15926
J. Phys. Chem. 1996, 100, 15926-15932
Energy and Photoinduced Electron Transfer in Porphyrin-Fullerene Dyads Darius Kuciauskas, Su Lin, Gilbert R. Seely, Ana L. Moore,* Thomas A. Moore,* and Devens Gust* Center for the Study of Early EVents in Photosynthesis, Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287-1604
Tatiana Drovetskaya and Christopher A. Reed* Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089-0744
Peter D. W. Boyd* Department of Chemistry, The UniVersity of Auckland, Auckland 1, New Zealand ReceiVed: May 3, 1996; In Final Form: July 8, 1996X
Time-resolved fluorescence and absorption techniques have been used to investigate energy and photoinduced electron transfer in a covalently linked free-base porphyrin-fullerene dyad and its zinc analog. In toluene, the porphyrin first excited singlet states decay in about 20 ps by singlet-singlet energy transfer to the fullerene. The fullerene first excited singlet state is not quenched and undergoes intersystem crossing to the triplet, which exists in equilibrium with the porphyrin triplet state. In benzonitrile, photoinduced electron transfer from the porphyrin first excited singlet state to the fullerene competes with energy transfer. The fullerene excited singlet state is also quenched by electron transfer from the porphyrin. Overall, the charge-separated state is produced with a quantum yield approaching unity. This state lives for 290 ps in the free-base dyad and 50 ps in the zinc analog. These long lifetimes suggest that such dyads may be useful as components of more complex light-harvesting systems.
Introduction A wide range of molecular dyads and more complex species that absorb visible light and undergo energy and photoinduced electron transfer have been prepared and studied. Many of these imitate photosynthetic reaction centers by using porphyrins as pigments and quinones as electron acceptors.1-6 However, synthetic photovoltaic or optoelectronic molecules can potentially employ a wide range of other chromophores and electron donor-acceptor systems. For example, the C60 moiety, with its low-energy first excited singlet state, is a good energy acceptor and in addition readily accepts multiple electrons, making it a potential electron accumulator. We recently reported the synthesis and study of a covalently linked porphyrinfullerene dyad that demonstrates extremely rapid interchromophore singlet-singlet energy and photoinduced electron transfer.7 This initial study has been followed by reports of molecules containing C60 covalently linked to other porphyrins,8-10 phthalocyanines,11 ruthenium complexes,12 aniline derivatives,13 and carotenoid polyenes.14 In this study, we have investigated the photochemical properties of two of the new porphyrin-C60 dyads, 1 and 2. Results Electrochemistry. In order to estimate the energies of charge-separated states formed after excitation, the first oxidation potential of the porphyrin moiety and the first reduction potential of the fullerene are required. These were determined by cyclic voltammetric experiments with 1 and 2 in benzonitrile solution. The experiments were carried out at ambient temperatures with ferrocene as an internal reference redox system. The reductive electrochemistry of 1 shows six one-electron processes in the X
Abstract published in AdVance ACS Abstracts, September 1, 1996.
S0022-3654(96)01274-9 CCC: $12.00
potential range -0.2 to -2.3 V (Figure 1). These occur at -0.994, -1.411, -1.649, -2.050, and approximately -2.26 V vs Fc/Fc+. Comparison with the reduction potentials of porphyrin 4 (-1.654 and -2.040 V) and fullerene 3 (-1.05, -1.44, -2.01, and -2.42 V)15 indicates that four of the reductions correspond to the formation of the fullerene mono-, di-, tri-, and tetraanions and two to porphyrin reductions. The second reduction of the porphyrin and the third reduction of the fullerene overlap at about -2.05 V. Oxidation of dyad 1 takes place at approximately +0.59 V, but the process is almost irreversible, owing, perhaps, to migration of H+ from the center of the porphyrin ring to the exocyclic tertiary amine. The first reduction of the fullerene moiety of zinc dyad 2 occurs at -1.031 V and the second at -1.457 V. Both are reversible. One-electron oxidation of the porphyrin moiety of 2 is reversible at +0.345 V, although the second oxidation step © 1996 American Chemical Society
Electron Transfer in Porphyrin-Fullerene Dyads
Figure 1. Cyclic voltammetry of dyad 1 and model porphyrin 4 in benzonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate at 50 mV/s.
Figure 2. Absorption spectra of free-base dyad 1 (s) and model compounds 3 (-‚-) and 4 (-‚‚-) in toluene solution.
(∼+0.63 V) is not. The fullerenes are not oxidized within the range spanned by the cited potentials. Steady-State Absorption Spectra. The absorption spectra of free-base dyad 1 and model compounds 3 and 4 in toluene solution are shown in Figure 2. In the dyad, the Soret band of the porphyrin has a maximum at 427 nm, and the Q-bands appear at 519, 555, 595, and 653 nm. These bands are shifted to longer wavelengths by 5-8 nm relative to those of model porphyrin 4. The zinc dyad 2 features maxima at 430, 555, and 592 nm (Figure 3). These maxima are also shifted to longer wavelengths relative to those of model zinc porphyrin 5. In both dyads, some of the absorption bands are slightly broadened relative to those of the model compounds. Similar effects have been observed in other porphyrin-fullerene dyads.7 These small spectral perturbations indicate a weak electronic interaction between the porphyrin and fullerene chromophores. In more polar solvents including benzonitrile and dichloromethane, the porphyrin absorption maxima of both dyads and of the model porphyrins are slightly red shifted by an additional 1-5 nm relative to toluene. Steady-State Fluorescence Spectra. The fluorescence emission spectra of 1-5 were measured in toluene solution. Figure 4 shows the corrected emission spectra of dyad 1 and model compounds 3 and 4 with excitation at 550 nm. The spectra reflect equal absorbances at the excitation wavelength. Porphyrin 4 has emission maxima at 651 and 718 nm and a fluorescence quantum yield of 0.10 (determined by the comparative method with 5,10,15,20-tetraphenylporphyrin, Φ ) 0.11, as the standard). Fullerene 3 emits at 715 and 796 nm
J. Phys. Chem., Vol. 100, No. 39, 1996 15927
Figure 3. Absorption spectra of zinc dyad 2 (s) and model compounds 3 (-‚-) and 5 (-‚‚-) in toluene solution.
Figure 4. Steady-state fluorescence emission spectra of dyad 1 (s, ×200) and model compounds 3 (-‚‚-, ×200) and 4 (-‚-) in toluene solution with excitation at 550 nm. The emission intensities reflect equal absorbance at the excitation wavelength. The shoulder in the 650 nm region of the spectrum of 4 is due to a small amount of porphyrin emission.
(Φ ) 0.001). In dyad 1, porphyrin emission is almost totally quenched, whereas that of the fullerene (Φ ) 0.0009) is essentially unaffected by the linked porphyrin. The quenching of the porphyrin singlet state is due entirely to singlet-singlet energy transfer to the fullerene, whose first excited singlet state lies at lower energy. Energy transfer in the dyad is confirmed by the corrected fluorescence excitation spectrum for fullerene emission measured at 800 nm, which is identical to the absorption spectrum within experimental error. Thus, the quantum yield of energy transfer is essentially unity. Similar results were obtained for the zinc-containing dyad 2 and model compounds 3 and 5. Zinc porphyrin 5 has emission maxima at 599 and 648 nm (Φ ) 0.03). Dyad 2 has emission maxima corresponding to the fullerene at 722 and 803 nm (Φ ) 0.0003). No significant emission from the porphyrin moiety was observed. Again, the corrected fluorescence excitation spectrum reveals energy transfer from the porphyrin to the fullerene with a yield approaching unity. The results in toluene show that, for both dyads, the porphyrin singlet states are strongly quenched by singlet-singlet energy transfer, but the fullerene singlet states are essentially unaffected. In the more polar dichloromethane and benzonitrile, however, no emission was observed from either chromophore in either dyad (Φ < 10-5 in benzonitrile). Thus, a new quenching pathway comes into play in these solvents.
15928 J. Phys. Chem., Vol. 100, No. 39, 1996
Figure 5. Decay-associated fluorescence emission spectra for a solution of dyad 1 in toluene with excitation at 590 nm. The two components have lifetimes of 22 ps (b) and 1.34 ns (9).
Time-Resolved Fluorescence Studies. Additional information concerning the fate of the excited states was obtained from time-resolved fluorescence studies carried out by the singlephoton timing method. A solution of dyad 1 in toluene was excited at 590 nm, and emission time profiles were collected at eight wavelengths in the 700-840 nm region. These were analyzed globally to yield the decay-associated spectra shown in Figure 5 (global χ2 ) 1.19). The two exponential decay components have lifetimes of 22 ps and 1.34 ns. The 22 ps component has a large positive amplitude at 700 nm, where porphyrin 4 emits strongly, and negative amplitudes (signaling an increase of fluorescence intensity with time) in the 740840 nm region, where the fullerene moiety emits. This is consistent with singlet-singlet energy transfer from the porphyrin to the fullerene, as detected in the steady-state experiments. The 1.34 ns component of the spectrum has the spectral shape of fullerene emission, and the component derives from the decay of PH2-1C60. Fullerene model 3 in toluene has a singlet lifetime of 1.31 ns. Thus, the fullerene singlet state in 1 is not quenched, in agreement with the steady-state fluorescence results. Similar experiments were carried out with zinc-containing dyad 2; kinetic traces at 10 wavelengths were collected in the 660-840 nm spectral region. Global analysis (χ2 ) 1.21) yielded two exponential components of 20 ps and 1.42 ns. The former component was associated with energy transfer from the porphyrin singlet state to the fullerene, and the lifetime of 1PZnC60 is therefore 20 ps. The 1.42 ns component has the emission profile of the fullerene singlet state, and the component confirms the absence of quenching of PZn-1C60 by the porphyrin moiety. Time-resolved fluorescence studies were also carried out in benzonitrile solution, where quite different results were obtained. Model fullerene 3 and free-base porphyrin 4 have singlet lifetimes of 1.36 and 9.6 ns, respectively, in this solvent. A solution of dyad 1 was excited at 590 nm, and decay profiles were measured at 12 wavelengths in the 600-840 nm region. Global analysis yielded the decay-associated spectra in Figure 6 (χ2 ) 1.29). The two significant exponential components have lifetimes of 6 and 74 ps. The component with the shorter lifetime is associated with emission from the porphyrin and some energy transfer to the fullerene, as indicated by the negative amplitude at 820 nm. The spectrum of the 6 ps component is distorted by Raman scattering from the solvent in the 720 nm region. The spectral shape of the 74 ps component identifies it as fullerene emission. Thus, both the porphyrin and fullerene
Kuciauskas et al.
Figure 6. Decay-associated fluorescence emission spectra for a solution of dyad 1 in benzonitrile with excitation at 590 nm. The exponential components have lifetimes of 6 ps (b), 74 ps (9), 820 ps (2), and 4.7 ns (1).
singlet states are strongly quenched in benzonitrile, as expected from the steady-state measurements. Zinc porphyrin 5 has a singlet lifetime of 1.58 ns in benzonitrile. Time-resolved fluorescence experiments with dyad 2 similar to those described above show only one significant decay component, with a lifetime of ∼1.5 ps (χ2 ) 1.13). This suggests that both 1PZn-C60 and PZn-1C60 have singlet lifetimes on the picosecond or subpicosecond time scale. It should be noted that lifetimes of ∼6 ps or less obtained by analysis of the global time-resolved fluorescence data sets are only upper limits for the actual lifetimes due to time resolution limitations of the apparatus (instrument response function of 35 ps). Dyad 1 was also investigated in dichloromethane solution. Excitation was at 590 nm, and 11 fluorescence time profiles were collected in the 610-840 nm region. Global analysis (χ2 ) 1.16) yielded a lifetime of 8 ps for 1PH2-C60, which decays in part by singlet energy transfer to the fullerene, and 91 ps for PH2-1C60. Subpicosecond Transient Absorption Studies. Transient absorption techniques were used to obtain improved time resolution and investigate nonemitting states. A benzonitrile solution of dyad 1 was excited at 590 nm with ∼200 fs laser pulses, and transient absorption spectra were measured using the pump-probe technique. Figure 7 shows the transients in the 930-1060 nm region where the fullerene radical anion absorbs.16,17 The transient absorption spectra indicate formation and decay of a fullerene radical anion. This is attributed to the PH2•+-C60•- charge-separated state generated by photoinduced electron transfer. The rise of the absorption is biexponential, with rise times of 4 and 75 ps, and is accompanied by a small shift of the absorption maximum to shorter wavelengths. The decay is exponential with a time constant of 290 ps. The results of similar experiments with detection in the 700 nm region appear in Figure 8. The spectra are characteristic of the porphyrin radical cation,18,19 with a minimum at ∼653 nm due to bleaching of the porphyrin ground state absorption. Thus, these spectra confirm the formation of the PH2•+-C60•- state. There is also some stimulated emission from the fullerene in the 690-760 nm region, which contributes to the apparent more rapid decay at these wavelengths. Global analysis of the data in the 625-670 nm range yields a biexponential rise consistent with that observed in the 1000 nm region. The decay was monoexponential, with a lifetime of 290 ps. Zinc-containing dyad 2 was studied under the same experimental conditions. In the 950-1060 nm region, the fullerene
Electron Transfer in Porphyrin-Fullerene Dyads
Figure 7. Transient spectra in the long-wavelength region arising from excitation of dyad 1 in benzonitrile at 590 nm with ∼200 fs laser pulses. The spectra were measured before the pulse (- -), and 1.5 ps (---), 4.5 ps (‚‚‚), 16.5 ps (-‚-), 63 ps (-‚‚-), and 108 ps (s) after excitation. The inset shows the rise and decay of the transient in the 1000 nm region and a simulation using the rate constants reported in the text.
Figure 8. Transient spectra in the short-wavelength region arising from excitation of dyad 1 in benzonitrile at 590 nm with ∼200 fs laser pulses. The spectra were measured before the pulse (- -), and 3.0 ps (‚‚‚), 12 ps (-‚-), 32 ps (-‚‚-), and 87 ps (s) after excitation. The inset shows the rise and decay of the transient in the 670 nm region and a simulation using the rate constants reported in the text.
radical anion transient absorption is again observed, signaling formation of PZn•+-C60•-. In this case, there is no initial shift of the spectral maximum. Global analysis shows the rise and decay to be monoexponential, with time constants of 1.9 and 50 ps, respectively. Examination of the transient absorption in the 625-755 nm range shows formation of the porphyrin radical cation with a time constant of 1.9 ps and decay of this species with a lifetime of 50 ps (Figure 9), as was observed in the longwavelength region. Nanosecond Transient Absorption Studies. In the nonpolar toluene, quenching of the fullerene excited singlet state by electron transfer was not observed. In order to investigate the fate of this state, transient absorption studies were carried out on the nanosecond time scale. Argon-purged solutions were excited at 550 nm with ∼5 ns laser pulses. Results for model fullerene 3 and dyads 1 and 2 (all solutions with equal absorbance at the pump wavelength) are shown in Figure 10. All three compounds show an absorption band at 690 nm which is assigned to the fullerene triplet state.7,13,20,21 Dyads 1 and 2 show an additional absorption below 500 nm that is indicative
J. Phys. Chem., Vol. 100, No. 39, 1996 15929
Figure 9. Rise (a) and decay (b) of the transient spectra at 690 nm arising from excitation of dyad 2 in benzonitrile at 590 nm with ∼200 fs laser pulses. The fits derived from kinetic analysis of the transient are also displayed.
Figure 10. Transient absorption spectra obtained 20 ns after excitation of deaerated toluene solutions of dyad 1 (b), dyad 2 (9), and model fullerene 3 (2) with ∼5 ns laser pulses at 550 nm. The absorbance of all three solutions was the same at the excitation wavelength.
of the porphyrin triplet state. In the case of 1, this absorption is substantial. The transient absorptions decay monoexponentially at all wavelengths with time constants of 70, 31, and 29 µs for 1, 2, and 3, respectively. In aerated solvent, the triplet states of all three compounds are rapidly quenched by molecular oxygen to generate singlet oxygen. The singlet oxygen was detected by emission at 1270 nm. The observed quantum yields of singlet oxygen for 1 and 2 were both 0.66 times that of model fullerene 3. Discussion Energetics. The energies of the porphyrin first excited singlet states of the dyads in toluene were estimated from the longestwavelength absorption band of the porphyrin moiety, as fluorescence from these species was not observed, and Stokes shifts in such porphyrins are usually small. On the basis of these estimates, 1PH2-C60 lies at 1.90 eV and 1PZn-C60 at 2.09 eV. The energy of the fullerene first excited singlet state is 1.74 eV, based on the absorption and emission spectra of model fullerene 3. The cyclic voltammetric data allow estimation of the energies of PH2•+-C60•- and PZn•+-C60•- as 1.58 and 1.38 eV above the ground states, respectively. Such estimates ignore any Coulombic effects in the dyads that differ from those
15930 J. Phys. Chem., Vol. 100, No. 39, 1996
Kuciauskas et al. of the rate constants for steps 1 and 2 in Figure 11, as the rate of step 5 is negligible by comparison. Computer simulation of the rise of the long-wavelength absorption (Figure 7) according to the scheme in Figure 11 gives a k2/k1 value of 2.5. Thus, k2 is 1.8 × 1011 s-1, and the quantum yield of photoinduced electron transfer by step 2, Φ2, is 0.71. The rate constant for singlet-singlet energy transfer step 1 is 7.1 × 1010 s-1, which is only slightly faster than the rate in toluene (Table 1). The quantum yield of energy transfer Φ1 is 0.29. The lifetime of the fullerene singlet state in the dyad τC is 74 ps, as determined by the time-resolved fluorescence study, and is given by eq 2. A value of 7.4 × 108 s-1 for k4 may be
1 ) k3 + k4 τC
Figure 11. Transient states of dyad 1 and their interconversion pathways. The broken lines indicate the energies of the corresponding states of zinc dyad 2. The energies of the charge-separated states are estimated from cyclic voltammetric studies in benzonitrile. The hollow rectangles represent triplet energy levels. The value shown for PH23C 60 is that for C60 itself (see text).
TABLE 1: Rate Constants for Photochemical Processes Shown in Figure 11 rate constant (s-1) k1 k2 k3 k4 k5 k6 a
toluene
dyad 1 benzonitrile
4.5 × 1010 7.5 × 108 1.0 × 108
7.1 × 1010 1.8 × 1011 1.3 × 1010 7.4 × 108 1.0 × 108 3.4 × 109
toluene
dyad 2 benzonitrile
4.9 × 1010 7.0 × 108 6.3 × 108 a
4.9 × 1010 b 4.8 × 1011 (∼5 × 1011)c 7.0 × 108 b 6.3 × 108 2.0 × 1010
Measured in benzonitrile. b Measured in toluene. c See text.
affecting the individual ions during the electrochemical experiments. These energies were used to construct the diagram in Figure 11, which shows energy levels and associated interconversion paths. Photoinduced Electron Transfer. The various pathways for decay of the singlet states of dyad 1 can be discussed with reference to Figure 11. The lifetime of 1PH2-C60, for example, is given by eq 1, where τP is the lifetime of the porphyrin first
1 ) k1 + k2 + k5 τP
(1)
excited singlet state, and the rate constants refer to the processes in Figure 11. In toluene, τP ) 22 ps and the rate of step 2, photoinduced electron transfer, is negligible, as the yield of singlet-singlet energy transfer to the fullerene is essentially unity. The rate constant for step 5 can be estimated as 1.0 × 108 s-1 on the basis of the 9.6 ns lifetime of the first excited singlet state of model porphyrin 4 in benzonitrile. Thus, k1 is 4.5 × 1010 s-1 (Table 1). In benzonitrile the lifetime of 1PH2-C60 is reduced, in part because photoinduced electron transfer to the fullerene, step 2, becomes significant. The most reliable lifetime for 1PH2-C60 is the τp value of 4 ps determined for the fast component of the rise of the fullerene radical anion. This compares well with the less accurate determination of 6 ps from the time-resolved fluorescence study. The measured rate constant for decay of 1P -C (1/τ ) 2.5 × 1011 s-1) must be a linear combination H2 60 p
(2)
estimated from the 1.36 ns lifetime of the first excited singlet state of model fullerene 3. Thus, k3 is 1.3 × 1010 s-1, and the quantum yield of step 3 is 0.95. The overall quantum yield for photoinduced charge separation based on total light absorbed by both pigments equals Φ2 + Φ1Φ3, or 0.99. The 290 ps decay of the transient absorbances reflects the lifetime of the PH2•+C60•- charge-separated state, and k6 in Figure 11 is therefore 3.4 × 109 s-1. These rate constants and the values for k1 and k2 discussed above were used to simulate kinetics for the transient absorptions in the 670 nm region. The results, which agree with the measured data within experimental error, are shown in Figure 8. They illustrate clearly that the biexponential rise of the charge-separated species originates from significant contributions of the two photoinduced electron transfer pathways. The fluorescence data for dyad 1 in dichloromethane can be used to perform a similar kinetic analysis, assuming that the value of k1 is the same as it is in benzonitrile. This gives a k2 value of 5.4 × 1010 s-1, a Φ2 value of 0.43, and a Φ1 value of 0.57. The 91 ps excited singlet lifetime of the fullerene moiety in this solvent yields a k3 of 1.0 × 1010 and a Φ3 of 0.93. The overall yield of charge separation is 0.96. The time-resolved fluorescence studies of zinc dyad 2 in toluene solution yielded a 20 ps lifetime for 1PZn-C60. Model zinc porphyrin 5 had a lifetime of 1.6 ns in benzonitrile. Using these data and the fact that singlet-singlet energy transfer in toluene occurs with a quantum yield of essentially unity, eq 1 gives a rate constant for singlet-singlet energy transfer, k1, of 4.9 × 1010 s-1. The subpicosecond transient absorption results for dyad 2 in benzonitrile show a 1.9 ps rise time for the PZn•+C60•- state. If we assume that the rate constants for steps 1 and 5 are approximately the same in this solvent as they are in toluene, eq 1 yields a rate constant for photoinduced electron transfer, k2, of 4.8 × 1011 s-1 and a quantum yield of 0.91. The quantum yield of singlet energy transfer Φ1 is only 0.09. At 590 nm, the majority of the excitation light is absorbed by the porphyrin, and singlet energy transfer in benzonitrile is inefficient. Thus, little fullerene singlet state is formed. If PZn1C is being detected at all in the time-resolved fluorescence 60 or transient absorption studies, then it also must decay by electron transfer (step 3) with a rate constant on the order of 5 × 1011 s-1, as no longer components were noted in fluorescence or in the rise of the radical anion absorption. The PZn•+-C60•state in benzonitrile decays with a k6 value of 2.0 × 1010 s-1, based on the observed lifetime of 50 ps. Triplet-Triplet Energy Transfer. In toluene, neither 1 nor 2 displays photoinduced electron transfer from either the porphyrin or the fullerene singlet state. The porphyrin singlet state is quenched by singlet-singlet energy transfer to the fullerene, and essentially no porphyrin triplet is therefore formed
Electron Transfer in Porphyrin-Fullerene Dyads by intersystem crossing. The fullerene singlet state in both molecules undergoes intersystem crossing to give the fullerene triplet in high yield. (The triplet quantum yield of C60 itself is close to unity.20-22) The resulting fullerene triplet gives rise to the 690 nm absorption band in Figure 10. The triplet character of this species is verified by its ability to sensitize singlet oxygen formation. The spectra for the dyads in Figure 10 feature absorption below 500 nm that is characteristic of the porphyrin triplet states. Since the porphyrin triplets cannot be formed by intersystem crossing, they must result from triplet-triplet energy transfer from the fullerene. As the spectra feature both porphyrin and fullerene triplets, and their ratio is not time dependent on the time scale of measurement, the triplet states must be in equilibrium. This is a reasonable conclusion, as the electronic coupling between the porphyrin and fullerene moieties is strong enough to promote rapid photoinduced electron transfer and therefore should also be strong enough to allow triplet-triplet transfer. The triplet state of a typical meso-tetraarylporphyrin such as 5,10,15,20-tetraphenylporphyrin lies at about 1.44 eV.23-25 The energy of the triplet state of C60 is 1.55 eV.22,26 Because the first excited singlet state of 3 is shifted to lower energy by ∼0.17 eV relative to that of C60, the triplet level of the fullerene moiety of dyads 1 and 2 is probably slightly lower than 1.55 eV. This similarity of triplet state energies explains the observation of the two states in equilibrium in dyad 1. This equilibrium also explains the lifetime of the triplet-triplet absorption in 1, which is greater than that for triplets of fullerenes such as 3, but substantially less than that typical of porphyrins. In the case of dyad 2, the lifetime of the triplet transient is similar to that of model compound 3 and the amount of absorption below 500 nm appears to be less (although the ground-state absorbance of the solution did not permit measurements at the porphyrin triplet absorption maximum in the 440 nm region). This suggests that the amount of porphyrin triplet contributing to the transient absorption is less for 2 than for 1. This is reasonable, as the energy of the zinc-porphyrin triplet state is about 1.59 eV,23-25,27 and less of this species would therefore be present at equilibrium. Lifetimes of Charge-Separated States. The long lifetime for charge separation in these molecules is consistent with the occurrence of charge recombination (step 6) in the Marcus inverted region, where increased thermodynamic driving force leads to smaller electron transfer rate constants. The PH2•+C60•- state has a lifetime of 290 ps, whereas PZn•+-C60•-, which has less driving force for recombination, lives only 50 ps. Any proton transfer from the free-base porphyrin to the tertiary amine could also affect the lifetime of charge separation in 1. Solvent Effects on Photoinduced Electron Transfer. In toluene solution, photoinduced electron transfer in 1 and 2 is not observed. The reason for this is probably thermodynamic. The free energy change for photoinduced electron transfer is not large enough to allow electron transfer to compete with other deactivation pathways. The cyclic voltammetry in benzonitrile indicates that there is only 0.16 eV of driving force for photoinduced electron transfer from PH2-1C60 to yield PH2•+C60•- and 0.36 eV of driving force in the case of the zinc analog. The energies of these charge-separated states are expected to increase substantially in nonpolar solvents. For example, application of the dielectric continuum model28 to these dyads using a porphyrin ionic radius of 4.8 Å, a fullerene radius of 3.5 Å, and a center-to-center separation of 9.9 Å yields endergonic free energy change values of 0.58 and 0.37 eV for formation of PH2•+-C60•- and PZn•+-C60•-, respectively, from
J. Phys. Chem., Vol. 100, No. 39, 1996 15931
Figure 12. Conformations of dyads 1 and 6 from molecular-modeling studies.29
the fullerene excited singlet state in toluene. Photoinduced electron transfers from the porphyrin singlet states are also predicted to be endergonic in toluene by this model. The Weller model is likely an oversimplification, and the exact values quoted above are probably not completely reliable, but they are consistent with a substantially reduced driving force for electron transfer in toluene which slows electron transfer to the point where it cannot compete with other deactivation pathways. It is interesting to compare the results for dyads 1 and 2 with those previously reported for porphyrin-fullerene dyad 6 (Figure 12).7 This molecule also demonstrates photoinduced singlet energy and electron transfer of the types shown in Figure 11. The rate constants for similar processes in 6 are generally comparable to or larger than those in 1. Singlet energy transfer in 6 occurs in 7 ps in toluene solution, whereas the time constant for the comparable process in 1 is 22 ps. In 6, photoinduced electron transfer from PH2-1C60 to form PH2•+-C60•- occurs in benzonitrile with a free energy change of -0.31 eV and a rate constant of ∼2 × 1011 s-1. In 1, photoinduced electron transfer from 1PH2-C60 to yield PH2•+-C60•- occurs in the same solvent with a free energy change of -0.32 eV and a rate constant of 1.8 × 1011 s-1. In addition, the porphyrin absorption bands of 6 and its zinc analog in toluene are shifted to longer wavelengths by 11-15 nm, relative to model porphyrins, whereas those for 1 and 2 are shifted by only 4-8 nm. All of these data are consistent with a smaller separation between the porphyrin and fullerene moieties in 6 than in 1, and stronger electronic coupling between them. In 1 and 2, the π-electron systems of the chromophores are separated by three single bonds, and the center-to-center separation is 9.9 Å. In 6, there are five single bonds separating the π-electron systems, but NMR and molecular-modeling evidence indicates that the molecule exists in a folded conformation where the fullerene resides over the center of the porphyrin ring (see Figure 12). The center-to-center separation is ∼7 Å, and the π-electron systems are essentially in van der Waals contact. These results suggest that the electronic interactions responsible for electron, triplet energy, and perhaps singlet energy transfer in 6 and its metalated analog are mediated by direct through-space interaction of the π-electron systems, rather than the linkage bonds. Conclusions This study shows that, in polar solvents such as benzonitrile, both the free-base and zinc porphyrin-fullerene dyads undergo very rapid photoinduced electron transfer to give chargeseparated states with overall quantum yields near unity. The porphyrin singlet states decay within a few picoseconds by a combination of photoinduced electron transfer and singlet energy transfer to the fullerene, whereas electron transfer within 100 ps is the only significant decay pathway for the fullerene excited singlet state. The resulting charge-separated states are relatively long lived for dyad systems (290 ps for 1 and 50 ps for 2), and
15932 J. Phys. Chem., Vol. 100, No. 39, 1996 this suggests that dyads such as these might be well suited for incorporation into molecular-scale photovoltaic or optoelectronic systems. Experimental Section Compounds 1-5 were prepared as described previously.9 Steady-state absorption spectra were measured on a Shimadzu UV2100 UV-vis spectrophotometer. Fluorescence emission and excitation spectra were determined on a SPEX Fluorolog and corrected. Fluorescence decay measurements were performed by the time-correlated single-photon counting method. The excitation source was a cavity dumped Coherent 700 series dye laser synchronously pumped by a frequency-doubled Coherent Antares 76 Nd:YAG laser. Detection was via a multichannel plate photomultiplier (Hamamatsu R2809U-11), and the instrument response time was ca. 35 ps.30 For the nanosecond transient absorption measurements an Opotek OPO pumped by the third harmonic of a Continuum Surelite Nd: YAG laser was used as the excitation source. The spectrometer has been described.31 For the subpicosecond pump-probe measurements excitation was at 590 nm with ∼200 fs pulses at a 540 Hz repetition rate. The signals of the continuum-generated probe beam were collected by an optical spectrometric multichannel analyzer with a dual diode array head.32 Cyclic voltammetric measurements were carried out with a Pine Instrument Co. Model AFRDE4 potentiostat and a Bioanalytical Systems BAS100W electrochemical workstation. All electrochemical measurements were performed in benzonitrile at ambient temperatures with a glassy carbon or platinum disk working electrode, a Ag/Ag+ reference electrode, and a platinum wire counter electrode. The electrolyte was 0.1 M tetra-nbutylammonium hexafluorophosphate, and ferrocene was employed as an internal reference redox system. Acknowledgment. This work was supported by the National Science Foundation (Grant CHE-9413084 to D.G. and T.A.M.) and the National Institutes of Health (Grant GM-23851 to C.A.R.). This is publication number 302 from the ASU Center for the Study of Early Events in Photosynthesis. References and Notes (1) Connolly, J. S.; Bolton, J. R. In Photoinduced Electron Transfer; Fox, M. A., Channon, M., Eds.; Elsevier: Amsterdam, 1988; Part D, pp 303-393. (2) Gust, D.; Moore, T. A. AdV. Photochem. 1991, 16, 1-65. (3) Gust, D.; Moore, T. A. Top. Curr. Chem. 1991, 159, 103-151. (4) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 1993, 26, 198-205.
Kuciauskas et al. (5) Gust, D.; Moore, T. A. In The Photosynthetic Reaction Center; Norris, J. R., Deisenhofer, J., Eds.; Academic Press: New York, 1993; pp 419-464. (6) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461. (7) Liddell, P.; Macpherson, A. N.; Sumida, J.; Demanche, L.; Moore, A. L.; Moore, T. A.; Gust, D. Photochem. Photobiol. 1994, 59S, 36S. (8) Imahori, H.; Hagiwara, K.; Akiyama, T.; Taniguchi, S.; Okada, T.; Sakata, Y. Chem. Lett. 1995, 265-266. (9) Drovetskaya, T.; Reed, C. A.; Boyd, P. D. W. Tetrahedron Lett. 1995, 36, 7971-7974. (10) Imahori, H.; Sakata, Y. Chem. Lett. 1996, 199-200. (11) Linssen, T. G.; Durr, K.; Hanack, M.; Hirsch, Q. J. Chem. Soc., Chem. Commun. 1995, 103-104. (12) Maggini, M.; Dono, A.; Scorrano, G.; Prato, M. J. Chem. Soc., Chem. Commun. 1995, 843-845. (13) Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 4093-4099. (14) Imahori, H.; Cardoso, S.; Tatman, D.; Lin, S.; Macpherson, A. N.; Noss, L.; Seely, G. R.; Sereno, L.; Chessa de Silber, J.; Moore, T. A.; Moore, A. L.; Gust, D. Photochem. Photobiol. 1995, 62, 1009-1014. (15) Maggini, M.; Karlsson, A.; Scorrano, G.; Sandona, G.; Farina, G.; Prato, M. J. Chem. Soc., Chem. Commun. 1994, 589-590. (16) Greaney, M. A.; Gorun, S. M. J. Phys. Chem. 1991, 95, 72417144. (17) Kato, T. Laser Chem. 1994, 14, 155-160. (18) Gasyna, Z.; Browett, W. R.; Stillman, M. J. Inorg. Chem. 1985, 24, 2440-2447. (19) Wasielewski, M. R.; Gaines, G. L., III; O’Neil, M. P.; Svec, W. A.; Niemczyk, M. P.; Prodi, L.; Gosztola, D. In Dynamics and Mechanisms of Photoinduced Transfer and Related Phenomena; Mataga, N., Okada, T., Masuhara, H., Eds.; Elsevier Science Publishers: New York, 1992; pp 87103. (20) Bensasson, R. V.; Hill, T.; Lambert, C.; Land, E. J.; Leach, S.; Truscott, T. G. Chem. Phys. Lett. 1993, 201, 326-335. (21) Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J. Phys. Chem. 1991, 95, 11-12. (22) Hung, R. R.; Grabowski, J. J. J. Phys. Chem. 1991, 95, 60736075. (23) Egorova, G.; Knyukshto, V.; Solovev, K.; Tsvirko, M. Opt. Spectrosc. (USSR) 1980, 48, 1101-1109. (24) Gouterman, M.; Khalil, G.-M. J. Mol. Spectrosc. 1974, 53, 88100. (25) Harriman, A. J. Chem. Soc., Faraday Trans. 1 1980, 76, 19781985. (26) van den Heuvel, D. J.; Chan, I. Y.; Groenen, E. J. J.; Schmidt, J.; Meijer, G. Chem. Phys. Lett. 1994, 231, 111-118. (27) Quimby, D. J.; Longo, F. R. J. Am. Chem. Soc. 1975, 97, 51115117. (28) Weller, A. Z. Physik. Chem. (Munich) 1982, 133, 93-98. (29) Structure obtained from molecular modeling using Biosym Technologies programs InsightII and Discover. (30) Gust, D.; Moore, T. A.; Luttrull, D. K.; Seely, G. R.; Bittersmann, E.; Bensasson, R. V.; Rouge´e, M.; Land, E. J.; De Schryver, F. C.; Van der Auweraer, M. Photochem. Photobiol. 1990, 51, 419-426. (31) Davis, F. S.; Nemeth, G. A.; Anjo, D. M.; Makings, L. R.; Gust, D.; Moore, T. A. ReV. Sci. Instrum. 1987, 58, 1629-1631. (32) Lin, S.; Chiou, H.-C.; Kleinherenbrink, F. A. M.; Blankenship, R. E. Biophys. J. 1994, 66, 437-445.
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