NOTES
447
tude was required using the resistance network designed by Rusche and Goodall On the other hand, our ratio transformer network is capable of reproducing the cell capacitance to within better than O . O l ~ owithout any correction a t conductances as high as 200 pmhos (specific conductance, 2 pmhos). The resulting capacitance of 678.97 pF results in a dielectric constant of water a t 25.00" of 78.40 + 0.005 in excellent agreement with our previous value2 of 78.41. Owing to the precision with which this value has been obtained, without the necessity of making any corrections, we have adjusted our data a t other temperatures2 and proposed for the dielectric constant of water log
m10
= 1.94409
- 1.991 X
10-3T
Acknowledgment. This work was supported by Contract No. 14-01-0001-1729 with the Office of Saline Water, U. S. Department of the Interior. We are indebted to Professors Good and Rusche for supplying us with the detailed information concerning their measurements. (11) Reference 7, p 78.
Photochemical Reactions of Fluorescein Dyes with H20z1
by Karl J. Youtsey and L. I. Grossweiner Physics Department, Illinois Institute of Technology, Chicago, Illinois 60616 (Received September g3, 2968)
Hydrogen peroxide is expected to be a product in dye-sensitized autooxidations. The precurser H02 can be formed either by the reaction of the sensitizer triplet state (De) with oxygen (Dt 0 2 +- D o x i d HO2) or
+
0.6
1 B E
8 0.4
(1)
where T is in degrees Centigrade. This gives b e ~ , o / dT = -4.585 X 10-3~H20, in excellent agreement with E H ~ Oobtained from the data of Rusche -4.587 X and Good,' the logarithmic plot of their data being completely linear and preferable to their quadratic equation. It should also be pointed out that our present data a t all temperatures are now identical with those of Lees4 to better than 0.01 unit. Significantly, Lees was able to completely compensate for the capacitance associated with his conductance network, as is the case with our measurements, by using a tungsten lamp whose resistance was varied by changing the dc current through the filament. Lees determined the constant capacitance of such a conductance network by using a cell of variable geometry. This fact and the excellent agreement with our data adds considerable weight to the reliability of Lees' data a t higher pressures. The data of Owen, et aLj6are only 0.02 unit lower than our own a t 0" and 0.05 lower a t 40".
+
0.8
0.2
0.0 100
300 600 Time, psea.
700
000
Figure 1. Growth and decay of erythrosin semiquinone in the presence of hydrogen peroxide after flash photolysis of a 5 p.44 deaerated solution at pH 9.3. HzOz concentrations ( p M ) : (a) 2, (b) 5, (c) 7, (49, ( e ) 15.
indirectly via the reaction of the excited sensitizer with an oxidizable substrate (Dt QHz Dred &He), followed by oxidation of the reduced dye intermediate (nred O2 D f H02). In the case of the fluorescein dyes, evidence for the first mechanism derives from the identification of H20zafter aerobic photobleaching of concentrated fluorescein or eosin aqueous solutions.2 The reduction of triplet eosin by aromatics has been studied by flash p h o t o l y ~ i s ,while ~ ~ ~ the oxidation of eosin semiquinone by oxygen was demonstrated with pulse radiolysi~.~The object of this work is to obtain information on the role of accumulated H202in sensitized autooxidation reactions by studying the flash photolysis of deaerated dye-H,Oz solutions. The apparatus and procedures are similar to those described in ref 4. The irradiation of a deaerated dye solution containing HzO2with a green light flash (460-600 nm) produces a
+
+
+-
+
+-
(1) Supported by the U. S. Public Health Service on Grant No, OM-10038 from the National Institute of General Medical Sciences. (2) Y . Usui, K. Itoh, arid M. Koizumi, Bull. Chem. SOC.Jap., 38, 1015 (1965). (3) L. I. Grossweiner, J . Chem. Phys., 34, 1411 (1961). (4) J. Chrysochoos and L. I. Grossweiner, Photochem. Photobiol., 8, 193 (1968). (5) J. Chrysochoos, J. Ovadia, and L. I. Grossweiner, J . Phys. Chem., 71, 1629 (1967). Volume YS, Number $2 February 1080
NOTES
448 Table I: Rate Constants for the First-Order and Second-Order Decay Processes of the Triplet Dye and the Reactions of Triplet Dye and the Dye Semiquinone with Hydrogen Peroxide 7 -
Dt
Fluorescein (pH 12.0) Dibromofluorescein (pH 9.3) Eosin (pH 9.3) Erythrosin (pH 9.3)
+ HzOz ...
x 2.4 x 3.3 x 7.8
----Dred
+ HZOZ
8.8 X 4.9 x 1.9 x 1.2 x
104 104 104
lo4 108 104
106
Rate constanta------Dt --* D b
2Dt 2Dred
+
+
+ OZ
(1)
+2D f 2H20
(2)
H202 +2 D r e d
H202
Measurements of the pseudo-first-order rates of triplet and semiquinone decay a t H20z concentrations up to 50 mM led to the rate constants in Table I. The firstorder and second-order triplet quenching rate constants obtained in this work and taken from the literature are given in Table I also. The low rate constants are typical of noncatalyzed redox reactions involving H202. It is probable that reaction 2 proceeds via a slow Fenton-type process Dred
+ HzOz
D f OH* f HzO
+
(3)
followed by the well-known OH, H02, H20z radical chain.* The energetics of reaction 3 can be estimated with the redox potentials given by Baxendale9 and taking the reversible polarographic half-wave potential of the dye as approximately equal to the redox potential of the couple D, H +/Dred. The calculation for the case of eosin based on Ell2 = -1.03 V (pH 10 us. sce)1° gives A.F'(pH 10) -30 kcal/mol a t 50% conversion of HzO2. Reaction 1 demonstrates the ability of dye triplet states to act as electron acceptors. It probably takes place in two stages with the intermediate slow step
-
Dt
-
+ HzO~
Dred
+ H+ +
0 2 -
(4)
noting that pK, of H02is 4.5.l' The energetics can be estimated for eosin, as in the above case, taking the The Journal of Physical Chemistry
+ Dt
1.1 x load 3.0 X 1 . 2 x l0Bc 6 . 0 X 106c
50d 1500 540' 5200
a Units are l./mol-sec unless indicated otherwise. * Units in this column are sec-l. expressed in terms of the unknown triplet molar absorbance at 600 nm. Reference 6. Photobiol., 4, 923 (1965).
broad, strong absorption in the blue region which grows in and decays more rapidly at higher Hz02concentrations. Typical data for erythrosin are shown in Figure 1. The flash spectra show that the transient bands correspond with the dianionic forms of the semiquinone for the case of fluorescein,6eosinJ3and erythrosin.' Kinetic measurements in the red region, where only the dye triplet states absorb, indicate that the triplet lifetime decreases with increasing H20z concentration. It is concluded that H202 both reduces the dye in its triplet state and oxidizes the corresponding semiquinone. The over-all reactions can be described as
Dt
e
Dt -k D
1 x 2.4 X 3.7 x 2.2 x
load lo8 108"
10s
The results obtained in this work are V. Kasche and L. Lindqvist, Photochem.
phosphorescence energy of 40 kcal/mol'2 as approximately the free energy change of Dt + D. It is found that AF'(pH 10) -3 kcal/mol a t 50% conversion of Hz02. Thus both the reduction (eq 4) and the oxidation (eq 3) are energetically feasible reactions. Permanent bleaching of the dye after several light flashes was observed except in the case of fluorescein, where the triplet yield was very low. A posszble explanation is the oxidation of the dye by OH formed in reaction 3, which has been shown to be fast.61' These results indicate that H202 should attain a steadystate Concentration in dye-sensitized autooxidations not involving the excited singlet oxygen mechanism, in which H2Oz is not an intermediate, when it will compete with oxygen and the substrate for triplet dye in the initial act.
-
(6) L. Lindqvist, Ark. K e m i , 16, 79 (1960). (7) .'I Cordier and L. I. Grossweiner, J . Phys. Chem., 72, 2018 (1968). (8) F. Haber and R . Willstatter, Ber. Bunsenges Phys. Chem., 648, 2844 (1931). (9) J. H. Baxendale, Radiat. Res. Supp., 4, 114 (1964). (10) T. Tani and S. I. Kikuchi, Rep. Inst. Ind. Sci. Univ. Tokyo, 18, 1 (1968). (11) G. Czapski and L. M. Dorfman, J . Phys. Chem., 68, 1169 (1964). (12) C. A. Parker and C. G. Hatchard, Trans. Faraday Soc., 57, 1894 (1901).
Vibrational Spectra of the Hydrogen Dihalide Ions. V.
BrHBr- at 20'1.
by J. C. Evans(and G. Y-S. Lo Chemical Physics Research Laboratory, T h e now Chemical Company, Midland, Michigan 48840 (Received September 1.9, 1968)
The previous paper in this series' contains a major printing error; the wave number scale was omitted from all the infrared absorption spectra. This missing scale, (1) J. C. Evans and G. Y-S. Lo, J . Phys. Chem., 71, 3942 (1907).
