Supramolecular Triads of Free-Base Porphyrin, Fullerene, and

links have been augmented by coordination, strong π−π interactions, base-pairing, and .... Figure 2 shows the optimized structures of the supr...
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J. Phys. Chem. C 2007, 111, 11123-11130

11123

Supramolecular Triads of Free-Base Porphyrin, Fullerene, and Ferric Porphyrins via the “Covalent-Coordinate” Binding Approach: Formation, Sequential Electron Transfer, and Charge Stabilization Francis D’Souza,*,† Suresh Gadde,† Amy L. Schumacher,† Melvin E. Zandler,† Atula S. D. Sandanayaka,‡ Yasuyuki Araki,‡ and Osamu Ito*,‡ Department of Chemistry, Wichita State UniVersity, 1845 Fairmount, Wichita, Kansas 67260-0051, and Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Sendai 980-8577, Japan ReceiVed: April 4, 2007; In Final Form: May 13, 2007

Supramolecular triads composed of free-base porphyrin, fullerene, and ferric porphyrin were constructed by using “covalent-coordinate” binding strategy. For this, a free-base porphyrin was covalently linked to a fullerene entity bearing a pyridine ligand, and was subsequently utilized to coordinate ferric porphyrins bearing different peripheral substituents. The newly formed triads were characterized by spectral methods and the structures were deduced from DFT B3LYP/3-21G(*) methods. Free-energy calculations performed with use of the redox potential and emission data suggested the occurrence of sequential electron transfer from singlet excited freebase porphyrin to the covalently linked fullerene, followed by an electron transfer from fullerene anion radical to ferric porphyrin, ultimately generating free-base porphyrin cation radical and ferrous porphyrin as the electron-transfer products, anticipating the generation of long-lived charge-separated species as a consequence of distant separation between the oxidized and reduced species. Time-resolved emission and nanosecond transient absorption techniques were used to obtain kinetic and spectral evidence of electron transfer. Attempts were made to obtain the lifetime of the final charge separated species by monitoring the decay of H2P•+ at 620 nm. Lifetimes of the order of 20 µs were obtained; however, they were found to be overlapped with the long-living triplet states of porphyrin of similar lifetimes at the monitoring wavelength.

Introduction The process of converting light energy into chemical energy in photosynthesis involves two major steps, namely, absorption and funneling of light energy of appropriate wavelengths by the antenna molecules to the reaction center, and photoinduced electron transfer (PET) to generate charge-separated states by using the electronic excitation energy.1 The antenna system consists of chromophore arrays which transport energy via singlet-singlet energy transfer mechanism among the chromophores. The reaction center, effectively coupled to the antenna, absorbs the excitation energy and converts it to chemical energy in the form of transmembrane charge separation via a multistep electron-transfer process. The stored energy in the form of charge-separated species (electrochemical energy) is later converted into other forms of biologically useful energy such as proton motive force.1 Mimicking the photosynthetic functions by using synthetic model compounds is important to further our understanding of the process of bioenergetics.2,3 Research in this area also holds promise for technological advances in solar energy harvesting,4-7 and building molecular optoelectronics such as photonic wires and switches.8 In this regard, several studies on artificial photosynthesis focused on PET have been reported and studied.3-7 The efficiency and quantum yield of charge separation in some of these systems were comparable to those found * Address correspondence to these authors. D’Souza: [email protected]. Ito: e-mail [email protected]. † Wichita State University. ‡ Tohoku University.

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in natural reaction centers. To mimic the antenna function, energy transfer between two or more covalently linked or selfassembled porphyrins, or other chromophores, have been studied extensively.6,7 In a few elegant systems, mimicry of both antenna and reaction center functionalities, that is, occurrence of sequential energy and electron transfer, has been demonstrated.9 Fullerenes and porphyrins possess unique photo and redox properties that make them suitable chromophores for building photosynthetic mimics. Studies on covalently linked fullereneporphyrin dyads demonstrated the occurrence of electron transfer from the excited singlet state of the porphyrin to the fullerene, with a high quantum yield.7,10-12 Owing to the small threedimensional reorganization energies of fullerenes, following rapid electron transfer, slow back-electron-transfer rates have been observed.13 Recently, different combinations of covalently linked porphyrin-fullerene conjugates, along with auxiliary donor and acceptor entities, have been constructed to generate long-lived charge-separated states with lifetimes comparable to the photosynthetic reaction center.6-13 In a few studies, the covalent links have been augmented by coordination, strong π-π interactions, base-pairing, and hydrogen-bonding methodologies, and energy and electron transfer have been probed.7,14 In the present study, we report construction of supramolecular triads composed of free-base porphyrin, fullerene, and ferric porphyrin by the “covalent-coordinate” bonding approach. As shown in Scheme 1, to accomplish this, a covalently linked freebase porphyrin-fullerene dyad, H2P-C60py, is newly synthesized. The pyrrolidine ring on the fullerene entity also possesses a pyridine ligand capable of axial coordination. The pyridine

10.1021/jp072657d CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007

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SCHEME 1: Structure of the Presently Investigated H2P-C60py:Fe(P)Cl Triads, C60py:Fe(P)Cl Dyads, and H2P-C60py Dyad

Figure 1. Absorbance changes observed during the titration of Fe(T(m-OCH3)PP)Cl by C60py in o-dichlorobenzene. The inset shows a Benesi-Hildebrand plot at 510 nm constructed to evaluate the binding constant: [C60] is the concentration of C60py, Ao is the intensity observed in the absence of C60py, and ∆A is the change in absorption upon the addition of C60py.

entity is utilized to axially coordinate a second electron acceptor, ferric porphyrin Fe(P)Cl, to form the supramolecular triads, H2P-C60py:Fe(P)Cl. Here, the employed ferric porphyrins are better electron acceptors than the fullerene.15,16 As a result, upon photoexcitation of the free-base porphyrin, an electron transfer to the closely disposed primary electron acceptor, fullerene, is expected. Further electron transfer (dark) from the fullerene anion radical to the ferric porphyrin is expected to occur through charge migration. A direct consequence of this would be slowing down the charge recombination process to yield long-lived charge-separated species. The axially coordinated ferric porphyrin-fullerene conjugate, C60py:Fe(P)Cl, in Scheme 1 is used to establish axial binding behavior of pyridine-functionalized fullerene to ferric porphyrin. Results and Discussion Formation of the Supramolecular Triads. To establish the axial coordination of H2P-C60py to Fe(P)Cl, and to determine the binding constants, a fullerene functionalized with a pyridine entity, C60py, was utilized. The reason for using C60py instead of H2P-C60py is to reduce the spectral complications due to strong absorption of H2P in the entire spectral range. Figure 1 shows the spectral changes during the titration of Fe(T(mOCH3)PP)Cl by C60py. It is well-known that ferric porphyrins form a 1:1 complex at low ligand concentration and a 1:2 complex at high ligand concentrations,16a,b and several studies

have reported binding constants for these complexes.17 The binding constants vary depending upon the substituents on the porphyrin ring periphery and nature of the axial coordinating ligand. The spectral changes observed during the titration of Fe(T(m-OCH3)PP)Cl by C60py are suggestive of a 1:1 complex formation.16a Additionally, isosbestic points at 493, 533, 640, and 708 nm were observed suggesting the existence of one equilibrium process in solution. A 1:2 complex was not observed perhaps due to the low concentration of employed C60py. Similar spectral changes were observed for the titrations involving Fe(TPP)Cl and Fe(T(F5)PP)Cl. Thus, the equilibrium process involving Fe(P)Cl and C60py could be represented by eq 1 where the symbol “:” represents a coordinate bond between iron and pyridine entities (see Scheme 1 for structures of individual species).

C60py + FeIII(P)Cl h C60py:FeIII(P)Cl

(1)

The binding constants were evaluated by constructing Benesi-Hildebrand plots.18 As shown in the Figure 1 inset, straightline plots were obtained in the employed concentration range of C60py. The binding constant, K1, was found to be 3.2((0.6) × 103, 1.4((0.4) × 103, and 1.2((0.3) × 103 M-1 respectively for Fe(T(m-OCH3)PP)Cl, Fe(TPP)Cl, and Fe(T(F5)PP)Cl binding to C60py in o-dichlorobenzene. These values suggest weak to moderate stability of the supramolecular complexes. B3LYP/3-21G(*) Geometry Optimizations. Further, to visualize the structure of the supramolecular triads, geometry optimizations were performed at the B3LYP/3-21G(*) level19,20 on the representative H2P-C60py:Fe(TPP)Cl triad. Earlier, the B3LYP/3-21G(*) methods were successfully used to predict the geometry and electronic structure of molecular and selfassembled supramolecular dyads and triads.20 For the present calculations, the starting molecules, H2P-C60py dyad and Fe(TPP)Cl, were fully optimized to a stationary point on the Born Oppenheimer potential energy surface and allowed to interact. Figure 2 shows the optimized structures of the supramolecular triad. Two structures, namely, “closed” and “extended” types, were conceivable; although owing to the very flexible nature of the systems, several other intermediates

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Figure 4. Cyclic voltammograms of Fe(T(F5)PP)Cl on 0.0 (black line), 1.0 (red line), 3.0 (blue line), and 10 equiv (magenta line) addition of C60py in o-dichlorobenzene, 0.1 M (n-C4H9)4NClO4. Scan rate ) 100 mV/s. Figure 2. B3LYP/3-21G(*) optimized structures of the supramolecular H2P-C60py:Fe(TPP)Cl triad in the (a) closed and (b) extended forms: C, gray; N, blue; O, red; Fe, sky blue; and Cl, green.

Figure 3. Cyclic voltammograms of (a) H2P-py, (b) H2P-C60py, (c) Fe(T(m-OCH3)PP)Cl, (d) Fe(TPP)Cl, and Fe(T(F5)PP)Cl in o-dichlorobenzene, 0.1 M (n-C4H9)4NClO4. Scan rate ) 100 mV/s. Colors of the curves show the range of scan.

TABLE 1: Electrochemical Redox Potentials (vs Fc/Fc+) of the Free-Base Porphyrin-Fullerene Dyad and Iron Porphyrins in o-Dichlorobenzene, 0.1 M (n-C4H9)4NClO4 Eox compd H2P-py C60py H2P-C60py Fe(T(m-OCH3)PP)Cl Fe(TPP)Cl Fe(T(F5)PP)Cl

2nd Ox

1st Ox

-1.77 -1.15 0.55 (H2P) -1.14 (C60) 0.65 (PFe) -0.87 (FeIII/II) 0.66 (PFe) -0.88 (FeIII/II) -0.61 (FeIII/II)

0.53 0.88 0.93

Ered 1st Red

2nd Red -2.06 -1.53 -1.53 -1.61 -1.62 -1.62

structures with local minima are possible but the ones shown in Figure 2 seem to be the low-energy structures. The extended

structure was energetically more stable by ca. 3 kcal/mol. The Fe-N axial bond distance (∼2.1 Å) was similar to that reported earlier for ZnP-pyC60 dyads.20 In the optimized “closed” structure, the center-to-center distance between free-base porphyrin and fullerene was found to be ∼11.5 Å while such distances between Fe(P) and C60, and Fe(P) and H2P were found to be 9.4 and 20.6 Å, respectively. For the extended structure, the center-to-center distances between H2P and C60, Fe(P) and C60, and Fe(P) and H2P were found to be ∼18, ∼9.3, and ∼24.9 Å, respectively. In either of the structures, no through-space π-π type interactions between different entities of the supramolecular triad were observed. Electrochemical Studies and Electron-Transfer Driving Forces. Detailed electrochemical studies with the cyclic voltammetric technique were performed to visualize the redox states and also to evaluate the energetics of electron-transfer reactions. Figure 3 shows cyclic voltammograms of the studied compounds, while Table 1 lists their redox potential values. The H2P-py, which was employed as a model of H2P-C60py in the present study, revealed two reversible one-electron reductions and one one-electron oxidation processes, as judged by their peak-to-peak separation and scan rate variation studies, similar to pristine meso-tetraphenylporphyrin, H2TPP.21 Scanning the potential beyond the first oxidation process revealed a second oxidation process that was irreversible. The dyad, H2PC60py, revealed peaks corresponding to both porphyrin and fullerene entities. The first anodic process corresponding to the oxidation of porphyrin ring was located at 0.55 V and was anodically shifted by 20 mV compared to H2P-py. During the cathodic scan, up to five reductions were observed within the potential window of the solvent, and by comparison with the potential values of H2P-py and C60py, the first two were assigned to the reductions of fullerene, the third corresponding to the reduction of porphyrin, and the forth, which was an overlap of two one-electron processes, was assigned to the third reduction of fullerene and the second reduction of porphyrin. In agreement with the results reported in the literature,15,16 the employed iron(III) porphyrins revealed three one-electron reductions and up to two one-electron oxidation processes. The potential of the first reduction process, which corresponds to the FeIII/FeII transition, was found to depend upon the substituents on the porphyrin macrocycle and followed the order: Fe(T(m-OCH3)PP)Cl > Fe(TPP)Cl > Fe(T(F5)PP)Cl. Importantly, these values were smaller than that of the first reduction

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TABLE 2: Energy Levels of the Charge-Separated States (∆GRIP) and Hole Shift (∆GHS) for Supramolecular Triads in o-Dichlorobenzene dyad/triad H2P-C60py H2P-C60py:Fe(T(m-OCH3)PP)Cl H2P-C60py:Fe(TPP)Cl H2P-C60py:Fe(T(F5)PP) Cl

∆GRIP(H2P-C)a/ eV

∆GRIP(H2P-Fe)a/ eV

-∆GHS(C-Fe)/ eV

1.42 1.43 1.16

0.27 0.26 0.53

1.69

a ∆GRIP ) e(Eox - Ered) + ∆GS, where ∆GS ) -e2/(4π0RRCt-Ct) and 0 and R refer to vacuum permittivity and dielectric constant of o-dichlorobenzene. b -∆GCS ) ∆E0-0 - ∆GRIP, where ∆E0-0 is the energy of the lowest excited state of H2P (1.90 eV).

Figure 6. Fluorescence decays (600-750 nm) of H2P-C60py:Fe(P)Cl (0.05 mM:0.07 mM) in o-dichlorobenzene (λex ) 410 nm): (i) H2PC60py, (ii) H2P-C60py:Fe(T(m-OCH3)PP)Cl, (iii) H2P-C60py:Fe(TPP)Cl, and (iv) H2PC60py:Fe((F5)TPP)Cl. Figure 5. Steady-state fluorescence spectra of (i) H2TPP, (ii) H2PC60py dyad, and (iii) H2P-C60py:Fe(TPP)Cl triad (obtained by coordinating Fe(TPP)Cl to the pyridine entity of H2P-C60py, 5 equiv of Fe(TPP)Cl was employed) in o-dichlorobenzene. The concentration of porphyrins was held constant at 20 µM. The samples were excited at 516 nm corresponding to the most intense visible band of the freebase porphyrin entity.

of C60py (Table 1). Similar trends in the redox potential values for the remaining oxidation and reduction processes of the iron porphyrins were observed; however, in the case of Fe(T(F5)PP)Cl due to an electron deficient porphyrin macrocycle, no oxidation process within the potential window covering 1.0 V vs Fc/Fc+ was observed. The value of reduction potential corresponding to the FeIII/FeII process is known to vary with the number of axially coordinated nitrogenous ligands. Generally, a mono coordination with nitrogenous ligand shifts the reduction process in anodic direction by few tens of millivolts while bis coordination with nitrogenous ligands shifts the reduction process up to 400 mV anodically (see the Supporting Information for representative CV for dimethylaminopyridine binding to the iron porphyrins and the corresponding potential shifts).16 That is, both mono and bis coordination of ferric porphyrins by nitrogenous ligands make the reduction process facile and hence better electron acceptors. As shown in Figure 4 for the representative Fe(T(F5)PP)Cl, addition of C60py makes the reduction quasireversible and facile by 20 mV suggesting both the coordination of the pyridine entity of C60py to the iron center (C60py:Fe(P)Cl) and better electron acceptor ability of the coordinated complex. Similar results were observed for the other two iron porphyrins whose reduction potential values for the FeIII/FeII process upon binding of C60py are given in Table 1. The energy levels of the charge-separated states (∆GRIP) were evaluated by using the Weller-type approach22 utilizing the redox potentials, center-to-center distance, and dielectric constant of the solvent as listed in Table 2. By comparing these energy

levels of the charge-separated states with the energy levels of the excited states, the driving forces (∆GCS) were evaluated (Table 2). The generation of H2P•+-C60•-py is exothermic either via 1H2P* or 1C60* in o-dichlorobenzene for the dyad, H2PC60py. Importantly, a hole shift from C60•- to FeIII(P)Cl, yielding the H2P•+-C60py-FeII(P) charge-separated state (the free energy change for this process is referred to as ∆GHS(C-Fe) in Table 2), is also found to be exothermic for all of the supramolecular triads. The overall effect of the hole shift is the distant separation of the cation and anion species that is eventually expected to slow down the charge recombination process. Steady-State Fluorescence Measurements. Figure 5 shows the steady-state fluorescence emission spectra of H2TPP, H2PC60py dyad, and H2P-C60py:Fe(TPP)Cl triad (obtained by coordinating Fe(TPP)Cl to the pyridine entity of H2P-C60py) in o-dichlorobenzene. The employed ferric porphyrins were found to be nonfluorescent in both the absence and presence of any coordinating nitrogenous ligands in o-dichlorobenzene owing to the presence of the paramagnetic metal center. The fluorescence spectrum of H2P-C60py was found to be quenched over 70% as compared to H2TPP bearing no fullerene entity. As will be demonstrated in the subsequent sections, this is due to the occurrence of efficient electron transfer from the singlet excited H2P to the covalently linked fullerene. Further quenching of the H2P emission in the H2P-C60py dyad was observed upon building the supramolecular H2P-C60py:Fe(P)Cl triads. This observation illustrates the indirect effect of the strong electron accepting Fe(P)Cl on the C60py moiety. Although direct interaction between 1H2P* and Fe(P)Cl is energetically possible23 in the present supramolecular systems, it is unlikely that 1H P* and Fe(P)Cl interact directly due to geometry consider2 ations from the stable structures shown in Figure 2. Further timeresolved emission and transient absorption spectral studies were performed to arrive at the kinetics and characterization of the electron-transfer products.

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TABLE 3: Time-Resolved Fluorescence Lifetimes (τF,H2P) and Rate Constants (kCS,H2P) and Quantum Yields (ΦCS,H2P) for Charge Separation in o-Dichlorobenzene dyad/triad H2P-C60py H2P-C60py:Fe(T(m-OCH3)PP)Cl H2P-C60py:Fe(TPP)Cl H2P-C60py:Fe(T(F5)PP)Cl a

τF,H2Pa,b/ps (fraction/%)

kCS,H2Pb/s-1

ΦCS,H2P

880 (30); 2750 (70) 550 (50); 3200 (65) 300 (80); 2500 (30) 190 (85); 2510 (13)

1.0 × 109 1.9 × 109 3.2 × 109 5.2 × 109

90 94 97 94

Goodness of the decay curve fit (χ2) was in the rage of 1.00-1.10. b τF for H2P-py was found to be 10 000 ps.

Figure 7. Nanosecond transient absorption spectra observed by 532 nm laser irradiation of H2P-C60py (0.1 mM) at 0.025 (O), 0.1 (b), and 1.0 µs (O) in Ar-saturated o-dichlorobenzene. Inset: Absorptiontime profiles at 1020 nm.

Fluorescence Lifetime Measurements. To probe the dynamics of 1H2P* in the dyads and triads, time-resolved emission spectral studies were performed. When the H2P moiety of the H2P-C60py dyad was excited at a wavelength of 410 nm, the time profiles for the decays of the H2P moiety at 650 nm were observed as shown in Figure 6. The fluorescence decay rate of 1H P*-C py shown in Figure 6 clearly was faster than that of 2 60 1H P* (H TPP or H P-py) demonstrating the occurrence of the 2 2 2 photoinduced electron-transfer process in 1H2P*-C60py. The decay could be fit satisfactorily to a biexponential curve as judged from the χ2 values. The lifetimes were found to be 880 (30%) and 2750 ps (70%) for the 1H2P*-C60py, which were faster than 10 000 ps (100%) for 1H2TPP* (Table 3). On addition of Fe(T(m-OCH3)PP)Cl, further quenching of the 1H2P* moiety was observed with lifetimes of 550 (50%) and 3200 ps (50%) for the 1H2P*-C60py:Fe(T(m-OCH3)PP)Cl, suggesting an increase in charge separation from 1H2P* to C60py by the indirect effect caused by coordination of Fe(T(m-OCH3)PP)Cl to C60py. Similar results were obtained for the triads formed by coordinating Fe(TPP)Cl or Fe(T(F5)PP)Cl to H2P-C60py. The rate constants (kCS,H2P) and quantum yields (ΦCS,H2P) for charge separation in 1H2P*-C60py and 1H2P*-C60py:Fe(P)Cl were calculated by using the standard procedure and are listed in Table 3. Nanosecond Transient Absorption Measurements. The nanosecond transient absorption spectra using 532 nm laser light recorded for the H2P-C60py dyad are shown in Figure 7. Absorption appeared immediately after laser pulse at 1020 nm corresponding to the radical anion of C60py (C60py•-). The absorption band around 600 nm observed at 100 ns time interval is attributed to the radical cation, H2P•+, although this band overlaps with the depletion of the H2P moiety. The intense absorption in the 700-800 nm region is attributed to the triplet states of H2P (3H2P*) and C60py (3C60py*).24 The time profile of the 1020 nm peak shown in the inset of Figure 7 revealed that the C60py•- moiety raises quickly, which may correspond to the fluorescence decay, suggesting charge separation from the singlet excited state generating H2P•+-C60py•- species. After

Figure 8. Nanosecond transient absorption spectra of H2P-C60py:Fe(T(m-OCH3)PP)Cl observed by 532 nm laser irradiation in at 0.1 (b) and 1.0 µs (O) in Ar-saturated o-dichlorobenzene. The concentrations of H2P-C60py and Fe(T(m-OCH3)PP)Cl were respectively 0.1 and 0.14 mM. Inset: Absorption-time profiles at 1020 nm.

reaching a maximum, C60py•- at 1020 nm revealed faster decay. This decay was attributed to the charge-recombination process of H2P•+-C60py•-, which resulted in a rate constant, kCR-dyad, of 2.6 × 107 s-1, from which the lifetime (τRIP-dyad) of H2P•+C60py•- was calculated to be 38 ns. After the initial decay of C60py•- at 1020 nm, a prolonged decay was observed, which was attributed to the tail of the 3H2P* and 3C60py* moieties. The faint rise observed near 200 ns suggests the chargerecombination process to the 3H2P* and 3C60py* moieties. Nanosecond transient absorption spectra of the supramolecular triads, H2P-C60py:Fe(P)Cl, were recorded to exploit the effect of the second electron acceptor, Fe(P)Cl, on the photophysical behavior of H2P-C60py. Figure 8 shows the transient absorption spectra, upon forming the triad by the addition of 1.4 equiv of Fe(P)Cl to H2P-C60py in o-dichlorobenzene, after the 532 nm laser irradiation. Transient absorption bands corresponding to C60py•- at 1020 nm were observed in the spectrum depicted at 100 ns, generating H2P•+-C60py•-:Fe(P)Cl. As shown in the time profile at 1020 nm in Figure 8, the observed decay of C60py•- in the supramolecular triad was slower than the decay of the dyad, H2P•+-C60py•-, in Figure 7. This observation suggests charge stabilization in H2P•+-C60py•-:Fe(P)Cl, which was supported by the more facile reduction of Fe(P)Cl than that of C60py. Therefore, one would anticipate the charge-recombination rates of H2P•+-C60py•-:Fe(P)Cl, kCR-triad, to be slower than that of the dyad, kCR-dyad. Thermodynamically, electron migration (EM) from C60py•to FeIII(P)Cl is possible in the supramolecular triads; therefore, the observed decays for triads may include both kCR-triad and EM rate (kEM). Interestingly, the rate constants (kCR-triad + kEM) estimated from the decay of the 1020 nm band revealed an increase with the increase in the electron-withdrawing ability of the FeIII(P)Cl moiety. That is, increasing the thermodynamic driving force for the electron migration process results in an increased decay rate constant of the C60py•- supporting the occurrence of electron migration in H2P•+-C60py•-:Fe(P)Cl to

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Figure 9. Long time scale profiles at 620 nm in Ar-saturated o-dichlorobenzene for (i) H2P-C60py:Fe(T(F5)PP)Cl, (ii) H2P-C60py: Fe(T(m-OCH3)PP)Cl, and (iii) H2P-C60py:Fe(TPP)Cl.

TABLE 4: Sum of the Rate Constants for Charge Recombination and Electron Shift (kCR-triad + kEM) of H2P•+-C60py•-:Fe(P)Cl with Longer Time Scale Decay Rate Constant at 620 nm in o-Dichlorobenzene

c

triad

kCR-triad + kEMa/s-1

kfinal-decayb,c/s-1

H2P-C60py:Fe(T(m-OCH3)PP)Cl H2P-C60py:Fe(TPP)Cl H2P-C60py:Fe(T(F5)PP)Cl

7.1 × 8.9 × 106 1.1 × 107

3.8 × 104 4.4 × 104 4.8 × 104

106

a From time profiles in Figure 8. b From time profiles in Figure 9. Involves both kCR and ktriplet decays.

form H2P•+-C60py:FeII(P)Cl. The rates of final chargerecombination (kCR-final) in the distantly charge-separated species, H2P•+-C60py:FeII(P)Cl to H2P-C60py:FeIII(P)Cl, were evaluated by monitoring the decay of H2P•+ at 620 nm in the long time scale are as shown in Figure 9, in which the initial rises can be attributed to the recovery of the depletion of the transient species to the ground state. From the decays, the final decay rate constants (kfinal-decay) at 620 nm were evaluated as listed in Table 4. Addition of oxygen to the solution decreased the amount of long-lived transients appreciably indicating the involvement of the triplet excited states with the radical ion species. Thus, the kfinal-decay values evaluated include the rate constants of the final charge-recombination (kCR-final) in the distantly charge-separated species, H2P•+-C60py:FeII(P)Cl to H2P-C60py:FeIII(P)Cl, in addition to the triplet decays. Although it was difficult to separate the two factors, kCR-final may be almost the same order as the reported triplet decay rate constant (ca. (3-5) × 104 s-1).14,15 It should be mentioned here that our efforts to monitor the decay of the ferrous porphyrin transient bands in the Q-band region were not fully successful due to the strong absorption bands of free-base porphyrin and fullerene entities. Energy Level Diagram. Figure 10 shows the energy level diagram showing the different photochemical events of the supramolecular triads in o-dichlorobenzene. The results of the present investigation along with all of the control experiments clearly show the occurrence of sequential electron transfer in the studied supramolecular triads. The combination of steadystate absorption and emission, molecular modeling, electrochemistry, and time-resolved emission and transient absorption studies has permitted the extraction of the needed energetic and kinetic information. Energies of the excited states of the different entities were calculated from the fluorescence peaks, while the triplet state energy of H2TPP was cited from the literature.24 The driving forces for the occurrence of electron transfer from the different excited chromophores of the triad are

Figure 10. Energy level diagram showing the different photochemical events of the supramolecular H2P-C60py:Fe(P)Cl triads after excitation of the H2P moiety in o-dichlorobenzene; the singlet and triplet energies were obtained from ref 24. In H2P•+-C60py•-:Fe(P)Cl, a maximum energy of 1.69 eV for H2P•+-C60py•- is employed as a reference.

cited from Table 2. Since the energy level of 3H2TPP* is lower than that of H2P•+-C60py•-:FeIII(P)Cl, a charge-separation process via 3H2P* was not possible. The electron migration process is slightly exothermic supporting the rates in the order of ca. 107 s-1. The energy of the final charge-separate states, H2P•+-C60py:FeII(P)Cl, was found to be ca. 1.4 eV. Due to the distant radical ions in the supramolecular triads, a considerable slowing down of the final charge-recombination was predicted. Conclusions A series of supramolecular triads were constructed via axial coordination of a pyridine appended fulleropyrrolidine covalently bonded H2P dyad to the metal center of ferric porphyrins. Free-energy calculations performed by Weller approach suggested the possibility of the occurrence of sequential electron transfer from singlet excited free-base porphyrin to the covalently linked fullerene, followed by an electron transfer from the fullerene anion radical to ferric porphyrin. Efficient electron transfer from the singlet excited free-base porphyrin to the fulleropyrrolidine within the dyad was observed. As revealed by the transient absorption spectral studies, in the supramolecular triad, the electron transfer between the freebase porphyrin and fullerene was much influenced by the Fe(P)Cl to create the prolonged charge-separated state. We also attempted to obtain the lifetime of the final charge-separated species by monitoring the decay of H2P•+ at 620 nm. Lifetimes of the order of 20 µs were obtained; however, the transient cation radical peak was overlapped with the triplet absorption band having a lifetime of the same order of magnitude. That is, it was difficult to dissect the lifetime of the radical ion-pair from that of the triplet absorption bands, although the transient spectral data along with the control experiments were suggestive of the existence of such long-lived charge-separated species due to the occurrence of sequential electron transfer in the studied supramolecular triads. Experimental Section Chemicals. Buckminsterfullerene, C60 (+99.95%), was obtained from SES Research (Houston, TX). o-Dichlorobenzene in a sure seal bottle was obtained from Aldrich Chemicals

Construction of Supramolecular Triads (Milwaukee, WI). Tetra-n-butylammonium perchlorate, (nC4H9)4NClO4, was obtained from Fluka Chemicals. All the chromatographic materials and solvents were procured from Fisher Scientific and were used as received. Syntheses and purification of pyridine appended fulleropyrrolidine,14a and the ferric porphyrins,23 starting from the respective free-base porphyrin derivatives, were carried out according to the literature procedures. Synthesis of 5-{4′′-Formyl benzoic acid-4′-phenyl ester}10,15,20-triphenylporphyrin. In a round-bottomed flask 100 mg of 5-(4′-hydroxyphenyl)-10,15,20-triphenylporphyrin (0.1583 mmol), 5 equiv of 4-carboxybenzaldehyde (119 mg, 0.79 mmol), and 5 equiv of DMAP (96 mg, 0.79 mmol) were added to 50 mL of dry CH2Cl2 then the solution was cooled to 0 °C and 5 equiv of DCC (163 mg, 0.79 mmol) was added. The reaction mixture was stirred for 2 h at room temperature and solvents were removed. Crude compound was washed with water several times and extracted with CHCl3. Further purification of the compound was carried out on a silica gel column with toluene and chloroform (90:10 v/v) as eluent. Yield 70%. 1H NMR in CDCl3, δ (ppm) 9.95 (1H, -CHO), 8.89 (m, 8H, β-pyrrole-H), 8.27 (m, 6H, o-phenyl-H), 7.55-7.75 (m, 9H, m-, p-phenylH), 7.90, 7.25 (d, d, 4H, substituted phenyl-H), 8.42-6.91 (d, d, 4H, phenyl-CHO), -2.81 (s, 2H, imino-H). Synthesis of 5-[4′′-(2-(N-Methyl)fulleropyrrolidine) benzoic acid-4′-phenyl ester]-10,15,20-triphenylporphyrin. In a 250 mL round-bottomed flask 5-{4′-(4-formylphenoxy)phenyl}10,15,20-triphenylporphyrin (85 mg, 0.11 mmol), 2 equiv of C60 (160 mg, 0.22 mmol), and 2 equiv of 4-pyridylalanine (37 mg, 0.22 mmol) were added to 100 mL of dry toluene and the solution was refluxed for 10 h. Solvent was removed under vacuum and the crude compound was adsorbed on silica gel and purified on silica gel column with toluene and ethyl acetate (85:15 v/v) as eluent; the yield was ca. 25%. ESI mass in CH2Cl2 matrix: calculated 1587.1, found 1588.0. 1H NMR (CS2: CDCl3 (1:1 v/v) δ (ppm) 8.81 (m, 8H, β-pyrrole-H), 8.168.20 (m, 6H, o-phenyl-H), 7.6-7.7 (m, 9H, m-, p-phenyl-H), 8.0, 7.58 (d, d, 4H, substituted phenyl-H), 8.35, 7.1 (d, d, 4H, phenyl-C60), 8.66, 8.22 (br, d, 4H, pyridyl), 6.75, 6.21, 5.75, 5.45, 5.51, 3.75, 3.92, 3.55 (s, s, s, d, d, d, t, t 4H, pyrrolidine-H and -CH2, three geometric isomers). Instrumentation. The UV-visible spectral measurements were carried out with a Shimadzu Model 1600 UV-visible spectrophotometer. The fluorescence emission was monitored by using a Spex Fluorolog-tau spectrometer. A right angle detection method was used. The 1H NMR studies were carried out on a Varian 400 MHz spectrometer. Tetramethylsilane (TMS) was used as an internal standard. Cyclic voltammograms were recorded on a EG&G Model 263A potentiostat using a three-electrode system. A platinum button electrode was used as the working electrode. A platinum wire served as the counter electrode and a Ag/AgCl electrode was used as the reference electrode. A ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were purged prior to electrochemical and spectral measurements with argon gas. The computational calculations were performed by DFT B3LYP/321G(*) methods with the GAUSSIAN 0319 software package on a high-speed computer. The graphics of HOMO and LUMO coefficients were generated with the help of GaussView software. The ESI-Mass spectral analyses of the newly synthesized compounds were performed by using a Fennigan LCQDeca mass spectrometer. For this, the compounds (about 0.1 mM concentration) were prepared in CH2Cl2, freshly distilled over calcium hydride.

J. Phys. Chem. C, Vol. 111, No. 29, 2007 11129 Time-Resolved Emission and Transient Absorption Measurements. The picosecond time-resolved fluorescence spectra were measured with use of an argon-ion pumped Ti:sapphire laser (Tsunami; pulse width ) 2 ps) and a streak scope (Hamamatsu Photonics; response time ) 10 ps). The details of the experimental setup are described elsewhere.24 Nanosecond transient absorption measurements were carried out with use of the SHG (532 nm) of an Nd:YAG laser (Spectra Physics, Quanta-Ray GCR-130, fwhm 6 ns) as excitation source. For the transient absorption spectra in the near-IR region (6001600 nm), the monitoring light from a pulsed Xe lamp was detected with a Ge-avalanche photodiode (Hamamatsu Photonics, B2834).24 Acknowledgment. This work is dedicated to Professor D. Paul Rillema on the occasion of his 65th birthday. This work is supported by the National Science Foundation (Grant 0453464 to F.D.), the donors of the Petroleum Research Fund, administered by the American Chemical Society, and Grants-in-Aid for Scientific Research on Primary Area (417) from the Ministry of Education, Science, Sport and Culture of Japan. Supporting Information Available: Cyclic voltammograms of Fe(TPP)Cl in the presence of dimethylaminopyridine. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press; San Diego, CA, 1993. (b) Koepke, J.; Hu, X.; Muenke, C.; Schulten, K.; Michel, H. Structure 1996, 4, 581. (c) McDermott, G.; Prince, S. M.; Freer, A. A.; Hawthornthwaite-Lawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N. W. Nature 1995, 374, 517. (d) Fleming, G. R.; van Grondelle, R. Curr. Opin. Struct. Biol. 1997, 7, 738. (e) Polivka, T.; Sundstro¨m, V. Chem. Rev. 2004, 104, 2021. (2) (a) Sutin, N.; Brunschwig, B. S. AdV. Chem. Ser. 1990, 226, 65. (b) Bolton, J. R.; Mataga, N.; McLendon, G., Eds. AdV. Chem. Ser. 1991, 228, 295. (c) Wheeler, R. A. Introduction to the Molecular Bioenergetics of Electron, Proton, and Energy Transfer. ACS Symp. Ser. 2004, 883, 1. (d) Leibl, W.; Mathis, P. Electron Transfer in Photosynthesis. Ser. PhotoconVers. Sol. Energy 2004, 2, 117. (3) (a) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265. (b) Marcus, R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111. (c) Bixon, M.; Jortner, J. AdV. Chem. Phys. 1999, 106, 35. (4) (a) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J. Am. Chem. Soc. 1984, 106, 3047. (b) Closs, G. L.; Miller, J. R. Science 1988, 240, 440. (c) Connolly, J. S.; Bolton, J. R. In Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, The Netherlands, 1988; Part D, pp 303-393. (d) Connolly, J. S., Ed. Photochemical ConVersion and Storage of Solar Energy; Academic: New York, 1981. (5) (a) Balzani V.; Scandola, F. Supramolecular Chemistry; Ellis Horwood; New York, 1991. (b) Schlicke, B.; De Cola, L.; Belser, P.; Balzani, V. Coord. Chem. ReV. 2000, 208, 267. (c) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (d) Kurreck, H.; Huber, M. Angew. Chem., Int. Ed. 1995, 34, 849. (6) (a) Gust D.; Moore, T. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: Burlington, MA, 2000; Vol. 8, pp 153-190. (b) Fukuzumi, S.; Guldi, D. M. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH; Weinheim, Germany, 2001; Vol. 2, pp 270-337. (7) (a) Imahori, H.; Sakata, Y. AdV. Mater. 1997, 9, 537. (b) Guldi, D. M. Chem. Commun. 2000, 321. (c) Guldi, d. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695. (d) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22. (e) Meijer, M. E.; van Klink, G. P. M.; van Koten, G. Coord. Chem. ReV. 2002, 230, 141. (f) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem. Photobiol. C 2004, 5, 79. (g) Imahori, H.; Fukuzumi, S. AdV. Funct. Mater. 2004, 14, 525. (h) D’Souza, F.; Ito, O. Coord. Chem. ReV. 2005, 249, 1410. (i) Sanchez, L.; Martin, N.; Guldi, D. M. Angew. Chem., Int. Ed. 2005, 44, 5374. (j) Bouamaied, I.; Coskun, T.; Stulz, E. Struct. Bonding 2006, 121, 1-147. (k) Verhoeven, J. W. J. Photochem. Photobiol. C 2007, 7, 40. (l) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (8) (a) Introduction of Molecular Electronics; Petty, M. C., Bryce, M. R., Bloor, D., Eds.; Oxford University Press, New York, 1995. (b) Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH GmbH: Weinheim, Germany, 2001. (c) Gust D.; Moore, T. A.; Moore, A. L. Chem. Commun. 2006, 1169.

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