J. Phys. Chem. 1994,98, 4854-4860
a54
Photoinduced Electron Transfer between Sulfur-Containing Carboxylic Acids and the 4-Carboxybenzophenone Triplet State in Aqueous Solution Bronislaw Marciniak,+v*Krzysztof Bobrowski,+*$ Gordon L. Hug,*v+and Jaroslaw RozwadowskF Radiation Laboratory, University of Notre Dame, Indiana 46556, and Faculty of Chemistry, A. Mickiewicz University, Poznan, Poland Received: February 3, 1994'
The mechanism of photoinduced electron transfer between sulfur-containing carboxylic acids and the 4-carboxybenzophenone (CB) triplet state in aqueous solution was investigated using laser flash photolysis and steady-state photolysis techniques. Bimolecular rate constants for quenching of the CB triplet state by six sulfur-containing acids, with varying numbers of COO- groups and varying locations with respect to the sulfur atom, were found to be in the range (0.3-2.1) X lo9 M-l s-l depending on the charge of the acid molecule. The observation of ketyl radical anions and intermolecular (S:.S)-bonded radical cations of some of the acids was direct evidence for the participation of electron transfer in the mechanismof quenching. An additional absorption band a t approximately 410 nm in the transient absorption spectra for some of the acids was assigned to intramolecularly (S:.O)-bonded species (for acids for which a five-member ring structure was sterically favorable). Quantum yields of formation of intermediates from flash photolysis experiments and quantum yields of C 0 2 formation and CB disappearance from the steady-state measurements were determined. The values of these quantum yields clearly indicated that the diffusion apart (escape of the radical ions) of the charge-transfer complex, formed as a primary photochemical step, is the main photochemical pathway (contribution of -90%). Competing processes of proton transfer and back electron transfer within the C T complex gave only minor contributions to these yields. A detailed mechanism of the CB-sensitized photooxidation of sulfur-containing carboxylic acids is proposed, discussed, and compared with that for sulfur-containing amino acids in aqueous solution.
Introduction The biological importanceof electron-transferreactions induced by the excited states of aromatic ketones,' with the participation of the sulfide function in sulfur-containing peptides and proteins,2 requires a detailed understanding of the mechanism of photoinduced electron transfer in less complicated systems. sensitized by aromatic ketones, photooxidation of sulfur-containingorganic compounds, e.g., thioethers and amino acids, has been a subject of several studies.3-10 Recently there have been detailed mechanistic investigationsof the 4-carboxybenzophenone (CB) sensitized photooxidation of sulfur-containing amino acids in aqueous solutions.8JO Electron transfer from the sulfur atom to the triplet stateof the ketone was determined to be a primary photochemical step. This was established by the direct observation of the free radical ions, Le., ketyl radical anions and sulfur-centered radical cations, and by the large values of quenching rate constants (k,'s in the range of 108-109 M-I s-l). Fast protonation of the ketyl radical anions" and intramolecular proton transfer within the initially formed radical ion pair are responsible for the observed rapid formation of ketyl radicals in neutral aqueous solution. An observed slow formation of ketyl radicals in neutral solution was shown to be due to the one-electron reduction of the CB ground state by the a-aminoalkyl radicals produced by decarboxylation of the sulfur-centered radical cations. In alkaline solution, the presence of a second electron-donatinggroup (-NH2) led to much more complex secondary reactions that depend markedly on the structure of the amino acids used.I0 Since the mechanism of secondary photoreactions for the sulfurcontaining amino acids in aqueous solutions was shown to be very complex, due to the involvement of the a-aminoalkyl radicals, we University of Notre Dame. t A. Mickiewicz University. t
8 On leave from the Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Warsaw, Poland. Present address: Institute of Nuclear Chemistry and Technology, Warsaw, Poland. Abstract published in Advance ACS Absrmcrs, April 1, 1994.
0022-3654/94/2098-4854S04.50/0
have decided to study some sulfur-containing carboxylic acids, containing no amino groups. The lack of an amino group in these reactants was expected to simplify the mechanism of primary and secondary photoreactionscompared to the analogoussystems containing amino acids. In this paper the results of nanosecond flash photolysis and steady-state photolysis studies of the CBsensitized photooxidation of various sulfur-containingcarboxylic acids, with different locations of the COOH groups with respect to the sulfur atom, in aqueous solution are presented.
Experimental Details Materials. 4-Carboxybenzophenone(Aldrich) and the sulfurcontaining acids obtained from Aldrich [2-(methy1thio)ethanoic acid (2-MTEA) and 2-((carboxymethy1)thio)succinic acid (2CMTSA)], Sigma [2,2'-thiodiethanoic acid (2,2'-TDEA) and 3,3'-thiodipropionic acid (3,3'-TDPA)], Lancaster [3-(methylthio)propionicacid (3-MTPA)], and Fluka [ 3-((carboxymethyl)thio)propionic acid (3-CMTPA)] as the best available grades were used without further purification (except 3-((carboxymethy1)thio)propionic acid that was recrystallized twice from water). Water was purified by a Millipore Milli-Q system. Laser Flash Photolysis Experiments. The nanosecond laser flash photolysis has been described in detail elsewhere.lZ Laser excitation at 308 nm from a Lambda-Physik EMG 101 MSC (Xe/HCl) excimer laser (operated at 4-6 mJ, pulse width 20 ns) was used in a right-angle geometry of excitation. Rectangular quartz cells (0.5 X 1.0 cm) with a path length of 0.5 cm for the monitoring beam were used. The transient absorbances at preselected wavelengths were monitored by a detection system consisting of a double monochromator, a photomultiplier tube, and a pulsed xenon lamp of 1 kW as the monitoring source. The signal from the photomultiplier was processed by a 7912 AD Tetronix transient digitizer controlled by an LSI 11/2 microprocessor. Cutoff filters were used to avoid spurious response from second-order scattering from the monochromator gratings. The concentrations of sulfur-containing acids were in the range 0 1994 American Chemical Society
Photoinduced Electron Transfer
The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4855
TABLE 1: Rate Constants for Quenching of the CB-Triplet State by the Sulfur-Containing Acids in Aqueous Solution at pH = 6.8 (Phosphate Buffer) no. acid formula abbreviation k, X 1 P (M-1 1 2-(methy1thio)ethanoicacid CH3-S-CHflOOH 2-MTEA 2.10 f 0.08 3-MTPA 1.71 f 0.12 2 3-(methylthio)propionicacid CH3S-CH2-CH2-COOH 2,2'-TDEA 0.95 f 0.05 HOOC-CH2S-CHrCOOH 3 2,2'-thiodiethanoic acid 3-CMTPA 0.94 f 0.04 HOOC-CH&-CH~CHZ-COOH 4 3-((carboxymethyl)thio)propionic acid 1.03 f 0.07 HOOC-CHrCHfi-CHz-CH2-COOH 3,3'-TDPA 5 3,3'-thiodipropionic acid 2-CMTSA 0.32 f 0.02 6 2 4 (carboxymethyl)thio)succinicacid HOOC-CH-(S-CHrCOOH)-CH2-COOH 7 acetic acid CH3-COOH s2xl e a Errors are twice the standard deviations from the least-squares fits of 10-4-10-3 M (in the quenching experiments), 2 X M in the quantum yield determination experiments, and 2 X 10-2 M when recording spectra in the time range following complete quenching of the CB triplet state. In all experiments, the CB concentration was in the range (1-2) X 10-3 M. The pH of the solutions was adjusted by adding sodium hydroxide, or the solutions were buffered in the presence of NaHzP04/Na*HP04 (0.025 M). All solutions were deoxygenated by bubbling highpurity argon through them. Steady-State Photolysis Experiments. The irradiations were performed in a 1 X 1 cm rectangular cell on an optical bench irradiation system. A high-pressure mercury lamp (HBO 200) with a combination of a BC-4 glass filter (Mashpriborintog, U.S.S.R) and a 313-nm interference filter (Zeiss) was used for excitation. A solution of 2-hexanone in cyclohexane was used as an a~tinometer,'~ and the intensity of the incident light was determined to be8.7 X l e e i n ~ t e i n d m s-l. - ~ Solutionscontaining CB (2 X 10-3 M) and carboxylic acid (2 X 10-2 M) a t an appropriate pH (6.3-6.4), adjusted with sodium hydroxide, were purged with oxygen-free argon and irradiated. The duration of irradiation times were chosen to cause about 15-25% CB conversion for the CO2 measurements and about 10-20% CB conversion for the determinations of the quantum yields of CB disappearance. The changes in the CB concentration during irradiation were determined spectroscopically using a HewlettPackard 8452A diode array spectrophotometer. The concentration of C 0 2was measured using gas chromatographic techniques. After irradiations, the solutions were acidified with concentrated HCl and the vapor phase in the reaction cell was analyzed on a Chromatron G C H F gas chromatograph with a thermal conductivity detector on a Porapak Q column. All experiments were performed a t room temperature (22 f 1 "C).
Quenching Rate Constants of the CB Triplet State by SulfurContaining Carboxylic Acids. Six sulfur-containing acids and acetic acid (Table 1) were used as quenchers of the CB triplet state in aqueous solution a t pH 6.8 (phosphate buffer). Since only solutions with p H >6.3 were studied, it will be understood that the CB triplet state is always in its carboxylate form,I4 (pK, = 4.5 for CB)15 and that the sulfur-containing acids are also in their ionic forms.1618 The quenching rate constants, k,, were obtained by monitoring the triplet-triplet (T-T) absorption decays (/cob)of CB a t fixed wavelengths for various quencher concentrations and by employing the Stern-Volmer relation
where rTis the lifetime of the CB triplet in the absence of quencher. A typical experimental trace for the triplet decay in the presence of 4 X 1 V M of 2,2'-TDEA is presented in the upper inset of Figure 1. The long-lived absorption has been attributed to the formation of ketyl radicals that was established by comparison of the observed transient absorption spectra with that from Inbar et al.14 (absorptionmaximum 570nm). The higher ratioof molar absorption coefficients of triplet vs ketyl radical absorption at
15.0
1I
e /
10.0
-
c v
)
.
g . r
Time. P
00
10
20
30
40
50
1 I I O ~ 103, X M-'
0
5
10
15
[aix 104, M Figure 1. Stern-Volmer plot for the quenching of CB triplet by 2,2'M, phosphate TDEA in aqueous solution at pH = 6.8 ([CB] = 2 X buffer). Upper insert: kinetic trace for the CB triplet decay at 480 nm in the presence of 4 X l e M of 2,2'-TDEA. Lower insert: plot of l / & vs the reciprocal of 2,2'-TDEA concentration (see text).
480nmcompared to thecorresponding ratioat 535 nm (maximum of the T-T absorption), made 480 nma more favorable monitoring wavelength for triplet decays. The pseudo-first-order rate constants, ko,, were calculated using eq 2 which takes into account a concomitant, underlying first-order growth of a photoproduct's absorptionI9
A-A" = -kObt AO - A" where AO, A, and A" are the absorbance changes a t time 0, f , and infinity, respectively. A typical plot based on eq 1 is presented in Figure 1, and the quenching rate constants obtained for all sulfur-containing acids and acetic acid are summarized in Table 1. Transient Absorption Spectra of Intermediates. The aqueous solutions of 4-carboxybenzophenone and acids used in the flash photolysis and steady-state photolysis experiments were first examined spectroscopically for any evidence of ground-state association. The absorption spectra of these mixtures were shown to be equal to those expected by adding the spectra from separate solutions of donor and quencher. No evidence for ground-state association was found under the experimental conditions used. Flash excitation of the solution of 4-carboxybenzophenone (1 X 10-3 M) and acid (0.02 M) a t pH = 6.3 resulted in the appearance of the absorption corresponding to various transients depending on the time delay and the structure of the acid used. High concentrations of acids were used to rapidly quench more than 99% of the CB triplets. In the case of the acids 2-MTEA, 2,2'-TDEA, 3-CMTPA, and 2-CMTSA, and transient spectra had spectral features reminiscent of the CB triplet, the ketyl radical anion, and the ketyl radical. Typical transient absorption spectra of this type, obtained for 2,2'-TDEA in the absence and presence of phosphate buffer, are presented in Figures 2 and 3. The transient at 535 nm is associated
In-
Marciniak et al.
4856 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994
I\
loo " I
a 0 02
0
0.02
u vu
I
I
I
T
0.00 400
500
600
700
800
Wavelength (nm) Figure 4. Resolution of the spectralcomponentsin the transientabsorption spectra following the quenching of CB triplet by (a) 3-MTPA and (b) 3,3'-TDPA in aqueous solution at pH = 6.3 (without buffer) taken -75 ns after the flash ([CB] = 1 X M, [acid] = 0.02 M). 0.01
Hurley et al.I5 The quantitative resolution of the observed spectra were done using a linear multiple regressionZoof the form
0.00
400
500
600
700
Wavelength (nm) Figure 2. Resolution of the spectralcomponentsin the transient absorption spectra following the quenching of CB triplet by 2,2'-TDEA (0.02 M) at pH = 6.3 (without buffer) taken (a) 900 ns, (b) 8 ps, and (c) 28 p s after the flash.
I - _
.
400
500
600
700
Wavelength (nm) Figure3. Resolutionof the spectral componentsin the transient absorption spectra following the quenching of CB triplet by 2,2'-TDEA (0.02 M) at pH = 6.3 (phosphate buffer) taken (a) -20 ns, (b) 390 ns after the
flash.
with the triplet of CB, and those at 570 and 660 nm are assigned to the ketyl radical (K) and the ketyl radical anion (CB*-), respectively, on the basis of data published by Inbar et al.l4 and
where M ( X j ) is the observed absorbance change of the composite spectrum and e&) is the molar absorption coefficients of the ith species a t the j t h wavelength of observation. Physically, the regression coefficients ai are equal to til, where ci is the concentration of the ith transient and I is the optical path length of the monitoring light. There are a total of n transients, typically numbering 1-5, in the spectral mixture and N observed absorbances, typically numbering 30-40. The details of the spectral resolution procedure has been described el~ewhere.~ The spectra of CB triplet, K, and CB*- were taken from ref 14 with renormalized values of molar absorption coefficientsI5.21 (es35 = 6250 M-I cm-l, €570 = 5200 M-l cm-l, and €660 = 7660 M-I cm-I for CB triplet, K,and CB*-, respectively). For the acids 3-MTPA and 3,3'-TDPA, two additional intermediates could be identified from the composite spectrum (Figure 4). The shoulder observed in the 480 nm region was assigned to the intermolecular S:.S bonded radical cation. Such an intermediate was recently seen in analogous systems, Le., CB/ sulfur-containing amino acids.5Ja9 As shown in Figure 4b, the presence of a second intermediate absorbing in the shortwavelength region around 410 nm is required to get a reasonably good composite fit with the experimental data. That new intermediateis suggested to bean intramolecularly (S:.O)-bonded species (Scheme 1). Formation of this intermediate requires interaction of the unpaired electron on the sulfur with a free electron pair on the oxygen atom of the carboxylate group and is assisted by a suitable steric arrangement (five-membered ring):
R R = -CH, ; -(CH2)2-COO-
I
Photoinduced Electron Transfer
The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4857
SCHEME 1
r \ f = O
R-
+
s -cH-(cH,),.,-coo-
CH;
The (S:.O)-bonded species cannot be formed for 2-MTEA and 2,2'-TDEA since formation of the intramolecular S:.O bond would require a thermodynamically unfavorable four-membered ring structure. Lack of formation of that intermediate for acids 3-CMTPA and 2-CMTSA may be explained by a fast concurrent reaction of decarboxylation of the sulfur-centered radical (intramolecular electron transfer from the COO- group, located at the -CH2- group linked to the oxidized sulfur function, followed by homolytic carbonxarboxyl bond breakage into C02 and the w(alky1thio)alkyl radical)." The appropriate spectra of (S:.S)+ dimers derived from 3-MTPA and 3,3'-TDPA were taken from ref 22 with molar absorption coefficients €480 = 7870 and 6550 M-1 cm-I, respectively. The spectral parameters for the (S:.O) intermediates of 3-MTPA and 3,3'-TDPA were taken from ref 22 with 6410 = 1890 and 2500 M-1 cm-l, respectively. Quantum Yields of Formation of Ketyl Radical, Ketyl Radical Anion, (S:.S)+, (S:.O), and Carbon Dioxide and Quantum Yields of CB Disappearance. Quantum yields of formation of transients and final photoproducts are important quantitative data for elucidating and describing the mechanisms of the reactions under study. In this work, these quantum yields were determined at concentrations of the acids that were large enough such that the CB triplets were almost totally quenched (>99%). The relative actinometry method was used in the laser flash measurements (see ref 23 for a critical review and exposition). The flash photolysis experiments were carried out in matched optically flat cells with identical absorbances due to ground-state CB at 308 nm. For samples of 4-carboxybenzophenone and the various acids in one of the matched cells, the absorption changes were measured at the spectral maxima of the ketyl radical, the ketyl radical anion, the (S:.S)-bonded radical cation, and the (S:.O) intermediate. These absorbance changes were compared with an absorption change in the actinometer, which consisted of a separate matched cellcontainingonly CB. In the actinometer, the CB triplet's absorbance was monitored at 535 nm immediately after the flash. The quantum yields were calculated according to eq 4,23
* = AAPtT/AATfP
(4)
where AA, is the product's absorption change extrapolated to the
TABLE 2 Quantum Yields of Product Formation in the CB-Sensitized Photooxidation of Sulfur-Containing Carboxylic Acids at pH = 6.3 in the Presence of Phosphate Buffer ([CB] = 1 X 10-3 M, [acid] = 2 X 10-2 M) no. acid Qtoul K a &yJ QCBb 1 2 3 4 5 6
2-MTEA 3-MTPA 2,2'-TDEA 3-CMTPA 3,3'-TDPA 2-CMTSA
0.98 0.79 1.01 0.92 0.87 0.93
C
C
0.39
0.28
C C
C C
0.04
0.78
C
C
0.99 1 .o 0.93 0.93 0.90
a Quantum yields were determined from the kinetic traces at 570,480, and 410 nm and eq 4 (see text); estimated error for 1,3,4, and 6, *lo%, for 2 and 5 , &15%, b Quantum yields of CB disappearance from the steady-statemeasurements (estimated error +lo%). e Not observed (see text).
"end-of-pulse" value under the conditions of nearly complete quenching, AAT is an absorption change immediately after the flash due to the CB triplet a t 535 nm measured under the conditions of no quenching. The molar absorption coefficients (e) of the CB triplet, K, and CB'- were renormalized as in ref 10, and the values for cp of the (S:.S)+ dimers and the (S:.O) intermediates of 3-MTPA and 3,3'-TDPA were taken from ref 22. Since the absorption spectra of K, CB*-, (S:.S)+ dimers, and (S:.O) intermediates overlap, the absorption changes (AA,) needed for eq 4 had to be computed. The calculation was done by solving a set of n linear equations (eq 3 with the regression coefficients replaced by the concentration times the path length) for the n concentrations, where n was the number of transients and also the number of observation wavelengths. The resulting concentrations were used directly in eq 4 ( A = ecl) since the path lengths (I) cancel for the actinometer and the sample cells. The quantum yields computed for the ketyl radicals, the ketyl radical anions, the (S:.S)-bonded radical cations, and the (S:.O) intermediates are summarized in Tables 2 and 3. The results presented in Table 2 were measured in the presence of a phosphate buffer where fast protonation of CB*-by the buffersdid not permit a determination of the quantum yields of ketyl radical anion formation. Under those experimental conditions, only the total quantum yield, of ketyl radical plus ketyl radical anion formation could be obtained. On the other hand, for the solutions
4858 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994
Marciniak et al.
TABLE 3 Quantum Yields of Product Formation in the CB-Sensitized Pbotoxidation of Sulfur-Containing Carboxylic Acids at pH = 6.3 without Buffer no. 1 2 3 4
5 6
acid 2-MTEA 3-MTPA 2,2'-TDEA 3-CMTPA 3,3'-TDPA 2-CMTSA
@cn-'
@K'
0.12 0.11 0.16 0.12 0.09 0.1 1
0.87 0.78 0.84 0.83 0.90 0.88
*total
K + CB-
0.99 0.89 1.oo 0.95 0.99 0.99
b
@s+
@so
@cot
C
c
0.40 c c 0.05
0.18
C
C
0.86 C0.03 0.80 0.77 k,(acid-2) > k,(acid-3). A similar dependence of the quenching rate constants on the reactant charges was found for quenching of the singlet state of the pyridinium cation by sulfur-containing amino acids in aqueous solution.2s On the other hand, for acetic acid, which lacks a sulfur moiety, the k, is at least 4 orders of magnitude lower than the k,'s obtained for the sulfur-containing acids. This supports the idea that the quenching of the CB triplet state by sulfur-containing acids involves an interaction a t the sulfur atom. Direct evidence for the participation of an electron transfer from the sulfur atom to the CB triplet state in the quenching was found by the observation of the 4-carboxybenzophenone ketyl radical anion in the transient absorption spectra at X = 660 nm (vide supra Figures 2-4) and by the detection of the (S2.S)-
bonded radical cations at X = 480 nm (Figure 4). The observation of the latter species for high concentrations of the sulfur-containing carboxylic acids is direct confirmation of the presence of >S+ radical cations in the photoreaction studied. Additional support for the formation of >S+ radical cations in the quenching events was found through the presence of another species that was assigned as the intramolecularly (S:.O)-bonded species. This species was observed at X = 410 nm in some of the transient absorption spectra (Figure 4). Such a species was not observed in the transient absorption spectra accompanying the quenching of the CB triplet state by sulfur-containing amino acids.8 On the basis of the above experimental observations, a reaction mechanism for the CB-sensitized photooxidation of sulfurcontaining carboxylic acids in aqueous solution is proposed (Scheme 1). The initial step in this mechanism is an electron transfer from a sulfur atom to the CB triplet state to form a C T complex (radical ion pair). The complex can disappear by the following processes: (1) diffusion apart of the CT complex to form sulfur-centered radical cations and ketyl radical anions, (2) intramolecular proton transfer within the complex to form a ketyl radical and an a-(alky1thio)alkyl radical, and (3) back electron transfer to regenerate the reactants. Further secondary reactions (Scheme 1) of the intermediates depend on the structure of acid used, the pH of the solution, and the presence or absence of a phosphate buffer (vide supra). The secondary reactions consist of protonation of ketyl radical anions, deprotonation of monomeric sulfur-centered radical cations, production of a-(alky1thio)alkyl radicals from a decarboxylation which was initiated by an intramolecular electron transfer, formation of (S:.S)-bonded radical cations, and cyclization to form an intramolecularly (S2.0)-bonded species. On the basis of Scheme 1, using the steady-stateapproximation, the following expression for 1 / @as ~ a function of the reciprocal of the acid concentration can be derived (modified Stern-Volmer relation) for solutions in the presence of phosphate buffer:
where ai"' is the limiting quantum yield of ketyl radical formation (the CB triplet state is totally quenched by the acid used). Typical double reciprocal plots of @K and [Q] show the linear relationship expected from eq 5 (see the lower inset of Figure 1). From the slope and intercept of these plots, the limiting quantum yields of ketyl radical formation and thequenching rate constants were estimated from eq 5 . These values for k, and slim, based on the modified Stern-Volmer equation for the mechanism in Scheme 1, are equal within experimental error to the values of @,t(otal determined from the relative actinometry method (Tables 2 and 3) and to the values of k, determined directly from time-resolved quenching experiments according to eq 1 (Table 1). This is additional justification for the mechanism of Scheme 1. Scheme 1 for the CB-sensitized photooxidation of sulfurcontaining carboxylic acids is a modified version of the analogous
The Journal of Physical Chemistry, Vol. 98, NO. 18, 1994 4859
Photoinduced Electron Transfer reaction with sulfur-containing amino acids.8 The primary photochemical steps are the same for both the amino and the non-amino acids, but thesecondary reactionsdiffer. The (S:.O)bonded species were observed in the quenching events involving some of the non-amino acids. Furthermore, in contrast to the amino acids: no secondary formation of ketyl radicals due to the reduction of CB by the a-aminoalkyl radicals was possible since a-aminoalkyl radicals cannot be produced in non-amino acids. The lack of a secondary formation of ketyl radicals for the nonamino acids, where decarboxylation leads to a-(alky1thio)alkyl radicals, indicates that the a-(alky1thio)alkyl radicals are much weaker reducing agents than the a-aminoalkyl radicals. Although the primary steps in the mechanism were the same for the amino and non-amino-sulfur-containingacids, the primary quantum yields of the various processes differed significantly between the two types of sulfur-containing carboxylic acids. In the non-amino sulfur-containing carboxylic acids, the total quantum yields of ketyl radical formation from Table 2 and from Table 3 are close to unity. From Scheme 1, these primary quantum yields represent combined yields of two processes, namely, the diffusion apart of the CT complex (radical ion escape) and the intramolecular proton transfer within the complex. Supplementary measurements of the quantum yields of the CB disappearance obtained in the steady-state photolysis are in very good agreement with the values of @Fa'(Table 2). This clearly indicates that, contrary to quenching by sulfurcontaining amino acids,8 back electron transfer is an inefficient process in quenching events with non-amino-sulfur-containing acids. Furthermore, the individual contributions to the large yield of primary photoproducts can be separately quantified as being mainly (>0.83) due to the escape of radical ions from the CT complex (see the values of @CB- in Table 3) and to a much lesser extent (
