J. Phys. Chem. 1994,98, 10659-10664
10659
The Effect of Anions on the Electrochemistry of Zinc Tetraphenylporphyrin G. R. Seely,* D. Gust,* T. A. Moore,* and A. L. Moore* Department of Chemistry and Biochemistry and Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287-1604 Received: April 28, 1994; In Final Form: July 22, 1994@
Accurate measurements of porphyrin redox potentials are essential for the prediction and rationalization of the rates of electron transfer reactions involving these biologically important electron-donating and accepting chromophores. It is well established that ligation of bases, including anions, to metalloporphyrins lowers the potential for one-electron oxidation of the porphyrin ring. Evidence has been accumulating that the anion of perchlorate electrolytes, commonly used in cyclic voltammetry, may also affect the potentials for oxidation of zinc tetraphenylporphyrin by ligation to the oxidized radical cation species. The present work describes a survey of redox potentials of zinc tetraphenylporphyrin obtained by cyclic voltammetry in dichloromethane, with tetrabutylammonium salts containing a variety of anions as electrolytes. Of the anions tested, hexafluorophosphate appears to have the least ability to ligate the metal, so that potentials measured in its presence as electrolyte should most closely approach those of the unligated porphyrin. With perchlorate electrolyte, the potential for one-electron oxidation is approximately 80 mV lower, enough to affect the interpretation of photochemical electron transfer rates. In general, anions bind much more strongly to the cation radical than to zinc tetraphenylporphyrin itself. The use of reference redox systems based on thymoquinone and ferrocene carboxylate enabled comparison of potentials measured with different electrolytes.
Introduction Porphyrins and related tetrapyrrole macrocycles are important mediators of biological oxidation and reduction. Being colored materials, they are involved in a variety of photochemical redox processes, including photosynthesis. A large number of synthetic model systems that mimic such biological functions have been reported, and many of these involve meso-tetraarylporphyrins. Studies of these models can allow interpretations based on theory, such as the rate vs free energy correlations made popular by the works of Marcus and of Weller, among others.'S2 For example, we have recently reported the results of a study of the free energy dependence of photoinduced electron transfer rates in a series of porphyrin dyads.3 Any such quantitative interpretation of electron transfer rates requires accurate knowledge of the relevant electrochemical potentials under the specific conditions of the reaction. Kadish et al.4 established significant correlations between the potentials for electrochemical oxidation of axially substituted zinc meso-tetraphenylporphyrin (ZnTPP), the basicities of the substituted pyridines ligated to the Zn, and the constants for the formation of complexes of the ligands with ZnTPP. However, their results include one curious consequence: for substituted pyridines less basic than pyridine itself, calculated constants for association of the ligand with the porphyrin radical cation, ZnTPP'+ @,)', are smaller than the corresponding &, whereas for more basic substituted pyridines, they are larger. one would intuitively expect p: always to be larger than because of the additional ion-dipole stabilization between ZnTPP+ and the ligand. This observation could be readily explained, at least qualitatively, if the anion of the electrolyte (C104-) competes with the pyridine in binding to the cation ZnTPP+ much more strongly than it does in binding to ZnTPP. Competition effectively reduces the amount of pyridine bound to ZnTPP+
@E)
@Abstractpublished in Advance ACS Abstracts, September 15, 1994.
0022-365419412098-10659$04.50/0
at a given ligand concentration, thus giving the appearance of a smaller value of p:. Coulombic interaction should increase the strength of ligation of ZnTPP+ by anions much more than ion-dipole interaction could for pyridines. The first reported evidence of ligation of c104- to ZnTPP+ was the crystal structure of ZnTPPfC104-.5 Electron spin resonance hyperfine coupling constants, however, showed little effect of C104- on ZnTPP+ in solution and led to the conclusion that either ligation was absent or its effect was too weak to be However, Kadish et al. postulated that C104- displaced solvent from the metal in ZnTPP+.8 Hinman and Pavelich inferred the presence of ligation in dichloroethane solution from the infrared spectrum9 and then estimated, from electrochemical measurements at different electrolyte concentrations, that the potential for oxidation to unligated ZnTPP'+ was at least 50 mV higher than that to C104--ligated ZnTPP+.l0 Thus opinion is presently divided as to the importance, if not the existence, of ligation of C104to the Zn of ZnTPP'+. The immediate occasion of the present undertaking was the realization" that differing rates of electron transfer in a monometalated porphyrin dyad in the presence and absence of pyridine in dichloromethane could be better explained if the potential for oxidation of the Zn tetraarylporphyrin moiety in the absence of pyridine was substantially greater than that implied by the potentials of Kadish et al? In fact, measurements on the model substance ZnTPP with tetrabutylammonium hexafluorophosphate (TBA PF6) as electrolyte gave a potential for one-electron oxidation about 90 mV greater than that with the perchlorate salt (TBA C104) as electrolyte.ii Addition of pyridine lowered the potential in TBA PF6 solution by about 110 mV, in harmony with the greater rate of photoinduced electron transfer. In view of these results, and the continuing necessity of obtaining accurate electrochemical potentials to compare with reaction rates, we decided that the time was right for a review of the effects of a variety of anions on the electrochemistry of ZnTPP in the commonly used nonligating solvent dichloromethane. 0 1994 American Chemical Society
10660 J. Phys. Chem., Vol. 98, No. 41, 1994 The present study differs from earlier, related ones in two significant respects. First, the anion in question, as the TBA salt, usually constitutes the electrolyte. The more common practice, especially with strongly binding anions, has been to add small quantities of a salt of the anion to the solution already containing a weakly ligating electrolyte, usually TBA c104.8,10 In a few cases, this must be done here also, as the results will show; however, since we are most interested in anions that may ligate weakly, this procedure is in general unsuitable. Of course, the range of anions that can be used in the electrolyte is restricted to those not too easily oxidized. The second respect is the choice and use of suitable intemal reference redox systems. This is necessary because the electrolyte is different in each set of measurements and it cannot be assumed that liquid junction potentials will be unchanged. We have used two such, thymoquinone (TQ) and ferrocene carboxylate (FcC02-), as detailed below. While the first one-electron oxidation of ZnTPP is the object of primary interest, information has been obtained by cyclic voltammetry on the second one-electron oxidation and on the first one-electron reduction whenever possible. Visible spectra characterizing ZnTPP in the solutions used for electrochemistry were also recorded.
Experimental Section Materials. Zinc tetraphenylporphyrin was purified of the chlorin by oxidation with 2,3-dichloro-5,6-dicyano1,6benzoquinone followed by chromatography. Dichloromethane was distilled from P205 immediately before use. Acetonitrile was purified by the method of Kiesele.12 Ferrocene carboxylic acid (Aldrich) and thymoquinone were recrystallized from aqueous ethanol. Commercial electrolytes were obtained from Aldrich Chemical Company. Tetrabutylammoniumhexafluorophosphate (TBA PF6) was recrystallized from methanol. TBA tetrafluoroborate (TBA BF4), TBA nitrate (TBA NO3), and TBA perchlorate (TBA C104) were recrystallized from ethyl acetate; TBA BF4 was additionally recrystallized from aqueous methanol. TBA NO3 was further prepared and purified as described later. Tetrabutylammonium chloride (TBA C1) was dried by bubbling N2 through a solution of the commercial hydrate in toluene at 100 "C until the mixture clarified; the recovered crystals were dried over P2O5 in vacuum. TBA thiocyanate (TBA SCN) was dried in vacuum at 60 "C, and TBA tetraphenylborate (TBA B P h ) was used as received. The TBA salts of trifluoroacetic acid (TBA CF3C02), p-toluenesulfonic acid (TBA C7H7S03), p-chlorobenzoic acid (TBA ClCfiC02), and 2,5-dichlorophenol (TBA DCP) were prepared by neutralization of the acid with 40% aqueous TBA hydroxide (TBA OH), extraction into CH2Clz, back-extraction with water to neutral reaction, and vacuum drying of the evaporated CHzClz phase over P2O-j. Reference Redox Systems. With the use of different electrolytes, it could not be assumed that junction potentials would not vary. The recommended reference system for comparing potentials in different solvents is ferrocene (Fc),I3 which has in fact been used in several studies of the effect of solvent on reduction of metalloporphyrin~,~J~J~ Fc is unsuited to the present work for a practical and a theoretical reason. The practical reason is that its redox potential is too close to that of the first oxidation of ZnTPP, especially when the latter is ligated. This could be obviated by running Fc separately, but the more compelling reason is that the oxidation of Fc
Seely et al. formally resembles that of ZnTPP, ZnTPP i=ZnTPP"
+ e-
and might also be subject to the influence of anions. There is no body of experimental evidence assuring that reaction 1 is immune to anion effects, and to assume so would be to beg the question of greatest importance to us, the potential of reaction 2. In hopes of avoiding this dilemma yet retaining the supposed advantages of Fc, we investigated the oxidation of commercially available ferrocene carboxylic acid (FcC02H). The reaction 3 FcC02- zz Fc'C02-
+ e-
(3)
of oxidation of the TBA salt to a zwitterion should be less susceptible to anionic influences than reactions 1 and 2. In dichloromethane with TBA PF6 as electrolyte, FcCOzH was oxidized reversibly at 0.770 V. This is 0.25 V greater than the redox potential for Fc under the same conditions, in close agreement with previous reports in acetonitrile. 16,17 Addition of base (TBA OH, 1 M in methanol) gave rise to a reversible wave at E112 = 0.39 V, corresponding to oxidation of FcCOz-. However, with slight excess base and under other circumstances also, the wave was not entirely reversible and the cathodic component often disappeared at low scan rates. Because FcC02- was not useful under all circumstances, a second reference redox system was needed. For this we have employed thymoquinone (2-isopropyl-5-methyl- 1,Cbenzoquinone), which has a well-defined reversible one-electron reduction wave well in advance of the first ZnTPP reduction. Although TQ would be a poor reference for comparison of potentials in different solvents, it should be satisfactory when the only variation is in the anion of the electrolyte, and the results appear to support this assumption. Data for both FcCOzand TQ are reported when possible. A stock solution of TBA FcC02- in dichloromethane was prepared as described above for organic electrolytes; however, only part of the FcC02H was found to be ionized. To avoid interferences, this stock solution and the stock solution of TQ in dichloromethane were added only after redox potentials for ZnTPP had been determined. The redox potential for FcCO2was obtained by scanning from 0 to 0.6 V and back; for TQ, scanning was reversed at -1.0 V, to avoid intruding on the second one-electron reduction wave of this substance. Apparatus. In the cell for cyclic voltammetry, the sample compartment (15 mL), containing electrolyte (0.10 M), ZnTPP (5 x M), and eventually reference redox system in dichloromethane, is connected to a saturated calomel electrode (SCE) in the reference compartment and to the counter electrode (Pt in aqueous hydrazine sulfate) through bridges of 0.10 M TBA PFs in acetonitrile. In this arrangement, there are no junctions between immiscible liquids and the only significant variable junction potential is that between the test electrolyte in dichloromethane and TBA PF6 in acetonitrile in the Luggin capillary tip. The working electrode was glassy C (Bioanalytical Systems). The solution was flushed before scanning with highpunty N2 bubbled through CH2C12. Potential was controlled with a Pine Instrument Co. AFRDW potentiostat. Current vs potential was traced on a JSipp and Zonen BD 90 X-Y recorder. Scanning rates were varied between v = 10 and 800 mV/s; wave peak voltages were plotted against v1I2 and extrapolated to v = 0 in order to correct for uncompensated resistance. All reported potentials were thus corrected.
Redox Potentials of Zinc Tetraphenylporphyrin
J. Phys. Chem., Vol. 98, No. 41, 1994 10661
TABLE 1: Spectral and Electrochemical Data for ZnTPP in DichJoromethane Solutions of Tetrabutylammonium Salts anion’ PF6PF6PF6PF6- SCN-, 1.2“ I PF6- H20,satd PFs- py, 0.82Md BF4BF4BF4- DCP-, 0.2M BPhclodclodc104c104OH-,-0.7 mM Clod- py, 0.82Md C7H7SO3N03- e NO3NO3CF3C02ClC6H4C02SCNc1-
++
+
+
+ +
hb(nm) -
12
(nm)
-
-
586 606
548 548
-
-
585 585 61 1 588
548 548 570 549
-
-
586 610
548 549
602 609 608 606 603 607 607 611
549 549 548 549 550 566 567 570
-
-
-
-
de2 -
-
0.169 0.145
-
-
0.168 0.171 0.662 0.186 -
0.169 0.268
-
0.191 0.276 0.227 0.196 0.314 0.561 0.592 0.678
$: (VI
Gi (VI
0.888 0.859 0.858 0.61 0.842 0.768 0.826 0.843
1.183 1.218 1.204
-
’0.7 0.798 0.787 0.787 0.561 0.767 0.580 0.544 0.547 0.564 0.559 0.502
0.504
1.193
-
1.133 1.116
-
1.094 1.089 1.094 -
-
-
-
E? (VI 1.211
-
1.234 1.00
-
E;: (VI -1.324 - 1.335 -1.327 -
-1.338 -
E@cCO;) -
+0.400 -
0.374
-
-1.34 -1.333
-
-
0.923 -0.85 0.879 -
-
-
-1.338 -1.351 -
-1.362 - 1.358 -1.358 -1.364 -1.360 -1.45 - 1.430 -1.447
-
-0.632 -0.636 -0.648
-
1.120 1.124 1.118 0.926 1.174
-0.626 -0.646
-1.338 -1.335 -
E d T Q ) (VI
0.384
-
1.162 1.148
-
(V)
-
-
0.366
-
0.332 0.347
-
0.354
-
0.334
-
-
-
-0.475(!) -0.636 -0.637 -
-0.647 -0.668
-
-0.659 -0.660 -0.660 -
-0.670
a As tetrabutylammonium salt at 0.10M concentration except as noted. DCP- = 2,5-dichlorophenolate; py = pyridine. Spectral data comprise wavelengths of the first and second Q-bands of ZnTPP and the intensity ratio between them. In some cases (see text), the bands are not representative of a single species. Electrochemical data tabulated are half-wave potentials for the first (E::) and second (E:;) reversible one-electron oxidations of the latter and the half-wave potentials for one-electron reduction (E,;) of ZnTPP, for one-electron of ZnTPP and the anodic wave peak oxidation of FcC02-, and for the first one-electron reduction of thymoquinone. Potentials are expressed relative to the saturated calomel electrode. was recorded at v = 100 mV s-l and corrected to Y = 0 to cancel uncompensated resistance. Data reported in ref 11. e The rows The value of present successively results for commercial TBA NO3 recrystallized from ethyl acetate, TBA NO3 prepared by combination of TBA OH and HN03, and commercial TBA NO3 purified by an ion exchange column.
(c)
Spectra of the test solutions of ZnTPP before voltammetry, and sometimes after, were taken on a Cary 219 spectrophotometer. The state of ZnTPP in solution was characterized by the positions and relative intensities of the first two Q-bands in the visible region.
Results Recorded spectral data and electrochemical potentials are summarized in Table 1. Certain potential differences, including corrections for junction potentials, are collected in Table 2. The anions are listed in perceived order of increasing ligation strength. Inspection of the spectral data of Table 1 shows that the anions fall into three groups. The first consists of anions that do not ligate ZnTPP: PFs-, BF4-, BPh-, and C104-. The absorption spectrum of ZnTPP in the presence of these ions is the same as that in dichloromethane alone. The first two bands in the Q-series are at 585-586 and 548-549 nm,and the ratio of intensities is about 0.17. The second group consists of anions that ligate ZnTPP fully at 0.1 M: C1-, SCN-, and clc6&c02-. To this group also belong OH- and DCP-, though they were not tested at 0.1 M, and the neutral base pyridine.4v1s,19The first two Q-bands are at 607-611 and 566-570 nm, and the ratio of intensities increases almost to 0.7. The third group consists of three ions that appear to ligate ZnTPP partially at 0.1 M: Nos-, C7H7S03-, and CF3C02-. In these cases, the visible absorption spectrum contains bands of both ligated and unligated species; the ca. 565 nm band of the ligated species appears as a shoulder on the 549 nm band of the unligated species, while the 585 band of the unligated species is submerged between bands of the ligated species. In these cases, the wavelengths reported in Table 1 are for the first band of the ligated species and the second band of the unligated one. Therefore the spectral data characterize the solution in which
voltammetry was carried out, rather than either the ligated or the unligated species of ZnTPP in it. Spectra for TBA PF6 or TBA C104 solutions with small additions of SCN- or OH- were similar in appearance and were similarly treated. It is somewhat surprising that NO3-, C7H7SO3-. and CF3C02do not ligate all of the ZnTPP even at 0.1 M. Perhaps, since dichloromethane is a not very polar solvent, so much of the electrolyte is in the form of ion pairs and clusters that the activity of the anion is much lower than its concentration and only the stronger anions ligate ZnTPP completely. The nature of available anions has a profound influence on the potential for the first one-electron oxidation of ZnTPP, which ranges from 0.86 V for TBA PF6 electrolyte to 0.50 V for TBA C1. The presence of anions of the third group, as well as of the second, favors potentials in the lower part of the range. Within the first group, the potential with TBA C104 averages almost 80 mV lower than with TBA PF6 as electrolyte. The second one-electron oxidation of ZnTPP is reversible only with electrolytes of PFs-, BF4-, and C104-, and the halfwave potential E t : decreases in that order. Other anions add to ZnTPP2+ at the ,8-pyrrolezo,21or meso22$23 position to form i s o p o r p h y r i n ~thereby , ~ ~ ~ ~eliminating ~ ~ ~ ~ the cathodic component of the wave. In these cases, as well as in others, Table 1 lists the peak of the anodic wave, ET2, recorded at 100 mV s-l scanning speed but corrected to zero speed to cancel uncompensated resistance. With TBA B P b , TBA SCN, or TBA C1 as electrolyte, oxidation of the anion prevented observation of this anodic wave. In contrast to the oxidation waves, the first one-electron reduction wave is little affected by electrolyte, unless ZnTPP is already ligated, and the potential E,: lies in the range -1.33 to -1.36 V. Ligation by C1-, SCN-, or clc6H4c02- lowers the potential by about 0.1 V. The value of E,: with TBA C104 is perhaps 50 mV less negative than that reported by
Z$i,
10662 J. Phys. Chem., Vol. 98, No. 41, 1994
Seely et al.
TABLE 2: Potential Differences Calculated from Table la
-0.626 (-0.636) (-0.646) -0.646
-
(-0.636) -0.632 -0.636 -0.648 -0.636 -0.637 (-0.636) (-0.636)
-
-0.647 -0.668 (-0.660) -0.659 -0.660 -0.660 (-0.660) (-0.67) -0.670
-
1.036
-
1.030
1.010
-
-
1.002 1.Ooo 1.007
-
1.014
-
1.01
-
1.514 1.495 1.504 1.26 1.478 1.400 1.462 1.491 1.434 1.424 1.423 1.197
0.295 0.359 0.346
-0.698 -0.699 -0.681
-
0.346 0.36
-
0.349
-
-0.702
0.307 0.273 0.296 0.302 0.307
0.306 0.275 0.292 0.307 0.301 0.335
-0.702 -0.687 -0.697
-
1.414 1.248 1.204 1.206 1.224 1.219 1.22
-
1.174
-
-
0.293
-
-
-
-
-
0.329 -0.26 0.288 -
-
-
-
-
-0.702 -0.715 -
-0.694 -0.698 -0.699 -0.704 -0.700 -0.79 -0.76 -0.777
2.212 2.194 2.185
-
0.867 0.62
-
2.180
0.838 0.763
2.164 2.178 2.129
0.854
8.2 x 104 (SCN-)
-
0.787
2.125 1.912
(ai = 2 2 0 ~
0.560
3.8 x 105 (OH-)
1.942 1.902 1.905 1.928 1.919 1.95
-
1.951
0.777 0.611
5.8 x 104
0.587 0.582 0.58
1.7 x 105 1.2 x 105 7.3 x 104
0.537
4.0 x 105
Most terms are defied in Table 1 footnotes as well as in the text. Order of rows is the same as in Table 1. Values in parentheses were not determined in the test solution but inferred from values in similar solutions and used to calculate differences in the same row. When there is a choice, only the value judged “best” by internal evidence is tabulated. Ratio of formation constants for adducts of anion to ZnTPP+ and ZnTPP as calculated in the Discussion. From ai[A-] divided by the concentration (0.1 M) of A-, “a:’ incorporates the activity coefficient of A-. Kadish et al? under ostensibly similar conditions but is in close agreement with the results of Kadish et a1.l5 when referred to Fc (0.520 V vs SCE). With one exception, potentials Eln(TQ) for the one-electron reduction of thymoquinone lie in the range -0.63 to -0.67 V, which suggests that liquid junction potential differences with different electrolytes are small. There appears to be a reaction between TQ- and BPb- which affects the reversibility as well as the potential. Fewer data are available for the oxidation of FcCOz-, but those that are show a trend with anion parallel to that of El/z(TQ). In Table 2, the potential differences listed are presumably independent of variations in junction potential. The redox potential difference between the two reference systems, Eln(FcCOz-) - El/z(TQ), averages 1.014 f 0.013 V, which tends to c o n f m the suitability of either as a reference system for the present purpose. Subtracting E~Iz(TQ)from li$ makes little change in the order of potentials listed in Table 1. The relative potential is still highest with TBA PF6 and distinctly the lowest with TBA C1. There is probably no significant difference between the potential E:; for TBA PF6 and TBA BF4. Neither anion appears to ligate ZnTPP and ZnTPP+, or to do so only very weakly. The potential with TBA Clod is still substantially below that of either. The value of E; - EL; is similar with TBA BF4 and TBA c104 and perhaps 50 mV less than with TBA PF6. Perhaps BF4- (and c104-) bind more strongly to ZnTPP2+ than to ZnTPP+, whereas PF6- is unbound or less strongly bound to both. The quantity E’: - 0.030 V closely approximates E;: - E; for the reversible systems and can be used to characterize some of the irreversible systems as well. Here, there is no such clear trend with anion type as there is with - E d T Q ) , implying no further change in the state of ligation on removal of the second electron.
