Supramolecular Donor–Acceptor Assembly Derived from

Publication Date (Web): May 17, 2012 ...... Porphyrin Science; Kadish , K. M. ; Guilard , R. ; Smith , K. M., Eds.; World Science Publishers: Singapor...
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Supramolecular Donor−Acceptor Assembly Derived from Tetracarbazole−Zinc Phthalocyanine Coordinated to Fullerene: Design, Synthesis, Photochemical, and Photoelectrochemical Studies Chandra K. C. Bikram, Navaneetha K. Subbaiyan, and Francis D’Souza* Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, Texas 76203-5017, United States S Supporting Information *

ABSTRACT: A functional photosynthetic antenna−reaction center mimicking donor− acceptor polyad has been newly designed, synthesized, and characterized. The polyad was comprised of four entities of carbazole covalently linked to the macrocycle periphery of a zinc phthalocyanine (ZnPc). Efficient singlet excitation transfer from the carbazole to zinc phthalocyanine has been witnessed from the emission studies. Axial coordination of phenylimidazole-functionalized fulleropyrrolidine to ZnPc served as an electron acceptor in the polyad. Optical absorption and emission along with computational studies revealed stable complex formation wherein the evaluated binding constant K was 7.7 ± 0.2 × 105 M−1, an order of magnitude higher than that observed earlier for similar complexes due to the electronic effect induced by the carbazole entities. From the free-energy calculations, photoinduced electron transfer from the 1ZnPc* to fullerene within the polyad was established to be an exothermic process. Kinetics of charge separation, kCS, monitored by time-resolved emission was found to be 2.8 × 109 s−1, indicating a relatively fast chargeseparation process. The electron-transfer products were characterized by nanosecond transient absorption spectroscopic technique; the presence of ZnPc+• radical cation at 890 nm and fulleropyrrolidine anion radical at 1000 nm was clear from this study. The kinetics of charge recombination, kCR, evaluated from the decay of either of the radical ions, was found to be 6.25 ± 0.2 × 107 s−1, revealing the persistence of the radical ion-pair species to some extent. Further, photoelectrochemical studies, performed by constructing photocells by electrophoretic deposition of the studied polyad on nanocrystalline SnO2 modified surface, revealed a higher value of incident photon-to-current conversion efficiency covering the wide visible−near IR spectral region and good on−off switchability. Better charge injection from the excited polyad to the conduction band of the semiconductor was evident from the electrochemical impedance spectral measurements of electron recombination resistance calculations. Porphyrins18 and phthalocyanines,19 the synthetic tetrapyrrole macrocycle analogs of natural chlorophyll, have extensively been used as photosensitizer electron-donor molecules in construction of donor−acceptor dyads and polyads (triads, tetrads, etc.) due to their intense absorption and emission and redox properties coupled with well-established synthetic protocols.1−12 In most of the successful models, threedimensional electron acceptors, fullerenes in general and C60 in particular, have proven to be superior electron acceptors due to their favorable reduction potentials and low reorganization energies in electron-transfer reactions.20 Accordingly, in fullerene-containing donor−acceptor dyads, forward electron transfer occurs in the top portion of the Marcus parabola while charge recombination occurs in the inverted region of the parabola, ultimately producing long-lived charge-separated states.4−10,20b Subsequently, several light energy-harvesting devices have been constructed using tetrapyrrole−fullerene donor−acceptor dyads, and high light-to-energy conversion

1. INTRODUCTION The photosensitizer entity of the electron donor−acceptor dyads when peripherally substituted with secondary electro- or photoactive entities serves as a functional molecule to mimic the intricate events of the photosynthetic antenna and reaction center.1−11 That is, when the sensitizer is functionalized with suitable redox-active molecules they engage themselves in the process of electron or hole migration depending on the position of redox states during photoinduced electron transfer leading to generation of long-lived charge-separated states,4,7−10 the key reaction center functionality of photosynthesis.12 Interestingly, when functionalized with suitable fluorophore molecules, they engage in the process of excitation transfer to the photoactive primary electron donor,7,11,12 thus carrying out the antenna functionality of photosynthesis.14 In some cases, the peripheral substituents also aid in forming redox-active conducting films without perturbing the electronic structure of the sensitizer segment, a key requirement of surface modification of donor−acceptor molecules.15 In recent years, such functional molecules have found wide applications ranging from fabricating energy-harvesting devices,16 optoelectronic devices, logic gates, and switches, sensors, etc.17 © 2012 American Chemical Society

Received: April 4, 2012 Revised: May 15, 2012 Published: May 17, 2012 11964

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efficiencies have been reported.16 Interestingly, although porphyrins have been widely utilized in construction of relatively sophisticated antenna−reaction center mimics, such studies incorporating phthalocyanines have been limited, primarily due to the associated synthetic challenges.1−12 Carbazole is a tricyclic molecule with two benzene rings fused on either side of a five-membered nitrogen-containing ring. Among vivid applications including pharmaceuticals, agrochemicals, dyes, pigments, etc., carbazoles have been successfully used as a light-emitting photosensitizer, charge/ hole transport material in OLED applications, and photovoltaic and display devices.21 However, application of carbazole as an energy relay (antenna) molecule in sophisticated donor− acceptor polyad assemblies has not been reported. This has been investigated in the present study by covalently linking four entities of carbazoles on the macrocycle periphery of zinc phthalocyanine,22 which was subsequently used to build a donor−acceptor system by axial coordination of phenylimidazole-functionalized fulleropyrrolidine,7 as shown in Scheme 1. Singlet excitation transfer from carbazole to zinc

Scheme 2. Synthetic Route Adapted for Preparation of Tetracarbazole-Appended Zinc Phthalocyanine, 1

Scheme 1. Structure of the Newly Developed Antenna− Reaction Center Mimicking a Donor−Acceptor Polyad Comprised of Four Entities of Carbazole and One Entity Each of Zinc Phthalocyanine- and PhenylimidazoleFunctionalized Fullerene (C60Im)a

trile obtained from the previous step was reacted with dimethylaminoethanol and ZnCl2 in o-dichlorobenzene (DCB) to obtain the tetracarbazole-substituted zinc phthalocyanine, 1. The two tert-butyl groups on the carbazole macrocycle ensured complete solubility of the compound in the investigated solvents, o-dichlorobenzene (DCB) and chloroform. The newly synthesized compounds were fully characterized by MALDI-mass, NMR, spectral, and electrochemical methods. Steady-State Absorption and Fluorescence Studies. Figure 1 shows the absorption spectrum of 1 along with the control compounds in chloroform. The Soret and visible bands of 1 were located at 349, 624, and 691 nm. The visible bands were found to be red shifted by about 12 nm compared to tertbutyl zinc phthalocyanine lacking the carbazole entities.

a

The envisioned route of photochemical events is shown by the red arrows.

phthalocyanine and photoinduced electron transfer from singlet excited zinc phthalocyanine (produced either by direct excitation or as a result of energy transfer from carbazole) to fullerene resulting into relatively long-lived charge-separated states have been observed. Further, photoelectrochemical devices have been newly constructed to demonstrate the ability of the present donor−acceptor polyad to harvest light energy into electricity in the wide panchromatic region.

2. RESULTS AND DISCUSSION Synthesis of Tetracarbazole-Appended Zinc Phthalocyanine, 1. The methodology developed for synthesis of 1 is shown in Scheme 2, while details are given in the Experimental Section. This involved, first, reacting carbazole with iodo phthalonitrile in the presence of copper oxide and N,Ndimethylacetamide. Next, the carbazole derivative of phthaloni-

Figure 1. Normalized to the Q-band absorption spectrum of (i) 1, (ii) tert-butyl zinc phthalocyanine, (iii) mixture of 4 equiv of carbazole and 1 equiv of zinc phthalocyanine, and (iv) 4 equiv of carbazole in chloroform. 11965

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1, as shown in Scheme 1. Figure 3 shows the spectral changes observed during binding of C60Im to 1 in DCB. Binding of

The absorption maxima of the carbazole entities were located at 295 nm as a main band and shoulder band to the Soret band at 335 nm. Interestingly, a new broad band was observed at 478 nm for 1. A control experiment performed by recording the spectrum of a mixture zinc phthalocyanine and 4 equiv of carbazole revealed the absence of such a band, suggesting the origin of this band in 1 is due to intramolecular interactions of the carbazole entities with the π system of the phthalocyanine macrocycle. Compound 1 when excited at either the Soret or the visible band positions of zinc phthalocyanine revealed a main singlet emission band at 707 nm and a weak band at 770 nm. These emission bands were found to be red shifted by about 17 nm when compared to zinc phthalocyanine devoid of the carbazole entities. Direct excitation at 295 nm of carbazole revealed an emission at 361 nm. When the carbazole entity of 1 was excited, this emission band was found to be absent; however, the emission of phthalocyanine at 707 nm was apparent, as shown in Figure 2. In a control experiment, an equimolar

Figure 3. UV−vis spectral changes observed during increasing addition of C60Im (2 μL of 6.85 × 10−5 M each addition) to a solution of 1 (3.4 μM) in DCB. (Inset) Benesi−Hildebrand plot constructed to obtain the binding constant. Absorption changes of the 696 nm band of 1 was utilized; I and Io corresponds to the absorbance of 1 in the presence and absence of C60Im.

C60Im resulted in blue-shifted absorption bands accompanied by isosbestic points at 501, 630, and 700 nm, indicating the presence of an equilibrium binding process. The molecular stoichiometry calculated by the mole ratio method was found to be 1:1, indicating formation of a 1:ImC60 supramolecular complex. The binding constant was obtained by constructing a Benesi−Hildebrand plot23 as shown in Figure 3 inset and found to be K = 7.7 × 105 M−1. This binding constant is an order of magnitude higher compared to similar C60Im binding to other metallotetrapyrroles7,24 due to the presence of four carbazole entities on the phthalocyanine macrocycle periphery. Further, 1H NMR spectral studies performed in CDCl3 also revealed binding of C60Im to the zinc center of 1. As shown in Figure S1, Supporting Information, increasing addition of 1 to a solution of C60Im revealed a systematic upfield shift of the pyrrolidine and phenyl ring protons of C60Im. The imidazole ring protons of C60Im in 1:C60Im revealed a large upfield shift due to the shielding effect caused by zinc phthalocyanine ring currents and appeared around 3.2 ppm, confirming axial coordination.24 To gain insight into the molecular geometry and electronic structure, computational studies were performed using the DFT B3LYP/3-21G(*)25 method. This moderate basis set is known to predict the geometry and electronic structures of large supramolecular systems accurately to some extent.26 In the present study, the self-assembled assembly was optimized to a stationary point on the Born−Oppenheimer potential energy surface. In the optimized structure of 1:ImC60 (Figure 4a), the center-to-center distance (dCC) between the Zn center of 1 and center of C60 was 13.2 Å while the edge-to-edge distance (dee) was 9.6 Å. Importantly, although sufficiently flexible, no intermolecular interactions between fullerene and the peripheral carbazole entities were observed. The molecular electrostatic potential map as shown in Figure 4b was able to track the electron-rich and -deficient sites of the supramolecule, that is, electron-rich portions at the carbazole sites (reddish) and the electron-deficient portion along C60Im (bluish) were observed. The frontier highest occupied molecular orbital (HOMO) was delocalized over the zinc phthalocyanine macrocycle with some

Figure 2. Fluorescence spectrum of (i) 1 (10 μM), (ii) tert-butyl zinc phthalocyanine (10 μM), (iii) mixture of 4 equiv of carbazole (40 μM) and 1 equiv of zinc phthalocyanine (10 μM), and (iv) 4 equiv of carbazole (40 μM) in chloroform. λex = 295 nm.

amount of zinc phthalocyanine was also excited at 295 nm; under such condition, the emission of zinc phthalocyanine at 692 nm was found to be about 1/3 of that of 1. In another control experiment, a mixture of zinc phthalocyanine and 4 equiv of carbazole was excited at 295 nm, and under these conditions, emission of carbazole at 361 nm and zinc phthalocyanine at 692 nm were observed, however, with reduced intensity. The excitation spectrum of 1 was also recorded by holding the emission monochromator to 715 nm and scanning the excitation wavelength. Such spectrum revealed peaks corresponding to both carbazole and zinc phthalocyanine entities including the new band at 478 nm (see Figure S2, Supporting Information). These results indicate the occurrence of efficient excited-state energy transfer from the singlet excited carbazole to the zinc phthalocyanine entity in 1. Supramolecular Donor−Acceptor Formation via Axial Coordination of Imidazole-Functionalized Fullerene. In recent years, construction of donor−acceptor dyads and polyads by a metal−ligand axial coordination approach to probe photoinduced energy and electron transfer has been one of the most successful methods, especially with fullerenes, due to its relative simplicity.8,9 In the present study, we extended this approach by axially coordinating a phenylimidazolefunctionalized fulleropyrrolidine, C60Im, to the zinc center of 11966

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entities. The first two reversible reductions of 1 were located at E1/2 = −1.54 and −1.95 V vs Fc/Fc+. Upon forming the supramolecular complex, 1:ImC60, the first oxidation of the zinc phthalocyanine entity of 1 was found to be cathodically shifted by 30 mV and appeared at EOX = 0.13 V vs Fc/Fc+ while the first reduction of C60Im was located at ERED = −1.16 V vs Fc/ Fc+, not significantly different from its reduction potential prior to forming the coordination complex. The small cathodic shift of the oxidation potential upon axial coordination is typical for zinc tetrapyrroles.7b The second and third reductions of fullerene were found to be overlapped with the first and second reductions of 1 in the supramolecular complex. The energies of the radical ion pair (RIPs), ZnPc•+− ImC60•−, in which one electron transfer from the central zinc phthalocyanine to the C60 entity was calculated as ERIP =1.29 eV using the first EOX of 1 and first ERED of fulleropyrrolidine according to Weller’s approach.27 In these calculations, the electrostatic term was neglected as a first approximation. Here, the free-energy changes for the charge-recombination (CR) process, ΔGCR, is equal to −ERIP. The free-energy change for charge separation (CS), the −ΔGCS value, was evaluated to be −0.46 eV as the difference between the singlet excitation energy of 1 being 1.75 eV (see the next section for the emission properties) and ERIP. In agreement with earlier reports, the negative ΔGCS values indicate the CS process to be exothermic, positioned near the top region of the Marcus parabola, while the ΔGCR value suggests the CR process belongs to the inverted region of the Marcus parabola.28 Photochemical Studies. The occurrence of efficient singlet excitation energy transfer from the appended carbazole to the central zinc phthalocyanine has been discussed in the previous section. When excited either at the Soret or the visible bands of 1, zinc phthalocyanine revealed an emission with a main band centered at 716 nm and a shoulder band located at 784 nm. Addition of C60Im quenched the emission intensity drastically, suggesting the occurrence of intrasupramolecular photochemical events (Figure 6). Quenching data was further used to evaluate the binding constant by constructing a Benesi−Hildebrand plot.23 A relatively large value of K = 7.5 × 105 M−1, the magnitude of which was in agreement with the earlier discussed K value from absorption studies, was obtained (Figure 6 inset). The observed quenching could result from the

Figure 4. B3LYP/3-21G(*)-optimized structure (a), electrostatic potential map (b), HOMO (c), and LUMO (d) of the optimized structure of 1:ImC60.

electron densities over the carbazole entities also (Figure 4c), while the lowest unoccupied molecular orbital (LUMO) was located fully on the fullerene spheroid (Figure 4d). However, the earlier discussed energy transfer from the singlet excited carbazole to zinc phthalocyanine indicates that the ZnPc and carbazole entities behave like independent units. These observations suggest the zinc phthalocyanine entity of 1 is an electron donor and C60Im is the electron acceptor, that is, formation of a ZnPc•+−ImC60•− radical ion pair during electron-transfer reactions could be envisioned. The cyclic voltammograms of 1 and 1:ImC60 are shown in Figure 5. Compound 1 revealed a reversible anodic process at

Figure 5. Cyclic voltammograms of 1 (dark and red lines) and 1:ImC60 (blue line) in DCB containing 0.1 M (n-Bu4N)ClO4. Scan rate = 100 mV/s. Complex concentration calculated to be 0.19 M.

EOX = 0.16 V vs Fc/Fc+ corresponding to oxidation of the zinc phthalocyanine macrocycle. The four carbazole entities on 1 revealed that a quasi-reversible oxidation whole peak position was located at Epa = 1.04 V vs Fc/Fc+. The current for this process was nearly four times as much as that of zinc phthalocyanine oxidation as a consequence of four carbazole

Figure 6. Steady-state fluorescence spectra of 1 (3.4 μM) upon increasing addition of C60Im (2 μL of 6.85 × 10−5 M each addition) in DCB, excited at 495 nm. (Inset) Benesi−Hildebrand plot constructed to obtain the binding constant; I and Io correspond to the emission intensity of 1 in the presence and absence of C60Im. 11967

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occurrence of either photoinduced energy transfer or photoinduced electron transfer. The lack of fulleropyrrolidine absorption at the emission wavelength of zinc phthalocyanine and higher exothermicity for photoinduced electron transfer suggested that the excited energy-transfer process is not a likely quenching mechanism.29 Further, the fluorescence lifetime of 1 in the presence of C60Im was measured to realize the dynamic and not the static nature of the quenching process. The time profile of pristine 1 revealed a monoexponential decay with a lifetime of 2.48 ns in DCB. Upon complexing C60Im by its increasing addition, the decay became biexponential with a fast decaying component of about 0.3 ns and a slow decaying component with a lifetime nearly equal to that of the pristine 1. The amplitude of the former increased with increased addition of C60Im while the latter decreased. These results suggest the existence of complexed and uncomplexed species in solution during the titration process. By assuming that the photoinduced electron transfer is the quenching mechanism, the kinetics of charge separation was evaluated using eq 1 below7f k CS = 1/τcomplex − 1/τref

(1)

where τcomplex and τref refer to the lifetimes of the complexed species (fast decay component) and pristine 1, respectively, Figure 7. The kCS value thus calculated was found to be 2.8 × 109 s−1, indicating a relatively fast charge-separation process in the assembled donor−acceptor system.

Figure 8. (a) Nanosecond transient absorption spectra observed by the 505 nm laser irradiation (ca. 3 mJ/pulse) of 1 (0.1 mM) in the presence of C60Im (1.0 equiv) in N2-saturated DCB. (b and c) Absorption time profiles of the 890 and 1000 nm bands; red lines are the first-order fitting curves.

rise of these bands was less than 7 ns, that is, within the time scale of the nanosecond laser pulse in accordance with the earlier discussed kCS values. The time constants for decay of the 890 and 1000 ns transient bands were almost the same. The first-order decay constant for both transient bands calculated was found to be 6.25 ± 0.2 × 107 s−1, indicating persistence of the radical ion species to some extent. It may be mentioned here that there was also a minor slow decay component (microsecond time scale); however, independent experiments performed on 1 revealed the presence of the tail of the triplet absorption band of 1 in this wavelength region with almost the same time constant. Photoelectrochemical Studies. The ability of the present donor−acceptor to harvest light energy was investigated by constructing photoelectrochemical cells. For this, the polyad and control compounds were electrophoretically deposited onto a nanocrystalline SnO2-modified fluorine-doped tin oxide (FTO) optically transparent electrode.30 We used nanocrystalline SnO2 instead of TiO2 as it is known for its little or no charge injection capability due to the high-lying conduction band of TiO2.30 However, although photocells built on wellengineered SnO2/dye/electrolyte surfaces are known to give very high IPCE values,31 they usually suffer from poor photovoltage, which is often ascribed to the presence of hydroxyl groups in SnO2.16 Thus, as the proof of concept solar cells described here are aimed at providing insights into solar materials, further surface engineering efforts are needed to improve the overall light energy conversion efficiency. The monochromatic incident photon-to-current conversion efficiency (IPCE), defined as the number of electrons generated by light in the outer circuit divided by the number of incident photons at individual wavelength, was determined according to eq 232a

Figure 7. Time-correlated single-photon counting (TCSPC) fluorescence emission decay of 1 (3.4 μM, blue line) on increasing addition of C60Im (0.4 equiv each addition) in DCB. Samples were excited using a 494 nm Nano-LED, and emission was collected at 715 nm corresponding to emission of the zinc phthalocyanine entity of 1. Time calibration factor was 0.439 ns/channel, and the nanoled sync delay was 60 ns.

In order to further establish the occurrence of photoinduced electron transfer, nanosecond transient absorption measurements were performed. Figure 8a shows transient spectral data in the near-IR region where the radical species of ZnPc•+ and ImC60•− are expected to occur and not the triplet excited states of the fluorophores (580 nm for the excited triplet state of zinc phthalocyanine and 700 nm corresponding to the triplet state of fullerene7f). Two transient absorption bands, one located at 890 nm and the second one centered at 1000 nm, diagnostic of zinc phthalocyanine cation radical and fullerene anion radical were, respectively, observed providing ultimate proof for formation of ZnPc•+:ImC60•− radical ion species.4,7,8 The time profile of these bands, as shown in Figure 8b and 8c, revealed almost the same rise and decay time constants. The 11968

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Figure 9. (a) Incident photon-to-current conversion and (b) photoswitching plots of FTO/SnO2 (dark line), FTO/SnO2/1:ImC60 (green), FTO/ SnO2/C60Im (blue), and FTO/SnO2/1 (red) electrodes in DCB containing I3−/I− (0.25 M BMII, 0.25 M LiI, and 0.05 M I2) redox mediator using an AM 1.5 simulated light source with a 350 nm UV cut off filter. (a) Difference spectrum obtained by subtracting photocurrents of FTO/SnO2/ C60Im from FTO/SnO2/1:ImC60 (magenta) to visualize enhanced photocurrents due to charge separation in the donor−acceptor dyad. (b, bottom) Picture of the electrodes modified by electrophoretic deposition of the investigated compounds.

IPCE(%) = 100 × 1240 × ISC(mA cm−2) /[λ(nm) × Pin(mW cm−2)]

(2)

where ISC is the short-circuit photocurrent generated by the incident monochromatic light and λ is the wavelength of this light with intensity Pin. The photocurrent action spectrum of the FTO/SnO2-modified electrodes in a mediator solution of 0.25 M BMII, 0.25 M LiI, and 0.05 M I2 in acetonitrile, with a Pt foil as the counter electrode, is shown in Figure 9a. The spectra resembled those of the absorption spectra shown in Figure 1 covering both the visible and the near-IR spectral region; however, relatively higher currents in the 400−500 nm region were observed for both 1 and 1:ImC60. This could be attributed to the presence of four carbazole entities of 1 that could also be involved in charge injection. The photoaction spectrum recorded for electrophoretically deposited pristine zinc phthalocyanine on FTO/SnO2 revealed no activity in the spectral region, and the IPCE value was less than 1%, in agreement with the results reported earlier by Kira et al.32b The employed electrophoretic deposition method was found to be especially suited here as good coverage/adhesion of the donor− acceptor material on the electrode surface was observed, as shown by the pictures of the electrodes in Figure 9b. The difference IPCE spectrum obtained by subtracting photocurrents of FTO/SnO2/C60Im from FTO/SnO2/1:ImC60 is shown in Figure 9a (magenta trace). The difference in photocurrent in this plot can be attributed to the effect of charge separation in 1:ImC60. A steady anodic photocurrent was observed when the electrodes were illuminated, and reproducible photocurrents were observed during on−off photoswitching, revealing the robustness of the dye material developed here (Figure 9b). The short-circuit current, ISC, ranged between 0.3 and 1.4 mA/cm2, while the open-circuit potential, VOC, was around 0.2 V. Importantly, the photocurrent obtained was three times as much as that obtained for either 1 or C60Im-modified electrodes. In order to further understand the FTO/SnO2/1:ImC60 interface, electrochemical impedance studies (EIS) were performed since this technique has been a useful tool to estimate electron recombination resistance and to understand the dye regeneration efficiency.33 Figure 10 shows EIS results; both FTO/SnO2/1:ImC60 and FTO/SnO2/1 electrodes

Figure 10. Impedance spectra (Nyquist plot) measured at the respective open-circuit potential (VOC) of FTO/SnO2/1:ImC60 (red and blue) and FTO/SnO2/1 (green and black) in the dark (red and green) and under AM1.5 light conditions (blue and black), respectively. (Inset) Equivalent circuit diagram used to fit the experimental data.

revealed high resistance under dark conditions compared to the values under illumination. The recombination resistance under dark conditions for FTO/SnO2/1:ImC60 and FTO/ SnO2/1 were 103 and 66 Ω cm2, respectively, while these values under AM1.5 light conditions at Voc were 86 and 62 Ω cm2, respectively. The increase in recombination resistance due to dyad formation under dark and decrease in recombination resistance under light can be attributed to the better performance of FTO/SnO2/1:ImC60, that is, photoregeneration of FTO/SnO2/1:ImC60 is much efficient compared to FTO/SnO2/1, a result that agrees well with the cell efficiency measured by the IPCE curves in Figure 9 and persistence of the charge-separated state by transient spectral studies in solution.

3. CONCLUSIONS The newly synthesized and assembled tetracarbazole−zinc phthalocyanine−fullerene polyad served as a photosynthetic antenna−reaction center mimic. For the first time, utilization of 11969

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1695.61 (see Figure S3, Supporting Information, for the MALDI-mass spectrum). Preparation of Nanocrystalline SnO2 Electrodes. A 10 mL amount of SnO2 colloidal solution (Alfa Aesar, 15%) was dissolved in 10 mL of ethanol; 0.5 mL of NH4OH was added to this solution for stability. About 2.5 mL of colloidal solution was placed on an optically transparent electrode, fluorine-doped indium tin oxide (FTO) (Pilkington TEC-8, 6−9 Ω/square), and dried in air. The electrodes were annealed in an oven for 1 h in air at 673 K. The estimated thickness of the electrode was around 5 μm.31a,34 Spectral Measurements. UV−vis spectral measurements were carried out either on a Shimadzu model 2550 double monochromator UV−vis spectrophotometer or on a Jasco V670 spectrophotometer. Steady-state fluorescence spectra were measured using a Horiba Jobin Yvon Nanolog UV−visible− NIR spectrofluorometer equipped with a PMT (for UV−vis) and InGaAs (for NIR) detectors. Lifetimes were measured with the Time Correlated Single Photon Counting (TCSPC) lifetime option with nano-LED excitation sources on the Nanolog. The time calibration factor for the system was 0.439 ns/channel. All solutions were purged using nitrogen gas prior to spectral measurements. A right angle detection method was used for emission measurements. 1H NMR studies were carried out on a Bruker 400 MHz spectrometer. Tetramethylsilane (TMS) was used as an internal standard. Cyclic voltammograms were recorded on an EG&G 263A potentiostat/ galvanostat using a three-electrode system. A platinum button electrode was used as the working electrode. A platinum wire served as the counter electrode, and an Ag/AgCl electrode was used as the reference electrode. The ferrocene/ferrocenium redox couple was used as an internal standard. All solutions were purged prior to electrochemical and spectral measurements using nitrogen gas. Computational calculations were performed by DFT B3LYP/3-21G* methods with the GAUSSIAN 03 software package25 on high-speed PCs. HOMO and LUMO orbitals were generated using the GuessView program. Photoelectrochemical and Electrochemical Impedance Measurements. Photoelectrochemical experiments were performed in a two-electrode configuration using donor−acceptor hybrids drop coated onto the FTO/SnO2 electrode and a platinized FTO electrode as the counter electrode in o-DCB solution containing (0.25 M 1-butyl-3methylimidazolinium iodide (BMII), 0.25 M LiI, and 0.05 M I2) for I−/I3− redox mediator. The photocurrent−photovoltage characteristics of the solar cells were measured using a model 2400 current/voltage source meter of Keithley Instruments, Inc. (Cleveland, OH) under illumination with an AM 1.5 simulated light source using a model 9600 of 150-W Solar Simulator of Newport Corp. (Irvine, CA). Incident photon-tocurrent efficiency (IPCE) measurements were performed under ∼4 mW cm2 monochromatic light illumination conditions using a setup comprised of a 150 W Xe lamp with a Cornerstone 260 monochromator (Newport Corp., Irvine, CA). Electrochemical impedance measurements were performed using a EG&G PARSTAT 2273 potentiostat. Impedance data were recorded under forward bias condition from 100 kHz to 100 mHz with an ac amplitude of 10 mV. Data were recorded in the dark and A.M 1.5 illumination conditions applying the corresponding open-circuit potential (Voc) for each electrode. Data were analyzed using ZSimpwin software from Princeton

carbazole as an antenna relay in photosynthetic model compounds has been established in the present study. The occurrence of photoinduced energy transfer from singlet excited carbazole to zinc phthalocyanine and photoinduced electron transfer from singlet excited zinc phthalocyanine to fullerene is established from time-resolved emission and transient absorption techniques. The measured kinetics of charge separation and charge recombination from these techniques revealed that the charge-separation state persists to some extent in this polyad, thus giving an opportunity to build light energy-harvesting solar cells. Subsequently, solar cells constructed on the FTO/SnO2 surface revealed a higher light energy conversion efficiency covering the wide panchromatic region and robust nature of the devices. Electron recombination resistance calculations based on electrochemical impedance spectral measurements revealed better charge injection from the excited polyad to the conduction band of the semiconductor. Further studies toward achieving improved charge stabilization in donor−acceptor polyads and accomplishing better light energy conversion efficiency of the built solar cells are in progress in our laboratory.

4. EXPERIMENTAL SECTION Chemicals. All of the reagents were from Aldrich Chemicals (Milwaukee, WI), while the bulk solvents utilized in the syntheses were from Fischer Chemicals. Tetra-n-butylammonium perchlorate, (n-Bu4N)ClO4, used in electrochemical studies was from Fluka Chemicals. Synthesis of C60Im is given elsewhere.24 Synthesis of 3,6-Di-tert-butylcarbazole−phthalonitrile. 3,6-Di-tert-butylcarbazole (300 mg, 1.07 mmol), 4iodophthalonitrile (305 mg, 1.20 mmol), Cu2O (320 mg, 2.23 mmol), and N,N-dimethylacetamide (10 mL, as a solvent) were added sequentially to a 100 mL round-bottomed flask under N2 and then heated at 170 °C on an oil bath for 20 h. Then the mixture was cooled at room temperature and filtered. The filtrate was treated with water and dichloromethane. The organic layer was separated and dried over sodium sulfate. The obtained organic layer was evaporated and purified using column chromatography on silica gel using hexane:chloroform (60:40 v/v) as eluent. Yield: 250 mg (57%). 1H NMR (CDCl3, 400 MHz), 8.14 ppm (dd, 2H, Ar−H), 8.09 (m, 1H, Ar−H), 8.01 (dd, 2H, Ar−H), 7.52 (dd, 2H, Ar−H), 7.42 (dd, 2H, Ar− H), 1.42 (s, 18H, −CH3). MS: found, 408.75 [M + H]; calculated, 407.55. Synthesis of 3,6-Di-tert-butyltetracarbazole− phthalocyaninatozinc(II). 3,6-Di-tert-butylcarbazolephthalonitrile (250 mg, 0.614 mmol) and ZnCl2 (85 mg, 0.615 mmol) were kept in a 100 mL round-bottomed flask under nitrogen for 20 min. Then dimethylaminoethanol (DMAE) and DCB (1:1, 2 mL each) were added into the round-bottomed flask and heated at 150 °C for 18 h. A dark-green-colored phthalocyanine was obtained. Then the mixture was cooled at room temperature and centrifuged upon addition of MeOH:H2O (80:20 v/v) for about 1 h. After removing the supernatant, dark green residue was dissolved in a minimum amount of chloroform and purified over silica gel column using hexane:CHCl3 (30:70 v/v) as eluent. Yield: 90 mg (34%). 1H NMR (CDCl3, 400 MHz), δ ppm: 1.40 (m, 72H, carbazole 24CH3), 7.15 (m, 8H, carbazole Ar−H), 7.40 (m, 16H, carbazole Ar−H), 7.90 (m, 4H, PcAr−H), 8.00 (m, 4H, PcAr−H), 8.10 (m, 4H, PcAr−H). MS: found, 1696.78 [M + H]; calculated, 11970

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Balzani, V., Ed.; Wiley-VCH: New York, 2001; Vol. 2, p 270. (e) Sgobba, V.; Rahman, G. M. A.; Ehli, C.; Guldi, D. M. In FullerenesPrinciples and Applications; Langa, F., Nierengarten, N. J., Eds.; Royal Society of Chemistry: Cambridge, U.K., 2007. (5) (a) Seth, J.; Palaniappna, V.; Wagner, R. W.; Johnson, T. E.; Lindsey, J. S.; Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 11194. (b) Hsiao, J.-S.; Krueger, B. J.; Wagner, R. W.; Johnson, T. E.; Delaney, J. K.; Mauzerall, D. C.; Fleming, G. R.; Lindsey, J. S.; Bocian, D. F.; Donohoe, R. J. J. Am. Chem. Soc. 1996, 118, 11181. (6) (a) Sessler, J. S.; Wang, B.; Springs, S. L.; Brown, C. T. In Comprehensive Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vögtle, F., Eds.; Pergamon: New York, 1996; Chapter 9. (b) Sessler, J. L.; Lawrence, C. M.; Jayawickramarajah, J. Chem. Soc. Rev. 2007, 36, 314. (7) (a) D’Souza, F.; Ito, O. Chem. Soc. Rev. 2012, 41, 86. (b) D'Souza, F.; Ito, O. In Handbook of Porphyrin Science; Kadish, K. M., Guilard, R., Smith, K. M., Eds.; World Science Publishers: Singapore, 2010; Vol. 1, Chapter 4, pp 307−437. (c) D’Souza, F.; Ito, O. Chem. Commun. 2009, 45, 4913. (d) D’Souza, F.; Sandanayaka, A. S. D.; Ito, O. J. Phys. Chem. Lett. 2010, 1, 2586. (e) D’Souza, F.; Ito, O. Coord. Chem. Rev. 2005, 249, 1410. (f) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem. Photobiol. C 2004, 5, 79. (g) D’Souza, F.; Ito, O In Multiporphyrin Array: Fundamentals and Applications; Kim, D., Ed.; Pan Stanford Publishing: Singapore, 2012; Chapter 8, pp 389−437. (8) (a) Fukuzumi, S. Org. Biomol. Chem. 2003, 1, 609. (b) Fukuzumi, S. Phys. Chem. Chem. Phys. 2008, 10, 2283. (c) Fukuzumi, S.; Kojima, T. J. Mater. Chem. 2008, 18, 1427. (d) Fukuzumi, S.; Honda, T.; Ohkubo, K.; Kojima, T. Dalton Trans. 2009, 3880. (e) Satake, A.; Kobuke, Y. Org. Biomol. Chem. 2007, 5, 1679. (9) (a) Delgado, J. L.; Herranz, M. Á .; Martín, N. J. Mater. Chem. 2008, 18, 1417−1426. (b) Sanchez, L.; Martín, N.; Guldi, D. M. Angew. Chem., Int. Ed. 2005, 44, 5374. (c) Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M. Chem. Rev. 2006, 106, 1105. (10) (a) Sakata, Y.; Imahori, H.; Tsue, H.; Higashida, S.; Akiyama, T.; Yoshizawa, E.; Aoki, M.; Yamada, K.; Hagiwara, K.; Taniguchi, S.; Okada, T. Pure Appl. Chem. 1997, 69, 1951. (b) Imahori, H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445. (11) (a) Schuster, D. I.; Li, K.; Guldi, D. M. C. R. Chimie 2006, 9, 892. (b) Ziessel, R.; Harriman, A. Chem. Commun. 2011, 47, 611. (c) Benniston, A. C.; Harriman, A. Coord. Chem. Rev. 2008, 252, 2528. (12) (a) Maligaspe, E.; Kumpulainen, T.; Subbaiyan, N. K.; Zandler, M. E.; Lemmetyinen, H.; Tkachenko, N. V.; D’Souza, F. Phys. Chem. Chem. Phys. 2010, 12, 7434. (b) D’Souza, F.; Wijesinghe, C. A.; ElKhouly, M. E.; Hudson, J.; Niemi, M.; Lemmetyinen, H.; Tkachenko, N. V.; Zandler, M. E.; Fukuzumi, S. Chem. Phys. Phys. Chem. 2011, 13, 18168. (c) Terazono, Y.; Kodis, G.; Liddell, P. A.; Garg, V.; Moore, T. A.; Moore, A. L.; Gust, D. J. Phys. Chem. B 2009, 113, 7147. (d) D’Souza, F.; Amin, A. N.; El-Khouly, M. E.; Subbaiyan, N. K.; Zandler, M. E.; Fukuzumi, F. J. Am. Chem. Soc. 2012, 134, 654. (13) (a) In The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R., Eds.; Academic Press: San Diego, 1993. (b) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol. Biol. 1984, 180, 385. (14) (a) In Phtosynthetic Light Harvesting; Cogdell, R., Mullineaux, C., Eds.; Sprigner: Dordrecht, The Netherlands, 2008. (b) In Handbook of Photosynthesis, 2nd ed.; Pessarakli, M., Ed.; CRC Press LLC: Boca Raton, FL, 2005. (c) In Light-Harvsting Antennas in Photosynthesis; Green, B. R., Parson, W. W., Eds.; Kluwer: Dordrecht, The Netherlands, 2003. (15) (a) In N4 Macrocyclic Metal Complexes; Zagal, J. H., Bedioui, F., Dodelet, J. P., Eds.; Springer: New York, 2006. (b) Subbaiyyan, N. K.; Obraztsov, I.; Wijesinghe, C. A.; Tran, K.; Kutner, W.; D’Souza, F. J. Phys. Chem. C 2009, 113, 8982. (16) (a) Kamat, P. V. J. Phys. Chem. C 2007, 111, 2834. (b) Umeyama, T.; Imahori, H. Energy Environ. Sci. 2008, 1, 120. (c) Hasobe, T. Phys. Chem. Chem. Phys. 2010, 12, 44. (d) Subbaiyan, N. K.; Wijesinghe, C. A.; D’Souza, F. J. Am. Chem. Soc. 2009, 131, 14646. (e) Imahori, H.; Umeyama, T.; Kei, K.; Yuta, T. Chem. Commun. 2012, 48, 4032. (f) Kamat, P. V.; Schatz, G. C. J. Phys. Chem.

Applied Research. The solution resistance (Rs), charge-transfer resistance (Rct), and capacitance due to the constant-phase element (Q) was deduced from the fitted data. CPE was considered as the capacitance component of the double-layer electrode interface due to roughness of the electrode. Laser Flash Photolysis. The studied compounds were excited by a Opolette HE 355 LD pumped by a high-energy Nd:YAG laser with second- and third-harmonics OPO (tuning range 410−2200 nm, pulse repetition rate 20 Hz, pulse length 7 ns) with powers of 1.0−8.5 mJ per pulse. Transient absorption measurements were performed using a Proteus UV−vis−NIR flash photolysis spectrometer (Ultrafast Systems, Sarasota, FL) with a fiber optic delivered white probe light and either a fast rise Si photodiode detector covering the 200−1000 nm range or a InGaAs photodiode detector covering the 900−1600 nm range. The output from the photodiodes and a photomultiplier tube was recorded with a digitizing Tektronix oscilloscope. Data analysis was performed using Surface Xplorer software supplied by Ultrafast Systems.

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ASSOCIATED CONTENT

S Supporting Information *

1 H NMR titration curves during 1:C60Im formation, excitation spectrum of 1, and MALDI-mass spectrum of the newly synthesized compound, 1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Prof. O. Ito and Drs. M. E. ElKhouly, A. S. D. Sandanayaka, and K. Yamanaka for helpful discussions on the nanosecond transient absorption spectrometer setup. This work was supported by the National Science Foundation (Grant No. 1110942 to F.D.).

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REFERENCES

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