NOTES
2058
Acknowledgment. One of the authors (11.K.) wishes to thank the Norwegian Cancer Society for a research fellowship.
Proton Ejection in Photoactive Quinone Systems
by Kenneth P. Quinlan Photochemistry Section, Energetics Branch, Space Physics Laboratory, Air Force Cambridge Research Laboratories, L. G. Hanscom Field, Bedford, Massachusetts 01780 (Received August 22, 1968)
Quinlan and Fujimori1,2have shown that proton ejection accompanies electron transfer in light-activated porphyrin-quinone systems. In those studies it was assumed that the proton originates from the porphyrin, Kinetic measurements of the decay of the quinone ion radial led Chibisov* to postulate a role for the solvent in these reactions. In the present study, intermittent irradiation of quinones in the presence of abstractable hydrogen atom substrates reveal that proton ejection and semiquinone formation also occur in these photoreactions. These findings not only shed light on the basic mechanism for the photoreactions of quinones but also may contribute to an understanding of the photooxidation of chlorophyll and chlorophyll derivatives by quinones. The photoreaction of quinones with abstractable hydrogen atom substrates have been studied both in the presenceks and in the absenceg-12of air. In deaerated alcoholic solutions, both the hydroquinone and the carbonyl compound derived from the alcohol are formed. The quinones can also act as efficient photosensitizers during the autooxidation of substrates. Cooper12 has shown that irradiation of sodium anthraquinone-2sulfonate with N-ethylacetamide in deaerated systems yields N-vinylacetamide and the hydroquinone. Bridge and Porter13 have studied the transient intermediates formed in these photoreactions using flash photolysis. They have shown in their studies of duroquinone in ethanol that the primary process of the photochemical reaction is hydrogen abstraction by duroquinone followed by dissociation of the semiquinone radical. The substrate-solvent systems reported in the present paper are representative of those investigated in either the photochemical studies of quinones or chlorophyllquinone systems.
Experimental Section The p-benzoquinone was purified by sublimation, and sodium anthraquinone-2-sulfonate by recrystallization from water. The solvents were purified by distillation either in vacuo or at atmospheric pressure. T h e Journal of Physical Chemistry
Esr spectra were obtained with a Varian V-4502 spectrometer utilizing 100-kc modulation. The quinone systems were irradiated in the cavity with a 1000W projection lamp whose beam was passed through a 1cm heat filter of 0.05% copper sulfate and a Corning CS1-61 blue filter. The CS1-61 filter is opaque to wavelengths below 310 mp and therefore little or no excitation of the substrate solvents occurs. The apparatus for proton ejection measurements using a Radiometer (Copenhagen) GK2021 combined electrode to measure the changes in apparent pH has been previously described.l 4 The investigations of various workers16-17 have shown that the glass electrode is not affected by quinone, ~ e m i q u i n o n e ,and ~~ hydroquinone. The systems were irradiated with a 500-W projection lamp whose beam was passed through 7 cm of water, 1 cm of 0.05% copper sulfate, and a Corning CS1-61 filter. In the case of ethanolic solutions, only white light was used. Titration experiments showed that the glass-calomel system in Nethylacetamide responds to increases in hydrogen ion activity.
Results and Discussion Figures 1 and 2 show the esr spectra and changes in the apparent pH of the quinone systems when exposed to intermittent light. The esr spectra do not correspond to the time coordinate but simply show those signals formed upon irradiation. The esr spectrum of the sodium anthrasemiquinone-2-sulfonate does not display any fine structure. Sharp, et ~ 1observed . ~ a~ broad singlet for anthrasemiquinone in methanol a t (1) K. P. Quinlan and E. Fujimori, J . Phys. Chem., 71, 4154 (1967). (2) K. P. Quinlan, ibid., 72, 1797 (1968). (3) A. K. Chibisov, A. V. Karyakin, N. N. Droadova, and A. A. Krasnovskii, Dokl. A k a d . A'aulz S S S R , 175, 737 (1967). (4) J. L. Bolland and H. R. Cooper, Proc. Roy. SOC. (London), A225, 405 (1954). (5) C. F. Wells, Nature, 177, 483 (1956). (6) N. K. Bridge, Trans. Faraday SOC.,56, 1001 (1960). (7) C. F. Wells, ibid., 57, 1703, 1719 (1961). (8) C. F. Wells, J . Chem. SOC.,3100 (1962). (9) B. Atkinson and M . Di, Trans. Faraday SOC.,54, 1331 (1958). (10) F. Wilkinson, J . Phys. Chem., 66, 2569 (1962). (11) K. Tickle and F. Wilkinson, Trans. Faraday Soc., 61, 1981 (1965). (12) H. R. Cooper, ibid., 62, 2865 (1966). (13) N. K. Bridge and G. Porter, Proc. Roy. SOC. (London), A244, 259, 276 (1958). (14) K. P. Quinlan and E. Fujimori, Photochem. Photobiol., 6, 665 (1967). (15) J. S. Blair, Trans. Electrochem. SOC.,74, 567 (1938). (16) C. Morton, J . Chem. SOC.,2469 (1932). (17) E. S. Amis and J. L. Gabbard, J . Am. Chem. Soc., 5 9 , 557 (1937). (18) H. S. Mason, G. Narni, and I. Yamazaki, Abstracts, International Biophysics Congress, Stockholm, 1961, p 328. These
authors have shown that semiquinone is present in neutral solutions of quinhydrone. (19) J. H. Sharp, T. Kuwana, A. Osborne, and J. K. Pitts, Jr.. Chem. I n d . (London), 508 (1962).
~
2059
NOTES H
IO GAUSS
7.338 10.221 IO. 201
7.328
rn
I n
IO. 181
$ IO.I61
7.318
W
a
a
zn
n d
a
a 10.427 7.508
10.407
6.488
10.381
6.468
IO.361
. l
0
3
6 9 12 15 TIME (SEC)
18 21 24 27
Figure 1. A. Irradiation of sodium anthraquinone-2-sulfonate in N-ethylacetamide: upward arrow, light on; downward arrow, light off; upper segment, esr spectra, [AQSOsNa] = 4.4 x 1 0 + M ; lower segment, apparent p H change, [AQSOaNa] = 2.3 X 10-2 M . B. Irradiation of sodium anthraquinone-2-sulfonate in 50y0 (v/v) aqueous 2-propanol: upper segment, esr spectra, [AQSOsNa] = 7.7 X M; lower segment, change of apparent pH, [AQSOaNa] = 1.0 x 10-2M.
-190". The changes in apparent pH demonstrate that, upon irradiation, a proton is ejected, and when illumination is terminated, the protons are taken up. Similar results were obtained using 50% (v/v) aqueous solutions of i\'-ethylacetamide. The increased acidity of the solutions obtained after irradiation is most likely due to the slight degree of dissociation of the hydroquinone formed. No change in the apparent pH is observed when only the substrate-solvents are irradiated. A possible source of the proton is the substratesolvent. If the substrate-solvent is the proton source, there remains the uncertainty whether the proton reverts back to the solvent radical or to some other species. These results do not preclude the existence of the dissociation of the semiquinone radical as postulated in the mechanism.la Hermann and Schenck20have shown the importance of the interaction between the excited quinone and the hydroquinone in the duroquinone system. This type of an interaction is plausible in the case of the anthraquinone system since increases in the quantity of protons ejected are observed during subsequent light intervals in the initial phases of the experiment. Extended continuous irradiation of the anthraquinone-K-ethylacetamide system does not eliminate proton ejection or the esr signal. Similar results have been observed for the duroquinone-methanol
0 3
6
9 12 15 18 21 24 27 TIME (SEC)
Figure 2. A. Irradiation of p-benzoquinone in N-ethylacetamide: upward arrow, light on; downward arrow, light off; upper segment esr spectra, [BzQ] = 2.3 X 10-2 M ; lower segment, apparent p H change, [BzQ] = 3.1 x 10-2 M . B. Irradiation of p-benzoquinone in dimethylformamide: upper segment, esr spectra, [BzQ] = 1.3 X 10-2 M ; lower segment, change of pH, [BzQ] = 1.9 x 10-2 M . Sensitivity of light-induced esr spectra of B is 0.5 that of A.
system where the durosemiquinone ion radical was observed. Figure 2 shows that irradiation of p-benzoquinone in the presence of either N-ethylacetamide or dimethylformamide forms the benzosemiquinone ion radical along with the ejection of a proton. Aqueous solutions of these substrates give similar results. Proton ejection was also observed in the case of p benzoquinone in methanol, ethanol, or 75% dioxane25% (v/v) water solution. Benzoquinone in either methanol or N-ethylacetamide after extended continuous irradiation exhibits little or no apparent pH change when exposed to intermittent light. Benzoquinone probably reacts with N-ethylacetamide in a manner similar to sodium anthraquinone-2-sulfonate where the attack is on the ethyl group of the nitrogen. One is hesitant to assume the locus of attack on dimethylformamide since the esr spectrum characteristic of the dimethylamine nitroxide radical2I was observed when these systems were irradiated in the presence of air. Acknowledgment. The author wishes to express his appreciation to Miss Maria Tavla for obtaining the esr data. (20) H. Hermann and G. 0. Schenok, Photochem. Photobiol., 8, 255 (1968). (21) J. Q.Adams, S. W. Nicksio, and J. R. Thomas, J . Chem. Phys., 45, 654 (1966).
Volume '73,h'umber 6
June 1960
