Ultrafast Electron Transfer Dynamics of a Zn(II)porphyrin−Viologen

J. Phys. Chem. B 2010, 114, 14329–14338

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Ultrafast Electron Transfer Dynamics of a Zn(II)porphyrin-Viologen Complex Revisited: S2 vs S1 Reactions and Survival of Excess Excitation Energy† Jonas Petersson, Mattias Eklund, Jan Davidsson, and Leif Hammarstro¨m* Chemical Physics Group, Department of Photochemistry and Molecular Science, Uppsala UniVersity, Box 523, SE-751 20 Uppsala, Sweden ReceiVed: December 10, 2009; ReVised Manuscript ReceiVed: February 11, 2010

The photoinduced electron transfer reactions in a self-assembled 1:1 complex of zinc(II)tetrasulphonatophenylporphyrin (ZnTPPS4-) and methylviologen (MV2+) in aqueous solution were investigated with transient absorption spectroscopy. ZnTPPS4- was excited either in the Soret or one of the two Q-bands, corresponding to excitation into the S2 and S1 states, respectively. The resulting electron transfer to MV2+ occurred, surprisingly, with the same time constant of τFET ) 180 fs from both electronic states. The subsequent back electron transfer was rapid, and the kinetics was independent of the initially excited state (τBET ) 700 fs). However, ground state reactants in a set of vibrationally excited states were observed. The amount of vibrationally excited ground states detected increased with increasing energy of the initial excited state, showing that excess excitation energy survived a two-step electron transfer reaction in solution. Differences in the ZnTPSS•3-/MV•+ spectra suggest that the forward electron transfer from the S2 state at least partially produces an electronically excited charge transfer state, which effectively suppresses the influence of the inverted regime. Other possible reasons for the similar electron transfer rates for the different excited states are also discussed. Introduction Porphyrins and their analogues are of great interest in both natural and synthetic photochemical systems, and therefore, much attention has been given to investigate the photophysics and dynamics of their electronically excited singlet states, S1 and S2.1–6 Ultrafast photoinduced electron transfer (ET) from unrelaxed states is an interesting violation of Kashas’s law of photochemistry and is attracting increasing attention.7 Accessing higher excited states opens up the possibility of not only using a wider range of energies for photoinduced reactions but possibly also for directing reactions to different desired outputs depending on the initial excitation. There has been a limited number of reports that concern electron transfer from the intrinsically very short-lived (τ ) 1-2 ps) higher excited porphyrin S2 state.8–15 The first report of S2 ET from the ZnTPP to the dichloromethane solvent was presented by Chosrowjan et al.,8,16 and following that, our group made the first studies demonstrating electron transfer from the S2 state of a porphyrin in porphyrin-acceptor assemblies.9,10 Among the more extensive investigations of S2 electron transfer are those by Mataga and co-workers, where the energy gap dependence on electron transfer from the S2 state in a series of porphyrin-imide complexes was studied.12 In ref 9, we studied a self-assembled 1:1 complex of zinc(II)tetrasulphonato-phenylporphyrin (ZnTPPS4-) and methylviologen (MV2+) in aqueous solution. The results and interpretations in that report are summarized in Figure 1. It was found that forward electron transfer (FET) from the S2 state was very rapid (τ < 200 fs), a process which we could not resolve, and generated the radical ion pair ZnTPP•3-/MV•+. Electron transfer was rapid also from the S1 state (τ < 200 fs), in agreement with earlier reports.17 In both cases the subsequent back electron transfer (BET) was also rapid, with τ ) 0.8 ps, †

Part of the “Michael R. Wasielewski Festschrift”. * To whom correspondence should be addressed. E-mail: Leif@fotomol. uu.se.

Figure 1. The states involved and the time constants for electron transfer and internal conversion processes of ZnTPPS4-/MV2+ in H2O in the interpretation model of ref 9. D+/A- denotes the radical ion pair state.

and showed no obvious probe wavelength dependence, nonexponentiality, or other signs of relaxation dynamics. We were surprised to find that the electron transfer dynamics seemed to be independent of the initial excitation, as electron transfer from the S2 state should have about 0.9 eV excess energy to dissipate before back electron transfer, compared to electron transfer from the S1 state. Our data suggested that this energy would be fully dissipated on a time scale shorter than 0.8 ps. Our previous study was made quite some time ago, with single- or two-color pump-probe at a few wavelengths only. Now, with a better detector system and time resolution, we decided to reinvestigate these processes to see if we could resolve the initial electron transfer, and detect any vibrational or solvent relaxation concomitant with electron transfer steps. Moreover, we wanted to examine how the observed dynamics depends on the identity and energy of the initially excited state. In a recent communication, concerning the same ZnTPPS4-/ MV2+ complex, we showed qualitatively that the BET led to a distribution of relaxed and unrelaxed ground state products.18 In the present report, we describe the electron transfer dynamics in more detail and present new findings. With a different instrumental setup, we have now resolved the forward electron

10.1021/jp911686z  2010 American Chemical Society Published on Web 03/01/2010

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transfer and will discuss the dependence of kFET on the initial state. The resolved FET together with global analysis has made it possible to resolve the more subtle differences of the charge transfer state, and also the unrelaxed ground state products. Comparisons of measurements in D2O vs H2O are also made. Experimental Section Samples. Sodium-zinc-meso-tetrasulfonatophenyl-porphyrin (Na4ZnTPPS, Porphyrin Products) and methyl viologen dichloride (MVCl2, Sigma Aldrich) were used without further purification and were dissolved in a 2 mM sodium phosphate buffer with either H2O or D2O at pH 7.0. The steady state absorption spectra were recorded on a Varian Cary 5000 UV-vis-NIR spectrophotometer, and the ZnTPPS4- emission spectrum was recorded on a Horiba Fluorolog. In order to avoid self-absorption in the emission measurement, a dilute (c ) 1.4 × 10-7 M) ZnTPPS4- sample was used, giving a maximum absorbance of 0.1.2a The ZnTPPS4- concentrations of the samples used in the transient absorption measurements were adjusted until an absorbance of 0.15-0.5 at the excitation wavelength was obtained (the higher absorbance is for excitations in the Soret band), giving concentrations of a few µM. The concentration of MVCl2 was 4-7 mM, in large excess of the porphyrin, resulting in 95% complex formation.19 All experiments were carried out at room temperature. The reference spectrum of MV•+ was obtained by addition of sodium dithionite to MV2+ in the buffer. The reference spectrum of ZnTPP•+ was obtained by spectroelectrochemical oxidation in acetonitrile (0.1 M TBAPF6) in a cell described previously.20 Spectroscopy. Time resolved measurements were done by means of transient absorption. Two different setups were used, one with a response width of about 130 fs and white light continuum (wlc) probe and the other with a response width of about 90 fs but single-color probe. Both setups use the same laser system and can be operated simultaneously. The laser system consists of a 1 kHz Ti:sapphire amplifier (Legend-HECryo, Coherent) pumped by a frequency doubled Q-switched Nd:YLF laser (Evolution-30, Coherent) and seeded by a modelocked Ti:sapphire oscillator (Vitesse-800, Coherent). The output is 800 nm pulses with a temporal width of about 100 fs. The beam is split in two parts to be used by the white light probe system and the single-color probe system. The 800 nm beam for the white light system is again split in two to be used for pump and probe. The pump light is directed to an optical parametric amplifier (TOPAS, Light Conversion). 426 nm light is obtained by tripling of the 1281 nm signal; the 560 and 600 nm light are obtained by doubling of the 1120 and 1200 nm signal. The pump light then passes through a mechanical chopper which blocks every second pulse and is later focused in the sample cell. Before the sample cell, part of the pump light is led to a diode to reject extreme pulses (1

m)1

(2)

Due to chirp in the wlc, t0 needs to vary with wavelength and one of the advantages of calculating the pure spectra according to eq 2 is that the resulting spectra then are free from chirp. Results and Discussion Reference Spectra. The ZnTPPS4- ground state absorption spectrum (Figure 2) consists of two bands denoted the Soret and Q-bands corresponding to the S2 r S0 and S1 r S0 transitions, respectively. The Q-band is further split in two due to different vibrational levels. The fluorescence Stokes shift for both the Soret and Q-band emission is small, 38 and 41 meV, respectively. Complexation of ZnTPPS4- and MV2+ leads to a small shift of the Soret-band maximum from 421.5 to 427 nm and some broadening, while the Q-bands are little affected. The small changes indicate that the excited singlet states in the complex are mainly localized on the porphyrin. No difference is seen when changing the solvent from H2O to D2O. We begin by first examining the free ZnTPPS4- in order to find reference spectra for the initially excited singlet states in the ZnTPPS4-/ MV2+ complex. Global fit of the transient absorption spectra for ZnTPPS4- in H2O excited at 420 nm showed that internal conversion (IC) from S2 to S1 occurred with a lifetime of τIC ) 1.3 ps, in excellent agreement with the previous report that presented the first S2 state transient absorption spectrum for a porphyrin.9 In addition, we also observe small absorption changes with a 5 ps lifetime which we attribute to vibrational relaxation in the S1 state, in agreement with ZnTPP3b and other metalloporphyrines.6,22 The spectra for ZnTPPS4- in D2O are also very similar, but the S1 relaxation time is longer, with τ ) 6.2 ps. Both S2 and S1 difference spectra (Figure 3) are in agreement with the ones seen for ZnTPP.1 The most important features of the S2 spectrum are S2 f S0 stimulated emission around 440-460 nm and the ground state bleach giving net negative difference absorption at 555 nm. The S1 spectrum instead shows stimulated emission around 640-670 nm and a stronger net transient absorption around 450 nm. The next state we need a reference spectrum for is the radical ion pair ZnTPPS•3-/MV•+. The spectrum is estimated by adding the spectrum of the chemically reduced MV2+ (with dithionite in phosphate buffer) and the difference spectrum for the

Figure 2. Ground state absorption spectra of ZnTPPS4- (gray line) and ZnTPPS4-/MV2+ (red line) in H2O and emission spectrum of ZnTPPS4- (dotted gray line) excited at 400 nm in H2O. Extinction coefficients are obtained by normalizing the ZnTPPS4- absorbance at 421 nm using ε ) 6.83 × 105 M-1 cm-1.21 MV2+ absorbs only in the UV region.

Figure 3. Reference spectra for ZnTPPS4-/MV2+: calculated difference spectra of the S2 (purple line) and S1 (green line) states from fit of transient ∆A spectra of ZnTPPS4- in H2O pumped at 420 nm; radical ion pair state (orange line) as the sum of difference spectrum for oxidized ZnTPP (in acetronitrile) and reduced MV2+ (in H2O) and S0v state (red line) as difference spectra of ground state ZnTPPS4-/MV2+ shifted 500 cm-1 and scaled by a factor of 0.3 to match the experimental yields (see text). Extinction coefficients for the excited singlet states and the radical ion pair state are calculated assuming that the ZnTPPS4Q(1,0) band bleach is responsible for the entire dip at 560 nm.

Figure 4. Difference absorption spectra of ZnTPPS4-/MV2+ in H2O excited at (a) 600 nm (S1(ν ) 0) state) and (b) 560 nm (S1(ν ) 1) state). Arrows indicate excitation wavelength.

electrochemically oxidized ZnTPP (in acetonitrile). The resulting difference spectrum is in good agreement with published spectra of MV•+ 23 and ZnTPPS•3+,24 and is shown together with the S2 and S1 spectra in Figure 3. The most important differences compared with the S2 and S1 spectra are the sharp peaks at 410 and 395 nm for the porphyrin and viologen radicals, respectively, the overall larger absorbance at wavelengths longer than 570 nm and the lack of net negative difference absorbance at 555 nm. Electron Transfer in ZnTPPS4-/MV2+. The aqueous ZnTPPS4-/MV2+ samples were excited at either 600, 560, or 427 nm, to the S1(ν ) 0), S1(ν ) 1), and S2 state, respectively. Transient absorption spectra are shown in Figures 4 and 5. The spectral shape, and in particular the evolution dynamics, is very different from the free ZnTPPS4-. For all excitations, the forward electron transfer is very fast and on the limit of the instrumental response (ca. 130 fs Gaussian). Features of the initially excited state are discernible at the earliest delay times. At 600 and 560 nm excitation (Figure 4), stimulated S1 emission is observed in the same region as for the uncomplexed ZnTPPS4-. At 427 nm excitation (Figure 5; both in H2O and D2O), they instead show stimulated S2 emission and net negative difference absorption at 555 nm. The most prominent indication that the radical ion pair states ZnTPPS•3-/MV•+ are being produced is the two peaks at 395

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Figure 5. Difference absorption spectra of ZnTPPS4-/MV2+ excited at 427 nm (S2 state), in (a) H2O and (b) D2O. Arrows indicate excitation wavelength.

Figure 6. Two-color transient absorption data of ZnTPPS4-/MV2+ in H2O, probed at 380 nm and pumped at 600 nm (blue), 555 nm (green), and 430 nm (red). Data is fitted with a sum of three exponentials, and the 430 nm trace also contains a Gaussian-shaped artifact (light blue line).

and 410 nm, maximizing after ca. 0.3 ps, which are characteristics of the MV•+ and ZnTPPS•3-, respectively. The broader features in the visible also confirm radical ion pair formation. From two-color measurements on a different setup with better time resolution, we can resolve the initial FET and subsequent BET rates. Figure 6 shows traces with excitation wavelengths of 600, 555, and 430 nm corresponding to the S1(ν ) 0), S1(ν ) 1), and S2 states, all probed at 380 nm. Interestingly, all three excitations give, within accuracy of the measurements, identical forward and back electron transfer rates of τFET ) 180 fs and τBET ) 700 fs; the individual time constants are given in Table 1. The fact that FET now can be resolved in all three cases gives important proof that the excitation is not a direct charge transfer transition but that the electron transfer to MV2+ in fact occurs directly from both the S2 and S1 states of ZnTPPS4-. Even though the stimulated S2 and S1 emission are visible, the FET processes cannot clearly be resolved in the white light data. That indicates that τFET are close to the instrumental response width (130 fs fwhm), which is consistent with the time constants obtained from the two-color measurements. The values of the ET rate constants given in Table 1 are in good agreement with our previous conclusion, τFET < 200 fs and τBET ) 800 fs.9 What is very different from before however is that we now see a strong absorption band just to the red side of the Soret band, growing in with the same time constant as the radical ion pair state decays. This new band then decays on a time scale of a

Petersson et al.

Figure 7. Transient absorption traces at probe wavelengths of 432, 436, and 440 nm of ZnTPPS4-/MV2+ in H2O excited at 427 nm (see Figure 5a). All traces are fitted with time constants of τFET ) 0.18, τBET ) 0.70, τv,1 ) 0.85, τv,2 ) 3.0, and τv,3 ) 5.0 ps.

few ps and has its maximum amplitude at 1.5-2 ps for all three excitation wavelengths. Interestingly, this spectrum closely resembles a ZnTPPS4-/MV2+ ground state spectrum that is shifted to the red, as already shown in Figure 3. As the S0v state decays, the absorption band shifts to the blue. These observations strongly suggest that the back electron transfer produces a vibrationally excited ground state (S0v) that then relaxes on a time scale of a few ps. In addition to the two ET time constants, three more time constants, τv,1-3, on the order of 0.85, 3, and 5 ps are needed to model the S0v relaxation after 427 nm excitation (Figure 7). At a probe wavelength of 440 nm, all three S0v components have positive amplitudes, but as the probe wavelength gets shorter, the faster components successively change to negative sign implying a rise-and-decay behavior. It is clear that the unrelaxed ground state products are formed from the back electron transfer, and not in parallel with formation of the ZnTPPS•3-/MV•+ state, since the kinetics of their appearance matches τBET. It is also not possible that impulsive Raman scattering of the excitation is responsible for the S0v absorption band, since it did not appear for the free ZnTPPS4-. The effects of the unrelaxed ground states are best seen when exciting to the S2 state because of the larger population of the S0v state but also notable with S1 excitation. For the S1 excitations, however, two time constants with τv,1 ) 0.9 ps and τv,2 ) 4 ps are enough to fit the data. It is interesting to note that the amplitude of this vibrationally excited ground state, relative to that of the preceding radical ion pair state, decreases with decreasing excitation energy, as seen in Figures 4 and 5. Since it is formed from the radical ion pair state, at least some of the excess energy obtained in the initial light excitation must be conserved during the forward and back electron transfer processes. To our knowledge, this has never been reported for a two-step charge transfer process of large molecules in solution, before our preliminary communication on this work.18 Measurements in D2O revealed similar kinetics for forward and back electron transfer, as shown in Table 1. However, the S0v relaxation is slower. Slower relaxation in D2O compared with H2O was also seen for the relaxation in the S1 state of ZnTPPS4- excited to the S2 state. The time constants obtained for the vibrational relaxation are given in Table 2. In Figure 8, the pure, species-associated difference spectra of each state involved in the reaction after excitation to the S2 state are calculated from a global fit using a consecutive reaction model, eq 2. The spectra are in good agreement with reference

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TABLE 2: Time Constants and Shifts for the Unrelaxed Ground States unrelaxed ground state, with ground state shifta initial state

τv,1/ps (R, shift/cm-1)

τv,2/ps (R, shift/cm-1)

τv,3/ps (R, shift/cm-1)

S1(ν ) 0) S1(ν ) 1) S2

0.9 (0.10, 460) 0.9 (0.14, 460) 0.85 (0.36, 510)

H 2O 4.0 (0.05, 340) 4.0 (0.09, 340) 3.0 (0.25, 380)

5.0 (0.24 110)

1.1 (0.40, 540)

D 2O 4.4 (0.29, 390)

7.2 (0.30, 160)

S2

The amplitude factor, R, and shift relative to the ground state obtained from a fit to eq 3 are shown in parentheses. The error in R is (0.01, and the error in shift is (30 cm-1. a

Figure 10. Schematic decay scheme of the observed dynamics in ZnTPPS4-/MV2+ excited to the S1(ν ) 0), S1(ν ) 1), and S2 states. D+/A-(S2/S1) denotes the radical ion pair state originating from S2 or S1 excitation.

The ground state shifts for the S0v relaxation given in Table 2 are obtained by a fit of the calculated S0v states with ground state absorption (GSA):

Sυ0¯ ,shift ) R × (GSA(υ¯ + υ¯ shift) - GSA(υ¯ ))

Figure 8. Calculated species-associated difference spectra according to eq 2 of ZnTPPS4-/MV2+ in H2O pumped at 427 nm (S2 state) with time constants according to Tables 1 and 2: S2 (blue), ZnTPPS•3-/MV•+ (yellow), S0v(1,2,3) (red lines with decreasing amplitude). Inset: a zoom of the ground state Soret-band region.

Figure 9. ZnTPPS•3-/MV•+ states calculated from a global fit of data for ZnTPPS4-/MV2+ in H2O after excitation to the S2 state (purple), S1(1,0) state (green), and S1(0,0) state (dotted light green). The spectra are normalized at the peak around 450 nm, and regions about 10 nm around the corresponding pump wavelengths are removed.

spectra (Figure 3). The peaks from ZnTPPS•3- at 410 nm and from MV•+ at 395 nm are clearly visible in the calculated ZnTPPS•3-/MV•+ spectra, although their amplitudes are understandably not as high as in the reference spectrum. This is presumably because of the lower spectral resolution of a transient spectrometer that broadens sharp bands and in addition the 410 nm peak is close to the strong Soret band. Also, because of the short time scale, the species are not fully relaxed, which may give a significant band broadening. Figure 9 shows the difference spectra of the radical ion pair states originating from the different initially excited states, S2, S1(ν ) 1), and S1(ν ) 0), in the ZnTPPS4-/MV2+ complex, and they are all similar to the ZnTPPS•3-/MV•+ reference spectrum. Some differences are seen, however, depending on the different initial excitation energy.

(3)

where υ j shift is the shift in wavenumber and R is an amplitude factor equal to the fraction of initially excited molecules that passes through the intermediate S0v state. The absorbance of the S0v state depends on the spectral shift but also on the population. As we do not know what fraction of the charge transfer state that recombines to the S0v states instead of directly to the fully relaxed ground state, we have to use both the shift and amplitude as free variables when we fit each spectrum of the S0v states. As was clearly seen already in the spectra (Figures 4 and 5), the amplitudes given in Table 2 are higher for excitation to higher lying initial states. We note that the same holds also for the shifts, although the uncertainties are too large to draw any conclusions based only on these shift values. Discussion of the Observed Dynamics. We have resolved the forward electron transfer from all initially excited states and are also able to observe stimulated emission from the porphyrin localized S2 and S1 states. This gives important evidence for the conclusions in our earlier report that forward electron transfer occurs directly from the S2 state. In that report, the FET process was not resolved (τ < 200 fs) and the conclusion was based solely on the observation that the ZnTPPS•3-/MV•+ state was formed significantly faster than internal conversion to the S1 state.9 Better temporal resolution now gave us the ability to observe additional dynamics, and based on our observations, we need to refine the reaction scheme (Figure 1) to that of Figure 10. We have confirmed that forward electron transfer occurs directly from all initially populated states. With τFET ) 180 fs, it is also among the fastest light induced electron transfer reactions reported for purely molecular systems. It is intriguing that the rate of initial electron transfer from the different porphyrin states is so similar, and reasons for this will be discussed. We have also found that the back electron transfer products carry parts of the excess energy from the initial excitation process. There is even a difference between excitation to the (ν ) 1) and (ν ) 0) vibrational level of the S1 state, indicating that loss of the excess vibrational quantum in the former case is not complete prior to charge transfer. That the excess excitation energy influences the back electron transfer products is clear evidence for forward electron transfer to excited ZnTPPS•3-/MV•+ states.

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Figure 11. Schematic diagram of the crossings between free-energy surfaces, *ZnTPPS4-(S2 or S1)/MV2+ and ZnTPPS•3-/MV•+ (D+/A-). Vibrational levels of D+/A- and S0 are included.

Electronically or Vibrationally Excited Radical Ion Pairs. Since different excitation wavelengths give different amounts of vibrationally excited S0v states, and with correspondingly increased shift, it is clear that the intermediate ZnTPPS•3-/MV•+ state also carries this excess energy. We therefore conclude that forward electron transfer occurs to excited radical ion pair states, but whether they are vibrationally or electronically excited is not yet clear. The exact character of the radical ion pair states ZnTPPS•3-/MV•+ is of great interest but difficult to determine from the transient absorption data in the visible region. MV•+ absorbs up to 770 nm (1.6 eV) and ZnTPP•+ up to 970 nm (1.3 eV) in acetronitrile, but their fluorescence spectra and therefore their E00 transition energies are not known. The driving force for the S2 state reaction (ca. 1.6 eV) is large enough to allow charge separation to electronically excited product states. Vauthey and co-workers have shown that, if electronic states are available, they efficiently suppress the influence of the inverted regime in back electron transfer reactions and reason that it should probably also occur for the forward electron transfer processes.15,25,37 The involvement of intramolecular high frequency modes in the electron transfer processes, on the other hand, modifies the classical high temperature limit (kBT . pω) Marcus equation for nonadiabatic electron transfer (eq 4) to the Jortner-Bixon version (eq 5).26,27

kNA )

kNA )

(

2πHAB2

exp -

p√4πλkBT

2πHAB2 p√4πλclkBT

∞

exp(-S)

(∆G0 + λ)2 4λkBT

)

n

S ∑ n!

n)0

(

exp -

(4)

(∆G0 + λcl + npω)2 4λclkBT

)

(5)

HAB is the electronic coupling between reactant and product states, ∆G° is the reaction free energy, and λ is the reorganization energy. In the latter version, the summation is over vibrational modes in the product state, with kBT , pω for those modes. S is the Huang-Rhy’s factor, S ) λν/pω, λν is the intramolecular reorganization energy, and λcl is the reorganization energy for the remaining classical coordinates. This

effectively reduces the influence of inverted region on the electron transfer rates and forms the products in a set of vibrationally excited states, as depicted in Figure 11. The effect of a nonequilibrated vibrational energy distribution on electron transfer dynamics has mainly been studied for charge recombination of direct (optical) charge transfer excitations.28–33 In addition, Kubo and co-workers observe vibrational coherence in the transient absorption data of a porphyrin-ferrocene complex, and their interpretation is that this regulates the charge separation.14a Many of the reports of photoinduced ET from the porphyrin S2 state in purely molecular systems discuss the involvement of an excited charge separated state9,12–14,34,35 but without having direct spectroscopic evidence.36 Mataga and coworkers observe S1 fluorescence rise faster than the S2 r S1 IC and with the same rate as the S2 fluorescence decay, even though the charge separated state lies below the S1 state.12 This is a clear indication for the involvement of an excited charge separated state. Similar conclusions were earlier drawn by Vauthey and co-workers (see above).15,37 It is also interesting to compare with a recent report by Wallin et al. where stateselective electron transfer in an unsymmetric acceptorZn(II)porphyrin-acceptor triad was studied.38 Excitation to the porphyrin S2 state did, in some polar solvents, lead to the formation of a charge separated state followed by recombination to the porphyrin S1 state, which once again led to formation of the charge separated state. It was then concluded that the S2 excitation led to electron transfer to hot charge separated states. For the ZnTPPS•3-/MV•+ data, a comparison of the difference spectra of the radical ion pair state for the three different excitations might give a hint whether the excess energy is electronic or vibrational. A previous report demonstrated a fast (τ ) 750 fs) electronic relaxation from the D1 to D0 state of MV•+ in acetonitrile followed by a slower (τ ) 1 and 16 ps) vibrational relaxation in the D0 state.20 As seen in Figure 9, the radical ion pair following S2 excitation has a lower absorbance between 500 and 700 nm and a higher absorbance at wavelengths shorter than 410 nm. This corresponds well to the difference between the D1MV•+ excited and the D0MV•+ ground states,20 and is an indication that forward electron transfer from S2 in the complex might at least partially produce an electronically excited ZnTPPS•3-/MV•+ state where the electronic excitation is mainly localized on the viologen radical. The radical ion pair spectrum formed from the S1(ν ) 1) state is also somewhat different compared to that from the S1(ν ) 0) state. This may be matched by the difference between vibrationally unrelaxed and relaxed D0MV•+ states, especially for the difference in absorbance below 410 nm where it has a sharp peak compared to the relaxed D0MV•+ state. The excited ZnTPPS•3-/ MV•+ state may also involve the porphyrin radical anion (ZnTPPS•3-) that has lower lying excited states. ZnTPP•+ has been shown to perform electron transfer under continuous illumination with 400 nm light,39 and Okhrimenko and coworkers have shown a 17 ps relaxation time of ZnTPP•+ excited at 400 nm which they interpreted as a D1 f D0 internal conversion and a conformational/vibrational relaxation process.40 The difference spectrum of the excited ZnTPP•+ in ref 40 shows a rather opposite behavior to the one seen for the excited MV•+, however, and cannot explain the differences seen in Figure 9. This suggests that charge separation products mainly involving the excited viologen radical are a more plausible interpretation. Even though the observed differences in the spectra for the radical ion pair states obtained after the different excitations match the excited MV•+, the assignment to excited MV•+ species should still be viewed as a tentative suggestion. Complexation

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of the porphyrin and viologen may affect the spectra, and there may also be other states involved that we do not have reference spectra for. It is also possible that electronic and vibrational relaxation occurs faster in the ZnTPPS•3-/MV•+ complex than in the separate porphyrin and viologen radicals, but the rate of back electron transfer is anyway clearly fast enough to compete with the reported relaxation processes. This enables at least parts of the initial excess excitation energy to survive also the back electron transfer reaction, which is evident from the unrelaxed ground state of the complex. Other Factors That May Be Responsible for the Identical Electron Transfer Rates. Surprisingly, FET occurred with the same time constant for all three excitation wavelengths, in spite of the large difference in ∆G°, and possibly other factors. Even though excited states, vibrational and/or electronic, clearly are involved, we still cannot exclude other factors contributing to this unexpected behavior. First, we consider the possibility that the rates are equal because of a common rate limiting factor. A more general expression of the electron transfer rate is given by Rips and Jortner:27,41

kET ) kNA /(1 + κ) κ)

4πJAB2τS pλS

(6)

where κ is the adiabacity factor and λS and τS are the solvent reorganizational energy and relaxation time, respectively. J is the effective, Franck-Condon-weighted electronic coupling JAB2 ) HAB2(Sn/n!) exp(-S).27 In the adiabatic limit, κ . 1, eqs 5 and 6 then give

kA )

1 τS



λcl × 16πkBT

∞

∑

n)0

(

exp -

(∆G0 + λcl + npω)2 4λclkBT

)

(7)

The pre-exponential factor will typically be very close to τs-1, which gives the upper limit for kA. Limiting solvent dynamics may thus give a rather constant ET rate over a wide range of driving forces, as seen for the S2 and S1 FET rates in the ZnTPPS4-/MV2+ complex. Although the longitudinal relaxation time of water is 0.4 ps,42 the main part of the solvent response to an instantaneous charge redistribution in water occurs on the time scale of ca. 20 fs,33 and is not likely to limit the rate in the present case. Slower, diffusional components with time constants of 150-900 fs exist however, and their contribution to the reaction coordinate may be required in the observed electron transfer, as was suggested for an ultrafast (τ ≈ 85 fs) RuIII-RuII intervalence transfer reaction in water.33 We add that lowfrequency intramolecular modes may also limit the electron transfer rate. Both of these modes and the diffusional motion of water must be sufficiently strongly coupled to the electron transfer process (i.e., contribute sufficiently to the total reorganization energy) to be rate limiting. Otherwise, ultrafast electron transfer can occur anyway without involving these modes in the reaction coordinate (see ref 32 for examples). The ET rates are, however, very similar in H2O as in D2O, which indicates that low-frequency solvent dynamics is not limiting the rates. Low-frequency intermolecular modes between the porphyrin and viologen will modulate the donor-acceptor distance and

thus the electronic coupling (HAB), which could in principle lead to a gating behavior where the reaction rate is limited by this vibrational frequency. It would require that the reaction is nonadiabatic (eqs 4 and 5) for the rate to depend on the magnitude of HAB, however, at least in the equilibrium position of the complex. We believe it is unlikely that the amplitude of these vibrational modes in the complex is large enough, given the moderate distance dependence of HAB on vibrational length scales,26,27 to sufficiently modulate the coupling for a gating behavior to be observed. Now instead, we consider the possibility that the similarity in rate for electron transfer from the S2 and S1 states is simply due to variation of two parameters that by coincidence give a compensatory effect. If not dynamically limited, the rate of both adiabatic and nonadiabatic reactions will depend on the Franck-Condon factors. It is difficult to evaluate the reaction free energy (∆G°) and reorganization energy (λ) for the tight complex, but the latter is likely to be much smaller than the typical 1.0 eV found in polar solvents due to the proximity of the reactants. Given the dependence of initial energy between S1(ν ) 1) and S1(ν ) 0) on the back electron transfer products, both reactions are, if not in the inverted region, probably at least close to the activationless region (as indicated in Figure 11). This would give an upper limit on the reorganization energy, λ < 0.74 eV. It should also be very similar for the two reactions (S2 and S1), as the fluorescence Stokes shift shows that the ground-excited state distortion is small for both the S1 and S2 states. On the basis of the redox potentials and excited state energies for the individual species involved, without correcting for their interaction, we obtain ∆G° ) -0.74 and -1.57 eV for electron transfer from the relaxed S1 and S2 states, respectively. The latter is likely to be far into the inverted region, and it is not very probable that both reactions happen to give the same activation energy. Furthermore, the electronic coupling could be different for electron transfer from the different states. The Soret band shifts significantly upon complexation with viologen, while the Q-band is much less affected. The shift is presumably due to a perturbative mixing with a charge transfer state. This implies a stronger electronic coupling for electron transfer from the S2 state. If at least electron transfer from the S1 state is nonadiabatic, and thus dependent on variations in electronic coupling, this may compensate for the presumably larger activation energy of the S2 reaction. To conclude, although it seems unlikely, we cannot exclude that the equal rates for the two electron transfer reactions are due to a compensatory effect of changes in two different parameters. We will attempt to determine the activation energies by varying the reaction temperature. Complexes of related porphyrins and viologen derivatives will also be used to vary the driving force and thus investigate the rate dependence on the Franck-Condon factors. Even though we cannot completely exclude the effect of factors discussed in this section, we believe that the main reason for the similarity in rates for the S2 and S1 electron transfer is due to the influence of excited states in the electron transfer products. It should also be pointed out that most electron transfer theory, including eqs 4 and 5, assumes thermally equilibrated reactant states, while the present reactions are clearly too fast for this assumption to be valid. Vibrationally Unrelaxed Back Electron Transfer Products. On the basis of the shape of the spectra and on the decay times, we have assigned the long-lived transient species to vibrationally unrelaxed ground states, S0v. The probe wavelength dependence on the average decay times also has the shape of a hot state; see Figure 12. The average decay time observed at longer probe

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Figure 12. Probe wavelength dependence of the amplitude weighted average decay times of the unrelaxed ground state in H2O after excitation to the S2, S1(ν ) 1), and S1(ν ) 0) states, respectively.

wavelengths is shorter than that observed at shorter probe wavelengths, as expected for vibrational relaxation.43 We should point out, though, that an alternative explanation could be a structural difference of the ZnTPPS4-/MV2+ pair due to charge redistribution and loss of donor-acceptor interaction in the ZnTPPS•3-/MV•+ state. This could also lead to a modulated ground state spectrum, if a return to the relaxed ground state geometry is slower than the back electron transfer. As the interaction would decrease, however, the electronic coupling would decrease and thus rather blue-shift the porphyrin spectrum toward that of its uncomplexed state, so we believe this explanation can be discarded. The back electron transfer from the radical ion pair ZnTPPS•3-/MV•+ to the vibrationally excited ZnTPPS4-/MV2+ ground state S0v is a new observation, reported first in ref 18. It gives a large transient absorption only in a narrow spectral region and was therefore not seen in the previous study. The fact that the excess energy survives two steps of electron transfer makes it different from the many cases of vibrationally excited products formed in one reaction step, or excitation-wavelengthdependent (that is, state-dependent) single-step electron transfer.44 The reason for the unusual behavior is most likely that both forward and back electron transfer are very fast reactions in the present system, which do not allow for all excess energy to relax. In large molecules, vibrational energy relaxation (VER) is often divided into two processes: intramolecular vibrational energy redistribution (IVR) between different modes of the excited solute that typically occurs on the time scale 10-100 fs, producing a “hot” species with a defined local temperature, and subsequent vibrational cooling (VC) that typically occurs on time scales from one to some tens of ps.22 However, Vauthey and co-workers have shown IVR processes on a 2 ps time scale and that the separation of IVR and VC in two distinct time scales is not always possible.45 The present electron transfer processes are clearly rapid enough to compete on these time scales. It would then be interesting to see if the shifts from the fitted S0v spectra in Table 2 correspond to specific vibrational modes in the porphyrin. Further studies including time-resolved vibrational spectroscopy will hopefully give more insight in the relaxation processes, and may also give proof of the tentative assignment that electron transfer from the S2 state generates electronically excited viologen radical. Conclusion To conclude, our study revealed some unusual electron transfer and relaxation dynamics of a porphyrin-acceptor complex, ZnTPPS4-/MV2+. By varying the excitation wave-

Petersson et al. length between the Soret band and the Q-bands, we study the electron transfer from the corresponding porphyrin localized excited states, S2, S1(ν ) 1), and S1(ν ) 0). In spite of the 0.8 eV excess energy in the S2 state compared to the S1 state, the electron transfer rates are similar, and unusually rapid (τFET ) 180 fs). The back electron transfer is also rapid and also occurs with the same time constant (τBET ) 700 fs) irrespective of which state is initially excited. However, the back electron transfer produced vibrationally excited ground states, observed as transiently shifted ground-state absorption spectra. The spectral shape depends on the initial excitation wavelength, showing that the excess energy survives both the forward and back ET steps. To our knowledge, this is the first example of this observation for large molecules in solution. Changing the solvent from H2O to D2O gave no notable effect on the ET rates, but the vibrational relaxation of the unrelaxed ground state products was slower in D2O. Both the dependence of initial excitation on the unrelaxed back electron transfer products and the shape of the ZnTPPS•3-/MV•+ spectra indicate the involvement of excited states also in the intermediate radical ion pair state. We have indications that the electron transfer reaction from the S2 state proceeds via an electronically excited radical ion pair state which in that case would be the first time it has ever been directly observed. In a wider perspective, this study gives an example of excitation wavelength control of an ultrafast reaction. We have shown a case where different outputs of a two-step electron transfer with direct UV-vis spectroscopic signatures can be selected by excitation to different initial states with well separated absorption bands. Acknowledgment. This work was supported by the K&A Wallenberg Foundation, The Swedish Foundation for Strategic Research, and the Swedish Energy Agency. L.H. acknowledges a Research Fellow position from the Royal Swedish Academy of Sciences. Supporting Information Available: Additional experimental data, procedures, and analysis. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Lukaszewicz, A.; Karolczak, J.; Kowalska, D.; Maciejewski, A.; Ziolek, M.; Steer, R. P. Photophysical Processes in Electronic States of Zinc Tetraphenyl Porphyrin Accessed on One- and Two-Photon Excitation in The Soret Region. Chem. Phys. 2007, 331, 359–372. (2) (a) Tripathy, U.; Kowalska, D.; Liu, X.; Velate, S.; Steer, R. P. Photophysics of Soret-Excited Tetrapyrroles in Solution. I. Metalloporphyrins: MgTPP, ZnTPP, and CdTPP. J. Phys. Chem. A 2008, 112, 5824– 5833. (b) Liu, X.; Tripathy, U.; Bhosale, S. V.; Langford, S. J.; Steer, R. P. Photophysics of Soret-Excited Tetrapyrroles in Solution. II. Effects of Perdeuteration, Substituent Nature and Position, and Macrocycle Structure and Conformation in Zinc(II) Porphyrins. J. Phys. Chem. A 2008, 112, 8986– 8998. (c) Liu, X.; Mahammed, A.; Tripathy, U.; Gross, Z.; Steer, R. P. Photophysics of Soret-Excited Tetrapyrroles in Solution. III. Porphyrin Analogues: Aluminum and Gallium Corroles. Chem. Phys. Lett. 2008, 459, 113–118. (3) (a) Baskin, J. S.; Yu, H.-Z.; Zewail, A. H. Ultrafast Dynamics of Porphyrins in the Condensed Phase: I. Free Base Tetraphenylporphyrin. J. Phys. Chem. A 2002, 106, 9837–9844. (b) Yu, H.-Z.; Baskin, J. S.; Zewail, A. H. Ultrafast Dynamics of Porphyrins in the Condensed Phase: II. Zinc Tetraphenylporphyrin. J. Phys. Chem. A 2002, 106, 9845–9854. (4) Mataga, N.; Shibata, Y.; Chosrowjan, H.; Yoshida, N.; Osuka, A. Internal Conversion and Vibronic Relaxation from Higher Excited Electronic State of Porphyrins: Femtosecond Fluorescence Dynamics Studies. J. Phys. Chem. B 2000, 104, 4001–4004. (5) Kumble, R.; Palese, S.; Lin, V. S.-Y.; Therien, M. J.; Hochstrasser, R. M. Ultrafast Dynamics of Highly Conjugated Porphyrin Arrays. J. Am. Chem. Soc. 1998, 120, 11489–11498. (6) Zhong, Q.; Wang, Z.; Liu, Y.; Zhu, Q.; Kong, F. The Ultrafast Intramolecular Dynamics of Phthalocyanine and Porphyrin Derivatives. J. Chem. Phys. 1996, 105, 5377–5379.

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