18868
J. Phys. Chem. 1996, 100, 18868-18874
Photoinduced Electron Transfer in Porphyrin Quinone Donor/Acceptor Pairs: pH-Dependent Free Ion Yield Tione Buranda,*,† Neil Soice,† Shui Lin,‡ Randy Larsen,‡ and Mark Ondrias*,† Department of Chemistry, The UniVersity of New Mexico, Albuquerque, New Mexico 87131, and Department of Chemistry, UniVersity of Hawaii at Manoa, Honolulu, Hawaii 96822 ReceiVed: June 19, 1996; In Final Form: August 29, 1996X
The protonation of 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UQ0) has been determined to improve the quantum efficiency of charge separation between a zinc meso-tetra(4-sulfonatophenyl)porphine (MP) donor and UQ0 (UQ0H+ upon protonation) acceptor system. The primary effect of protonation is to increase the driving force of the initial photoinitiated electron transfer (ET). Although this does not alter the diffusionlimited rate of forward ET, it may have a substantial effect on the nature of the ion pair formed by the initial ET and the subsequent competition between free ion separation and back electron transfer (BET). Transient absorption data show that although the rates of ET from photoexcited 3MP* to UQ0 or UQ0H+ are diffusion limited, there is a >3-fold increase in free ion product yields upon UQ0 protonation. The results are assessed in terms of current electron transfer theories. It is argued that the increase in product yield is a result of BET for the reactions of UQ0H+ being deeper in the normal Marcus region. The importance of these results for understanding trends in the rates of BET is discussed.
I. Introduction The primary step in photoinduced electron transfer (PET) processes is the conversion of electronic energy of an excited state into redox energy.1-7 In order to retain this energy in chemically useful forms, it is necessary to suppress back electron transfer within the original photoexcited donor/acceptor radical ion pair (D•+/A•-). Hence, an often stated goal of photoinduced electron transfer studies in molecular systems is the efficient creation of charge-separated donor/acceptor pairs whose lifetimes are sufficiently long to allow for useful subsequent processes.1-4 One strategy for the generation of long-lived, charge-separated species is to suppress the back electron transfer (BET) rate by the tuning of Franck-Condon factors and donor/ acceptor coupling.2,3 Considerable effort has been expended in synthesizing molecular mimics of the photosynthetic reaction center.1-4 Much attention has focused on covalently linked systems, which enable some control of donor/acceptor separation as well as relative orientation.1 An alternative approach is to create PET systems from independent, diffusionally interacting donor/acceptor pairs, where forward electron transfer (ET) is quite efficient and D•+/ A•- dissociation is competitive with BET.5,6 The appealing features of bimolecular systems as potential agents for use in photomolecular devices2b are simplicity, ready availability, and durability. In addition, confinement of donor/acceptor pairs in microheterogeneous media such as micelles or microemulsions can allow flexible control of reactants in a potential solar energy conversion device.7 Metalloporphyrins and quinones play an essential role in electron transport in natural systems.4,5,8-10 Thus, these molecules have been the subject of extensive work designed to understand and ultimately mimic their electron transfer proclivities in model systems.1 The versatility of quinone reactivity in respiration and photosynthesis has been attributed to the sensitivity of quinone redox properties to structural and envi†
The University of New Mexico. University of Hawaii at Manoa. X Abstract published in AdVance ACS Abstracts, November 1, 1996. ‡
S0022-3654(96)01827-8 CCC: $12.00
ronmental influences.9 In the living cell for instance, accessibility of protons to quinones is regulated by the protein environment, thus modulating different redox characteristics on the quinone species.10 Protonation of quinones and the ensuing effect on quinone redox potentials have been the subject of numerous investigations employing electrochemical, NMR, or CIDNP techniques.11,12 Here, we report the results of our investigation of the kinetics and quantum yields of charge-separated radical ion species obtained by using light-promoted reactions to generate excited electron transfer transients from diffusive encounters between the 3MP* donor and the ubiquinone acceptor.11 Zinc mesotetra(4-sulfonatophenyl)porphine (MP) (Porphyrin Products) and 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UQ0) (Aldrich) were used as a prototypical electron transfer system, and UQ0 was protonated through addition of trifluoroacetic acid to the solvent.
Higher quantum yields of charge-separated radical ion species were observed upon protonation of UQ0. II. Experimental Section Materials. Zinc meso-tetra(4-sulfonatophenyl)porphine (Pophyrin Products; Logan, Utah) and 2,3-dimethoxy-5-methyl1,4-benzoquinone (Aldrich), trifluoroacetic acid (Aldrich), absolute ethanol (Midwest), deuterated ethanol (Aldrich), and dimethylformamide (Fisher) were used without further purification. © 1996 American Chemical Society
Porphyrin Quinone
J. Phys. Chem., Vol. 100, No. 48, 1996 18869 SCHEME 1: Qualitative Energy Diagram for Photoinduced Electron Transfer
Figure 1. Ground state absorption spectra showing the difference between (broken line) metalated and (solid line) demetalated porphyrin.
A. Electrochemistry. Electrochemical measurements were performed in a single compartment cell housing a reflective platinum disk (area ) 0.33 cm2) working electrode with a silver wire reference electrode and a platinum wire spiral counter electrode. A Princeton Applied Research (PAR) Model 173 potentiostat-galvanostat along with a PAR 175 universal programmer was used for electrochemical measurements. Data collection was performed in 0.01 M ethanolic and DMF solutions of 2,3-dimethoxy-5-methyl-1,4-benzoquinone with vacuum-dried electrochemical grade tetra-n-butylammonium perchlorate (TBAP) (Southwestern Analytical) as the supporting electrolyte. The solutions were deaerated with ultrapure nitrogen gas. MP data were collected in DMF solution. Ferrocene was used as an internal standard in all the solutions to ensure a consistent data set. B. Transient Absorption. Transient absorption spectroscopy can yield quantitative information about the free ion yields, provided that the extinction coefficients of the transient species are known. In this work, the ET dynamics and product yields were monitored via absorption bands corresponding to the triplet and radical ion species of the MP. In order to make accurate comparisons of free ion yields at high and low pH, it is necessary to ensure uniform absorption behavior of the porphyrin under acidic and neutral solution conditions. Demetalation of the metalloporphyrin is a common problem in aqueous solutions at low pH.5 However, this problem is minimized in ethanol. When demetalation of the MP occurs, it is clearly manifested by a solution color change from light purple to green, the latter being the color of the free-base porphyrin (Figure 1). Thus, absorption spectra of the MP under acidic conditions were taken before and after each experiment to check for sample degradation, and under our experimental conditions, the MP absorptions at high and low pH remained the same. A further advantage of using ethanol over aqueous solutions is that porphyrins generally tend to aggregate in water but not in organic solvents.5 The output from the second harmonic of a Spectra Physics DCR-2 YAG laser was used as the pump (10 ns, 10 Hz) and a 75 W xenon lamp (Photon Technologies International) as the probe. The probe beam was focused into the middle of the sample where it was crossed with the pump beam. The transmitted probe beam was focused onto a slit of a Spex Triplemate spectrograph coupled to a gated EG&G Princeton Applied Research 1420 diode array (1024 channels, with 800 intensified) detector equipped with a 1461 controller interfaced (National Instruments GPIB) to a 486 PC computer (Equus).
OMA2000 software was used for data collection. Macros written within the OMA2000 software were used to semiautomate the collection process. Processing of raw data to obtain transient absorption profiles was done through a program written by Dr. Ian Gould (Imaging Research Labs, Eastman Kodak Company). Typically, for each absorption profile, intensities were measured for the background, the ground state (probe only), excited state (pump/probe), and pump only. The background and pump-only data sets were used for dark and emission or scattered laser light corrections, respectively. A typical transient absorption spectrum using a 300 g/mm grating present in our setup encompasses an ∼100 nm range. Time resolution was achieved by using a PAR 1304 gate pulse amplifier (range 50 ns to 10 ms). Millimolar solutions of UQ0 were analyzed in 1 mm cuvettes equipped with long necks for nitrogen or argon purging to remove dissolved oxygen. Sufficient amounts (∼10-5 M) of MP to give an optical density of about 1.0 at the Soret absorption maximum were added to the UQ0 solution. Ground state UV-vis absorption spectra were taken on a HP8452A diode array spectrometer as well as on the transient absorption spectrometer. III. Results A. Transient Absorption Studies. Absorption Spectra of Transient Species. As shown in Scheme 1 below, MP, 3MP*, and MP•+ can all be expected to contribute to the net absorption spectra subsequent to photoexcitation. Although a fraction of the photoexcited 1MP* returns to the ground state via radiative and nonradiative processes (internal conversion, intersystem crossing, and ET quenching), this occurs too rapidly for detection in the present study.17 In order to make straightforward comparisons of experimental results at high and low pH, it is necessary to characterize the behavior of these species over the range of solution pH conditions employed. Excitation of MP is manifested by bleaching of the ground state 424 nm Soret band of the MP and the appearance of a broad absorption band around 400 nm indicating the formation of the triplet state (3MP*).14 In the absence of a quencher, 3MP* is formed within the instrument rise time (∼50 ns) and decays on a millisecond time scale. The excited state dynamics as shown by the bleaching and triplet absorption at low and high pH indicate uniform excited state behavior in both solutions (Figure 2A). Spectra of MP•+ were reconstructed from transient difference and ground state spectra using the method reported by Orbach and Ottolenghi.13 Absorption spectra of the photogenerated MP•+ radical ions (absorption maximum 410 nm) in neutral and acidic solutions (Figure 2B) confirm that the absorption characteristics of the radical ions are independent of pH, like those of the ground state. Photoinduced Electron Transfer. Diffusive encounter between 3MP* and UQ0(H+) leads to electron transfer quenching
18870 J. Phys. Chem., Vol. 100, No. 48, 1996
Figure 2. Control experiments: (A) difference transient absorption spectra of excited state 3MP* showing invariance of triplet decay kinetics in the absence of Q at neutral and low pH between 0 ns and 10 µs; (B) normalized absorption spectra of MP•+ at high and low pH. The spectra are displayed to demonstrate the pH independence of the absorption characteristics of radical cations.
of the donor excited state. The time-resolved evolution of the MP•+ radical cation, measured at 410 nm, was of the same kinetic origin as the disappearance of 3MP* absorption at 440 nm. This is summarized in Figure 3. Quenching rates were measured at different concentrations, and the average rate constants were very similar regardless of solution conditions, implicating diffusion-limited kinetics15 (Vide infra). Thus, the intrinsic forward ET rates in all cases exceeded kq[Q] (Q ) UQ0 or UQ0H+(D+)). The data are summarized in Table 1. Quantum Yields (φion). The range of [UQ0] or [UQ0H+] was such that the quenching of 3MP* was diffusive (kq[Q] ≈ 106 s-1), notwithstanding the fact that some amount of static quenching of 1MP* was observed.16 However, singlet-correlated ET products were not observed owing to spin-allowed ultrafast BET.17 Hence, the observed yields were strictly those of triplet origin. By evaluating the free ion quantum yields relative to the initial triplet concentrations (Table 1, note 3), we avoid the need to assess the amount of loss due to singlet quenching. Free ion yields were obtained from the highest concentration (5 × 10-3 M) of the UQ0(H+) samples to ensure complete interception of the triplet state. We have used an extinction coefficient for MP•+ of 2 × 105 M-1 cm-1 obtained from electrochemical data.18 Unlike the diffusive quenching rates, the free ion yields related to the UQ0H+/3MP* pair were 3 times greater than those of the UQ0/3MP* dyad. For the experiments conducted in
Buranda et al. deuterated ethanol, minimal deuterium isotope effects were observed, suggesting the lack of a H-bonding role in the ET process between 3MP* and UQ(H+). B. Determination of ET Free Energies (∆Get, ∆GBET). The free energies of electron transfer can be estimated from redox potentials determined from cyclic voltammetry. The reduction potentials for 0.01 M UQ0 in ethanol and DMF are shown in Table 2. The E1/2 values are in good agreement with those in the literature.19 The differences in the potentials in ethanol and DMF arise from protonation reactions of quinones in protic solvents. Thus, quinone electrochemistry in protic solvents is usually resolved in terms of a set of electron transfer and proton transfer subreaction equilibria.20 The reduction of quinones in protic media (pH < 6) is generally considered to proceed via an electrochemical (Q + e h Q•), chemical (Q•- + H+ h QH•), and electrochemical (QH• + e h QH-) (ECE) scheme. Within this framework the electrochemical reduction of ubiquinone in DMF forms the semiquinone radical, UO0•-, during the cathodic sweep, which is followed by the reverse process via the anodic sweep. In ethanol, however, the low sweep rates used (50 mV/ s) in this work enables the semiquinone radical anion to undergo protonation before or during the anodic sweep. As a result, the electron (E) and proton (C) transfer sequence is kinetically coupled to the extent that the electrochemical waves for the electron and proton transfer appear as one. The net result is a larger anodic-cathodic peak separation (∆Ep ≈ 200 mV) and a more positive potential due to UQ0H•. Thus, the reduction potential measured in ethanol may not represent a useful thermodynamic quantity.19 Therefore, values measured in DMF will be used as reasonable estimates in the free energy calculations. The presence of a strong acid such as TFA leads to protonation of UQ0 prior to electrochemical reduction.20-22 This is manifested by the appearance of a new anodic peak at about 0.6 V. However, the separation between the anodic and cathodic peaks (∆Ep ≈ 1 V) indicates an irreversible process otherwise complicated by the various protonation and electron transfer equilibria. Such effects have been reported and widely discussed.20 Electrochemical characterization of protonated quinones using rotating-disk electrode voltammetry, cyclic voltammetry, etc. have enabled other workers to fully characterize the protonated quinones.20 Reversible reduction of protonated p-benzoquinone has been reported in the literature at 0.2 eV,22 and this allows us to set an upper limit for the free energies associated with ET to the UQ0H+. The Rehm-Weller equation has been commonly used to estimate free energy changes for photoinduced electron transfer between donor (D) and acceptor (A) species in a solvent with a static dielectric constant, s, and is given in eq 1.23
∆GET ) e[EDox - EAred] - Etriplet +
(
)
(
)
1 1 1 1 e2 1 e2 + + (1) r 2r 4π0s 2rA 2rD rDA 2r 4π0s A D The ionic radii rA and rD are those of the donor (MP) and acceptor (UQ0), which are separated by a distance rDA. EAred and EDox are the respective reduction and oxidation potentials of the acceptor and donor as determined in a reference solvent of dielectric constant sr, and Etriplet is the energy of the 3MP* state (∼1.6 eV).14 e and 0 are the electronic charge and permittivity of free space, respectively. Equation 1 describe the free energy of the forward electron transfer. However, deletion of the excited state energy term gives an expression for the driving
Porphyrin Quinone
J. Phys. Chem., Vol. 100, No. 48, 1996 18871
Figure 3. Representative transient absorption difference spectra at 300 ns, after 532 nm laser excitation of ∼10-3 M UQ0 (or UQ0H+) and 10-6 MP, of argon-purged solution showing 3MP* absorption ∼440 nm. Addition of UQ0 to solution results in electron transfer quenching, as shown by bleach recovery of Soret absorption at 424 nm and growth of MP•+ absorption at 410 nm. Solid line indicates transient difference spectrum of 3MP* alone; broken line represents the spectrum showing the quenching of the 3MP*. Part A shows the time evolution of MP•+. mOD∞ indicates the absorbance at maximum growth and mOD refers to optical density (in milli OD units) at each time delay. Part B indicates the decay of the triplet state. Here, mOD∞ is the product absorption (if any) at the end of the quenching process.
TABLE 1: Photoinduced ET Quenching of 3MP* with Protonated and Unprotonated Ubiquinone kq b 109 M-1 s-1 [Q]a × 10-3 M
UQ0c
UQ0H+ d
5.0 2.5 1.5 1.0 0.5 0.25 average
2.5 2.4 2.3 3.1 2.3 3.0 2.6 ( 3
3.2 3.0 2.9 3.0 3.6 3.0 3.1 ( 0.2
UQ0D+ e 3.1 2.8 3.0 2.9 ( 1
φionf 0.17
0.51
0.41
a
Ubiquinone concentrations in ethanol. b Rate constant for diffusive quenching by UQ0, UQ0H+, and UQ0D+. c Neutral ubiquinone solution. d Protonated ubiquinone, 0.25 M TFA added. e Protonated ubiquinone in deuterated ethanol. f φion ) ([MPi•+]t)∞/[MPi*]t)0), i ) UQ0, UQ0H+, 3 •+ UQ0D+, and [3MP* i represents initial concentration of MP*, MPi indicates the concentration of MP•+ at t ) ∞ (Figure 3).
TABLE 2: Summary of E1/2 Values of Ubiquinone in Ethanol and DMFc solvent
[TFA] M
∆Epa mV
E1/2b V
ethanol
0 0.25 0
200 1000 80
-0.36 irrev -0.58
DMF a
Anodic and cathodic peak separation. b Standard redox potentials determined the usual way by averaging cathodic and anodic peaks.c All experiments performed with 0.05 M solutions of UQ0 at 50 mV/s vs Ag/AgCl.
force for BET, -∆GBET. Application of (1) requires the problematic assumption of spherical radii to characterize the clearly nonspherical donor/acceptor pair and some reasonable estimate of their mutual separation. Nevertheless previous estimates of rD and rA have been demonstrated to work quite well for a set of porphyrin quinone species when the radii were ∼7 and 3 Å, respectively.1 For the bimolecular system used here, consideration will be given to a range of rDA values (Vide infra).
IV. Discussion In this study we have used transient absorption spectroscopy to examine charge separation dynamics in a simple bimolecular ET model system. Irradiation at the porphyrin Q band (532 nm) yields the initial singlet state (τ ≈ 2 ns), which rapidly undergoes intersystem crossing (Φ ≈ .8) 5 to form the triplet state (τ ≈ 1 ms).5 Diffusive quenching of the triplet excited state by the redox partner leads to primary charge-separated species such as exciplexes and solvated geminate radical ion pairs. In the absence of chemical reactions or other processes within the geminate radical pair, the quantum yield of free ion formation is affected by competition with first-order back electron transfer (kBET in Scheme 1) and separation (ksep in Scheme 1) to regenerate the starting materials.23 Upon protonation of UQ0, the quantum yield of free ion formation is increased by a factor of 3. Scheme 1 shows that ET initially produces a geminate radical ion pair. In this instance forward transfer, kET, is limited by diffusion. Thus, in both cases φion must be a function of different competitive efficiencies of ksep and kBET at high and low pH, respectively. It is reasonable to expect that these processes are strongly influenced by the structural nature of the geminate pairs. The geminate radical ion pair produced by the forward ET process can take the form of contact (CRIP) or solvent-separated radical ion pairs (SSRIP).24 The mechanism by which these ET intermediates are formed has been the subject of considerable discussion.24,25 Recently, a systematic study of ET reactions in organic aromatic systems by Gould et al.26 has shown an increasing dominance of the SSRIP-forming pathway with increasing driving force. Similar arguments can be used to explain the differences in radical yields obtained at low and high pH. Semiclassical electron transfer theories3 allow us to clarify the roles of several factors that may influence the mechanisms of the forward and back electron transfer. A. Diffusion-Limited Electron Transfer and the Nature of the Ion Pair. Consideration of electron transfer as a nonradiative process and separation of nuclear and electronic motions lead to rate constant expressions that involve the product of an electronic matrix element squared (V 2), which is related
18872 J. Phys. Chem., Vol. 100, No. 48, 1996
Buranda et al.
to the electronic coupling between the electron donor and electron acceptor, and a nuclear factor, commonly approximated as a thermally weighted sum of Franck-Condon factors (FCWD) (eq 2).
k) ∞
FCWD ) ∑(4πλskBT)
2π 2 |V| FCWD p
-1/2
j)0
(
Fj exp -
(2a)
)
(g + jhν + λs)2 4λskBT
(2b)
λν Sj Fj ) exp(-S) , S ) ; g ) ∆Get or ∆GBET j! hν Equation 2b reproduces the basic elements of classical theories3a with the advantage of quantitative distinction between classical solvent and quantum mechanical (hν g 200 cm-1) vibrational reorganization energies. The FCWD term contains the dependence on the reaction exothermicity. The modes associated with molecular vibrational reorganization energies (λv) are treated quantum mechanically. The frequencies of these modes are commonly treated as a single averaged frequency ν. The rearranged modes connected to the solvent reorganization energy (λs) are treated classically. λs is estimated as in eq 3 using the dielectric continuum model of Marcus3b, where n is the index of refraction of the solvent and the rest of the parameters have been previously defined.
λs )
(
)(
)
∆e2 1 1 1 1 1 + 4π0 2A 2D rDA n2 s
(3)
Electron transfer may occur while the donor and acceptor approach each other. If so, the dynamics of the ET process is strongly dependent on the distance (rDA), or range of distances, over which ET occurs.15,25a,28 We believe that at large separations kET (rDA) is larger for UQ0H+ than for UQ0. In principle rDA may take a range that spans separations typically attributed to a CRIP (∼4 Å 24) to ∞. However, the upper limit of rDA is clearly curtailed by V 2 and FCWD. We have used eqs 1 and 3 to calculate the values of ∆GET and λs as functions of rDA. We have taken the assumption that rDA typically ranges from ∼4 Å (CRIP) to ∼8 Å (SSRIP).24 However, we have extended the separation to >10 Å to account for the edge to edge distance in our system. The results are summarized in Figure 4A. Some important inferences can be made from Figure 4A. To do so, it is most convenient to turn to classical Marcus theory.3 If we assume that λs . λv, or more suitably hν . λv, eq 2b simplifies to the classical descriptions of Marcus theory. The rearranged activation energy expression, ∆GqET) (λ/4)(1 + ∆GET/λ)2, conveys a decrease in kET with increasing exorthermicity (the Marcus inverted region) for reactions that are more exothermic than the sum of λs and λv (total reorganization energy, λ). λs(rDA) increases with increasing distance of separation and as -∆GET(rDA) + λs(rDA) f 0, and kET(rDA) becomes larger. Figure 4A shows that the sum of the reorganization energy and the driving force is smallest at a much higher value of rDA (>10 Å) for UQ0H+ than UQ0 ( λs(rDA)) and the -∆GET(rDA) and λs(rDA) curves representing UQ0H+ become more divergent. Addition of λv to λs(rDA) illustrates the net effect of truncating rDA at which transfer may occur. Deuteration is expected to lead to a priori lowering of the high-frequency modes associated with ET (ν in eq 2b) and possible correspondingly lower values of λv. This might lead one to expect faster forward ET rates
Figure 4. (A) Plots of calculated values of electron transfer driving force ∆Get and solvent reorganization energy λs as a function of donor/ acceptor separation distance. 3MP*‚‚‚UQ0 (circles) and 3MP*‚‚‚UQ0H+ (crosses) curves represent unprotonated and protonated encounter complexes, respectively. λs is assumed to be unaffected by protonation. λs + λv curve represents λv ) 0.2 eV. (B) Calculated FCWD curves (eq 2b) for ET representing protonated (crosses) and unprotonated (circles) ubiquinone. Each point was calculated using the appropriate ∆GET(rDA) and λs (rDA). The sum was evaluated for j ) 0-20. The deuterium isotope experiments infer negligible effects on the internal vibrational frequencies associated with ET. Thus, the same values of hν ) 1500 cm-1 and λv ) 0.2 eV were assumed for the UQ0 and UQ0H+.
and lower BET, rates leading to even higher free ion yields upon deuteration. Nevertheless, since this is not the case, we conclude that the internal reorganizations associated with electron transfer to UQ0H+ do not involve the exchangable proton. The differences in the interplay between -∆GET(rDA) and λs(rDA) have a substantial effect on the Franck-Condon factors for forward ET. Figure 4B depicts net effects of -∆GET(rDA) and λs(rDA) on the separation distances at which diffusional ET occurs in the MP/quinone system. The FCWD(rDA) curves were calculated for UQ0 and UQ0H+ using typical1,24e hν and λv values (1500 cm-1 and 0.2 eV, respectively) for similar systems. Thus, for sufficient donor/acceptor coupling1-4,29-33 the trends in -∆GET(rDA) and λs(rDA) indicate how FC factors may conspire to govern the average separation distances at which electron transfer may occur in neutral or TFA-containing solutions.
Porphyrin Quinone
J. Phys. Chem., Vol. 100, No. 48, 1996 18873 charge recombination is enhanced by favorable Franck-Condon factors, as Figure 5 shows.35,36 V. Summary
Figure 5. Calculated FCWD curves representing BET for protonated (crosses) and unprotonated (circles) ubiquinone as a function of rDA (cf. Figure caption 4B).
The general picture that emerges can be summarized as follows: (1) forward ET is so fast that it occurs every time a 3MP* meets UQ or UQ H+, (2) ET will occur at some 3MP*/Q 0 0 separation distance, dependent on the ET driving force, (3) from the respective average driving forces of ∼0.45 eV (UQ0) and ∼1.25 eV (UQ0H+) we infer that ET occurs from a distribution25a of donor/acceptor separation distances and geometries with, on average, the shorter distances being associated with UQ0 and those at longer distances corresponding to UQ0H+,26 and (4) the invariance of both the quenching rate and relative free ion yields to deuteration strongly suggests that high-frequency internal modes and/or hydrogen bonding plays no significant roles in the reorganizational energies associated with the transfer process. B. Free Ion Yields and Back Electron Transfer. The relative free ion yield results, φion(UQ0H•) > φion (UQ0•-), implicate less efficient charge recombination (kBET) upon ubiquinone protonation. Consequently, the pathway through which separation (ksep) occurs, at the expense of charge recombination, produces a higher product yield at low pH. Coulombic effects on ksep are expected to favor repulsion between the UQ0•- radical and the negatively (due to sulfonatophenyl groups) charged porphyrin over the neutral UQ0H• species. Thus, the higher yield at low pH suggests ksep may not be the dominant factor determining φion. Moreover, the similarity of forward diffusive encounter rates strongly suggests that ksep may also be independent of pH.34 It is then most likely that the differences in φion arise from different BET rates. The conceptual view of the respective encounters of 3MP* with UQ0 and UQ0H+ leading to forward ET (Vide supra) has profound consequences for the relative efficiencies of back electron transfer for the two systems. Figure 5 depicts FCWD(rDA) curves corresponding to BET for MP•+/UQ0•- and MP•+/ UQ0H• ion pairs (using the same assumptions as in Figure 4B). Some important conclusions can be drawn from the relative ranges of rDA over which transfer is favorable for the forward and reverse processes. FCWD(rDA) in the rDA < 5 Å domain favor BET for MP•+/UQ0H•. The experimental evidence, which shows more free ions at low pH, suggests that contact radical ion pair formation is not a major pathway in this case. This is consistent with the previous assertion that the ET process involving UQ0H+ occurs at larger separation distances. In the case of UQ0 it is clear that forward transfer (Figure 4B) is likely to occur in the range spaning 4 Å < x < 6 Å. In this region,
The results of this study demonstrate that solution conditions can be used to regulate the net yield of separated ions produced by PET in a simple bimolecular system. The experimental evidence suggests that protonation of a quinone acceptor has little or no effect on the net initial electron transfer in dilute solutions. However, the net yield of charge separation is substantially increased in the protonated acceptor. A probable mechanism of this phenomena lies in the substantial influence of the Franck-Condon factors on the relative efficiencies of forward and back electron transfer. We conclude that variations in FC factors produce qualitatively different ion pair structures for the UQ0H+ and UQ0 acceptors, which in turn exhibit distinctive dynamics subsequent to the initial ET. The larger rDA of the MP•+/UQ0H• transients favors more efficient free ion separation (versus BET) than the “CRIP” formed with UQ0 acceptors. This work has demonstrated a relatively simple means by which ET reactivity can be controlled by manipulating the solvent conditions, such as pH to improve charge separation in bimolecular ET systems. Acknowledgment. This work was supported by the NIH to M.R.O. (GM 33330) and PRF 2851064 to R.L. We are grateful to Dr. Su Moon Park for providing us access to the electrochemical equipment and Ms. Bertha Ortiz for assisting us with the cyclic voltammometric data collection. We thank Dr. Ralph Young for insightful criticisms of this paper from the initial draft to the present. We also acknowledge several useful discussions with Drs. Ian Gould, John F. Endicott, Peter Ogilby, and Martin Kirk. References and Notes (1) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198. (b) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (2) For reviews, see the following. (a) Fox, M. A., Chanon, M., Eds. Photoinduced Electron Transfer. Parts A-D; Elsevier: Amsterdam, 1988. (b) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Simon and Schuster: Hempstead, England, 1990. (3) (a) Newton, M. D.; Sutin, N. Annu. ReV. Chem. 1984, 35, 437. (b) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (a) Newton, M. D. Chem. ReV. 1991, 91, 767. (d) Buranda, T.; Lei, Y.; Endicott, J. F. J. Am. Chem. Soc. 1992, 114, 6916. (e) Kestner, N. R.; Logan, J.; Jortner, J. J. Phys. Chem. 1974, 78, 2148. (4) (a) The Photosynthetic Reaction Center Complex: Structure and Dynamics; Breton, J., Vermiglio, A., Eds.; Plenum Press: New York, 1988. (b) Plato, M.; Mobius, K.; Michel-Beyerle, M. E.; Bixon, M.; Jortner, J. J. Am. Chem. Soc. 1988, 110, 7279. (c) Franzen, S.; Goldstein, R. F.; Boxer, S. G. J. Phys. Chem. 1993, 97, 3040. (d) Beratan, D. N.; Onuchic, J. N. Photosynth. Res. 1989, 22, 173. (5) For extensive literature on related bimolecular ET processes, see the following. Kalyasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: New York, 1992, Chapters 1216, and references therein. (6) (a) Endicott, J. F.; Ramasami, T. J. J. Phys. Chem. 1986, 90, 3790. (b) Gould, I. R.; Ege, D.; Moser, J. E.; Farid, S. J. Am. Chem. Soc. 1990, 12, 4290. (7) Clapp, P. J.; Armitage, B.; Roosa, P.; O’brien, D. F. J. Am. Chem. Soc. 1994, 116, 9166, and references therein. (8) (a) Dolphin, D., Ed. The Porphyrins; Academic Press: New York, 1978; Vols. 1-5. (b) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. III, p 1. (9) (a) Burie, J.; Boussac, A.; Boullais, C.; Berger, G.; Mattioli, T.; Mioskowski, C.; Nabedryk, G.; Breton, J. J. Phys. Chem. 1995, 99, 4059. (b) Gunner, M. R.; Dutton, P. L. J. Am. Chem. Soc. 1989, 111, 3400. (10) (a) Functions of Quinones in Energy ConserVing Systems; Trumpower, B. L., Ed.; Academic Press: 1982. (b) Morton R. A. Biochemistry of Quinones; Academic Press: London, New York, 1965.
18874 J. Phys. Chem., Vol. 100, No. 48, 1996 (11) (a) The Chemistry of Quinonoid Compounds Part 1; Patai, S., Ed.; Wiley: New York, 1974; Vols. I and II. (b) The Chemistry of Quinonoid Compounds Part 2; Patai, S., Rappaport, Z., Eds.; Wiley and Sons: NY, 1988; Vols. I and II. (12) (a) Craw, M. T.; Depew, M. C.; Wan, J. K. S. Can. J. Chem. 1986, 64, 1414. (b) Gust, D.; Moore, T. A.; Moore, A. L.; Ma, X.; Nieman, R.; Seely, G. R.; Belford, R. E.; Lewis, J. E. J. Phys. Chem. 1991, 95, 4442, and references therein. (13) Orbach, N.; Ottolenghi, M. In The Exciplex; Gordon, M., Ware, W. R., Eds.; Academic Press: New York, 1975; p 75. (14) (a) See ref 5, chapter 12. (b) Rodriguez, J.; Kirmaier, C.; Holten, D. J. Am. Chem. Soc. 1989, 111, 6500. (15) Hug, G. L.; Marciniak, B. J. Phys. Chem. 1995, 99, 1478-1483, and references therein. (16) 1MP* has a lifetime of about 2 ns.14a Ubiquinone (10-3 M range) concentrations quenched some of the singlet state fluorescence due to donor/ acceptor preassociation. (17) (a) Holten, D.; Gouterman, M.; Parson, W. W.; Windsor, M.; Rockley, M. G. Photochem. Photobiol. 1976, 23, 415. (b) Gouterman, M.; Holten, D. Photochem. Photobiol. 1977, 25, 85. (18) Fajer, J.; Borg, D. C.; Forman, N. A.; Dolphin, D.; Felton, R. H. J. Am. Chem. Soc. 1970, 92, 3451. (19) Bauscher, M.; Mante¨le, W. J. Phys. Chem. 1992, 96, 11101. (20) (a) Chambers, J. In The Chemistry of Quinonoid Compounds Part 2; Patai, S., Rappaport, Z., Eds.; Wiley and Sons: New York, 1988; Vol II, p 719. (b) Chambers, J. In The Chemistry of Quinonoid Compounds Part 1; Patai, S., Eds.; Wiley and Sons: New York, 1974; Vol I, p 737, and references therein. (21) Cauquis, G.; Marbach, L. Biochim. Biophys. Acta 1972, 283, 239. (22) Eggins, B. R.; Chambers, J. Q. J. Chem. Soc., Chem. Commun. 1969, 232. (23) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259. (24) (a) Beens, H.; Weller, A. In Organic Photochemistry Molecular Photophysics; Birks, J. B., Ed.; Wiley: London, 1975; Vol. 2, Chapter 4. (b) Weller, A. Z. Phys. Chem. (Wiesbaden) 1982, 130, 129. (c) Weller, A. Z. Phys. Chem. (Weisbaden) 1982, 133, 93. (d) Masuhara, H.; Mataga, N. Acc. Chem. Res. 1981, 14, 312. (e) Gould, I. R.; Young, R. H.; Moody, R.; Farid, S. J. Phys. Chem. 1991, 95, 2068. (25) (a) Kakitani, T.; Yoshimori, A.; Mataga, N. J. Phys. Chem. 1992, 96, 5385. (b) Li, B.; Peters, K. S. J. Phys. Chem. 1993, 97, 7648. (c) Shannon, C. F.; Eads, D. D. J. Chem. Phys. 1995, 103, 5208. (26) Gould, I. R.; Young, R. H.; Mueller, L. J.; Farid, S. J. Am. Chem. Soc. 1994, 116, 8176. (27) No reference given. (28) Tachiya, M.; Murata, S. J. Phys. Chem. 1992, 96, 8441. (29) V varies exponentially with rDA, the distance of donor/acceptor separation. Hence, the upper limits of rDA at which ET is deemed possible
Buranda et al. based on favorable Franck-Condon factors alone may not be tenable (especially for UQ0H+). From Figure 4A, as rDA gets smaller for UQ0H+, ket approaches/enters the inverted region while the UQ0 reaction remains in the normal region. In some cases the forward rates for the two species may be equal but on opposite sides of the Marcus curve. (30) (a) Endicott, J. F. Acc. Chem. Res. 1988, 21, 59. (b) Mclendon, G. Acc. Chem. Res. 1988, 21, 160, and references therein. (31) (a) Newton, M. D. Chem. ReV. 1991, 81, 767. (b) Newton, M. D. In The Challenge of d and f Electrons; Salahub, D. R., Zerner, M. C., Eds.; ACS Symposium Series 394; American Chemical Society: Washington, DC; 1989; p 378. (c) Fischer, G. Vibronic Coupling; Academic Press: London, 1984. (32) Baede, A. P. M. AdV. Chem. Phys. 1975, 30, 463. (b) Kuznetzov, A. M.; Ulstrup, J.; Vorotyntsev, M. A. In The Chemical Physics of SolVation; Dogonadze, R. R., Kalman, E., Kornyshev, A. A., Ulstrup, J., Eds.; Elsevier: New York, 1988; Part C. (33) (a) Kuki, A.; Wolynes, P. G. Science (Washington D.C.) 1987, 93, 3030. (b) Kumar, K.; Lin, Z.; Waldeck, D. H.; Zimmt, M. B. J. Am. Chem. Soc. 1996, 118, 243. (34) (a) Standard diffusion-limited expressions predict an exponential dependence on the charges at the donor and acceptor. See the following. Steinfeld, J. I.; Fransisco, J. S.; Hase. W. L. Chemical Kinetics and Dynamics; Prentice Hall: New Jersey, 1989; Chapter 4. In our experiment MP4-/Q0, zAzB ) 0 and MP4-/QH+, zAzB ) 4 in principle predict a faster quenching rate for the latter pair. Since we observe similar forward rates, however, we have chosen to forgo this analysis in discussing ksep. Furthermore, the respective ET products, MP3-/Q0•- and MP3-/QH• would favor a larger ksep for the pair based on MP3-/Q0•- Coulombic repulsion. Our observation of fewer free ions in this case suggests that ksep may not be dominant in determining the relative yields. (b) See Vauthey, E.; Parker, A. W.; Bohdana, N.; Phillips, D. J. Am. Chem. Soc. 1994, 116, 9182, for a discussion that includes experimental determination of ksep. (35) The MP•+/UQ0•- curve indicates overwhelmingly favorable FCWD(rDA) for BET, and the survival of a reasonable population of free ions in this instance, and is testimony to a small value of V connected to 3(MP•+/ UQ0•-) f 1(MP/UQ0) charge recombination. (36) Based on exchange interaction energy considerations, BET to the neutral ground state involves the spin forbidden triplet to singlet pathway, rather than intersystem crossing, to the singlet state, prior to charge recombination. See the following. (a) Molin, Yu. N., Ed. Spin Polarization and Magnetic Effects in Radical Reactions; Elsevier: Amsterdam, 1984; Chapter 2. (b) Turro, N. J.; Buchachenko, A. L.; Tarasov, V. F. Acc. Chem. Res. 1995, 28, 69.
JP961827B
