J. Phys. Chem. 1993,97, 13637-13642
13637
Effect of Coordinated Ligands on Interporphyrin Photoinduced-Electron-Transfer Rates Devens Gust,' Thomas A. Moore,' Ana L. Moore,. Ho Kwon Kang,? Janice M. DeCraziano, Paul A. Liddell, and Gilbert R. Seely Department of Chemistry and Biochemistry, Center for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287- I604 Received: July 20, 1993; In Final Form: September 28, 1993"
The effect of coordination of various pyridines on the rate of photoinduced interporphyrin electron transfer in a molecular dyad consisting of a zinc porphyrin covalently linked to a free base porphyrin moiety has been investigated using time-resolved fluorescence techniques. Coordination of pyridine itself to the zinc ion results in a nearly 30-fold increase in the rate constant for one of the photoinduced-electron-transfer reactions. In similar studies with a series of pyridine ligands bearing substituents with electron-donating or -accepting properties, the data correlate well with the substituent constant under the Hammett linear free energy relationship. The reaction constant p has a value of -0.35, which indicates that the rate of electron transfer is increased by electron donation to the zinc porphyrin. Electrochemical experiments with a poorly-coordinating electrolyte suggest that this donation stabilizes the interporphyrin charge-separated state, in which the zinc porphyrin moiety is positively charged, and that this stabilization in turn leads to a larger rate constant for electron transfer. Effects of this type undoubtedly play a role in metalloproteins in which a porphyrin metal ion is ligated to the protein. They may also affect the interpretation of solvent effects noted in electron-transfer reactions involving metallated porphyrins.
Introduction Metalloporphyrinsplay a variety of important roles in biological electron transport. The initial photoinduced electron transfer in photosynthesis involves magnesium-containing chlorins in the reaction center, whereas iron-bearing porphyrins are found in the various cytochromes that take part in the dark reactions of photosynthesis and in respiration. The porphyrin metal ions in these proteins are coordinated to additional ligands that are often provided by amino acid side chains. Such coordination certainly plays a structural role in the protein. In addition, it is likely that it significantly affects the rates of electron transfer to or from the porphyrin moiety in question. Numerous model systems for photosynthetic electron transfer employ synthetic porphyrins containing zinc or other metal ions as electron donors or acceptor^.^-^ For example, we have recently reported on a series of porphyrin dyads6 and carotenoid-diporphyrin that undergo photoinduced interporphyrin electron transfer and a carotenoid-diporphyrin-diquinone molecular pentad that features interporphyrin electron transfer as a 'dark" reaction following a photoinduced electron-transferevent.sv8 Coordinationof solvent or adventitious ligands to the metals in these porphyrins might affect the rate constants for, and quantum yields of, electron transfer. In order to investigate the effects of such ligands, we have prepared porphyrin dyad 1 and studied its photoinducedelectron-transfer properties in dichloromethane in the presence of a series of substituted pyridine ligands to zinc. The electrontransfer results have been correlated with ligand structure using the Hammett linear free energy relationship and the results of electrochemical measurements of oxidation potentials.
Results Photochemistry of Dyad 1 in the Absence of Pyridines. The photochemistry of 1 in dichloromethanesolution has been reported previously7 but will be summarized here in order to contrast this behavior with that in the presence of pyridine ligands. The absorption spectrum of 1 in dichloromethane (Figure 1) is + On leave from the Department of Agricultural Chemistry, Sunchon National University, Sunchon 540-742, Korea. 0 Abstract published in Advance ACS Abstracts. November 15, 1993.
essentially a linear combinationof the spectra of model porphyrins 2 and 3. It features a broad band at 422 nm than encompasses the Soret absorptions of both the zinc and free base moieties. The Q-band absorption at 550 nm arises mainly from the zinc porphyrin, whereas those at 510 and 640 nm are due mostly to the free base. The 588-nm band consists of approximately equal contributions from both porphyrins.
3
)$I: F
The fluorescence emission spectrum of 1 in dichloromethane is shown in Figure 2. The fluorescence maxima at 643 and 710 nm are similar to those of the free base porphyrin 3. The zinc porphyrin moiety, which emits at 600 and 648 nm, based on the emission spectrum of 2, contributes only slightly to the emission of the dyad, even though excitation was at 560 nm, where this moiety absorbs most of the light. This weak emission is due in part to quenching of the zinc porphyrin first excited singlet state through singlet-singlet energy transfer to the attached free base porphyrin. Fluorescence excitation studies have shown that this transfer occurs with a quantum yield of 0.77. This singlet-singlet
0022-36541931209713637%04.00/0 0 1993 American Chemical Society
Gust et al.
13638 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 2.5
2.0
1
c
i
0.09 O’l2
500
450
0.0 0 400
500
550
. 550
600
600
650
650
700 5
Wavelength (nm) Figure 1. Absorption spectra of porphyrin dyad 1 in dichloromethane in the absence (-) and presence (- - -) of 2.0 M pyridine. The inset is a vertical expansion of the Q-band region.
I
I
2 has a fluorescence lifetime of 1.6 ns in dichloromethane, and the excited singlet state of free base 3decays in 8.5 ns. The decay of the fluorescence of 1, however, shows two components with time constants of 0.082 and 2.7 ns. Decay-associated spectra resulting from global analysis of the decays at seven different wavelengths showed that the shorter component represents the lifetime of the zinc porphyrin first excited singlet state (IPznPF), whereas the longer lifetime is that of Pz,,-IPF. Thus, both porphyrin first excited singlet states are quenched, and an intramolecular process other than singlet energy transfer must be playing a role. The quenching is assigned to electron transfer to form Pzn*+-P~*-.This conclusion is supported by the spectroscopicobservation of a long-lived charge-separated state formed via photoinduced electron transfer in a closely related carotene-diporphyrin~ triad molecule’ and of a photogenerated charge-separated state in a related diporphyrin using transient absorption techniques on the picosecond time scale? The fluorescence results allow calculation of the rate constants for the various pathways shown in Scheme I. The rate constant k, for singlet-singlet energy transfer is found to be 9.4 X lo9 s-l using eq 1, where r is the 0.082-nslifetime of IPz,,-PF and 0 1 is the singlet energy transfer quantum yield of 0.77.
a, = rk, (1) The rate constant kg for electron transfer from the zinc porphyrin first excited singlet state to the free base porphyrin, determined from eq 2, is 2.2 X lo9s-l based on the values for r and kl given above and a value for k3 of 6.3 X lo8,as estimated from the 1.64s lifetime of the first excited singlet state of 2. l / r = k,
+ k3 -tk,
(2) The rate constant for electron transfer in Pz,,-IPF to yield Pzn*+-P~*-,ks, is given by eq 3 1/r‘ = k,
Wavelength (nm) Figure 2. Corrected fluorescence emission spectra of dyad 1 (-), zinc porphyrin 2 (- -), and free base porphyrin 3 (- -) in dichloromethane solution in the absence of pyridine. The sample was excited at 560 nm. The intensities have been normalized to allow comparison of the spectral features.
-
SCHEME I 2.0
eV
I.o
0.0
_I
‘Zn.‘F
transfer is indicated as step 1 in Scheme I, which shows the relevant transient species of 1 and their interconversion pathways. Time-resolved fluorescence studies of 1 yield information concerning the rates of the energy transfer and other pathways for decay of the porphyrin excited singlet states.’ Zinc porphyrin
+ k,
(3) where 7‘ is the 2.7-11s lifetime of PZ,,-,PF and kq is estimated to be 1.2 X lo8 s-* from the 8.5-ns lifetime of free base porphyrin 3. The calculated value of k5 is 2.5 X lo8 s-I. Photochemistry of Dyad 1 in the Presence of Pyridine. It is well-known that zinc porphyrins possess a fifth coordination site on the metal and form 1:l complexes with organic amines? including pyridine derivatives.I2 The complexes with pyridine ligandsare quite stable. In benzene solution, the stability constant for the complex between pyridine itself and zinc tetraphenylporphyrin is 6.03 X lo3 M-I at 25 O C , as determined from spectrophotometric titrations.I2 No evidence for coordination of a second pyridine was reported. Spectrophotometric studies of a 1.3 X M solution of 1 in dichloromethane containing 2.0 M pyridine showed shifts in the absorbance bands of the dyad, as illustrated in Figure 1. As expected from the equilibrium constant reported above, complex formation was complete, and additional pyridine did not lead to further changes in the absorption spectrum. In the presence of pyridine, the Soret region displays two maxima at 415 and 431 nm. The 431-nm band is due to the zinc porphyrin moiety, whose absorption has been red shifted. Figure 3 shows the absorption spectra in the Q-band region. The absorption of the dyad 1 is a linear combination of those of model compounds 2 and 3 in the presence of pyridine. The absorption of the free base porphyrin moiety, with maxima at 5 10,540,586,and 640nm, is unchanged from that in the absence of ~ y r i d i n e .However, ~ the two major zinc porphyrin absorptions have been red shifted by about 15 nm to 564 and 605 nm (from 550 and 589 nm in the absence of pyridine). Thus, the pyridine has indeed coordinated to the zinc porphyrin moiety, and this has left the free base porphyrin unaffected in terms of absorption. This emission spectra of 1-3 in the presence of 2.0 M pyridine (with excitation at 560 nm) are shown in Figure 4. In the emission
The Journal of Physical Chemistry, Vol. 97, No. 51, 1993 13639
Interporphyrin Photoinduced Electron Transfer
0.3
100
l A
"."
I
600
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550
600
700
650
Wavelength (nm) Figure 3. Absorption spectra in dichloromethane solution containing2.0 M pyridine of the Q-band region of dyad 1 (-), zinc porphyrin 2 (- - -), and free base porphyrin 3 (- -). Also indicated (- * -) is the linear combination of the spectra of 2 and 3 which approximates the spectrum of the dyad.
.-0 v)
C
0 C
-
600
650
700
750
Wavelength (nm) Figure 4. Corrected fluorescence emission spectra of dyad 1 (-), zinc porphyrin 2 (- - -), and free base porphyrin 3 (- -) in dichloromethane solution containing 2.0 M pyridine. The sample was excited at 560 nm. The intensitieshave been normalized to allow comparison of the spectral features. spectrum of 1, the free base bands at 645 and 71 1 nm are still present and are not shifted relative to those in the absence of pyridine. However, the emission from the zinc porphyrin moiety now contributes proportionately much more to the emission of the dyad, and its maxima are red shifted from their values in the absence of pyridine to 617 and 670 nm (Figure 2). From these results it is clear that the pyridine ligand to the zinc porphyrin affects the shape of the emission spectrum of only that moiety; that of the free base porphyrin is unaltered in form. It is also clear that the pyridine ligand remains coordinated to the zinc porphyrin throughout the lifetimeof the first excited singlet state, as it affects the emission spectrum as well as the absorption properties. Similar shifts in the absorption and emission maxima of zinc porphyrins upon coordination of pyridine have been noted by other investigators.I1-l3 The fluorescence excitation spectrum of 1in dichloromethane solution in the presence of 2.0 M pyridine was obtained with detection at 710 nm, where emission is due almost entirely to the free base porphyrin moiety. The excitation spectrum was corrected for the output of the excitation source as a function of wavelength and normalized to the absorption spectrum in the 640-nm region where absorption by the zinc porphyrin moiety is minimal. Comparison of the excitation and absorption spectra
620
640
660
680
700
720
Wavelength (nm) Figure 5. Decay-associated spectra derived from global analysis of 13 time-resolved fluorescence measurements performed on 1.3 X lC5M
dyad 1in dichloromethane solution containing 2.0 M pyridine. The global x 2 value was 1.28. The two major lifetime components are 0.032ns ( 0 ) and 0.144 (#). The sample was excited at 590 nm. in regions where both porphyrin moieties absorb light allowed calculation of the efficiency of singlet-singlet energy transfer from the zinc porphyrin to the free base as 0.24. Time-resolved fluorescence studies were performed on 1-3 in the presence of pyridine at concentrations similar to that given above. The fluorescence decays of 2 and 3,with excitation a t 590 nm, were single exponentials with lifetimes of 1.24 (xz = 1.20) and 8.9 (x2= 1.09) ns, respectively. Thus, the excited singlet state lifetimes of the monomeric porphyrins are little affected by pyridine. Dyad 1 in dichloromethane containing 2.0 M pyridine was excited a t 590 nm, and decays were measured at 13 wavelengths. The decay data were fit by a global analysis,14 whose results are shown as a decay-associated spectrum in Figure 5 . Five exponential components were used, and a global xzvalue of 1.28 was obtained. There are two major components with lifetimes of 0.032 and 0.144 ns and three very minor components that will be disregarded. The 0.032-ns component has a strong positiveamplitudeat 616 and 670nm, wherethepyridine-bearing zinc porphyrin moiety emits, and a negative amplitude in the 710-nm region, where emission is due mainly to the free base porphyrin. The positive amplitude regions correspond mainly to decay of the emission of the zinc porphyrin moiety and the negative amplitude a t 710 nm to an increase in the intensity of emission from the free base porphyrin with time due to singlet-singlet energy transfer from the zinc species. Thus, the lifetime of IPZ,,(~~)-PF is 0.032 ns. The 0.144-11s component of the decay has the spectral characteristics of the free base porphyrin and represents the decay of PZ,,(~~)--IPF. Similar experiments with a lower pyridine concentration (0.1 M) gave lifetimes of 0.030 and 0.163 ns for the decay of 'PZn(py)-PF and PZn(py)-'PF. Thus, the shortened lifetimes in the presence of pyridine are due to coordination of pyridine to the zinc porphyrin rather than to a general solvent effect. As was the case for 1 in the absence of pyridine, the fluorescence properties of this dyad in the presence of pyridine indicate the Occurrence of both energy- and electron-transfer processes as illustrated in Scheme I. The data may be analyzed in a similar fashion. From the singlet-singlet energy transfer quantum yield of 0.24 and the 0.032-11slifetime of 'PZ,,(~~)-PF, eq 1 may be used to calculate a value for kl of 7.5 X lo9s-I. The energy of ] P Z , , ( ~ ~ ) PF may be estimated as 2.03 eV, taken from the wavenumber average of the longest-wavelength absorption maximum and shortest-wavelength emission maximum of the zinc porphyrin moiety. The energy of PZ~(,,~)-IPF is 1.93 eV, calculated in the same way. Thus, in the presence of pyridine, the thermodynamic driving force for step 1 in the scheme is 0.10 eV. If one assumes that the interporphyrin singlet-singlet energy transfer is an
13640 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
Gust et al.
k6. However, the analysis of k6 is complicated by two factors. In the first place, the measurements of the singlet-singlet energy transfer quantum yields by steady-state fluorescence excitation spectroscopy and use of the resulting values of kl introduced errors into the calculation of kg that were relatively large, compared to those in ks and the magnitudes of the substituent effects to be discussed below. Second, for a few of the pyridines, the lifetime of I P Z ~ ( ~ ~ )was - P Ftoo short to measure accurately on the instrumentation available (the instrument response function was ca. 35 ps), due to rapid electron transfer to the free base 4.64 x 109 0.37 0.210 3-c1 porphyrin and quenching of the zinc porphyrin first excited singlet 4.96 x 109 0.23 0.197 4x1 state by the pyridine ligand. For example, in the case of the 3.49 x 109 -0.16 3-NHz 0.277 4-formylpyridyl complex of zinc porphyrin 2, the first excited 0.290 3.33 x 109 0.66 4-CN singlet state lifetime was less than 7 ps. The PZn(py)-'PF excited Lifetime of the PZ,,(~~)-'PF state, as determined from fluorescence state, however, was insulated from the effects of energy transfer decay studies. Rateconstant for electron transfer from the zinc porphyrin moiety to the free base first excited singlet state, yielding Pz~(~~)'+-PF'. and any direct interactions with the pyridine ligands, as shown by the fact that the spectra and excited singlet state lifetime of This value was calculated from the lifetime in the adjacent column using eq 3 and a value of 1.2 X lo8s-l for kq. u meta or para value taken from 3 are unaffected by addition of a pyridine to the solution. Thus, McDaniel and Brown.16 the rate constant for electron transfer from the zinc porphyrin moiety to the excited free base singlet state could be readily equilibrium process, then the equilibrium constant is 49, and k2 calculated using eq 3. Table I shows the measured lifetimes of equals 1.5 X 108 s-1. As will be shown below, this rate is much P z ~ ( ~ ~ ) in - ~the P F presence of the 10 pyridine ligands and the slower than those of the other processes that lead to deactivation values for ks calculated from these lifetimes. of the porphyrin first excited singlet states and may be ignored Hammett Analysis of the Electron-Transfer Data. It is clear for the purposes of the analysis that follows. from the results for dyad 1in the presence and absence of pyridine Equation 2 may be used to calculate the rate constant for that the fifth ligand to zinc can have a substantial effect on the electron transfer from to yield Pzn(py)'+-P~'-. The rates of electron transfer from the zinc porphyrin first excited values of T and kl are given above, and k3 equals the reciprocal singlet state and to the free base porphyrin first excited singlet of the 1.24-11s lifetime of the first excited singlet state of 2 in the state. Table I shows that simply varying the nature of the presence of pyridine. Thus, kg equals 2.3 X 1Olos-I. Similarly, substituent on pyridine can change the value of k5 by a factor of eq 3 yields a value of 6.8 X IO9 s-I for ks, as calculated from the more than 3. It might be expected that electron-transfer rate T' value of 0.144 ns and a kq of 1.1 X lo8s-l as determined from constants could correlate with the electron-donating or -withthe fluorescence lifetime of 3 in the presence of pyridine. drawing ability of the pyridine substituent in 1. Earlier work has Photocbemistry of Dyad 1 in the Presence of Substituted shown that the stability constants for complexes of zinc tetraPyridines. It is clear from the last section that coordination of arylporphyrins with pyridinemay be correlated using a Hammett pyridine to the zinc porphyrin moiety of 1 has a substantial effect linear free energy relationship.I2 Similarly, we reported some on the rateconstants for photoinduced electron transfer. In order years ago that nitrogen-1 5 chemical shift changes in zinc to further quantify this effect, the study was extended to a total tetraphenylporphyrin may be correlated with the electronof 10 pyridine derivatives substituted in the 3- and 4-positions donating ability of the pyridine substituent using a Hammett (Table I). In all cases, enough of the substituted pyridine was plot.15 We therefore investigated the existence of such an added to a 1.3 X It5M solution of dyad 1 in dichloromethane association in 1. to ensure complete complexation of the porphyrin.'* The The relationship employed is given by eq 4, where k is the absorption and emission spectra of all of the complexes featured value of k5 for the complex bearing the pyridine ligand in question red shifts of the zinc porphyrin absorptions of essentially the and ko is the value of k5 for pyridine itself (6.8 X lo9 s-l). samemagnitude as noted for pyridine itself. The emission spectra for all of the complexes showed maxima at essentially the same (4) wavelengths as shown in Figure 4. However, the relative The reaction constant p is a measure of the effect of the electron contributions of the zinc and free base moieties to the emission density at the reaction site on the electron-transfer reaction. The spectrum varied according to the particular pyridine employed. Hammett substituent constant (a) set chosen for the correlation Excitation spectra of the complexes indicated varying amounts was that of McDaniel and BrownI6 as selected by Ritchie and of singlet-singlet energy transfer from the zinc porphyrin to the Sager.17 Other sets of uvaluesgivesimilarcorrelations. Although free base moiety. these u values were derived for benzene-based systems, successful Time-resolved fluorescence data with excitation at 590 nm applications to pyridines have been reported.12 Figure 6 shows were also obtained in each case. The free base monomer 3 had a plot of eq 4 using the data in Table I. The line is the linear a fluorescence lifetime of approximately 8.5 ns in the presence least squares best fit to the data, excluding the 3-amino-substituted of the substituted pyridines. In the case of the zinc porphyrin 2, pyridine. The slope, p, is -0.35, and the correlation coefficient the lifetime was about 1.25 ns for some of the pyridines but was is 0.97. strongly quenched for a few, such as the formyl and acetyl The results for the complex with 3-aminopyridine do not derivatives. In all cases, the major component of the fluorescence correlate well with the other data. This might be due to some of dyad 1 at 615 nm had a lifetime of a few picoseconds and was specific interaction of the amino group with the porphyrin or ascribable to IPZ~(~~)-PF, whereas the major component at 710 metal ion. nm had a lifetime on the order of 100-300 ps and was assigned to the emission of PZn(py)-'PF. Additional minor components Discussion similar to those shown in Figure 5 were also noted in each case. In all cases, the decays were analyzed either at single wavelengths The findings for 1 in the presence and absence of pyridine or globally at 615 and 710 nm, and goodness-of-fit criteria were demonstrate that coordination of pyridine has a large effect upon satisfied. the rates of both electron-transfer processes. The rateof electron In principle, the data for each substituted pyridine could be transfer from lPZn(py)-PF to yield Pz~(~~)*+-PF*is about 10 times analyzed as described above for pyridine itself to extract ks and faster than the corresponding rates in the pyridine-free dyad. TABLE I: Electron-Transfer Rate Constants and u Values for Complexes of Various Substituted Pyridines and Porphyrin Dyad 1 pyridine substituent T' (ns)O ks (ss')~ d 1.06 X l O l o -0.66 4-NH2 0.093 8.97 x 109 -0.37 0.1 10 4-OH 0.126 7.82 x 109 -0.17 4-CH3 0.144 6.82 x 109 0.0 4-H 0.170 5.76 x 109 0.22 4-CHO 5.26 x 109 0.50 0.186 4-CH3CO
Interporphyrin Photoinduced Electron Transfer
The Journal of Physical Chemistry, Vol. 97, No, 51, 1993 13641
containing 0.1 M tetra-n-butylammonium perchlorate are 0.78 V in the solvent alone and 0.76 V in the presence of 1.0 M pyridine.z1 This very small difference would suggest that changes in the thermodynamic driving force are not in fact responsible for the observed effects. In fact, a similar argument has been used to suggest that redox potential changes do not play a role x" in pyridine-assisted electron transfer from the triplet state of zinc B tetraphenylporphyrin to 1,4-benzoq~inone.~~ ul -0 A key to this riddle is the observation that although perchlorate ion does not bind strongly to zinc tetraphenylporphyrin itself, it does bind to the zinc tetraphenylporphyrin radical cation and can therefore affect redox potential measurements performed using perchlorates as supporting e l e c t r ~ l y t e s . Perhaps ~ ~ ~ ~ ~ in the photoinduced-electron-transfer experiments described above, the coordination of pyridine to the zinc ion does indeed significantly lower the first oxidation potential of the zinc porphyrin moiety. Figure 6. Hammett plot of electron-transfer rate data for dyad 1 However, in the electrochemical experiments, perchlorate coorcoordinated to 10 substituted pyridines (Table I). The electron-transfer dination with the radical cation may fortuitously lower the process is step 5 in Scheme I. The solid line is the least squares best fit observed oxidation potential by about as much as pyridine, and tonineofthedatapoints(0) andyieldsaslopepof-0.35andacorrelation thus it appears that pyridine coordination has little effect. coefficient of 0.97. The result for 3-aminopyridine (m) is not included In order to investigate this possibility, we have undertaken in the fit. cyclic voltammetric experiments on 5 X 10" M solutions of The rate of electron transfer from the coordinated zinc porphyrin chlorin-free zinc tetraphenylporphyrin in dichloromethane. Poto the first excited singlet state of the free base porphyrin, PZ,,(~~)- tentials were measured in nitrogen-purged solutions against an IPF, to yield the same charge-separated state is increased by a SCE reference electrode under conditions similar to those factor of 27 relative to the pyridine-free case; it is also sensitive employed by Kadish and co-workers.21 Thymoquinone was to the nature of the substituent on the pyridine ligand. The rate employed as a reference redox system in order to correct for of singlet-singlet energy transfer from the zinc porphyrin to the relatively small variations in junction potential. With a supporting free base moiety, on the other hand, is only slightly altered by electrolyteof 0.1 M tetra-n-butylammonium perchlorate, the first coordination of pyridine. oxidation potential was found to be0.79 V vsSCE. In the presence In principle, the presence of the pyridine ligand on the zinc of 0.83 M dry pyridine, the potential was 0.77 V. These results porphyrin could affect electron-transfer rates in several ways. are comparable to those obtained by Kadish and co-workers. The size, shape, and energy of each of the zinc porphyrin molecular Similar experiments were carried out with 0.1 M tetra-norbitals will change upon coordination, and such changes will butylammonium hexafluorophosphate as an electrolyte. The affect electron donor-acceptor properties and orbital overlap. A measured potentials were 0.88 and 0.77 V in the absence and suggestion as to the major effect of coordination can be drawn presence of 0.83 M pyridine. Thus, in the presence of hexaflufrom the Hammett study. The relatively good fit to eq 4 (Figure orophosphate ion, which is expected to coordinate relatively weakly 6 ) shows that the rate of electron transfer from the zinc porphyrin with the zinc porphyrin and its radical cation, coordination of moiety to PZ,,(~~)-IPF is sensitive to the electron-donating or pyridinelowers the first oxidation potential by 0.1 1 V. This effect -withdrawing propertiesof the pyridine substituents. The negative is masked by perchlorate electrolyte, which coordinates to the value of p shows that the rate of electron transfer is increased by zinc porphyrin radical cation. Similar cyclic voltammetric studies electron donation to the zinc porphyrin. Thus, a reasonable of model free base 3 with tetra-n-butylammonium hexafluorointerpretation of the effect of the ligand is that the coordination phosphate show identical first reduction potentials in the presence of the electron-donating pyridine helps stabilize a positive charge and absence of 0.82 M pyridine. on the zinc porphyrin moiety and thus lowers the energy of the Given the interpretation that the coordination of pyridines to PZ,,(~~)'+-PF'charge-separatedstate. A report that pyridine binds 1 affects electron transfer via step 5 mainly through thermodyto the zinc tetraphenylporphyrin radical cation18 is consistent namic stabilization of the charge-separated state, it is interesting with this interpretation. Binding of a pyridine would therefore to note that the effect of the coordination of pyridine on the rate increase the thermodynamic driving force for electron-transfer constant for step 6 (a factor of 10) is only about one-third as large step 5 in Scheme I. If the stabilization were greater than the loss as the effect on step 5 . This is the case even though the final of singlet excitation energy resulting from the red shift of the zinc PZ~(~~)*+-PF'species is the same in both cases, and therefore the porphyrin absorption spectrum upon coordination, pyridine stabilization of that state should be identical for both reactions. binding would increase the driving force for step 6 as well. In This effect is due mainly to the decrease in thermodynamic driving the pyridine-free case, the free energy change has been found to force for step 6 resulting from the red shift of the absorption and be -0.32 eV for step 5 and -0.48 eV for step 6 , based on emission spectra of the zinc porphyrin upon binding pyridine spectroscopic and cyclic voltammetric studies of the dyad.' As (0.06 eV). No such shift was observed for the free base porphyrin, has been shown previously for other porphyrin-containing speand the driving force for step 5 therefore increases more than cie~,491~.~O an increase in thermodynamic driving force in this region does that for step 6 upon coordination of pyridine. Other effects of reaction exergonicity should lead to an increase in electronmay be operative as well, because it is evident from the spectral transfer rate, as is observed. shifts that the coordination has different effects upon the highest Consistent with this interpretation is the fact that the oxidation occupied and lowest unoccupied molecular orbitals of the zinc potentials of zinc and other metallated tetraarylporphyrins are porphyrin. sensitive to solvent and other coordinating ligands including substituted pyridine^.^^-^' However, it has been pointed out in Conclusions the literature that coordination of pyridine itself to zinc tetIt is clear from these results that coordination of ligands to raphenylporphyrin has no apparent effect on electrochemically metal porphyrins can have large effects on interporphyrin electronmeasured oxidation potentials.21x26 For example, Kadish and cotransfer rates, including those of light-initiated processes. The workers reported that the half-wave potentials us SCE for the results of the Hammett study indicate that the magnitude of the first oxidation of zinc tetraphenylporphyrin in dichloromethane h
13642 The Journal of Physical Chemistry, Vol. 97, No. 51, 1993
effect correlates with the electron-donating ability of the substituent, and this in turn suggests that a major portion of the effect may be ascribed to stabilization of a positive charge on the metallated porphyrin. The cyclic voltammetric results are consistent with this interpretation. It is reasonable to suppose that in natural protein systems coordination of an amino acid side chain to a metallated porphyrin can both perform a structural role and tune redox potentials to control the rates of various electron-transferevents. With regard tomodel systems in solution, the present results indicate that care must be taken to ensure that coordination with solvent molecules, ions, or other adventitious ligands does not obscure or interfere with trends under study. For example, solvent effects on electron-transfer rates in molecules containing metallated porphyrins are commonly interpreted in terms of general solvent properties such as dielectric constant. The present work shows that solvents capable of donating an electron pair may serve as ligands to the central metal ion and thus exert very specific effects that are not accounted for by the bulk properties of the solvent. Experimental Section The preparation of dyad 1 and model compounds 2 and 3 has been reported elsewhere.’ The pyridine ligands were obtained commercially and purified before use. The dichloromethane solvent was purified by distillation from calcium hydride and treatment with anhydrous potassium carbonate in order to remove water and traces of acid. Steady-state fluorescence and fluorescence excitation spectra were measured using a SPEX Fluorolog-2. Excitation was produced by a 450-W xenon lamp and single grating monochromator. Fluorescence was detected at a 90° angle to the excitation beam via a single grating monochromator and an R928 photomultiplier tube having S-20 spectral response operating in the photon counting mode. Fluorescence decay measurements were made using the timecorrelated single photon counting method. The excitation source was a frequency-doubled, mode-locked Nd:YAG laser coupled to a synchronously pumped, cavity-dumped dye laser with excitation at 590 nm. Detection was via a microchannel plate photomultiplier (Hamamatsu R2809U-1 l ) , and the instrument response time was ca. 35 ps. The apparatus has been described in detail elsewhere.29 Cyclic voltammetric measurements were performed using a three-electrode system and a Pine Instruments Model AFRDE4 potentiostat. The cell featured a glassy carbon working electrode and salt bridges to an SCE reference electrode and a platinum wire counter electrode. Electrolytes were recrystallized and dried before use, and the cell was kept under an atmosphere of nitrogen. Acknowledgment. This research was supported by the National Science Foundation (CHE-8903216). This is publication 173 from the Arizona State University Center for the Study of Early
Gust et al. Events in Photosynthesis. The Center is funded by US. Department of Energy Grant DE-FG02-88ER13969 as part of the US. Department of Agriculture-Department of EnergyNational Science Foundation Plant Science Center Program. References and Notes ( I ) Connolly, J. S.;Bolton, J. R. In Photoinduced Electron Transfer, Part D Fox, M. A., Channon, M., Eds.; Amsterdam: Elsevier, 1988; pp 303-393. (2) Gust, D.; Moore, T. A. Advonces in Photochemistry 1991,16, 1-65. (3) Gust, D.; Moore, T. A. Topics in Current Chemistry 1991,159,103151. (4) Wasielewski, M. R. Chem. Rev. 1992, 92, 435461. ( 5 ) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198-205. (6) Gust,D.;Mwre,T.A.;Moore,A.L.;Leggett,L.;Lin,S.;DcGraziano, J. M.; Hermant, R. M.; Nicodem, D.; Craig, P.; Seely, G. R.; Nieman, R. A. J . Phys. Chem. 1993, 97, 79267931. (7) .Gust, D.; Moore, T. A.; Moore, A. L.; Gao, F.; Luttrull, D.; DeGraziano, J. M.; Ma, X. C.; Makings, L. R.; Lee,S.-J.; Trier, T. T.; Bittersmann, E.; Seely, G. R.; Woodward, S.;Bensasson, R. V.; Rougk, M.; De Schryver, F. C.; Van der Auweraer, M.J. Am. Chem. Soc. 1991, 113, 3638-3649. (8) Gust, D.; Moore, T.A.; Moore, A. L.; Lee, S.-J.; Bittersmann, E.;
Luttrull,D.K.;Rehms,A.A.;DeGraziano,J.M.;Ma,X.C.;Gao,F.;Belford, R. E.; Trier, T. T. Science 1990, 248, 199-201. (9) See, for example, refs 10 and 11. (10) Scheer, H.; Katz, J. J. Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; New York: Elsevier, 1975; Chapter 10. (11) Nappa, M.;Valentine, J. S.J . Am. Chem. SOC.1978, 100, 50755080. (12) Kirksey, C. H.; Hambright, P.; Storm, C. B. Inorg. Chem. 1969, 8, 2141-2144. (13) Whitten, D. G.; Lopp, I. G.; Wildes, P. D. J. Am. Chem. Soc. 1968, 90, 71967200. (14) Wendler. J.; Holzwarth, A. Biophys. J . 1987, 52, 717-728. (15) Gust, D.; Neal, D. N. J . Chem. SOC.,Chem. Commun. 1978,681682. (16) McDaniel, D. H.; Brown, H. C. J. Org. Chem. 1958, 23,420. (17) Ritchie, C. D.; Sager, W. F. Prog. Phys. Org. Chem. 1964,2,323400. (18) Fujita, I.; Hanson, L. K.; Walker, F. A.; Fajer, J. J. Am. Chem.Soc. 1983, 105, 3296-3300. (19) Harrison, R. J.; Pearce, B.; Beddard, G. S.;Cowan, J. A.; Sanders, J. K. M. Chem. Phvs. 1987.116. 429448. (20) Joran, A. D.;Leland, B. A,;Felker, P.M.; Zewail, A. H.; Hopfield, J. J.; Dervan, P. B. Nature (London) 1987,327, 508-51 1. (21) Kadish, K. M.; Shiue, L. R.; Rhodes, R. K.;Bottomley, L.A. Inorg. Chem. 1981, 20, 12741277. (22) Kadish, K. M.; Bottomley, L. A.; Kelly, S.;Schaeper, D.; Shiue, L. R. Bioelectrochem. Bioenergetics 1981, 8, 213-222. (23) Constant, L. A.; Davis, D. G. J . Electroanal. Chem. 1976,74,85-94. (24) Walker, F. A.; Beroiz, D.; Kadish, K. M. J . Am. Chem. Soc. 1976, 98. 34843489. (25) Kadish, K. M.; Cornillon, J.-L.; Yao, C.-L.; Malinski, T.; Gritzner, G. J. Electroanol. Chem. 1987, 235, 189-207. (26) Manassen, J. Isr. J. Chem. 1974, 12, 1059-1067. (27) Hinman, A. S.;Pavelich, B. J. J. Electroonal. Chem. 1989, 269, 53-61. (28) Seki, H.; Hoshino, M.;Shizuka, H. J . Phys. Chem. 1989,93,36303634. (29) Gust, D.; Moore, T. A.; Luttrull, D. K.; Seely, G. R.; Bittersmann, E.; Bensasson, R. V.; Rougk, M.; Land, E. J.; De Schryver, F. C.; Van der Auweraer, M. Photochem. Photobiol. 1990, 51, 419426.
