1797
LIGHT-INDUCED PROTON EJECTION AND ELECTRON TRANSFER effect owing to the decrease in activation energy can be observed only in the case of reactions 4, 5 , 7, 8, and 10, but for the other reactions no catalysis of this type appears. Thus, in the latter case, the acceleration arises from the increase in the entropy of activation only: L e . , either the electrostatic repulsion and the free-energy change of the complex formation have no marked effect or they are opposite and compensate each other. As can be concluded from the present treatment, the electrostatic and polarization interactions are not the most important effects, unlike in the theories of Eyring and coworkers,ll Libby,12 and Marcus,13which, there-
fore, do not seem t o be easily extendable to the innersphere mechanism. On the other hand, this model does not contain such complicated parameters and computing processes as the conjugation theoryl47l6 does. (11) R.J. Marcus, R. J. Zwolinsky, and H. Eyring, J . P h y s . Chem., 58, 432 (1954). (12)W.F. Libby, ibid., 56, 863 (1952). (13) R. A. Marcus, J . Chem. Phys., 24, 966 (1956); 26, 867 (1957); Disc. Faraday Soc., 29, 21 (1960); J . Phys. Chem., 67,853 (1963). (14) J. Halpern and L. E. Orgel, Discussions Faraday SOC.,29, 32 (1960). (15) P.V. Manning, R. C. Jarnagin, and M. Silver, J . Phys. Chem., 68, 265 (1964).
Light-Indulced Proton Ejection and Electron Transfer in the Zinc Tetraphenylporphin-Benzoquinone System by Kenneth P. Quinlan Photochemistry Section, Energetics Branch, Space Physics Laboratory, A . F . Cambridge Research Laboratories, L. G. Hanscom Field, Bedford, Massachusetts 01730 (Received December 6 , 1967)
Excitation of zinc tetraphenylporphin in the presence of p-benzoquinone activates a reversible proton ejection and an electron transfer attended by the formation of the benzosemiquinone ion radical. The reaction is slightly quenched by air and inhibited by large concentrations of benzoquinone. The results are similar to those found with the chlorophyll-benzoquinone system.
Introduction I n a recent communication, the chlorophylls were shown to eject, a proton during the light-activated single-electron transfer with p-benzoquinone. Possible sources of the origin of the proton from chlorophyll based upon previous work2p3 are the methine and the C-10 positions. A study of the zinc tetraphenylporphin (ZnTPP)-benzolquinone system offered an opportunity to see whether the presence of hydrogens at these particular sites is necessary for proton ejection. Two additional significant observations were also noted in the study of the chlorophyll-benzoquinone system. Proton ejection was in the same apparent proton-activity range whether air was present or not,' and high concentrations of benzoquinone inhibited proton ejection." Results are presented in the present paper to show that the light-induced reaction between zinc tetraphenylporphin and benzoquinone is slightly quenched by air and inhibited by large concentrations of benzoquinone.
Experimental Section The zinc a,P,y,G-tetraphenylporphin was kindly supplied by G. Sherman of Brandeis University. It was further purified by chromatography on a column of a mixture of magnesia and Hy-Flo Super Cel using the method of Seely and Calvin.6 Benzoquinone was purified by sublimation. Methanol was Baker Analyzed reagent spectrophotometric grade. Dimethylformamide was purified by distillation from calcium hydride under vacuum. The method of measuring the apparent pH values of the solutions has recently been described.6 I n the (1) K. P. Quinlan and E. Fujimori, J . P h y s . Chem., 71, 4154 (1967). (2) R. C. Dougherty, H. H. Strain, and J. J. Katz, J . A m e r . Chem. Soc., 87, 104 (1965). (3) R. B. Woodward and V. Skaric, ibid., 83, 4676 (1961). (4) K. P. Quinlan, unpublished results. Electron spin resonance studies of the same system in dimethylformamide showed a large decrease in the esr signal with increasing amounts of pbenaoquinone. (5) G.R. Seely and M. Calvin, J . Chem. Phys., 23, 1068 (1955). (6) K. P. Quinlan and E. Fujimori, Photochem. Photobiol., 6, 665 (1967). Volume 78, Number 6 M a y 1968
1798
KENNETH P. QUINLAN
I :i i
0.71
I
A, NE
8. AIR
I
I
I
I
i-\l4 I I I I
0.6 -
>
c
v)
2
-I
I I
0.5 -
0.4 -
s 0.3P
a
I
I I
1 I
I'
0.2-
'\
"'ir"---' O*O
WAVELENGTH (m, )
Figure 1. Absorption spectra of 2.88 X loTGM zinc tetraphenylporphin in methanol, , and the CS3-69 filter, - - - -.
-
present study, the solutions were irradiated with a 500-W projection lamp setup consisting of a collimating lens, a Corning CS3-69 filter, a 1-cm heat filter of 0.05% copper sulfate, and a 7-cm filter of distilled water. Tha light intensity was 3.5 X lo4 ergs/cm2 see. The epr spectra were obtained with a Varian V-4502 spectrometer utilizing 100-kc modulation. The samples were irradiated with light from a 1-kW projection lamp. The light was passed through a 1-cm heat filter of 0.05% copper sulfate and the CS3-69 filter. The test solutions were prepared a t the time of determination. Air-free systems were obtained by flushing with nitrogen which was previously scrubbed with either. Alkaline pyrogallol or chromous chloride solutions. Methanol was used directly for systems studied in the presence of air. The adsorption spectrum of ZnTPP in methanol and the CS3-69 filter is shown in Figure 1.
Results and Discussion Figure 2 shows the esr spectra of the ZnTPPbenzoquinone system in the presence and absence of air when ZnTPP is excited with light. The figure illustrates the characteristic five-line spectrum of the p-benzosemiquinone formed in the deaerated system as a result of an electron transfer from the ZnTPP. Benzoquinone in methanol gives a diminutive signal but is in no way affected by yellow light. The small light-induced signal obtained at a modulation amplitude of 7.0 g in the presence of air indicates that there is an interaction between the ZnTPP and benzoquinone. The Journal of Physical Chemistry
I Figure 2. Effect of air on the light-induced esr spectra of the zinc tetraphenylporphin-p-benzoquinonesystem in methanol: A, air free (1.8 X M ZnTPP and 1.5 X lo-+ M quinone); B, air (1.8 X M ZnTPP and 1.3 X 10-8 M quinone).
The decrease in signal height may be because of an interaction of oxygen with the formed radicals. No signal is observed when ZnTPP is excited alone in methanol, whether methanol is air free or not. Porton ejection accompanying electron transfer for the ZnTPP-benzoquinone system in methanol is illustrated by the typical results shown in Figure 3. Similar observations were also noted in aqueous dimethylformamide solutions. Figure 3 shows that no change in apparent pH is observed when either ZnTPP or quinone is irradiated with yellow light. Table I shows the results of a series of measurements performed in the absence and presence of air. I n each case, proton ejection was observed in the presence of air. These results are similar to the chlorophyll system where proton ejection is observed in the presence of air. These values were calculated using pK = 16.7' for the autoprotolysis constant of methanol, and it was assumed that there was no buffering action. Table I: Effect of Quinone Concentration on Proton Ejection from the Zinc Tetraphenylporphin-Benzoquinone System in Methanol ZnTPP oonon, M
2.88 X 2.88 X 2.88 x 2.88 X 10-6 2.88 X 2.88 X
Quinone ooncn,
-8pparent proton ejection, M-. Aerated Deaerated
M
4.0 X 4.2 X 1 . 0 X 1O-O 5.1 X 2.7 X No ejection
2.3 X 6 . 8 X lo-* 1.2 X 6 . 6 X 10-3 2.0 X 2 . 0 X 10-1
2.0 X 2.7 X 6.0 X 3 . 3 X 10-"' 2 . 1 X 10-l" No ejection
The above results can be summarized by the following simplified equations hv
ZnTPP -+ ZnTPP* ZnTPP* --+ZnTPP' (ZnTPP*) ZnTPP'
+ Q -+
ZnTPP.
+ Q * - -I- H+
(7) E. J. King, "Acid-Base Equilibria," The Maomillan Co., New York, N. Y . , 1965, p 298.
LIGHT-INDUCED PROTON EJECTION AND ELECTRON TRANSFER
I
n 8.543
8,4451r
8,387
-y--i-;--;-:--, -.-.-.-.-.-.-
.-.-.-.-.-.-.-I
0
3
6 9 12 15 I8 21 TIME ( SEC I
Proton ejection from the ainc tetraphenylporphin-p-benzoquinone system in aerated Figure 3.
methanol (upwaird arrows represent light on; downward arrows represent light off): -zinc tetraphenylporphin (2.88 X M ) and p-benzoquinone (6.6 X M); M ) ; - - - - quinone l ZnTPP (2.88 X (1.4x 10-3 A [ ) .
-.-.-.
where ZnTPP* and ZnTPP' are the excited singlet and triplet state of ZnTPP. In the presence of air, quinone and oxygen compete for the excited states of ZnTPP. This is shown by the small decrease in proton ejection in aerated solutions. The triplet state of ZnTPP and its quenching have been studied by Pekkarinen and Linschitz.8 The above results show that neither the methine nor the C-10 hydrogen is necessary for proton ejection. Comparison of the chlorophylls and ZnTPP indicates that no specific type of hydrogen is required for proton ejection from porphyrins during light-activated electron
1799
transfer. These conclusions are based upon the assumption that the solvent plays no role in proton ejection. A study of the effect of quinone concentration on proton ejection is shown in Table I. The results illustrate that an optimum concentration of quinone exists for maximum proton ejection. Proton ejection decreases as the quinone concentration increases beyond this point. At a concentration of 2 X lo-' M, quinone, proton ejection is no longer detected. A solution containing 1.8 X M ZnTPP with 1.9 X lo-' M quinone in air-free methanol exhibited no light-induced esr signal. Evstigneev and coworkersg also showed in their photopotential study of the chlorophyll-benzoquinone system that a high concentration of quinone reduced the magnitude of the photopotential. These results indicate that the interaction between chlorophyll or ZnTPP with benzoquinone cannot be simply restricted to an electrontransfer process. The results reported in the present paper not only substantiate those obtained with the chlorophyll system, but also may shed light on the mechanism of the electron-transfer process in photosynthesis.
Acknowledgment. The author wishes to thank Dr.
E. Fujimori for many helpful discussions. (8) L. Pekkarinen and H. Linschitz, J . Amer. Chem. SOC.,82, 2407, 2411 (1960).
(9) V. B. Evstigneev, V. A. Gavrilova, and N. -4.Sadovnikova, Biokhimiua, 3 1, 1229 (1966).
Volume 72, Number 6 May 1068
