The Journal of
Physical Chemistry
Registered in U.S. Patent Office
0 Copyright, 1981, by the American Chemical Society
VOLUME 85, NUMBER 2
JANUARY 22, 1981
LETTERS Hydrogen Abstraction by Benzophenone Studied by Holographic Photochemistry Chr. Brauchle,t D. M. Burland;
and G. C. Bjorklund
IBM Research Laboratory, San Jose. California 95193 (Recelved: September 8, 1980; I n Final Form: December 1, 1980)
Intermolecular hydrogen abstraction by benzophenone in a poly(methy1 methacrylate) host is studied by using the new technique of holographic photochemistry. In this technique the temporal course of a photochemical reaction is followed by followingthe growth of a hologram resulting from the photochemistry. The photochemistry is found to involve two steps, each involving the absorption of two laser photons. In the first step a benzophenone triplet state abstracts a hydrogen atom from the host to produce a ketyl radical. This ketyl radical reacts with the host, producing a long-lived intermediate that is involved in the second step. The hydrogen abstraction reaction is found to occur only from a higher na* triplet state. No irreversible hydrogen abstraction occurs from the lowest n?r* or a ~ triplet * state.
Introduction The photochemistry of ketones in general and benzophenone in particular has been very thoroughly studied.’V2 This interest has in part been due to the wide variety of synthetically useful photochemical reactions undergone by ketones and the easy accessibility of the reactive excited states in the near-UV. Of particular interest has been the reactivity of these molecules in the lowest na* triplet state. In this state the molcule can undergo a cleavage, chargetransfer complexation, or, as will be the subject of this paper, hydrogen abstraction. Most of the previous works have concentrated on the solution chemistry of benzophenone. Here we concentrate on its reaction in the solid state. Hydrogen abstraction by benzophenone yields several different ultimate photochemical products. One such pathway is as follows: t Institute of Physical Chemistry, University of Munich, D-8000Munich-2, West Germany.
Ph,C=O
2hv
step I
(Ph,C=O)*
- LAT
2hv
step I1
products
(1) In the first step, benzophenone is excited indirectly to the lowest triplet state by direct absorption into the singlet state. The triplet state reacts with its host environment, in the present case a poly(methy1 methyacrylate) (PMMA) polymer matrix, to produce what has been called among other things a “light absorbing transient (LAT)”.3-6 This intermediate further reacts upon irradiation to yield products. Chilton et a1.6 have shown that reaction I is a minor pathway in solution. Our results indicate that in PMMA, where diffusion together of ketyl radicals is in(1) J. C.Scaiano, J.Photochem., 2, 81 (1973/74). (2) P.J. Wagner, Top. Curr. Chem., 66, 1 (1976). (3) G. 0.Schenck, M. Cziesla, K. Eppinger, G. Matthias, andM. P a p , Tetrahedron Lett., 193 (1967);G. 0.Schenk and G. Matthias, rbrd., 699 (1967). (4)N. Filipescu and F. L. Minn, J.Am. Chem. SOC., 90,1554 (1968). (5) J. Chilton, L. Giering, and C. Steel, J.Am. Chem. SOC., 98, 1865 (1976).
0022-3654/81/2085-0123$01.00/00 1981 American Chemical Society
124
The Journal of Physical Chemistty, Vol. 85, No. 2, 1981 Mirror
7
Photodiode
Focusing
Signal In
616
Signal Out
Strip Chart Recorder
Computer
Figure 1. A schematic representation of the experimentalsetup used to record the holographic growth curves. The dashed lines indicate the llght paths and the solid line the paths of the electrical signals. The angle 8 is referred to in eq 1.
hibited, reaction I is the predominant pathway. The general features of the reaction scheme have been pieced together by other^.^-^ Here we will concentrate on using the newly developed technique of holographic photochemi~trf*~ to examine some of the details of the reaction in a solid polymer matrix. In particular, we will show that steps I and I1 each involve the sequential absorption of two photons, that is, they are quadratic in laser intensity. Furthermore, we will demonstrate that the hydrogen abstraction reaction of step I involves excitation of a higher n7r* triplet state. The experiments reported here, in addition, indicate how easily one can investigate a rather complicated reaction scheme using the technique of holographic photochemistry. Experimental Section
Sample Preparation. The benzophenone used was obtained from J. T. Baker Chemical Co. (Baker grade) and purified by recrystallization from pentane. Two different techniques were used to prepare the samples of benzophenone-doped PMMA. In the most frequently used method, PMMA (Aldrich, secondary standard) was dissolved in acetone to form a viscous solution. The benzophenone was added to this solution. Thin (-250 pm thick) samples of doped polymer were then made by casting the mixture in a mold and allowing the solvent to evaporate slowly. To determine if the presence or absence of oxygen in the samples had anything to do with the photochemical properties being observed, several degassed samples were made in the following way. The benzophenone was dissolved in distilled methyl methacrylate monomer to which a small amount (-0.05 g) of ad-azodiisobutyronitrile had been added. A 0.5-mm thick cell was then filled with this solution and outgassed by repetitive freeze-pump-thaw cycles. After degassing, the cell was sealed under vacuum and placed in an oil bath at 60 O C until the monomer had thermally polymerized, a process taking at least 24 h. Measurements on samples of this type were made without exposing them to air. The Holographic Technique. The technique of holographic photochemistry has been discussed in detail e l ~ e w h e r e . ~Here ? ~ we will give a brief description of the apparatus with particular emphasis on the differences between the system currently being used and the one previously de~cribed.~ The experimental set-up used to produce and monitor the holograms is shown in Figure 1. The holograms were produced by the multiline output of an argon laser with UV optics. This output consisted of discrete laser lines (6) D. M. Burland, G. C. Bjorklund, and D. C. Alvarez, J.Am. Chem.
Letters
at 363.8, 351.4, 351.1, and 333.6 nm. The laser beam, polarized perpendicular to the plane of the figure, was split into object and reference beams. By placing a focusing lens before the beam splitter, both beams could be brought to a focus at the sample. The angle of interference, 20 in the figure, was on the order of 0.02 rad. The samples were exposed to an overall optical flux of between 0.0064 and 6.4 W/cm2. The distribution of the light power among the laser lines was 46.8% for 363.8 nm, 47.5% for 351.4 and 351.1 nm together, and 5.7% for 333.6 nm. The hologram is produced in the sample by the interference of the two intersecting beams. As photochemistry goes on, the interference fringes are impressed in the sample as modulations of the index of refraction and extinction coefficient? In actuality, the sample records the superposition of the interference fringes produced by each of the laser lines. However, the general theoretical conclusions of ref 7 are still valid. The object and reference beam continue beyond the sample. If one blocks the object beam before the sample, one observes a holographic image of this beam produced by the deflection of a fraction of the reference beam intensity into the object beam direction by the interference fringes in the sample. It is the growth of this holographic image that we monitor to follow the temporal course of the photochemistry. The growth of the hologram is monitored by chopping the reference beam at about 200 Hz. The reference signal from this chopper is used to trigger a boxcar integrator. The boxcar integrator then samples the signal from a photodiode placed in the reference beam. The signal is sampled over a time interval of 0.5 ms at a fixed delay time after the triggering pulse. In this way the signal from the photodiode is only detected when the chopper has blocked the reference beam. The output signal from the boxcar integrator thus records the growth of the first-order hologram. This output signal is recorded either on a strip chart recorder or digitally by using an IEiM Series/l computer. The hologram efficiency, q, that is the fraction of the incident object beam that is diffracted by the hologram, is given during the initial stages of hologram formation bf
x cos 0
2 cos 0
Here d is the sample thickness, the wavelength of the laser light, and a the spatially averaged absorption coefficient for the material. The most important parameters in eq 1 for our present purposes are nl and cy1. nl is the amplitude of the modulation of the index of refraction in the sample as a consequence of the photochemistry and a1is the corresponding amplitude of the modulation in the absorption coefficient. In another publication7we have related the quantities nl and cy1 to the photochemical parameters. We have shown that the temporal growth of the hologram can be simply expressed by the relationship
a(I)t2 = bPt2
q =
where b is a proportionality constant that contains among other parameters information about the quantum yields of the photochemical reaction involved, I is the total intensity of the laser light incident on the sample, and t is the time of irradiation. n is the order of the photochemical reaction and is equal to one for a one-photon process and
Soc., in press.
(7) G. C. Bjorklund, D. M. Burland, and D. C. Alvarez, J.Chem. Phys., in press.
(8) H. Kogelnik, Bell Syst. Tech. J.,48, 2909 (1969).
The Journal of Physical Chemistry, Vol. 85, No. 2, 1981 125
Letters
Time (sec.)
Flgure 2. The holographic efficiency as a function of time recorded by using the UV lines of an Ar laser with a power density of 1.9 W/cm2. The numbers next to the curve refer to points at which the spectra in Figure 3 were taken.
two for a two-photon process.
Results In Figure 2 we show an experimentalholographic growth curve. Actually one can observe two stages in the growth curve. In the first stage, the hologram grows very rapidly and then decays back to zero. In the second stage the hologram again begins to grow, but at a much slower rate. Complex growth curves of this type are characteristic of systems in which more than one step is involved in the photochemistry. This can be seen by referring to eq 1. Consider for simplicity the case where the hologram is produced predominantly by changes in index of refraction. Holograms of this kind are called phase holograms and may be considerably more efficient than amplitude holograms due to changes in the a b s o r p t i ~ n . ~ ~The ' ~ efficiencies of the holograms in our samples (10-15%) are greater than could be obtained from a pure absorption hologram. In addition, the fact that the diffraction by the first step hologram is almost completely cancelled during the growth of the second step hologram indicates that one has either a nearly pure phase or amplitude hologram. Equation 1 then indicates that During the initial phases of a two-step process the change in n, is due entirely to the conversion of starting material, in the present case benzophenone, to products. The hologram thus begins to grow. However, if the photoproducta themselves can react to produce still other products, then the growth rate changes. In particular, if the index of refraction change from this second set of products is opposite in sign from the change produced in the first step, the hologram intensity actually decreases, goes through zero, and then begins to increase again. In the case where the sign of the change in index of refraction is the same for both reaction steps, one expects an increase in the quadratic growth rate ( b in eq 2) as the second step begins to contribute. In Figure 2 one sees the case where the index of refraction change has a different sign for the two reaction steps. The figure illustrates a significant advantage of the holographic technique; in a single experiment lasting a few minutes one is able to determine the number of steps involved in the reaction. In should also be pointed out that the almost complete cancellation of the first hologram by the second reaction step indicates that reaction scheme is the major pathway for disappearance of benzophenone unlike the situation in s ~ l u t i o n .If~ an additional parallel reaction were oc(9)J. C. Urbach and R. W. Meier, Appl. Opt., 8, 2269 (1969). (10)W.J. Tomlinson and E. A. Chandross, Adu. Photochem., 12,201 (1980).
Wavelength (nm.)
Figure 3. The absorption spectra of benzophenone in PMMA taken at various points during the growth of the hologram. The numbers next to the spectra refer to the times indicated in Figure 2. The vertical axis for the short wavelength portion of the spectrum is at the left of the figure and for the long wavelength portion at the right.
curring the hologram produced by it would persist even in the presence of step 11. In a future paper a more detailed theoretical discussion of holographic growth when more than one reaction step is involved will be given." Various parts of the holographic growth curve can be correlated with changes in the absorption spectrum of the sample. This can be seen by comparing Figures 2 and 3. Initially, before any photochemistry has occurred, one sees two bands corresponding to a *a* (249 nm) state and an nr* (338 nm) state of benzophenone in PMMA. As the hologram grows, the benzophenone spectrum disappears. This can be seen by following the disappearance of the AT* absorption. The benzophenone spectrum is replaced by a strong absorption in the vicinity of 333 nm. This spectral change has been observed by a number of other workers in a variety of host m a t e r i a l ~ ~ , ~and J ~ -seems '~ to be a general feature of benzophenone photochemistry. The growth of the second step in the hologram corresponds to a decrease in the absorption of this intermediate. The behavior described here was independent of whether a degassed sample or one prepared in air was used. It should also be noted that the product of the first reaction step underwent no detectable dark reaction in 24 h. This is in contrast to the results in solution where a dark reaction is ~ b s e r v e d . ~ One can, using eq 2, determine the order of the photochemical reaction by measuring the holographic growth A log-log plot of the curve as a function of laser parameter a(I) defined by eq 2 vs. I enables one to determine n. A plot of this type is shown in Figure 4 for both steps in the hologram formation. A value of n = 1.89 f 0.07 is obtained for the first step, indicating that this step requires the absorption of two laser photons. This should be compared with a value of 1.4 obtained by Murai and Obi16 by following the increase of the EPR signal of the ketyl radical as a function of light intensity. Since the measurement of integrated EPR absorption spectra yields large error margins in the determination of n, these workers inferred as we have in fact shown that the photochemistry was biphotonic. The second step in the photochemical process is also quadratic as shown in Figure 4, where measurements on two different samples yield n = 1.99. It is possible in the (11)D. M. Burland, Chr. Briuchle, and G. C. Bjorklund, to be published. (12)S. A. Weiner, J. Am. Chem. SOC.,93,425 (1971). (13)J. H.Hutchison, M. C. Lambert, and A. Ledwith, Polymer, 14, 250 (1973). (14)P. Colman, A. Dunne, and M. F. Quinn, J. Chem. SOC.,Faraday Trans. 1, 72,2605 (1976). (15)H. Murai and K. Obi, J.Phys. Chem., 79,2446 (1975).
128
The Journal of Physicai Chemistry, Vol. 85,No. 2, 1987 10.0 " A1.0
1 .o
ns *
1
T3
/o.ol
SO 0.01
t-.;/L
4
t
-
Letters
LAT
1
Figure 5. An energy level scheme for benzophenone.
0.001
simple single-photon process from the lowest n?r* triplet state.'619 The explanation for the two-photon nature of the reaction in the solid state can be understood by considering the energy level scheme of Figure 5. The first I I 1 I 1 ' ' 0.0001 0.001 absorbed laser photon excites the system into the lowest 0.2 0.3 0.4 0.5 0.7 0.9 na* singlet SI.This state is very rapidly converted via Relative Intensity intersystem crossing to TI with a quantum yield close to Figure 4. Plots of the relative value of a(1) vs. the relative laser unity.21 Hydrogen abstraction can occur from this state. intensity. In all cases a(Io)Is the value of a ( I )at the maximum laser In solution the two radicals formed,in this reaction have intentisy I, for a given experimental run. 0 , A,and refer to values a finite probability of diffusing apart. One thus observes of a(r) determined for the growth of the first hologam in three different irreverisble photochemistry in solution. samples. 0 and A refer to corresponding values for the second Murai and co-workers22have considered the reaction of hologram. The vertical scale for the first hologram is at the left and for the second hologram at the right. The slopes of these two lines benzophenone in an ethanol glass. In this system there according to eq 2 are 2n where n is the order of the photochemical is no evidence of irreversible hydrogen abstraction from reaction. the lowest triplet state below about 100 K. The glass transition temperature for ethanol is 90 K.n They explain present case to determine separately the intensity depenthese results by assuming that below the glass transition dences of the two reaction steps because of the great temperature diffusion is too slow to prevent the radical disparity in the time scales of the two steps. Step I is pairs from recombining and thus no overall reaction is finished in seconds while step I1 requires minutes for observed. Above this temperature the two species are completion so that uncertainty in setting the time zero for capable of diffusing apart. Triplet lifetime measurementa step I1 due to interference from step I is small compared in isopentane solutions as a function of temperature and to the total time involved in step 11. hence viscosity'* also indicate that diffusion is important Discussion for irreversible photochemistry. The glass transition temperature of PMMA is 378 K,23 The overall reaction scheme for the first step in the hydrogen abstraction has been worked out in s o l ~ t i o n . ' ~ ' ~ well above room temperature where the measurements reported here were made. As in the ethanol glass, our The first part of the reaction involves the absorption of results support the assumption that for PMMA there is a photon by the benzophenone to eventually produce an no irreversible hydrogen abstraction from the lowest triplet excited na* triplet. This triplet state is the reactive species state. This is consistent with the results of Melhuish,16 that acts to abstract a hydrogen atom from, for example, who could find no evidence in the transient absorption alcohol^,'^ alkanes.20or, as in the present case, a polymer spectrum for significant production of the ketyl radical. matrix.13 The products are a ketyl radical and a radical Melhuish used flashlamp excitation of low intensity and formed from the solvent or matrix that we will denote Re. could not have observed significant two-photon processes. If the ketyl radical and R. are hindered in their abilities The lack of irreversible photochemistry may be due to the to escape each other, then geminate recombination occurs, inhibition of diffusion as the above results suggest. We re-forming the benzophenone molecule. If the two fragcannot, however, rule out the possibility that no hydrogen ments can diffuse apart or reorient with respect to each abstraction at all occurs from the lowest triplet state of other, stable photoproducts can form. For example, the benzophenone in PMMA. absorbing species responsible for the growth of the holoWhen higher light intensities are used, as in our work gram in step I is believed to be either and the work of Murai and Obi,15 irreversible photochemOH istry can occur as a result of a two-photon process. The second photon takes the system from the lowest triplet to a higher triplet from which irreversible photochemistrycan or its ortho a n a l ~ g u e . ~ - ~ occur, The additional energy provided by this second The surprising feature of the photochemidal reaction in photon provides sufficient energy either for the radical PMMA is that it requires two photons. In solution the species produced in the solid to rotationally or translahydrogen abstraction reaction appears to proceed by a tionally diffuse or to undergo hydrogen abstraction reac-
r
@"a
(16) W. H. Melhuish, Trans. Faraday Soc., 62, 3384 (1966). (17) H. Tsubomura, N. Yamamoto, and S. Tanaka, Chem. Phys. Lett., 1, 309 (1967). (18) T. S. Godfrey, J. W. Hilpern, and G. Porter, Chem. Phys. Lett., 1, 490 (1967). (19) M . R. Topp, Chem. Phys. Lett., 32, 144 (1975);39, 423 (1976). (20) M. A. Winnik, J. Polym. Sci.: Polym. Symp., 62, 283 (1978).
(21) P. M. Rentzepis and C. J. Mitschele, Anal. Chem., 42,20A (1970); D. E. Damschen, C. D. Merritt, D. L. Perry, G. W. Scott, and L. D. Talley, J. Phys. Chem., 82, 2268 (1978). (22) H. Murai, M. Jinguji, and K. Obi, J. Phys. Chem., 82, 38 (1978). (23) J. Brandrup and E. H. Immergut, Ed., "Polymer Handbook, Interscience, New York, 1966.
J. Phys. Chem. 1981, 85. 127-129
tion from the higher triplet state by a different mechanism, such as, for example, by initial charge transfer. The triplet-triplet absorption spectrum of benzophenone is w e l l - k n ~ w n . ~ ~ -It' ~consists ,~~ of two broad bands, a weak one with a maximum at 526 nm and a stronger one with a maximum at 317 nm. The long wavelength absorption has been assigned to the Tl(nir*) T,(ira*) transition, while the short wavelength band is T,(nir*) transition. The short wavedue to Tl(nir*) length transition is the one reached by the second UV laser photon. To see if excitation of Tz alone could produce photochemistry, a three-beam experiment was performed. In this experiment the benzophenone sample was excited by the light from a 200-W mercury lamp. This beam could excite the molecule to TI and it was even intense enough (102 W/cm2) to produce two-photon photochemistry by itself. If could not, however, produce a hologram because the light was incoherent and there was no second beam with which it could interfere. If the T1 Tz transition is capable of producing irreversible photochemistry then, by interfering two additional beams of coherent light of the appropriate wavelength at the point where the UV lamp light hits the sample, one should observe the growth of a hologram. The UV lamp photons excite the benzophenone to its lowest triplet state and the laser photons
-
-
-
(24) D. S. McClure and P. L. Hanst, J. Chem. Phys., 23, 1772 (1955).
127
excite the molecule to a higher triplet state. Experiments of this type were performed by using 442-nm HeCd, 488- and 514-nm Ar ion, and 565-600-nm R6G dye laser lines. In none of these cases could hologram growth be detected that would indicate that T1 Tz excitation could produce photochemistry. To be sure that enough UV light for the population of T1was supplied, we replaced the Hg lamp by a UV beam from an Ar ion laser (power densities up to 120 W/cm2) that by itself could not produce a hologram. No hologram formation could be observed in these experiments as well. It thus seems that the triplet AT* state is much less reactive than the second nir* triplet state. This is consistent with measurements of hydrogen abstraction rates for ketones where the lowest triplet state is mr* is nature.'J Much less is known about the photochemistry involved in the second step of hologram formation. It presumably involves the photoreaction of the LAT produced by recombination of host and ketyl radical. What is certain is that the overall reaction requires two laser photons. The product formed in this second step has a weakly structured absorption spectrum beginning on the long wavelength side around 280 nm. This absorption resembles in vibrational spacing and wavelength the absorption spectrum of benzene and may indicate the presence of a benzene-like group in the final product. Acknowledgment. The authors express their indebtedness to D. C. Alvarez for his able technical assistance.
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Mechanism of Cholesterol Gallstone Dissolution. Analysis of the Kinetics of Cholesterol Monohydrate Dissolution in Taurocholate/Lecithin Solutions by the Mazer, Benedek, and Carey Model W. I. Higuchl,' C. C. Su, J. Y. Park, M. H. Alkan,+ and E. Guiari The College of Pharmacy and Chemical Engineering DepaHment, The University of Michigan, Ann Arbor, Michigan 48109 (Received: September 17, 1980)
The recent publication by Mazer, Benedek, and Carey (MBC) of a physical model for the micellar species distribution in the taurocholate/lecithin (TC/L) systems provides a basis for a mechanistic analysis of cholesterol monohydrate (C) dissolution behavior in bile acid/L solutions. The previously published data by Kwan et al. for compressed pellet dissolution rates (G) in TC/L solutions have been analyzed by this model. In the working range reported by Kwan, the MBC model assumes a mixture of three species, i.e., the simple TC micelles (concentration Cis), the mixed micelles of L and TC (concentration C&,), and the TC monomers (concentration CLs). A kinetic rate expression for this system is thus proposed, G = ksC;, + kMCys + kICbs, where the K's are the rate constants of dissolution involving each species. Good agreement was found between Kwan's data and the above equation when only the simple TC micelle is assumed to be involved in the rate-determining step.
A recent publication by Mazer, Benedek, and Carey (MBC)'l2 describes a model for species distribution in bile acid/lecithin solutions suggested by their quasielastic light-scattering (QLS) experiments. In the case of the taurocholate/lecithin (TC/L) system, extensive QLS data were used by the authors for proposing that in the region of high bile acid-to-lecithin ratio, y,simple micelles of a 'Ankara Universitesi, Eczacilik Fakultesi, Farmasotik Teknoloji Kursusu, Tandogan/ Ankara, Turkey. 0022-365418112085-0127$01 .OO/O
relatively constant size may co-exist with mixed bile acid/lecithin micelles of a relatively constant size. This interpretation is consistent with that which may be deduced from the equilibrium dialysis experiments of Duane3 which demonstrated the presence of substantial concentrations of simple micelles in equilibrium with the mixed (1)N. A. Mazer, G. B. Benedek, and M. C. Carey, Biochemistry, 19, mi _ _ _(iwnl. ~ (2) N. A. Mazer, M. C. Carey, R. F. Kwasnick, and G. I 3 Benedek,
Biochemistry, 18, 3064 (1979). (3) W. C. Duane, Biochem. Biophys. Res. Commun., 74, 223 (1977).
0 1981 American Chemical Society
