J. Phys. Chem. 1981, 85, 810-813
810
Kinetics of Photobleaching Recovery in the Iron(11)-Thionine System P. V.
Kamat,t M. D. Karkhanavala,* and
P. N. Moorthy'
Chemistty Division, Bhabha Atomic Research Centre, Bombay 400 085, Indla (Received: July 18, 1980; I n Final Form: November 3, 1980)
Employing a cross-beam illumination kinetic spectrophotometry technique, we studied the oxidation of leucothionineby Fe(II1) in an aqueous medium. The specific rate of oxidation of leucothionine by Fe(III), monitored as photobleaching recovery, was found to follow pseudo-first-order kinetics. This specific rate was found to attain a limiting value with increasing Fe(II1). The influence of varying concentrations of Fe(I1) and TH+ was found to be marginal. The observed results supported the approach of complex formation between leucothionine and Fe(II1). The activation energy for the oxidation of leucothionine by Fe(II1) was 69 kJ mol-'.
Introduction diation of the sample was uniform, and the intensity of the photolyzing light (400-800 nm) at the cell position was 190 The iron(I1)-thionine photoreversible redox system is mW cm-2 as measured by a calibrated Epley thermopile, of considerable importance because of its potential usethe output of which was read on a Philips GM 6020 mifulness in photogalvanic cells for conversion of visible light crovoltmeter. The analyzing light which traversed a into electrical Since its d i s ~ o v e r y many ~,~ pathlength of 0.44 cm in the cell was too weak to cause any workers have studied the kinetics of this system both by photobleaching. This was verified by checking the optical steady-state illumination7-" and flash photolysis12-18 density of the sample at 600 nm with that for no incident techniques. Hardwick7assumed semithionine as the main photolyzing light. The photolyzing light intensity was reduced species in the photobleached solution. However, varied by varying the operating voltage of halogen lamp. it is now e s t a b l i ~ h e d ~ J ~that - ' ~ leucothionine which is From the intensity profile monitored at these operating formed by the dismutation of semithionine is the prevoltages it was confirmed that the ratio of the thionine dominant photoproduct. Ainsworthgand Haveman et al.'O absorption band (560-610 nm) to the total intensity reproposed that the photoreduction process involved a Femained constant (17.2 f 0.2%). Other experimental de(11)-thionine complex (2TH+.FeC12.2HC1) in an HC1 metails are given el~ewhere.~ dium. Haveman et al." also attempted to explain the Thionine was purified by reprecipitation from an first-order recovery from photobleaching on the basis of aqueous solution. Ferrous sulfate, ferric sulfate, and other complex formation between leucothionine and Fe(II1). chemicals were analytical reagents. All the measurements Hatchard and Parker,12who established a detailed mechwere carried out in an aqueous medium with S0?-/HSO4anism for the photoreduction of thionine by Fe(I1) ions, anion at constant pH and ionic strength. Solutions were observed that the reaction between leucothionine and made with triple distilled water. Before carrying out any Fe(II1) ions is pseudo-first order at high Fe(II1) concenexperiment the solution was purged with oxygen-free argon trations. However, in contrast to the conclusion of gas and an inert atmosphere was maintained above the Haveman et al." they concluded that, at equimolar consolution. centrations of Fe(II1) and leucothionine, second-order kinetics were applicable. Lichtin and his c o - ~ o r k e r s ~ ~ - ~ " have interpreted the photogalvanic effect on the basis of the kinetic results. In a partially bleached concentrated (1)W. D. K. Clark and J. A. Eckert, Solar Energy, 17, 147 (1975). (2)E. Rabinowitch, J. Chem. Phys., 8, 551,560 (1940). solution of thionine in 50% (vol/vol) acetonitrile they (3)Y.Suda, Y.Shimoura, T. Sakata, and H. Tasubomura, J. Phys. found14the recovery from photobleaching to be first order. Chem., 82,268 (1978). Based on their flash photolysis studies in aqueous aceto(4)P. V. Kamat, "Photoelectrochemical Study of Dye-Redox and Semiconductor-RedoxSy&ms", Ph.D. Thesis, Bombay University, 1978. nitrile medium Osif et al.17 have proposed complex for(5)K. Weber, 2.Phys. Chem., B15, 18 (1931). mation between leucothionine and Fe(II1) ions. (6)J. Weiss, Nature (London),136, 794 (1935). In view of these divergent interpretations, we have (7)J. Hardwick, J. Am. Chem. Soc., 80,5667 (1958). carried out a detailed study of the Fe(I1)-thionine system. (8)J. Schlag, 2.Phys. Chem. (Frankfurt am Main), 20, 53 (1959). The dependence of photogalvanic effect on the extent of (9)S.Ainsworth, J . Phys. Chem., 64,715 (1960). (10)R.Haveman and H. Pietsch, 2.Phys. (Leipzig),208,98 (1957). steady-state photobleaching has been discussed earlier.lg (11)R. Haveman and K. G. Reimer, 2.Phys. Chem. (Leipzig),211,26 The results on the kinetics of the reaction between Fe(II1) (1959). and leucothionine which partly governs the steady-state (12)C.G.Hatchard and C. A. Parker, Trans. Faraday Soc., 54,1093 photobleaching are presented in this paper. (1961). (13)S.N. Guha, P. N. Moorthy, and K. N. Rao, Mol. Photochem., 9, Experimental Section 183 (1979). (14)P. D. Wildes, N. N. Lichtin, and M. Z. Hoffman in "Solar The experimental setup employed for illumination and Energy", J. B. Berkowitz and 1. A. Lesk, Ed., The Electrochemicd-Sokinetic spectrophotometry is shown in Figure la. The ciety, Princeton, NJ, 1976,pp 128-138. sample cell consisted of 5.0 X 1.0 X 0.44 cm quartz rec(15)P. D. Wildes, N. N. Lichtin, M. Z. Hoffman, L. Andrews, and H. Linschitz, Photochem. Photobiol., 25,21 (1977). tangular cuvette fitted with a Pyrex jacket having quartz (16)P. D. Wildes and N. N. Lichtin, J. Phys. Chem., 82,981 (1978). windows (Figure lb). This design permitted circulation (17)T. L. Osif, N. N. Lichtin, and M. Z. Hoffman, J.Phys. Chem., 82, of water from a thermostat through the cell jacket. Irra1778 (1978). 'Department of Chemistry, Boston University, Boston, MA 02215. t Deceased Nov 17, 1979. 0022-365418112085-0810$01.25/0
(18)M. D. Archer, M. I. C. Ferreira, G. Porter, and C. J. Tredwell, Nouu. J. Chem., 1, 9 (1977). (19)P. V. Kamat, M. D. Karkhanavala, and P. N. Moorthy, Ind. J. Chem., 184,206 (1979).
0 1981 American Chemical Society
Kinetics of the Fe(I1)-Thionine
System
The Journal of Physical Chemistry, Vole85, No. 7, 198 1 811
Light absorption at 600 nm was monitored with an RCA 1P28 photomultiplier. At this wavelength thionine is the only light-absorbing species in the system. A typical decay curve (Figure 2) shows the variation of the photomultiplier output with time-before, during, and after stopping the illumination. The concentration of the reduced dye was calculated from the steady signal observed during steady illumination. The overall order of the photobleaching recovery was determined graphically by fitting the data read from the decay curve to different rate equations as shown in Figure 3. A good first-order fit was obtained under all conditions.
Results and Discussion From the known standard potentials of the relevant redox couples it has been shown2 that in the absence of light the equilibrium TH+ + 2Fe(II)
hv + 3H+ 5 TH42++ 2Fe(III)
TABLE I: Effect of Different Parameters Influencing the Photobleaching Recovery in the Fe*+,-TH+ Systema
0.35 0.55 1 .oo
6.50 10.90 16.00 35.00 68.00 200.00 PHd
2.8 2.3 1.8 1.6
mol dm-
0.0500 0.0530 0.0575 0.2000 0.6900 2.0000 11.4000 14.2000 16.0000
4.60 4.80 4.95 4.90 3.20 0.97 0.35 0.30 0.28
0.1570 0.0658 0.0345 0.0300
4.98 4.96 4.20 3.60
0.0970 0.0640 0.0480 0.0460
2.00 4.80 3.60 3.50
0.0540 0.0650 0.0601 0.0710
1.00 2.50 7.25 9.40
0.0587 0.0552 0.0560 0.0575
3.30 4.45 4.90 4.95
mol dm-3)c
0.25 1.03 7.58 13.80 thionine 1 .oo
mol dm-3)d
2.5 7.5 10.0
TH+
10 [ L i,,,p
Fe(II1) (lo+ mol dm-3)c
Fe(I1)
lies far to the left. When the necessary free energy deficit is supplied via the photoexcitation of thionine (TH+),the
h , s-l
varying parameter
incident light intensity (mW c m - 2 y
25 50 107 130 temperature ( K)d TH~"
latter is reduced to semithionine (TH2+),the metastable species, which undergoes dismutation to form leucothionine (THd2+).The main reactions governing the photobleaching and its recovery are14-.17 TH+
hv
1TH+*
H+
3TH22+*
3TH22+*---* TH+ + H+
--
3TH2+*+ Fe(I1) 2TH2+ + H+
+ TH42++ Fe(II1)
-
TH2+ Fe(II1)
?H2+ + Fe(II1)
TH42++ TH+
(1)
309 318 3 28 338
0.0145 4.90 0.275 4.35 0.640 3.50 1.830 1.75 The experimental conditions were 5 x mol dm'3 Fe(II), mol dm-3 Fe(III), 5 x mol dm-3 TH', pH 2 at incident light intensity 190 mW cmT2and temperature 298 K. For evaluating the effect of each component five of these were kept constant a t the level indicated
(2)
while one parameter was varied. [:LIS, = steady-state p = 0.3. fi = 0.05. leucothionine concentration.
(3)
If the recovery of photobleachin,g was via the slow reaction step ( 6 ) followed by the fast dismutation reaction (4),it should be kinetically second order when [Fe(III)] = [THt+]. However, experimentally it was observed to be first order over the entire range of concentrations of Fe(II1) and T H t + thus implying this mechanism is incorrect. The first-order kinetics can be satisfactorily explained on the basis of complex formation between THd2+and Fe(II1). Consider the equilibrium TH:+.Fe(III) + TH42++ Fe(II1) (7)
(4)
TH+ + Fe(I1) + H+
(5)
TH2++ Fe(I1) + 2H+
(6)
From the absorbance measurements made at the semithionine absorption maximum (780 nm13) in the same apparatus (an RCA 7102 photomultiplier tube was used for this measurement), the semithionine concentration under steady illumination was found to be about lo-" mol dm-3 in a completely bleached solution of 5 X 10" mol dm-3 TH+, 5 X mol dm-3 Fe(II), lod mol dm-3 Fe(III), and 5X mol dm-3 H2S04. The low concentration of TH2+ is due to the high rate of dismutation to form TH+ and TH,2+ (h4 = 2.9 X lo9 dm3 mol-' s-l in an aqueous H2S04 system20). Hence the predominant reduced species in the photobleached solution is leucothionine which on stopping the illumination is oxidized by Fe(II1) to form qHz+. The oxidation of TH2+to TH+ is mainly by reaction 4 as reaction 5 is very slow (h5 = 1.8 X lo4 M-l s-l ).l7 (20) P.D.Wildes, N. N. Lichtin, and M. 2.Hoffman, J. Am. Chem. SOC.,97, 2288 (1975).
-
followed by reactions 8 and 4
+
+
TH42+*Fe(III) TH2+ Fe(I1) 2H+ (8) If Bo and 8 are the thionine conce:ntrations initially and in the bleached solution, the total concentration of leucothionine [L] would be (8, - 6) so that [L] = [TH42+l+ [C] where the term [C] refers to the concentration of TH42+.Fe(III). As the steady-state semithionine concentration is negligibly small, i.e., [TH2+]
