidation of the destruction products (Fig. 12). This type of thermal destruction is characteristic of olefines [17]. Benzaldehyde as the second fragment is probably adsorbed on the surface in benzoate form. Therefore, the substitution of a hydroxy group by a methoxy group in stilbenes changes the photolysis mechanism of stilbenoids on a TiO2 surface. The effect of the methoxy group is due to the stereometric change of the molecule, which is already not planar like that of hydroxystilbene [19]; as a result, the degree of conjugation, donor–acceptor, and photochemical properties of methoxystilbene change, as does the molecular packing density in monolayers [20].
CONCLUSION The adsorption interaction of stilbenne and its hydroxyand methoxyderivatives with nanodispersed TiO2 aerogel surface was studied by means of Fourier IR radiosity spectroscopy. Our studies showed that trans- and cis-isomers of stilbene are weakly adsorbed on a TiO2 surface, forming hydrogen bonds. UV irradiation at wavelengths of 285– 305 nm results in the partial destruction of adsorbed stilbenes, yielding aldehydes, ketones, and carboxylates. The photolysis products of stilbene are strongly adsorbed on the TiO2 surface, but not desorbed with solvent (diethyl ether). The modification of the TiO2 surface with transhydroxystilbenes is characterized by the formation of various hydrogen bonds and colored, quinoid-type surface compounds. The interaction of TiO2 with trans-methoxystilbenes changes the state of surface hydroxyl groups and forms surface carboxylate compounds. UV irradiation results in the partial destruction of adsorbed stilbenoids, yielding alhehyde, ketone, and carboxylate compounds on the surface. ACKNOWLEDGMENTS This study was supported by the Russian Fund for Basic Research (project nos. 06-03-33185 and 09-08-01231) and by the Fund for Aid to Homeland Science.
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES
Vol. 45
No. 6
2009
692
POLUNIN et al.
REFERENCES 1. Diebold, U., Surface Sci. Reports, 2003, vol. 48, no. 5—8, p. 53. 2. Ermilov, P.I., Indeikin, E.A., and Tolmachev, I.A., Pigmenty i pigmentirovanie (Pigments and Pigmenting), Leningrad: Khimiya, 1987. 3. Thompson, T.L. and Yates, J.T., Chem. Rev., 2006, vol. 106, p. 4428. 4. Guo Z., Ma R., and Li G., Chem. Eng. J., 2006, vol. 119, no. 1, p. 55. 5. Polunin, K.E., Sokolova, N.P., Gorbunov, A.M., et al., Fizikochim. Poverkhn. Zashch Mater., 2008, vol. 44, no. 4, p. 378. 6. Kolotilov, P.N., Polunin, K.E., Polunina, I.A., and Larin, A.V., Fizikochim. Poverkhn. Zashch Mater., 2009, vol. 45, no. 1, p. 22. 7. Polunina, I.A., Polunin, K.E., Buryak, A.K., et al., Sorbtsionye i khromatografisheskie protsessy, 2004, vol. 4, issue 2, p. 233. 8. Polunina, I.A., Polunin, K.E., Buryak, A.K., et al., Sorbtsionye i khromatografisheskie protsessy, 2004, vol. 4, issue 6, p. 787. 9. Beilstein, F., Handbuch der organischen Chemie (Handbook of Organic Chemistry), E II–E VI, Frankfurt am Main: VCH, 1992.
10. Tyukavina, N.A., Gromova, A.S., Lutskii, V.I., and Voronov, V.K., Khimiya prirod. Soedinenii, 1972, vol. 8, no. 5, p. 600. 11. Davydov, A.A., IK-spektroskopiya v khimii poverkhnosti okislov (IR Spectroscopy in Oxide Surface Chemistry), Novosibirsk: Nauka, 1984. 12. Seo, Y.S., Lee, C., Lee, K.H., and Yoon K.B., Angew. Chem. Int. Ed. Engl., 2005, vol 44, no. 6, p. 910. 13. Nurmukhametov, R.N., Pogloshchenie i luminsetsentsiya aromaticheskikh soedinenii (Absorption and Luminescence of Organic Compounds), Moscow: Khimiya, 1971 14. Waldeck, D.H., Chem. Rev., 1991, vol. 91, no. 3, p. 415. 15. Vibrational Spectra and Structure, During, I.R., Ed., New York: Elsevier, 1986, vol. 16. 16. Bellamy, L., Novye dannye po IK spektram slozhnykh molekul (New IR Spectral Data of Complex Molecules), Moscow: Mir, 1971 17. Nesmeyanov, A.N. and Nesmeyanov, N.A., Nachala organicheskoi khimii (Principles of Organic Chemistry), Moscow: Khimiya, 1974, vol. 2. 18. El’tsov, I.Yu. Studzinskii, O.P., and Grebenkina, V.M., Uspekhi Khimii, 1977, vol. 46, no. 2, p. 185. 19. Potapov, V.M., Stereokhimiya (Stereochemistry), Moscow: Khimiya, 1988, p. 116. 20. Lin, C.H., Huang, J.M., and Wang, C.S., Polymer, 2002, vol. 43, p. 2959.
PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES
Vol. 45
No. 6
2009