J. Phys. Chem. 1985,89. 5815-5821 individual vibronic transitions can be unambiguously assigned. Futhermore, spectroscopic studies have suggested that vibrational relaxation rates are relatively low. Thus in argon matrices two low-lying vibronic levels denoted A (31 1 cm-’) and B (386 cm-’) were found to exhibit lifetimes of 250 and 350 ps, respectively. These lifetimes are certainly remarkably long for a molecule of this size embedded in a condensed medium. They become even more extraordinary and unexpected when one considers that the relaxed lifetime is approximately the same, namely, only 370 ps. Other lifetime-measuring experimental systems fail to detect these differences probably because they are based on the observation and measurement of an “overall” relaxation rate of many vibronic levels which provides very little information and understanding of the details of individual vibrational level energy dissipation mechanism and the processes involved. Excitation to higher vibrational levels showed that the relaxation follows specific preferential channsls and is not statistical as might be expected from some radiationless theory models for large dense level molecules. For example, the F level at 0; + 484 cm-’ and 630 cm-’ were found to have lifetimes of 40 the H level at 0: and 45 ps, respectively. Their relaxation proceeds to the B level and, subsequently, to the vibrationless level bypassir level X situated at 0: 201 cm-I and several others level Similar relaxation behavior is observed for other levels, Le., 1( i K at 0; 1275 cm-’ and level J at 0; 1084 cm-’ which bo1 decay to the A and B levels located at 0; + 3 11 cm-’ and 0; + 386 cm-’, respectively, and subsequently, to the 0; state with little population reaching several intermediate levels. Such specific relaxation pathways would not be expected from the statistical radiationless transition models. The present and previous experimental data suggest that the relaxation lifetime of at least the upper vibronic levels studied show a general decrease with increasing excess vibrational energy. However, this does not explicitly mean that each vibrational level of lower energy decays with a lifetime longer than energy levels above it with higher vibrational energy. This is exemplified by
+
+
+
+
5815
the F state which relaxes with a lifetime of 100 ps while the H state which is located below it exhibits a lifetime of 7.‘ s (Figures 3 and 4). Even more noteworthy is the lifetime bel ior of the lowest observed state X which is situated more tha, !OOO cm-I below the F and H states and relaxes in less than 5 ps. Deuteration Effects Previous reports have shown by means of intensity and quantum yields that substitution of the hydroxyl protons with deuterium in naphthazarin has resulted in the increase in the relaxed fluorescence lifetime by a factor of 6-8. The present experiments support this conclusion by the direct measurement of the decay = 77Nph; and T N lifetimes in Nphz and Npd,. We find that = 2 . 5 ~ This ~ ~unusually ~ ~ . large effect on the decay lifetime by deuterium substitution may suggest that the radiationless transition is influenced by intramolecular proton transfer which might be controlled at lower temperatures by quantum mechanical tunneling. In the case of unrelaxed fluorescence originating from the F and H levels, we measure lifetimes near 100 ps for both protonated and deuterated naphthazarin; yet, the relaxed fluorescence rise time is measured to be 100 ps for Nphz and -300 ps for Npd,. These results suggest that the F and H vibronic levels relaxed to a “bottleneck” state located between the F and H state, and the vibrationless level of SI. This “dark state” must possess a small fluorescence oscillator strength but be coupled to and decay into the vibrationless level with a lifetime characterized by the rise time of the relaxed fluorescence. It is particularly interesting to note that this intermediate dark “bottleneck” state, if present, is very short lived in normal naphthazarin but must have a lifetime of -200-300 ps in the deuterated compound. While in the absence of detectable emission from this state it is difficult to determine its identity, the large effects of deuteration upon its lifetime might lead one to speculate that it could be one of the “tunneling states” associated with the double minimum hydrogen-bonding potential.
-
Surface Photochemistry: CdS Photoinduced Cis-Trans Isomerization of Olefins’ Hussain Ai-Ekabi and Paul de Mayo* Photochemistry Unit, Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 (Received: June 17, 1985)
The CdS powder photomediated cis-trans isomerization of six styrene derivatives has been demonstrated. The reaction with cis-stilbene is quenched by electron donors (methoxybenzenesand pyrene) and is affected by the amount of CdS used, light intensity, and the temperature. A minimal simple scheme involving the radical cation is proposed, as a basis for further work, to which Langmuir-Hinshelwood and Freundlich kinetic treatments are applied, the latter somewhat more successfully. The presence of oxygen inhibits the reaction.
The use of semiconductor particles for the light-induced utilization of solar energy is an area of active current research interest. Far less application has been made with other photosynthetic objectives, and these have consisted, for the most part, of chemical oxidation or reduction., Among the few examples not so designated are cases of CdS photocatalyzed retrocycloaddition, valence isomerization, and [ 1,3]-sigmatropic rearrangement react i o n ~the , ~ dimerization ~~ of phenyl vinyl ether,4 and the gas-phase (1) Publication No. 353 from the Photochemistry Unit, The University of Western Ontario. (2) (a) Fox, M. A. Acc. Chem. Res. 1983, 16, 314. (b) Bard, A. J. Science 1980, 207, 139. (3) (a) Okada, K.; Hisamitsu, K.; Mukai, T. J. Chem. Soc., Chem. Commun. 1980,941. (b) Okada, K.; Hisamitsu, K.; Takahashi, Y.;Hanaoka, T.; Miyashi, T.; Mukai, T. Tetrahedron Lett. 1984, 25, 5311.
0022-3654/85/2089-5815$01.50/0
photoisomerization of butenes over ZnO and TiO, powder^.^ The purpose of the present report is to describe our studies concerning the CdS photocatalyzed cis-trans isomerization of some styrene derivatives (compounds 1-6) previously described, in part, in preliminary form.6 Experimental Section High purity CdS powder (>99.99; Aldrich Gold label, lot no. 0897) of surface area 20 m2/g was used for the photo(4) Barber, R. A,; de Mayo, P.; Okada, K. J. Chem. Soc., Chem. Commun. 1982, 1073.
(5) Kodama, S.; Yabuta, Y.; Kubokawa, Y. Chem. Lett. 1982, 1671. ( 6 ) AI-Ekabi, H.; de Mayo, P. J. Chem. Soc., Chem. Commun. 1984, 1231. (7) Taylor, T.W.; Murray, A. R. J . Chem. SOC.1938, 2078. (8) Kistiakowsky, G. B.; Smith, W. R. J. Am. Chem. Soc. 1936,58, 2428.
0 1985 American Chemical Society
~
~
5816 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
AI-Ekabi and de Mayo TABLE I: Isomeric Composition of the Thermodynamic Cis-Trans Equilibria of 1-6 Initiated by CdS. I , " or Heat
R'
olefin
1 2
3 4 5 6
RZ
H H H
H H
OMe
H H H
H H
CN
R3 Ph COMe Ph Me
COOMe CN
stationary-state experiments as well as for the isomerization of the cis isomers of 1 and 6 as a function of time of irradiation. CdS (>99%; Fisher, lot no. 792913) of surface area 12 m2/g was used for the rest of this work. Solvents used were all of spectroscopic grade (Baker or Fisher) and were used without further purification as was trans-stilbene (scintillation grade, Eastman Kodak Co.). Cis-stilbene (Aldrich), trans-cinnamonitrile (Eastman Kodak Co.), and 1,2,4-trimethoxybenzene (Aldrich) were purified by fractional distillation under reduced pressure. Trans-a-cyanostilbene was prepared by condensation of benzaldehyde with benzyl cyanide in pyridine and purified by recrystallization from methylene dichloride-hexane, Trans-benzalacetone was prepared by condensation of benzaldehyde with acetone in a basic aqueous medium and purified by fractional distillation under reduced pressure. With the exception of cis-stilbene, the rest of the cis isomers were prepared either by direct irradiation or via triplet sensitization of the corresponding trans isomers at X > 300 nm. flash column chromatography (silica gel: Merck, 230-400 mesh) was used to separate the cis-trans mixtures. 1,4-Dimethoxybenzene and 1,2,3and 1,3,5-trimethoxybenzeneswere all of high purity (Aldrich) and were used without further purification. Pyrene was purified by recrystallization from methanol and then by sublimation. Isomerization of 1-6. In order to obtain the photostationary state a dilute solution of olefin in methylene dichloride (typically M) in the presence of CdS (Aldrich Gold label; 45 mg/5 mL) was irradiated at X > 430 nm by using a 1-kW PRA Xenon lamp run at 840 W. Prior to irradiation the mixture was sonicated for 10 min. The mixture was constantly stirred during the course of irradiation. Similar conditions were used for the isomerization of the cis isomers of 1 and 6 as a function of time of irradiation. For the rest of the kinetic studies another set of conditions was used. Olefins (0.15 M) in methylene dichloride in the presence of CdS (Fisher; 65 mg/6 mL) were irradiated at X > 430 nm (Corning Filter 3-72) by using either a 1-kW PRA xenon lamp or a 150-W PRA xenon lamp run at 140 W; the output of both lamps was periodically checked with a Scientech 364 power energy meter. Compressed air was used as coolant for both lamps and a water filter was used to remove IR radiation. Except for degassed samples the reaction vessels were open to the atmosphere via a reflux condenser. For degassed samples three freeze-thaw cycles were used before sealing to a residual pressure 99 >99 98 91 >99 75
>99 >99 98 97 >99
96c
2 3 3Id
-
63d
"Equimolar solutions (0.01 M) of olefin and iodine in methylene dichloride were irradiated a t h > 430 nm by using a I-kW xenon lamp. * 2 were irradiated a t h > 460 nm. C T h ethermodynamic equilibrium of cis- and trans-1 has been estimated in the liquid phase a t 200 "C (ref 7). d T h e thermodynamic equilibrium of cis- and trans-6 has been estimated in the gas phase at 352 'C (ref 8). SCHEME I I Y
;c;.4
SC t c-D*j
SC
+
C-D
+
f-D
C-Q
both sides, lay heavily on the side on the trans isomers. Appropriate control experiments indicated that both light and CdS were required. The initial step (Scheme I) (in Scheme I, SC* is the excited semiconductor and c-D and t-D are the cis and trans olefins, respectively) was presumably the formation of electronhole pairs9 which could lead, after migration to the surface, to fluorescence,I0or capture by an appropriate surface ~ t a t e ~ - ' ~and " the ultimate formation of the olefin radical cation intermediate (path b). Annihilation of an electron-hole pair may also generate tripletsI2 of energy equivalent to the band gap (ca. -55 kcal/mol) which could, in principle, compete with a radical ion pathway for isomerization (path a, Scheme I). However, such sensitization is known, in the case of stilbene (l), to favor the cis isomer.I3 For this reason, and for its compatibility with other observations to be detailed below, the radical ion pathway (path b, Scheme I) is indicated. In the formation of the photostationary state the excited CdS may be regarded as a catalyst. If this be so, then the photocatalyst provides a route for the equilibration of ground-state isomers, in the manner of the iodine equilibration of olefinsI4and the Schenck isomerization of the same species." Under these circumstances the photostationary state will approximate the composition of the thermodynamically equilibrated mixture. For comparison, the composition of the thermodynamically equilibrated mixture was examined. The iodine-catalyzed procedureI4 was used to obtain (9) (a) Gerischer, H. J . In 'Physical Chemistry-an Advanced Treatise"; Eyring, H., Henderson, D., Jost, W., Eds.; Academic Press: New York, 1970; Vol. 9A. (b) Harris, W.; Wilson, R. Annu. Reu. Mater. Sei. 1978,8, 99. (c) Nozik, A. Annu. Reu. Phys. Chem. 1978, 29, 189. (d) Wrighton, M. S. Acc. Chem. Res. 1979, 12, 303. (10) Rossetti, R.; Brus, L. J . Phys. Chem. 1982,86,4470. (b) Kuczynski, J.; Thomas, J. K. J . Phys. Chem. 1983, 87, 5498. (c) Ramsden, J.; Gratzel, M. J. Chem. SOC., Faraday Trans J 1984.80, 919. (d) Becker, W. G.; Bard, A. J . J . Phys. Chem. 1983, 87, 4888. (e) Henglein, A. Pure Appl. Chem. 1984, 56, 1215. ( I 1) For semiconductor photoinduced reactions see: (a) Bard, A. J . J . Photochem. 1979, 10, SO. (b) Reference 2. (c) Kanno, T.; Oguchi, T.; Sakuragi, H.; Tokumaru, K. Terrahedron Lett. 1980,21,467. ( d ) Chum, H. L.; Ratacliff, M.; Posey, F. L.; Turner, J. A,; Nozik, A. J. J . Phys. Chem. 1983, 87, 3089. (12) (a) Roth, H. D.; Schilling, M. L. M. J . Am. Chem. SOC.1979, 101, 1898. (b) Zbid. 1980, 102, 4303. (c) Mattes, S. L.; Farid, S. Org. Photochem. 1983, 7 , 233. (13) Saltiel, J.; Agostino, J. D.; Megarity, E. D.; Mett, L.; Neuberger, K. R.; Wrighton, J.; Zafidou, 0. C. Org. Photorhem. 1973, 3, 1. (14) Fischer, G.; Muszkat, K. A,; Fischer, E. J . Chem. SOC.E. 1968, 1156. ( 1 5 ) (a) Saltiel, J.; Neuberger, K.; Wrighton, M. J . Am. Chem. SOC.1969, 91, 3658. (b) Moussebois, C.; Dale, J. J . Chem. SOC.C 1966, 260. (c) Dale, J.; Moussebois, C. J . Chem. SOC.C 1966, 264.
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5817
Photoinduced Cis-Trans Isomerization of Olefins TABLE 11: Rates of Trans Isomer Formation under Aerated and Degassed Conditions of 1-6 Initiated by CIS'
comud
trans % aerated ( 1 5 % ) degassed (*5%)
26 36 4' 5d
6c
degassed aerated
69
24 32 13 11 46 48
lb
M-lL 500
2.9 2.9 1.8 1.9 I .3 1.1
92 24 21
59 54
'0.01 M of olefin in methylene dichloride: X > 430 nm; the output of the lamp was the same for aerated and degassed samples of each substance but it was different from one substance to another. bThe samples were irradiated for 1 h. 'The samples were irradiated for 15 min. dThe samples were irradiated for 30 min.
the data contained in Table I. Only with 6 did the iodine technique fail; such failures have been previously recorded for some stilbene derivative^.'^ It is evident from these results that the photostationary state compositions and those of the thermodynamic equilibrium mixtures are close. Effect of Oxygen. In CdS photocatalyzed processes (retrocy~loaddition,'-~ valence isomerization,' [ 1,3]-sigmatropic rearrangement,) and dimerizationI6) previously examined it was found that oxygen enhanced the rate of reaction. This was rationalized as the trapping of the conduction band electron of CdS by adsorbed oxygen as surface-bound superoxide ion (or equivalent), thus prolonging the life of the hole by delaying the collapse of the electron-hole pair. Conversely, it was reported" that trapping M) enhanced the rate of of the hole by triethanolamine reduction of heptylviologen by the conduction band electron of CdS by a factor of -50. It was also reportedI8 that addition of EDTA (ethylenediaminetetraacetic acid) to an aqueous CdS dispersion caused a substantial increase in the photoproduction of 02-.That, indeed, a conduction band electron is trapped on CdS by adsorbed oxygen as superoxide ion was confirmed by a spin trapping technique.18 In the present study, as shown in Table 11, the reverse has been found. A degassed solution of 1, for instance, reacted 2.9 times faster than the aerated solution. The effect diminishes along the series 1-6. The data are, perhaps, best discussed for stilbene in terms of the steps shown in Scheme 11. Reactions b, c, e, i, and SCHEME I1
A + CdS(h+) + C-D CdS(h+,e-) + c-D CdS(e-) + O2
--
CdS CdS(h+,e-) CdS(h+,e-) O2 CdS(h+)
c-D+.
+
0 2 - 0
0 )
+ c-D+* CdS(e-) + c-D+. CdS + 02-.
(4
t-D+.
(f)
CdS
+ c-D+. CdS(e-) +. t-D+. 0 2 - * + c-D'. 0 2 - * + t-D+.
CdS(e-)
(a)
-+
+ t-D CdS + t-D C-D + 0 2 t-D + 0 2
(c)
(e)
CdS
0 ) (9
ci)
j occur only in the presence of oxygen. Reactions b and c, by prolonging the lifetime of the hole, increase the formation of c-D+.. On the other hand the consistently formed superoxide anion, by electron transfer, competes with (f) in reaction i to suppress the isomerization. On the basis that the presence of molecular oxygen inhibits the radical cation isomerization of ~is-stilbene'~ and in(16) Draper, A. M.; Ilyas, M., de Mayo, P.; Ramamurthy, V. J . Am. Chem. SOC.1984,106, 6222. ( 1 7 ) Saeva, F. D.; Olin, G. R.; Harbour, J. R. J . Chem. Soc., Chem. Commun.
1980,401.
(18) Harbour, J. R.; Hair, M. L. J . Phys. Chem. 1977,81,1791.
I
I
I
1
40
80
120
160
I
2WM-
1fC
Figure 1. Application of the reciprocal Langmuir-Hinshelwood kinetic treatment (eq 2) to the isomerization of cis-1 (0)and cis-5 (0).
itiates the photooxidation of cis- and t r a n s - ~ t i l b e n e s ~and ~ ~in '~,~~ the absence of appreciable amount of oxidation products,21we assume that the lifetime of c-D+. is long enough to make the c-D+. + 02-- c-D O2 process compete with the isomerization of c-D'.. The overall outcome depends on the relative rates. Scheme I1 does not in any way imply that the species involved are necessarily free on the surface: they are so written for the sake of clarity. Unlike in homogeneous solution, the radical ion pairs on the surface have much less opportunity to separate into free species, depending on their respective adsorption nature. That the effect of oxygen diminishes along the series 1-6 can be rationalized in terms of an increasing adsorptive ability thus decreasing the amount of molecular oxygen on the surface. The following data may reflect the role of adsorbed molecular oxygen in retarding the rate of reaction through the above proposed mechanism: the ratio of the reaction rate of degassed sample to the reaction rate of undegassed sample decreased from 2.9 to 1.6 when 0.15 M cis-1 was used instead of 0.01 M. To minimize the effect of oxygen, this higher concentration of 1 was used unless otherwise stated, for subsequent kinetic studies. Kinetic Analysis. It was desirable to ascertain whether reaction was taking place in the adsorbed state, or whether the surface merely provided an active species which was desorbed into solution, and therein reacted; whether the product, if formed on the surface, desorbed into solution or remained on the surface in competition with starting material. We have attempted to discriminate among these and other possibilities. The Langmuir-Hinshelwood and Freundlich kinetic equations are known to be good models for the description of solid-gas reactions;22 they seem to be good approximations for the description of solid-liquid systems as ~ e l l . ~ * 3 The ~ ' LangmuirHinshelwood kinetic treatment22s23assumes that a reaction surface is perfectly smooth (homogeneous), that the rate of reaction is proportional to surface coverage, and that adsorption is independent of surface coverage; then, the rate of a unimolecular
-
+
~
~~~
(19) Lewis, F. D.; Petise, J. R.; Oxman, J. D.; Nepras, M. J. J . Am. Chem. SOC.1985,107,203. (20) Eriksen, J.; Foote, C. S . J . Am. Chem. SOC.1980, 102, 6083. (21) Benzoic acid, a possible oxidation product, was tested as an inhibitor, as which it could function by occupying the active sites on the surface of the semiconductor; it showed no significant effect. (22) (a) Adamson, A. W. "Physical Chemistry of Surfaces" 4th ed.;Wiley: New York, 1982; p 374. (b) Jaycock, M. J.; Parfitt, G . D. "Chemistry of Interfaces"; Ellis Horwood: New York, 1981; p 259. (c) Parfitt, G. D.; Rochester, C. H. In "Adsorption from Solution at the Solid Liquid Interface"; Parfitt, G.D., Rochester, C. H., Eds.; Academic Press: London, 1983; p 4. (d) Laidler, K. J. In "Catalysis"; Emett, P. E.; Ed.; Reinhold: New York, 1961; Chapter 4, p 119. (23) (a) Hsiao, C. Y.; Lee, C. L.; Ollis, D. F. J . Cafal. 1983,82,418. (b) Bruden, A. L.; Ollis, D. F. Ibid. 1983,82,404. (c) See ref 16.
AI-Ekabi and de Mayo
5818 The Journal of Physical Chemistry, Vol. 89, No. 26, 1985
01
1
I
20
0
LO
I
-In C
Figure 2. Application of Freundlich equation (eq 4) to the isomerization of cis-1 (0)and c i s 4 (0). The data are taken from Figure 1.
surface reaction, where the reactant is significantly more strongly adsorbed than the product, will obey equations of the forms (1, 2)
-1= - -1+ - - 1 1 R
k,
k,K c
where R is the rate of reaction,24 k, is the reaction rate constant, K is the adsorption coefficient, and c is the initial concentration of the reactant. - (2.7 X The effect of the concentration ( ( 5 X M) on the rate of reaction was investigated. Plotting the data, according to eq 2 , in Figure 1 gave straight lines with nonzero intercepts. The points belonging to the lower concentrations of either reactant deviated, however, significantly from the straight lines. This deviation suggested that the surface is heterogeneous and that unmodified Langmuir-Hinshelwood kinetics do not apply over the whole concentration range. It is generally agreedz2that the adsorption on a heterogeneous surface takes place, preferentially, on the most active sites; the adsorption on these sites needs least activation energy. Accordingly, the fraction of molecules adsorbed on the most active sites to the total adsorbed molecules at low initial concentration of either reactant is larger than that at higher concentration. Since the reaction on the most active sites should occur with higher catalytic activity than that on the less active sites, the rate of reaction is expected to be faster on the former than the latter. If this be the case, then the Freundlich equation should fit the data better. According to the Freundlich kinetic treatment a reaction surface is not necessarily smooth (homogeneous), the adsorption is dependent on the surface coverage, and the rate of reaction is proportional to the surface coverage. The rate of a unimolecular surface reaction will then obey equations of the forms R = k,kc"
In R = In k,k
+ n In c
(3) (4)
where R is the rate of reaction, c is the initial concentration of the reactant, k, is the reaction rate constant, k is a constant reflecting the adsorbent capacity, and n is a constant reflecting the strength of adsorption (0 < n < 1); lower n values indicate stronger adsorption. (24) (a) The rates, R , were taken throughout this study as the amount of product produced after irradiation for particular time periods (indicated in the captions to the plots) with constant light flux. (b) Since the concentration of the solvent is much higher than that of the olefin and remains constant, the part of CdS surface covered by the solvent is unchanged at all olefin concentrations used.
Figure 3. Disappearance of the cis isomers of 1 (0)and 6 (0) and appearance of the trans isomers of 1 (m) and 6 ( 0 )as a function of time of irradiation.
Plotting In R vs. In c should give a straight line with a slope n. Indeed, plotting the data in Figure 1 according to eq 4 gave straight lines (Figure 2 ) with nowero intercepts (r = 0.999). The values of n for cis-1 and cis-5 were 0.71 and 0.42, respectively. Thus the Freundlich equation fits the data better than the Langmuir-Hinshelwood equation and we conclude, as required by these equations, that the reactions occur on the surface. The fact that the isomerization of cis-stilbene does not occur when acetone or methanol was used as solvent supports the view that the reaction occurs on the surface as the more polar solvents seems to prevent the reaction via occupying the surface sites completely. The experiments just mentioned were performed on one particular type of CdS (Fisher). As recent work has i n d i ~ a t e d , ~ ~ . ~ ~ the surface area and the surface nature of CdS may vary considerably. We have, therefore, studied the isomerization of cis-1 and cis-6 on CdS of higher purity (Aldrich, Gold label) and higher surface area (20 m2/g)-a polytype crystalline formZ6-as a function of time (Figure 3). The isomerization of cis-6 showed that the Concentration of 6 decreased linearly with time down to the photostationary state. The linear behavior corresponds to a surface which is fully covered by adsorbed reactant. Accordingly, the term Kc in eq 1 is large compared with unity, and the rate expression becomes eq 5 integrating to eq 6 which is a zero-order R = -dc/dt = k,
c=
CO
- k,t
(5)
(6)
rate equation where co is the initial concentration of the reactant and c is the concentration of the reactant at time t. This behavior shows that the product does not inhibit the reaction (see later, however). The concentration of cis-1, on the other hand, diminished exponentially with time (Figure 3). This behavior corresponds to a surface which is sparsely covered by adsorbed reactant. Hence, the Kc term in eq 1 can be neglected in comparison with unity, and t h e r a t e expression becomes R = -dc/dt = k,Kc (7) which integrates to a first-order rate equation (8) In co/c = k,Kt
(8)
A good straight line with zero intercept was obtained by plotting In co/c vs. time (Figure 4). The fact that cis-6, under our conditions, fully covers the CdS surface whereas cis-1 does not finds some support from the fol( 2 5 ) Mau, A. W.-H.; Huang, C.-B.;Kakuta, N.; Bard, A. J.; Campion, S. E. J . Am. Chem. SOC.1984, 106, 6537. ( 2 6 ) Ilyas. M.; de Mayo, P. J . Am. Chem. SOC.1985, 107, 5093.
A.; FOX,M. A.; White, J. M.; Webber,
Photoinduced Cis-Trans Isomerization of Olefins
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5819
"fr 0
25
50
75
10 0
1
/
TIME (HRS)
Figure 4. Plot of the data in Figure 3 (isomerization of cis-1 as a function of time of irradiation) according to eq 8.
25
OO
50
75
100
LIGHT INTENSITY l%l
Figure 6. Effect of variation of light intensity upon the isomerization of cis-1. (0) Variation with light intensity (I); (0) variation with Illz. Each sample was irradiated for 30 min with a 150-W xenon lamp.
5c
/
mg is presented in Figure 5. The reaction rates were linearly dependent on the catalytic loading up to -70 mg and then gradualy levelled off. Similar observations have been reported as, for instance, in the selective photocatalytic oxidation of alkanes on T i 0 2 9 where the mass of catalyst was spread out in the reactor as a thin homogeneous layer and in the oxidation of propan-2-01 either as a liquid phase on suspended Ti0;O or as a vapor phase on a thin uniform film of Ti02.31 This behavior was explained in terms of the complete absorption of radiation near the surface of the s e m i c o n d u ~ t o r . ~With ~ - ~ ~large masses the rate would be expected to decrease since the reactant will be adsorbed on semiconductor particles which are shielded from the light flux by other particles of semiconductor. The intensity of light was reduced by calibrated metal gauze filters to examine its effect on the rate of reaction. It was found that the reaction rate decreased nonlinearly with reduction in light intensity down to a few percent of full illumination, as shown in Figure 6. However, the rate of reaction is approximately proportional to the square root of light intensity I. Similar observations have been reported for at least the following photocatalytic reactions: (a) oxidation of vapor propan-2-01 on a thin uniform film of TiOz.31(b) oxidation of liquid propan-2-01 on suspended TiOz,30(c) gas-phase photo-Kolbe reaction of acetic acid over Pt/Ti02,32and (d) a photoelectrochemical cell system when a Pt electrode coated with TiOz powder was used as a p h ~ t o a n o d e . ~ ~ By analogy with the l i t e r a t ~ r e ~this l - ~behavior ~ may be interpreted on the basis that under weak illumination the band bending in the space charge layer of powdered CdS is enough to separate all hole-electron pairs. However, with increasing light intensity a strong competition between processes involving hole-electron recombination and those involving participation of photogenerated hole in surface photooxidation may occur and the band bending tends to be decreased. Accordingly the quantum efficiency for the formation of cis-stilbene radial cation by the excited CdS is very low. Effect of Temperature. Investigation of the effect of temperature over the range of 293-343 K on the reaction rates show an increased rate with increased temperature. An Arrhenius-type plot of log [rate] against 1 / T yields an apparent activation energy of 10.4 kcal mol-' (Figure 7). Similar behavior with other reactions on other semiconductors have been observed by several research g r o ~ p s . ~ ~Davidson , ~ ~ - ~et~al.33explained this behavior
,
OO
50
100
MASS
150
2w
OF CdS lrngs)
Figure 5. Influence of the mass of CdS (Fisher) on the rate of isomerization of cis-1. Each sample was irradiated for 30 min with a 150-W xenon lamp.
lowing observation^:^' it was found possible to separate a mixture of the trans isomers of 1 and 6 on a CdS column using methylene dichloride as an eluent. Under these conditions, it was found that 1, because of its weak adsorption on CdS surface in comparison with 6, was eluted from the column earlier than 6. While the applicability of the Freundlich equation suggested, as discussed above, that the surface of CdS (Fisher, cubic crystallinez6) is heterogeneous, the present good fitting for the two extreme cases of Langmuir-Hinshelwood kinetics suggests that the surface of the particular CdS used (Aldrich, Gold label) is less hetergeneous than that of Fisher, and much closer to homogeneous. However, clearly, further work is needed to understand better the factors that govern the behavior of the different crystalline forms of CdS. To test directly, in the case of stilbene itself, whether the product (trans-stilbene) desorbs into solution or is adsorbed on the surface, the effect of added trans-stilbene on the rate of reaction was studied. The results indicated an inverse relationship between the rate of isomerization and the amount of trans-stilbene added. This clearly requires that the cis- and trans-stilbenes adsorb on the same sites on the surface. The reaction may be inhibited by the following pathways: (a) the donation of an electron from the product to the surface hole, competitively with the reactant, and (b) the competition of the product and the reactant for the surface sites. The retardation of the isomerization by the trans-stilbene is plausible in view of the less positive oxidation potential of trans-stilbene ( E , 2Ox = 1.49 V vs. than cis-stilbene (El,? = 1.63 V vs. SCk).2s Effect of CdS and Light Intensity. The effect of increasing amounts of CdS on the rate of reaction over the range 10-233
(29) Djeghri, N.; Formenti, M.; Juillet, F.; Teichner, S . J. Faraday Discuss. Chem. SOC.1974,58, 185.
(30) Harvey, P. R.; Rudham, R.; Ward, S. J . Chem. Soc., Faraday Trans. 1 1983, 79, 1391.
(31) Cunningham, J.; Hodnett, B. J . Chem. SOC., Faraday Trans. 1 1981, 77, 2717.
(27) We thank Mr. P. Carson for these observations. (28) Spada, L. T.; Foote, C. S . J . Am. Chem. SOC.1980, 102, 391
(32) Sato, S. J. Phys. Chem. 1983, 87, 3531. (33) Davidson, R. S.; Slater, R. M.; Meek, R. r. J . Chem. SOC.,Faraday Trans. 1 1979, 75, 2507.
5820
The Journal of Physical Chemistry, Vol. 89, No. 26, I985
Al-Ekabi and de Mayo
-"A -1.2
3 -1.6 40 A
e
L
3 -22
'
'
'
3.0
0 '
I
36
10'K
Figure 7. Arrhenius plot of log [rate] against 1/T for isomerization of cis-1. Each sample was irradiated 1 h with a 150-W lamp.
SCHEME I11 SC
SC-./Qad+-
SC*
SC
+
c-Dad
SC-*/c-Dad+*
+
c-Dad
c-Dad
+
e
*E
I
I
I
I
I
I
40
80
12 0
160
200
25 0
( i a i ~ ~io3) Figure 8. Effect of various quenchers on the isomerization of cis-1. (A)
34 (1;:,
e
+
t-Dad
SC-*/t-Dad+.
1,2,4-Trimethoxybenzene (0, experimental points; solid curve was obtained from eq 16 by using a = 53.9, b = 210.7, and n = 0.50; dashed curve was obtained from eq 14 by using a = 10, b = 115, and c = IO). (B) Pyrene ( 0 ,experimental points; solid curve was obtained from eq 16 by using a = 25.5, b = 145.5, and n = 0.50). (C) 1,4-Dimethoxybenzene (m, experimental points; solid curve was obtained from eq 16 by using a = 17.0, b = 36.9, and n = 0.50). (D) 1,2,3-Trimethoxybenzene (A, experimental points; solid curve was obtained from eq 16 by using a = 15.6, b = 29.5, and n = 0.50). (E) 1,3,5-Trimethoxybenzene ( 0 ,experimental points). Each sample was irradiated for 30 min with a I-kW xenon lamp.
respective ions) are cis-stilbene, quencher, and trans-stilbene (and their ions) adsorbed on the surface, respectively.) Application of the steady-state hypothesis leads to the quantum yield expression, in the absence of quencher, eq 9, where Or is the
SC-./Qad+*
on the basis that an increase in temperature will help the reaction to compete more efficiently with hole-electron recombination. Clearly, increase in temperature will increase desorption by physisorbed adsorbates, but this need not be the case for chemisorbed species: the activation energy for reaction may be lower than the activation energy for conversion to physisorbed material or desorption. Gratzel et al.34 have suggested that the rate of formation of electron-hole pairs depends among other factors upon the temperature of the semiconductor which controls its Fermi band position. Rudham et have reported that for pure anatase, five doped rutiles, and coated anatase and rutile pigments, the activation energy under standardized conditions ranged from 7.4 to 21.7 kcal mol-'. They, therefore, considered these energies to reflect the solid-state properties of the catalyst rather than the chemistry of propan-2-01 oxidation. The results contained in Figure 5-7 emphasize the requirements of constant catalyst weight, light intensity, and temperature if reproducible activities are to be achieved. Stilbene Quenching Studies. The semiconductor photoinduced reaction of cis-stilbene may be quenched by (a) donation of an electron by the quencher, Q, to the photogenerated hole competitively with reactant, c-D, and (b) interception of the radical cation, c-D+, by the quencher on the surface. On the assumption that reaction (isomerization of c-D+) is faster than desorption, that quenching involves surface-bound quencher, and that there is a single route to product we arrive at Scheme 111 as a minimal representation of the processes involved. Oxygen is omitted from Scheme I11 because, under our conditions, we believe that it has little inhibitory effect on the rates of isomerization. (In Scheme 111, SC is the semiconductor, and c-Dad,Qad,and ?-Dad(and their (34) Kiwi, J.; Borgarello, E.; Pelizzetti, E.; Visca, M.; Gratzel, M. "Photogeneration of Hydrogen"; Harrimann, A., West, M. A,, Ed.: Academic Press: London, 1982; p 129. (35) Lehn, J.-M.: Sauvage, J.-P.; Ziessel, R. N o w . J . Chern. 1980, 4, 623.
(9) fraction of the surface covered by the reactant, c-D, and k l , kZ, k j , and k4 are the rate constants for electron-hole recombination, donation of an electron from the reactant to the hole, recombination of the SC-./c-D+. ion pair, and isomerization of the intermediate c-D+., respectively. It is important to note that the quantum yield for the conversion of the final radical cation, t-D+., to product is unity; Le., no chain reaction," with electron transfer from cis-stilbene to the trans cation, is involved. In addition, the reversal of the trans cation to give the cis isomer is observably very slow. The quantum yield expression, in the presence of the quencher, is shown in eq 10 where Os is the fraction of the surface covered
k20,
= kl
k4
+ kzO, + k50, k 3 + k4 + k60q
(10)
with the quencher, Q, and k , and k6 are the quenching constants for the hole and the radical cation, c-D+., on the surface, respectively. ( 3 6 ) Very recently Lewis et aI.l9 have reported an elegant study of the solution-phase photoisomerization of cis-stilbene via a cation radical chain mechanism. In this study an electron transfer from cis-stilbene to the trans-stilbene cation radical is proposed. However, on the basis of our low quantum yield" (4 = 0.05 f 0.01 at 0.14 M) this mechanism seems not to be important in the CdS photoinduced isomerization of cis-stilbene. Presumably the surface concentration of the latter is too low; and the counter radical anion 02-and/or CdS(e-) continuity to the trans cation on the surface renders its lifetime too short for effective participation in a chain reaction. (37) The quantum yield for CdS photoinduced isomerization of cis-stilbene was determined by using potassium ferrioxatate. In calculating the quantum yield it was assumed that all radiation entering the reaction vessel was absorbed by the CdS (is., scattering was ignored). The concentration of the actinometer (0.15 M) and the Corning Filter (CS 5-58; transmitted at 360-470 nm with a maximum at -405 nm) were chosen to assure almost complete absorption of the radiation by the actinometer. In addition the low light intensity was chosen to minimize the effect of the electron-hole recombination.
The Journal of Physical Chemistry, Vol. 89, No. 26, 1985 5821
Photoinduced Cis-Trans Isomerization of Olefins Dividing eq 9 by eq 10 gives eq 11. Since the reactant (constant
initial concentration) and the quencher (variable concentrations) adsorb independently on a heterogeneous surface 0, is required to be constant. Accordingly, the conversion in quenching experiments was kept 510%. According to the Langmuir-Hinshelwood isotherm, 0, can take the form
where K, and [Q] are the adsorption coefficient and the initial concentration of the quencher and K, and [c-D] are the adsorption coefficient and the initial concentration of the reactant. However, since the reactant and the quencher adsorb independently on a heterogeneous surface (Le., no significant mutual displacement), then 0, can take the modified form38
in Figure 8; in all cases convex curves were obtained. 1,3,5Trimethoxybenzene (a poor electron-transfer quencher) quenched the reaction inefficiently. The data in Figure 8A (dashed curve) could not be fitted adquately by selection of values of a, b, and c to the requirements of eq 14. This suggests that the surface of the Fisher CdS is heterogeneous. According to the Freundlich isotherm, Os has the form shown in eq 15 where k and n are as in eq 3 and [Q] is the concentration Oq
-4O=
4
I+-
aNQ1 + 1 + b[Q1 1
cbZ[Ql2
+ 2b[Q] + b2[QI2
(14)
where a=-+- k5 kl k20r
+
k6
k3
+ k4
b = K, k5k6
C =
(kl
+ k2°r)(k3 + k4)
1,4-Dimethoxybenzene (EIIzoX = 1.35 V vs. SCE),39 1,2,3trimethoxybenzene (EII2OX= 1.42 V vs. SCE),391,3,5-trimeth= 1.49 V vs. SCE),391,2,4-trimethoxybenzene oxybenzene (El,20X ( E I I 2 O x = 1.12 V vs. and pyrene (EII2OX= 1.27 V vs. SCE)Ik were tested as quenchers. Plots of ~ O / $ I vs. [Q] are shown (38)Mouanega, H.; Hermann, J.-M.; Pichat, P. J. Phys. Chem. 1979,83, 2251. (39)Zweig, A,; Hodgson, W. G.;Jura, W. H. J . Am. Chem. SOC.1964, 86,4124.
(15)
of the quencher. Substitution of eq 15 in eq 11 and rearrangement of the resulting equation gives eq 16
do -= 1 0
+ a[Q]" + b[Q]2fl
(16)
where a=-+- k5k kl k20,
+
b= Substitution of eq 13 in eq 11 and rearrangement of the resulted equation gives eq 14
= k[QI"
k6k
k3
+ k4
k5k6k2 ( k ~+ k,Or)(k, + k4)
The data in Figure 8 fit nicely curves (solid curves in Figure 8) obtained by use of appropriate sets of constants for each quencher in eq 16. This suggests that the proposed mechanism in Scheme 111, involving but a single route to product, and surface-bound quenching,40and the assumptions used throughout the derivation of eq 14 are compatible with the data. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for support of this work. We are grateful to Dr. M. Ilyas for the surface area determination of CdS samples and fruitful discussion. Thanks to Dr. T. Hasegawa for the quantum yield determination of isomerization of cis-stilbene and to him and to Mr. A. Draper, Dr. J. Miranda, and Mr. P. Carson for many helpful discussions throughout the course of this study. Registry No. cis-1, 645-49-8;trans-1, 103-30-0;cis-2, 937-53-1; trans-2, 1896-62-4; cis-3,61 14-57-4; trans-3, 16610-80-3; cis-4, 25679trans-5, 1754-62-7; cis-6, 28-1;trans-4, 4180-23-8;cis-5, 19713-73-6; 24840-05-9;trans-6, 1885-38-7;CdS, 1306-23-6;02,7782-44-7;I,, 7553-56-2;1,4-dimethoxybenzene, 150-78-7;1,2,3-trimethoxybenzene, 634-36-6;1,3,5-trimethoxybenzene,621-23-8;pyrene, 129-00-0; benzaldehyde, 100-52-7; benzyl cyanide, 140-29-4; acetone, 67-641.
(40)An alternative kinetic quenching analysis will be presented in a forthcoming publication.
