J. Phys. Chem. 1981, 85, 1474-1479
7474
-BOo with slightly different D and E values. The field strengths of ;he stationary points are tabulated in Table 111. Since H is parallel to the L axis of the durene at a dial reading of nearly Oo in Figure 7, the y axes of the bimane molecules are rotated by 40' in the opposite sense. Since the x direction is parallel to the C=O direction, the probable orientations of two bimane molecules with respect to durene are shown in Figure 8. The angular dependences obtained by solving the above spin Hamiltonian are given by the solid lines in Figure 7. Although the agreement between the experimental values and the calculated ones is not perfect, possibly because of errors in mounting, the
agreement is considered to be sufficient to conclude that these two differently oriented bimanes give rise to the site I and site I1 phosphorescence and ODMR spectra. The bimanes in these two orientations have nonequivalent environments which produce slightly different zfs and phosphorescence spectra. We have attempted to observe hyperfine splittings, but so far we have not succeeded.
Acknowledgment. We thank Hanna Dodiuk, Hanna Kanety, and Joshua Hermolin (Tel-Aviv University) for their contributions to this study.
Disproportionation of Semimethylene Blue and Oxidation of Leucomethylene Blue by Methylene Blue and by Fe(II1). Kinetics, Equilibria, and Medium Effects David W. Hay, Stephen A. Martln, Sugata Ray,+ and Norman N. Lichtin" Department of Chemistry, Boston Universw, Boston, Massachusetts 022 15 (Received:March 3, 1980; In Final Form: January 29, 198 1)
The dependence on reaction medium of the kinetics of three ground-state elementary reactions occurring in the iron-methylene blue photoredox system has been investigated by studying the relaxation of the photostationary state and by flash photolysis. The rate constants which have been evaluated include 2k6, for disproportionation of semimethylene blue (S),k,, for syn proportionation (the oxidation of leucomethylene blue (L) by methylene blue (MB)),and klo, for the oxidation of leucomethylene blue by ferric ion. Variations of media include nature of the solvent and anions, ionic strength, and concentration of acid. Values of the equilibrium constant & = k4/2k6 = [S]2/[L][MB]have been derived from the kinetic data and used in conjunction with potentiometric data to determine values of the one-electron standard reduction potentials, cotMBlS and eo'sIL in several media. As in the iron-thionine photoredox system, the half-reduced dye, S, is a minor component of the photostationary state and oxidation of leuco dye by ferric ion appears to proceed via a metastable association complex of the reactants. Mechanistic interpretations of some of the medium effects are suggested.
Introduction The photophysics, photochemistry, and subsequent ground-state chemistry in solutions containing methylene blue, MB+, and Fe"(H,O):+ are similar to the corresponding phenomena in solutions containing thionine and hexaaquoferrous.1-9 Thus, semimethylene blue, which is produced with high efficiency in the quenching of protonated triplet dye, 3MBH2+,in acid solution by hexaaquoferrous,loundergoes reversible disproportionation, eq 6,11and the resulting leucomethylene blue, MBHQP+ at the pH values of this research (pK,, = 4.5, pKa2 = 5.9 in water),12 is oxidized by Fe"'(H20):+. Knowledge of photostationary state composition, of the kinetics of bulk back-reaction of MBHQP' with Fe(III), of standard oneelectron reduction potentials of methylene blue and semimethylene blue, and of the dependence of these quantities on the medium are necessary for a quantitative understanding of photogalvanic cells employing the ironmethylene blue photoredox system. We have measured specific rates of disproportionation of semimethylene blue, 2k6, in various media by means of conventional or laser flash photolysis-kinetic spectrophotometry and specific rates of oxidation of leuco dye, MBHQ2+,by methylene blue, i.e., syn proportionation, k,, and apparent specific rates of oxidation of MBH3'+ by Fe1rr(H20)63'or related +Deceased Sept 25, 1979. 0022-365418112085-1474$0 1.2510
labile complexes, klo, in various media by a photochemical perturbation technique.13 Values of the equilibrium constant K6 = k+/2k6 and of standard one-electron reduction potentials of MB+ and semimethylene blue have been calculated. Results are compared with the corresponding values for thionine and its reduction products.13J4 Equations 1-10 represent established and assumed elementary steps in the acidified iron-methylene blue (1)Parker, C. A. J. Phys. Chem. 1959, 63, 26. (2) Kato, S.; Morita, M.; Koizumi, M. Bull Chem. SOC. Jpn. 1964, 37, 117. (3) Danziger, R. M.; Bar Eli, K. M.; Weiss, K. J . Phys. Chem. 1967, 71, 2633. (4) Faure, J.; Bonneau, B.; Joussot-Dubien, J. Photochem. Photobiol. 1967, 6, 6 , 331. (5) Bonneau, R.; Fornier de Violet, P.; Joussot-Dubien, J. Photochem. Photobiol. 1974, 19, 129. (6) Wildes, P. D.; Lichtin, N. N.; Hoffman, M. Z.; Andrews, L.; Linschitz, H. Photochem. Photobiol. 1977, 25, 21. (7) Ohno, T.: Osif, T. L.: Lichtin, N. N. Photochem. Photobiol. 1979, 30, 541. (8) Osif, T. L.; Lichtin, N. N. Photochem. Photobiol. 1980, 31, 403. (9) Osif, T. L.; Lichtin, N. N.; Hoffman, M. Z.; Ray, S. J.Phys. Chem. 1980, 84,410. (10) Ohno, T.; Lichtin, N. N. J. Am. Chem. SOC. 1980, 102, 4636. (11) Equations are numbered to correspond to numbering of corresponding reactions in the iron-thionine system in ref 15. (12) Obata. H. Bull. Chem. SOC.J D ~1961. . 34. 1057. (13) Wildes, P. D.; Lichtin, N. N. 3. Phys. 'Chem. 1978, 82, 981. (14) Wildes, P. D.; Lichtin, N. N.; Hoffman, M. Z. J.Am. Chem. SOC. 1975, 97, 2288.
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 11, 1981 1475
Kinetics and Equilibria of Iron-Methylene Blue
photoredox system at pH >O. MB+(So)-% MB+(SJ MB+(SJ
ka
MB+(So)
-
(2)
-
quenching
(2a)
MB+(T1)-% MBH2+(T1)
(3)
+
MB+(S1) Fe”(H20),2+ MB+(S1)
(1)
MBH2+(T1)
k4
+
kza
MB+(So) + H+
+
MBH2+(T1) Fe11(H20)62’-% MBH’.
(4)
+ Fe111(H20)63+ (5)
MB+
+ MBHS2+z 2MBH+. + H+ 2ks k4
(6)
K?
+ Fe111(H20)63+ z“complex”) (7) ks MBH’. + 2H+ + Fe11(H20)62+)(8) (“complex” MBH+. + Fe111(H20)2+ -% MB+ + H+ + Fe11(H20)62+ (MBH:+
+
-
-+
(9)
k 10
MBHS2+ Fe111(H20)63+ MBH’.
Fe1r(H20)62+ + 2H+ (10)
MB’, = 665 nm Equations 7 and 8 are written by analogy to the reaction of leucothionine with Fer11(H20)63+.15i16 Investigation of the complexation of leucomethylene blue with ferric ion does not appear to have been reported. Complexation of semithionine with ferric ion has been inferred15J7but information on the analogous complexation of semimethylene blue is also lacking.
Experimental Section Materials. All reagents were used as supplied and were ACS Reagent grade unless otherwise specified. Methylene blue chloride.3H207 (mol wt, 374) was Fluka “puriss” grade. Distilled water was further purified with a Millipore Milli-Q water system to a resistivity of 10 MO cm. Acetonitrile was Burdick and Jackson UV grade. Absolute ethanol was supplied by US. Industrial Chemicals. Sulfuric, acetic, and perchloric acids and ferric sulfate (Analyzed) were supplied by Baker. Acetone, hydrochloric and nitric acids, most inorganic salts, and mossy zinc were obtained from Fisher. Chromium perchlorate hydrate (99%) and 1,lO-phenanthroline (96%) were from Aldrich. Primary standard potassium acid phthalate was from Mallinckrodt. Triple-distilled instrument grade mercury was from Bethlehem. Nitrogen was prepurified grade from Union Carbide. Stock solutions of acids were standardized indirectly against primary standard potassium acid phthalate. Concentrations of ferrous ion were determined spectrophotometrically as the 1,lO-phenanthroline complex in acetate buffer at pH 4, ~ 5 1 =0 11100 ~ M-l cm-l. Impurity (15) Osif, T.L.; Lichtin, N. N.; Hoffman, M. Z. J. Phys. Chem. 1978, 82,1778. (16) Havemann, R.;Reiner, K. G. 2.Phys. Chem. (Leipzig) 1959,211, 26. (17) Guha, S. N.;Moorthy, P. N.; Rao, K. N. Mol. Photochem. 1979, 9,183.
ferric in ferrous was determined in HSOL/SO:- media by 1:50 dilution of a stock solution with sulfuric acid and measurement of absorbance, caMnm = 2174 M-’ cm-l. Impurity ferric in ferrous was determined in chloride media by making up stock solution to 50% (vol/vol) aqueous acetone, 0.10 M in HC1, and 0.30 M in KSCN and measuring the absorbance, 6480nrn = 16700 M-l cm-’. Apparent molar absorbancy indices of MB+ at 665 nm at all concentrations used were determined in water, 50% (vol/vol) aqueous CH&N and 50% (vol/vol) aqueous EtOH by normalization to E ~ , , , , , = 78000 M-l cm-l for 3.0 pM solutions of MB+ in water. Nitrogen used to deaerate stock and test solutions and to protect stock ferrous solutions during storage was deoxygenated by passage through a 15 wt % solution of chromous perchlorate in 3 M perchloric acid over amalgamated mossy zinc and then through the same solvent used in the test or stock solution. Test solutions were deaerated for 20 min. Apparatus and Measurements. Static spectra were measured with a Cary Model 118C spectrophotometer. Most flash photolysis experiments employed a Xenon Corp. Model 720 unit with a xenon-filled flash lamp (180 J/pulse, pulse width 100 ps) and a 5-cm cell length. The monitoring beam from a 150-W xenon lamp was reduced in intensity by passage through several neutral density filters and one interference filter so that test solutions were not detectably bleached. After passing through the solution, the focussed monitoring beam was passed through a Schoeffel CMlOO miniature grating monochromater and into an RCA 1P28 photomultiplier. The output of the photomultiplier tube was recorded on a Tektronix Model 564 storage oscilloscope. Measurements of the dependence on acidity of the specific rate of disproportionation of semimethylene blue, 2k6, were performed with a 694.3-nm Q-switched ruby laser flash apparatus and associated monitoring system which have been described.’ Flash photolysis was performed at 24 f 2 OC. Photochemical perturbation measurements were performed in a crossillumination apparatus which has been described elsewhere13J8J9and which was thermostatted at 25.0 f 0.1 OC. Absorption was monitored at 665 nm. Electrical potentials were measured with a Leeds and Northrup Model 8687 potentiometer.
-
Results Kinetics. Values of I t , and Itlo were determined from the kinetics of decay of the photostationary state of ironmethylene blue by using an analysis of the data which has been de5~ribed.l~”As with iron-thionine solution^,^^ dark recovery of photobleached iron-methylene blue solutions from the bleached photostationary state was complete and followed pseudo-first-order kinetics, i.e., -In ( [MB+l0[MB+],) = It,&. Stoichiometric concentrations of unbleached dye in water solution were calculated from absorbances at 665 nm by using experimental values of E matched to the actual concentration of unbleached dye by successive approximations to allow for variation of the extent of dimerization of the dye with its stoichiometric concentration. (Dimerization was significant only in water.) Analogously to the behavior of the iron-thionine (18) Wildes, P. D.;Lichtin, N. N.; Hoffman, M. Z. “Proceedings of the International Symposium of Solar Energy”;Berkowitz, J.; Lesk, I., Ed.; The Electrochemical Society: Princeton, NJ,1976; pp 128-138. (19) Wildes, P.D.;Brown, K. T.; Hoffman, M. Z.; Lichtin, N. N.; Hall, D. E. Sol. Energy 1977, 19, 579. (20) Numbering of rate and equilibrium constants in this paper is related to the numbering used in ref 13 as follows, where present numbering is given first: 2k6 = 2kz; k4 = k-2; k9 = ks;klo = kl;K6 = K2.
1476
The Journal of Physical Chemistry, Vol. 85, No. 11, 198 1
Hay et al.
TABLE I: Effect of Variation of Solvent on k,,, k-,, 2k6, and K , for Methylene Blue and Thionineavb solvent
H2O 50%' aqueous EtOH 50%' aqueous CH,CN
HZO. 50%' aqueous CH,CN
k,o,C l o 3 M-I s-' 0.02-0.04 (0.1-0.28) 2.1-4.2 (2.0-4.7) 0.80- 1.6 (0.78-1.7) 0.36-0. 72h 0.3-0.6h
k-,,C l o 3 M - I s-l 2k 69 10' M-I s-l Methylene Blue 4.1-8.2 1.5 i 0.2 (3.9- 8.6) 4.9-9.8 0.31 t 0.03 (4.5-10) 1.1-2.2 0.76 t 0.05 (0.9- 2.6) Thionineg 0.8-1.6 0.5-1.0
2.0 0.28
106K,e
Z,f kcal/mol
2.7-5.4
94.6
16-3 2
-
89.5
1.4- 2.8
87.5
0.4-0.8 1.8-3.6
94.6 87.5
Values in parentheses are All solutions contained 0.010 M FeSO,, 0.0100 M H,SO, with no added salt; p 0.05 M. limits including probable errors. T = 25.0 f 1 "C. T = 24 1 "C. e K , = kM6/2k,= [ SI2/[L] [D]. f Kosower's solvent k , , = (0.42 t 0.06) x l o 3M - l s - ' i n water and (0.36 t 0.04) x l o 3 M-' polarity parameter, ref 22. g Data from ref 14. s-' in 50%(vol/vol) aqueous CH3CN was determined by flash photolysis under the conditions of Table I. See ref 15. ' Volume/volume.
system,lBk,, in the iron-methylene blue system varied linearly with the initial stoichiometric concentration of methylene blue in the unbleached solution, [MB+],, at constant initial concentration of Fe1I1(H20):+, [Fe(III)lo, and with [Fe(III)loat constant [MB+l0. Typical data are displayed in Figure 1. Analogously to the behavior of iron-thionine solution^,'^ semimethylene blue was not detectable in the photostationary state under any of the conditions employed, using optical absorbance in a 1-cm cell at 420 nm as the criterion. Because the pKa of MBH?+. has values lying within the range of values of pH used in this research, €420 varied with the medium.21E.g., pKa of MBH2"+. is 1.86 in 5% ethanol-95% water at 25 "C with p = 0.4 M and 1.15 in 50% (vol/vol) aqueous CH3CN under the same conditions.21 At pH -2, €420 of semimethylene blue is -7000 in 5% EtOH-95% water and -8200 in 50% (vol/vol) aqueous CH3CN. Since a change in absorbance under illumination equal to 0.004 would have been detectable and ~ 4 2 of0 MB+ ~ = 440 M-l cm-l, it is readily calculated that the photostationary state concentration of semimethylene blue was less than 6 X lo-' M under all conditions and therefore a minor component of the sum of dye, leuco dye, and semireduced dye. We have previously shown that, under the latter condition, the first-order decay of the photostationary state can be represented by eq 11, where L is leuco dye and D -In [L], = -In ([D+Io- [D]J = F(klo[Fe(III)lo+ k+[D10)t (11) is dye (methyleneblue or thionine) and 1 / 2 IF I1is given by eq 13, where S representa semireduced dye in whatever (12) k,, = F ( ~ ~ o [ F ~ ( I I+I )kI-o0 1 0 ) F = (kg[Fe(III)It + ks[sIt/2)/(kg[Fe(III)It + k6[slt) (13) state of protonation is present.13J8 The excellent linearity of all our data when analyzed in the form of eq 11indicated that F changes little if at all during decay of a given photostationary state, in agreement with the previously reported behavior of the iron-thionine system.13 Comparison of values of klo for oxidation of leucothionine obtained with the aid of eq 11-1313 with values measured directly by flash photolysis15 indicates that F was 1under the conditions of measurement, i.e., k9[Fe(III)It>> k6[S']t, where S' represents semithionine. Values of klo and k 4 which are reported below were calculated by means of computerized least-square analysis of sets of data in the (21) Kamat, P. V.; Lichtin, N. N. Photochem. Photobiol. 1981,33,109.
Dependence of the Observed Rate Constant for Dark Reoxidation of Photobleached Iron-Methylene Blue Solutions, hex, on Total Methylene Blue Concentration
5.88xl0-6M Fe(llll,,,
p: .04M
4 h
I
I
I
I
I
I
5
IO
15
20
25
30
1
[MBI,, rM Figure 1. Dependence of the observed rate constant for dark reoxldatlon of photobleached Iron-methylene blue solutlon, k,, on total methylene blue concentration, [MB],: (0)50% (vol/vol) aqueous CH,CN, 0.010 M HCI, 0.010 M FeCI,, 5.88 pM Fe(IIQ0,p = 0.04 M; (0) 50 % (vol/vol) aqueous CH,CN, 0.100 M HCI, 0.010 M FeCI,, 7.51 pM Fe(III)o, p = 0.40 M; ( 0 ) 50% (vol/vol) aqueous CH,CN, 0.010 M H2SO4,0.010 M FeSO,, 18.2 pM Fe(III)o, p = 0.40 M.
form of eq 12 with [Fe(III)loconstant and [MB+l0as the independent variable. Most values of 2k6 for disproportionation of semimethylene blue were determined by conventional flash photolysis by monitoring the second-order recovery of methylene blue at 665 nm. It was found that the amount of dye regenerated during this stage of relaxation was equal to half the amount bleached by the initial reaction of Fe11(H20)62+ with 3MBH2+. The slower subsequent relaxation of the solution, during which leucomethyleneblue was oxidized by Fe111(H20)63+ or related labile complex, resulted in complete regeneration of the dye under the conditions employed. The dependence of 2k6 on acidity was studied by means of laser flash phot~lysis.~ Absorbance of semimethylene blue was monitored at 880 nm and concentrations were calculated by using previously determined pH-dependent values of the molar absorbancy index.21 Values of klo, k4, 2k6, and K6 = k4/2k6 = [SI2/[L][D] at 25 OC and pH 2 in water, 50% (vol/vol) aqueous ethanol, and 50% (vol/vol) aqueous CH3CN with HS04-/ S042-as anion and p 0.05 M are summarized in Table I. Dependence of these quantities on salt concentration in 50% (vol/vol) aqueous CH3CN is presented in Table 11. Information on the dependence of klo and k4 on [H+] in 50% (vol/vol) aqueous CH3CN with chloride as the anion and p = 0.4 M is given in Figures 2 and 3. Analo-
-
Kinetics and Equilibria of Iron-Methylene Blue
The Journal of Physical Chetnistty, Vol. 85,No. 11, 198 1 1477
TABLE 11: Effects of Ionic Strength and Nature of Anions on k,,, 50% (vol/vol) Aqueous CH,CNaib
k-,, 2k6, and K, for Methylene Blue in
!J,M
h,,,'
anion
-0.05
HS0,-/SO... -
-0.40f
HSO,-/SO,Z-
-
0.40g
s-'
0.8-1.6 ( 0.7 8-1.7) 0.18-0.36 (0.13-0.46) 4.0- 8.0 (3.5-9.0) 28-56 (25-60)
,
aa-
0.04
l o 3 M-I
k-,,'
l o 3 M-I s - '
10' M-Is-'
2k,,d
1.1-2.2 (0.9-2.6) 0.4 5- 0.90 (0.43- 0.92 1.8-3.6 (1.7-4.0) 12- 24 (11-26)
106K,e
0.76 r 0.05
1.4-2.8
1.61 i 0.09
0.27-0.54
1.27
1.4-2.8
i
0.06
2.5 r 0.2
5-10
Values in parentheses are limits including probable errors. ' T a All solutions contained 0.0100 M acid, 0.01 M Fe(I1). = 25.0 i 0.1 "C. T = 24 i 1"C. e K = k-,/2k6 = [SI a/[L][D]. f 0.0793 M NaHSO, and 0.0896 M Na,SO, added to obtain desired P without altering pH. 20.36 KCl added to obtain desired fi. TABLE 111: Standard One-Electron Reduction Potentials for Methylene Blue and Thioninea in Various Media P,
M
acid
-0.05 ,0.40' 0.40' 0.40'
0.0100 M H,SO, 0.100 M HCl 0.0100 M HCl 0.00100 M HC1
-0.05 -0.05
0.010 M H,SO, 0.010 M H,SO,
solvent Methylene Blue water 50%d aqueous CH,CN 50%d aqueous CH,CN 50%d aqueous CH,CN
Eo'D/S,
v
v
E0'S/L,
0.187-0.197 0.3 01-0.3 10 0.203-0.212 0.148-0.157
0.507-0.516 0.605-0.614 0.522-0.531 0.397-0.405
0.192-0.200 0.17 2-0.181
0.566-0.575 0.525-0.535
Thioninea
a
Data from ref 13. 60
I
Vs. NHE.
water 50%d aqueous CH,CN
' Made up with KCl.
Volume/volume. DEPENDENCE
I
OF
OF METHYLENE
k-6
BLUE ON -log[Hq
I
I
I
I
1.0
1.5
2.0
2.5
40
.5 10
5
15
20
25
30
-log [ti': /range of kI0values A limits of range including probable error
:::.
Medium 50v/v%aq CH3CN, ionic strength 4h3 with chloride (15 the anion
Figure 2. Dependence of klo for methylene blue on -log [H'] in 50% (vol/vol) aqueous CH3CN with p = 0.4 M and chloride as the anion.
gous information for 2k6 is given in Table IV. Dependence of these rate constants on [H+] in aqueous solution (where the pK,'s of MBH2+,MBH?'., MBH32+,and MBH2+are all known) could not be measured by the method employed in this work because under most conditions in this solvent the extent of bleaching in the photostationary state was too small to permit accurate determination of the kinetics of its relaxation. Standard One-Electron Reduction Potentials of Methylene Blue, to'MB,s and to'sIL, in Various Media (Table 114. Values of to m/s and where MB, S, and L refer to methylene blue, semimethylene blue, and leucomethylene blue in their states of protonation appropriate
3.0
-lag [H*] Arrange of k-6 values
...'...'limits
of range including probable error
Medium: 5Ov/v% CH,CN/H,O,
ionic strength .4h3 with
chloride as the anion
Figure 3. Dependence of k4 for methylene blue on -log [H'] in 50% (vol/vol) aqueous CH3CN with p = 0.4 M and chloride as the anion.
to the conditions of measurement, were determined by the same procedure which was used to determine one-electron standard reduction potentials for thionine and semithionine.13 This procedure is based on the fact that t0'm/s + =2 ~and ~ -' =~0.06 log ~ &, where ~ to'm/L is the two-electron standard reduction potential of methylene blue in the solvent and at the acid concentration values (vs. and ionic strength of measurement. to'mvIBIL normal hydrogen electrode) in dilute acidic aqueous solu,~~ tion at 298 K are available from the l i t e r a t ~ r eto'm/L (22) Kosower, E. M. "An Introduction to Physical Organic Chemistry"; Wiley: New York, 1968; pp 296-304. (23) Clark, W. M. "Oxidation-Reduction Potentials of Organic Systems"; Williams and Wilkins: Baltimore, 1960; p 32.
1470
The Journal of Physical Chemistty, Vol. 85, No. 11, 1981
Hay et al.
TABLE IV: Effect of Variation of [ H+]on 2k, for Methylene Blue in 50% (vol/vol) Aqueous CH,CNa
K6, k4, 2k6, and klqprovides information essential to interpretation of variation of efficiency of photogalvanic conversion with medium. A t the same time, medium ef-log [H'] 2k,, 10' M-' S-' nc fects on K6, k4, 2k6,and klo serve as probes of the mech0.72 2.08 t 0.16b 6 anisms of reactions 6, -6, and 10. 1.01 2.30 5 0.24 8 The following sections discuss most of the observed 1.21 2.85 t 0.28 10 medium effects. In several cases, mechanistic inferences 1.60 2.55 t 0.20 6 1.95 2.24 t 0.21 10 are drawn, some of them speculative. 2.50 1.14 5 0.08 5 Effects of Variation of Solvent o n klO,k4, 2k6, and K6 for Methylene Blue and Thionine with HS04-/SOt- as a 0.01 M Fe(II), anion = Cl-, p = 0.40 M made up with Anion (Table I ) . Values of 2k6, the specific rate of disKCI. Uncertainties are standard deviations. Number proportionation of half-reduced methylene blue or thioof replicate measurements. nine, are not subject to uncertainty relating to the value = 0.532 - 0.09pH. Values of EO'MB/L in 50% (vol/vol) of F. The solvent dependences of 2ks are quite different aqueous CH3CN with p = 0.04 M made up with KC1 were for semimethylene blue, mostly MBH+. at the pH of determined by measuring the potential of a platinum probe measurement,21and semithionine, believed to be TH2+electrode immersed in a solution containing equimolar around pH 2F5 We have previously reported that log 2k6 concentrations of MB and L (-3 X 10" M) under N2 vs. for TH2+.varies linearly with Kosower's solvent polarity a saturated KC1 standard calomel electrode to which it was parameter, 2,in 0.01 M F3CS03H,p = 0.1 M.14 Correlation connected by a KCl/agar bridge. The observed depenwith 2 is less apparent in the values of 2k6 for MBH+*given dence of (vs. normal hydrogen electrode) on [H'] in Table I. in this medium for 1.0 I -log [H+] I 3.6 is given in eq 14. Values of k4 are not strongly solvent dependent. The direction of dependence of K6 on solvent appears to be t o ' ~ ~ /=L0.547 + 0.09 log [H'] (14) opposite for methylene blue from its direction with thioLeucomethylene blue solutions were prepared by the nine when data in water and in 50% (vol/vol) aqueous photoreduction of methylene blue with ferrous acetate in CH3CN are compared. As expected from our failure to the same way as has been described for leu~othionine.~~ detect semimethylene blue in the photostationary state, One-electron reduction potentials, t o ' ~ / and s ~ O ' S / L , in it can be calculated from the values of K6 that this species several media are presented in Table I11 along with coris a minor component of the photostationary state under responding data for thionine.13 to'ILIB/S and to's/L are senthe conditions of measurement. sitive to [H+] in the medium employed. The value of Values of klo for thionine are essentially the same in increases by 0.055 V with increase in [H+] from water and in 50% (vol/vol) aqueous CH3CN with [H+] = 0.001 to 0.01 M and by 0.098 V with increase in [H'] from while the magnitude of klo for methylene blue is 0.01 to 0.1 M. These results are consistent with the strongly solvent dependent. The small value of the latter half-cell reactions MB+ e- H+ MBH'. and MB' + in water would favor the use of the iron-methylene blue e- + 2H+ MBH22+.and the value of pKa1of MBH2"+. system in water for photogalvanic conversion if dimeriequal to 1.15 which has been observed in 50% (vol/vol) zation of the dye could be suppressed. aqueous CH&N with p = 0.4 M and C1- as the anion.21 The possibility exists that the solvent dependence of klo The value of for methylene blue increases by 0.125 may be caused by changes in the composition of the inner V as [H+] increases from 0.001 to 0.01 M but only 0.083 coordination sphere of Fe(II1). Such changes would be V as [H+]increases from 0.01 to 0.1 M. These results are expected to have similar effects on the oxidation of leuconsistent with the half-cell reactions MBH+. + e- + 2H+ cothionine and leucomethylene blue by Fe(II1) if the MBH;+ and MBH22+. e- H+ MBHQP+and the mechanisms of the two reactions were identical. The fact value of pKal of MBH22+-indicated above. that the solvent dependences of klo are different for the two leuco dyes does not, however, completely exclude Discussion variation of the ligands bound to Fe(II1) as the solvent Significance of the Data. The photoredox chemistry varies. Significant variation of the ligands bound to Fe(II1) of the iron-methylene blue system which is summarized with variation of solvent over the range reported in Table in eq 1-10 and the analogous behavior of iron-thionine are I (water, 50% (vol/vol) aqueous CH,CN, 50% (vol/vol) fairly complicated. As indicated in the Introduction, aqueous EtOH) does appear to be excluded by spectral quantitative characterization of many dynamic and data. The absorption spectrum of Fe(II1) is virtually the equilibrium properties of these systems is necessary for same under all the conditions of Table I. It should be a full understanding of photogalvanic cells which employ noted that the mole fraction of H20 is at least 0.70 in each these systems. Perhaps the most useful quantities are solvent. It is accordingly assumed that the results reported values of K6 (Tables I and 11) from which relative proin Table I refer to reactions of Fe11'(H20)63+. portions of D, S, and L in the photostationary state can Effects of Variation of the Nature of Anions and Ionic be calculated and the standard one-electron reduction Strength on kl0, k4, 2k6, and K6 for Methylene Blue in potentials (Table 111) which are determined with the aid 50% (vol/uol) Aqueous CH3CN (Table II). Values of klo, of values of K,. Values of K6 are derived here from values k4, and 2ks in acidified 50% (vol/vol) aqueous CH3CN are of k4 and 2k6. In addition, the bulk solution lifetimes of greater in chloride than in sulfate media and in every case principal charge carriers L and Fern depend on k4 and kl@ the difference is greater at higher ionic strength. The This is important because of the requirement for photoobserved effect on klo can be associated, at least in part, galvanic conversion that these charge carriers diffuse to with replacement of one or more molecules of water in the electrodes. by chloride, a inner coordination sphere of Fe111(H20)63+ Knowledge of how variation in medium, i.e., solvent process which has been studied quantitatively in water (Table I),nature of counteranions(Table 11), ionic strength solution.26 The occurrence of this substitution in 50% (Table 11), and acidity (Figures 2 and 3, Table IV), affects
-
-
+ +
+ +
-+
-
(24) Wildes, P. D.; Lichtin, N. N. J. Am. Chem. SOC.1978,100,6568.
(25) Bonneau, R.; Faure, J.; Joussot-Dubien, J. Ber. Bunsenges. Phys. Chem. 1968, 72,263.
Kinetics and Equilibria of Iron-Methylene Blue
(vol/vol) aqueous CH3CN 0.01 N in acid was documented in the work reported here by observing a shift in ,A, of of the ferric complex from 304 nm (the same as A,, Fe111(H20)63+ in water) in sulfate media to 340 nm in chloride media, even with y N [Cl-] = 0.04 M. With p N [Cl-] = 0.4 M, the intensity of absorption at 340 nm increased by 70%. Such replacement of coordinated water by chloride may increase klo by reducing electrostatic repulsion between the ferric complex and MBHB2+and, possibly, by bridging.27 The smaller accelerations of reactions 6, disproportionation of semimethylene blue, and reaction -6, its reversal, by chloride may possibly be a consequence of bridging.27 For klO,k+, and K6, increasing chloride concentration increases the magnitude of the constant while increasing bisulfate-sulfate decreases it. In contrast, 2ks is similarly influenced (increased) by increase in the concentration of either salt. Effects of Variation of p H on kl0, k+, and 2k6 for Methylene Blue in 50% (uol/vol) Aqueous CH&N with Cl- as the Anion (Figures 2 and 3, Table IV). Values of klo and kb in 50% (vol/vol) aqueous CH3CNwith y = 0.4 M, chloride as the anion, and -log [H+]varying from 1.0 to 3.0 are summarized for methylene blue in Figures 2 and 3. Similar information is given for 2k6 in Table IV. In water, pKal and pKaz of MBH32+are 4.5 and 5.9, respectively.28 The values of these quantities in 50% (vol/vol) aqueous CH,CN are not known but the observed adherence of P‘MB,Lto eq 14 through -log [H+] = 3.6 assures that pKa1of MBH?+ in this solvent is >3.6. The M decrease in klo with decreasing [H+] at [H+] C which is illustrated in Figure 2 cannot, therefore, be ascribed to variation in the state of protonation of leucoto methylene blue. I.e., as [H+] decreases from M, the fraction of leuco dye present as MBH2+increases but remains less than 0.5. Under these circumstances, even if MBH2+were much less reactive than MBH?+, increase in the fraction of leuco dye present as MBH2+could not account for decrease in klo by at least 90%. Furthermore, it is probable that MBH2+is a more reactive reductant than is M B H P . Similarly, pKal and pKazof Fem(HzO),3+ in water solution are 3.05 and 3.26, respe~tively,~~ but neither they nor the acid dissociation constants of Fe”‘(H20)6-nCln(3-n)+ species are known in 50% (vol/vol) aqueous CH3CN. Although it is possible that protonic equilibria of the labile complexes of iron play some role in the variation of klo with [H+], these processes are ~
~~
(26) Rabinowitch, E.; Stockmayer, W. H. J.Am. Chem. SOC. 1942,64, 335. (27) Haim, A. Acc. Chem. Res. 1975,8, 264. (28) Clark, W. M.; Cohen,B.; Gibbs, H. D. Public Health Rep. 1925, 23., 1131. (29) Cotton, F. A.; Wilkinson, G. “Advanced Inorganic Chemistry”; Wiley: New York, 1972; 3rd ed, p 863. ~~
The Journal of Physlcal Chemistry, Vol. 85,No. 11, 198 1
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probably not responsible for the maximum observed with [H+] 0.01 M since deprotonation of the ferric complex with decreasing [H+]could only decrease klo. The maxis, imum in klo for methylene blue around [H+] = however, consistent with a two-step mechanism for reaction of ferric ion with leucomethylene blue like that indicated by eq 7 and 8. This mechanism is shown for the case of mixed hydrate-chloride complexes in eq $5 and 16.
__ K7
+ MBHB2+ Fe111(H20)6-nCln(3-n)+ ‘‘complex”(5-n-r)+ + rH+ (15)
ks
“c~mplex”(~-~-‘)+ Fer1(H20)6-mClm(2-m)+ 4MBH+. (n - m)CF + (2 - r)H+ (16)
+
The product of association of Fer1’(H20)6-nCln(3-n)+ with MBHQB+ is expected to be more acidic than either of its components because of its high positive charge. The increase in klo (= K,k8) as [H+] decreases from lo-’ to can be ascribed to shift of the equilibrium of reaction 16 to the right with decreasing [H+]. The decrease in klo as can be ascribed [H+] decreases further from loF2to to decrease in k8 as r, the number of protons dissociated from the “complex”, increases. The data of Table IV reveal a maximum in 2k6 with -log [H+] at or near 1.21. This behavior is consistent with a pKal of MBH?+. = 1.15 in 50% (vol/vol) aqueous CH&N with y = 0.4 M and chloride as the anion which we have reported21if the rate constant for oxidation of MBH’. by MBH2”+.is greater than the rate constants for either “homogeneous” disproportionation, Le., reaction of two MBH?+. or two MBH+. ions. The increase in k4 with decreasing [H+]shown in Figure 3 is consistent with the reasonable assumption that the rate constants for oxidation of leucomethylene blue by MB+ decrease in the order k4(mH) > k+(mH +) > k+(m As indicated above, values of pKal and pka2of MB27’ are not known in 50% (vol/vol) aqueous CH3CN but pKal is probably more than 3.6. As also noted above, we have determined that pKal of semimethylene blue, MBH22+., is 1.15 in 50% (vol/vol) aqueous CH3CN,considerably less than its value of 1.8 in 5% ethanol-95% water.21 By analogy, it can be expected that pKal of leucomethylene blue, MBH?’, is also significantly smaller in 50% (vol/vol) aqueous CH3CN than in water where its value is 4.5.12 Thus, for MBHC+ in 50% AN, it can be estimated that 3.6 C pKal C 4.5, reasonably consistent with the variation of k, with -log [H+] recorded in Figure 3. Acknowledgment. This work was supported by D.O.E. contract EY-764-02-2889and, in part, by a grant to David W. Hay from the Alumni Fund of the College of Liberal Arts of Boston University. We thank Dr. T. Ohno for many helpful discussions.
