J . Phys. Chem. 1984, 88, 683-688
683
Factors Affecting the Efficiency of Photochemical Water Cleavage Systems. The Reaction between 0, and the Reduced Electron Acceptor Thomas W. Ebbesen, Bhalachandra L. Tembe, and John J. Kozak* Department of Chemistry and Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: May 16, 1983; In Final Form: July 13, 1983)
In an effort to identify and assess the possible experimental problems that arise in effecting a photochemical water cleavage system, we have undertaken the analysis of a prototype water splitting system in the absence and presence of an undesired side reaction. The specific experimental system studied is the R~(bpy),~+/Ru(bpy),~+, MV+/MV2+,system (with dispersed metal/metal oxide catalysts) discovered by Gratzel and co-workers. Their experiment is modeled first as a cyclic system; then, we explore the consequences of considering the side reaction between O2 and the reduced electron acceptor, MV+. The kinetic analysis draws attention to the means by which the system can be optimized and the back-reactions suppressed, specifically by adjusting concentration levels of the majority species (here, R ~ ( b p y ) , ~MV2+, +, and the catalysts). Further, it clarifies the role of heterogeneous environments in water cleavage systems, emphasizing their importance in influencing cage escape yields. It becomes evident that a major problem here is the presence of undesired side reactions. Accordingly, numerical simulations were performed to study the ideal cyclic system in the presence of the reaction between O2and MV+. We compare the results obtained from a steady-state kinetic analysis and from numerical simulation of the evolution profiles for O, H2, and 02-and quantify the extent to which the side reaction involving O2compromises the efficiency of H2 production in the system.
I. Introduction It is becoming apparent from recent reports in the literatureI-l6 that the experimental realization of an efficient cyclic (nonsacrificial) system for cleaving water into H2 and O2using visible light is exceedingly difficult to achieve. For that reason studies are concentrating on the detailed examination of the experimental systems that have been reported. A number of groups have been repeating crucial experirnent~,l-~ weighing the importance of parameters thought to determine the outcome of individual steps,"' examining the details of individual reaction^,^^^ or identifying undesired side reactions that might affect the overall performance of the system under continuous irradiation.lO-ll Only a few studies to date have analyzed the kinetics of evolution of a cyclic system by taking into account not only the reactions initiated under flash photolysis but also those that occur under continuous irradiation of the system.I2-l5 A very obvious problem that has not been studied thus far is the presence of O2 produced in the water cleavage system. Molecular oxygen reacts competitively with many radicals and triplet states and thus, should it accumulate in solution, its presence will lead to serious problems.I6 In this paper we assess the consequences of considering explicitly one important side reaction (1) M. Gohn and N. Getoff, Z. Narurforsch. A, 34, 1135 (1979). (2) P. Keller and A. Moradpour, J. Am. Chem. SOC.,102, 7193 (1980). (3) A. Harriman, G. Porter, and P. Walters, J . Phorochem., 19, 183 ( 1982). (4) M. H. Dung and J. J. Kozak, J . Photochem., 16, 121 (1981). (5) A. Harriman, G. Porter, and M. C. Richoux, J. Chem. Soc., Faraday Trans. 2, 77, 833 (1981). (6) B. L. Gore, A. Harriman, and M. C. Richoux, J . Phorochem., 19,209 ( 1982). (7) Kitamura, S. Okano, and S.Tazuke, Chem. Phys. Lett., 90, 13 (1982). (8) E. Amouyal, D. Grand, A. Moradpour, and P. Keller, Nouv'.J . Chim., 6, 241 (1982). (9) M. Kaneko, N. Awaya, and A . Yamada, Chem. Lett., 5, 619 (1982). (10) 0. Johansen, A. Launikonis, J. W. Loder, A. W. H. Mau, W. H. F. Sasse, J. D. Swift, and D. Wells, Ausr. J . Chem., 34, 981 (1981). ( I 1) 0. Johansen, A. Launikonis, J. W. Lcder, A. W. H. Mau, W. H. F. Sasse, J. D. Swift, and D. Wells, Aust. J. Chem., 34, 2347 (1981). (12) S. F. Chan, M. Chou, C. Creutz, T.Matsubara, and N. Sutin, J . Am. Chem. SOC.,103, 369 (1981). (13) W. J. Albery, A. W. Foulds, and J. R. Darwent, J. Photochern., 19, 37 - (19x2). (14) M. Rougee, T.W. Ebbesen, F. Ghetti, and R. Bensasson, J. Phys. Chem., 86, 4404 (1982). (15) M. Kirch, J. M. Lehn, and J. P. Sauvage, Helv. Chim.Acra, 62, 1345 (1 979). (16) J. R. Darwent, P. Douglas, A. Harriman, G. Porter, and M. C. Richoux, Coord. Chem. Reu., 44, 83 (1982).
0022-3654/84/2088-0683$01.50/0
involving molecular oxygen. W e do so within the context of the cyclic water cleavage system discovered by Gratzel et al.,17*18 for which these authors propose the following sequence of reactions:
s
-s* hu
-s
s*
-
IadT (= g[sl)
(a)
ko (s-1)
(b)
ko
+ A S+ + AS+ + A- 2 S + A ki
S*
with S+
k3
-
k , (M-I s-I), dC k2 (M-l s-I)
+ H+ + 1/402 k3 (s-l) A + OH- + 1/2H2 k4 (s-I) catalyst. H p L catalyst. H20
S
k4
A-
(c) ( 4
(e) (f)
Although proposed originally to describe their specific system, we may regard the above scheme as an idealization of cyclic water cleavage systems in general. In this broader sense, S is a photosensitizer which may be excited to a triplet state S* with a rate equal to the product of the number of photons absorbed (in einsteins L-' s-l) by the system, Ia,times the quantum yield dT of intersystem crossing to the triplet state. The species S* can either return to the ground state S (reaction b) or be quenched by the electron acceptor A to give the radicals S+and A- with cage escape yield dC (reaction c). In the Lausanne S is Ruand the steps e and f a r e mediated by the [ b ~ y ] ~A~is+ MV2+, , dispersed metal/metal oxide catalysts, RuO, and P t 0 2 , respectively. To the above scheme, we should like to consider as well the effect of the further reaction A-
+ O2
ks
A
+ 0,-
k5 (M-I
S-I)
(g)
Estimates of the various rate constants appearing in the chemical network a-g are available in the literature for the Lausanne system and these will be given explicitly later in this paper when we present the results of our evolution studies. In the following section, we present a kinetic analysis of the system a-f in the steady-state approximation. Regarding the (1 7) K. Kalyanasundaram and M. Gratzel, Angew. Chem., Znt. Ed. Engl., 18, 701 (1979). (18) K. Kalyanasundaram, J. Kiwi, and M. Gratzel, Helo. Chim. Acta, 61, 2720 (1978).
0 1984 American Chemical Society
684
Ebbesen et al.
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984
network a-f as a general scheme for producing H 2 and 0,via a photoinitiated electron transfer event, we identify certain general problems facing the experimentalist, problems which appear already even in the case of an ideal, nonsacrificial system. We are able to sort out and identify some of the variables that influenced the results obtained in an earlier numerical study.21 In so doing, it becomes clear that one of the most significant problems confronting the experimentalist here is that of undesired side reactions. We therefore take up the further problem of considering an important side reaction (reaction g) between two of the species MV+ and 0, produced in the network a-f. The steady-state analysis is presented in section I1 and a kinetic analysis is presented in section 111. The results of numerical simulations are presented in section IV where contact will be made with earlier numerical analyses of the same system. The conclusions that are drawn from this combined analytic and numerical study will be presented in section V.
The experimental implications of results presented above will be examined in the following two sections.
111. Analysis of the Steady State To facilitate discussion of the steady-state problem, we organize our discussion around a number of specific issues. A . Optimization of the Cyclic System via Concentration Balance. We define a majority species as a component whose concentration is much larger than any intermediate species produced in the reaction sequence. In contrast, reaction intermediates will be called minority species. In the water cleavage experiment, S, A, and the catalysts are majority species and A-, S', and S* are minority species. After excitation of the photosensitizer (reaction a), there are three sets of two competing reactions: (b, c), (d, e), and (d, f). It is seen that each set of reactions involves a competition between a majority species and a minority species for another minority species. Since the concentrations of the majority species are controlled by the experimentalist, the rates 11. Kinetic Analysis in the Steady-State Approximation of the reactions involving the majority species can be increased We consider first the prototype system, reactions a-f, and apply so as to make them much larger than the rates of the competing an analytical approach that uses features of two earlier s t u d i e ~ . I ~ * ~ reactions between two minority species. For example, if we wish Assuming the steady state has been achieved, we write to suppress reaction b (since it is a nonproductive step), we can adjust the concentration of species A so that kl[A] >> ko. d[S*l/dt = Ia4T - ( k ~+ k,[Als)[Sls* = 0 (1) Similarly, if the undesired reaction d is to be suppressed, then the d[S+l/dt = ~ c k l [ A l s [ ~ *-l ,(k,[A-Is + k3)[S+ls = 0 (2) catalyst concentrations (and type) can be chosen such that k4 >> k2[S+] and k3 >> k2[A-]. Thus, if the experimentalist knows the d[A-l/dt = d~cki[Al,[S*ls- (kz[S+l, + kd[A-Is = 0 (3) values of the various rate constants in the system, the concentration of the majority species can generally be adjusted to make the two d[H21/dt = 1/2k4[A-l (4) ratios in eq 6, viz., kl[A]/(ko k,[A]) and k4/(k2[S+] k4) d[021 /dt = f/4k3[S'] (5) approximately equal to unity. Then, the yield of H2 will depend only on the amount of light absorbed by the system, and the where [S*],, [S'], [A],, and [A-1, refer to the steady-state conproduct &dC: centrations of the species indicated. Then, from eq 1-5 we find that in the steady state
+
Similarly we determine that the [A-1, and [S'], are solutions to the following quadratic equations: [A-]?k4k2 + [A-Isk4k3 - k3K = 0 (8)
[S'],Zk3k2
+ [S+],k4k3 - k4K = 0
(9)
where
Since we require positive concentrations, only the positive root of each quadratic needs to be considered., When the system a-f is augmented by the reaction g, an analysis similar to the one presented above may be carried out. If a steady state has been reached for A-, the results are
d[Ozl/dt = )/4k3[Sf1 - k,[A-I[Ozl or, explicitly, in the steady-state approximation for [S'] and [A-] for a given O2 concentration
(19) J. Willner, J. W. Otvos, and M. Calvin, J . Am. Chem. Soc., 103, 3203 (1981). (20) P. A. Brugger and M. Gratzel, J . Am. Chem. Soc., 102, 2461 (1980). (21) M . H. Dung and J. J. Kozak, J . Chem. Phys., 77, 3246 (1982).
+
Thus, for example, in the system studied by Gratzel et al.," where the values ko = 1.7 X lo6 s-I and kl = 5 X lo8 M-I s-l pertain" and the concentration [A] = 2 X M was reported, one determines that the reported H2 yield is only 37% of what might have been realized if the system were optimized by increasing the concentration of the majority species, [A]. Similar remarks pertain if one considers the 0, evolution. B . Role of Heterogeneous Environments. To suppress the undesired, back-reaction between S' and A- (Le., the electrontransfer reaction d) in the cyclic system, the desirability of designing appropriate heterogeneous environments (such as micelles, vesicles, microemulsions) has been s t r e ~ s e d From . ~ ~ ~the ~ ~discussion presented above, when considering a water cleavage system described by the general scheme a-f, simply adjusting the concentration of majority species (here the dispersed metal/metal oxide catalysts) will, in most cases, be sufficient to make insignificant the rate of reaction d between two minority species, as compared to the rates of reactions e and f. In particular, if k4 >> k,[S'], and k 3 >> k,[A-],, eq 12 would still be valid, thereby making the yield of H, again independent of reaction d. Thus the primary reason for introducing a heterogeneous environment must be other than the suppression of the back-reaction d, per se. From eq 6 it is seen that the efficient generation of S' and Ais dependent on the cage escape yield, &, and the consequent expression 12 for the rate of production of H, incorporates this dependence on &. Now, a heterogeneous environment can be useful in increasing the cage escape yield & (see ref 19). While reaction d occurs by encounter of the species S+ and A- after their escape from the initial cage, both 4c and kz can be modified by the heterogeneous medium (e.g., r$cincreased and k2 decreased) if both reactions c and d involve a [S'A-] cage whose dissociation is the limiting step and if reactions c and d occur in the same microenvironment of the medium. C. Dependence on the Light Intensity, I,. We observe that if already k3 >> k2[A-], and k, >> k2[S'], then increasing k3 and k4 further will not improve in any significant way the efficiency of the system, inasmuch as k4/(k4 + k2[S+],) and k3/(k3+ k2-
Efficiency of Photochemical Water Cleavage Systems
[A-I,)are already close to unity in eq 6 and 7. This explains why in the simulations reported by Dung and Kozak2’ on the Lausanne system, a plateau was realized in the efficiency of the prototype system a-f. These authors also determined that, whereas the plateau region itself was independent of Za, prior to reaching the plateau the efficiency decreased as Za increased. This effect can also be understood by analyzing the ratios, k4/(k4+ kz[S+],)and k 3 / ( k 3 k 2 [ A - ] , )of, eq 6 and 7. Firstly, the steady-state concentrations, [ S + ] ,and [A-],,are dependent (among other things) on Z,, as can be seen from eq 8 and 9. When k4 >> k2[S+],and k3 >> k,[A-I,, then the ratios are not sensitive to small changes in [S’], and [A-1,caused by variations in I,. However, when k3 ko and
k4 >> k 2 [ S + ] ,
we find
As the concentration of O2increases in the system, the term k 5 [ 0 2 ] increases and the ratio k 4 / ( k 4 k 5 [ 0 , ] )decreases from unity; consequently, the yield of H2 drops precipitously. [Notice, by the way, that we have not considered explicitly the reactivity of the species 0,- produced in reaction g; as a reactive intermediate, 02produced in reaction g; as a reactive intermediate, 02-is likely to introduce even more problems as regards H 2 production than O2 itself.] Given the availability of reliable values for the rate constants k4 and k5,one can generate, using eq 13, quantitative estimates of the effect of O2on the H2 yield. As well, estimates of H z production can be obtained by following the evolution of the chemical network a-g to the steady state. The latter calculations are also of interest in exposing characteristic features of the evolving system. These will be reported in the following section where the long time results will be compared with estimates based on eq 13.
+
IV. Evolution of the Water Cleavage System The results of the steady-state analysis (section 11) and their implications (section 111) can be extended considerably by studying, via numerical simulation, the dynamics of the water cleavage system. To study the evolution of cyclic vs. sacrificial water cleavage system, we adopt as a model for our simulations the system, eq a-f, proposed by Gratzel et al.,” to which we add the further side reaction g. The following species were assumed to A, = MV2+, and heterobe present initially: S = R ~ ( b p y ) , ~ + geneous catalysts. The values of the various rate constants needed to specify the model were taken from the literature: ko = 1.67 X lo6 SKI(ref 22); h 1 (ref 23); k, = 5 X IO8 M-’ s-l (ref 17)); (22) C. Creutz, M.Chou, T. L. Netzel, M.Okumura, and N. Sutin, J . Am. Chem. Soc., 102, 1309 (1980).
(23) R. V. Bensasson, C. Salet, and V. Balzani, C. R. Acad. Sci., Ser. B, 789, 41 (1979).
686 The Journal of Physical Chemistry, Vol. 88, No. 4, 1984
Ebbesen et al.
TABLE I: H , Yields in the Absence of Reaction g after lo3 sa [ M V 2 + ] ,M
lH21, M
ratiob
ratioc
5 x lo-* 5 x 10-3
1.187 X 7.811 x 10-5 1.772 X
1 0.66
0.64
0.15
0.14
5
x 10-4
1
a The calculations reported in this table were performed for g = 2 . 7 6 X 1 0 ~ 3 M s ~ ' , k , = 1 . 6 7 X 1 0 6 ~ ~=15, kx10'M-l l s",k2= 2.4 X lo9 M-' S K I k, , = 1.41 X 10.' s - ' , and k , = 5.7 X 10, s-'. The values of k and k , correspond to M = 3 and N = 3. resprespectively. b3Ratio computed from the simulation results of column 2, normalized with respect t o the Concentration [ H , ] = 1.187 X Mat [ M V 2 * ] = 5 X M. Ratiocomputed by using eq 11 of the steady-state analysis.
k2 = 2.4 X IO9 M-' s-l (ref 17); @c = 0.25 (ref 12, although in our simulations we adopted the value unity); k3 = 1.41 X IOM s-I, k4 = 57 X loN s-I, k5 = 6 X lo8 M-' s-l, and g = 2.76 x 10-3 S-1 ( I , = SI). The exponents M and N , referred to p r e v i ~ u s l yas~ ~en~~ hancement exponents, calibrate the enhancement in the rate constants k3 and k4, respectively, owing to the catalytic activity of the colloidally dispersed catalysts. The value (0,O) of ( M , N ) corresponds to a lower bound on the catalytic activity while the value (3,3) refers to, approximately, the highest rates reported in the literature. (Note: The exponent N of the present study is related to the exponent N of ref 4 and 21 as follows: NETK = NDK+ 3.) In previous a broad range of the enhancement exponents was investigated in order to understand the role of nonlinearities in influencing the evolution of the set of coupled rate equations a-f and also to explore the onset of synergetic behavior. In the present paper, we have restricted the values of (M,N) to lie between (0,O) and (3,3) in order to correspond most closely with ongoing experimental investigations. In a first set of simulations, the initial concentration of MVZ+ was varied between 5 X and 5 X M. The initial concentration of sensitizer was set at M. We display in Table I the concentration of H2 after lo3 s of simulation as a function of the initial MVZ+concentration. We anticipate from eq 6 that, when kl[MV2+] >> ko, the rate of H2 production should be maximized and this is confirmed by the present simulations as well as those reported earlier.4*21 In Table 11, we record the steady-state concentrations of the various species involved in the water cleavage system a-g. It is interesting that the steady-state concentrations of S, S*, and S+ change by small amounts when the reaction of MV+ with O2 is included in the reaction scheme. The steady-state concentration of A- decreases due to the presence of an additional positive term in the denominator: [S*ls[Als
@&I
[*-Is
+ k4 4- k5[O2ls
= k2[S+Is
On the other hand, the steady-state concentrations of 0,depend markedly on the values of the enhancement exponents. The highest value of [O,], is 0.317 X M for the value of (3,3) for (M,N). The yields of H2 as a function of time are also shown in Table 11. The decrease in the Hz yield after lo4 s of simulation time due to the side reaction is about 13% for (M,N) = (1,l) and (1,3), 16%for (M,N) = (3,1), and 25% for (M,N) = (3,3). From eq 6 and 10, we see that the rate of H2 production in the presence of the bleed term is scaled by a factor of
k2[S+ls + k4 k2[S+Is + k4 + k,[O2ls Qualitatively, then, one anticipates that the concentration [O,], must be suppressed in order to maximize the generation of H2, and the simulations reported here quantify the importance of this factor in influencing the efficiency of H2 production. In Figures 1-4, we display the evolution profiles for different species assuming the following enhancement exponents: ( l , l ) , (1,3), (3,1), and (3,3). Dotted lines in these figures refer to the case when the bleed term (g) is absent. In all these figures, we
-
-5.001
I -7.50
.-c e c
5 -10.00 0
A-I
9
0
-12.50 0
01 0
-
-I 5 . 0 0
-20.00[ A L', /I I I I -10.00 -7.50 -5.00 -2.50 0.00 2.50 5.00
log,o (timelsec) Figure 1. Evolution curves for the water cleavage system. Here log [concentration/M] vs. log [time/s] is shown for the species S, S*, S+, A, A-, 0,, 02-, and H,. The initial concentrations of S and A are and 0.05 M, respectively. The enhancement exponents (M,N) are (1,l). Dotted lines for H,, OIrand A- refer to concentrations in the absence of the side reaction g.
O.Or
I
A
-17.5
/
/
I
'
/
,
I
L'I
I
I
1
I
Efficiency of Photochemical Water Cleavage Systems
The Journal of Physical Chemistry, Vol. 88, No. 4, 1984 687
TABLE 11: Steady-State Concentrations and the Concentrations of H, as a Function of Time for Different Enhancement Exponents (M,N)' species
(1,1)
S
0.92 0.96 0.72 0.13 0.32
s* S+ A-
0,
(1J)
(0.94 X
X
0.56 X
x 1 0 - 1 (0.97 ~ x 10-l~) x lo-' (0.63 x l o + ) X X
0.58 x 0.44 x 0.81 x 0.32 x
l o - ' ' (0.16 X 10"O)
(3,1)
(3,3)
Steady-State Concentrations lo" (0.60 X 0.99 X (1.0 X 1 0 - l ~(0.62 x i o - l 4 ) 0.10 x 1 0 . ~ 3 (0.10 x 10-13) 1 0 . ~(0.41 x 1 0 . ~ ) 0.62 x i o - 6 (0.55 x 10-6) l o - ' , (1.0 x 0.11 x 1 0 . ~(0.14 x 1 0 . ~ ) 10-4 0.32 X
0.98 x 0.10 x 0.17 x 0.32 x
1 0 . ~(0.98 x 10-4) 10-13 (0.10 x 10-13) 10-5 (0.17 x 10-5) lo-" (0.42 x 1 0 - l l ) 0.32 x 10-4
time, s
1 50
0.93 X 0.11 x 0.16 X 0.41 X 0.62 X
(0.93 X 10-5 (0.11 x 10.' (0.17 X IO-' (0.41 X 10.' (0.63 X 0.10 X (0.11 X 0.14 x (0.15 x 0.40 x 1 0 . ~(0.46 x
100 500 1000 2000 3000
io4
lo-') 10.~1
lo-') 10.') 10.~1 10.~1
Concentrations of H, a i a Function of Time 0.13 X (0.13 X 0.94 X (0.94 X 0.51 x 10-5 (0.51 x 10-5) 0.19 x 10-5 (0.21 x 10-5) 0.87 X (0.87 X 0.36 X (0.40 X 10.') 0.26 X 10.' (0.26 X 0.17 X (0.20 X 0.41 X (0.42 X 10.') 0.33 X (0.39 X IO-') 0.67 X (0.70 X 0.65 X 10.' (0.78 X 10.") 0.92 x (0.99 x 10-4) 0.98 x 10-4 (0.12 x 10-3) 0.26 x 10-3 (0.30 x 10-3) 0.33 x 10-3 (0.39 x 10-3)
0.13 0.59 0.12 0.54 0.10 0.20 0.29 0.92
(0.13 X
X
x 10-5 (0.60 x 1 0 - 9 x 10-4 (0.12 x 1 0 4 )
x x x x x
10-4 (0.59 x 10-3 (0.12 x 10-3 (0.24 x 10-3 (0.36 x 10-3 (0.12 x
10-4) 10-9 10-3) 10.~1 io-')
The values in parentheses indicate the concentrations in the absence of side reaction g.
O.Or
I
-
O.Or
A
I
A
-5.0-
f
C
.o c
e
c
-7.5-
.P e
e
e
s
C
c
-10.0-
8 8
0 V
-10.0 [A-I
Y
2 -12.5-
s
0 -12.5
0 0
-
-7.5
c
0 0
-1 5.0 -
[s*Is -I 5.0
-17.5-
, -20.01 /I I I -10.0 -7,5 -5.0 -2.5 #(
I 0.0
I 2.5
I
5.0
log,o (time/sec) Figure 3. Evolution curves for the water cleavage system with the enhancement exponents ( M , N ) = ( 3 , l ) . (For conventions, see Figure 1.)
same magnitude as that of H2 itself. Suppose that at some initial time (i.e.,, before the lamp is turned on) the initial concentration of oxygen is zero. Suppose further that the reaction vessel is constructed so that there is an accessible free volume above the aqueous phase. Assuming O2to be an ideal gas and using Henry's law, one can estimate the equilibrium fraction of O2in the aqueous phase; given the amount of O2produced, this fraction will be 0.032 ( T = 20 "C), assuming the gas and liquid phase occupy nearly equal volumes.26 Considering an extreme case, it has been reported that rate of excitation might reach 10 s-' in full ~unlight.~' In this case the optimal rate of production of O2could be as high as 1 s-I in solution; consequently, under continuous irradiation, the O2distribution may be quite different from the equilibrium partition estimated above and, consequently, O2 may achieve proportionally higher concentration levels in the aqueous phase. A partition calculation, similar to the one reported in the previous paragraph for 02,can be carried through for H2. Given the consequent and coincident distribution of O2 and H2 in the gas phase, serious questions of maintaining the stability of a (24) J. Kiwi and M. Gratzel, J . Am. Chem. SOC.,101, 7214 (1979). (25) L. K. Patterson, R. D. Small, and J. C. Scaiano, Radiat. Res., 72, 218 (1977). (26) T. W. Ebbesen, manuscript in preparation. (27) R. T. Ross,R. J. Anderson, and T. L. Hsiao, Photochem. Photobiol., 24, 261 (1976).
J
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I
/I
I
I
I
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J. Phys. Chem. 1984, 88, 688-702
We have demonstrated that a water cleavage system described by the prototype reaction sequence a-f can be optimized by an appropriate pairing of majority vs. minority species with emphasis on controlling their respective concentrations. In particular, we have given estimates on the extent to which the H, yields may be improved upon by taking advantage of this optimization procedure; we anticipate an enhancement of 25% for the network described by the sequence a-f, given the rate constants reported for the Lausanne system. An interesting point which emerges from our analysis has to do with the role of heterogeneous environments (micelles, vesicles, and microemulsions) in optimizing water cleavage systems. Since we have seen that the rates of the undesired, back-reactions can be made insignificant via competition between majority and minority species, their principal usefulness in water cleavage systems of the sort described by the general scheme a-g is not in minimizing the importance of back-reactions but rather in enhancing the cage effect yields and in minimizing, via compartmentalization, the importance of other side reactions that may compromise the efficiency of the systemaZ8 To exemplify the problems that arise upon considering side reactions which convert a cyclic system into a sacrificial one, we have focused on the reaction between O2 and MV+, for the pragmatic reason that this reaction has already been studied in some detail and the governing rate constant has been determined. The analysis and simulations show that the oxygen concentration (28) M. Almgren, 'Solar Energy-Photochemical Conversion and Storage", S. Claesson and L. Engstrom, Ed., National Swedish Board for Energy Source Development, Stockholm, 1979, Supplement I, Chapter VII.
level will begin to stabilize at the steady-state value of 3.2 X M after lo3 s of evolution for the enhancement exponents (3,3), with the consequence that the theoretically possible H 2 yields will decrease to -75% of the optimal value if the evolved oxygen remains dissolved in the aqueous phase. If the experimental apparatus is designed in such a manner that there exists a free volume above the aqueous phase then, owing to the possibility of partitioning 0, between the two phases via convection, the evolution to the steady state will take place on a longer time scale than that noted above. On intermediate time scales, the O2levels will tend to be lower than those predicted for a single-phase experiment, and this will tend to reduce the effect of reaction g and presumably the further problems introduced by the reactive product of reaction g, namely, the species 02-.Although the conversion of a closed thermodynamic system to an open one may the tend to alleviate problems arising from the presence of 02-, reactivity of this species is so pronounced that even trace amounts are likely to slow the production of H,. More quantitative estimates are needed here, however, and these will be reported later. Acknowledgment. The research described herein was supported in part by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2409 from the Notre Dame Radiation Laboratory. T.W.E. dedicates the kinetic analysis presented in this paper (section 111) to the teaching of Professor M. RougeC; B.L.T. and J.J.K. thank Dr. Mei Hsu Dung for assistance in the early stages of the numerical analysis. Registry No. Tris(2,2'-bipyridine)ruthenium(2+), 15 158-62-0;methylviologen, 1910-42-5;hydrogen, 1333-74-0;water, 7732-18-5;oxygen, 7782-44-7;ruthenium dioxide, 12036-10-1;platinum dioxide, 1314-15-4.
Vibrational Analysis of 13-cis-Retinal Bo Curry, Ilona Palings, Albert Broek,' Johannes A. Pardoen,' Patrick P. J. Mulder,' Johan Lugtenburg,' and Richard Mathies* Department of Chemistry, University of California, Berkeley, California 94720 (Received: June 9, 1983)
We have obtained preresonance Raman and IR spectra of 13-cis-retinal, its 7-, 8-, lo-, 11-, 12-, 14-, 1 5 , 7,8-, 11,12-, 12,14-, 19-, 20-, and 10,19-deuterio,and its lo-, 12-, 13-, 14-, 15-, 19-, 20-, 10,ll-, and 14,15-I3Cderivatives. The major lines are assigned to specific vibrations on the basis of their shifts in these derivatives, and by analogy with the assignments of all-trans-retinal. Our modified Urey-Bradley force field for all-trans-retinal has been adapted and refined to accurately reproduce the 13-cisvibrational frequencies and isotopic shifts. This analysis allows us to explain the major differences between the spectra of the 13-cis and all-trans isomers. The increased C14H in-plane rocking frequency and the decreased CI4H out-of-plane wagging frequency of the 13-cis isomer are shown to result from direct kinetic effects of isomerization. Furthermore, steric interaction between the protons on CI2and ClSshifts the CI2Hrock up to 1311 cm-I and increases the mixing of both the CI2Hand CISHrocks with CC stretches, accounting for their increased Raman intensity. In 12-deuterio-13-cis-retinal the CI2Drock appears at an unusually high frequency (1006-1040 cm-') compared with that observed for the 12-deuterio all-trans isomer (972 cm-I). This elevated CI2Dfrequency is most apparent in the 12,14-dideuterioderivative, whose C12D + C14D rock combination appears at 936 cm-' in 13-cis-retinal,compared with 901 cm-' in all-trans-retinal. The cis bond also causes significant changes in the ground- and excited-state electron distributions,which result in a characteristic increase in the relative Raman intensity of the c14-cIs and C12-C13 stretches. Comparison of these results with spectra of all-transand 13-cis-retinal Schiff bases (SB) and of bacteriorhodopsin intermediates containing all-trans and 13-cis chromophores allows us to identify key vibrational features which are diagnostic for a 13-cis configuration in pigments.
Introduction The interconversion of the all-trans and 13-cis isomers of retinal is of fundamental importance in the proton-pumping photocycle of bacteriorh&opsin.2 Resonance Raman (RR) spectroscopy is
a useful noninvasive technique for studying chromophore structure For example, resonance Raman studies in bacteriorhodop~in.~,~ have Shown that the K, L,M, and BRsso intmnediates of hacteriorhodopsin contain 13-cis-retinal chromophores,s-8 while the
(1) Department of Chemistry, Leiden University, 2300 RA Leiden, The
(3) Abbreviations used: RR, resonance Raman; HOOP, hydrogen outof-plane; SB, Schiff base. (4) For resonance Raman reviews see: (a) R. Mathies, Chem. Biochem. Appl. Lasers, 4, 55 (1979); (b) A. Warshel. Annu. Rev. Biophys. Bioeng., 6 , 273 (1977); (c) R. Callender and B. Honig, ibid., 6, 33 (1977); (d) A. Lewis, Methods Enzymol., 88, 561 (1982).
Netherlands.
(2) For general reviews see: (a) W. Stoeckenius and R. A. Bogomolni, Annu. Reo. Biochem., 51, 587 (1982); (b) R. R. Birge, Annu. Rev. Biophys. Bioeng., 10, 315 (1981); (c) M. Ottolenghi, Adv. Photochem., 12, 97 (1980); (d) B. Honig, Annu. Rev. Phys. Chern., 29, 31 (1978).
0022-3654/84/2088-0688$01.50/0
0 1984 American Chemical Society
