Synthesis, Photophysics, and Photovoltaic Studies of Ruthenium

Article pubs.acs.org/JPCC

Synthesis, Photophysics, and Photovoltaic Studies of Ruthenium Cyclometalated Complexes as Sensitizers for p‑Type NiO DyeSensitized Solar Cells Zhiqiang Ji,† Gayatri Natu,† Zhongjie Huang,† Oleksandr Kokhan,‡ Xiaoyi Zhang,*,§ and Yiying Wu*,† †

Department of Chemistry & Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States ‡ Chemical Sciences and Engineering Division and §X-ray Sciences Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: We report the first application of cyclometalated ruthenium complexes of the type Ru[(N∧N)2(C∧N)]+ as sensitizers for p-type NiO dye-sensitized solar cells (NiO p-DSCs). These dyes exhibit broad absorption in the visible region. The carboxylic anchoring group is attached to the phenylpyridine ligand, which results in efficient hole injection. Moreover, the distance between the Ru[(N∧N)2(C∧N)]+ core and the carboxylic anchoring group is systematically varied by inserting rigid phenylene linkers. Femtosecond transient absorption (TA) studies reveal that the interfacial charge recombination rate between reduced sensitizers and holes in the valence band of NiO decreases as the number of phenylene linkers increases across the series. As a result, the solar cell made of the dye with the longest spacer (O12) exhibits the highest efficiency with both increased short-circuit current (Jsc) and open-circuit voltage (Voc). The incident photon-to-current conversion efficiency (IPCE) spectra match well with the absorption spectra of sensitizers, suggesting the observed cathodic current is generated from the dye sensitization. In addition, the absorbed photon-to-current conversion efficiencies (APCEs) display an increment across the series. We further studied the interfacial charge recombination of our solar cells by electrochemical impedance spectroscopy (EIS). The results reveal an enhanced hole lifetime as the number of phenylene linkers increases. This study opens up opportunities of using cyclometalated Ru complexes for p-DSCs.

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than that in a conventional n-DSC.5−7 Recently Hammarström and co-workers8 found that this charge recombination lies in the Marcus normal region with an extremely large reorganization energy, and suggested that the recombination may occur through intrabandgap states. The results imply that the sensitizer−NiO interface behaves differently from the TiO2 interface; however, the exact reason remains unclear. Accordingly, to design efficient sensitizers, one has to consider strategies to suppress the undesired detrimental interfacial charge recombination. Stimulated by the initial work of Sun and co-workers,9−12 some push−pull organic sensitizers have been synthesized, usually using triphenylamino and dicyanovinyl as the electron donor and acceptor, respectively. High incident photon-tocurrent conversion efficiencies (IPCEs) were achieved due to the properly polarized charge separation by attaching a carboxylate anchoring group on an electron donor.9 Other

INTRODUCTION NiO p-type dye-sensitized solar cells (NiO p-DSCs) are attracting increasing research attention as they can be used as the photocathode in tandem dye-sensitized solar cells (tDSCs)1 and artificial photosynthesis devices.2 In a t-DSC, both anode and cathode are sensitized and photoactive. Ideally, the anodic sensitizer and the cathodic sensitizer have complementary light absorption. Therefore, a t-DSC can absorb a broader range of the solar spectrum and achieve a higher photovoltage than the conventional TiO2- or ZnO-based n-type DSCs (n-DSCs).3 However the performance of the current tDSCs is still hindered by the relatively low photocurrent generated at the p-DSC side.4 Sensitizers play a crucial role in boosting the photocurrent. In addition to the general requirements for the light absorption and the proper energy alignment, the molecular design of the sensitizers should also facilitate the directional electron transfer from NiO to the photoexcited sensitizers, and suppress the recombination between the holes in NiO and the reduced sensitizers. Previous works indicate that the charge recombination occurs at the picosecond time scale, which is much faster © 2012 American Chemical Society

Received: April 23, 2012 Revised: July 13, 2012 Published: July 26, 2012 16854

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Scheme 1. Structures of Sensitizers with the Distance between Ru Metal Center and the Carbon Atom of Carboxylate Group (Blue Arrow) Estimated by Gaussian 03

away from the NiO surface for suppressing the interfacial charge recombination. Their photophysical and electrochemical properties, and the solar cell devices incorporating these sensitizers were studied. The interfacial electron transfer and recombination was studied by transient absorption (TA) spectroscopy, and the recombination rates of the photovoltaic devices were studied by electrochemical impedance spectroscopy (EIS). Our results show that, as the length of the phenylene linker increases, the interfacial charge recombination rate decreases, and the efficiency of our solar cells increases.

sensitizers include a perylenemonoimide−naphthalenediimide (PMI−NDI) dyad13,14 and push−pull dyes with π-conjugated thienyl units linked to the NDI acceptor.15 All these sensitizers exhibit a long-lived charge separated state, and possess structure features that can effectively suppress recombination between holes in NiO and reduced sensitizer post excitation. On the other hand, Ru(II) complexes have been rarely explored for p-DSCs despite their popular uses in conventional n-DSCs.16−18 In general, Ru(II)-based organometallic sensitizers have advantages in their stability, structural flexibility, and synthetic accessibility over organic dyes.3,19 The intensive metal-to-ligand charge transfer absorption (MLCT) and long lifetime of the 3MLCT excited state partially fulfill the design requirement of sensitizers for p-DSCs. In addition, their structural flexibility results in tunable photophysical and electrochemical properties. Especially, the optical absorption of ruthenium complexes can be extended down to ca. 800 nm, which cannot be easily obtained by most of the organic dyes.20,21 There has been a recent attempt of using ruthenium polypyridine complexes as the sensitizers for NiO p-DSCs.22 However, the anchoring groups were attached to the bipyridine ligand, where the lowest unoccupied molecular orbital (LUMO) of the molecules is located. Such an arrangement is not optimized for the vectorial hole transfer from the sensitizer to the NiO support and resulted in the low photocurrent of the solar cells. In the present work, we report the first use of cyclometalated Ru(II) complexes as the sensitizers for NiO p-DSCs. The complexes are represented as Ru[(N∧N)2(C∧N)]+, where N∧N represents 2,2′-bipyridine, and C∧N represents bidentate phenylpyridine derivatives. As noted earlier by Berlinguette and other groups, the highest occupied molecular orbital (HOMO) in these cyclometalated Ru(II) complexes is extended over the metal center, and the anionic phenyl ring of the phenylpyridine ligand.17,18,23−27 Therefore, in our design of the sensitizers, the carboxylic anchoring group is attached to the phenyl ring (Scheme 1) to position the HOMO region close to the NiO surface and keep the LUMO region away. This design should facilitate the hole injection from the photoexcited dyes to NiO and slow down the charge recombination as discussed above. This is opposite to the design of the cyclometalated Ru(II) sensitizers for conventional n-DSCs.16,28 We have designed and synthesized three sensitizers: O8, O11, and O12 (Scheme 1). They have different numbers of phenylene spacer units between the Ru[(N∧N)2(C∧N)]+ core and the carboxylic anchoring group. The aim of the additional phenylene spacer units is to keep the photoreduced dye far

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RESULTS AND DISCUSSION Synthesis. A facile one-step cross-coupling method involving ruthenium cyclometalated Suzuki coupling reagent [Ru(bpy)2(bpp)]+ (where bpy is 2,2′-bipyridine, and bpp is 2(3-4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenylpyridine) was developed (Scheme 2). [Ru(bpy)2(bpp)]+ was synthesized Scheme 2. Synthetic Procedures for Sensitizers O11 and O12

by complexation of bpp ligand to commercial available Ru(bpy)2Cl2 with the aid of silver trifluoromethanesulfonate (AgOTf), and then reacted with brominated precursors in the presence of Pd(dppf)Cl2 catalyst and K3PO4 in the mixture of dimethylformamide (DMF) and water. The crude products were acidified and purified by chromatography. Sample purity was confirmed by 1H NMR, electrospray ionization mass 16855

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spectrometry (ESI-MS), and elemental analysis. O8 was prepared by following the literature procedure.18 Electronic Absorption. Figure 1 shows the absorption and emission spectra of three sensitizers. They display broad and

Figure 2. Cyclic voltammograms of O8, O11, and O12 in CH3CN/ (nBu)4NClO4 (TBAP); the potentials are reported versus NHE.

Figure 1. Absorption spectra of sensitizers O8 (black line), O11 (red line), and O12 (green line) in DMF, and emission spectra of O8 (black dash), O11 (red dash), and O12 (green dash) in a mixture of methanol and ethanol (4/1, v/v) at 77 K.

Table 1. The Electrochemical Data of O8, O11, and O12 in Dry CH3CN

intensive absorption throughout the UV and visible region extending to ca. 650 nm. The intense absorption in the region below 350 nm is attributed to the ligand-centered π−π* transition. The broad and moderately intense absorption in the visible region is ascribed to the MLCT transition, which involves the transfer of electron density from the d-orbitals of Ru together with significant cyclometalated ligand contribution to the bpy ligands as indicated by our density functional theory (DFT) calculations (see below). The absorption is red-shifted with respect to that of [Ru(bpy)3]2+ because the presence of phenylpyridyl ligand increases the electron density of the Ru metal center, and lowers the energy of the MLCT transition. The broad feature of absorption is due to the low molecular symmetry and thus the multiplicity of the excited states. All sensitizers exhibit similar spectra with only notable differences between 300 and 450 nm, where the absorption extinction coefficient (ε) increases as the number of phenylene units increases. For instance, the ε value at 370 nm is 29860, 21150, and 15640 L·mol−1·cm−1 for O12, O11, and O8, respectively. The same trend has been observed in other Ru2+ sensitizers.29 The emission spectra of all complexes at 77 K in the ethanolic solution are also shown in Figure 1(dotted lines). The E00 for O12, O11, and O8 is 1.78, 1.79, and 1.83 eV, estimated by the energy at the cross-section of the normalized absorption and emission spectra. Electrochemical Studies. The electrochemical properties of our sensitizers were examined by cyclic voltammetry in dry CH3CN (Figure 2). The results are summarized in Table 1. All sensitizers exhibit one reversible oxidation potential at 0.83, 0.72, and 0.72 V for O8, O11, and O12, respectively (all potentials are reported versus normal hydrogen electrode (NHE)), together with two reversible bipyridine-based reduction potentials. The oxidation potential is due to the Ru(III)/(II)-based redox process. Due to the strong σ-donating ability of the phenylpyridyl ligand, the oxidation potential occurs at more negative values compared with their polypyridine analogues,22 which accounts for the bathochromic absorption for cyclometalated sensitizers. The excited-state

O8 O11 O12

E0/− (V)

E+/0 (V)

E00 (ev)

E*/− (V)

−1.31, −1.62 −1.29, −1.57 −1.32, −1.61

0.83 0.72 0.72

1.83 1.79 1.78

0.52 0.50 0.46

reduction potential (E*/−) is 0.52, 0.50, and 0.46 V for O8, O11, and O12 respectively, calculated from the equation: E*/− = E00 + E0/−. The values are close to each other (within ∼60 mV), suggesting that the difference among the sensitizers in their driving force for hole injection is small and thus is unlikely to cause the significant difference in solar cell performance (vide infra). DFT Calculations. DFT and time-dependent DFT (TDDFT) were performed by using the B3LYP/LANL2DZ basis with Gaussian 03 for O8, O11, and O12. Figure 3 shows the isodensity plots of frontier HOMO and LUMO orbitals. The simulated absorption spectra are shown in Figure S1 of the Supporting Information. The closely spaced transitions reflect their asymmetric structures. According to the calculation, the HOMO is primarily localized on the Ru metal center with contributions from the oligophenylene groups, while the LUMO is exclusively located on the bipyridine ligands for all of the sensitizers. Our calculation is in accordance with previous studies on the related cyclometalated ruthenium complexes, which also indicate the delocalization of HOMO onto the carbonionic group.16,18 When the dye molecules are attached to the NiO surface with the carboxylic anchoring group, photoexcitation of the dye molecule will shift the electron density away from the surface. This arrangement should facilitate the hole injection from the HOMO into NiO. In addition, as the number of the phenylene unit increases, the electron tunnelling distance between the electrons in LUMO post photoexcitation and the holes in NiO is increased, which is expected to exert a profound effect on suppressing the charge recombination between electron−hole pairs. TA Spectroscopy. Ultrafast TA spectroscopy was used to study the dynamics of hole injection and charge recombination 16856

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Figure 3. The isodensity plots of HOMOs and LUMOs of O8, O11s and O12 calculated by TDDFT.

Figure 4. TA spectra of O8/NiO film (left), O11/NiO film (middle), and O12/NiO film (right) at different delay times pumped at 400 nm. The insets present the TA spectra of dyes in CH3CN solution.

deprotonation of the carboxylate group, which is strongly involved in the HOMO orbital according to our DFT calculation.31 No obvious shift of absorption spectra is observed for either O11 and O12 when adsorbed on NiO films. Therefore, we expect the electronic structures of all the sensitizers to remain the same both in solution and on films. On the other hand, the absorption around 650 nm is very similar to the π − π* absorption of free 4,4′-bipyridine radical anion32 and the absorption features obtained for ruthenium complexes bearing bipyridine derivatives under electrochemical reduction.33 To confirm the above assignment, we recorded the absorption of the reduced sensitizers in DMF solution by applying a bias at ca. 100 mV more negative than the first reduction peak. The absorption changes are shown in Figure S10. The bleach is in line with the ground-state absorption, and the positive signals correspond to the absorption of reduced sensitizer, which is bpy radical anion. The bpy radical anions in all sensitizers exhibit broad absorption in the spectra region beyond 600 nm with a distinct peak around 630 nm for O8 and O12, which is very similar to the spectra observed in femtosecond TA measurements (vide infra). Accordingly, we assign the positive TA of the sensitized film to that of bipyridine radical anion (bpy•−), which is the product of holeinjection from an excited dye to NiO.

of O8, O11, and O12 adsorbed to NiO electrodes. For reference, the excited-state dynamics of sensitizers in acetonitrile were also studied (Figure 4 insets). The TA spectra of sensitizers in acetonitrile exhibit a ground-state bleaching between 450 to ca. 600 nm, accompanied by a broad, structureless excited-state absorption extending past 760 nm (insets of Figure 4 and Figure S2). The structureless absorption is tentatively assigned as the absorption of the MLCT excited state based on previous studies of other Ru polypyridyle complexes.30 In addition, this absorption persists within the experimental time window (∼3 ns), which is typical for the long-lived 3MLCT excited state (Figure S2). Figure 4 also displays representative TA spectra of dyesensitized NiO films. All sensitized films exhibit similar spectra features including a ground state bleaching between 450 to 600 nm, and an absorption band peaked around 650 to 670 nm. The absorption band is different from the broad and structureless absorption of dyes in solution, indicating different origins of electronic states. Such differences can be caused by either NiO that modifies the electronic structure of the sensitizer, or by hole-injection that creates different excited species. The UV−vis absorption spectra of sensitizers adsorbed on NiO films and in DMF solution exhibit similar features (Figure S3). Compared to the absorption of dyes in DMF, the lower-energy MLCT transition of O8 adsorbed to the NiO surface is red-shifted by ∼80 nm, possibly due to the 16857

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After the TA peak was assigned, the next thing was to examine the kinetics of the TA decay. As shown in Figure S4, the absorption band peak between 650 nm and 670 nm from the reduced dyes forms immediately after photoexcitation, which indicates that the hole injection from excited-state dye to the valence band of NiO happens within the instrumental response function (180 fs). Other relaxation processes such as vibrational cooling (VC) and intersystem crossing (ISC) from 1 MLCT to 3MLCT also happen on this ultrafast time scale. With a close look at the transient spectra in the sub-picosecond regions, the spectra shape changes in the first several hundred femtoseconds. The spectra evolution at early time within ∼1 ps is shown in Figure S4. As can be seen, after 0.4−0.5 ps, the amplitude of the TA spectra decays, while the shape stays the same, with an isosbestic point at ΔA ∼ 0. These results suggest that hole injection, VC, ISC, and other ultrafast processes compete with each other and complete within a sub-picosecond time window. The fast hole injection kinetics is in agreement with the reported time constant of the sensitizer/NiO interface,6−8 and also consistent with the rate constant of electron injection at the N3/TiO2 interface.34,35 The fast hole injection is remarkable for O11 and O12, considering the long tunneling distance for electron transfer. According to our DFT calculations, HOMOs of sensitizers are delocalized to the phenyl spacers, which might effectively enhance the electronic coupling, and therefore facilitate the hole injection. After 0.5 ps, the amplitude of the TA spectra decays and exhibits an isosbestic point at ΔA ∼ 0, as shown in Figure 4. The appearance of the isosbestic point clearly shows that the decay is caused by the recombination process, namely, the back electron transfer from the reduced sensitizers to the valence band of NiO. An excitation-power-dependent study was performed on the O12/NiO film. The time constant of decay kinetics at 681 nm is independent of the laser power from 0.1 to 0.4 and to 0.9 mW. In addition, the difference absorption amplitude varies linearly with the laser power (Figure S5). These indicate that the charge injection is a first-order process within our experimental conditions. Figure 5 shows representative kinetic traces of the three sensitizers adsorbed to NiO thin films probed at the absorption

Table 2. The Fitted Time Constants, Relative Amplitude (A), and Attenuation Factor β of All Sensitizer/NiO Films τ1/ps, τ2/ps, τ3/ps, τ4/ns,

(A) (A) (A) (A)

O8

O11

O12

β (A−1)

0.14−0.15 (0.69) 8.91 (0.18) 248.4 (0.09) >3 ns (0.04)

0.43 (0.40) 24.12 (0.24) 629.4 (0.21) >3 ns (0.12)

2.63 (0.22) 52.87 (0.36) 1135 (0.28) >3n (0.13)

0.20 0.17

assigned to the charge recombination. The fastest time constant spans from 0.15 to 2.6 ps, and the cause may be a combination effect of charge injection, VC, ISC, and charge recombination. Our observed charge recombination times are similar to those using perylene imide as sensitizers.8 The picosecond charge recombination kinetics is much faster than the microsecond to millisecond recombination rate of ruthenium sensitizer/TiO2 interface.36−38 We speculate that the fast recombination rate at the dye/NiO interface may come from the slow hole mobility in NiO due to the localized Ni 3d band. The injected holes cannot quickly migrate away from sites of the reduced dye molecules. Other factors such as the different driving forces and the lower dielectric constant of NiO may also be responsible for the fast recombination. Our results together with other studies6−8 clearly imply a major limitation of using NiO in p-DSCs. Therefore, designing sensitizers that can slow down charge recombination and finding alternative p-type wide-bandgap semiconductors are crucial issues for making efficient p-DSCs. When we compare the different dyes, it can be clearly seen from Table 2 that the charge recombination rates decrease across the series with increasing the phenylene linkers. In addition, the magnitude of absorption after ∼3 ns follows the same trend as O8 < O11< O12. The correlation of charge recombination rate constants and the distance is further analyzed in term of the attenuation factor β, which is obtained by fitting the recombination rate constant versus the spacer length by the following equation: ket = k 0 exp( −βr )

where ket is the electron transfer rate, k0 is the rate constant, and r is the distance for electron transfer. The spacer length is estimated by the distance from ruthenium metal center to carbon atom of the carboxylate anchoring group. The value of β is listed in Table 2, and the fitting curves are shown in Figure 6. The observed β values of 0.20 and 0.17 are comparable with the values reported for many donor-bridge-acceptor systems in solution,39,40 revealing a strong dependence of charge recombination on the spacer distance. The result implies that our sensitizers are strongly attached on the NiO surface, and most of electron transfer is through bond. It is interesting to note that our results are different from those of previous studies on TiO2.31 They have not been able to clearly correlate spacer length and charge transfer rate, and have concluded that the dyes lie down on the surface. However, we have correlated well the recombination rate with the spacer distance, and the large attenuation factor obtained from this analysis strongly suggests that back electron transfer is more likely through bond. The difference between our work and the previous work most likely comes from the different binding geometry between the dyes and the oxide surface. The decay residues in Figure 5 (4 −10% for O8−O12) indicate that a small fraction of long-lived excited species remains over the full delay range of the femtosecond TA

Figure 5. The kinetic traces of O8/NiO, O11/NiO, and O12/NiO probed at 681 nm, 679 nm, and 684 nm, respectively.

peak. The kinetic traces were best fitted by a global fitting over four characteristic wavelengths between 640 to 700 nm with three time constants and a constant residue. The fitting results are summarized in Table 2. The two slow time constants are in the range of picoseconds. On the basis of the previous discussion of the isosbestic point, these two time constants are 16858

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Figure 6. The dependence of recombination kinetics on spacer distance. The recombination rate is calculated from τ2 (A) and from τ3 (B).

Figure 7. (A) J−V characteristic curves and (B) APCE spectra of solar cells made of O8, O11, and O12. Inset in B: IPCE spectra of the corresponding solar cells.

example, the normalized IPCE spectrum of O12 in the wavelength longer than 450 nm resembles its absorption spectrum (Figure S8) very well, with peaks around 480 and 550 nm. The highest IPCE value for solar cells sensitized with O12, O11, and O8 in the wavelength longer than 450 nm is 9.08%, 5.49%, and 2.02%. The trend is consistent with the value of Jsc obtained in the J−V measurement. In order to find out the reason of the drastic enhancement of Jsc from O8 to O12, we divided the IPCE by the absorption of the dye-sensitized NiO films to calculate the absorbed photonto-current conversion efficiencies (APCEs) of our solar cells. As shown in Figure 7B, the APCE values display a noticeable increment from O8 to O11 and to O12. For instance, the values of APCEs for O8, O11, and O12 solar cells at 520 nm are 19.4%, 52.7%, and 65.6%, respectively. All three dyes have similar structures and driving forces for hole injection (see results from the Electrochemical Studies section above); however, the distance for hole recombination between the reduced sensitizer and holes in NiO is systematically varied. The femtosecond TA studies reveal that hole injection is within ∼1 ps, however the recombination kinetics is distancedependent. Specifically, as the number of phenylene linkers increase from O8 to O12, the recombination rate decreases. APCE is determined by the product of the hole injection efficiency and the hole collection efficiency. Therefore, the enhanced Jsc should mainly result from the enhanced collection efficiencies across the series. The large APCE value obtained for our sensitizers and the picosecond recombination rate imply that the dye regeneration, which is the reduction of triiodide (I3−), competes favorably with recombination. The ultrafast regeneration has also been observed for both n-DSCs and p-

measurements (3 ns). The properties of the long-lived species were further studied by nanosecond flash photolysis experiment. All sensitizers adsorbed on NiO films exhibit spectra features similar to those we observed in the femtosecond TA measurements with two broad absorption bands: one is around 350−450 nm, and the other is above 600 nm (Figure S6). Therefore, the TA in the nanosecond TA studies is probably due to the very slow recombination between h+/dye− pairs, presumably through space. All three sensitizer/NiO films exhibit similar decay kinetics which can be best described as a biphasic process with τ1 ∼ 8 μs and τ2 ∼ 80 μs (Figure S7). The small amplitude of this slow charge recombination in the microsecond time scale should cause negligible effect on the solar cell performance. Photovoltaic Studies. Our sandwiched solar cells consist of a NiO film41 (600 nm thick) sensitized with O8, O11, or O12, a CH3CN electrolyte solution containing 1.0 M LiI/0.1 M I2, and a platinized counter electrode. The J−V characteristic curves are shown in Figure 7A, and the solar cell performance data are shown in Table 3. With the increased number of phenylene units from O8 to O11 and O12, the Jsc increases by 1.6- and 3.2-fold, respectively, and the Voc increases by 25% and 30%, respectively. In combination, the efficiency increases by 2.7- and 4.6-fold. The IPCE (Figure 7B inset) spectra indicate that the current is indeed generated from excited dyes. As an Table 3. DSC Performance of O8, O11, and O12

O8 O11 O12

Jsc (mA/cm2)

Voc (mV)

FF

η (%)

0.44 1.16 1.84

63 79 82

0.36 0.36 0.34

0.009 0.033 0.051 16859

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The Journal of Physical Chemistry C DSCs with organic sensitizers, which has been ascribed to preassociation of redox mediator (here I3−) with sensitizer.42 Electrochemical Impedance Measurement. In order to evaluate the collection efficiency of the hole carriers in NiO, EIS was used to determine the charge carrier’s lifetime in the working cell. EIS was performed both under 1 sun at Voc and in the dark at different bias. The Nyquist plots of all solar cells under 1 sun conditions at their own Voc are shown in Figure 8A. The detailed analysis of the EIS study of the NiO-based p-

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CONCLUSION

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EXPERIMENTAL SECTION

Article

We have successfully designed three cyclometalated ruthenium complexes as sensitizers for NiO p-DSCs. Femtosecond TA measurements indicate that hole injection from the excited dye to NiO occurs within ∼0.5 ps, and the recombination is also fast in the picosecond time scale. The recombination rate is suppressed successfully by increasing the length of the bridging oligophenylene linker between the Ru[(N ∧ N) 2 (C ∧ N)] + chromophore and the carboxylic anchoring group. The solar cell performance also depends on the length of the bridging oligophenylene linker. EIS results show that, as the length of the phenylene linker increases, the interfacial charge recombination rate decreases. Our work demonstrates that these cyclometalated ruthenium complexes with properly designed molecular structures are promising sensitizers for NiO p-DSCs.

General Information. All reagents and solvents (analytical grade) were purchased from VWR Scientific Company and used without further purification. The silica gel (60−200 μm) and neutral alumina was purchased from Fisher Scientific. cisBis-(2,2′-bipyridine)dichlororuthenium(II) dihydrate and 4bromobenzoic acid was purchased from Alfa Aesar. All products were characterized by 1H NMR, and high-resolution mass spectrometry (HRMS), and elemental analysis. All NMR spectra were obtained using a Bruker 250 or 400 MHz spectrometer. HRMS was conducted on a Bruker Daltonics BioTOF system with an ESI source. Elemental analyses were conducted by NuMega Resonance Laboratories, Inc. in San Diego, California. 2-(3-4,4,5,5-Tetramethyl-1,3,2-dioxaborolan2-yl)phenyl)pyridine,44 4′-bromobiphenyl-4-benzoic acid, and O818 were prepared following the literature procedure and obtained with satisfactory purity. The absorption spectra in solution were measured by a Lamda 950 spectrophotometer (Perkin-Elmer), and the absorption of films was measured by the same spectrophotometer with an integrating sphere detector. The emission spectra were obtained with a Fluoromax 4 spectrofluorometer (Jobin Yvon). Cyclic voltammetry was measured by using a CV50W electrochemical workstation. The electrochemical cell consisted of a Pt working electrode, a Ag wire as reference electrode, and a Pt wire auxiliary electrode in a single compartment. (TBA)ClO4 (recrystallized in EtOH twice before use, 0.1 M) in degassed dry CH3CN was used. The scan rate was 100 mV/s. Ferrocenium-ferrocene (Fc+/0) was used as an external reference, and all potentials were reported relative to NHE using Fc+/Fc couple (0.64 V versus NHE) as a reference. The spectroelectrochemistry was performed in DMF solution containing 0.1 M TBAP in a thin layer cell (0.5 mm) from ASL in Japan. The concentration of sensitizers was 1 mM. Platinum mesh, platinum wire, and Ag/AgCl aq. were used as working, counter, and reference electrodes, respectively. Potentials at −1.5, −1.6, and −1.6 V were applied for O8, O11, and O12. Femtosecond TA Measurement. The femtosecond TA measurements were carried out at the Center of Nanoscale Materials (CNM) of the Argonne National Laboratory. A commercial transient spectrometer (Ultrafast systems, Hellos) was used. The ultrafast TA system consists of a Ti:Sapphire regenerative amplified laser system with a 1.7 kHz repetition rate, and the output of the Ti:Sapphire amplifier was split into two beams. The majority part (95%) was used to pump to an optical parametric amplifier (OPA) to generate tunable pump

Figure 8. (A) The Nyquist plots of solar cells made of O8, O11, and O12 at open-circuit voltage under 1 sun conditions. (B) The plot of charge recombination rate versus open circuit voltage by varying the light intensity.

DSCs has been reported in our recent paper.43 The recombination rate was obtained from the reciprocal of recombination lifetime and plotted versus Voc by varying the light intensity (Figure 8B). Under illumination, the Voc is the difference of redox potential of electrolyte and the quasi Fermi level in NiO, which is determined by the charge carrier densities. This plot allows us to compare the recombination rate of solar cells at the same charge carrier density. Apparently, the carrier lifetime increases as the length of the phenylene spacer increases. For instance, at the Voc of 67 mV, an increase of charge recombination rate of approximate 14% and 75% is observed for the O11 and O8 solar cells compared to the O12 solar cell. The observed trend in charge recombination rate is consistent with the obtained Voc. Additionally, we performed EIS in the dark, and plotted the recombination rate against the applied bias. The result is shown in Figure S9. A trend similar to that studied under 1 sun conditions was observed: O8 exhibits faster recombination than O11 and O12. Since the dye molecules stay at the ground state during the measurements in the dark, and the EIS lifetimes (milliseconds to seconds) is much longer than the recombination lifetime between the holes in NiO and the reduced dyes (picoseconds), the EIS charge recombination is between holes in NiO and I− in the electrolyte. The differences for the different dyes could be due to different surface blocking effects by the larger dyes. 16860

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atom groups were calculated from this data. TDDFT calculations were performed to obtain low-energy excitations at the ground-state geometry. Synthesis. [Ru(bpy)2(bpp)]+. Ru(bpy)2Cl2·2H2O (312 mg, 0.6 mmol) and AgOTf (309 mg, 1.2 mmol) were added to 100 mL MeOH. 2-(3-4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)pyridine (850 mg, 3.0 mmol) was added to the mixture. The mixture was refluxed for overnight under Ar. After reaction, white solid was filtered out. The filtrate was evaporated. The crude product was purified on Alumina column (from 2/1 (v/v) Hexane/EtOAc, to 3% MeOH/ dichloromethane (DCM)). The major dark band was collected. The pure product was collected after recrystallization in DCM/ hexane and washed with large quantity of hexanes. After ion exchange in saturated aq. NH4PF6, red solid was collected. Yield (84 mg, 51%). 1H NMR (d-CDCl3, 400 MHz): 1.22 (12 H, s), 6.40 (1H, d, J = 10 Hz), 6.83 (1H, t, J = 5.0 Hz), 7.12 (4H, m), 7.35 (1H, t, J = 7.5 Hz), 7.46 (1H, m), 7.87 (7 H, m), 7.95 (3H, m), 8.09 (1H, s), 8.30−8.49 (4H, m) ppm. ESI_Mass (m/z): [M-PF6]+ = 694.2. Anal. Calcd for C38H39BF6N5O3PRu: C, 52.42; H, 4.52; N, 8.04. Found: C, 53.00; H, 4.21; N, 8.35. O11. [Ru(bpy)2(bpp)]+ (42 mg, 0.05 mmol), 4-bromobenzoic acid (11 mg, 0.06 mmol), Pd(dppf)2Cl2 (1 mg), and 1 mL 1 M K3PO4 were added in 5 mL of DMF. The mixture was heated under Ar for 48 h. Solvent was removed. Water was added. Dark solid was filtered out and dried. Product was purified on a silica gel column eluted by CH3CN/KNO3 aq. (7/ 1, v/v). Red solid was collected after ion exchange (21 mg, 50%). 1H NMR (d6-CD3COCD3, 400 MHz): 6.68 (1H, d, J = 8.0 Hz), 7.09 (1H, t, J = 8.0 Hz), 7.27 (1H, d, J = 8.0 Hz), 7.39−7.44 (3H, m), 7.61 (1H, t, J = 6.0 Hz), 7.78 (1 H, d, J = 4.0 Hz), 7.83 (3H, m), 7.92−8.02 (5H, m), 8.07 (3H, m), 8.14 (1H, m), 8.24 (1H, m), 8.36 (1H, s), 8.42 (1H, d, J = 8.0 Hz), 8.62 (2H, m), 8.7.0 (1H, m), 8.77 (1H, d, J = 8.0 Hz) ppm. ESI_Mass (m/z): [M-PF6]+ = 688.1. Anal. Calcd for C38H38F6N5O2PRu. H2O: C, 53.65; H, 3.55; N, 8.23. Found: C, 53.35; H, 3.26; N, 8.08. O12. [Ru(bpy)2(bpp)]+ (42 mg, 0.05 mmol), 4-bromophenylbenzoic acid (16.7 mg, 0.06 mmol), Pd(dppf)2Cl2 (1 mg), and 1 mL 1 M K3PO4 were added in 5 mL of DMF. The mixture was heated under Ar for 48 h. Solvent was removed. Water was added. Dark solid was filtered out and dried. Product was purified on a silica gel column eluted by CH3CN/KNO3 aq. (7/1, v/v). Red solid was collected after ion exchange (31 mg, 68%). 1H NMR (d3-CD3OD, 400 MHz): 6.60 (1H, d, J = 8.0 Hz), 7.01 (1H, t, J = 8.0 Hz), 7.20 (1H, m), 7.27−7.33 (3H, m), 750 (1H, t, J = 8.0 Hz), 7.63 (1 H, d, J = 4.0 Hz), 7.76 (6H, m), 7.80 (6H, m), 8.10 (4H, m), 8.22 (3H, m), 8.50 (2H, t, J = 12.0 Hz), 8.56 (1H, d, J = 8.0 Hz), 8.64 (1H, d, J = 8.0 Hz) ppm. ESI_Mass (m/z): [M-PF6]+ = 764.1. Anal. Calcd for C44H32F6N5O2PRu.CH3CN: C, 58.17; H, 3.71; N, 8.85. Found: C, 58.30; H, 3.18; N, 8.55. O8. 1H NMR (d6-CD3COCD3, 250 MHz): 6.50 (1H, d, J = 7.8 Hz), 6.86 (1H, t, J = 6.0 Hz), 7.10 (3H, m), 7.29 (2H, m), 7.49 (1H, d, J = 6.0 Hz), 7.73 (7H, m), 7.83 (2H, m), 8.02 (1H, d, J = 8.0 Hz), 8.22 (5H, m). ESI_Mass (m/z): [M-PF6]+ = 612.1. Anal. Calcd for C32H24F6N5O2PRu.0.5H2O: C, 50.20; H, 3.29; N, 9.15. Found: C, 50.27; H, 3.02; N, 9.26.

pulse while the left 5% beam was focused onto a sapphire disk to generate a white light contimuun probe. The timedependent TA signals at different pump and probe wavelengths are fit to a multiexponential function, f(t) = ∑iAi exp(−t/τi) convoluted with a Gaussian instrument response function of 180 fs fwhm. The TA spectra are chirp corrected to within 100 fs. The TA measurements were performed on both solution and thin film samples. The samples were excited at 400 nm and probed between 450 to 760 nm. The pump power was between 0.3 to 0.4 mW except for the excitation-power dependence study on the O12/NiO sample, where the power was varied from 0.1 to 0.9 mW. The solution samples were prepared in dilute CH3CN with o.d. ∼ 0.4 at 400 nm. The thin film samples were made by coating three layers of NiO film (total thickness ∼ 600 nm) onto glass slides using the doctor blading method, and then the NiO films were immersed in the dye solutions overnight to achieve o.d. ∼ 0.3−0.4. The sample does not decay during the measurements, suggesting the superior stability of ruthenium cyclometalated complexes. Nanosecond TA Measurement. Samples were excited at 355 nm with ∼5 ns fwhm pulses from the third harmonic output of a NdYAG laser (Surelite-II, Continuum). The laser shot frequency was 10 Hz, and beam power was attenuated to ∼3 mW. Spectra at selected time delays before and after laser flashes were obtained with 2 ns exposures near the peak of a white Xe flash lamp (EG&G, τ ∼ 1 μs) used as the probe source and an intensified gated CCD camera with monochromator (PI-MAX2, Princeton Instruments) as the detector. Time delays between pump and probe pulses were controlled using a DG-535 delay generator (Stanford Research Systems). Solar Cell Fabrication and Measurement. The NiO paste was prepared using polymer template methods adopted from the literature.41 Double layers of NiO film were deposited on fluorine-doped tin oxide (FTO) with modification from Sun’s method.45 The film area is proximately 0.5 × 0.5 cm2. The film was soaked in dye solution (0.1 mM in CH3CN) for 16 h. After being washed with CH3CN and dried under air, the NiO electrode and platinized counter electrode was sealed by placing Surlyn 60 film in between. A CH3CN electrolyte solution of 1.0 M LiI and 0.1 M I2 was filled through the holes predrilled on the counter electrode by applying vacuum. The holes were sealed afterward with a glass cover slide. The J−V curve was measured under 1 sun AM 1.5 G simulated sunlight (Small-Area Class-B Solar Simulator, PV Measurements) and recorded with a CV-50W voltammetric analyzer. The IPCE spectra were recorded by the QEX7 quantum efficiency measurement system from PV Measurements using silicon diode as the reference cell. The EIS spectra were measured by an EIS600 potentiostat (Gamry Instruments, Warminster, PA) in the frequency range of 3 × 105 to 0.05 Hz at open circuit conditions and in the dark at different bias. The light intensity was varied by using neutral density filters. The AC amplitude was 10 mV. DFT Calculations. All computations were carried out with the Gaussian 03W46 software package at the Ohio supercomputer center. DFT calculations were carried out using the B3LYP functional with the LanL2DZ basis set. The geometry was optimized in the ground state (charge 1, singlet spin). A single-point SCF energy calculation was performed (with the Gaussian keyword pop=full) on the optimized structure in order to obtain the Mulliken population analysis, and the contributions to the frontier molecular orbitals from different 16861

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(20) Chang, W.-C.; Cheng, H.-S.; Li, T.-Y.; Hsu, N.-M.; Tingare, Y. S.; Li, C.-Y.; Liu, Y.-C.; Su, C.; Li, W.-R. Angew. Chem., Int. Ed. 2010, 49, 8161−8164. (21) Chen, B.-S.; Chen, K.; Hong, Y.-H.; Liu, W.-H.; Li, T.-H.; Lai, C.-H.; Chou, P.-T.; Chi, Y.; Lee, G.-H. Chem. Commun. 2009, 5844− 5846. (22) Pellegrin, Y.; Le Pleux, L.; Blart, E.; Renaud, A.; Chavillon, B.; Szuwarski, N.; Boujtita, M.; Cario, L.; Jobic, S.; Jacquemin, D.; Odobel, F. J. Photochem. Photobiol., A 2011, 219, 235−242. (23) Collin, J.; Kayhanian, R.; Sauvage, J.; Calogero, G.; Barigelletti, F.; De Cian, A.; Fischer, J. Chem. Commun. 1997, 775−776. (24) Collin, J.-P.; Kayhanian, R.; Sauvage, J.-P.; Calogero, G.; Barigelletti, F.; De, C. A.; Fischer, J. Chem. Commun. 1997, 775−776. (25) Wadman, S. H.; Lutz, M.; Tooke, D. M.; Spek, A. L.; Hartl, F.; Havenith, R. W. A.; van Klink, G. P. M.; van Koten, G. Inorg. Chem. 2009, 48, 1887−1900. (26) Wadman, S.; van, L. Y. M.; Havenith, R. W. A.; van Klink, G. P. M.; van Koten, G. Organometallics 2010, 29, 5635−5645. (27) Bomben, P. G.; Thériault, K. D.; Berlinguette, C. P. Eur. J. Inorg. Chem. 2011, 1806−1814. (28) Bomben, P. G.; Gordon, T. J.; Schott, E.; Berlinguette, C. P. Angew. Chem., Int. Ed. 2011, 50, 10682−10685. (29) Myahkostupov, M.; Piotr, P.; Wang, D.; Galoppini, E. J. Phy. Chem. C 2007, 111, 2827−2829. (30) McCusker, J. Acc. Chem. Res. 2003, 36, 876−887. (31) Kilsa, K.; Mayo, E.; Kuciauskas, D.; Villahermosa, R.; Lewis, N.; Winkler, J.; Gray, H. J. Phys. Chem. A 2003, 107, 3379. (32) Kihara, H.; Gondo, Y. J. Raman Spectrosc. 1986, 17, 263−267. (33) McCusker, C.; McCusker, J. Inorg. Chem. 2011, 50, 1656−1669. (34) Asbury, J. B.; Ellingson, R. J.; Ghosh, H. N.; Ferrere, S.; Nozik, A. J.; Lian, T. J. Phys. Chem. B 1999, 103, 3110−3119. (35) Hagfeldt, A.; Grätzel, M. Acc. Chem. Res. 2000, 33, 269−277. (36) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Stipkala, J. M.; Meyer, G. J. Langmuir 1999, 15, 7047−7054. (37) Yan, S. G.; Prieskorn, J. S.; Kim, Y. J.; Hupp, J. T. J. Phys. Chem. B 2000, 104, 10871−10876. (38) Kuciauskas, D.; Freund, M. S.; Gray, H. B.; Winkler, J. R.; Lewis, N. S. J. Phys. Chem. B 2001, 105, 392−403. (39) Smalley, J. F.; Sachs, S. B.; Chidsey, C. E. D.; Dudek, S. P.; Sikes, H. D.; Creager, S. E.; Yu, C. J.; Feldberg, S. W.; Newton, M. D. J. Am. Chem. Soc. 2004, 126, 14620. (40) Eng, M. P.; Ljungdahl, T.; Martensson, J.; Albinsson, B. J. Phys. Chem. B 2006, 110, 6483. (41) Sumikura, S.; Mori, S.; Shimizu, S.; Usami, H.; Suzuki, E. J. Photochem. Photobiol., A 2008, 199, 1−7. (42) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595−6663. (43) Huang, Z.; Natu, G.; Ji, Z.; Hasin, P.; Wu, Y. J. Phys. Chem. C 2011, 115, 25109−25114. (44) Su, S.; Takahashi, Y.; Chiba, T.; Takeda, T.; Kido, J. Adv. Funct. Mater. 2009, 19, 1260−1267. (45) Li, L.; Gibson, E. A.; Qin, P.; Boschloo, G.; Gorlov, M.; Hagfeldt, A.; Sun, L. Adv. Mater. 2010, 22, 1759−1762. (46) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; M. A. Al-Laham, Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,

ASSOCIATED CONTENT

S Supporting Information *

[The simulated TDDFT calculation results, nanosecond TA spectra and kinetics, and 1H NMR spectra of O8, O11, and O12. 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] (Y.W.); xyzhang@aps. anl.gov (X.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering, under Award DE-FG02-07ER46427. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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REFERENCES

(1) He, J.; Lindström, H.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. B 1999, 103, 8940−8943. (2) Li, L.; Duan, L.; Wen, F.; Li, C.-Y.; Wang, M.; Hagfeldt, A.; Sun, L. Chem. Commun. 2012, 48, 988−990. (3) O’Regan, B.; Grätzel, M. Nature 1991, 353, 737−40. (4) Odobel, F.; Le Pleux, L.; Pellegrin, Y.; Blart, E. Acc. Chem. Res. 2010, 43, 1063−1071. (5) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarström, L. J. Phys. Chem. C 2008, 112, 9530−9537. (6) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarström, L. J. Phys. Chem. B 2005, 109, 19403−10. (7) Borgstrom, M.; Blart, E.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A.; Hammarstrom, L.; Odobel, F. J. Phys. Chem. B 2005, 109, 22928−34. (8) Smeigh, A.; Le Pleux, L.; Fortage, J.; Pellegrin, Y.; Blart, E.; Odobel, F.; Hammarstrom, L. Chem. Commun. 2012, 48, 678−680. (9) Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. J. Am. Chem. Soc. 2008, 130, 8570−8571. (10) Qin, P.; Linder, M.; Brinck, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Adv. Mater. 2009, 21, 2993−2996. (11) Ji, Z.; Natu, G.; Huang, Z.; Wu, Y. Energy Environ. Sci. 2011, 4, 2818−2821. (12) Qin, P.; Wiberg, J.; Gibson, E. A.; Linder, M.; Li, L.; Brinck, T.; Hagfeldt, A.; Albinsson, B.; Sun, L. J. Phys. Chem. C 2010, 114, 4738− 4748. (13) Le Pleux, L.; Smeigh, A. L.; Gibson, E. A.; Pellegrin, Y.; Blart, E.; Boschloo, G.; Hagfeldt, A.; Hammarstroem, L.; Odobel, F. Energy Environ. Sci. 2011, 4, 2075−2084. (14) Morandeira, A.; Fortage, J.; Edvinsson, T.; Le Pleux, L.; Blart, E.; Boschloo, G.; Hagfeldt, A.; Hammarström, L.; Odobel, F. J. Phys. Chem. C 2008, 112, 1721−1728. (15) Nattestad, A.; Mozer, A. J.; Fischer, M. K. R.; Cheng, Y. B.; Mishra, A.; Baeuerle, P.; Bach, U. Nat. Mater. 2009, 9, 31−35. (16) Bessho, T.; Yoneda, E.; Yum, J.-H.; Guglielmi, M.; Tavernelli, I.; Imai, H.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. J. Am. Chem. Soc. 2009, 131, 5930−5934. (17) Wadman, S. H.; Kroon, J. M.; Bakker, K.; Lutz, M.; Spek, A. L.; van Klink, G. P. M.; van Koten, G. Chem. Commun. 2007, 1907−1909. (18) Bomben, P. G.; Robson, K. C. D.; Sedach, P. A.; Berlinguette, C. P. Inorg. Chem. 2009, 48, 9631−9643. (19) Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.; Balzani, V. Top. Curr. Chem. 2007, 280, 117−214. 16862

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W.; Wong, M. W.; Gonzalez, C.; Pople, J. A., Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004.

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dx.doi.org/10.1021/jp303909x | J. Phys. Chem. C 2012, 116, 16854−16863