2222
J. Phys. Chem. 1981, 85, 2222-2226
bond, which nearly corresponds to the value of AVO,* in the micelle. In nonmicellar systems, the small negative values of AVO,* for hydrolysis of PNPA and P N P P are explained by Neuman et al.29to be the result of a balance between the negative volume change for the bimolecular reaction and the positive volume change for the difference of the solvation effect between reactants and the activation
complex. The difference in AVO,* between the more negative value of -10 cm3/mol for trimethylacetateZ9and the less negative value of -3 cm3/mol for PNPV in this work showing the same hydrophobic effect would not come from the hydrophobic (hydration) effect but from the steric effect, so that the reason for the steric effect on the AVO,* values will be clarified by further study of the steric effect on the base hydrolysis reactions.
Electron-Transfer Quenching of Excited Ru( bpy),2+ Ions Adsorbed on Ion-Exchange Surfaces Anny Slama-Schwok, Yehuda Feltelson, and Joseph Rabani" Energy Research Center and the Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem, 9 1000, Israel (Received: December 9, 1980: In Final Form: March 27, 198 1)
Both the decay of the 3CT Ru(bpy)$+ emission at 600 nm and the bleaching of Ru(bpy)$+ ground-state optical absorption have been investigated. The ionic species were adsorbed on a negative ion exchange resin (Sephadex ~~+ SP-C50) which is a tridimensional dextran gel with sulfopropyl groups attached. The 3CT R ~ ( b p y ) ions are quenched by Cu2+,Fe3+,ferric nitrilotriacetate (FeNTA), 02,benzoquinone, and Cr(acacI3 in a dynamic process. Except for 0 2 and benzoquinone, electron-transfer products are produced with quantum yields ranging from 0.1 for FeNTA and C r ( a ~ a cto ) ~0.68 for Cu2+and 1.0 for Fe3+. The rates of quenching for the positive ions, based on the total volume of the swollen resin, are higher by about one order of magnitude as compared to the appropriate rates in water. Rates of quenching by the uncharged species seem to be comparable to the rates in polyelectrolyte solutions. The reversed electron-transfer reactions were also measured for Fe3+,Cu2+, and FeNTA. In all three cases, the back-reaction is very fast, obeying fairly closely a second-order rate law (deviations indicating a possible first-order contribution are observed after >70% have reacted back). Reaction rate constants are 3.4 X los, 4.1 X lo9,and 4 X lo9M-' s- for Fe3+,Cu2+,and iron(II1) nitrilotriacetate,respectively. At sufficiently high coverage by Ru(bpy)p and sufficiently high laser pulse intensities,triplet-triplet annihilation was observed for 3CT R ~ ( b p y ) ~similarly ~+, to the effect measured previously in micelle solutions and in a polyelectrolyte solution. Unlike the complicated nature of the enhanced decay of 3CT R ~ ( b p y ) , ~in+ the polyelectrolyte,the enhanced decay in the ion-exchange resin obeys a simple second-order rate law. The results are discussed in the light of previous work in water, polyelectrolytes, and an ion exchanger.
'
Introduction The so-called triplet state of tris(2,2'-bipyridine)ruthenium(I1) ions (3CTRu(bpy)z+) has been extensively investigated in the last decade.'+ Electron-transfer reactions from 3CT Ru(bpy)3z+to various ions, including Fe3+ the effects of and Cu2+,have been s t ~ d i e d . ~ -Recently, '~ p~lyelectrolytes'~-~~ and ion-exchange resin16 on several (1) F. E. lytle and D. M. Mercules, J . Am. Chem. SOC.,91,253 (1969). (2) D. M. Klassen and G. A. Crosby, J . Chem. Phys., 49, 1853 (1968). (3) J. N. Demas and A. U. Adamson, J . Am. Chem. SOC.,93, 1800 (1971). (4) F. Bolleta, M. Maestri, and L. Moggi, J . Phys. Chem., 77, 861 (1973). (5) J. N. Demas and A. W. Adamson, J . Am. Chem. Soc., 95, 5159 (1973). (6) G. Navon and N. Sutin, Inorg. Chem., 13, 2159 (1974). (7) H. D. Gafney and A. W. Adamson, J. Am. Chem. SOC.,94, 8238 (1972). (8)G. S. Lawrence and U. Balzani, Inorg. Chem., 13, 2976 (1974). (9) C. R. Bock, T. S. Meyer, and D. G. Whitten, J . Am. Chem. SOC., 96,4710 (1974). (10) C. T. Lin and N. Sutin, J. Phys. Chem., 80, 97 (1976). (11) C. T. Lin and N. Sutin, J . Am. Chem. SOC.,97, 3543 (1975). (12) J. N. Demas, J. W. Addington, S. H. Peterson, and E. W. Harris, J . Phys. Chem., 81, 1039 (1977). (13) D. Meisel and M. S. Matheson, J . Am. Chem. Soc., 99, 6577 (1977). ' (14) D. Meisel, J. &bani, D. Meyerstein, and M. S. Matheson, J. Phys. Chem., 82, 985 (1978). (15) D. Meyerstein, J. Rabani, D. Meisel, and M. S. Matheson, J. Phys. Chem., 82, 1879 (1978). 0022-3654/81/2085-2222$O1.25/0
3CT Ru(bpy),2+ reactions have been investigated. The triplet-triplet annihilation of 3CT Ru(bpy),2+ has been reported to be observed in both micelles17 and polyelectrolytels aqueous solutions. The absorption spectrum of R ~ ( b p y ) , ~the + , relatively ps), and its highly long lifetime of 3CT R ~ ( b p y ) , ~(-0.6 + negative redox potential made the Ru(bpy)gP+ a popular compound for the investigation of photochemical conversion and storage of solar energy. Recently, water splitting to hydrogen and oxygen using Ru(bpy),2+systems has been r e p ~ r t e d . ' ~ Among the major obstactles in the way of photochemical storage of solar energy are the following: (a) the short lifetime of most excited states; (b) the inability, in most cases, to achieve an efficient redox reaction without losing too much of the excess free energy which was obtained by the primary photolytic process; and (c) the so-called "back-reactions'' in which the ground-state reactants are regenerated from photochemical transients. Environmental effects of polyelectrolytes and micelles have been (16) A. T. Thornton and G. S. Lawrence, J . Chem. SOC.,Chem. Commun., 408 (1978). (17) U. Lachish, M. Ottolenghi, and J. Rabani, J. Am. Chem. SOC., 99, 8062 (1977). (18) S. Kelder and J. Rabani, J . Phys. Chem., submitted for publica-
tion. (19) K. Kalyanasundaram and M. Gratzel, Angew. Chem., 41, 759 (1979).
0 1981 American Chemical Society
The Journal of Physical Chemistry, Vol. 85, No. 15, 1981 2223
Electron Transfer Quenching of Excited Ru(bpy),*+ Ions
observed on both the rates and yields14of photoelectrontransfer reactions as well as on the charge separation and rates of b a c k - r e a c t i ~ n s .Ion-exchange ~~~~~ resins are also expected to affect the rate of photoelectron transfer and of the back-reactions between ionic products. In a preliminary report16 it has been reported that dynamic photoreduction took place between materials adsorbed at the surface of ion-exchange resin. The purpose of the present study is to report extended quantitative work on reactions of ,CT Ru(bpy),2+ in a highly swollen resin. These results may be useful in the assessments of various alternatives (polyelectrolytes, micelles, vesicles, microemulsions) as means of affecting yields and rates of photochemical redox reactions.
Experimental Section The laser excitation and detection systems have been previously describedaZ1The dye laser, Molectron DL-200, with a half-width pulse of 10 ns was used at 421 or 457 nm. The exciting light intensity was determined from the bleaching of the Ru(bpy),2+ absorbance at 480 nm in a solution containing R ~ ( b p y ) , ~(usually + 4X M) or Ru(bpy)Ba+and 10” M Fe(C104)3.In both cases the results were identical, as the quantum yield for electron transfer from the excited ,CT R ~ ( b p y ) to ~ ~Fe3+ + is unity, and neither ,CT R ~ ( b p y ) nor ~ ~ the + product Ru(bpy),,+ significantly absorb light at 480 nm. Due to light scattering by the ion-exchange resin, which was kept and used as an aqueous slurry, the geometry of both the laser and the analyzing light beams in the presence of the resin was different from that in its absence. Therefore, comparison of the bleaching of Ru(bpy)gl+upon excitation in water to the bleaching in the resin can be used for the calculation of quantum yields only in relatively high concentrations of Ru(bpy)2+, where the scattering of the laser light is small in comparison with the absorption by R ~ ( b p y ) , ~ +A. short light path, 1 mm, was used for these measurements. Under such conditions we determined the quantum yield of electron transfer in a resin containing R ~ ( b p y ) , ~and + Fe3+ (perchlorate) as 1.0. In all other experiments with the resin, actinometry was carried out by using resin solutions containing Ru(bpy);+ at the same concentrations as the test solutions, in the presence of absorbed Fe3+ (used as perchlorate, at pH 2). The light scattering by the resin considerably decreased the intensity of the analyzing light that reached the photomultiplier. This was compensated for by using a large load resistor (e.g., 20 kQ) which resulted in poor time resolution when the bleaching was measured. This did not affect the time resolution when emission was measured. In addition, the overlap between the exciting light and the analytical light was tested for every set of experimental runs. This was carried out by measuring the effect of an aperture between the lamp and the cell on the optical density change observed in the test solution as a result of a laser pulse. Changing the aperture so that the analyzing light intensity varied -5-fold did not have any effect on the absorbance change, as might be expected when there was good overlap. Note that, unlike in the earlier work,21 the laser light and the analyzing light were in opposite directions, in order to reduce the effects of “scattered” laser light. The ion-exchange resin, SP-C50 Sephadex, is a poly(sulfopropyl) dextran cation exchanger. It is stable in the pH range above pH 2 and has a low degree of cross linking. ~
~
~~
~~
~~
~
(20) R. E. Sassoon and J. Rabani, J. Phys. Chem., 84, 1319 (1980). (21) D. J. Lougnot, G. Dolan, and C. R. Goldschmidt, J . Phys. E , 12, 1057 (1979).
Swelling in water increases the volume of the resin by about 30-fold and the resin matrix permits even very large molecules (molecular weight up to 200 000) to gain access to the internal functional groups. The resin was allowed to swell in triply distilled water for at least 48 h at room temperature prior to its use. No additional swelling could be observed after the first 30 h under water. The effective capacity of the ion exchanger was measured with the aid of Cu2+ions. Excess of l M Cu(C104)z was passed through an ion-exchange column. The resin was washed with water and the CuZf removed by Ba2+ (used as perchlorate) and analyzed by atomic absorption. The results showed that 100% coverage of the resin by the Cu2+corresponded to (2.2 f 0.2) X lo-, mol equiv Cu2+ per 1 g of dry resin. This agrees well with a value of 2.3 X mol equiv of active sites per 1g of dry resin, a value that can be calculated from the molecular weight of the repeating group in the polymer. In order to obtain a homogenous distribution of ions in the ion exchanger we added samples that were taken for pulsed laser tests, stock solutions of the ionic materials (Fe3+,Cu2+,and R ~ ( b p y ) , ~ +dropwise ), to stirred swollen resin particles in water. The resultant slurry was mixed and left to equilibrate for 24 h. Ions which remained in solution were washed out. The amount of ions adsorbed was determined as the difference between the amount of ions added and the amount washed out. Ferric perchlorate stock solutions were kept at pH 2 (HC104). R ~ ( b p y ) , ~was + added only after H+ was completely removed from the solution in equilibrium with the resin. Excess H+ interfered with the adsorption of Ru(bPY)?+. In order t o check that 24 h was sufficient to achieve equilibration, we mixed two equal volumes of the swollen resin, one containing Ru(bpy)gS+and the other Cu2+ions. After 24 h the emission of the ,CT R ~ ( b p y ) , ~was + measured with a spectrofluorimeter and it was found that the Cu2+ quenched the emission to the same degree as observed when the two ions were slowly added as described above and produced the same concentrations of adsorbed ions. Similar tests were carried out in the Fe3+-R~(bpy)3z+ system. Unless otherwise stated, O2was eliminated from the ion exchanger by bubbling Nz through the water-resin mixture. O2quenches the emission of ,CT R ~ ( b p y ) , ~so+ that removal of the O2is expected to increase the intensity of the steady-state emission. Indeed, bubbling of the Nz through the water-resin mixture resulted in an increase of the emission intensity by 20% after 20 min and 25% after 40-180 min. This was taken to mean that O2 was removed after 40 min of bubbling which was used as a standard time for all the OZ-freeexperiments. Materials. Ru(bpy),C12 from K & K was recrystallized from triply distilled water. The purified product had an extinction coefficient = 14000 M-l cm-l at 453 nm. C U ( C ~ Oand ~)~ Fe(C104), were from G. Frederick Smith (reagent grade) and from City Chemicals, respectively. Ferric nitrilotriacetate (FeNTA) was prepared as before15 by mixing a ferric perchlorate solution with 5% excess nitrilotriacetic acid (Sigma grade) and titrating the solution with 0.1 N NaOH to pH 2.6. Concentration of Ions Adsorbed on the Resin. The concentration of adsorbed ions was determined from the difference between the concentrations of the stock solutions and the concentrations of the ions which were removed by washing the resin with water. Knowing the weight of the resin in a given test, we were able a obtain
2224
The Journal of Physical Chemistry, Vol. 85, No. 15, 1981
Slama-Schwok et al.
A
0.9
c
0
A
1.0
2.0
[Fe3' 1
1
-I
Flgure 1. Dependence of the bed volume of the resin on the concentration of Fe3+ ions adsorbed. Bed volume in units of mL/20 mg of resin. [Fe3+] in units of mmole/20 mg of resin.
directly the number of moles of ion per standard quantity of the dry resin (20 mg of dry resin was a convenient standard amount). The swelling of the resin depended on the nature and the concentration of ions used. To convert moles per 20 mg to molar units, we measured the volume of the swollen resin for each experiment and assumed a close packing of the resin grains that contributed to 2/3 of the volume, the rest was water which filled the space between the resin grains. (The resin-water mixture was allowed to settle for a t least 30 min before any measurements were carried out). In Figure 1 we demonstrate the effect of Fe3+ions on the swelling properties of the resin. No effect of [Fe3+]on the volume of the swollen resin is observed above [Fe3+] = 0.6 ~ m o l / 2 0mg of resin, after an initial decrease of 15% in the polymer volume upon increasing the [Fe3+]from zero to 0.6 pmol/20 mg of resin. Cu2+ions have an insignificant effect on the volume of the swollen resin, when the Cu2+ion concentration was less than mo1/20 mg of resin (9.34 mM). R ~ ( b p y ) , ~ + , under our conditions, had no effect on the swelling.
time(ns)
Flgure 2. Typical oscilloscope traces and kinetic plots for the decay of 3CT Ru(bpy),*+ emission. The effect of Fe3+ions. Excitation at 421 nm, emission (in arbitrary units) at 600 nm. The straight lines on the left drawn on a semilogarithmic scale: R~(bpy),~+= 4.2 X lom5M; (A)no Fe3+ (lower trace); (0)2 mM Fe3+ (upper trace).
TABLE I: Quenching Rate Constants of the Emission of 'CT Ru(bpy),2' by Various Quenchers ( Q ) in Sephadex SP-C 50
Q
[&I, mM
~(C10,)Z Fe(ClO,), (adsorbed at pH-2 (HCIO,)) FeNTA (pH 2.65) 0, BQ Cr(acac),
h,,= M-' s-'
2.0-4.7 0.3-2.8
(2.81: 0.3)X (2.5 f 0.2) x
lo8 lo9
2.00-4.0 0.25-1.25 1-4
(1.11: 0.3) X (1.2 i 0.11 X
lo9 lo9
0.25-0.8
(5.0 1: 0.5)X
(2.6 1: 0 . 2 j x 109
lo9
a Average of 4-7 runs at various concentrations. Calculated from the quenching kinetics of the 3CT Ru(bpy),2* emission at 600 nm.
TABLE 11: Comparison between Quenching Rate Constants in Various Media
Results and Discussion When 3.8 X M deaerated Ru(bpy),2+ (-0.2% coverage) was excited in the resin (Aex = 421 nm) an emission peaking at 600 nm was observed (reaction l), using either 3CT Ru(bpy),2+-,Ru(bpy)?+ + hv (1) laser or steady-state illumination. The decay of the emission following a laser pulse obeyed a first-order rate law, kl = (1.54 f 0.10) X lo6 s-l, as compared with kl = (1.67 f 0.10) X lo6 s-l in water (no resin). Thus, the emission spectrum and the decay kinetics of 3CT Ru( b ~ y ) in ~ ~the + resin were the same as in water and in polyelectrolyte solution.18 Upon addition of quenchers, the lifetime of the 3CT R ~ ( b p y ) became ~ ~ + shorter, without affecting the first-order nature of the decay kinetics. The quenchers employed in this work include Fe3+,Cu2+ (asperchlorates), FeNTA, 02,benzoquinone, and Cr(acacI3. For Fe3+, Cu2+,and 02,the dye laser was used for excitation a t 421 nm. In the presence of FeNTA, benzoquinone, or Cr(acacI3 the dye laser was used a t 457 nm, to minimize the absorption of the excitation light by the quenchers. In the presence of a quencher, Q, competition between reaction 1 and the quenching of 3CT Ru(bpyIs2+ takes place. Quenching rates by all the quenchers were found to be pseudo-first order with rates proportional to [Q]. This was measured by the effect of the quencher on the rate of the decay of the emission of 3CT R ~ ( b p y )at ~ ~600 + nm. The effects of Q on the emission does not provide information on the nature of the quenching reaction, namely, whether it is an electron-transfer or an energy-
1
200 600 1000
Q 0
2
FeNTA BQ Cr(acac), cU(cio,), Fe(C10,)
k,(water) 3.7 x 3.7 x
h,(PVS)
io9=
2.7 x i o g a 2.0 x l o g a
6.2 x 109 b 6.6 X l o 9
4.2 x 4.3 X
3.3 x 2.1 x
1x
lose
2.2 x
io9 b lo9 io7
h,(ion exchanger) 1.2 x 1 0 9 b 1.1x l o 9 2.6 x i o 9 b 3.9 X l o 9 2.8 x i o 8 2.5 x l o 9
a Reference 13. This work. Essentially the same values were obtained whenever measured both by the pulsed laser method and by the steady-state (spectrofluorimeter) technique. Reference 15. Reference 13. e Reference 1 4 , measured at pH = 2.5. Reference 14, valid for low [Fe3+],taking into account that both reactants are confined to the polymer volume.
transfer reaction. However, the addition of quenchers which affect the rate of decay of emission but not its initial intensity shows that the quenching is a dynamic effect, in agreement with previous conclusions.16 In Table I we present quenching rate constants for reaction 2: 3CT Ru(bpy)gz++ Q
-
R ~ ( b p y )+ ~ ~Q+
(2a)
or 3CT R ~ ( b p y ) 3 ~++Q
Ru(bpy)S2++ Q
(2b) In Figure 2 we present typical oscilloscope traces and pseudo-first-order plots for the calculation of k 2 values which are reported in Table I. Comparison of the quenching rate constants which we measured in the ion exchanger with values obtained in water and in a poly(vinylsulfate) (PVS) solution shows that the rates for the +
Electron Transfer Quenching of Excited Ru(bpy),'+
Ions
The Journal of Physical Chemistry, Vol. 85,No. 15, 1981 2225
TABLE 111: Quantum Yields of Electron Transfer to Q neutral quenchers are slowest in the ion-exchange systems at Various Concentrationsa (Table 11). The effect of PVS on decreasing the reactivity of 3CT Ru(bpy)gP+ ions toward neutral quenchers was Q concn, mM @etb a t t r i b ~ t e d to ' ~ the fact that the 3CT R ~ ( b p y ) ions ~ ~ +are cu2 5.0-17 0.68 k 0.07 confined to the polymer volume and cannot diffuse to the Fe3t 0.3-3.0 1.03 * 0.15 bulk. The diffusion rate of the neutral quencher to the FeNTA 1.0-4.0 0.15 t 0.05 polymer containing 3CT Ru(bpy)gP+determines the rate
