865
DYE-SENSITIZED PHOTOOXID.4TIONS
REFEREKCES (1) BELL,K. K., CARR,C. J . , I':VANY,W. E., JR., AND I ~ R A N TJZ. ,C., J R . : J. Phys.
Chem. 4!2, 507 (1938). (2) FAUCONNIER, A , : Bull. soc. chim. 41, 119 (1884). (3) FISCHER, E., AND ZaCH, K.:Ber. 46,156 (1912). (4) KRANTZ, J. C., JR.,OAKLEY, M., A N D CARR,C. J . : J. Phys. Chem. 20, 151 (1936). (5) VAN ROMBURGH, G., AND V A N DER BERG,J. H. N . : Proc. Aead. Sci. Amsterdam 26, 335 (1922). (6) WILSON~ J . A . : Ind. Eng. Chem. 17, 71 (1925).
THE QUANTUhl YIELDS OF SOME DYE-SENSITIZED PHOTOOXIDATIONS FRANK HURD
~ N W ROBERT
LIVINGSTON
School of Chemzstry, Instztute of Technology, Cnzverszty of Minnesota, Mznnesota
11.i inneapnlis,
Recezved January 18, 1940
The photosensitization of oxidation-reduction reactions by dyes has been extensively studied. Since 1910 about fifty papers. have appeared on this or closely related subjects. However, relatively little quantitative work has been reported. This lack of quantitative information has seriously handicapped the establishment, upon a sound basis, of a theory or mechanism of these reactions, as is demonstrated by the fact that a t least four mutually inconsistent theories have been proposed in the recent literature. Since these reactions have important biological analogs and are not without intrinsic interest, it appeared to the authors that they were worthy of a careful quantitative study from the physicochemical viewpoint. The present paper reports the results of a series of determinations of the quantum yields of photooxidations of several reducing agents sensitized by various dyes. The results of a detailed investigation of one of these systems,-solutions of eosin and potassium iodidej--will be published shortly. EXPERIMENTAL PROCEDURE
Materials a
The rose bengal, phenosafranine, and safranine T were Grubler preparations. The fluorescein was a Kahlbaum product. The neutral red was A product of the Xational Aniline and Chemical Company. The indigosulfonates and one sample of methylene blue were LaMotte indicators.
866
FRANK HURD AND ROBERT LIVINQSTON
The second sample of methylene blue was a zinc-free product of the du Pont Company, prepared for biological use. These dyes were used without , further purification. The eosin was purified by precipitating it with dilute hydrochloric acid, washing it, dissolving it with an equivalent amount of base, and crystallizing it from the resulting solution. All other reagents were of analytical g r d e and were used without further purification. Conductivity water was used in preparing all solutions. Analytical methods Oxalate was determined with standard 0.01 N potassium permanganate, following the standard procedure. In a few cases where the dye caused an erratic consumption of potassium permanganate, the oxalate was precipitated as calcium oxalate, filtered off, washed, dissolved in acid, and then titrated with potassium permanganate (reference 10, page 325). Solutions of thiosulfate, arsenite, and those containing both iodide and arsenite were titrated with a standard iodine solution (reference 10, pages 590-6). In certain experiments, particularly those with eosin, the end point was determined by the polarized-electrode method of Faulk and Bowden (5). Iodine formed in arsenite-free solutions of iodide was titrated with standard thiosulfate. A calibrated microburet was used in all of these titrations, except in the oxalate determinations, where a weight buret was used. Apparatus The light source was a water-cooled quartz-capillary mercury arc of the atmospheric pressure type (4, 11, 14). Glass color filters were used to isolate the desired spectral regions. Energy measurements were made with a Moll surface thermopyle and a Leeds and Northrup galvanometer, provided with a suitable shunt and series resistance. The galvanometer thermopyle system was calibrated with a radiation standard lamp from the United States Bureau of Standards, following the explicit directions furnished with the lamp. These calibrations were checked with the uranyl oxalate actinometer of Leighton and Forbes (12). Two different optical systems were used in these measurements. The system used in the final series of measurements, which includes all of the experiments using I- and AsOs- as acceptors, was similar to that described by Livingston (15). In the experiments in which either oxalic acid or thiosulfate was used as the acceptor, a spherical reaction cell of about 4-ml. capacity was used. This cell, like the cylindrical cell used in the other series of measurements, was fitted with a gas inlet tube in the bottom, by means of which oxygen could be forced into the solution through a sintered-glass plate. A large glsas condensing lens waa used to bring the
DYE-SENSITIZED PHOTO~XIDATIONS
867
light to focus at the center of the cell. A surface thermopyle waa placed directly behind the cell and intercepted all of the emergent light. The whole system was mounted rigidly on an optical bench. The cell waa protected from stray light and waa so arranged that it could be moved in or out of the light beam in a reproducible manner (8). While the use of this sytem resulted in a high intensity of absorbed light (quanta per second per milliliter of solution), it had the disadvantage that the distribution of intensity in the cell was far from uniform. The experiments on methylene blue were performed separately by a different method.' The light sources used in these experiments were a ZOO-watt projection lamp and a 500-watt carbon 'arc. The rates of the reactions were followed manometrically, using a standard Warburg respiration apparatus. The energy absorbed was not measured directly, but the quantum yields were estimated by comparison with certain convenient known reactions. Routine procedure and computations Coincident with each experiment, two blank analyses were made, using samples which had the same volume as the reacting solution. The first of these titrations was made at the beginning of the run; the second, at the conclusion. The dark reactions were negligible in all of the experiments reported here. The number of molecules which reacted photochemically was computed in the usual way:-from the blank titre, the change in titre, the concentration of the blank, and the volume of the reacting solution. During the course of each experiment, galvanometer deflections, corresponding to the light transmitted by the reaction cell as well as that transmitted by the water cell, were observed a t 15-min. intervals. The total time of irradiation for a single experiment was from 0.5 to 3 hr. Comparison with the uranyl oxalate actinometer (12)showed that the measurements of the absorbed energy were correct within 3 or 4 per cent when the cylindrical cell was used but were about 42 per cent low when the spherical cell and strongly convergent light were used. EXPERIMENTAL RESULT8 AND CONCLUSION8
The quantum yields of the photooxidation of I-, using several dyes as sensitizers, are listed in table 1. All of these experiments were performed with the cylindrical cell and slightly convergent beam of light. Oxygen was passed through the cell at the rate of 30 bubbles per minute. A monohydrogen phosphate-dihydrogen phosphate buffer was used in all These experiments were performed in the Jones Chemical Laboratory of the University of Chicago by R. Livingston while he was a holder of a Lalor Fellowship. The apparatus used waa kindly placed at his disposal by Professor J. Franck.
868
FRANK IIURD AND ROBERT LIVINCSTON
of these exprriments, tho total phosphntc concentration being kept constant) a t 0.005 M . 111addition to the dye, potassium iodide, and buffer, a small amountJof sodium metarsenite was added to each solution. The sodium metarsenite prevented the formation of detectable concentrations of Is-, which would have interfered with the measurements by absorbing light.2 Monochromatic filtered light was used in all of these experiments, except the one with rose bengal and the 0.05 M potassium iodide run with eosin. In these two cases, the light source was a 1000-watt projection lamp, provided with a cupric sulfate solution filter. In the experiment with fluorescein, the wave length of the light was 3650 A. In the third TABLE 1 The photoozidation of iodide INITIAL MOLARITY
DYE
lab..
I-
'M
Fluorescein
i
i
Eosin
1x
Rose bengal Neutral red Phenosafranine Safranine T Indigodisulfonate Indigotetrasulfonate
5x 5x 2x 2x
1 I
i
5x 5x 5x 1x 5 x
M
10-6 0.4 10-6 0.05 10-6 1.00 10-6 3.00 0.05 10-6 10-6 1.00 10-8 1.00 10-6 0.50 10-6 1.00 10-6 1.00 10-6 1 .00 10-6 0.05 10-8 1.00
5x 8x 8x 7 x 10-6 7 x 10-6 7 x 10-
NaAsOn
x
M
4x 5x 5x 1x 5x 5x 5x 3x 4x 4x 3x
10-4 10-3 10-4
4
10-4 lo-' 10-4 10-4 10-4
x x 0.50 x 0.50 3 x 1.oo 4 x 4 3
10-3
10-3 10-4 10-4
10-4 10-4 10-4 10-4
1G-8
0.65 3.7 1.4 1.9 2.9 10.0 2.3 0.83 2.74 1.82 1.6 0.8 . 0.96 1.01 0.88 1.00
7 7 6.7 6.8 7 6.7 6.7 7.0 6.6 6.3 6.9 6.1 6.8 7.0 7.2 6.7
0.055 0.011 0.29 1.05 0.021 0.11
0.13 0.036 0.10
0.18 0.15 0,0014 0.0035 0.011 0.011 0.068
experiment with indigotetrasulfonate, light of X 5780 A. was used. I n all of the other experiments, light of X 5460 A. was used. The quantum yields are expressed as moles of iodide ion oxidized per Einstein absorbed. The results of a few experiments on the photooxidation of 5 2 0 3 - - and of C104-- are listed in table 2. All of these experiments were performed at a p H of approximately 7. Oxygen was bubbled slowly through the solutions during their irradiation. The light filter used transmitted both X 5460 and X 5780 A. Since the spherical cell and strongly convergent beam of light were used in these experiments, the tabulated intensities
* 1;
could act either as an internal filter (18)or as a photosensitizer (2).
#
DYE-SENSITIZED
869
PHOTOOXIDATIONS
(column 4) are spatial average values. The quantum yields are expressed as moles of reducing agents oxidized per Einstein absorbed. The upper limits of the quantum yields in the oxalate experiments were determined by the error of the potassium permanganate titration; there is no evidence that these reactions proceed with detectable speeds. The value for the upper limit of the quantum yield of the photooxidation of oxalate sensitized by methylene blue is 0.0005. This value is an estimate based upon a comparison with the ferric-sensitized oxidation of oxalate, making a semi-quantitative allowance for the difference in absorption of the incident light. The dye concentration in these experiments was 10-6M. Two different samples of methylene blue were used in preparing the solutions. Experiments were performed with 0.2 M oxalic acid, with 0.8 M oxalic acid, and with a solution which contained both sodium oxalate and oxalic acid a t concentrations of 0.10 M and 0.01 M , respecTABLE 2 The photodzzdation of thiosulfate and of ozalate DID
I
T
INITIAL YOLABlTY
Aooeptor
M
Eosin . . . . , . . . . . . . . . 5 X 10-6 0.05 M NazSrOa Rose bengal. . . . . . . . 1 X 1F6 0.003 N NazC104 N HzC204 0.05 M NarSzOs 0.003 N NazCrO, N HGOi Phenosafranine . . . . 5 X 10-6 0.003 N Na&ZO, N HrCz04
+
0.097
+ 0.097 + 0.097
14 2.1
