Langmuir 1997, 13, 3787-3793
3787
Ultraviolet-Visible and Fourier Transform Infrared Diffuse Reflectance Studies of Benzophenone and Fluorenone Adsorbed onto Microcrystalline Cellulose Laura M. Ilharco,* Ana R. Garcia, J. Lopes da Silva, M. Joa˜o Lemos, and L. F. Vieira Ferreira Centro de Quı´mica-Fı´sica Molecular, Complexo I, Instituto Superior Te´ cnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal Received January 21, 1997X Benzophenone and fluorenone, which have a nonrigid and a rigid structure, respectively, were used as probes to study the nature of the adsorption process onto microcrystalline cellulose. Diffuse reflectance techniques were used in the UV-vis and infrared regions. Luminescence studies revealed that whenever fluorenone or benzophenone are entrapped into the natural polymer chains and in close contact with the substrate, a strong quenching effect exists for both probe’s luminescence at room temperature. For fluorenone, the fluorescence quantum yields (ΦF) determined were about 0.10 when dichloromethane, cyclohexane, and benzene (solvents which do not swell cellulose) were used for sample preparation, while for dioxane, acetone, ethanol, and methanol (solvents which efficiently swell cellulose) ΦF was approximately 0.01. These values are about 1 order of magnitude higher than those obtained in solution, showing the importance of the rigid dry matrix in reducing the nonradiative pathways of deactivation of the (π,π*) fluorenone first excited singlet state. Complementary, infrared studies showed that the carbonyl group of benzophenone is affected by entrapment (when the solvents used induce the swelling of cellulose), whereas in fluorenone the same band is insensitive to the adsorption process, not allowing the differentiation between entrapped molecules and surface crystallites of this ketone. These observations implied that benzophenone is entrapped between the chains of the polymer forming hydrogen bonds between the carbonyl and the hydroxyl groups of the glycosidic chains, while the rigidity of fluorenone apparently restrains the ketone-substrate interactions to the aromatic rings. Through the modifications observed in the carbonyl stretching band of benzophenone, it was possible to establish a swelling effect scale for the solvents, which is compared with previous results.
1. Introduction In the last few years, some of us have concentrated our efforts on photochemical and photophysical studies of several probes, including ketones, adsorbed onto an almost unexplored host: microcrystalline cellulose.1 Adsorption of probes onto this solid substrate was achieved by the use of a solution of the probe in polar protic (alcohols) or aprotic (e.g., acetonitrile, acetone, dioxane) solvents. When microcrystalline cellulose is added to this solution, cellulose to cellulose hydrogen bonds are replaced by cellulose to solvent bonds and the matrix exhibits a certain degree of swelling which depends on the solvent used for sample preparation.1a,c,2 Probes can then penetrate into submicroscopic pores of the solid substrate and stay entrapped between the natural polymer cellulose chains after solvent removal. From these studies, a large amount of information concerning triplet-triplet energy transfer,3 fluorescence and phosphorescence,1a,c,2,4 absorption spectroscopy,1c,5 and electron and hydrogen transfer6 was obtained. All these processes depend on the interactions between probes and matrix, the latter providing a rigid environment strongly affecting the properties of the guest molecules. Aromatic * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, June 15, 1997. (1) (a) Vieira Ferreira, L. F.; Netto-Ferreira, J. C.; Khmelinskii, I. V.; Garcia, A.R.; Costa, S. M. B. Langmuir 1995, 11, 231. (b) Vieira Ferreira, L. F.; Oliveira, A. S.; Wilkinson, F.; Worrall, D. J. Chem. Soc., Faraday Trans. 1996, 92, 1217. (c) Vieira Ferreira, L. F.; Freixo, M. R.; Garcia, A. R.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1992, 88, 15. (d) Netto-Ferreira, J. C.; Vieira Ferreira, L. F.; Costa, S. M. B. Quı´m. Nova 1996, 19, 230. (2) Vieira Ferreira, L. F.; Garcia, A. R.; Freixo, M. R.; Costa, S. M. B. J. Chem. Soc., Faraday Trans. 1993, 89, 1937. (3) (a) Wilkinson, F.; Leicester, P.; Vieira Ferreira, L. F.; Freire, V. M. M. R. Photochem. Photobiol. 1991, 54, 599. (b) Wilkinson, F.; Kelly, G. P.; Vieira Ferreira, L. F.; Freire, V. M. M. R.; Ferreira, M. I. J. Chem. Soc., Faraday Trans. 1991, 87, 547.
S0743-7463(97)00060-7 CCC: $14.00
ketones are a very useful tool to study the nature of the guest-host interactions. Benzophenone (BZP) is a nonrigid ketone (Chart 1), whose adsorption onto microcrystalline cellulose has been recently studied by the use of UV-vis ground-state diffuse reflectance spectroscopy, as well as steady-state and transient phosphorescence measurements.1a Diffuse reflectance laser flash photolysis transient absorption studies showed that both triplet BZP and the corresponding ketyl radicals are formed following laser excitation, when the probe was entrapped and in close contact with the cellulosic polymer chains, enabling triplet molecules to efficiently abstract hydrogen from the matrix.1a It is well-known that ketones with a low-lying 3(n,π*) state abstract hydrogen from the solvent in a highly efficient photochemical reaction,7 since the carbonyl oxygen atom is electron-deficient. On the other hand, 3(π,π*) states are less reactive in hydrogen-abstraction reactions, since the excitation energy is partially delocalized into the aromatic π-systems, while in the 3(n,π*) states it is localised in the carbonyl group.7 Under the point of view of infrared spectroscopy, benzophenone may be an interesting probe, as the carbonyl (4) (a) Vieira Ferreira, L. F.; Netto-Ferreira, J. C.; Costa, S. M. B. Spectrochim. Acta, Part A 1995, 51, 1385. (b) Vieira Ferreira, L. F.; Oliveira, A. S.; Khmelinskii, I. V.; Costa, S. M. B. J. Lumin. 1994, 60 & 61, 485. (5) Wilkinson, F.; Worrall, D. R.; Vieira Ferreira, L. F. Spectrochim. Acta 1992, 48A, 135. (6) (a) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B. Chem. Phys. Lett. 1990, 173, 277. (b) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B.; Katalnikov, I. V. Chem. Phys. Lett. 1992, 193, 461. (c) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B. Langmuir 1993, 9, 1001. (d) Levin, P. P.; Costa, S. M. B.; Vieira Ferreira, L. F. J. Photochem. Photobiol., A 1994, 82, 137. (7) Turro, N. J. Modern Molecular Photochemistry; Benjamin Cummings: New York, 1978.
© 1997 American Chemical Society
3788 Langmuir, Vol. 13, No. 14, 1997 Chart 1
stretching band is very strong and sensitive to any interactions in which it may be involved. For the planar molecule (crystalline phase), the conjugated π system is extended to the whole molecule and the wavenumber of the carbonyl stretching vibration is particularly low (1650 cm-1).8 When rotation is free around the C-(CdO) bonds (in solution or molten phase), the molecule loses coplanarity, the CdO double bond character is reinforced, and the correspondent mode shifts to higher wavenumbers (1667 cm-1 in cyclohexane, for instance). However, if the carbonyl group is involved in strong interactions, such as hydrogen bonds, this effect is partially compensated, and the band shifts back a few wavenumbers.9 Fluorenone (FLN) is a planar and rigid ketone10 (Chart 1), which, contrarily to BZP, fluoresces in certain solvents at room temperature.11 Phosphorescence was reported only to occur from this molecule in ethanol and EPA (diethyl ether/2-methylbutane/ethanol, 5:5:2) at low temperatures.12 The photochemical and photophysical behavior of fluorenone in solution is strongly solvent dependent. The first excited singlet state of FLN in cyclohexane or methylcyclohexane is 1(n,π*) with the second singlet of 1(π,π*) character, which is relatively close in energy to the lowest one. There are two triplet levels 3(π,π*) and one 3(n,π*) triplet level, which lie below the 1(n,π*) state.13 In moderately polar toluene, the order of 1(n,π*) and 1(π,π*) states is probably reversed and with increasing polarity of the solvent the transition energy of the 1(n,π*) state increases while 1(π,π*) decreases. Acetone, acetonitrile, dioxane, and ethanol are examples of solvents which produce this latter effect. Fluorescence quantum yields, fluorescence lifetimes, and triplet yields have been measured as a function of solvent polarity and as a function of temperature11a,b,14 and interpreted in terms of the change in the character of the lowest excited singlet state described above. At the same time, fluorescence of fluorenone is quenched by oxygen as well as by water and by ethanol.14,15 The short lifetime of fluorenone measured in alcohols (1-2 ns) as compared to the ones measured in acetonitrile or dimethylformamide (∼20 ns) was considered to be due to OH-containing solvents.11a,15 This quenching effect and the strong temperature dependence of the emission observed in these solvents were attributed to hydrogen bonding of the excited singlet fluorenone with the solvent molecules.12 (8) Mathur, M. S.; Frenzel, C. A.; Bradley, E. B. Spectrochim. Acta 1970, 26A, 451. (9) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 2nd ed.; Chapman and Hall: London, 1980; Vol. 2. (10) (a) Kuboyama, A. Bull. Chem. Soc. Jpn. 1964, 37, 154. (b) Hunter, T. F. Trans. Faraday Soc. 1969, 66, 300. (11) (a) Yoshihara, K.; Kearns, D. R. J. Chem. Phys. 1966, 45, 1991. (b) Biczo´k, L.; Be´rces, T. J. Phys. Chem. 1988, 92, 3842. (c) Biczo´k, L.; Be´rces, T.; Ma´rta, F. J. Phys. Chem. 1993, 97, 8895. (d) Arathi Rani, S.; Sobhanadri, J.; Prasada Rao, T. A. J. Photochem. Photobiol., A 1996, 94, 1. (12) Huggenberger, C.; Labhart, H. Helv. Chim. Acta 1978, 61, 250. (13) Kobayashi, T.; Nagakura, S. Chem. Phys. Lett. 1976, 43, 429. (14) Andrews, L. J.; Deroulede, A.; Linchitz, H. J. Phys. Chem. 1978, 82, 2304. (15) Monroe, B. M.; Groff, R. P. Tetrahedron Lett. 1973, 40, 3955.
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The carbonyl stretching mode of crystalline fluorenone absorbs at ∼1715 cm-1,16 a wavenumber characteristic of aliphatic ketones, where the CdO bond order is two. This indicates that, although fluorenone has a more extended conjugated system than benzophenone, the participation of the carbonyl group is reduced. It is known that, for cyclic ketones, the ν(CdO) frequency increases as the C-CO-C bond angle falls bellow 120°, a shift of 25-40 cm-1 being observed when passing from six- to fivemembered rings.9 Upon molecular interactions involving the carbonyl group, there is no reason to expect different effects to those observed in benzophenone. In this paper, and following recent studies on BZP and other ketones adsorbed onto microcrystalline cellulose1a,d or other supports,4a we are trying to get some insight into the nature of the ketones-polymer chains interactions, namely, the importance of hydrogen bonding of the probe to the substrate, which is of major importance in the immobilization process and in minimizing the nonradiative transitions of the excited states. The effectiveness of different solvents to induce swelling of cellulose has been compared. The effect of the rigidity of the probe was considered by the use of a nonplanar and structurally flexible ketone, benzophenone, and, for comparison, a planar and relatively rigid one, fluorenone. The possibility of obtaining additional information on the adsorption process of these ketones through their vibrational spectra suggested the extension of previous studies to the infrared range, using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). This technique has established its effectiveness in qualitative and semiquantitative analysis of finely divided powders.17 It has been used to characterize the structure of surface derivatized silica,18 polystyrene,19 and cellulose.20,21 Recently, it has been extended to the study of adsorption of an aromatic ketone (benzophenone) onto cellulose.22 Besides UV-vis and FTIR ground-state diffuse reflectance measurements, some steady-state luminescence experiments were performed, providing a close insight into the nature of the interactions between the cellulosic substract and the ketones. 2. Experimental Section 2.1. Materials and Sample Preparation. Benzophenone was purchased from Koch-Light Laboratories (Scintillation grade), and fluorenone was purchased from Aldrich in the highest purity available. Both compounds were used without further purification after checking their purity by the use of UV-vis absorption spectra as well as thin-layer chromatography. Ethanol, methanol, acetone, and dichloromethane were HPLC grade from Romil Chemicals. Benzene, 2-propanol, and acetonitrile were from Merck (Uvasol grade). 2-Methyl-2-butanol (tertamyl alcohol) was Merck pro-analysis grade. All these solvents were used as received, after checking their purity by UV and visible optical absorption spectrophotometry. Molecular sieves (3 and 4 Å, 4-8 mesh, Aldrich, previously activated by slow heating up to 250 °C under vacuum) were used in some cases for sample preparation, when very well dried solvents were needed. (16) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 4th ed.; Wiley: New York, 1981. (17) (a) Griffiths, P. R.; Fuller, M. P. Advances in Infrared and Raman Spectroscopy; Hester, R. E., Ed.; Heyden: London, 1982. (b) Christy, A. A.; Kvalheim, O. M.; Velapoldi, R. A. Vib. Spectrosc. 1995, 9, 19. (18) Boroumand, F.; van den Bergh, H.; Moser, J. E. Anal. Chem. 1994, 66, 2260. (19) Christy, A. A.; Liang, Y. Z.; Hui, C.; Kvalheim, O. M.; Velapoldi, R. A. Vib. Spectrosc. 1993, 5, 233. (20) Hulleman, S. H. D.; van Hazendonk, J. M.; van Dam, J. E. G. Carbohydr. Res. 1994, 261, 163. (21) Kondo, T.; Sawatari, C. Polymer 1996, 37, 393. (22) Ilharco, L. M.; Garcia, A. R.; Lopes da Silva, J.; Vieira Ferreira, L. F. Langmuir, in press.
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Microcrystalline cellulose (Fluka DSO), with 50 µm average particle size, was dried under vacuum (ca. 10-3 mbar) at 60 °C for at least 24 h before sample preparation. In some cases, Aldrich cellulose, 20 µm average particle size, was also used for benzophenone samples. No differences were detected in the samples prepared with these two cellulose grades. Samples were prepared as described in refs 1a-c, except for the solvent removal step, which was performed in an acrylic chamber with two electrically heated shelves (Heto, Models FD 1.0-110 and FC-2R/H) with temperature control (40 ( 1 °C) and reduced pressure (ca. 10-3 mbar, g12 h). For comparison purposes, several samples were prepared by mechanically mixing well ground benzophenone or fluorenone crystals with microcrystalline cellulose powder, using an agate pestle and mortar. 2.2. Ground-State Absorption Spectra in the UV-VisNear-IR Regions and Steady-State Emission Experiments. Ground-state absorption studies of benzophenone and fluorenone adsorbed onto microcrystalline cellulose were performed using a OLIS 14 UV-vis-near-IR spectrophotometer with a diffuse reflectance attachment. The integrating sphere is 90 mm in diameter and internally coated with a standard white coating. The standard apparatus was modified to include the possibility of using short-wave-pass filters that exclude luminescence of benzophenone and fluorenone from reaching the detector (Hamamatsu, Model R955, Corion UG5 filters). Further experimental details and a description of the system calibration used to obtain accurate reflectance measurements are given in refs 1a-c. Solution measurements were made using the same apparatus in the normal transmission mode. Corrected steady-state fluorescence and phosphorescence emission and excitation spectra of the ketones under study included within microcrystalline cellulose were obtained using a home-made fluorometer previously described.23 2.3. Infrared (DRIFT) Spectra. The IR spectra were recorded in diffuse reflectance mode (DRIFT) with a Mattson Research Series 1 FTIR spectrometer, using a wide-band mercury-cadmium-telluride (MCT) detector. The DRIFT accessory was a Graseby/Specac Selector. For the DRIFT analysis, the original samples were diluted in KBr (from Aldrich, FTIR grade), in ≈4% (w/w), and ground to a finely divided powder. Spectra were recorded at 4 cm-1 resolution in the range 4000 to 500 cm-1 and were obtained as a ratio of 500 scans to the same number of background scans for pure cellulose/KBr. No baseline corrections were made. The diffuse reflectance spectra were transformed to Kubelka-Munk units using the FIRST software.
Figure 1. Remission function for 0.25 mmol of fluorenone adsorbed onto 1 g of microcrystalline cellulose. The solvents used for sample preparation were (1) methanol, (2) ethanol, (3) dioxane, (4) acetone, (5) dichloromethane, (6) benzene, and (7) cyclohexane. Curve 8 is a mechanical mixture of fluorenone crystals with cellulose. Spectra are normalized to unity at 396 nm.
3. Results and Discussion 3.1. Ground-State Absorption Spectra of Fluorenone and Benzophenone. Ground state absorption spectra of benzophenone adsorbed onto microcrystalline cellulose (using different solvents for sample preparation) were previously reported.1a Figure 1 plots F(R)Probe, the remission function of the probe as defined by eq 1,1,2 versus wavelength for fluorenone adsorbed onto microcrystalline cellulose.
F(R)Probe ) F(R)total - F(R)cell.
(1)
where F(R)cell. is the remission function of the blank (microcrystalline cellulose), R is the experimentally determined reflectance, and
F(R)total )
(1 - R)2 K ) 2R S
(2)
where S is the scattering coefficient and K is the absorption coefficient. (23) (a) Vieira Ferreira, L. F.; Costa, S. M. B.; Pereira, E. J. J. Photochem. Photobiol., A 1991, 55, 361. (b) Vieira Ferreira, L. F.; Costa, S. M. B. J. Lumin. 1991, 48 & 49, 395.
Figure 2. Room temperature (20 ( 1 °C) absorption spectra for fluorenone in (1) methanol, (2) ethanol, (3) dioxane, (4) acetone, (5) dichloromethane, (6) benzene, and (7) cyclohexane. The dashed line is the remission function of fluorenone on cellulose from ethanol. Spectra are normalized to unity at 370 nm.
These equations apply to optically thick samples, where any further increase in thickness does not affect R. For an ideal diffuser
K)
∑i 2iCi
(where i is the Neperian absorption coefficient and Ci is the concentration). Figure 2 plots absorbance versus wavelength for fluorenone dissolved in the same solvents used for solid sample preparation as in Figure 1. Solution absorption spectra are very similar to those published long ago by Kuboyama10a for nonpolar, polar protic, and aprotic solvents and give clear evidence for an assignment of the 390 nm absorption band as a π,π* transition in spite of the fact that the transition is somewhat weak (log ∼ 2.5).10a,11a,13 The dashed line is
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the remission function of fluorenone on cellulose from ethanol as in Figure 1, curve 2. The red shift exhibited by fluorenone onto cellulose is even more evident than in polar protic solvents, as a comparison with data from Figure 1 shows. This and also the fact that the absorption band becomes broad and less structured indicate a strong interaction of the guest molecules with the host and also the polar nature of the substrate adsorption sites, as well as the inhomogeneity of the surface. Different adsorption sites provide different interactions via hydrogen bonding for the π-electron system or the nonbonding electron pairs of polar groups of the adsorbed molecules. In the case of fluorenone, hydrogen bonding probably occurs between surface hydroxyl groups and the probe. The first candidate for hydrogen bonding formation is the carbonyl group of the ketone, but an interaction hydroxylaromatic ring in the ketone cannot be obviously excluded. Further information from DRIFT studies will be presented in section 3.3. The interaction of surface -OH groups of Vycor glass and the carbonyl group of alkyl ketones was supported by Anpo et al.24 A nice correlation between the blue shift of the n,π* absorption transition of 2-pentanone and methyl ethyl ketone and increasing number of OH groups on the surface and polarity was established (surface polarity decreases with decreasing concentration of surface OH groups). For oxazine-1 we reported a significant red shift and broadening of the absorption spectra of the dye adsorbed onto cellulose, relative to an ethanolic solution, which reflects both the heterogeneity of the host adsorption sites as well as its high polarity.3a Similar effects were reported for different aromatic molecules adsorbed onto silica.25 For an aryl ketone, a strong interaction of this probe with the hydroxyl groups of cellulose was recently reported,1a and later in this paper we will show its connection with the -O-H‚‚‚OdC< interactions and conformational constraints. At the same time, the π-electronic systems of the aromatic group of aryl ketones may strongly interact with hydroxyl groups as in the case of aromatic molecules as guests in porous silica as hosts.25 In this case, absorption and emission spectra are considerably broadened with loss of structure as compared to solution spectra or spectra obtained with “wet” surfaces (with adsorbed water) or with physi- and/or chemisorbed methanol. Ground-state association is certainly present in mechanical mixtures (see Figure 1, curve 7), where small crystals of fluorenone absorb the excitation light, whereas in samples prepared with aprotic solvents (benzene, dichloromethane, and cyclohexane) light is absorbed by both small crystallites formed on the surface of microcrystalline cellulose after solvent evaporation and also fluorenone molecules adsorbed at the cellulose surface (see curves 4, 5, and 6). A red shift is noted as compared to samples prepared with ethanol, methanol, and dioxane (solvents which efficiently swell cellulose). However, the differences in what concerns new bands are not as large as in the case of rhodamine, oxazine, or cyanine dyes adsorbed onto the same substrate, where a severe change in the absorption spectra was recently reported by us,1b,3a,26 indicating the formation of ground-state aggregates (dimers and larger aggregates) in some of the cases. (24) (a) Anpo, M.; Wada, T.; Kuboyama, Y. Bull. Chem. Soc. Jpn. 1975, 48, 2663 and references therein. (b) Anpo, M.; Wada, T.; Kuboyama, Y. Bull. Chem. Soc. Jpn. 1975, 48, 3085. (25) de Mayo, P.; Natarajan, L. V.; Ware, W. R. Organic Phototransformations in Nonhomogeneous Media; Fox, M. A., Ed.; American Chemical Society: Washington, DC, 1985; p 1. (b) de Mayo, P. Pure Appl. Chem. 1982, 54, 1623.
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Figure 3. Room temperature corrected fluorescence emission spectra of fluorenone adsorbed onto microcrystalline cellulose excited at 405 nm. The concentration of fluorenone is 0.25 mmol/g of cellulose. The solvents used for sample preparation are (1) methanol, (2) ethanol, (3) dioxane, (4) acetone, (5) dichloromethane, (6) benzene, (7) cyclohexane, and (8) mechanical mixture. All the spectra are presented for the same amount of absorbed light.
3.2. Luminescence Studies of Benzophenone and Fluorenone Adsorbed onto Microcrystalline Cellulose. We recently reported benzophenone phosphorescence studies at room temperature when this ketone is adsorbed onto microcrystalline cellulose.1a When cellulose was treated with ethanol or acetonitrile, solvents which efficiently swell the natural polymer, the probe becomes entrapped and less luminescence was detected when compared to samples prepared with dichloromethane or benzene, in which case the probe is adsorbed on the surface as an isolated molecule or in the form of small crystallites also deposited on the surface. From diffuse reflectance transient absorption and emission studies it was concluded that whenever ketyl radical was formed, a smaller phosphorescence emission of the sample was detected.1a As benzophenone penetrates into cellulose by using a polar solvent, more benzophenone ketyl radical is formed and, consequently, less phosphorescence is detected. Figure 3 shows the corrected fluorescence steady-state emission spectra of fluorenone adsorbed onto microcrystalline cellulose, excited at 405 nm at room temperature and using different solvents for sample preparation. No significant phosphorescence as compared to fluorescence emission was detected for any of the samples at room temperature, as data in Figure 3 clearly show. Excitation at higher energies (380, 290, and 230 nm) produced the same spectra as in Figure 3 and did not give evidence for any anomalous phosphorescence emission as previously reported in one case.11a Table 1 presents the fluorescence quantum yields for the solid powdered samples under study and for comparison purposes the solution fluorescence quantum yields were also determined and are also presented. These later values are in very good agreement with literature.11b,14 As previously found for the phosphorescence of benzophenone on cellulose at room temperature, fluorenone samples with the probe entrapped into the polymer chains (26) (a) Vieira Ferreira, L. F.; Oliveira, A. S.; Wilkinson, F.; Worrall, D. J. Chem. Soc., Faraday Trans. 1996, 92, 1217. (b) Oliveira, A. S.; Vieira Ferreira, L. F.; Wilkinson, F.; Worrall, D. J. Chem. Soc., Faraday Trans. in press.
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Figure 4. DRIFT spectra of benzophenone and fluorenone crystals in a KBr matrix: carbonyl stretching regions. The spectra were normalized to the maximum of the ν(CdO) band. Table 1. Room Temperature (T ) 20 ( 1 °C) Fluorescence Quantum Yields of Fluorenone When Adsorbed onto Microcrystalline Cellulose and in Solution adsorbed fluorenone
ΦF (%)
ethanol methanol dioxane acetone cyclohexane mechanical mixture dichloromethane benzene
1.4 2.2 2.3 2.9 7.7 8.3 9.9 10.4
fluorenone solutions
ΦF (%)
methanol ethanol cyclohexane dioxane benzene dichloromethane acetonitrile
0.07 0.09 0.1 1.0 1.1 2.3 2.5
(see curves 1, 2, and 3 for ethanol, methanol, or dioxane, respectively, in Figure 3) also exhibit a strong quenching of its fluorescence. Since the nature of the excited singlet states is π,π*, the origin of the quenching process is certainly different in this case. 3.3. DRIFT Spectra of Benzophenone and Fluorenone Adsorbed onto Microcrystalline Cellulose. The carbonyl stretching regions of crystalline benzophenone and fluorenone in a KBr matrix are shown in Figure 4. The wavenumbers of maximum absorption are, respectively, 1650 and 1715 cm-1 for benzophenone and fluorenone as expected for a diaryl ketone8 and a ketone strained by a five-membered ring.9 Small bands assigned to the CdC/CdO coupled stretching modes are observed at ∼1627 cm-1 (for benzophenone) and as a doublet at ∼1670 cm-1 (for fluorenone). In the case of fluorenone, the stronger doublet at 1611 and 1599 cm-1 is assigned to skeletal ring breathing modes.16 In Figure 5, the DRIFT spectra of benzophenone adsorbed onto microcrystalline cellulose using different swelling solvents are shown, as well as the spectrum for a mechanical mixture (which is very similar to crystalline benzophenone). A first analysis shows that the carbonyl band maximum shifts to successively higher wavenumbers as the solvent
Figure 5. Carbonyl stretching region of DRIFT spectra of benzophenone adsorbed onto microcrystalline cellulose, for concentration 750 µmol of BZP/g of cellulose: (1) methanol; (2) acetone; (3) dichloromethane; (4) mechanical mixture of BZP crystals and cellulose. The spectra were normalized to the maximum of the ν(CdO) band. Table 2. Wavenumbers of Maximum Absorption of the Carbonyl Stretching Band of Adsorbed BZP onto Microcrystalline Cellulose for Different Swelling Solvents
a
solvent
ν˜ max/cm-1
Ra
methanol ethanol acetonitrile acetone amyl alcohol benzene dichloromethane mechanical mixture
1658 1658 1658 1654 1653 1652 1653 1650
0.34 0.35 0.42 0.50 0.79 1.01 1.25 2.70
Values of R ) area1650/area1658.
is changed from dichloromethane to acetone and to methanol. The complete list of wavenumbers of maximum absorption is indicated in Table 2. The sequence of solvents is the same as previously found for increasing swelling capacity.1a It has been shown that this effect is stronger when the solvent is polar (protic or aprotic) and with the possibility of forming nonsterically hindered strong interactions with the hydroxyl groups of cellulose, namely, hydrogen bonds. Upon solvent removal, benzophenone partially replaces the solvent in those interactions, remaining entrapped between the chains, which gives rise to the ν(CdO) at 1658 cm-1. As hydrogen bonds are highly directional, this effect is favored by the possibility of BZP losing planarity. When a solvent such as benzene or dichloromethane, which does not swell cellulose, is used, benzophenone adsorbs mainly as crystallites on cellulose surface. Additionally, in curve 3 of Figure 5, the band at ∼1627 cm-1 assigned to CdC/CdO coupled stretching modes in crystalline phase is strong enough to be identified, confirming the presence of a significant amount of surface crystallites. A more thorough analysis, taking also into account band shape modifications, was made by deconvoluting the carbonyl band into the crystalline and entrapped benzophenone components. These were determined by a nonlinear least-squares fit to a sum of four Gaussians, as detailed elsewhere.22 The wavenumbers corresponding to the four band components were determined as 1665,
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Figure 6. Deconvolution of the carbonyl band of BZP adsorbed onto microcrystalline cellulose (same concentration as in Figure 5): (a) mechanical mixture; (b) methanol; (c) acetonitrile; (d) acetone; (e) tert-amyl alcohol; (f) dichloromethane.
1658, 1650, and 1643 cm-1. A few examples are shown in Figure 6. The relative intensities of the bands at 1650 cm-1 (the main band for mechanical mixture) and 1658 cm-1 (the main band for only entrapped benzophenone) change from system to system, clearly inverting when comparing mechanical mixture to mostly entrapped benzophenone (using methanol, for instance). In this cellulose, the ratio R (area1650/area1658) varies from 2.70 (in mechanical mixture) to 0.33 (methanol). The complete set of results is summarized in Table 2 (third column). The ratio R may be interpreted as a measure of the fraction of benzophenone adsorbed as crystallites, the maximum value of 2.70 corresponding to 100% of crystals. The evolution of R values shows that this fraction is strongly dependent on the solvent, and therefore, for each cellulose, they allow the ordering of solvents according to their swelling capacity. The DRIFT spectra of adsorbed fluorenone in the region 1800-1550 cm-1 are shown in Figure 7 for a few of the solvents used as swelling agents. The spectrum of a mechanical mixture is included for comparison and once again it matches that of fluorenone crystals. There is no observable shift of the carbonyl band when comparing the mechanical mixture with fluorenone adsorbed onto cellulose using different solvents. An attempt to deconvolute the band was made, but no modifications
in the relative intensities of the components could be observed. The only striking difference between the carbonyl band of these spectra is the larger full width at half height for mechanical mixture. It was attributed to a different crystal size obtained when grinding the samples and when the crystals are obtained by recrystallization, upon solvent evaporation. Although the carbonyl group would be the first plausible candidate to form hydrogen bonds with the hydroxyl groups of cellulose, these results point elsewhere: either fluorenone does not remain entrapped into cellulose chains, even when the swelling solvent is efficient, and this hypothesis is not acceptable taking into account the UV-vis results or the interactions between fluorenone and the polymer chains do not involve the carbonyl group. In fact, other details in the DRIFT spectra corroborate the last hypothesis: the bands assigned to ν(CdC/CdO) coupled modes (∼1670 cm-1), present in crystalline fluorenone, mechanical mixture and when the solvent used does not swell cellulose, are absent from the spectra regarding good cellulose solvents. Hydrogen bonds through interactions of the π cloud with hydroxyl groups of cellulose chains would be responsible for the decoupling of these modes, not producing significant alterations in the CdC skeletal breathing modes, at 1611 and 1599 cm-1.9 This is exactly what curves 2 and 3 in Figure 7 suggest, in good agreement with UV-vis results. The rigidity of fluorenone is responsible for the different
Adsorption onto Microcrystalline Cellulose
Langmuir, Vol. 13, No. 14, 1997 3793
4. Conclusions
Figure 7. Carbonyl stretching region of DRIFT spectra of fluorenone adsorbed onto microcrystalline cellulose, for concentration 250 µmol of FLN/g of cellulose: (1) ethanol; (2) acetone; (3) dichloromethane; (4) mechanical mixture of FLN crystals and cellulose. The spectra were normalized to the maximum of the ν(CdO) band.
adsorption process onto microcrystalline cellulose identified in this work.
A compared study of two structurally different ketones adsorbed onto microcrystalline cellulose was made by diffuse reflectance techniques. Complementary information obtained from diffuse reflectance studies in different spectral ranges has contributed to a better understanding of the adsorption processes involved. It has been proved that benzophenone remains entrapped in the substrate mainly by hydrogen bonds between the carbonyl group and the hydroxyl groups of the cellulose chains, whereas fluorenone’s entrapment involves the π conjugated system but not the carbonyl group. In this case, a hydrogen bond like interaction of the hydroxyl groups with the π cloud has been proposed. The differences observed were correlated with the rigidity of fluorenone, which is responsible for its planarity in any environment, in contrast with the free rotation around the C-(CdO) bonds in benzophenone, which are responsible for its adaptability to the cellulose structure, enabling the carbonyl group to participate in hydrogen bonds with cellulose chains. Finally, a confirmation of the relative capability of different solvents to swell microcrystalline cellulose was possible using modifications observed in the carbonyl stretching band of adsorbed benzophenone. Acknowledgment. This work was financially supported by JNICT, Project PRAXIS XXI/2/2.1/QUI/22/94. LA970060R
