J. Phys. Chem. 1984,88, 4931-4937
4931
The Photochemistry of 9,1O-Anthraquinone-2-sulfonate in Solution. 2. Effects of Inorganic Anions: Quenching vs. Radical Formation at Moderate and High Anion Concentrations I. Loeff, A. Treinin,* Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel
and Henry Linschitz* Department of Chemistry, Brandeis University, Waltham, Massachusetts 021 54 (Received: February 28, 1984)
The chemical aspects of the interactions between excited 9,lO-anthraquinone-2-sulfonate (AQS) and various inorganic anions are examined. The anions which quench triplet AQS can be divided into two groups: Cl-, Br-, I- and NCS- (group I) photoreduce AQS to AQS- radical anion only at concentrations higher than that required for complete triplet quenching. The effect increases with concentration and passes through a maximum, with highest quantum yields of radical formation reaching 1 for C1- and NCS-; on the other hand, NOz-, SO3*-,and N; (group 11) give AQS- in parallel to triplet quenching. The nature of the high-concentration effect shown by group I is analyzed. Some results obtained with mixtures of anions support the conclusion that triplet AQS is also responsible for this effect, and it is suggested that triple exciplexes of the type 3(AQS-.X;) are involved. With this view and the recently proposed intraradical spin-orbit-coupling (IRSOC) model, a quantitative interpretation of the results is presented. N
In part 1 of this study, we described transient intermediates in the photohydroxylation reaction of 9,lO-anthraquinone-2sulfonate (AQS) in aqueous media.' It was shown that the AQS triplet reacted with water to give two transients (probably hydrated forms of AQS) denoted by B and C, with characteristic absorptions around 480 and 600 nm, respectively. Transient C reacts with ground-state AQS to give photoproducts while B reverts back to starting material. The effects of simple inorganic anions on the reaction were also reported, specifically with regard to their influence on the decay of the several intermediates involved and on the final yield of AQS hydroxylation. To follow these effects, it was sufficient to work at anion concentrations lower than -0.1 M. With the halide ions and NCS-, triplet AQS is the only intermediate whose decay rate was found to be enhanced. In all cases the quenching rate increases, up to a limiting value controlled by diffusion, with decrease in the ionization potential of the anion in solution. Thus, the charge-transfer nature of the primary interaction between triplet AQS and the anions was established.' It was clearly shown that in the case of OH- the triplet is quenched with no free OH radicals produced; Le., the charge-transfer interaction does not lead to net electron transfer. In part 2, we now examine in detail the chemical aspects of the anion-triplet interactions, extending our observations up to highly concentrated salt solutions. Some early studies of this subject were published: but their reliability is questionable because they were based on erroneous identification of triplet AQS or AQDS (9,10-anthraquinone-2,6-disulfonate).' Moreover, no systematic study of the effect of anion concentration was conducted and only in one case (C03,-) was the effect of 0, utilized to separate the semiquinone absorption from that of the inorganic radical. A recent report3 on the photoreduction of 9,lO-anthraquinone-2-methylsulfonateby C1- and Br- gave results limited mainly to acidic solutions and to transient absorptions measured long after the flash (600 p s ) . The main goals of the present work are as follows: (a) The first is to examine the factors which control formation of separated radicals from charge-transfer interactions of inorganic anions with excited molecules. The theoretical features of this subject were discussed elsewhere," with some reference to the results described (1) Loeff, I.; Treinin, A.; Linschitz, H. J . Phys. Chem. 1983, 87, 2536. (2) (a) Kuzmin, V. A.; Chibisov, A. K.; Karyakin, A. V. In!. J . Chem. K i m . 1972, 4 , 639. (b) Kuzmin, V. A.; Chibisov, A. K. Dokl. Akad. Nauk SSSR 1973, 212, 1146. (3) Metcalfe, J. J. Chem. SOC.,Faraday Trans. 1 1983, 79, 1721.
0022-3654/84/2088-493 1$01.50/0
here. A simple model (the IRSOC model) was outlined which emphasizes the role of spin-orbit coupling in an incipient product radical in controlling the radical yield. In a large number of cases, only this parameter and neither "heavy-atom" effects nor hyperfine interactions were found to correlate with efficient quenching of the triplet directly to the ground state.4 The IRSOC model is somewhat modified in the present work and further evidence for it is presented. (b) The second goal is to examine the mechanism by which anthraquinones act as catalysts in the aerobic photooxidation of halides to free halogens. The case of C1- has attracted special attention for its possible application to energy storages,6 and has been studied by steady photochemical methods. Although the evolution of Clz m u r s in highly acidic it is important to first understand the elementary interactions involved in neutral solution,
Experimental Section The experimental procedures were essentially the same as described in part 1. Flash excitations were carried out with either a nitrogen or frequency-doubled ruby laser, at 337 or 347 nm, respectively. The "initial transient absorptions" were derived by extrapolation to zero time (the beginning of pulse) from the digitized and averaged kinetic traces. For 0,-free solutions the extrapolation was carried out from times longer than -200 ns, so that any contribution of a short-lived triplet absorption was eliminated. For 0, (1 atm)-saturated solutions the traces were extrapolated from times longer than -5 ps, where the contribution of semianthraquinone was practically eliminated by Oz. In all cases the absorbance values were corrected for variations in the incident light intensity of the laser. No difficulties were encountered with instability of concentrated KI solutions. Actinometry. The R ~ ( b p y ) , ~ + / F esystem ~ + was used for actinometry at 3371 A: the photochemical reduction of Fe3+ by excited R ~ ( b p y ) , ~in+ acidic solution has a quantum yield of 1.0 f 0.1 for full quenching.8 The actinometric solution contained M Ru(bpy),,+, lo-, M Fe(C104)3, and 0.1 M ca. 3.5 X (4) Treinin, A.; Loeff, I.; Hurley, J. K.; Linschitz, H. Chem. Phys. Lett. 1981 95 3 3 3
( 5 ) Eckert, A. Ber. Dtsch. Chem. Ges. 1925, 58, 313; 1927, 60, 1691. (6) Scharf, H. D.; Weitz, R. Tetrahedron 1979, 35, 2255; In "Catalysis in Chemistry and Biochemistry, Theory and Experiment"; Pullman, B., Ed.; Reidel Publishing Co.: Dordrecht, The Netherlands, 1979; p 355. (7) Our unpublished results. (8) Rosenfeld-Griinwald, T.; Rabani, J. J . Phys. Chem. 1980,84,2981 and references cited therein.
0 1984 American Chemical Society
4932
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 400
430
5c0
,
I
Loeff et al.
503
? -
0.lZt
J
fi
CI-
r
0.041
t L
_tc
_-
i
10.10
4, J
L
X, nm Figure 1. Spectra of transients produced by laser photolysis of AQS in presence of halides and NCS- a t high concentrations, in oxygen-free ( 0 ) and oxygen-saturated (1 atm, A) aqueous solutions. Curves 3: difference spectra DN2- Do,. Curves 2: normalized spectra of X1-(ref 10; normalized relative to the absorbance a t X as close as possible to Amax). C1-: AQS, 0.53 M NaCl, 10 ps after pulse. Br-: 2 X M AQS, 2 M KBr, 5 ps after pulse. I-: 2 X lo4 M AQS, 2 M KI, 5 ps after pulse. M AQS, 2 M KNCS, 5 ps after pulse. The complex NCS-: 5 X structure with NCS- has been confirmed on both N, and ruby laser flash instruments.
HC104. The depletion of R ~ ( b p y ) ~at~452 + nm was measured 50 ns after pulse (when the kinetic trace reaches a plateau), taking for the extinction coefficient of Ru(bpy);+ t = 1.43 X lo4 M-' cm-'. The overall conversion was kept below 10% by using gray filters. Corrections were introduced for the absorption of the gray filter and of the Fe3+component in the actinometric solution, and the absorbance of Ru(bpy),*+ was taken as the average between its values before and after the pulse. With this actinometer, the quantum yield of semianthraquinone produced from AQS/2MClwas determined (see Results) by using the value,,E = 8200 M-' cm-' for AQS- radical at 500 nmS9 This much simpler system was then used for routine actinometry to measure quantum yields in other systems and to correct for variations in incident light intensity. Of all the anions examined only NO2- has some absorption at the photolyzing wavelength. In this case the concentration ratio [AQS]/[NO,-] was adjusted so that most of the light (395%) was absorbed by AQS. N o extra buffers were added to any of the solutions.
Results The transient absorptions produced by laser photolysis of AQS in presence of various anions at various concentrations were measured in oxygen-free and O2(1 atm)-saturated solutions. The anions examined could be divided into three groups according to their effectiveness in quenching the triplets, photoreduction of AQS, and the dependence of these phenomena on anion concentration. Only groups I and I1 are highly effective triplet quenchers. A . Group I: Halides and NCS-. Two distinct concentration regions could be discerned: (a) the moderate-concentration region ([X-] 10-3-5 X M), where the transient absorptions observed are essentially the same as produced with AQS alone (B and C)l but their i:itensities decrease with increasing [X-1; (b) the high-concentration region (above -0.1 M) where new absorptions are produced which increase with anion Concentration. In the intermediate zone (-5 X 10-2-0.1 M) the transient absorptions are minimal and appear to be concentration-dependent mixtures of the absorptions displayed in the moderate- and high-concentration regions.
-
(9) Hulme, B. E.; Land, E. J.; Philips, G. 0. J . Chem. SOC.,Faraday Trans. I 1972, 68, 199.
01
IO
IO
[ x - l , M (logarithmic scale) Figure 2. The dependence of initial transient absorption a t constant wavelength on anion concentration, in oxygen-free (continuous curves) and oxygen-saturated (1 atm, dashed curves) solutions. C1-: 2 X lo4 M AQS with NaCl (A) and 5 X M AQS with LiCl (A), 490 nm. NCS-: 5 X M AQS with KNCS, 400 nm. Br-: 2 X M AQS with KBr ( 0 )and 5 X low5M AQS with LiBr (M), 490 nm. I-: 2 X lo4 M AQS with KI, 490 nm.
Figure 1 shows the transient spectra produced in the highconcentration region, measured 5-10 ps after pulse, in presence and absence of 02. The effect of O2is very pronounced, and in the case of the halides most of the absorption around 500 nm is removed. Figure 1 also records the difference spectra DN2-Do, for the various anions: with some discrepancies they are similar and resemble the well-known spectrum of semianthraq~inone~ (AQS-) with peaks near 400 and 500 nm. To support this identification, the kinetics of the oxygen effect were determined: M AQS, M 02,and various solutions containing 3 X concentrations of NaCl (in the range 0.4-1.2 M) were pulsed, and the decay rate at 490 nm was measured. It was found to be first order with rate constant, k = (4.3 f 0.2) X lo5 s-l, independent of chloride concentration. From this value, the secondorder rate constant of the reaction with O2was derived, k = (4.3 f 0.2) X lo8 M-'s-l , in good agreement with that reported for AQS-, k(AQS-+02) = 4.6 X lo8 M-' s-'.~,~' The residual absorptions left in the presence of O2at t 3 5 ps (Le. at times sufficient for effective removal of the AQS- absorption) closely resemble those of the inorganic radicals X2- as reported in the literature" (reproduced in Figure 1). Further evidence was obtained in the case of C12- by measuring the effect of N3- on its decay. For this purpose, solutions containing 2 X M 02,and various concentrations M AQS, 1 M NaC1, of N3-were pulsed and the decay rate a t 380 nm was measured. M, so that it could (The azide concentration was kept below not interact directly with triplet AQS). N3- was found to enhance the decay, and from the pseudo-first-order plots a rate constant, 1.9 x 109 M-' s-1 , was derived for the reaction of C l c with N3-, in fair agreement with the reported value, 1.2 X lo9 M-' s-l.12 The effect of X- concentration on the initial transient absorption (extrapolated as described above) at fixed wavelength, in absence and presence of oxygen, is shown in Figure 2. The decrease in absorbance in the region of moderate [X-] corresponds to triplet quenching by X- and resulting inhibition of formation of transient B. The increase in absorbance at high [X-] corresponds to formation of AQS- and X2- radicals. From these data, the initial absorbance of AQS- (DN, - Do,) as function of [X-] was obtained (Figure 3). With the exception of NCS-, the fixed wavelength chosen was 490 nm where AQS- has its first absorption peak. In (10) However, higher values up to 2 X IO9 M-' sK1 w ere also obtained for this reaction in other AQS/X- systems. The reason is not clear. (11) Hug, G . L.Natl. Stand. Ref. Data Ser. (US., Natl. Bur. Stand.) 1981, NSRDS-NBS 69. (12) Ross, A. B.; Neta, P. Natl. Stand. Ref. Data Ser. (US., Natl. Bur. Stand.) 1979, NSRDS-NBS 65.
The Journal of Physical Chemistry, Vola88. No. 21, 1984 4933
Photochemistry of 9,1O-Anthraquinone-2-sulfonate
1
0.4
, I
C I -, 490 n m
a ' \A
I i
[x-1, M Figure 3. The dependence of initial absorbance of semianthraquinone on
[x-J,Io-~M Figure 4. Stern-Volmer plots for the quenching of initial transient (B) absorption at 490 nm by halides and NCS- in the moderate-concentration region (2 X M AQS, oxygen-free solutions): CI- (A),Br- (a), I(0),NCS- (A).
anion concentration. the case of NCS-, the measurements were made at 400 nm, close to the second AQS- peak, in order to reduce the contribution of (NCS), to the total absorption. To make sure that even at highly concentrated salt solutions these data reflect appropriately specific interactions of excited AQS (or any subsequent excited species, e.g. exciplexes) with the quenching anion and not some other effects, the following experiments were conducted: (a) None of the added salts (up to 2 M) were found to change the ground-state absorption spectrum of AQS appreciably. (b) By use of a series of gray filters to attenuate the laser beam intensity, the transient absorptions produced from the AQS/Cland AQS/Br- systems were shown to be proportional to light intensity. This implies that no complications arise due to ground-state depletion or inner filter effects involving absorption of photolytic light by the intermediates produced. (c) N o specific effect of cation on transient absorption was observed. For example, within limit of error the same initial optical density was obtained on using NaCl, KCl, or CsCl, all at 0.6 M. (d) To exclude the possibility that environmental effects (solvation or gegenions around AQS) play herea major role, we confirmed that addition of up to 4 M NaC104 to 5 X M AQS 0.5 M NaCl hardly affects the transient absorption. In general, the dependence of semiquinone yield, 4AQs-, on anion concentration shows the common features of increasing $AQs- with [X-1, leading to a broad maximum at some high salt concentration followed by a decline (Figure 3). However, the several anions vary considerably in both the magnitude of ,,$ , and the concentration at which it is attained. 'With C1-, &,,, as determined by the Ru(bpy)?+ actinometer, is 1 .O f 0.1 and is reached slightly above 1 M NaCl. The yield then remains constant up to about 2 M salt and decreases at higher concentrations. With NCS-, ,,$ , 1 .O is also reached, but at higher concentration (4-6 M). In the case of Br-, &,, 0.2, and the decline begins at 3 M. For I-, #IAQS- levels off around 0.15 and no final decrease is seen. The extrapolated absorptions remaining in 02-saturated sblutions (Figure 2) appear to increase parallel to 4AQS- up to the region of b,,, and increase further as $AQS- declines. The behavior of the system AQS/Cl- made it a convenient actinometer for determining $AQs- in all other systems: $AQs- = DAQS-/DAQS(2 M Cl-), where D A Q g is the initial absorbance of AQS- in the given solution and D,(2 M Cl-) is the absbrbance produced under the same conditions by replacing the given salt by 2 M NaC1. From the initial absorptions of AQS- (at 500 nm) and X, (at X as close as possible to their peaks) together with their extinction coefficients at given X as recorded in the literature (for X2- see ref 1 l), the ratio +X2-/4AQS- could be determined. The ratios 1.1, 1.1, 1.2 (all *lo%), and -1.4 for C12-, Br2-, (NCS),-, and 12-, respectively, were obtained each as an average of four measurements. The value for I, is uncertain because it was derived from low absorptions and their small difference (see e.g. Figure 2). If one ignores 12-, the ratios obtained are close to the expected 1:l ratio. (Error in extinction coefficients may explain the systematic deviations.)
+
-
-
--
-.
I
NCS-
cl-
7
I / [ x - l , M-' Figure 5. Stern-Volmer plbts for the effect of halides and NCS- on the primary quantum yield of AQS- in the high-concentration region. Data taken from Figure 3 together with some data obtained by replacing KBr (K) by NaBr (0).
TABLE I: Rate Constants (10-9k,, M-' s-') for the Quenching of Triplet A Q S CLmparison of Different Methods
k,
XCl-
from inhibition of photohydroxylation"
k, = Ksv/r< 1.1
0.5
0.5
N3NO*-
3.i 3.2
3.8 3.2d
Br-
3.8
3.4
NCS-
3.9 4.2
3.9
1-
from suppression of transient (B) yield,
from kinetics of triplet decay"
4.6
k x'/ k>'b 7.1 (6.2)c
-8.5' 5.6 5.1 5.8
2.6 ( 7 . 6 ) 9.8 (8.4)
" Referenceid1. From the mixed anions experiments (see text). Numbers in parentheses are from direct measurements (column 2). 'Up to -0.15 M N3-. dPresent work. It was also verified that the Stern-Volmer codstant does not depend on AQS concentration. eCompared with the other values in this column, the error here may be larger because NOT also shortens the lifetime of B. /Present work. Quantitative Correlations. (a) Moderate Concentrations. In part 1 we have shdwn that anions inhibit the formation of transient B by competitive quenching of the AQS triplet.' Figure 4 shows good Stern-Volmer correlitions of the yield of transient B against [X-] for group I anions. Table I (column 4) records the values , K , is the Stern-Volmer constant (the slope of of K s v / r T where the line) and T~ = 91 ns is the lifetime of the triplet in water. ( b ) High Concentrations. Figure 5 shows the dependence of &QS-;' on [X-]-l in the concentration range below that corresponding to &,. B. Group II: N3-, NOT, and S032-,Moderate Concentrations. For N3- and NOz-, photoreduction clearly accompanies triplet quenching and reaches its maximum when most of the triplets are quenched, with 4AQS1 for NO2- and -0.25 for N3-. The
-
4934
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984
o-
....*
Loeff et al.
20nsec e-.
,
1
1
1
I .5
3.0
O2 (t-0,ex‘rapalated 1 tN3-1, M @a/~;-i.4-i-4_o
450 X,nm
I
550
Figure 6. The transient spectra produced by laser photolysis of 6 X lo4 M AQS in presence of 6 X lo-, M NaN02, oxygen-free solutions at
various times after pulse. Under these conditions, NOT absorbs less than 5% of the laser light and quenches -65% of triplet AQS. The lowest curve is the spectrum in presence of 1 atm of O2 extrapolated to zero
0.02 m c 0
?
2 0.cI U
time. 400
500
X , nm
Figure 8. The primary effects of Nj- at high concentrations on the photolysis of AQS in oxygen-free solution (curves 1) and oxygen-saturated (1 atm, curves 2) solutions, with curves 3 being difference ab- Do2: (a) transient spectra ( M AQS, 2 M NaN,, 5 sorptions ps after pulse); (b) initial transient absorption at 490 nm against azide concentration (2 X M AQS); (c) Stern-Volmer plot for the effect of N3- on the quantum yield of AQS- (2 X IO4 M AQS).
X , nm
Figure 7. Spectra of transient produced by laser photolysis of 2 X M AQS in presence of 6 X M Na2S03,5 ps after pulse, in oxygen-free (curve 1 ) and in oxygen-saturated (1 atm, curve 2) solutions. Curve 3 is the difference spectrum DN2- Do2.
formation of AQS- in the primary reaction with NO2- is unambiguously shown in Figure 6 in which the initial triplet absorption near 450 nm is gradually replaced by the characteristic doublepeaked spectrum of the AQS- radical. The effect of NO2-on transient B was studied in 02-saturated solutions. NO2-shortens the lifetime of B, and a value, k = 6.8 X lo6 M-’ s-l, was derived for the reaction of NO2- with B. The initial yields of B, obtained from these measurements by extrapolation to zero time, were found also to follow a Stern-Volmer correlation (like that observed with group I) with K,/rT = 8.5
x 109 M-1
s-1.
High yields of AQS- were also obtained with S032-(Figure 7), but two experimental difficulties hindered the quantitative treatment of these results. First, like other basic anions (more so than NO2- or N3-),S032-shortens the lifetime of transient B until it approaches that of the triplet. Therefore, the rate constant for triplet quenching by SO?- could not be determined.I3 Second, the semiquinone absorption could not be completely removed by O2 (1 atm) as in other cases, in particular at [S032-]> M.I4 Therefore, our method of determining c $ ~(see ~ Experimental ~ (13) An attempt to determine k, by studying the inhibition of photohydroxylation’ by SOa2-also failed: contrary to other anions examined (Brand NO,-), the Stern-Volmer constant K,, appears to depend on AQS concentration, decreasing with increasing [AQS]. This suggests a competition between S032-and AQS on an intermediate involved in the photohydroxylation (not on the triplet whose decay rate is not affected by [AQS]’). This point, which is still under investigation, is not relevant to the present problem since below pH 12, the quantum yield of photohydroxylation is rather low (+,, 0.07; Clark, K. P.; Stonehill, H. I. J. Chem. SOC.,Faraday Trans. I 1972, 68, 577), and therefore a reaction which involves such an intermediate cannot account for the high yield of AQS-. (14),We believe that O2is consumed by the autoxidation of SO3*-which is a chain reaction. As in other cases (see e.g.: Hayon, E.; Treinin, A,; Wilf, J. J . Am. Chem. SOC.1972, 94, 47), the chain is probably initiated by the radical SO3-which is here produced by the photoreduction of triplet AQS by SO?-. Owing to the dependence of chain length on SO?- concentration, the consumption should increase with [SO3*-].
- -
Section) could not be employed. Nevertheless, there is no doubt ~ high and remains so also at high SO3” concentrations that c $ ~ is (studied up to 1 M). High Concentrations. This region could not be studied with NO2- because it absorbs some of the laser light (see Experimental Section), and [AQS] could not be further increased because of its limited solubility in the salt solutions. Therefore, the highconcentration region was studied in detail only with N3- (Figure 8). The typical spectrum of AQS- is clearly observed, and O2 eliminates most of the absorption from 360 to -450 nm (Figure 8a). (The nature of the residual absorption is not clear.) The anion concentration effect here is quite different from that of group I: the semiquinone quantum yield decreases with N3- concentration (Figure 8b) and follows a Stern-Volmer correlation (Figure 8c). C. Group IIZ C104-and SO:-. No influence on triplet decay and only little production of AQS- could be detected up to 1 M. However, above -0.1 M there is a gradual decrease in the subsequent overall transient absorption which results from the interaction of triplet AQS with water.’ The shape of the transient bands is also somewhat modified. These changes, while interesting, evidently do not involve direct interaction of triplet AQS with the anions. Borax can also be grouped here although like other basic anions, it markedly enhances the decay rate of transient B.’ Up to 6 X 10” M no effect on triplet decay and no semiquinone was detected. On the other hand, when 2 M NaCl was added to 6 X M borax solution, the yield of semiquinone was the same as in absence of borax. This observation demonstrates that in this particular case the enhanced decay of B does not lead to semiquinone and that the interaction between AQS and C1- is not affected by changing pH from -6 to 9.2. Mixtures of Anions. Solutions containing fixed concentrations of AQS and C1- (0.53 M) and various concentrations of an additional anion were laser photolyzed. Under the conditions employed, practically all the triplets were interacting with the anions. Compared with the added anions (Br-, I-, and N3-), C1- is most effective in reducing AQS (Figure 3) and least effective in quenching the triplet (Table I). Therefore, increasing the concentration of the added anion from zero to nearly that of C1markedly decreased the yield of AQS- (Figure 9). The added anion thus acted as an inhibitor of AQS reduction by C1-. This inhibition offers a means to determine the nature of the excited species responsible for the redox reaction (see Discussion).
The Journal of Physical Chemistry, Vol. 88, No. 21, 1984 4935
Photochemistry of 9,10-Anthraquinone-2-sulfonate
as function of X- concentration, is expected to obey eq l', which can be arranged as a Stern-Volmer correlation:
D o / D = 1 + (kT/kd)[X-]
0.2r-
[ K Br:,
M
Figure 9. Initial transient absorptions at 490 nm in mixtures of 0.53 M NaCl with another anion X- (curves 1) and X- alone (curves 2), with the dashed curves being difference absorptions, (1) - (2). For the plots of (Der - D)/(D- Dx-) against [X-1, see explanation in the text.
Discussion The discussion is confined to those anions which strongly quench triplet AQS (groups I and 11). A. Moderate Concentrations. Considering their quenching rate constants (Table I) and the rate of decay of triplet AQS in water,' kd = 1.1 X lo7 s-l, it is clear that effective quenching by both groups occurs in the moderate-concentration region and is almost complete (-90%) around 0.2 M for C1- and -0.03 M for the other anions.15 This charge-transfer quenching proceeds in parallel to reactions of the triplet with water to produce transients B and C.I Thus, the following set of reactions is considered to occur at pH 511:1*4
3AQS*
k0
+ H20
+
kC
B
(la)
C
(1b)
(kd = kB kc = 1.1 lo7 s-l, with kc
