PHOTOREDUCTION OF METHYL REDSENSITIZED BY CHLOROPHYLLIDE a
periniental values for GHE,obtained from the use of eq. I11 and the estiniate of GC2HIE, supports the applicability of the reaction mechanism employed.
82 1
Acknowledgment. The authors wish to express their gratitude to R. B. Wilkin for his diligent assistance in the collection of experimental data.
Photoreduction of Methyl Red Sensitized by Ethyl Chlorophyllide a'
by G. R. Seely Charles F . Kettering Research Laboratory, Yellow Springs, Ohio
(Received July 16, 1964)
The chlorophyllide-photosensitized reduction of methyl red by ascorbic acid, hydrazobenzene, and niercaptosuccinic acid in ethanol has been studied over a wide range of reducing agent concentration. Evidence is presented that the primary photocheniical reaction of photoexcited chlorophyllide is with methyl red. Even with a deficiency of reducing agent, reduction is practically complete to the four-electron stage, at least in weakly acidic or neutral solution. As the reaction progresses, the quantum yield changes in a complex way peculiar to the reducing agent; the reaction products responsible for this have thus far eluded identification. Empirical expressions are obtained for the initial quantum yield as a function of reagent concentrations, and a rather general expression is derived for the high reducing agent concentration region, based on the postulate that the reducing agent may react either with an initially produced ion pair Chl.+MR.- or with the separated radical ions.
Introduction In their quantitative study of the chlorophyll-sensitized reduction of methyl red by phenylhydrazine2 Livingston and Pariser suggested a mechanism whereby photoexcited chlorophyll reacts first with methyl red to form a complex which then reacts with the reducing agent. On the basis of later results3 they rejected this mechanism in favor of ones in which photoexcited chlorophyll reacts first with the reducing agent. A similar order of events was favored by Evstigneev and Gavrilova. Our present interest in the reaction arose in connection with our investigation of the photoreduction of ethyl ~hlorophyllide,~ through the hope that methyl red niight intercept the priniary product of the reaction between chlorophyllide and ascorbic acid. Although on investigation the sensitized reduction of methyl red by ascorbic acid showed superficial simi-
larities to the reduction of chlorophyllide itself, as for example the apparent inactivity of ascorbate ion and retardation by malic acid or MgC12, there were important differences, the chief being that the reduction of methyl red proceeded most rapidly in ethanol without the addition of any base, whereas chlorophyllide is very slowly, if at all, reduced under these conditions. Re-examination of the reduction of methyl red therefore seemed justified, and in the present paper results with ascorbic acid, hydrazobenzene, and mercapto(1) Contribution No. 158 from the Charles F. Kettering Research Laboratory.
(2) R. Livingston and R. Pariser, J . A m . Chem. Soc., 7 0 , 1510 (1948). (3) R. Livingston and R. Pariser, ibid., 78, 2948 (1956). (4) V. B. Evstigneev and V. A. Gavrilova, Dokl. Akad. S a u k S S S R , 98, 1017 (1954). ( 5 ) G. R. Seely and A. Folkmanis, J . A m . Chem. Soc., 8 6 , 2763 (1964).
Volume 69, ,Vumber 3
March 1966
G. R. SEELY
822
, ~ chlorosuccinic acid are presented. As b e f ~ r e ethyl phyllide a was used in place of chlorophyll. In ethanol none of the reducing agents which are effective reducing agents for methyl red have any comparable ability to reduce chlorophyllide in the absence of methyl red; in the absence of a reducing agent there is no visible reaction between chlorophyllide and methyl red.
Experimental Ethyl chlorophyllide a (Chl) was prepared from the leaves of Datura stramonium and purified chromat~graphically.~ Methyl red (MR) was recrystallized from ethanol. The extinction coefficient was 3.19 X lo4 l./mole-cm. for the 494-mp band of the neutral form in ethanol. In the presence of ascorbic acid (Asc) or mercaptosuccinic acid (Aha) methyl red was entirely in the neutral form in ethanol; with hydrazobenzene (Hzb) it was necessary to add a small amount of benzoic acid to put the methyl red entirely in the neutral form, which, according to previous work, is the only reducible form.2 The reducing agents were recrystallized from ethanol or water, and their stock solutions were stored cold under nitrogen. Anthranilic acid and azobenzene were recrystallized, but p-carotene (Nutritional Biochemicals) and the other reagents were used as received. Ethanol was distilled from magnesium ethoxide. The contents of the reaction tubes were flushed with nitrogen, purified by bubbling through a solution of sodium benzophenone ketyl in anisole. The reaction was sensitized by light from a 750-w. projector lamp passed through a Baird-Atomic 6600-A. interference filter of band pass 90 A. This light is absorbed only by chlorophyllide; there is no reaction when only methyl red absorbs light. The rate of light einstein/l. absorption was generally around 4 X sec. Light intensity was measured with an Eppley thermopile, and the rate of fall of the methyl red band at 494 n ~ pwas followed with a Beckman DU spectrophotometer Quantum yields were determined as averages over intervals of usually 10 sec. until the methyl red was exhausted.
Results Carotene Retardation. We had previously found5 M that at an ascorbic acid concentration of 1.84 X M the and a p-carotene concentration of 3 X quantum yield for reduction of chlorophyllide was reduced to about one-fifth of the value obtained in the absence of carotene. The quantum yield for photoreduction of methyl red at the same ascorbic acid concentration is reduced only by half a t a carotene conThe Journal of P h y a k d Chemistry
centration of 3 X lop5 M. If the function of the carotene is to quench triplet state chlorophyllide by energy transfer,’ it is most unlikely that the primary photochemical reaction is with ascorbic acid, and the most obvious alternative, primary reaction with methyl red, is to be preferred. A similar conclusion has recently been reached by Evstigneev, Gavrilova, and Savkina on other grounds.s If carotene (Q) competes with inethyl red for photoexcited chlorophyllide (Chl*) and the spontaneous rate of decay of Chl* is comparatively small, the rate constant kl for the reaction between Chl* and methyl red can be calculated from the equation
k i / k ~=
@Q[&]/(@
-
@Q)[J~R]
(1)
in which @Q and CP are the quantum yields in the presence and absence of the quencher carotene. With the known9 value k~ = 1.3 X lo9, the third carotene run of Table I gave kl = 1.4 X lo9. According to this, methyl red ranks among the best quenchers of triplet state chlorophyllide. The conclusion that photoexcited chlorophyllide reacts first with methyl red is supported by the ascorbic acid concentration dependence of the quantum yieldefficient reduction of methyl red persists at ascorbic acid concentrations 0.01 times those required for efficient photoreduction of ~hlorophyllide.~ Ascorbic Acid. The variation of quantum yield during runs at different ascorbic acid levels is shown in Figure 1. At the lower ascorbic acid concentrations the yield decreases gradually with extent of reaction, but a t higher concentrations it increases almost to the end of the run. The drop-off in the yield in the neighborhood of low6M methyl red remaining is conl./mole sec. and a Chl* lifetime sistent with K1 = of about sec. The initial quantum yield creeps upward with ascorbic acid concentration (Figure 2 ) . Points below 2X M ascorbic acid were calculated from the two lowest curves of Figure 1, on the assumption that reaction stopped when ascorbic acid was exhausted and consumption of ascorbic acid was proportional to consumption of methyl red. Above [Asc] = lo-‘ M the initial quantum yield increases with [Asc]according to Qo =
0.055
+ 0.86[Asc]”*
(2)
(6) A. S. Holt and E. E. Jacobs, Am. J . Botany, 41, 710 (1954). (7) H. Claes, 2. Naturforach., 16b, 445 (1961). (8) V. B. Evstigneev, V. A. Gavrilova, and I. G. Savkina, Aiokhimiya, 151, 227 (1963). (9) E.Fujimori and R. Livingston, Nature, 180, 1036 (1957).
PHOTOREDUCTION OF METHYL REDSENSITIZED BY CHLOROPHYLLIDE u
823
-
Table I : Effect of Various Substances and Conditions on the Initial Quantum Yield for Sensitized Photoreduction of Methyl Red. Ethanol Solution a t 25" (light absorption rate 4 X 10-6 einstein/l. sec., except as noted. (Chl] 5 X 10-6 M)
-
Reducing agent and concn., 10-aM
[MRlo, 10-6M
Other conditions; concn. in mole/l.
Reducing agent and concn., [MRlo, IO-rM 10-M
10'L
Ascorbic acid
0
1.57 1.89 1.84 1.86 1.59 1.58 1.58 4.61 1.55 1.86 1.61 1.55 0.26 1.57 0
3.08 3.13 3.05 3.09 0.27 0.66 7.73 7.50 22.8 3.06 3.10 2.65 0.24 3.08 3.11
1.61 0.16 1.86 1.86 0.21 0.21 2.08
3.03 3.14 3.05 3.05 1.60 1.58 1.58
8-Carotene, 0.60 X @-carotene,1.17 X 8-Carotene, 3.55 X 10-6
... ... ... I, = 0.22 xJ0-6 I, = 1.8 x 10-6 I, = 7.5 x 10I, = 0.88 x 10-6 Pyridine, x p y = 0.108d Sodium ascorbate, 0.70 X 10-3 Water, 1.4y0 (vol.) Benzoic acid, 1.74 X 10-2 Malic acid, 1.13 X 10-2 MgC12'6H20, 7.4 X lo-' KI, 4.6 x 10-3 DMPD, 1.37 x 10-3 Dehydroascorbic acid, 1.3 X
9.2 8.1 7.8 4.4 4.9 6.0 11.5 11.3" 11.5b 5.OC 9.4 8.5 5.1 5.8
14.5
lOZ*
Dehydroascorbic acid, 1.9 x 10-3
0
Hydrasobenzene
0.36 0.47 0.37 0.36 0.35
1.73 1.61 1.61 1.59 1.52
0.36 0.37 0.36 0.36
1.59 1.69 1.71 1.67
. . .0 ..,
I. I.
0.27 X 10-6 1.6 X 10-6 Tetrabutylammonium hydroxide, 1.03 X 10-3 Anthranilic acid, 5.9 X 10-3 Azobenzene, 1.73 X lo-' DMPD, 1.37 x 10-3 Benzoic acid, 9.75 X 10-3 = =
10.7 11.4 7.3 7.4 0.2h 7.4 10.9 3.6 13.5
Cysteine
0.70 1.7O 9.4' 1.2 4.1 14.8 6.0 6.4
1.73
Other conditions; concn in mole/l.
3.30
...
0.26
Mercaptosuccinic acid
1.47 1.47 1.48 1.82 1.44 9.18
1.75 1.79 1.76 0.31 7.07 1.44
Cyclohexene, 0.136 Cyclohexene, 0.272 Cyclohexene, 0.264 Cyclohexene, 0.265 Cyclohexene, 0.27; reducing agent 10% oxidized/12
0.115 0.175 0.18 0.45i
0.22' 6.61
'For first interval; quantum yield rose to 0.15 then a Quantum yield rose to 0.18 and remained there for most of run. gradually declined through remainder of run. Quantum yield rose steadily during reaction until equal to yield a t higher light. inAt t.his mole fraction, methyl red is not entirely in the anionic form.2*6' Initially methyl red was not entirely in tensities. t,he anionic form, perhaps, because of an ascorbic acid impurity in the sodium ascorbate. The quantum yield quickly fell to zero from this value for the initial period as the solution became more basic, and methyl red was converted entirely to the anionic form. The inactivity of ascorbate is only apparent and arises from conversion of methyl red to the anionic form in its presence. All hydrazobenzene runs contain 3.9 X Yield rose to 0.125 by the middle of the run. M benzoic acid, except where Benzoic acid absent. Reaction apparently reversible and self-retarding. In 40 min. only 10% of t,he met.hy1 red had noted. been reduced (?), and on standing in the dark most of this was regenerated, judging by the absorption increase at 406 mp, There have been ot.her indications that the anionic form of methyl red is reducible but that the reduction is shallow and reversible. Methyl red was reduced during intervals of darkness but more slowly than during intervals of For second light interval. light. The dark reduction probably did not begin before some photoreduction had occurred.
'
At lower ascorbic acid concentrations the yield can be represented by 9 0
= 0.055[A~~]/(1.15 X
+ [ASCI)
(3)
Curves for eq. 3,0.86[Asc]"', and their sum are plotted in Figure 2. The effects of other reagents are summarized in and M the Table I. Between [R/LR]o = yield increases approximately as [MR]o"'. The yield is almost independent of light intensity over a 30-fold range. The accelerating effect of dehydroascorbic acid is surprising and may be the basis of the rise in
quantum yield with conversion at high ascorbic acid concentrations. Dimethyl-p-phenylenediamine (DMPD) does not affect the quantum yield when present together with ascorbic acid, nor does it support reduction of methyl red in the absence of ascorbic acid. Hydruzobenzene. In contrast with ascorbic acid, the quantum yield declines immediately and rather steeply with extent of reduction (Figures 3 and 4). The rate of decline is smaller a t higher hydrazoberizetie concentrations. Again the initial quantum y~cld creeps upward with increasing hydrazobetizeric and initial methyl red concentrations (Figures 4 and 5 ) . Volume 69. Number 9 March 1,966
G. R. SEELY
824
25 14
-
[Hzbl 8.6 x 10-3 x 10-3 4:7 10-4 o 1.6 x 10-4 3.8 x 10-6
0
A 4
s
* 3.6
V
10
x 10-4
20
2 -
0 1.0
0.5
0
IO5 x
[MR],
-
[MR]
1,
-
1.5 I
MOLAR
Figure 3. Variation of quantum yield for reduction of methyl red with extent of reduction and hydrazobenzene concentration. [MR]o 1.8 X V : contained DMPD, 1.37 X .&.
-
I
0
IO5 x
3
2
[ [MRIo -
M
[MR]
1,
MOLAR
Figure 1. Variation of quantum yield for reduction of methyl red with extent of reduction and ascorbic acid concentration. [?*lR]o 3 X M.
-
0
1
2 IO5
10-31
lo-6
I
1
10-5
I
10-3
I
10-2
10-1
ASCORBIC ACID CONC., M
Figure 2. Variation of initial quantum yield with ascorbic acid concentration. Curves plotted according to equations of text: - - ., eq. 2; ---, eq. 3 ; , sum of ( 2 ) and ( 3 ) ; -- - - -, eq. 2 7 ; 0 - - - 0 , unites points of the same run at low ascorbic acid concentration; A : contained pyridine, z P y= 0.108.
D l I P D reduces both yield and slope but leaves their ratio about the same (Figure 3). Azobenaene, anthranilic acid, and light intensity have no effect. The The Journal oj' Physical Chemistry
3 X
[ [MR], -
4
[MR]
5
1,
6
7
MOLAR
Figure 4. Variation of quantum yield for methyl red reduction with extent of reduction and initial methyl red concentration. [Hzb] = 4.7 X Curve through uppermost set of points is the hyperbola of eq. 4, with constants cited in text.
presence of 25 times the usual amount of benzoic acid increased the yield by 30y0 but did not affect the rate of decline of yield with conversion. The simplest curve fitting points for the run of Figure 4 made at highest [MRIois a hyperbola of the form B%A
+ CW + E9 = 1
(4)
PHOTOREDUCTION OF METHYL REDSENSITIZED BY CHLOROPHYLLIDE a
I
I
a somewhat more conventional form for the quantum yield gives @ =
~~
10-6
10-3
10-4
10-5
10-2
10-1
CONCENTRATION OF REDUCING AGENT, MOLAR
Figure 5. Variation of initial quantum yield with hydrazobenzene and mercaptosuccinic acid concentration. Curves plotted according to equations of text. For hydrszobenzene: - - - -, eq. 11; - - - - -, eq. 12; - - , eq. 13, -__ , eq. 27 with parameters cited in text; runs designated by 0 contained 3.9 X lO-‘M benzoic acid; a contained no benzoic acid; V contained 9.8 x 10-3 M benzoic acid. For mercaptosuccinic acid: , eq. 15; -, eq. 27 with parameters cited in text; 0 0 connects points of a run deficient in mercaptosuccinic acid; runs designated by o contained cyclohexene; A did not.
+ +E2 + 4C)/2C
(d@/dA)o = -B@o/(E
+ 2C90)
=
2.64/ [ M R ] ~ [ H z ~ ] ‘ ”
c = 0.98 x E
=
10-413
-0.071C
(10)
= 0.0355 (1
+ dl + 3.04 X 106[MR]o[Hzb]”’)
For small values of [Hzb] and [MR],, = 1.6 X (11) may be approximated by @o =
(5)
(6)
Over the range 0.4 to 5 X lo-* M for [Hzb] and 1.6 to 8 X M for [MR], the constants in (4) were related to each other and the concentrations by L3
[MR]o[Hzb]”’ - 1.85 X
+ 2.6 X
(11)
( A = [MRl0 - [AlR]), having asymptotes on the Aaxis and at an angle tan-’ ( - B / C ) to the A-axis, with B = 1.45 X lo6,C = 140, and E = - 13.7. In search of clues to the reason for the behavior of the quantum yield in the hydrazobenzene runs, hyperbolas of the same form have been assumed for other runs. Values of B , C, and E were calculated when sufficient data were available from the extrapolated values of Oo, (d@/dA)o,and the quantum yield a t a point near the end of the run with the aid of (4), (5), and (6).
(-E
2.64A
This is, no doubt, only an approximation to the true and probably rather complicated form, but it suggests that there are at least two retarding agents-one having a concentration proportional to A, the other a transient with concentration proportional to CP. The numerator in (10) would probably also contain a term in acid concentration had this been taken into account. Solving (10) for @a gives @O
---
CPo =
825
(7) (8) (9)
with an average deviation of about 20%. The greatest deviation occurred at the lowest hydrazobenzene concentration, where the effect of depletion of hydrazobenzene was felt. Inserting (7), (8), and (9) into (4) and rewriting it in
0.071
+ 0.87[Hzb]”’
M, (12)
Equations 11 and 12 are plotted through the points for hydrazobenzene in Figure 5 ; (12) represents the initial yields somewhat better at high hydrazobenzene concentration, where direct evaluation of B , C , and E was impossible. Equation 12 is remarkably like eq. 2 for ascorbic acid though it was arrived at in a much inore roundabout way. Because of the self-retardation it proved impossible to determine the form of the @ us. [Hzb] curve at very low [Hzb] from runs deficient in hydrazobenzene. From the two lowest initial quantum yields and with consideration of (11) or (12), the relationship =
0.071[Hzb]/(l.O X
+ [Hzb])
(13)
is estimated for this region. Mercaptosuccinic Acid. In these runs radicals produced by oxidation of mercaptosuccinic acid attacked chlorophyllide. The extent of the chlorophyllide loss was reduced but not entirely eliniinated by incorporation of olefins such as allyl alcohol, cyclohexene, cainphene, and anethole. All were effective, and purified cyclohexene was included in most reaction mixtures. Quantum yields were sometimes increased by inclusion of an olefin but by no more than a factor of 3/2. Quantum yields with inercaptosuccinic acid were much lower than those with ascorbic acid or hydrazobenzene. At low [Msa] the yield increases linearly as the reaction progresses; at high [Msa] the yield is initially high and decreases as the reaction progresses (Figure 6). Initial yields show a !inear increase with [Msa] M according to eq. 14. above Volume 69, Number 9
March 1965
G. R. SEELY
826
a,, = 0.0014
+ 0.48[Msa]
(14)
From the niercaptosuccinic acid-deficient run, as a very rough estimate at low [Msa]
a0 = 0.0014[Msa]/(2.5 X 10-6
+ [Msa])
(15)
Equation 15 is plotted in the lower part of Figure 5. A run in which the niercaptosuccinic acid had been partially oxidized (by 12, the H I being neutralized with PbCOa) showed a comparatively large quantum yield, and a dark reaction ensued. Cysteine reduced methyl red with about the same quantum yield as niercaptosuccinic acid ; the yield dropped slowly during the reaction.
io
I\\
0 2 . 6 x 10;: A 1.04 X 1 0
. e 1.48 x 1013
I 5 0 x 104
0
+3k
x
10-5
8
0
0.5
1.0
IO5 x [.[MRl0
1.5
-
[MR]
2.0
1,
MOLAR
Figure 6. Variation of quantum yield for reduction of methyl red with extent of reduction and mercaptosuccinic acid concentration. [MR]o 1.8 x 10-6 M ; [cyclohexene] = 0.27 M.
-
Discussion In two runs with ascorbic acid-one with hydrazobenzene and one with mercaptosuccinic acid in which insufficient reducing agent was present to reduce all the methyl red-the ratios of reducing agent initially present to methyl red consumed were 1.52, 1.82, 1.91, and 3.90, respectively. Even when the reducing agent is deficient, methyl red is apparently reduced beyond the hydrazo stage to products at the oxidation level of anthranilic acid and DMPD. No niethyl red is regenerated on exposure to air or benzoquinone, even when the reducing agent is exhausted. The variation of quantum yield during the runs, which depends SO much upon the nature of the reducing agent, shows that it is difficult to separate the early steps in the reduction from the follow-up steps that complete the four-electron The Journal of Physical Chemistry
reduction of methyl red. For example, the comparatively low quantum yield with mercaptosuccinic acid may be a consequence of the inability of its thiyl radical to act as a reducing agent under the conditions of the reaction. The products of the chemical reduction of methyl red have been identified as anthranilic acid and DMPD. Presumably, these are the products of the four-electron photochemical reduction also, but the evident complexity of the follow-up reactions and relative inertness of added anthranilic acid and DMPD give rise to the suspicion that other products may also be formed. In one run ([Asc] = 2.0 X lo-*, [RfR],, = 3.2 X changes in the near-ultraviolet were followed in hopes of detecting products of the reaction. The bands of methyl red were replaced by a band with E 12,500 l./niole-cm. at 320 mc(, 10,200 at 330, 8300 a t 340, and 5100 at 350. Anthranilic acid and DMPD both absorb in this region, but their combined extinction coefficient is only about half that observed. Unfortunately, attempts to prepare larger amounts of the reduced products failed because of self-retardation. l 3 The evident complexity of the reaction makes it impossible to account for the rate once reaction products have accumulated, but if attention is confined to initial rates it appears from Figures 2 and 5 or eq. 2, 12, arid 14 that reactions with the three reducing agents have one feature in common-an increase in the quantum yield with reducing agent concentration in the neighM . Although these increases could borhood of be the result of unrelated reactions between the reducing agents and later products of reaction, it is also likely that a coninion step, early in the reaction, is involved. The following mechanisni is proposed as being able to account in a general way for the variation of initial quantum yields with concentration. It is based on the assumption that the initial reaction between triplet excited chlorophyllide and methyl red produces a radical-ion pair. Chl
2 Chl' (singlet)
Chl' -% Chl* (a < 0.7)
(16) (17)
(10) K.Fukui, Y.Inamoto, H. Kitano, and C . Nagatu, J. Oru. Chem., 26, 1394 (19fil).
(11) S. Kubota, T.Akita, and T. Yokoshima, Yakugaku Zasshi, 78, 1194 (1958); Chem. Abstr., 53, 5162 (1959). (12) W.C. J. Ross and G. P. Warwick, J . Chem. Sac., 1724 (1956). (13) Attempts to generate Wtlrster's red from DMPD after reduction by exposure to air or quinone have never succeeded, but failure may be due to interference by some other product and need not imply absence of DMPD.
PHOTOREDUCTION OF METHYL REDSENSITIZED BY CHLOROPHYLLIDE a
I, is the rate of light absorption in einstein/l. sec. The fraction Q of singlet excited chlorophyll that crosses over to triplet is less than (1 - fluorescence yield) about 0.7. Chl* -% Chl Chl*
+ RlR -%
[Chi.+
+ AIR.-] kr_ Chl + MR [Chl-+ + MR.-] -% Ch1.f + MR.Chi.+ + MR.- kr_ Chl + MR [Chi.+
(19)
(20) (21) (22)
At low concentration, a reducing agent RH competes with M R . - for Chi.+.
+ R H -% Chl + Re + H +
(23)
The reduction is completed by any of a variety of possible reactions of radicals with each other or with the reducing agent. The above sequence of reactions leads to a quantum yield expression of the following form, in which y is the probability that (23) will ultimately lead to reduction of a molecule of methyl red.
The interpretation of K A , which is 1.15 X for ascorbic acid, 1.0 X for hydrazobenzene, and perhaps 2.5 X for niercaptosuccinic acid, depends on assumptions about the rates of the various radical disproportionation reactions and is too involved to warrant further discussion without more information. Equation 24 is applicable up to reducing agent conM . The continued increase centrations around of quantum yield at higher RH concentration needs to be accounted for. I t is possible that the reducing agent reacts with MR - or rather I I R H . directly.
-
MRH.
+ RH +MRHZ + R .
+ MRe-1 + R H + [Chl + MR.- + R. + H + ] k6
[Chl*+
Recent meas~rementsl~ suggest a value of about 1.5 X lo3 set.--' for k d in ethanol; from carotene retardation, kl = 1.4 X logl./mole sec. The pair of ions may or may not separate, but, in the absence of a reducing agent, there is no net reaction. l6
Chi.+
be predicted if (25) competes with radical recombination reactions. If the product of (19) is an ion pair with a long enough life, reaction with reducing agent may compete with (20) and (21) as well as with (22).
(18)
+ MR.-]
827
(25)
Against this is the lack of any distinct increase of quantum yield with decreased light intensity, as would
(26)
If the probability of net reduction of methyl red after (26) is y’, the quantum yield at high reducing agent concentration can be approximated by
a = - [ ark3 kz
+
k3
+ (~’ke/ykd[RHl ] 1 + [ k ~ / ( k+ z $)][RH] 1
(27)
Curves of (27) have been fitted to the experimental points of Figures 2 and 5. Taking a! = 0.7 the curves have the parameters y’ = 0.32, 0.20, and 0.20, and k6/(kZ k3) = 780, 2900, and 360 for ascorbic acid, hydrazobenzene, and mercaptosuccinic acid. l6 The curves, plotted in Figures 2 and 5, fit the data fairly well for hydrazobenzene and mercaptosuccinic acid, but data for ascorbic acid are better represented by (2). Equation 27 is versatile enough to account for power law expressions like (2), (12), and (14). In passing, it may be noted that, for the yield to be greater a t high reducing agent concentration, it is not necessary that y’ be greater than y, but only that it be greater than r k d ( k 2 kP). The data of references 2 and 3 were obtained a t high reducing agent concentrations, where the sequence of reactions 16, 17, 19, and 26 dominates, and the meaning of the constants in their rate expressions should be reinterpreted in that light.
+
+
Acknowledgments. The assistance of Messrs. J. Miller, A. Folkmanis, and D. Stoltz is appreciated. (14) R. Livingston and P. J. McCartin, J. Phys. Chem., 67, 2511 (1963). (15) In the absence of reducing agent, methyl red does, however, suppress a very slow degradation of chlorophyllide by ethanol or an impurity in the ethanol, which presumably goes via the triplet state of chlorophyllide [R. Livingston and D. Stockman, ibid., 66, 2533 (1962) 1. (16) The bracketted expression of (27) is a family of curves of the form (1 pz)/(l z), where z = ks[RH]/(kz k3) and p = y’(kz k a ) / y k 3 . If the curves are plotted logarithmically for several values of the parameter p and comparison is made with experimental quantum yield curves, ordinate and abscissa give values of a y k s / ( k t ks) and ks/(kz k3). Values of a y a l ( k z k3) are the first term on the right in (2), (12), and (14); combining with the values of p for the curve which seems best to fit the experimental points gives values for y ’ , but y by itself cannot be obtained.
+
+
+
+
+
+
+
Volume 69. Number 8 March 1966
