2710
J. Phys. Chem. lB82, 86, 2710-2714
Light-Driven Electron Transfer from Tetrathiafulvaiene to Porphyrins and Ru(bpy):+. Charge Separation by Organized Assemblies Carole K. Gratzel" and Michael Gratzel Instnut de Chlmie Physique, Ecole Polytechnique FMrale, I n Final Form: March 5, 1982)
CH-10 15 Lausanne, Swlt2erlend (Received: December 15, 1981;
The photoinduced electron transfer from tetrathiafulvalene ('M'F) to ZnTPPS' and Ru(bpy)gO+was investigated both in homogeneous media and in CTAC micellar and microemulsion solutions. It was found that electron transfer to Ru(bpy)3z+from ?TF in methanol occurs readily while charge transfer in the ZnTPPS'/TTF system in alcoholic solution is inhibited. This lack of free ion production in the latter system is attributed to a small thermodynamic driving force for the redox reaction as well as to Coulombic effects. Efficient charge separation is, however, achieved for the ZnTPPS'ITTF system when the experiments are performed in micellar and/or microemulsion systems. Production of the ZnTPPS" anion and TTF+ radical cation is observed due to efficient charge ejection of the TTF+ from the assemblies' interiors, thus thwarting rapid back-reaction. The ZnTPPS" anion, observed at 910 nm in micellar solution, has a long decay time of 1.6 ms with a second-order rate of disappearance, kz N 5 X lo7 M-'s-'. While cationic microemulsions afford light-induced charge separation also, the rate of the back-reaction of the anion is 30 times faster than in micellar solution, thus approaching the diffusion-controlled limit. This shorter lifetime is attributed to the relatively low surface charge density of the microemulsion droplets. The study of the one-electron oxidation of TTF to its radical cation, TTF+, is an important contribution to the understanding of its participation in the conductance of organic metals such as TTF+-TCNQ-.
Introduction Many photoinduced electron-transfer reactions of Ru(bpy)32+and various metallated porphyrins have been investigated previously; however, most investigations have been of the oxidative quenching reactions of the excited sensitizers to form the cations Ru(bpy)$+ and porphyrin(+) and the corresponding reduced Only a few of these experimental studies have involved the formation of the cation Ru(bpy),+, a very powerful reductant,6-'0 and even fewer have been concerned with the formation of the reduced p~rphyrin.'Jl-~~ In general, when isolable, only the doubly reduced product, e.g., the chlorin species, is found and no intermediate radicals can be detected. Thus, although the existence of the zinc tetrakis(p-sulfonatopheny1)porphyrin anion (ZnTPPS") has been postulated,' its absorption spectrum has not previously been characterized. Since the cation Ru(bpy)$+ and monooxidized porphyrin ZnTPPS3-, and the cation Ru(bpy)3+and radical anion ZnTPPS" (the ligand carries the negative charge) are powerful oxidants and reductants, respectively (see Table I), they have often been eluded to (1)K. Kalyanasundaram and M. Griitzel, Helu. Chim. Acta, 63, 478 (1980). (2)N. Sutin, J.Photochem., 10, 19 (1979). (3)D. G. Whitten, Rev. Chem. Zntermed., 2, 107 (1978). (4)H. D.Gafney and A. W. Adamson, J. Am. Chem. SOC.,94,8238 (1972). (5)P.-A. Brugger and M. Gritzel, J. Am. Chem. Soc., 102,2461(1980). (6)M. Maestri and M. Gritzel, Ber. Bunsenges. Phys. Chem., 81,504 (1977). (7)C. Creutz and N. Sutin, Znorg. Chem., 15,496(1976). (8)C. Creutz and N. Sutin, J. Am. Chem. SOC.,98,6384 (1976). (9)P. J. DeLaive, T. K. Foreman, C. Giannotti, and D. G. Whitten, J. Am. Chem. SOC.,102,5627(1980). (10)C. P. Anderson, D. J. Salmon, T. J. Meyer, and R. C. Young, J. Am. Chem. SOC.,99,1980 (1977). (11)D. G. Whitten, P. J. DeLaive, T. K. Foreman, J. A. Mercer-Smith, R. H. Schmehl, and C. Giannotti in 'Solar Energy: Chemical Conversion and Storage", R. R. Hautala, R. B. King, and C. Kutal, Ed., Humana Press, 1979,pp 117-40. (12)G. R.Seely and M. Calvin, J. Chem. Phys., 23, 1068 (1955). (13)P. A. Carapellucciand D. Mauzerall, Ann. N.Y. Acad. Sci., 244. 214 (1975). (14)G. L. Closs and L. E. Closs, J. Am. Chem. SOC.,85,818 (1963). (15)G. R. Seely, Photochem. Photobiol., 27, 639 (1978). (16)M. Neumann-Spallartand K. Kalyanasundaram, 2.Naturforsch. B, 36, 596 (1981). 0022-3654/82/2086-2710$01.25/0
TABLE I: Redox Potentials of Sensitizers and Substrates redox reaction Ru(bpy),P+ t e- + Ru(bpy),l+* Ru(bpy),Z+* t e" -+ Ru(bpy),* Ru(bpy),l+ + Ru(bpy),l+* Z n T P P S - t e- -+ ZnTPPS4-* ZnTPPS4-* t e- -+ ZnTPPSSZnTPPS" + ZnTPPS4'* TTF' + e- -+ TTF
Ea: V
A E , eV
-0.83f t0.84f -0.93c + 0.52c
+ 0.42e,g
+ 2. l b + 1.8d
All redox potentials are relative to the NHE. Charge-transfer excited-state formation. The redox potential for ZnTPPS4- *I5 - has been estimated by adding - 5 0 mV per sulfate group, i.e., -200 mV and the triplet formation energy to the known value for ZnTPP.'I The redox potential f o r ZnTPPS3-'4'* was calculated by using the literature value for the potential of ZnTPPSs-'4and subtracting the known triplet energy from it. Tripletstate formation. e This value is the average of Hunig's Value and the literature value referenced therein. (Dr. Mark Eddowes in our laboratory has measured the same value in CTAB micelles using cyclic voltammetry.) f Reference 7. g Reference 17.
as good candidates for use in the photocatalytic splitting of ~ a t e r . ' , ~ J The ~ J ~zinc tetraphenylporphyrins are especially attractive as sensitizers in this sense due to their ease of synthesis, their relative inexpensiveness, and their broad and strong absorption in the visible domain of the electromagnetic spectrum. Although the present studies are interesting from this point of view, the most noteworthy aspect of this investigation stems from the photoinduced one-electron oxidation of the substrate, tetrathiafulvalene, TTF, to form its radical cation ("semiquinone", SEM ~ t a g e ) . ' ~ ~ ~ ~ ~ ~ ' (17)S.Hhig, G. Kiesslich, H. Quast, and D. Scheutzow, Justus Liebigs Ann. Chem., 310 (1973). (18)K. Kalyanasundaram and M. Gratzel, Angew. Chem., Znt. Ed. Engl. 18, 701 (1979). (19)P. J. DeLaive, B. P. Sullivan,T. J. Meyer, and D. G. Whitten, J . Am. Chem. SOC.,101,4007 (1979). (20)R. Zahrednik, P. Carsky, S.H h i g , G. Kiesslich, and D. Scheutzow, Int. J. Sulfur Chem., C, 6,109(1971). (21)K. Deuchert and S. Hiinig, Angew. Chem.,Znt. Ed. Engl., 17,875 (1978).
0 1982 Amerlcan Chemical Society
Photoinduced Electron-Transfer Reactions
Tetrathiafulvalene and other thioethers have been shown to be capable of producing organic conducting salts22v23 (classified as "organic metals" by P e r l ~ t e i n )in~ ~the presence of the appropriate counterion, e.g., TTF+TCNQ-. The knowledge of T T F s physical properties is important in the understanding of ita donor characteristics in these conducting salts. This study deals with the light-driven electron transfer from TTF to two sensitizers, i.e., R ~ ( b p y ) , ~and + ZnTPPS". An important aspect of this investigation is to map out the influence of molecular assemblies formed by micelles and microemulsions on the dynamics of the formation and yield of redox products. Experimental Section Material Preparation. ZnTPPS", zinc tetrakis(psulfonatopheny1)porphyrinwas prepared and purified as explained previously.' Ru(bpy),ClZfrom Strem Chemical and tetrathiafulvalene ("F) from Fluka AG (puriss) were used as supplied. The surfactant cetyltrimethylammonium chloride (CTAC) was recrystallized twice from purified acetone (95%) and deionized water (5%). The deionized water used in all experiments was refluxed with potassium permanganate and then distilled twice in a quartz still. The 1-pentanol, Fluka puriss PA, used in the preparation of the microemulsion, was distilled once before use. Hexadecane from Fluka (purum) was used as supplied. Organized Assembly Preparation. The pure CTAC microemulsion used in these studies was prepared by mixing (all percentages by weight) water (80%), CTAC (8%), l-pentanol(9%), and hexadecane (3%). The order of mixing was unimportant, and a clear solution resulted within a few minutes of stirring. The donor (TTF) and acceptor (ZnTPPS") solubilized microemulsions used in the laser experiments were prepared as follows. The appropriate quantities of CTAC, 1-pentanol, and pure hexadecane were mixed after which the ZnTPPS" dissolved in H20 was added. This solution was immediately covered to avoid ambient light, and degassing by Ar was started. After approximately 15 min of degassing in the laser cell, a concentrated solution of TTF in hexadecane (also kept in the dark) was added to the mixture while bubbling, and the degassing of the solution continued for another 10 min. Many precautions were taken to avoid the contact of TTF with H 2 0 in the presence of O2 and light which has been found to cause a rapid degradation of the molecule.26 Precautions were likewise taken to avoid the degradation of the zinc tetraphenylporphyrin in solution in the presence of O2 and ambient light. Concentrations of TTF in solution were changed by varying the ratio of pure hexadecane to concentrated stock of TTF in hexadecane. All stock solutions were freshly prepared and their concentrations verified before each group of experiments. The micellar solutions were all prepared by adding 2.5 X M CTAC to a solution of ZnTPPS4- (5 X M) in deionized H20. A concentrated solution of TTF in tetrahydrofuran was slowly injected (microsyringe) after originally degassing the micellar solution and while continuing to degas with mild heat applied to ensure the evaporation of all tetrahydrofuran. Apparatus. A Perkin-Elmer Hitachi 340 absorption spectrometer was used in all concentration determinations. A J K frequency-doubled, 530-nm, 25-1-15 pulse width, neodynium laser was used in all of the flash photolysis experiments with the detection system as described else(22) J. P. Ferraris, T. 0. Poehler, A. N. Bloch, and D. 0. Cowan, Tetrahedron Lett., 27, 2553 (1973). (23) J. B. Torrance, Acc. Chem. Res., 12, 79 (1979). (24) J. H. Perlstein, Angew. Chem., Znt. Ed. Engl., 16, 519 (1977). (25) Y. Tricot, C. Gritzel, and M. Gritzel, unpublished results.
The Journal of Physical Chemistty, Vol. 86, No. 14, 1982 2711 Ru (bipy)j
I
400
500
Cnml
-
600
Figure 1. Difference spectrum taken immediately after laser excitation (A = 530 nm) of Ru(bpy),'+ (lo-' M) and TTF M) in methanol; [,TF+] = 434 and 580 nm. A,[R~(bpy)~+] = 510 nm, XT
where.26 Cutoff filters appropriate for the analyzing wavelengths used were placed in front of the analyzing light and monochromator. Results and Discussion In methanolic solution a discernible quenching of the excited states of R ~ ( b p y ) , ~and + ZnTPPS4- by tetrathiafulvalene was observed. The laser-excited (530 nm) charge-transfer state of the polypyridyl ruthenium complex, Ru(bpy)gP+*,accepted an electron from tetrathiafulvalene to yield its reduced form, Ru(bpy),+ (d6 metal center reduction), and the radical cation of tetrathiafulvalene, TTF+, according to eq 1 and 2. The formation hu
R u ( ~ P Y ) ~+ ' + * tRu3+(bp~)3-1 * R ~ ( b p y )+ ~ ~TTF +
kb
Ru(bpy),+ + TTF+
(1)
(2)
of Ru(bpy),+ and TTF+was substantiated by the transient spectrum taken immediately after laser pulse excitation, cf. Figure 1, between 400 and 600 nm. This difference spectrum (uncorrected for bleaching) exhibits two small peaks corresponding to the tetrathiafulvalene radical cation at 434 nm (e = 1.86 X 104 M-' cm-' in acetonitrile) and 580 nm (e = 5.03 X 102 M-l cm-' in acetonitrile).'" A large peak due to the absorption of Ru(bpy),+ (ligand-to-metal transition) at 510 nm (e = 1.8 X lo4M-' cm-' in methanol) was observed as noted by previous authors.6 A SternVolmer plot, Figure 2, yields a quenching constant for the R ~ ( b p y ) ~emission ~+' at 620 nm of k, = 1.1 X 1O1O M-' s-l. A rapid back-reaction with kb N 8 X lo9 M-' s-l was obtained as expected for homogeneous media. Figure 3 displays the growth and decay of the cation, Ru(bpy),+ (A = 510 nm), and radical cation, TTF+(A = 430 nm, X = 580 nm), obtained upon laser excitation. Experimental studies of the electron-transfer reaction between Ru(bpy)gP+and TTF in organized assemblies were not performed since the partitioning of Ru(bpy)Sa+between the organizate and bulk medium is unknown. Instead, a tetraphenylpophyrin which would reside in a certain foreseen location in the interior or at the interface of the organized assembly was used as an acceptor. Use of ZnTPPS4- in place of Ru(bpy),2+allowed the kinetics to be easily evaluated by known models. Contrary to the R U ( ~ ~ ~ ) ~ ~ system, +/TFF in the ZnTPPS4-/TTF system, although there is triplet quenching of the ZnTPPS4-, there is negligible or no product formation observed. Thus, even 10 ps after the (26) G. Rothenberger, P. P. Infelta, and M. Gratzel, J.Phys. Chem.,
83, 1871 (1979).
2712
"ir,,
32
.\:430r n
E
Gratzei and Gratzei
The Journal of phvsical Chemistty, Vol. 86, No. 14, 1982
;,
"[
.1:430nm
,
Y
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0.0
2 8
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0
0
400 800 1200 1600 2000
TIME
TIME (nanoseconds)
,.
.i : 580 nm
20
40
60
80
100
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(microseconds)
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(microseconds)
.\ :510
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0
' ' ' ' ' ' ' ' " '0 0 20 40 60 80 I00 400 800 1200 1600 2000 T IHE (microeeconds) TIME (nanoseconds)
Figure 2. Growth and decay of TTF+ ([TTFi = 5 X lo4 M) at A = 430 and 580 nm and of Ru(bpyh+ ([Ru(bpyh* ] = lo4 M) at A = 510 nm in methanol as observed after laser excitation (A = 530 nm): (a and b) TTF' absorption spectra at A = 430 nm; (c and d) TTF' absorption spectra at A = 580 nm; (e and f) Ru(bpy),+ absorption spectra at A = 510 nm.
E
Flgure 4. Difference spectra of ZnTPPSC and TTF in various medii. (0)Znm and C in methad; spectnm taken = l o p s after iaser pulse; normalized absorbance scale at left-hand ordinate axis; maximum corresponds to Mplet absorption, A = 840 nm; [ZnTPPSC] = 5X M, [ l l F ] = 2.5 X lo3 M. (0) ZnTPPSC and TTF in CTAC mlcroemulsbn; spectrum taken =20 ps after laser pulse; normalized absorbance scale at rtght-hand ordinate axis; maximum corresponds to ankn (ZnTPPS") absorption, A = 910 nm; [ZnTPPSC] = 5 X M, [TTF] = 2.5 X lo3 M, composition of microemulslon described In text. (A)ZnTPPSC and lTF in CTAC mioeles; spectrum taken =10 ps after laser pulse; normeHzsd absort>ance scale at ri@-hand ordinate axis;maxknum ccfresponds to anbn (ZnTPPS") absorption, A = 910 nm; [ZnTPPSC] = 5 X M, [TTF] = 2.5 X lo3 M, [CTAC] = 2.5 x 10-3 M.
media. Practically all of the radical ions (TTF+/ ZnTPPS") apparently undergo back-reaction in the solvent cage (see eq 3 and 4). ZnTPPS4ZnTPPS4-'
+ TTF
ZnTPPS4-'
(3)
[ZnTPPS5--TTF+]
(4)
In striking contrast to methanolic solvent, one observes quenching of the ZnTPPS" triplet as well as definite product formation (cf. eq 5 ) in organized assemblies, such ZnTPPS4-'
+ TTF & ZnTPPS5- + TTF'
(5)
kb
BTF]~
~ 1 0 3
Figure 3. Stern-Voimer plots for the quenching of the Ru(bpy),*+' emlssion and ZnTPPSC triplet by tetrathiafulvaienein methanol: (0) k, (s-') for ZnTPPSC triplet quenching; left-hand ordinate axis for k, scaling; k , N 1.1 X lo8 M-' s-'; (0)k, (s-') for R~(bpy)~*'' emission uenchin : right-hand ordinate axis for k, scaling; k , = 1.1 X 10'% M-l s-!
laser excitation only a broad peak maximizing between 840 and 860 nm (corresponding to the triplet) is observed in the difference spectrum, see Figure 4. (The anion's absorption maximizes around 900 nm as will be discussed later.) In the absence of quencher the lifetime of the ZnTPPS" triplet is T = 80 w in methanol. The presence of 2.0 X M TI'F diminishes the lifetime to T = 6.5 w. A Stern-Volmer plot yields a value of k, = 1.1X lo8 M-' s-l (cf. Figure 2). I t is consequently postulated that the quenching reaction leads only to a negligible yield of redox products which cannot be detected within experimental limits, the charge transfer being inhibited in homogeneous
as micellar and microemulsion media. The decay of the ZnTPPSC triplet (observation wavelength is 840 nm) and of ZnTPPS" (A = 893 nm) in both CTAC micellar and CTAC microemulsion media are depicted in Figure 5. A difference spectrum taken approximately 10 ps after the laser pulse illustrates the existence of the zinc tetraphenylporphyrin anion at 910 nm in CTAC micellar solution (cf. Figure 4). (Absorption maxima in the IR of the ZnTPP anion have been observed at 700 and 910 nm.)14 This absorption is attributed to the free radical, ZnTPPS", the unpaired electron residing in the extended ?r* antibonding orbitals of the pyrrole rings. At shorter times, Le., approximately 1 ps, one can still observe a small contribution of the ZnTPPSG triplet at these wavelengths, Le., 890-920 nm. Likewise, a small constant contribution from the product anion is suspected at wavelengths where the triplet maximizes. However, all of these spectral overlaps can be minimized through the appropriate choice of time scales for regarding the kinetic events. The model for treatment of intramicellar kinetics developed by Maestri et al.n was applied to the triplet quenching of ZnTPPS& by "TF in CTAC micellar media. The hydrophilic ZnTPPS4- is expected to reside at the interface while TTF is solubilized in the hydrophobic core of the micelle. Figure 6 displays both the experimental ~
~~
(27) M.Maeetri, P.P. Infelta, and M.Grhtzel, J.Chem. Phys., 69,1522 (1978).
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982 2713
Photoinduced Electron-Transfer Reactions
0
2 4 6 8 10 TIME (microseconds)
2
1 C C 200 300 430 ::5 T. ME ~ ~ I C ~ O S ~ C O ~ ' C
. S I
-2.01 0
'
'
2
' TIME
7c
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d
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I , , . , . , , , , , G
10 2C 30 40 50 T I M E ImiCrosecondsI
0
10 TIME
20
30
40
5c
(microseconds
Flgure 5. Absorption decay curves of ZnTPPSC triplet ( h = 840 nm) and ZnTPPS" anion (A = 893 nm) in CTAC micellar and microemulsion media: (a) CTAC micelles, [CTAC] = 2.5 X loP3M, [ZnTPPSC] = 4 X 10" M, [TTF] = 2.5 X M, ZnTPPSC triplet decay at h = M, [ZnTPPSC] 840 nm; (b) CTAC micelles, [CTAC] = 2.5 X M, ZnTPPS5- (anion) decay at h =4 X M, [TTF] = 2.5 X = 893 nm; (c) CTAC microemulsion, [ZnTPPSc] = 5 X lob M, [STF] = 2.5 X M, ZnTPPSC trlplet decay at h = 840 nm. (d) CTAC M, [TTF] = 2.5 X microemulsion, [ZnTPPSC] = 5 X M, ZnTPPS" (anion) decay at h = 893 nm.
time course of the triplet absorbance decay at 840 nm and the simulated data (both at several concentrations) obtained by plotting
In this equation A*(O) is the initial absorbance of the ZnTPPS" triplet, A*(t),the triplet's absorbance at time, t, kT is the rate constant for triplet decay in CTAC micellar media with no quencher present, ri is the average number of TTF molecules in the micelles, and k, is the quenching rate constant to be calculated. f is the fraction of ZnTPPS5 contribution to the baseline of the triplet decay curve, i.e., f = A(m)/A*(O). A ( = ) is the absorbance taken from the experimental triplet decay curves at long times and attributed to the anion, ZnTPPS". The concentration of tetrathiafulvalene in the CTAC micelles is calculated by using cmc = M and aggregation number v 100, and the total concentration of CTAC and TTF in solution, i.e. ri = [TTF]/[M] (7) [MI = [CTAC] - cmc/v A Poisson distribution of solubilizate, TTF, molecules in the micelles is assumed. An optimum computer fit of the experimental to the simulated curves was made by varying k, until the best fit of all curves a t four different concentrations resulted. The fraction, f , was taken, as explained previously, from
'
4
'
'
6
'
. -
'
8
'
'
10
( m i oromroondd
Figure 6. Experimental absorption decay curves (-) and simulated decay curves (---) at h = 840 nm of ZnTPPSC triplet quenchin by TTF in CTAC micelles: (a) [TTF] = 0, [ZnTPPSC] = 5 X 10- M, [CTAC] = 2.5 X M; (b) [TTF] = 2.5 X lo-' M, [ZnTPPSC] = 5 X M, [CTAC] = 2.5 X M; (C) [TTF] = 5 X M, [ZnTPPS4-] = 5 X M, [CTAC] = 2.5 X M; (d) [TTF] = 1 X M, [ZnTPPSC] = 5 X lo5 M, [CTAC] = 2.5 X M; (e) M, [CTAC] = 2.5 X M, [ZnTPPSC] = 5 X [TTF] = 2.5 X 10-3 M.
2!
the experimental data. A reasonable fit with k, = 7 X lo4 s-l was obtained in support of the validity of the application of this intramicellar kinetic model to the present experimental data. Such a rate constant is approximately a factor of 20 less than the value to be anticipated in a diffusion-controlled intramicellar reaction.28 This variation can most likely be accounted for by the small driving force for the redox reaction in eq 5. It is thermodynamically only a marginally favored reaction, as noted from Table I. The ZnTPPS" anion (A = 893.nm) has a relatively long decay time of N 1.6 ms. The second-order rate constant for its disappearance is k / ( t l ) N 1.3 X lo4s-l. (1 N 0.5 cm, and e, the extinction coefficient of ZnTPPS" at 893 nm, is between lo3 and lo4 M-l cm-I.)l4 Thus, the disappearance of the anion is much slower than a diffusioncontrolled reaction. The longevity of this anion is due to rapid ejection of the TTF' from the interior of the micelle which thwarts rapid back-reaction with ZnTPPS". This process is an expected phenomenon due to the highly charged surface of the micellar entity. Thus, efficient charge separation is achieved in these micelles. As in the micellar solutions, a quenching of the ZnTPPS" triplet by TTF with the concomitant formation of the reduced porphyrin, ZnTPPS", and the TTF radical cation can be seen in the CTAC microemulsion system. The formation of ZnTPPS" can be quite easily observed as a broad peak at 910 nm in the difference spectrum taken approximately 20 after excitation by the laser (cf. Figure 4). As previously, the judicious choice of time scales for observing the kinetic events avoids the undesirable contribution of product anion and/or triplet to the respective decay curves. Since the microemulsion droplet is larger and its charge density at the interface is smaller than for a micelle, exchange of ZnTPPS4- triplets between droplets is fast. The observed electron-transfer events occur on a slower time scale than triplet exchange between different assemblies. Thus, the conditions necessary for treatment of the kinetic events by the previously mentioned intramicellar model are not fulfilled, and homogeneous rate laws should be applied. The addition of 2.5 X M TTF to (28) M. D. Hatlee, J. J. Kozak,G. Rothenberger, P. P. Infelta, and M. Gratzel, J.Phys. Chem., 84, 1508 (1980).
J. Phys. Chem. 1982, 86. 2714-2717
2714
the ZnTPPS solubilized CTAC microemulsion media decreases the porphyrin's triplet lifetime from 50 to 5.6 ps. The second-order rate constant for the anion decay is k / ( t l ) N 5.7 X lo6 s-l. The decay time of ZnTPPS" is -450 ps, approximately 30 times shorter than in micellar solution and, thus, a diffusion-controlled back-reaction. Charge separation in microemulsions is consequently not as efficient as in micelles due to the lower charge density at the interface, although charge ejection from the interior is apparently achieved.
Conclusion This paper reports for the first time the photoinduced oxidation of tetrathiafulvalene to its radical cation by two metal complexes, Ru(bpy)gP+ and ZnTPPS4-. As has previously been mentioned, both the reduced species, R ~ ( b p y ) and ~ + ZnTPPS" are powerful reductants, and, thus, could afford the reduction of water to hydrogen or could promote other desired reductions. An important result of this study is the achievement of charge separation with organized assemblies.w31 The photoinduced electron transfer from T T F to ZnTPPS4- to form redox products does not occur in methanol. ZnTPPS" formation in micellar and microemulsion media is, however, quite sub(29)M. Griitzel in 'Micellization, Solubilization and Microemulsions", Vol. 2,K. L. Mittal, Ed., Plenum, New York, 1977,pp 531-48. (30)M. Gritzel, Zsr. J. Chem., 18, 364 (1979). (31)We wish to draw attention to two recent publications on lightinduced charge separation in colloidal assemblies that appeared after completion of this work, i.e., (a) I. Willner, J. W. Otvos, and M. Calvin, J. Am. Chem. SOC.,103,3203 (1981);(b) S. S. Atik and J. K. Thomas, ibid., 103, 3550 (1981).
stantial. This aspect is important for light energy conversion devices where the formation of long-lived redox products in high yield is desirable. On thermodynamic grounds alone there is much more driving force for the formation of redox products in the R u ( ~ ~ ~ ) , ~ + /system T T F than in the ZnTPPS"/TTF system. Electrostatic factors may also influence the course of these two reactions. Since both redox products in the light-induced reduction of Ru(bpy),2+ by TTF to Ru( b ~ y )and ~ + TTF+are positive ions, static repulsion aids the completion of the charge-transfer process, whereas the Coulombic attraction between ZnTPPS" and TTF+ inhibits the formation of redox products. Other authors have noted the quenching of a zinc porphyrin triplet with no evident free ion production. These reacitons were explained in the light of exciples formation in nonpolar media.32 However, this study hypothesises that the two species stay in a caged ion configuration from whence a rapid back-reaction can take place in a relatively polar solvent such as methanol. The present study of the light-induced one-electron oxidation of tetrathiafulvalene makes a contribution to the vast effort presently underway in the field of conducting salts. Acknowledgment. Support of this research was provided by the European Research Standardization Group of the United States Army. We also express our appreciation to Dr. P. P. Infelta for his aid in the evaluation of the intramicellar kinetic events. (32)I. G. Lopp, R. W. Hendren, P. D. Wildes, and D. G. Whitten, J. Am. Chem. SOC.,92,6440 (1970).
Effect of Support Material on Rh Catalysts S. D. Worley,*t C. A. Rlce,t 0. A. Maltson,' C. W. Curtls,t J. A. Guln,t and A. R. Tarred Depertmnt of Chemistry and Department of Chemical Engineering, Auburn Univm/ty,Auburn Uniwdty, Alabama 36849 (Received:January 5, 1982: I n Final Form: February 26, 1982)
The effect of support material on Rh/X catalysts has been studied by using CO as a probe molecule for chemisorption and infrared spectroscopy as the analytical method. Support materials including TiOz,AlZO3, SO2,kaolinite, and montmorillonitehave been compared as to their tendencies to produce the various CO/Rh/X species generally attributed to this catalytic system. The TiOzand SiOzsupports enable the most facile reduction of a rhodium precursor material to rhodium metal. Although all of the Rh/X catalysts contain some Rh(1) sites, alumina-supported ones contain the most nonmetallic rhodium. Kaolinite and montmorillonitemay contain impurities which tend to poison the Rh sites for CO chemisorption. The CO/Rh/SiOz infrared bands are much less intense than for alumina- or titania-supported rhodium. This is probably due to weak metal/support interaction for Rh/Si02 because the silica employed was of high purity. Introduction Recent work in these laboratories has focused on the use of infrared spectroscopy as a means of probing the interaction of CO with supported Rh catalysts. Our first paper on the subject reported the identification of eight different CO/Rh/Alz03species for catalysts containing various Rh loadings and following different reduction conditions;' the primary species are I-III.233 In that work infrared spectroscopy was also used as a means of identifying the oxidation state of Rh in its various site distributions on A1203 by use of CO as a probe adsorbate molecule.' A second 'Department of Chemistry. Department of Chemical Engineering. f
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paper has recently appeared which discusses an extension of the work to a systematic study of the effect of rhodium precursor material on the CO/Rh/AlZO3~ y s t e m . ~Ex(1) C. A. Rice, S.D. Worley, C. W. Curtis, J. A. Guin, and A. R. Tarrer, J . Chem. Phys., 74,6487(1981). (2) A. C. Yang and C. W. Garland, J. Phys. Chem., 61, 1504 (1957). (3)J. T. Yates, T. M. Duncan, S. D. Worley, and R. W. Vaughan, J . Chem. Phys., 70,1219 (1979).
0 1982 American Chemical Society
