Langmuir 1996,11, 231-236
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Photochemistry on Surfaces: Matrix Isolation Mechanisms Study of Interactions of Benzophenone Adsorbed on Microcrystalline Cellulose Investigated by Diffuse Reflectance and Luminescence Techniques L. F. Vieira Ferreira,"??J. C. Netto-Ferreira,S I. V. Khmelinskii,?>O A. R. Garcia, and Silvia M. B. Costa" Centro de Quimica Fisica Molecular, Complexo I, Instituto Superior Ticnico, 1096 Lisboa Codex, Portugal, Departamento de Quimica, Universidade Federal Rural do Rio de Janeiro, Rio de Janeiro, Brasil CEP 23851, and Centro de Quimica Estrutural, Complexo I, Instituto Superior Ticnico, 1096 Lisboa Codex, Portugal Received June 20, 1994. In Final Form: September 21, 1994@ The swelling of microcrystalline cellulose by the use of polar protic solvents such as ethanol or methanol enables the penetration of benzophenone into submicroscopicpores of the natural polymer, while solvents such as benzene or dichloromethane do not open the polymer chains, thus not producing any entrapped benzophenone. Ground-state diffuse reflectance studies revealed a dramatic blue shift in the 350-nm absorption of benzophenone in the former case, in accordance with a strong interaction of the hydroxyl groups of cellulose with the ketone. Diffise reflectance laser flash photolysis studies of benzophenone adsorbed on microcrystallinecellulose showed, in cases where benzophenone is entrapped in the polymer chain, the formation of a transient which decays nonexponentially and exhibits a maximum absorption at about 530 nm, assigned to triplet benzophenone.After ca. 25ps, this transient generates another species with an absorption maximum at 545 nm. We assigned this new species to the diphenylketyl radical. In all caseswhere the solvent does not swell cellulose,a differentbehavior was observedtypical for benzophenone microcrystalstriplet decay. The ketyl radical formationis greatly reduced in this case. Triplet benzophenone decays by complex kinetics and lives about 10ps when adsorbed onto microcrystalline cellulose,while the ketyl radical, when formed, lives 1order of magnitude longer than the triplet. Samples which exhibit a high yield of ketyl radical formation also have a smaller phosphorescence emission in accordance with the fact that large amount of triplet molecules were consumed in the process of hydrogen abstraction from the matrix, involving hydrogens linked to carbons bearing a hydroxyl group.
1. Introduction The recent development of diffuse reflectance laser flash photolysis (DRLFP) made possible the study of transients in a variety of opaque ~ o l i d s l or - ~a t interfaces of powders, microcrystals, and polymer f i l m ~ . ~ -Kinetic l~ and spectroscopic information on the behavior of excited state of
* To whom correspondence should be addressed. Centro de Quimica Fisica Molecular, Complexo I, Instituto Superior TBcnico. *Departamento de Quimica, Universidade Federal Rural do Rio de Janeiro. On leave from the Institute of Chemical Kinetics and Combustion of the Russian Academy of Sciences, 630090, Novosibirsk, Russia. II Centro de Quimica Estrutural, Complexo I, Instituto Superior TBcnico. Abstract published in Advance ACS Abstracts, December 1, 1994. (1) Wilkinson,F.;Kelly, G.P. InHandbook oforganicphotochemistry, Scaiano, J. C., Ed.; CRC Press, Boca Raton, FL, 1989; Vol. 1, p 293. (2) Wilkinson, F. J. Chem. Soc., Faraday Trans. 2, 1986,82,2073. (3) Wilkinson, F.; Willsher, C. J. Tetrahedron 1987,43, 1197. (4) Wilkinson, F.; Willsher, C. J. J. Chem. SOC.,Chem. Commun. 1985, 142. (5) Oelkrug, D.; Honnen, W.; Wilkinson, F.; Wilsher, C. J. J.Chem. Soc., Faraday Trans. 2 1987,83,2081. (6) Wilkinson, F.; Willsher, C. J. Chem. Phys. Lett. 1984,104,272. (7) Wilkinson, F.; Willsher, C. J. Appl. Spectrosc. 1984,38, 897. (8) Kelly, G. P.; Leicester, P. A.; Wilkinson, F.; Worrall, D. R.;Vieira Ferreira, L. F.; Chittock, R.; Toner, W. Spectrochim. Acta 1990,46A, 975. (9) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B.; Katalnikov, I. V. Chem. Phys. Lett. 1992,193, 461. (10)Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B. Chem. Phys. Lett. 1990, 173, 277. (11) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B. Langmuir 1993, 9, 1001. (12) Wilkinson, F.; Worrall, D.; Vieira Ferreira, L. F. Spectmchim. Acta 1992,48A, 135. @
0743-746319512411-0231$09.0OlO
molecules adsorbed on microcrystalline cellulose was obtained by the use of this technique, either by direct excitationEJ2 or by the use of triplet energy transfer between molecules coadsorbed on these substrate^.'^-'^ In the case of Oxazine l14J6 important information on the adsorbed dye was obtained by the use of triplet benzophenone sensitization, since direct excitation of the dye did not produce a detectable amount of triplet Oxazine 1. I t was also demonstrated that restrictions on the mobility enhance the transient lifetimes14 with respect to those observed in solution.16 Recently we have shown that in Auramine 0, fluorescence emission a t room temperature increases 2-4 orders of magnitude when this molecule is adsorbed onto microcrystalline cellulose (relative to solution behavior), thus reaching a limitingupper value of 0.35.l' The fluorescence quantum yield also varies with the residual degree of humidity of the sample since the alteration in the rigidity of the environment greatly affects the internal conversion rate in Auramine 0. Rhodamine 101 and 6G can be used as fluorescence standards for adsorbed molecules on highly scattering materials provided dye aggregation is taken into account.18 We have recently presented a new simple method for fluorescence quantum yield (&) determinations of lumi(13) Wilkinson, F.; Vieira Ferreira, L. F. J.Luminesc. 1988,40&41, 704. (14) Wilkinson, F.; Leicester, P. A.; Vieira Ferreira, L. F.; Freire, V. M. M. Photochem. Photobiol. 1991, 54, 599. (15) Vieira Ferreira, L. F.; Oliveira,A. S.; Khmelinskii, I . V.; Costa, S. M. B. J.Luminesc. 1994, 60&61, 485. (16) Wilkinson, F.; Kelly, G. P.; Vieira Ferreira, L. F.; Freire, V. M. M.; Ferreira, M. I. J. Chem. SOC.,Faraday Trans. 2 1991,87, 547. (17) Vieira Ferreira, L. F.; Garcia, A. R.; Freixo, M. R.; Costa, S. M. B. J. Chem. Soc., Faraday Trans. 1993,89, 1937.
0 1995 American Chemical Society
232 Langmuir, Vol. 11, No. 1, 1995
Vieira Ferreira et al.
nescent molecules adsorbed onto powdered microcrystalcellulose chains or by room temperature fluorescence line cellulose.ls The quantum determination of& is based quantum yields or lifetimes.17 on the ratio of the slopes of curves which correlate In this paper we extend our studies to a triplet excited fluorescenceintensity and absorbed light for both standard state probe-benzophenone-which can be incorporated and unknown sample. in cellulose using polar protic (alcohols)or aprotic solvents Recent DRLFP studies of radical ions pair or radical (acetonitrile, acetone, and dichloromethane). We also used pairs on surfacesg-ll gave evidence for multiple adsorption a nonpolar solvent (benzene) as swelling agent for celsites on cellulose and silica surfaces. Time resolved spectra lulose. are usually similar to those obtained for the same species The penetration of benzophenone into submicroscopic in solution, although they are usually broader and exhibit pores of the solid substrate (cellulose) is evaluated from a nonexponential decay kinetics.10~11~17~19-22 ground-state diffise reflectance spectra and steady-state Cellulose is a n important solid substrate since it can be and transient phosphorescence data, as well as by timeused to obtain room temperature phosphorescence (RTP) resolved absorption spectral studies, showing its photofrom species adsorbed on it (usually spotted and dried on chemical reactivity toward H-abstraction. filter paper), which did not show any RTP in s o l ~ t i o n . ~ ~ ~ ~ ~ It was also reported that species adsorbed onto cellulose 2. Experimental Section are protected from quenching by molecular oxygen,14J7Ja,25 2.1. Materials. Benzophenone was purchased from since the mobility of the latter is highly reduced when Aldrich (Gold label grade) and was used without further cellulose is well dried. purification. Microcrystalline cellulose (Aldrich) with 20 X-ray studies of native or fibrous cellulose have shown that this medium acts as a two-phase s y ~ t e m :less ~ ~ * ~ ~pm average particle size was dried in a vacuum oven (ca. 1mTorr) a t 70 "C during 48 h before sample preparation. ordered and less compact amorphous regions mainly Ethanol, methanol, acetone, and dichloromethane were located on surface of the elementary fibrils and wellHPLC grade from Romil Chemicals. Benzene, 1-propanol, ordered regions (crystallites) where cellulose molecules and acetonitrile were from Merck (Uvasol grade). 2-Methexist in a definite crystal pattern closely packed together yl-2-butanol (tert-amyl alcohol) was Merck pro-analysis. with maximum hydrogen bonding between adjacent All these solvents were used as received, after checking cellulose chains.26 purity by W and visible optical absorption. In some cases Microcrystalline cellulose results from hydrolysis of solvents were dried prior to sample preparation by the purified cellulose after 15 min in 2.5 N HC1 at 105f 1"C, use of molecular sieves (3 and 4 A, 4-8 mesh, Aldrich) as described in ref 28 and patents quoted there, and is a previously activated by slowly heating up to 250 "C under very pure form of cellulose. At this point, hydrolysis has vacuum. Accordingto the producer's technical information slowed down and LODP ("levelled o f f degree of polybulletin, 44, molecular sieves were used as universal merization) cellulose is produced.28 In this severe acid drying agents for polar and nonpolar solvents. However, hydrolysis the amorphous regions are attacked and for methanol and ethanol, 3-A sieves are needed since transformed into a very highly crystalline residue, and these solvents have small critical diameters that may fit the final cellulose has a n unusually high degree of inside the 4-i%cavity. crystallinity.28 2.2. Sample Preparation. Samples were prepared The hydroxyl groups of cellulose have a strong affinity by adding a solution of benzophenone to the previously for polar solvents and solutes that can reach them. Water dried substrates in a beaker. The resulting suspension is a n example of a good swelling agent for cellulose, was stirred periodically and also allowed to stay 10 min although it still occurs to varying extent with other polar in a n ultrasonic bath a t room temperature to completely solvents such as methanol, ethanol, propanol, and others.29 desegregate the solid agglomerates. M e r slow evaporaRecently we have reported a matrix-isolation mechation of the solvent in a fume extractor, the final traces nism by using powdered samples of a fluorescent probe, were removed by placing the samples into a vacuum oven auramine 0 adsorbed onto microcrystalline cellulose. a t 60 "C for 24 h. Further drying of the samples did not Indeed, different degrees of swelling were obtained when produce any change in ground-state reflectance spectra polar protic or aprotic solvents were used to adsorb or or in transient absorption or emission decays. entrap the dye into c e l l ~ l o s e . The ~ ~ differences were A n alternative procedure was tested for initial solvent evaluated from changes of ground-state reflectance spectra evaporation, by using a rotary evaporator. No differences of auramine 0 entrapped into the natural polymer in ground-state diffuse reflectance spectra were found between the samples prepared by the two different (18)Vieira Ferreira, L. F.; Freixo, M. R.; Garcia, A. R.; Wilkinson, methods within experimental error. The simplest proF. J. Chem. SOC.,Faraday Trans. 1992, 88, 15. (19) Oelkrug, D.; Reich, S.; Wilkinson, F.; Leicester, P. A. J. Phys. cedure, slow solvent evaporation in the fume extractor, Chem. 1991,95,269. was adopted as standard in the present work. (20) Kelly, G. P.; Willsher, C. J.; Wilkinson, F.; Netto-Ferreira, J. C.; Olea, A.; Johnston, L. J.; Scaiano, J. C. Can. J. Chem. 1990, 68, 812. For comparison purposes several samples were also (21) Pankasen, S.; Thomas, J. K. J.Phys. Chem. 1991, 95, 6990. prepared by mechanically mixing of benzophenone crystals (22) Albery, W. J.;Bartlet, P. N; Wilde, C. P.; Darwent, D. R. J.Am. with microcrystalline cellulose, using a n agate pestle and Chem. SOC.1986,107, 1854. mortar. (23) Hurtubise, R. J. Phosphorimetry. Theory, Instrumentation and Applications; VCH Publishers: New York, 1990. 2.3. Ground-State Absorption and Steady-State (24) Vo-Dinh, T. Room Temperature Phosphorimetry for Chemical Emission Experiments. Ground-state absorption studAnalysis; Wiley-Interscience: New York, 1984. (25) Murtagh, J.; Thomas, J. K. Chem. Phys. Lett. 1988, 148, 445. ies of benzophenone adsorbed onto microcrystalline cel(26) Krassig, H.; Steadman, R. G.; Schliefer, K;Albrecht, W. Cellulose, lulose were performed using a OLIS 14 UVMS/NIR in Ullmann's Encyclopedia of Industrial Chemistry; VCH Publishers: spectrophotometer with a diffuse reflectance attachment New York, 1986; Vol. 5, p 375. based on a 90 mm diameter integrating sphere. The (27)Casey, J. P. In Pulp and Paper: Chemistry and Chemical Technology; Interscience: New York, 1966; Vol. 1,p 8. standard apparatus was modified to include the possibility (28) Battista, 0. A. Microcrystalline Cellulose. In Encyclopedia of of using short-wave-pass filters that stop the emission of Polymer Science and Technology;Mark, H. F., Gaylord, N. G., Bikales, benzophenone from reaching the detector (Hamamatsu, N. M., Eds.; Wiley: New York, 1965; Vol. 3, p 285. (29) McAlleese, D. L.; Dunlap, D. L. Anal. Chem. 1984, 56, 2246. Model R955). In this case we used a Corion UG5 filter.
Photochemistry on Surfaces Experimental details to obtain accurate ground-state absorption spectra of powdered samples are given elsewhere.17J8 Steady-state phosphorescence emission spectra of powdered samples of benzophenone adsorbed onto microcrystalline cellulose were performed a t room temperature (20 f 1 "C) using a home-made fluorometer, specially designed for front-surface studies. A detailed description of this apparatus as well as details of the method adopted to obtain fully corrected emission or excitation spectra are given in ref 30. 2.4. Time-ResolvedDiffuse Reflectance Transient Absorption and Emission Studies. Time-resolved absorption and emission spectra and decay kinetics of benzophenone adsorbed onto microcrystalline cellulose or benzophenone crystals were obtained by the use of diffuse reflectance laser flash photolysis technique developed by Wilkinson and co-workers in the last decade.l+ The equipment used for nanosecond laser flash photolysis in diffuse reflectance mode is identical to that used in transmission mode, except for the geometry for collecting the analyzing light. The setup used for laser flash photolysis experiments in our laboratory allows the use of the system either in transmission or in diffuse reflectance mode. The system is based on a Nd:YAG laser (SpectraPhysics, Quanta-Ray GCR-3) as a n excitation source (-8 ns pulse width a t 355 nm) the fundamental from which may be doubled, tripled, or quadrupled. A maximum energy pulse of 25 m J a t 355 nm was used in this work. Spectra were recorded by using a 2430A Tektronix digital oscilloscope interfaced to a PDP 11/73 microcomputer in order to acquire and store data. The detector was a Hamamatsu R928 photomultiplier, sensitive up to 900 nm, and the monitoring source was a 250-W xenon lamp from Applied Photophysics, pulsed to obtain ca. 0.5 ms light pulses. Kinetic traces were averaged over eight laser pulses. Data are reported as percentage of absorption by the transient, lOOAJdJ0 = (1- Jt/J0)100, where JOand Jtare diffuse reflected light from sample before exposure to the exciting laser pulse and a t time t afier excitation, respectively. In all samples, except for benzophenone microcrystals, the initial transient absorption (520%) increased proportionally with laser intensity, giving evidence to the validity of this treatment to analyze the transient decay, rather than the Kubelka-Munk a n a l y ~ i s . ~ , ~ ~ Special care was taken to ensure that the analyzing light was only incident on the part of the sample surface which was excited by the laser beam (a circle of ~6 mm diameter in our setup). Specular reflection of the laser from cell windows was excluded from the analyzing monochromator entrance slit. 3. Results and Discussion 3.1. Ground-State Absorption Spectra of Benzophenone Adsorbed onto Microcrystalline Cellulose. Ground-state absorption spectrum of benzophenone adsorbed onto microcrystalline cellulose is shown in Figure 1,in the case where dichloromethane was used as solvent for sample preparation. Figure 1also shows the room temperature steady-state phosphorescence emission from the same sample, as well as the emission of benzophenone crystals, both samples being in contact with the air. Very similar results were obtained when benzene was used as solvent for sample preparation. The remission curve (F(R)= (1 (30) VieiraFerreira,L.F.;Costa, S. M. B.; Pereira,E. J. J.Photochem. Photobiol., A: Chem. 1991,55, 361.
Langmuir, Vol. 11, No. 1, 1995 233
Wavelength 1 nm
Figure 1. Remission function for 3 mmol of benzophenone adsorbed onto 1 g of microcrystalline cellulose from dichloromethane (1)and room temperature phosphorescenceemission of same sample (2)compared to the benzophenone microcrystals emission (3).
1
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0.60.6
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300
,
,
320
340
360
380
400
Wavelength I nm
Wavelength I nm
Figure 2. Remission function for 0.75 mmol of benzophenone adsorbed onto 1g of microcrystalline cellulose from: (a, top) 1, methanol, 2, ethanol, 3, propanol, 4, tert-amyl alcohol; (b, bottom) 1, acetonitrile, 2, acetone, 3, benzene, 4, dichloromethane; (a, b) 5, remission function of the mechanical mixture of benzophenone crystals with cellulose.
- R)2/2Rvs wavelength, where R is the reflectance1 as determined according to refs 17 and 18 for benzophenone is similar to the published absorption spectra in solution (see refs 31 and 32 and shows the n-n* orbital transition of benzophenone maximizing a t -350 nm and also the n-n* orbital transition which occurs at shorter wavelength and peaks a t -250 nm. Figure 2a shows the remission curves obtained (using methanol, ethanol, propanol, and tert-amyl alcohol for sample preparation), for a 750;umol loading of benzophenone/gram of cellulose. Similar data are presented in (31) Murov, S.L.Handbook ofPhotochemistry; Marcel Dekker: New York, 1973. (32) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings: Menlo Park: CA, 1978.
Vieira Ferreira et al.
234 Langmuir, Vol. 11,No. 1, 1995 Figure 2b, but now the solvents used for sample preparation are acetone, acetonitrile, benzene, and dichloromethane. All curves are normalized to unity a t 340 nm. In both figures, the remission curve for mechanical mixture of benzophenone and microcrystalline cellulose, with the same loading of the ketone, is presented for comparison. It is well-known from solution studies that solvents like ethanol and other polar protic solvents promote a "blue shift" in the n-n* absorption ofthe benzophenone carbonyl due the fact that the oxygen of the carbonyl group 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 solvent interaction than in the excited state. Data from Figure 2a clearly show that cellulose promotes a considerable "blue shift'' in benzophenone n-n* absorption when methanol, ethanol, and propanol were used for sample preparation. tert-Amyl alcohol curve is clearly deviating from the other and approaches the behavior showed by the mechanical mixture. Data from Figure 2b also show that the magnitude of this "blue shift" effect decreases in the sequence acetone, acetonitrile, dichloromethane, and benzene. The use of different polar and nonpolar solvents allowed us to evaluate their respective capacities as swelling agents for cellulose, since they seem to open the way for benzophenone to penetrate into submicroscopic pores of cellulose to different degrees, as shown by different interactions of benzophenone with cellulose. The solvent is removed after drying the samples, and benzophenone is entrapped between cellulose chains. For good swelling agents, solvent-to-cellulose hydrogen bonds are replaced by cellulose-to-benzophenonehydrogen bonds, providing a strong stabilization of the adsorbate as shown by the large "blue shift" in the n-n* orbital transition. From data presented in Figure 2, we conclude that solvents like methanol, ethanol, propanol, acetone, and acetonitrile promote a considerable swelling of the predried microcrystalline cellulose, opening the way for benzophenone to penetrate into the submicroscopicpores of cellulose which were not accessible before this solvent-matrix effect occurred. Benzene and dichloromethane, which cannot form hydrogen bonds with the hydroxyl groups of cellulose, do not open the polymer chains; hence the cellulosebenzophenone interaction is similar to the one existing in the mechanical mixture. tert-Amyl alcohol is a n intermediate case, as we have seen in a previously study of Auramine 0 entrapped into cellulose polymer chains.l' Although it has a hydroxyl group, the steric factors prevent this solvent from penetrating into microcrystalline cellulose crystallites in a degree comparable to that for methanol, ethanol, or propanol. Benzophenone only penetrates into cellulose microcrystals after swelling by solvent treatment with good swelling solvents. Indeed, tert-amyl alcohol is not a good swelling agent for microcrystalline cellulose as can be seen from its remission curve in Figure 2a which resembles that of the mechanical mixture. Steady-state and transient emission spectra presented later will further justify this interpretation. The use ofwell-dried cellulose(with or without methanol o r ethanol solvent pretreatment) for sample preparation provides the same benzophenone ground-state absorption spectra within experimental error, showing that there is no significant solvent traces left after the final drying procedure. However, the use of wet cellulose (with different degrees of humidity) or undried solvents for
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650
660
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Figure 3. Time-resolved absorption spectrum of 0.75 mmol of benzophenone adsorbed onto 1g of microcrystalline cellulose from ethanol. Curves 1, 2, 3, 4, and 5 were recorded 1.0, 4.0, 10.0, 25.0,and 80.0ps after laser pulse. 98-
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Figure 4. Time-resolved absorption spectrum of 0.75 mmol of benzophenone and 1 g of microcrystalline cellulose from benzene. Curves 1,2,3, and 4 were recorded 1.0,4.0,10.0,and 80.0 ps afier laser pulse.
sample preparation affects the degree of benzophenone entrapment. 3.2. Triplet-Triplet Absorption Spectra of Benzophenone Adsorbed onto Microcrystalline Cellulose. Figures 3 and 4 show the absorption spectra of benzophenone adsorbed onto microcrystalline cellulose (750pmol g-' of cellulose in all solvents)from two different solvents used for sample preparation: ethanol and benzene, respectively. The latter is identical to the T-T' absorption spectra from a mechanical mixture of benzophenone and cellulose, also containing 750 pmol g-l of cellulose, as well as with the spectrum from benzophenone microcrystals, finely ground using agate vial and ball pestle. In order to enable spectral comparisons, data were recorded within the same experimental conditions in all cases and samples were air equilibrated and recorded in a 100-ps range time scale of the transient digitizer, using about 25 m J per excitation pulse. Clearly, two different groups of spectra can be distinguished. Into the first we include samples prepared from ethanol, methanol, propanol, acetonitrile, and acetone, where the maximum percentage of absorption varies between 21% and 12%,decreasing in the order listed above (see Table 1). The initial transient absorption maximum a t 530 nm was recorded with a -1 ps delay after laser excitation. Into the second group we include samples from benzene, dichloromethane, and also the one prepared from
Photochemistry on Surfaces
Langmuir, Vol. 11,No.1, 1995 235 Table 1. Comparison of Transient Absorption Results for Various Samplee solvent used for sample preparation
ethanol methanol propanol acetonitrile acetone tert-amyl alcohol benzene dichloromethane mechanical mixture of benzophenone with cellulose benzophenone crystals
d0
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3
E 8
E
4
Time,
s
Figure 5. Transient absorption decay at 530 nm from 0.75 mmol of benzophenone adsorbed onto 1 g of microcrystalline and 5 correspond to ethanol,propanol, cellulose. Curves 1,2,3,4, acetone, benzene, and dichloromethane,respectively, as solvents used for sample preparation.
tert-amyl alcohol. The mechanical mixture of benzophenone and cellulose also belongs to this group, as well as the pure crystals of benzophenone, although in this case the maximum percentage of absorption is much higher, about 40% at the excitation energy of 25 m J per pulse. All other samples in this group also exhibit a maximum absorption a t 530 nm, but only within a range of 5 4 % . The two groups are also clearly distinct from the point of view of the decay of the transient, as can be seen in Figure 5. In group I, the triplet state absorption decays to reveal the presence of a much longer-lived transient, as we obtained previously for benzophenone adsorbed on cellulose from acetonitrile, which we assigned to the ketyl radical of benzophenone,14 due to its similarity with the solution spectra.33 We show now that the same radical is also formed when using ethanol, methanol, propanol, and acetone for sample preparation. However, the radical formation is not detected when using tert-amyl alcohol, benzene, and dichloromethane, or in the mechanical mixture of benzophenone and cellulose, samples included in group 11. These results can be explained by assuming that for the case of solvents of group I, the triplet benzophenone molecules are entrapped in such way that the cellulose hydrogens which can lead to formation of stable radicals (i.e., the ones linked to a carbon bearing a hydroxyl group) are easily abstracted. Also, hydrogen abstraction reaction is faster for entrapped vs adsorbed benzophenone molecules due to the fact that there are more hydrogen atoms in contact with the entrapped one. Such a n arrangement is not possible when this ketone is adsorbed exclusively on the cellulose surface, which is the case for solvents of group 11. All the transient absorption decays of benzophenone adsorbed on microcrystalline cellulose were nonexponential as r e p ~ r t e d . ~Figure J ~ 5 shows transient absorption decay measured a t 530 for samples belonging to groups I and 11, in a compressed time scale (% absorption is plotted as a function of log t) in order to make evident the short and long time behavior. When no ketyl radical is formed (group I1 samples) the decay exhibits at early times a second-order component but, after several microseconds, becomes a first-order decay, as reported in refs 6 and 14. A simple kinetic analysis using a biexponential fit was used showing that the fast component has tl 1.2-3 ps and the slower one t2 10-25 ps for all samples, in
-
-
(33)Porter, G.;Wilkinson, F. Trans. Faraday SOC.1961,57,1686.
maximum % maximum % absorption of the absorption of benzophenone tripletb the ketyl radicalc 18-21 8 18-20 8 13-17 3-6 15 5 12-17 3-5 8
