648
The Journal of Physical Chemistry, Vol. 83,
TABLE 111: Effect of Added quenching rate constant
quencher (cm3molecule-' s-') O,a
SFeC
(0.1-7.8) x
No. 6, 1979
V i b r a t i o n a l Quenchers
concn d
range, (3-11)
x 10l4
(4-9) X 10"
1.0 ?: 0.1 1.0 i 0.05
a [HI,< 1 x lo1' ~ m -6~x ; l o i 2 G [ O , ] < 1 6 x 10l2 ~ m - ~b References . 6, 7, and 8. [HI, < 1 x 10" ~ m - ~ ; 7.6 x 10l2 < [ O , ] < 9.6 x 10l2 ~ m - d~ k,(298 . K ) with quencher/k,( 298 K ) without quencher.
to the reaction mixture. Four runs were carried out with each added quencher at several ozone concentrations. The results are summarized in Table 111. In the presence of high concentrations of vibrational quenchers, hl observed agrees with the values obtained without added quenchers to within f10% which is well within expected experimental error. In addition, no trends with increasing quencher concentration were observed. These results show that reactions of vibrationally excited OH are not important under the experimental conditions used in the present study. The absence of any significant effect with added quencher implies that k2b[OH*]is considerably less than 0.5 s-l, otherwise interference from reaction 2b should have been observed. As an additional check for possible interference from secondary reactions, the initial hydrogen atom concentration was varied from 0.5 to 8 X 10" cm-3 with the ozone concentration held constant a t 1.3 X 1013~ r n - ~The . observed value for kl was found to be independent of [HI, at concentrations below about 2 X 10l1~ m - However, ~, the observed hl decreased at higher [HI,; for example, at [HI, = 8 X lo1], h l = 2.1 X 10-l' cm3 molecule-I which is approximately 30% lower than the value observed a t [HI, 5 2 X lo1' ~ m - Small ~ . corrections were made for the loss of ozone a t the higher hydrogen atom concentrations. To avoid operating in the nonlinear region of the fluorescent intensity vs. [HI curve, the runs were carried out a t sufficiently long reaction times to allow [HI to fall below 1 X 10l1 ~ m - The ~ . reason for the decrease in hl a t [HI,
P. Bortolus and
S.Monti
> 2 X 1011 cm-3 is not clear at this time. Initial model calculations of the system suggest that reactions of vibrationally excited OH may be responsible. Additional model calculations and experiments with added vibrational quenchers at high [HI, are planned in the future to check this point. However, the results should have no bearing on the present rate constant measurements which were carried out a t [HI, 5 1.2 X lo1' where hl was found to be independent of [HI, and of added vibrational quenchers. Acknowledgment. The research described in this paper was carried out a t the Jet Propulsion Laboratory, California Institute of Technology, under NASA Contract NAS7-100.
References and Notes (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (171 (l8j (19) (20) (21) (22)
A. B. Meinel, Astrophys. J., 111, 555 (1950). D. R. Bates and M. Nicolet, J . Geophys. Res., 55, 301 (1950). G. Herzberg, J . Roy. Astr. SOC.Can., 45, 100 (1951). W. F. J. Evans and E. J. Llewellyn, J. Geophys. Res., 78, 323 (1973). (a) A. M. Bass and D. Garvin, J . Mol. Spectrosc., 9, 114 (1962); (b) D. Garvin, H. P. Broida, and H.J. Kostkowski, J . Chem. Phys., 32, 880 (1960), and earlier references cited there. (a) T. E. Kleindienst and B. J. Finlayson-Pitts, Abstracts of the 175th National Meeting of the American Chemical Society, Anaheim, Calif., March, 1978. (b) B. J. Finlayson-Pitts, to be published. G. E. Striet and H. S.Johnston, J . Chem. Phys., 64, 95 (1976). S. D. Worley, R. N. Coltharp, and A. E. Potter, Jr., J . fhys. Chem., 76, 1511 (1972), and earlier references cited there. J. C. Polanyi and J. J. Sloan, Int. J. Chem. Kinet. Symp., 1, 51 (1975). P. E. Charters, R. G. Macdonald, and J. C. Polanyi, Appl. Opt., 10: 1747 (1971). L. F. Phillips and H. I. Schiff, J . Chem. Phys., 37, 1233 (1962). M. A. A. Clyne and P. B. Monkhouse, J. Chem. SOC.,Faraday Trans. 2, 73, 298 (1977). J. H. Lee, J. V. Michael, W. A. Payne, and L. J. Stief, J. Chem. mys., 69, 350 (1978). L. F. Keyser, J . Chem. Phys , 69, 214 (1978). Reviewed by R. D. Hudson, Can. J . Chem., 52, 1465 (1974). H. Gg.Wagner, U. Welzbacher, and R. Zellner, Ber. Bunsenges. Phys. Chem., 80, 902 (1976). F. Kaufman. Proa. React. Klnet.. 1. 1 119611. R. V. Pokier and 6 . W. Carr, Jr., J: fhys: Cheh., 75, 1593 (1971). P. J. Ogren, J . Phys. Chem., 79, 1749 (1975). W. B. DeMore, J . Chem. fhys., 47, 2777 (1967). G. D. Downey and D. W. Robinson, J. Chem. fhys., 64, 2858 (1976). J. E. Spencer and G. P. Glass, Chem. fhys., 15, 35 (1976).
Cis-Trans Photoisomerization of Azobenzene. Solvent and Triplet Donor Effects' Pietro Bortolus" and Sandra Monti Laboratorio di Fotochimica e Radiazioni di Alia Energia of the Consiglio Nazionale delle Ricerche, 40 126 Bologna, Italy (Received August 14, 1978: Revised Manuscript Received December 11, 1978) Publication costs assisted by Consiglio Nazionale delle Ricerche (Rome)
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The quantum yields for trans cis (&) and cis trans (&) photoisomerization processes for azobenzene have been determined at a A,, of 317 and 439 nm in solvents of different polarity. At both irradiation wavelengths, & increases and 4tdecreases with increasing polarity of the solvent; the sum (& + &), however, remains fairly constant. For the triplet sensitized reaction, +t 'v 1 and the ratio &/& 65. The data suggest that upon direct photolysis isomerization proceeds in the singlet manifold. lntroduction Azobenzene, in solutions of nonpolar solvents, undergoes photochemical cis G trans isomerization with different quantum yields following excitation in the n,r* or in the T,T* The photoprocess mechanism is still a subject of debate in spite of the investigations carried out on substituent, temperature, and viscosity effects5 on the photoisomerization yields. The reasons are manifold. In 0022-3654/79/2083-0648$01 .OO/O
photosensitization experiments by triplet energy donors, it is very difficult to avoid direct adsorption of light from azobenzenes owing to the absorption spectrum of both isomeric compounds which have high molar extinction coefficients up to -500 nm. Therefore, results obtained in different laboratories are in sharp Moreover, the excited state properties of azobenzene are difficult to investigate because both isomers fail to exhibit detectable 0 1979 American Chemical Society
Cis-Trans Photoisomerization of Azobenzene
The Journal of Physical Chemistry, Vol. 83,
No. 6, 1979 649
incident light. Photostationary compositions and confluorescence and phosphorscence emission, at least with version percentages were determined by UV spectroconventional e q ~ i p m e n t . ~ JOnly ~ very recently an ulphotometry in the 300-330-nm region after suitable ditrashort emission (-25 ps) attributed to the l(n,n*) lution of the irradiated solution. Double-beam Perkinfluorescence of trans-azobenzene has been detected by Elmer 356 and single-beam Hitachi Perkin Elmer 128 picosecond fluorescence spectroscopy.l' Triplet states have spectrophotometers were used. In all solvents, good not been directly observed either by conventional flash cis-trans isosbestic points were obtained for irradiations photolysis or by laser flash photolysis. Finally, no calprolonged far after the achievement of photostationary culations, with a satisfactory treatment of n electrons, of state composition, to assure the absence of side reactions. the energies of the 3 ( n , ~ *and ) 3 ( ~ , ~ *states ) have been For the determination of the quantum yields at 317 nm, performed. Because of this shortage of experimental data, owing to the high cis percentage at the photostationary it has been suggested that the excited state responsible for state (-90% cis), irradiations were carried out up to photoisomerization is either a triplets or a singlet state.7 N 15% conversion for the trans cis process and 1-2 % Recently, on the basis of experiments on photoisomerfor the cis trans one. In experiments a t 439 nm, t,he ization sensitized by triplet donors with E T 1 55 kcal/mol time of irradiation was such that 10% of isomerizatiion and by dyes with low lying triplet states (33 5 1?T I47 was achieved. Photosensitized cis trans isomerizations kcal/mol), it has been proposed that direct photoisomwere performed in benzene solution under the condition erization occurs exclusively in the lowest triplet state of the cis and trans isomers with a quantum yield of ~ 0 . 5 . ~ of total absorption of light by the sensitizer (cbenzil = 3.14 X M; Cacrldine = 1.74 X M); the solutions were Moreover, it has been suggested that an upper triplet state deoxygenated by bubbling with pure nitrogen. The seis involved in the photoprocess. The upper triplet state lected irradiation light was 380 nm, near a minimum in of the cis isomer deactivates quantitatively to the lowest the absorption spectrum of cis-azobenzene. To minimize isomerizable triplet so that the two triplet levels are exthe effects of the direct process, the maximum concenperimentally indistinguishable. On the contrary, the upper tration of cis-azobenzene was such that less than 3% of triplet state of the trans form deactivates practically the incident light was absorbed by azobenzene. Conversiion quantitatively to the trans ground state. The differences percentages were evaluated directly on the irradiated in the photoisomerization quantum yields with A,, have solutions by visible spectrophotometry in the 420-460-nm been explained, in this model, with different intewystem region. Control experiments were performed to ascertain crossing yields from S1and S2to the reactive triplet of each the absence of chemical interaction between the sensitizer isomer. and azobenzene in the ground state and the photostability The present paper reports a detailed study of the solvent of the sensitizer. The occurrence of a photoreaction of the effect on the quantum yields of the direct cis 7 3 trans excited sensitizer with azobenzene was ruled out on the photoisomerization both in the P,T* and n,T* absorption basis of the fact (i) that the spectrum of the cis-trans bands thus investigating an aspect of the problem that has mixture a t the photostationary state remains practically received little attention in the literature. Sensi tization unaltered for irradiations prolonged far after the photomeasurements have been also performed using nonstationary state composition was reached and (ii) that, in fluorescent sensitizers, which appear more reliable than approaching the photostationary composition, the trans dyes in assuring pure triplet energy transfer without side percentages, calculated from the spectral variations at photoprocesses. Solvent effects and sensitization results various wavelengths, are the same in the explored spectral allow criticism to be made of the previously proposed range. The reported values of both direct and sensitiz,ed models and contribute in clarifying the role of the various yields, corrected for the contribution of the back reaction,15 excited states of azobenzene in the photoreaction. are the mean of five different determinations whose reExperimental Section producibility was f 7 % . trans- Azobenzene and benzil were Aldrich, gold label Results and Discussion products and were used as purchasedl. Purified acridine was a gift of Dr. Arlette Kellmann. 4,4'-DiimethylSolvent Effects. The trans cis (&) and cis trains aminobenzophenone was purified according to the pro(dt) photoisornerization quantum yields in solvents of cedure in ref 12. cis-Azobenzene was prepared photodifferent polarity are collected in Table I. The values in chemically from the trans isomer13and purified by column n-hexane, a nonpolar solvent, are in excellent agreement chromatlography (A1203support). The final product was with the data reported in the literature2-6 regarding the estimated to contain less than 1% of the trans isomer on values of the photoisomerization quantum yields obtained the basis of the extinction coefficients reported in the by irradiating in the n,+ band at 439 nm; for irradiations 1 i t e r a t ~ i - eand ~ ~ from gas-chromatographic analysis. All in the T,T* band, while the & value is also in agreement solvents were Carlo Erba; n-hexane, benzene, and ethanol with the literature value, dt is about one-half of that re(all RS grade) and ethyl bromide and ethyl iodide (both ported. This noticeable discrepancy is probably due to the R P grade) were used without further purification. Acedifferent method used to obtain the $ value. On account tonitrile (RP grade) was repeatedly refluxed over Pz05, of the unfavorable photostationary composition, the & washed with Na2C03solution and twice distilled water to value is generally ~ a l c u l a t e dfrom ~ , ~ the following relaneutrality, dried on MgS04, and finally distilled; the tionship: middle fraction was used. The light source was a stabilized high-pressure xenon arc (Osram XBO 150 W). In all experiments, the exciting wavelengths were isolated by narrow band Balzers interference filters. Ferrioxalate actinometry was used.14 where the photostationary composition 4c and molar Direct trans @ cis, photoisomerization yields were extinction coefficients (e) of the two isomers are the values measured by irradiating air-saturated solutions of the pure experimentally determined. We have, instead, directly cis or trans isomer in a 1-cm thick spectrophotometric cell; measured dt by irradiating solutions of the pure cis dissolved oxygen was not observed to affect the yields. The compound (see Experimental Section). A plausible exconcentrations were such to ensure total absorption of the planation for this discrepancy between the & values at 317
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650
P. Bortolus and S. Monti
The Journal of Physical Chemistry, Vo/. 83, No. 6, 1979
) Cis + Trans (&) Direct Photoisomerization Quantum Yields for TABLE I: Solvent Dependence of Trans + Cis ( G ~ and Azobenzene Irradiated in the T , T * (hirr 317 nm) and in the n,T* ( h i r r 439 nm) Bands
solvent n -hexane ethyl iodide ethyl bromide ethanol acetonitrile H,O/EtOH (80120) v/v a
dielectric const 1.9 6.5 9.2 24.3 37.5 74a
4%
@c
317 0.ll2
439 0.25 0.24 0.26 0.28, 0.31 0.35,
O.1ls 0.15 0.15, 0.21,
@*,I@,,, 2.23
317 0.27
2.32 1.90 1.96 1.64
0.25, 0.24 0.21 0.15
439 0.56 0.69 0.58 0.51 0.46, 0.41,
0439/@311
2.07 2.27 2.12 2.21 2.76
Estimatedfrom dielectric constant values and molar fractions of the two components.
nm calculated according to (I) and that obtained by us could be t h a t the light used in the irradiation experiment^"^ had a monochromaticity different from that used for the determination of molar extinction coefficients. 4 values obtained from (I) are correct only if t values are measured with the same experimental arrangement used for irradiations. This holds true expecially when irradiations are carried out at the edge of a band where t varies greatly with X (as for cis-azobenzene in the 300-350-nm region).16 To test the reliability of the 4t values obtained by irradiating at 317 nm, & and 4,were also determined by irradiating with 280-nm light. At this wavelength the photostationary composition is richer in the trans isomer because the ratio (etrans/ecis)280 N 1 compared with (ttrans/ccis)317 N 10. Both +t and & at 280 nm are, within experimental uncertainty, the same as the values at 317 nm. This confirms the finding of Zimmermann3 that, within a band, the conversion yield is not dependent on the ,,A, and also the reliability of the $t value found by irradiating a t 317 nm. The 4, values at both irradiation wavelengths increase with increasing polarity of the solvent; an opposite effect was observed for &values. It is interesting to note that the polarity of the solvent has, instead, little influence on cis and cis trans the ratio qb439/&17 for both trans processes. This coupling between the quantum yields in the two bands suggests that the wavelength dependence of the photoisomerization yield is not the result of processes occurring in different excited states having unlike reactivity, but, rather, that only one excited state, the singlet n,r* or a state reached more easily from the n,r* than from the r,r* singlet state, is responsible for the photoisomerization. Another noteworthy feature is that the polarity of the solvent, while strongly influencing the values of 4t and &, varies only slightly the value of the sum (4t + 4,) which is 0.38 for irradiation in the r,r*and 0.80 for irradiation in the n,r* band. The constancy of ($t + &) with the polarity of the solvent could be a coincidence, but it could indicate that the population of the reactive state is not influenced by the polarity of the solvent. The polarity of the solvent seems to be the only perturbing agent on the photoisomerization. No effect on the quantum yields of ISC perturbing agents, such as Xe bubbled in n-hexane solutions17or high concentrations of Br- or I- ( c = 1 M) in water/EtOH (80/20 v/v),l* was observed. The variation of isomerization yields in heavy-atom containing solvents is satisfactorily accounted for by their polarity. The increase of $t in ethyl iodide could be attributed to an increased ISC (@t in the triplet state is 1, see later); however, a catalysis of the cis trans process by iodine atoms1* seems a more probable process. In fact, no effect of CzHBIwas observed on 4,and, moreover, solutions in ethyl iodide of the cis isomer kept in the dark show thermal isomerization with a rate significantly higher than that observed in all other solvents reported in Table I. A plausible explanation of this lack
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of effect may be that the triplet state of isomeric azobenzenes is extremely short lived as is their singlet state. This, in turn, may explain the fact that the triplet state of azobenzenes has never been observed by flash-photolysis experiments. Sensitized Photoisornerization. Some information about the nature of the excited state responsible for the direct isomerization comes from experiments carried out in the presence of triplet energy donors. Rate constants for energy transfer to either cis or trans azobenzene from triplet naphthalene ( E T = 60.9 kcal/mol), pyrene (48.7 kcal/mol), and acridine (45.3 kcal/mol) determined by flash photolysisz0 are, within experimental uncertainty, equal to the value for a diffuse-controlled process (6 X lo9 M-l s?), indicating that sensitizers with E T 2 45 kcal/mol behave as high energy donors with respect to energy transfer to isomeric azobenzenes. The cis percentage at the photostationary state obtained in benzene solution using 4,4'-dimethylaminobenzophenone (ET= 61 kcal/ mol), benzil (ET= 53 kcal/mol), and acridine as sensitizers is 1-2%. These results, obtained with nonfluorescent sensitizers which do no abstract hydrogen at a significant rate in benzene, are in complete agreement with the results of Jones and H a m m ~ n d .The ~ following kinetic scheme has been suggested7 for the photoisomerization of azobenzene sensitized by triplet energy donors:
S
-s + + -+ + hu
+
'S*
h c
3S*
ki
3s*
3S* trans-Azo
3S* cis-Azo X
k4
k2
k3
StX
S
X
trans-Azo
k5
X cis-Azo where X is an isomerizable form of azobenzene. According to the scheme, the initial quantum yield for the photosensitized cis trans process is hl h4 k5 h4 + kj 1/4t = k3 k 4 @ i s c [ ~ i ~ l k44isc and the photostationary state composition is
-
+
+----
Flash photolysis experiments have demonstrated that, for sensitizers with ET 2 45 kcal/mol, hz = h3;so, it follows from photostationary composition, that k5 is low with respect to k4 and that, at high cis-azobenzene concentrations, l / & = 1/41sc. In Figure 1 is plotted l / &vs. l/[cis-Azo] obtained using acridine and benzil as sensitizers; the value of the intercept is 1.38 ($Isc = 0.72) for
The Journal of Physical Chemistry, Vol. 83,No. 6, 1979 651
Cis-Trans, Photoisomerization of Azobenzene
-- --ACRIDINE
I
TRiPLET
/
tls
TRANS
REACTION COORDINATE Figure 2. Schematic potential energy curves for the ground state and
--2000
[cis
-
Figure 1. Plot of sensitized cis solution.
LOWEST
4000
- Azo]-'
lowest lying triplet state of azobenzene (a,b) which could account for the results of triplet sensitized photoisomerization.
6000
(mol-'. 1)
l l 4 , vs.
l / [ c i s ] for the benzil (A) and acridine (0) trans photoisomerization of azobenzene in benzene
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energy donors with ET2 45.3 kcal/mol sensitize with unit efficiency the cis trans photoisomerization, while they cis plhoare practically unable to sensitize the trans toprocess; (2) that the solvent polarity influences both1 indicating and cjt but does not affect (i) the ratio 4439/~317, that the same excited state responsible for the photoisomerization is reached by irradiating in the P,T* and in the n,x* absorption bands of azobenzene, nor (ii) the value of the sum (& $J, indicating that an excited state common to both isomers is involved in the photoprocess. On the basis of these results, the following changes should be made in the model for the direct photoisolmerization proposed by Ronayette et a1.6 (widely described in the Introduction). The nonisomerizable upper triplet state of trans-azobenzene has an energy lower than 45.3 kcal/mol, rather than an energy between 47 and 54 kcal/mol, so that this nonisomerizable triplet is still populated in our experiments. The quantum yield of the isomerizable triplet of cis-azobenzene is N 1 and not 0.5 as is assumed in the model. In the direct cis trans photoprocess the isomerizable triplet is not reached quantitatively from SI and S2 states but with quantum yields which depend on the excitation wavelength. In our opinion the changes that must be made in the two-triplet model are substantial. According to the present data an alternative mechanism can be proposed. If in the direct photoisomerization an excited state common to both isomers is involved, this state cannot be the triplet, because the trans cis sensitized yield is -0.015 while the direct one is 0.1-0.2. Therefore, in agreement with the suggestion of Jones and Hammond7 we conclude that the dirlect photoisomerization of azobenzene occurs in the singlet manifold.
-
the reaction sensitized by acridine and 1 for the process sensitized by benzil. These values are, indeed, in excellent agreement with the values reported in the literature for +Isc of the two sensitizer^.^^^^^ From the triplet sensitized & value and photostationary composition, it follows that the sensitized value is -0.015. In processes of cis-trans isomerization about the double bond a similar highly selective deactivation of the triplet state to one isomer has been found for the benzophenone sensitized isomerization of azoisopropane22but is not experienced for the photoisomerization of the class of the more widely investigated stilbene-like compounds. The reactivity of triplet azobenzene can be accounted for by two types of potential energy surfaces: (a) a surface with a minimum along the reaction coordinate whose position with respect to the ground state is such to deliver triplet azobenzeine preferentially to the ground trans form; or (b) a surface with an energy barrier easily surmountable, at room temperature, on going from cis to trans, but hardly Surmountable in the reverse process. A sketch of the postulated triplet energy curves in the two cases is given in Figure 2. If an energy curve of type b is followed, an increase in & and, consequently, an enrichment of cis percentage at the photostationary state with increasing temperature, should be observed. Unfortunately, the increasing rate of the thermal cis trans process does not allow reliable results for the temperature effect to be obtained. A theoretical study could, perhaps, aid in determining Acknowledgment. The authors are indebted to llr. the shape of the triplet energy surface. This approach has Arlette Kellmann (University of Paris-Sud, Orsay) for 1,he been applied te diimide, the azo compound parent molkind gift of a sample of purified acridine and to Profesisor ecule, arid curves of type a or b have been calculated by Guido Galiazzo (University of Padova) for the preparation assuming as isomerization coordinates the dihedral twisting and purification of cis-azobenzene. angle or the bending angle N-N-II, r e s p e ~ t i v e l y . ~ ~ ~ ~ ~ References and Notes However diimide is probably an inadequate model for describing the photochemistry of an aromatic a17,>ocom(1) Presented, in part, at the XI1 National Symposium of the Associazione Italiana di Chimica Fisica, Bologna, Oct 1977. pound, so specific calculations on azobenzene are needed.
+
-
-+
+
Conclusions The relevant results of this work are (1) that triplet
(2) P. P. Birnbaum and D. W. G. Style, Trans. Faraday Soc., 50, 1'192 11954). (3) b.-Zimmermann, C. Y. Chow, and U. J. Paik, J . Am. Chem. Sac., 80, 3528 (1958).
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The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
(4) S. Yamashita, H. Ono, and 0. Toyama, Bull. Chem. SOC. Jpn., 35, 1849 (1962). (5) S. Malkin and E. Fischer, J. Phys. Chem.,66, 2482 (1962); D. Gegiou, 90, 3907 (1968); K. Muszkat, and E. Fischer, J . Am. Chem. SOC., D.Gegiou, K. A. Muszkat, and E. Fischer, ibid., 90, 12 (1968). (6) J. Ronayette, R. Arnaud, P. Lebourgeois, and J. Lemaire, Can. J . Chem., 52, 1848 (1974); J. Ronayette, R. Arnaud, and J. Lemaire, ibid., 52, 1858 (1974). (7) L. 8. Jones and G. S. Hammond, J . Am. Chem. Soc., 87, 4219 (1965). (8) E. Fischer, J. Am. Chem. SOC.,90, 796 (1968). (9) P. S. Engei and C. Steel, Acc. Chem. Res., 6, 275 (1973). (IO) H. Rau, Angew. Chem., Int. Ed. Engl., 12, 224 (1973). (11) W. S. Struve, Chem. Phys. Lett., 46, 15 (1977). (12) D. I. Schuster, M. D. Goldstein, and P. Bane, J . Am. Chem. SOC., 99, 187 (1977). (13) G. S. Hartley, Nature(London), 140, 281 (1937); J . Chem. Soc., 633 (1938).
J. R. Harbour and M. L. Hair (14) C. G. Hatchard and C. A. Parker, Proc. R. SOC.London, Ser. A , 235, 18 (1956). (15) A. A. Lamola and G. S. Hammond, J . Chem. Phys., 43, 2129 (1965). (16) For the importance of light monochromaticity in the calculation of 4 values using relationship I see, e.g., G. M. Wyman, Mol. Photochem., 6, 81 (1974). (17) A. R. Horrocks, A. Kearvell, K. Tickle, and F. Wilkinson, Trans. Faraby Soc., 62, 3393 (1966). (18) P. Bortolus, G. Bartocci, and U. Mazzucato, J . Phys. Chem., 79, 21 (1975); G. Bartocci, U. Mazzucato, and P. Bortolus, J. photochem., 6, 309 (1976-1977). (19) R. Arnaud and J. Lemaire, Can. J . Chem., 52, 1868 (1974). (20) E. Amouyai and S. Monti, to be published. (21) A. Kellmann, J . Phys. Chem., 81, 1195 (1977). (22) L. D. Fogel and C. Steel, J . Am. Chem. Soc., 98, 4859 (1976). (23) N. C. Baird and J. R. Swenson, Can. J . Chem., 51, 3097 (1973). (24) M. S. Gordon and H. Fischer, J . Am. Chem. Soc., 90, 91 (1968).
Radical Intermediates in the Photosynthetic Generation of H202with Aqueous ZnO Dispersions John R. Harbour" and Michael L. Hair Xerox Research Centre of Canada Limited, Mississauga, Ontario L5L 1J9, Canada (Received September 18, 1978) Publication costs assisted by Xerox Research Centre of Canada
The technique of spin trapping has been applied to a study of the photosynthesis of hydrogen peroxide in aqueous zinc oxide dispersions. In additive-freesystems, the hydroxyl radical was detected whereas in systems containing either formate or oxalate, the .CQ; radical was observed. The measurement of oxygen uptake was also accomplished on these same systems. Comparison of the number of radicals with the amount of H2Q2formed and of the quantum efficiencies determined by both electron spin resonance and oxygen uptake strongly suggest that these radicals are major participants in the mechanism of hydrogen peroxide photosynthesis. A mechanism is in fact suggested which is consistent with the observation of these radical species. It appears that additives such as formate or oxalate function by acting at least partially as hydroxyl radical scavengers. The conversion ratio (defined as the number of hydrogen peroxide molecules formed per one molecule of oxygen consumed) is introduced and is observed to be a function of initial oxygen concentration in the additive free systems. This reaction also proceeds in the presence of a surfactant.
the reaction nor whether the radicals produced are of Introduction sufficient number to be major participants in the overall I t is well known that when ZnO powder is dispersed in H202synthesis. I t is the goal of this paper to determine an oxygenated aqueous medium it will generate hydrogen whether radicals are generated in zinc oxide dispersion peroxide (H202)upon illumination with light of wavelength upon illumination, to identify the specific radicals, and to less than 380 nm.ld5 Addition of certain compounds such determine their concentrations. The technique of spin as formate, oxalate, and phenol greatly increase the trapping7 was employed for radical detection and repreamount of H202produced. The quantum efficiency of this sents an extension of our earlier work on radical detection reaction with additives usually approaches its proposed in pigment dispersions.s-10 maximum of 0.5. This high quantum efficiency coupled Systems containing only ZnO, water, and O2 have been with the absence of side reactions has made this system a model for the study of heterogeneous p h o t o c a t a l y ~ i s . ~ ~ ~studied as well as systems to which compounds such as formate or oxalate have been added. In both of these cases (In fact, the use of the term photocatalysis is apparently it is known that the oxygen which is incorporated into the inaccurate, as discussed later.) H202comes from dissolved molecular O2and that light of Various mechanisms have been proposed for these reenergy 23.2 eV (band gap energy) is r e q ~ i r e d .A~ limiting actions. In fact recent studies by Dixon and Healy6 have concentration of H202of M has been observed in the cast doubt on whether this system is photocatalytic. They additive-free case and this increases to M when argue that photocorrosion is occurring with interstitial zinc formate or oxalate is added.* Carbon dioxide is liberated ion (Zn') being oxidized to Zn2+followed by hydrolysis. when formate or oxalate is present. Nevertheless all the mechanisms which have been proposed to date invoke radical intermediates. However, the Experimental Section only evidence for radical involvement is indirect, through Zinc oxide powder was obtained from Fisher Scientific, observation of a ZnO photoinduced polymerization of both BDH Chemicals (Analar), and Ventron (ultrapure). All methyl methacrylate and acrylonitrile.2 This observation the powders were effective in the photosynthesis of H2Oz does not define which particular radicals are involved in 0022-3654/79/2083-0652$0 1.OO/O
1979 American Chemical Society
