Photoinduced Electron Transfer Dynamics of Cyclometalated

Article pubs.acs.org/JPCC

Photoinduced Electron Transfer Dynamics of Cyclometalated Ruthenium (II)−Naphthalenediimide Dyad at NiO Photocathode Zhiqiang Ji and Yiying Wu* Department of Chemistry & Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States S Supporting Information *

ABSTRACT: Both forward and backward electron transfer kinetics at the sensitizer/NiO interface is critical for p-type dye-sensitized photocathodic device. In this article, we report the photoinduced electron transfer kinetics of a Ru(II) chromophore−acceptor dyad sensitized NiO photocathode. The dyad (O26) is based on a cyclometalated Ru(N∧C∧N)(N∧N∧N) (Ru[II]) chromophore and a naphthalenediimide (NDI) acceptor, where N∧C∧N represents 2,2′-(4,6-dimethylphenylene)-bispyridine and N∧N∧N represents 2,2′,6′,6″terpyridine ligand. When the dyad is dissolved in a CH3CN solution, electron transfer to form the Ru(III)−NDI− occurs with a rate constant kf = 1.1 × 1010 s−1 (τf = 91 ps), and electron− hole pair recombines to regenerate ground state with a rate constant kb = 4.1 × 109 s−1 (τb = 241 ps). When the dyad is adsorbed on a NiO film by covalent attachment through the carboxylic acid group, hole injection takes place first within our instrument response time (∼180 fs) followed by the subsequent electron shift onto the NDI to produce the interfacial charge-separated state [NiO(h+)−Ru(II)−NDI−] with a rate constant kf = 9.1 × 1011 s−1 (τf = 1.1 ps). The recovery of the ground state occurs with a multiexponential rate constant kb = 2.3 × 109 s−1 (τb = 426 ps). The charge recombination rate constant is slightly slower than a reference cyclometalated ruthenium compound (O25) with no NDI group (τb = 371 ps). The fast formation of interfacial charge separated state is a result of ultrafast hole injection resulting in the reduced form of sensitizer, which provides a larger driving force for NDI reduction. The kinetic study suggests that Ru(II) chromophore−acceptor dyads are promising sensitizers for the NiO photocathode devices.

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INTRODUCTION P-type dye-sensitized solar cells (p-DSSCs) have gained considerable research interest recently.1−3 The operation of a p-DSSC is opposite to the traditional n-type TiO2 DSSCs.3,4 Photoexcitation of sensitizers results in a hole injection into the valence band of a p-type semiconductor, and the sensitizer is then regenerated by the oxidized form of the redox couple. The reduced species of the electrolyte is regenerated at the counter electrode. The research of p-DSSC is motivated to integrate it with a traditional n-type TiO2 DSSC into a tandem DSSC, which can provide a larger voltage and potentially high efficiencies.3,5 In addition to conversion of solar energy to electrical energy, the p-type dye-sensitized photocathode can also be used in a photosynthetic device to convert solar energy to produce chemical fuels.6−8 For example, solar driven hydrogen production using a NiO photocathode by artificial photosynthesis (p-DSPEC) has been demonstrated.9,10 The practical application of p-type dye-sensitized photocathode is currently limited by its low photoconversion efficiencies.11−13 Therefore, fundamental research in photoinduced interfacial electron-transfer dynamics is of great importance to gaining insight into the factors limiting the conversion efficiency and designing of new materials to optimize the device performance. The most widely studied p-DSSCs are based on p-type semiconductor NiO.14,15 In a typical TiO2 DSSC, it is © XXXX American Chemical Society

commonly observed that a long-lived interfacial charge separated state (TiO2(e−)/S+) is formed due to the intrinsic electronic property of semiconductor TiO2.16 Conversely, the interfacial charge separated state at NiO interface (NiO(h+)/ S−) formed following photoexcitation and hole injection is usually short-lived.17 Consequently, NiO DSSCs exhibit low incident-photon-to-current conversion efficiency (IPCE) as a result of germinate recombination of electron/hole pairs and inefficient dye regeneration. The fast charge recombination kinetics at NiO interface has been known to be a major limiting factor of NiO DSSCs. Hammarström and co-workers have studied the electron transfer kinetics of coumarin, porphyrin, and peryleneimide on NiO and observed similar ultrafast recombination on time scales from 10s ps to 1 ns.18−20 Recently, we have observed a similar ps-scale charge recombination in our cyclometalated ruthenium-sensitized NiO.21 We think this fast rate of charge recombination stems from the low hole mobility in NiO and the small dielectric constant of NiO. An effective approach to extending the lifetime of the interfacial charge separated state is to attach an additional Received: June 7, 2013 Revised: August 7, 2013

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dx.doi.org/10.1021/jp405659m | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Scheme 1. Synthetic Route for O25 and O26 Compounds

electron transfer kinetics at dyad/NiO interface by femtosecond transient absorption spectroscopy. The family of cyclometalated ruthenium complexes denoted as Ru(N∧C∧N)(N∧N∧N) precludes the possible formation of enationmers and diastereomers and allows facile and precise structure characterization. The carboxylic acid anchoring unit is attached at the para position of the ruthenium−carbon bond for strong electronic coupling between sensitizers and NiO. Femotosecond transient absorption were performed for sensitizers in solution and adsorbed at NiO film. By following the spectroscopy signature of NDI unit, we were able to obtain the electron transfer dynamics at NiO interface. We found that when the dyad is adsorbed on NiO film, the forward electron transfer to yield fully charge separated state as NiO(h+)/ NDI(e−1) is very fast with a rate constant kf = 9.1 × 1011 s−1 (τf = 1.1 ps), which is about 2 orders of magnitude faster than the intramolecular electron transfer kinetics of the dyad in dilute CH3CN solutions (kf = 1.1 × 1010 s−1). The recovery of the ground state for the dyad/NiO film occurs with a multiexponential rate constant kb = 2.3 × 109 s−1 (τb = 426 ps), slightly slower than a reference cyclometalated ruthenium compound (O25) with no NDI group (τb = 371 ps). The ultrafast formation of interfacial charge separated state is a result of ultrafast hole injection process resulting in the reduced form of sensitizer that holds larger driving force for NDI reduction. Our studies suggest that Ru(II) chromophore−

electron acceptor to the chromophore forming a chromophore−acceptor dyad. Electron is shifted away from NiO surface, and the increased distance between electrons and holes results in the enhanced interfacial charge separated state lifetime. For example, Odobel et al.20 have shown higher IPCE when an organic PDI-NDI dyad is used as the sensitizer. Hammarström and co-workers22 have also synthesized a donor−acceptor ruthenium polypyridyl complex and observed long-lived charge separation between the reduced dye and the holes in NiO. Previously, we have reported a few tris(bidentate) cyclometalated Ru(II) complexes represented as Ru[(N∧N)2(C∧N)]+ for NiO p-DSSCs, where N∧N represents 2,2′-bipyridine and C∧N represents bidentate phenylpyridine derivatives.21 They have broad absorption spectra due to the destabilized metal orbital by strong σ carbon anion and the desired dipolar alignment for efficient hole injection due to delocalization of HOMO onto the C∧N ligand, which is close to NiO surface providing efficient electronic coupling for hole injection. However, the possible formation of enationmers and diastereomers of tris(bidentate) complexes can complicate the forward and backward electron transfer at the semiconductor/ dye interface due to the uncontrolled alignment of the sensitizers at the oxide interface. The low molecular symmetry also complicates structure characterization. Herein, we report a tridentate cyclometalated ruthenium complex−naphthalenediimide (O26) dyad (Scheme 1) and the B



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Figure 1. Cyclic voltammogram (black, scan rate is 100 mV/s) and differential pulse voltammogram (blue, the step is 4 mV) of O25 (left) and O26 (right) in dry DMF with 0.1 M TBAP as supporting electrolyte.

Table 1. Photophysical and Electrochemical Data of O25 and O26 O25 O26

λabs (nm) (ε (M−1 cm−1))a

λem (nm)b

Eox (V)c

Ered (V)c

E00 (eV)d

293(46030), 501 (13220) 287 (73310), 357 (32820), 379 (38230), 510 (16240)

739 764

0.69 0.68

−1.33 −0.33, −0.84, −1.33

1.7 1.7

All absorptions were measured in dilute DMF solution with concentration ∼1 × 10−5 M. bThe emission spectra were taken in a 4/1 mixture of ethanol and methanol at 77 K. cThe data were from half-wave potentials of CV waves. dE00 is determined from the absorption onset. a

and O26, assigned as Ru(III)/Ru(II) couple. The low oxidation potential is due to the perturbation of the strong σ donating ability of carbon anion. One reversible reduction potential assigned as terpyridyl ligand reduction is observed at −1.33 V for O25. The first and second cathodic peaks at −0.33 and −0.84 V obtained at half-wave potentials of CV of O26 are consistent with two reductions of NDI moieties.26,27 Both the oxidation potentials and reduction potentials of tpy moieties of O26 are almost unchanged relative to O25 suggesting negligible ground state interactions between the ruthenium chromophore and the NDI acceptor. Electronic Absorption and Emission. The absorption spectra of O25 and O26 in DMF solutions are shown in Figure 2. The absorption maxima and absorption coefficients are

acceptor dyads are promising sensitizers for NiO photocathode devices for solar cells and solar fuels.

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RESULTS Design and Synthesis. The syntheses of ligand NDIttpy,23 1,3-di(2-pyridyl)-4,6-dimethylbenzene (N∧CH∧N),24 and dichloro(η6-benzene) ruthenium (II) dimer25 [RuCl2Bz]2 were based on literature procedures. The synthetic routes of O25 and O26 are depicted in Scheme 1. Ru(N∧C∧N)(CH3CN)3PF6 was prepared by the reaction of [RuCl2Bz]2 dimer with (N∧CH∧N) ligand in CH3CN in the presence of KPF6 and NaOH at 45 °C at moderate yield after chromatographic purifications on silica gel and used as a key intermediate in the preparation of the titled compounds. The heteroleptic ruthenium complexes Ru(N∧C∧N)(tpy)PF6 and Ru(N∧C∧N)(NDI-ttpy)PF6 were obtained by the addition of 1 equiv of corresponding ligand to the intermediate in ethanol. Electrophilic bromination of the cyclometalated phenyl ring allows further functionalization of the complexes with anchoring groups. A solution of Ru(N∧C∧N)(N∧N∧N)PF6 in CH3CN was treated with 1 equiv of NBS at room temperature. The reaction occurs explicitly at the para position of ruthenium− carbon bond of the cyclometalated phenyl ring and completes within 1 h as evidenced by 1H nuclear magnetic resonance (NMR) and electrospray ionization−mass spectrometry (ESI− MS) measurements. A subsequent palladium catalyzed Suzuki coupling reaction with 4-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-benzoic acid produced O25 and O26 in very high yield. Both complexes were characterized by 1H NMR and ESI-MS. Electrochemical Behavior. Cyclic voltammetry (CV) and differential pulse voltammetry (DP) of O25 and O26 were studied in dried DMF solutions with 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte. All potentials were converted and reported versus NHE using Fc redox couple as reference. Figure 1 shows the CV and DP voltammograms of both complexes, and the results are summarized in Table 1. Both complexes exhibit one reversible oxidation potential at 0.69 and 0.68 V, respectively, for O25

Figure 2. Absorption spectra of O25 and O26 in DMF solutions.

shown in Table 1. O25 features ligand-centered π−π* absorption in the UV region and a broad and moderately intense absorption in the visible region extending to 700 nm assigned to 1MLCT transitions with significant contributions from cyclometalated N∧C∧N ligand.27 For O26 an additional NDI absorption in the range of 340−400 nm was observed. O26 exhibits an enhanced absorption coefficient and the absorption maximum of 1MLCT red shifts by ∼10 nm relative to O25, suggesting there is an electronic coupling between C



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selectively excite the lower-energy MLCT charge transfer band of O25 and O26. The power of laser excitation remains at 500 μW for all measurements. In our previous study, we found that the difference absorption amplitude varies linearly on the laser power lower than 900 μW.21 This indicates that our experiments are in the first order process. Figure 4 shows the transient difference absorption spectra of O25 at different delay times and the decay kinetics at the excited state absorption (660 nm) and the ground state bleaching (550 nm). After excitation to the MLCT charge transfer transition, a broad transient absorption beyond 618 nm extending to near IR region with a small positive absorption below 437 nm is discernible instantly. The transient absorption features remain almost unchanged over our experimental window (3 ns). The transient absorption is assigned to 3 MLCT excited state, the fast formation of which is a result of strong spin orbital coupling caused by the ruthenium atom. Because of the slow decay, no reliable fitting of the kinetics can be obtained. However, the lifetime of the 3MLCT excited state is expected to be longer than 3 ns. Instantaneously after photoexcitation of O26 at 500 nm, the transient difference absorption spectra feature a positive absorption beyond 618 nm extending to near IR region and an absorption band near 400 nm, showing the production of 3 MLCT excited state similar to O25 (Figure 5). Following the signature of the absorption of NDI− at 472, 623, and 677 nm, the charge separated state RuIII−NDI− is formed in the following hundreds of ps. The transient absorption is in good agreement with our spectroelectrochemical (SEC) measurement of reduced O26, and also consistent with literature observations.23 Note the absorption at 605 nm is red shifted relative to SEC result, and the absorption is very small, which is because of overlapping with the ground state bleaching of the MLCT excited state. Afterward, the charge separated state recombines to reproduce the ground state within our experimental window, as no transients can be seen after ∼1 ns. The forward and backward electron transfer rate can be determined by fitting the kinetic traces at 474, 609, and 594 nm. The kinetic traces of 474 nm were fitted with two lifetimes with a formation lifetime τf = 91 ± 6 ps and a decay τb = 241 ± 17 ps. The fitting of kinetic traces of 609 nm gave the comparable values of τf = 113 ± 27 ps and τb = 282 ± 68 ps. The ground state bleaching at 594 nm can be fitted with monoexponential by a lifetime τb = 300 ± 14 ps. The kinetic traces and fitting curves are shown in Figure 5 (right panel).

ruthenium chromophore and NDI units. However, this electronic coupling should be small because NDI unit twists out of the plane of phenyl ring.23 The methylene linker and the attachment on the nitrogen atom of a nodal plane of the NDI HOMO and LUMO also contribute to the weak coupling.28 Both complexes are none emissive at room temperature consistent with the literature observation.27 The emission spectra were collected at 77 K in alcoholic solutions (Figure S1, Supporting Information). Both complexes exhibit structureless emission centered at 739 and 764 nm for O25 and O26, assigned to 3MLCT transitions. The red shift emission for O26 suggests weak electronic coupling between the ruthenium chromophore and NDI unit, consistent with the absorption measurement. Spectroelectrochemistry of O26 in Solution. The difference absorption spectrum of reduced O26 in DMF solution was collected by using spectroelectrochemical setups. The O26 was reduced by applying a bias at −0.7 V versus Ag/ AgCl reference, exhibiting a broad absorption in the visible region with a distinct peak at 475 nm, and a weak and broad absorption peak at 605 nm (Figure 3). This spectrum feature is

Figure 3. Dfference absorption spectrum of O26 upon applying bias at −0.7 V (vs Ag/AgCl).

in good agreement with the reported NDI− absorption in the literature and confirms that the first reduction of O26 is on NDI unit.23,26 The spectroelectrochemical result assists interpretation of the transient absorption results. Femtosecond Transient Absorption in Solution. Femtosecond transient absorption has been carried out to study the electron transfer process intramolecularly in CH3CN solutions (OD = 0.4). An excitation of 500 nm is employed to

Figure 4. (left) Transient difference absorption spectra of O25 in CH3CN at different delay times. (right) Corresponding kinetic trances at 550 nm (red) and 660 nm (black). D



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Figure 5. (left) Transient difference absorption spectra of O26 in CH3CN at different delay times. Inset shows the expended spectra between 580 and 640 nm. (Right) Corresponding kinetic trances at 474 nm (red), 594 nm (black), and 609 nm (green).

Figure 6. (left) Transient difference absorption spectra of O25/NiO at different delay times. (right) Corresponding kinetic trances at 658 nm (red) and 540 nm (black).

are shown in Figure 6. The kinetic traces were best fitted with two time constants and a residue. The fitting results are summarized in Table 2. Both lifetimes are in ps time region, consistent to our and other results.18,19,21,31

In considering the relatively long lifetime of control compounds O25 (τ > 3 ns) and rapid formation of NDI− (τ ≈ 91 ps), RuIII−NDI− is formed in quantitative yield. According to Hammoström,23,26 the charge separated state can also be calculated from the maximum absorption of NDI− (ε407nm = 8000 M−1 cm−1)26 and the ground state absorption of ruthenium chromophore (ε594nm = 10000 M−1 cm−1). This calculation results in 100% formation of charge separated state. Femtosecond Transient Absorption of Dye-Sensitized NiO Films. NiO films were prepared as stated in previous reports.29,30 The thickness of NiO films is 1.5 μm. The fs transient absorption spectra of sensitized films were pumped at 500 nm with power ∼500 μW. The difference absorption spectra of the O25/NiO film and kinetics traces at 540 and 658 nm are shown in Figure 6. The absorption spectra feature a ground state bleaching between 450 to 610 nm and a transient absorption band extending to near IR region with a maximum at 660 nm. The transient absorption forms immediately after the laser pulse. The amplitude of transient absorption decays and ground state bleaching recovers, while the spectra shape remains the same, with an isosbestic point at ΔA ≈ 0. According to our previous discussions, the transient absorption is assigned to an interfacial charge separated state with holes localized on NiO and electrons residing on the ruthenium complexes. The rapid formation of the transient signal indicates that the hole injection from sensitizers to VB of NiO is fast within our instrument response (∼180 fs). The representative kinetic traces of both ground state bleaching recovery (540 nm) and excited state decay (658 nm)

Table 2. The fitted time constants and amplitudes of the recombination kinetics of O25/NiO and O26/NiO. τavg is the average lifetime calculated by the equation: τavg = (∑aiτ2i )/(∑aiτi) τ1 (ps) (a1) τ2 (ps) (a2) τ3 (ns) τavg

O25/NiO (540 nm)

O26/NiO (594 nm)

11.4 ± 0.5 (0.37) 377.4 ± 21.8 (0.63) >3 ns 371 ps

20.8 ± 2.0 (0.50) 445.4 ± 28.6 (0.50) >3 ns 426 ps

Figure 7 shows the transient difference absorption spectra of an O26/NiO film pumped at 500 nm. Immediately after photoexcitation, the 3MLCT excited state of ruthenium chromophore dominates the difference absorption spectra with a ground state bleaching between 500−603 nm and a positive absorption with a maximum position at 660 nm similar to the absorption features of O25/NiO film. The spectra evolve after 0.5 ps with positive peaks at 474, 614, and 681 nm and with the ground state bleaching band remaining the same. After 3.8 ps, these absorption features dominate the whole difference absorption spectrum and start decay with a discernible E



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Figure 7. (Left) Transient difference absorption spectra of O26/NiO at different delay times. (right) Corresponding kinetic trances at 594 nm (red) and 474 nm (black). Inset: the expanded kinetic traces at 474 nm.

attachment is by a methylene group to the nitrogen atom at the nodal plane of the NDI HOMO and LUMO. Therefore, this electronic coupling is expected to be very weak. No room temperature emission can be observed for ruthenium cyclometalated complexes, due to the stabilized triplet excited stated obeying energy gap law.32 However, the strong σ donating ability of carbon anion ligand lifts up the ligand field and decreases the possibility of nonradiative decay pathway through thermal population of ligand field; therefore, a longer lifetime of 3 MLCT than Ru(tpy)2 complexes is observed. In the transient absorption measurement, we observed that the lifetime of 3 MLCT of O25 is longer than 3 ns, which is much longer than Ru(tpy)2 (250 ps).33 The emission of both complexes was observed at 77 K at 739 and 764 nm. The broad and structureless feature suggests the emission is from 3MLCT excited state, and the excitation spectrum confirms that the emission is from the absorption of the complexes. We note that no emission at ∼610 nm was observed for O26, indicating that no 3NDI is populated. The energy of 3MLCT of both complexes is 1.7 eV, estimated from the peak position of emission spectrum, which occurs at lower energy than 3NDI, which makes the energy transfer an uphill reaction and unlikely to happen. Femtosecond transient absorption allows us to study the electron transfer dynamics from ruthenium chromophore to NDI electron acceptor. First, the redox potentials of O25 and O26 were measured by electrochemical measurement. Both complexes exhibit very comparable Ru(III)/Ru(II) oxidation potential and tpy/tpy− reduction potential, suggesting the ground state electronic coupling is weak. The electron transfer of O26 is first studied in a dilute CH3CN solution, and the result is compared with O25. In previous studies, Hammarströ m has found that charge separation from Ru(tpy) 2 chromophore to NDI electron acceptor is unlikely to happen because of its short excited state lifetime; however, electron transfer and energy transfer takes place quickly from Ru(bpy)3 chromophore to NDI unit, and electron transfer rate depends on the distance and the bridge between the two.23 In our study, we observed that the charge separated state Ru(III)−NDI− is formed with a very fast rate kf = 1.1 × 1010 s−1 (τf = 91 ps), despite that NDI unit is separated relatively far from the tpy ligand. The charge separated state decays back to the ground state with a rate constant of kb = 4.1 × 109 S1− (τb = 241 ps), which is comparable with Ru(bpy)3−NDI dyad, and much slower than the complex with methyl viologen (MV) used as the electron acceptor.34 The long charge separated state

isosbestic point at 605 nm. These positive peaks can be assigned to the absorption of NDI−, suggesting that the interfacial charge separated state with a hole localized at NiO semiconductor and an electron transferred to NDI unit is formed. According to the transient absorption of O26 in a CH3CN, the forward electron transfer rate from the ruthenium chromophore to NDI can be determined by fitting the kinetic traces at 474 and 609 nm. The growth of the kinetic trace of 474 nm (Figure 7 inset) indicating the formation of NDI− was fitted with a formation lifetime τf = 1.1 ± 0.6 ps. The decay of the kinetic trace of 474 nm and the ground state bleaching at 594 nm can be fitted with the sum of two time constant and a residue. The fitted decay lifetimes and the corresponding residue are shown in Table 2.

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DISCUSSION Previously, we have shown that cyclometalated Ru(II) complexes are promising sensitizers for NiO dye-sensitized solar cells.21 These complexes are represented as Ru[(N∧N)2(C∧N)]+, where N∧N represents 2,2′-bipyridine and C∧N represents bidentate phenylpyridine derivatives. The carboxylic anchoring group is attached at the para position of the ruthenium−carbon bond. As an extension, in the present study, we intend to investigate a family of complexes denoted as Ru(N∧C∧N)(N∧N∧N). This family of complexes precludes the formation of enantiomers and diastereomers of the previous bistridentate heleroleptic complexes and the symmetric nature allows facile structure characterization. Facile electrophilic bromination explicitly occurs at the para position of the Ru− C bond of the N∧C∧N ligand, and a subsequent palladiumcatalyzed cross-coupling reaction allows convenient functionalization with anchoring groups. The terdentate tpy ligand can be further functionalized for the purpose of enhancing the light harvesting and changing electronic properties of the sensitizers. In the present study, we synthesized the titled compound with a NDI electron acceptor to explore the interfacial electron transfer at NiO interface. The ground state absorption spectrum of O26 is slightly redshifted relative to O25, and the absorption extinction coefficient of MLCT transition is enhanced, which is mainly caused be the extended conjugation in the functionalized terpyridyl ligand. The ground state electronic coupling between NDI and ruthenium chromophore can also account for this spectrum change. However, as discussed by Hammarström,23 the NDI unit is twisted out of the tpy conjugation plane, and the F



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relative to the rate in solution is a result of ultrafast hole injection leading to formation of the reduced Ru sensitizer, which holds higher driving force for NDI reduction. In solution, the driving force for the reductive quenching of 3MLCT excited state of Ru(II) chromophore by NDI can be calculated by the Rehm−Weller equation: ΔG(ET) = E 1/2 (Ru 3+/2+ ) − E1/2(NDI0/−)−E00(MLCT), where E1/2 is the electrochemical half-wave potentials from the electrochemical measurements, and E00 is the zero−zero transition of the 3MLCT excited state. From the calculation, −ΔG (ET) is 0.7 eV. When O26 is adsorbed on NiO film, photoexcitation leads to ultrafast hole injection resulting in the formation of the reduced sensitizer. The driving force for electron transfer from the reduced sensitizer to NDI acceptor can be calculated by ΔG(ET) = E1/2(Ru2+/1+) − E1/2(NDI0/−), where E1/2 is the reduction potentials from the electrochemical measurements. From the calculation, −ΔG (ET) is 1.0 eV. It is clear that the enhanced driving force is responsible for the enhanced electron transfer rate. Considering both forward and backward electron transfer rates, the charge separated state is formed in a very high yield. Finally, we would like to compare the charge recombination kinetic of O25 and O26 at NiO interface. As shown in Table 2, the averaged lifetime of O26/NiO is slightly longer than that of O25/NiO. For O26/NiO, the fully charge separated state with holes in NiO and electrons located at NDI is obtained in high yield. This imposes two different influences on the back electron transfer rate: (1) the distance for electron transfer is increased, as NDI is located further away from NiO surface, and (2) the driving force for electron transfer is lowered to 0.8 eV (1.8 eV for O25/NiO). The increased distance should decrease the back electron transfer rate constant as observed previously for Ru(C∧N)/NiO.5,21 However, the effect of driving force is unclear. Hammarströ m and co-workers found that the germinate recombination lies in the Marcus normal region with an uncommonly large reorganization energy and suspected that the electrons in the reduced sensitizer recombine to the interband states of NiO.17 Despite the increased tunneling distance, we observed a very slight change of the recombination rates from O25/NiO to O26/NiO film. The result suggests that the driving force may play an adverse effect on the charge recombination rate. Namely, for O26/NiO with a lower reduction potential and a lower driving force, the electron residing at the sensitizer recombines with holes or the surface states in NiO faster. Investigations into the back electron transfer mechanism at dye/NiO interface and the electronic properties of NiO will be highly important for NiO photocathode devices. The fast formation, high yield of charge separated state, and slower back electron transfer rates make Ru(II) chromophore−acceptor dyad promising as sensitizers for achieving high efficiencies of NiO DSSC.

lifetime is essential for application in solar energy conversions. The quantum yield of the charge separated state has been estimated from the lifetime measurements and also from the absorption extinction coefficient of the charge separated species. The quantitative formation of charge separated state is a consequence of longer excited state lifetime of Ru(N∧C∧N)(N∧N∧N) in comparison with Ru(tpy)2. In addition, because prior studies have shown that the forward charge separation reaction occurs in the Marcus normal region,35 a higher reduction potential of Ru(N∧C∧N)(N∧N∧N) gives a larger driving force for charge separation and thus increases the forward charge separation reaction rate. The long-lived charge separated state and high quantum yield suggests that Ru(II) chromophore−acceptor dyads are very promising for solar energy conversions. The goal of this study is to explore the electron transfer dynamics at NiO interface. Along this line of inquiry, femtosecond transient absorption measurements of O26/NiO and O25/NiO films were carried out. Photoexcitation of O25/ NiO leads to the formation of interfacial charge separated state with holes localized at NiO and O25 reduced to [NiO(h+)− Ru(tpy−)]. We observed that the transient absorption features form immediately after laser excitation, the amplitude of positive absorption decays and the ground state bleaching band recovers with an isosbestic point at ΔA = 0. There is no other spectra change except for the amplitude at different delay time. Figure S2, Supporting Information, exhibits the comparison of difference absorption spectra of O25/NiO film at 513 ps and the difference absorption spectrum of reduced O25 in DMF solution obtained by SEC by applying a bias of −1.8 V (vs Ag/ AgCl reference), showing negligible difference. Since the absorption of holes in NiO has very a small extinction coefficient, which is hardly observed in the measurement,36 this result suggests that the obtained spectra feature in transient absorption is caused by the reduced O25, supporting the fast formation of interfacial charge separated state. This charge separated state decays with multiexponential rate constants fitted with two lifetimes. The multiexponential rate constants are frequently observed for dye/oxide interfaces,16 which might be caused by heterogeneous localized environment of the anchored dye molecules. The averaged back electron transfer rate constant is 371 ps, consistent with our previous observations of Ru(bpy)(C∧N)/NiO films,21 but much faster than the typical Ru(II)/TiO2 interface commonly occurring in ns−μs.16 The fast back electron transfer has also been observed for coumarin/NiO,19 PNI/NiO,20 and organic donor−acceptor sensitizer/NiO interface.37 The distinctive absorption feature of NDI− makes it easy to understand the spectra evolution of dyad. Immediately after laser excitation, the MLCT excited state is formed and interfacial electron transfer takes place, as evidenced by positive absorption with a maximum around 474 nm. After ∼1 ps, interfacial charge separated state [NiO(h+)−Ru(II)−NDI−] is formed with absorption peak at 474 and 609 nm. After several ps, the absorption of NDI− dominate the whole spectra and decays to the ground state with concomitant ground state bleaching recovery with an isosbestic point at 599 nm with ΔA = 0. The formation of NDI− is obtained by fitting the growth of ΔOD at 474 nm. Surprisingly, a forward electron transfer rate constant of kf = 9.1 × 1011 s−1 (τf = 1.1 ps) is obtained, which is much faster than 91 ps in CH3CN solutions. The about 2 orders of magnitude increase of electron transfer rate from Ru(II) chromophore to the NDI acceptor at NiO interface

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CONCLUSIONS We have synthesized and characterized a dyad (O26) based on a tridentate cyclometalated Ru(N∧C∧N)(N∧N∧N) chromophore and a naphthalenediimide (NDI) acceptor. In comparison with our prior tris(bidentate) cyclometalated Ru(II) complexes,21 the tridentate complex possesses a more symmetric structure that precludes the possible formation of enationmers and diastereomers and allows facile and precise structure characterization. We have systematically studied the electron transfer dynamics of the dyad in solution and adsorbed on NiO films. We have found that when the dyad is adsorbed on NiO film, the forward electron transfer is about 2 orders of G



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were added to 50 mL of ethanol. The mixture was heated to reflux for 3 h under Ar. After the mixture was cooled to room temperature, the solvent was removed. The product was purified on a silica gel column eluted with a mixture of CH3CN/KNO3 (aq) to give the titled production of 26.4 mg. (72%). 1H NMR (400 MHz, CD3CN): 2.77 (6H, s), 6.93 (1H, s), 7.38 (2H, t, J = 12 Hz), 7.52 (2H, t, J = 8 Hz), 7.55 (2H, t, J = 8 Hz), 7.83 (4H, m), 8.03 (2H, t, J = 8 Hz), 8.22 (2H, d, J = 8 Hz), 8.35 (4H, m), 8.78 (2H, d, J = 8 Hz) ppm. ESI_MS([M−PF6]+): 594.1. Ru(N∧C∧N)(tpy)BrPF6. Ru(N∧C∧N)(tpy)PF6 (62.5 mg, 0.085 mmol) and NBS(15 mg, 0.1 mmol) were added in 20 mL of CH3CN. The mixture was stirred overnight. After solvent was evaporated, the mixture was purified on a silica gel column eluted with 5/1 (DCM/CH3CN, v/v). Yield: 57 mg. (82%) 1H NMR (400 MHz, CD3CN): δ 3.20 (6H, s), 6.67 (2H, m), 7.03 (2H, t, J = 8 Hz), 7.08 (2H, d, J = 8 Hz), 7.13 (2H, d, J = 8 Hz), 7.62 (2H, d, J = 8 Hz), 7.33 (2H, t, J = 8 Hz), 8.30 (3H, m), 8.43 (2H, d, J = 8 Hz), 7.40 (2H, d, J = 8 Hz) ppm. ESI_MS([M−PF6]+): 674.0 O25. Ru(N∧C∧N)(tpy)BrPF6 (13.3 mg, 0.016 mmol) and 4carboxylphenylboronic acid (8 mg, 0.03 mmol) were added into DMF (10 mL). The mixture was degassed for 20 min. Pd(PPh3)4 (2 mg) and K2CO3(22.5 mg/0.1 mL H2O) were added. The mixture was heated at 85 °C under Ar for 24 h. After the mixture was cooled to room temperature, the mixture was filtered. The filtrate was collected, and the solvent was evaporated. The crude product was recrystallized in CH2Cl2/ Et2O to give pure product 10 mg (81%). 1H NMR (400 MHz, CD3CN): δ 2.61 (6H, s), 6.75 (2H, t, J = 8 Hz), 7.02 (2H, d, J = 4 Hz), 7.11 (2H, d, J = 4 Hz), 7.15 (2H, t, J = 6 Hz), 7.60 (4H, m), 7.82 (2H, t, J = 8 Hz), 8.18 (2H, d, J = 6 Hz), 8.27 (2H, d, J = 6 Hz), 8.37 (1H, t, J = 4 Hz), 8.75 (2H, d, J = 6 Hz), 9.07 (2H, d, J = 6 Hz) ppm. ESI_MS([M−PF6]+): 714.1 Ru(N∧C∧N)(NDI-ttpy)PF6. Ru(N∧C∧N)(CH3CN)3PF6 (31.5 mg, 0.05 mmol) and NDI-ttpy (35 mg, 0.05 mmol) were added to 50 mL of ethanol. The mixture was heated to reflux for 3 h under Ar. After the mixture was cooled to room temperature, the solvent was removed. The product purified on a silica gel column eluted with a mixture of CH3CN/KNO3 (aq) to give the titled product 40.5 mg. (67%). 1H NMR (400 MHz, CD3CN): 1.0 (6 H, m), 1.5 (8H, m), 1.8 (1H, m), 2.98 (6H, s), 4.10 (2H, m), 5.54(2H, s), 6.10 (2H, t, J = 6 Hz), 6.98 (2H, t, J = 6 Hz), 7.00 (3H, m), 7.08 (2H, d, J = 6 Hz), 7.58 (2H, t, J = 8 Hz), 7.60 (2H, t, J = 8 Hz), 7.14 (2H, t, J = 8 Hz), 7.82 (2H, d, J = 8 Hz), 8.16 (2H, d, J = 8 Hz), 8.26 (2H, d, J = 8 Hz), 8.55 (2H, d, J = 8 Hz), 8.79 (4H, m), 8.99 (2H, s) ppm. ESI_MS([M−PF6]+): 1060.2. Ru(N ∧C ∧ N)(NDI-ttpy)BrPF 6 . Ru(N ∧ C ∧ N)(NDI-ttpy)PF6 (36.1 mg, 0.03 mmol) and NBS(5.4 mg, 0.03 mmol) were added in 20 mL of CH3CN. The mixture was stirred overnight. After solvent was evaporated, the mixture was purified on a silica gel column eluted with 5/1 (DCM/CH3CN, v/v). 1H NMR (400 MHz, CD3CN): δ 1.0 (6 H, m), 1.5 (8H, m), 1.8 (1H, m), 2.98 (6H, s), 4.10 (2H, m), 5.54(2H, s), 6.64 (2H, m), 7.14 (6H, m), 7.65 (2H, t, J = 8 Hz), 7.71 (2H, t, J = 8 Hz), 7.77 (2H, t, J = 8 Hz), 7.84 (2H, d, J = 8 Hz), 8.20 (2H, d, J = 8 Hz), 8.28 (2H, d, J = 8 Hz), 8.57 (2H, d, J = 8 Hz), 8.82 (4H, m), 9.03 (2H, s) ppm. ESI_MS([M−PF6]+): 1140.1 O26. Ru(N∧C∧N)(NDI-ttpy)BrPF6 (12.8 mg, 0.01 mmol) and 4-carboxylphenylboronic acid (5 mg, 0.02 mmol) were added into DMF (10 mL). The mixture was degassed for 20 min. Pd(PPh3)4 (2 mg) and K2CO3(13.8 mg/0.1 mL H2O)

magnitude faster than in solutions. The increase of electron transfer rate from Ru(II) chromophore to NDI acceptor at NiO interface is a result of ultrafast hole injection leading to formation of the reduced Ru sensitizer, which provides a higher driving force for NDI reduction. The charge recombination rate constant is slightly slower than a reference cyclometalated ruthenium compound (O25) with no NDI group. The fast formation and high yield of the charge separated state, along with slow back electron transfer rates make the Ru(II) chromophore−acceptor dyad promising as sensitizers for NiO p-DSSCs.

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EXPERIMENTAL SECTION General Information. All reagents, solvents, and the silica gel (60−200 μm) were purchased from Fisher Scientific Company and used without further purification. All products were characterized by 1H NMR on a Bruker instrument and high-resolution mass spectrometry (HRMS) on a Bruker Daltonics BioTOF system with electrospray ionization (ESI) source. Electrochemistry. Cyclic voltammetry (CV) and differential pulse voltammetry (DP) were conducted on a CV50W electrochemical workstation. All electrochemical measurements were performed in a homemade three-electrode cell consisting of a Pt working electrode, an Ag/AgCl reference electrode in saturated KCl, and a Pt wire auxiliary electrode. Then, 0.1 M tetrabutylammonium perchlorate (TBAP) was used as supporting electrolyte. All measurements were performed in DMF. The scan rate for CV is 100 mV/s, and the step for DP is 4 mV. The ferrocenium−ferrocene (Fc+/0) redox couple was used as internal reference. The potentials were converted and reported versus NHE by using Fc+/Fc couple (0.64 V versus NHE) as reference. Spectroelectrochemistry. The measurements were carried out in a 0.2 cm path-length quartz cuvette consisting of a 0.3 M Ag/AgCl reference electrode and a Pt flag working electrode, and a Pt wire counter electrode in 0.1 M TBAP DMF solutions. A CV 50 potentiostat was used to control the applied potential, and a UV spectrometer was used to monitor the absorbance changes at different applied potentials. The concentration of all solutions was adjusted to obtain less than 0.1 O.D. Femtosecond (fs) Transient Absorption Measurement. The fs TA measurements were carried out at the Center of Nanoscale Materials (CNM) of Argonne National Laboratory. A commercial transient spectrometer (Ultrafast systems, Hellos) was used. The detailed experiments and data analysis can be found in our previous report.21 The pump wavelength is 500 nm for all measurements, and the power is ∼500 μW. Synthesis. Ru(N∧C∧N)(CH3CN)3PF6. [RuCl2Bz]2 (125.5 mg, 0.25 mmol) and N∧CH∧N ligand (130 mg, 0.25 mmol) were added in 20 mL of CH3CN. KPF6 (188 mg, 1 mmol) and NaOH (25 mg) were added. The mixture was stirred at 45 °C for 15 h. After solvent was removed, the residue was purified on a Silica gel column using CH2Cl2 as the eluent. The title compound was purified by recrystallization in CH2Cl2/Et2O to give purple product. Yield: 101 mg, 64%. 1H NMR (400 MHz, CD3CN): δ 1.73 (6H, s), 2.79 (6H, s), 7.11 (1H, s), 7.44 (2H, t, J = 8 Hz), 8.02 (2H, t, J = 8 Hz), 8.25 (2H, m), 9.65 (2H, d, J = 8 Hz) ppm. Ru(N∧C∧N)(tpy)PF6. Ru(N∧C∧N)(CH3CN)3PF6 (31.5 mg, 0.05 mmol) and 2,2′:6′,2″-terpyridine (14 mg, 0.05 mmol) H



Article

were added. The mixture was heated at 85 °C under Ar for 24 h. After the mixture was cooled to room temperature, the mixture was filtered. The filtrate was collected, and the solvent was evaporated. The crude product was recrystallized in CH2Cl2/Et2O to give pure product 10 mg (75%). 1H NMR (400 MHz, CD3CN): δ 1.0 (6 H, m), 1.5 (8H, m), 1.8 (1H, m), 2.98 (6H, s), 4.10 (2H, m), 5.54(2H, s), 6.63 (2H, t, J = 6 Hz), 7.11 (4H, m), 7.26 (2H, m), 7.59 (4H, t, J = 8 Hz), 7.74 (2H, t, J = 8 Hz), 7.84 (2H, t, J = 8 Hz), 8.18 (2H, d, J = 8 Hz), 8.25 (4H, m), 8.57 (2H, d, J = 8 Hz), 8.79 (4H, m), 9.00 (2H, s) ppm. ESI_MS([M−PF6]+): 1180.2.

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(9) Li, L.; Duan, L.; Wen, F.; Li, C.-Y.; Wang, M.; Hagfeldt, A.; Sun, L. Visible Light Driven Hydrogen Production from a Photo-Active Cathode Based on a Molecular Catalyst and Organic Dye-Sensitized pType Nanostructured NiO. Chem. Commun. 2012, 48, 988−990. (10) Tong, L.; Iwase, A.; Nattestad, A.; Bach, U.; Weidlener, M.; Gotz, G.; Mishra, A.; Bauerle, P.; Amal, R.; Wallace, G.; Mozer, A. J. Sustained Solar Hydrogen Generation Using a Dye-Sensitised NiO Photocathode/BiVO4 Tandem Photo-Electrochemical Device. Energy Environ. Sci. 2012, 5, 9472−9475. (11) Qin, P.; Zhu, H.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A.; Sun, L. Design of an Organic Chromophore for p-Type Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2008, 130, 8570−8571. (12) Xiong, D.; Xu, Z.; Zeng, X.; Zhang, W.; Chen, W.; Xu, X.; Wang, M.; Cheng, Y. Hydrothermal Synthesis of Ultrasmall CuCrO2 Nanocrystal Alternatives to NiO Nanoparticles in Efficient p-Type Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 24760−24768. (13) Yu, M.; Natu, G.; Ji, Z.; Wu, Y. p-Type Dye-Sensitized Solar Cells Based on Delafossite CuGaO2 Nanoplates with Saturation Photovoltages Exceeding 460 mV. J. Phys. Chem. Lett. 2012, 3, 1074− 1078. (14) Gibson, E. A.; Smeigh, A. L.; Le Pleux, L.; Fortage, J.; Boschloo, G.; Blart, E.; Pellegrin, Y.; Odobel, F.; Hagfeldt, A.; Hammarström, L. A p-Type NiO-Based Dye-Sensitized Solar Cell with an Open-Circuit Voltage of 0.35 V. Angew. Chem., Int. Ed. 2009, 48, 4402−4405. (15) Srinivasan, R.; Chavillon, B.; Doussier-Brochard, C.; Cario, L.; Paris, M.; Gautron, E.; Deniard, P.; Odobel, F.; Jobic, S. Tuning The Size and Color of the p-Type Wide Band Gap Delafossite Semiconductor CuGaO2 with Ethylene Glycol Assisted Hydrothermal Synthesis. J. Mater. Chem. 2008, 18, 5647−5653. (16) Ardo, S.; Meyer, G. Photodriven Heterogeneous Charge Transfer with Transition-Metal Compounds Anchored to TiO2 Semiconductor Surfaces. Chem. Soc. Rev. 2009, 28, 115−164. (17) Smeigh, A.; Le Pleux, L.; Fortage, J.; Pellegrin, Y.; Blart, E.; Odobel, F.; Hammarstrom, L. Ultrafast Recombination for NiO Sensitized with a Series of Perylene Imide Sensitizers Exhibiting Marcus Normal Behaviour. Chem. Commun. 2012, 48, 678−680. (18) Borgstrom, M.; Blart, E.; Boschloo, G.; Mukhtar, E.; Hagfeldt, A.; Hammarström, L.; Odobel, F. Sensitized Hole Injection of Phosphorus Porphyrin into NiO: Toward New Photovoltaic Devices. J. Phys. Chem. B 2005, 109, 22928−34. (19) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarström, L. Coumarin 343-NiO Films as Nanostructured Photocathodes in DyeSensitized Solar Cells: Ultrafast Electron Transfer, Effect of the I3−/I− Redox Couple and Mechanism of Photocurrent Generation. J. Phys. Chem. C 2008, 112, 9530−9537. (20) Morandeira, A.; Fortage, J.; Edvinsson, T.; Le Pleux, L.; Blart, E.; Boschloo, G.; Hagfeldt, A.; Hammarström, L.; Odobel, F. Improved Photon-to-Current Conversion Efficiency with a Nanoporous p-Type NiO Electrode by the Use of a Sensitizer-Acceptor Dyad. J. Phys. Chem. C 2008, 112, 1721−1728. (21) Ji, Z.; Natu, G.; Huang, Z.; Kokhan, O.; Zhang, X.; Wu, Y. Synthesis, Photophysics, and Photovoltaic Studies of Ruthenium Cyclometalated Complexes as Sensitizers for p-Type NiO DyeSensitized Solar Cells. J. Phys. Chem. C 2012, 116, 16854−16863. (22) Freys, J. C.; Gardner, J. M.; D’Amario, L.; Brown, A. M.; Hammarström, L. Ru-Based Donor−Acceptor Photosensitizer That Retards Charge Recombination in a p-Type Dye-Sensitized Solar Cell. Dalton Trans. 2012, 41, 13105−13111. (23) Johansson, O.; Borgstrom, M.; Lomoth, R.; Palmblad, M.; Bergquist, J.; Hammarstrom, L.; Sun, L.; Åkermark, B. Electron Donor−Acceptor Dyads Based on Ruthenium(II) Bipyridine and Terpyridine Complexes Bound to Naphthalenediimide. Inorg. Chem. 2003, 42, 2908−2918. (24) Whittle, V. L.; Williams, J. A. G.; New, A. Class of Iridium Complexes Suitable for Stepwise Incorporation into Linear Assemblies: Synthesis, Electrochemistry, and Luminescence. Inorg. Chem. 2008, 47, 6596−6607. (25) Johnson, B. F. G.; Lewis, J.; Twigg, M. V. Kinetic Studies on The Reactions of The Tricarbonyl(1-3,6-η-cyclo-octadiene)−iron and

ASSOCIATED CONTENT

S Supporting Information *

Emission spectra at 77 K of O25 and O26 in alcoholic solutions; the absorption spectrum of single reduced O25 in a DMF solution. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(Y.W.) E-mail: [email protected]. 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. DyeSensitized Nanostructured p-Type Nickel Oxide Film as a Photocathode for a Solar Cell. J. Phys. Chem. B 1999, 103, 8940−8943. (2) Odobel, F.; Le Pleux, L.; Pellegrin, Y.; Blart, E. New Photovoltaic Devices Based on the Sensitization of p-Type Semiconductors: Challenges and Opportunities. Acc. Chem. Res. 2010, 43, 1063−1071. (3) Odobel, F.; Pellegrin, Y.; Gibson, E. A.; Hagfeldt, A.; Smeigh, A.; Hammarströ m, L. Recent Advances and Future Directions to Optimize the Performances of p-Type Dye-Sensitized Solar Cells. Coord. Chem. Rev. 2012, 256, 2414−2423. (4) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal Titanium Dioxide Films. Nature 1991, 353, 737−40. (5) Nattestad, A.; Mozer, A. J.; Fischer, M. K. R.; Cheng, Y. B.; Mishra, A.; Baeuerle, P.; Bach, U. Highly Efficient Photocathodes for Dye-Sensitized Tandem Solar Cells. Nat. Mater. 2010, 9, 31−35. (6) Song, W.; Brennaman, M.; Concepcion, J.; Jurss, J.; Hoertz, P.; Luo, H.; Chen, C.; Hanson, K.; Meyer, T. J. Interfacial Electron Transfer Dynamics for [Ru(bpy)2((4,4′-PO3H2)2bpy)]2+ Sensitized TiO2 in a Dye-Sensitized Photoelectrosynthesis Cell: Factors Influencing Efficiency and Dynamics. J. Phys. Chem. C 2011, 115, 7081−7091. (7) Young, K. J.; Martini, L. A.; Milot, R. L.; Snoeberger, R. C.; Batista, V. S.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W. Light-Driven Water Oxidation for Solar Fuels. Coord. Chem. Rev. 2012, 256, 2503−2520. (8) Youngblood, W. J.; Lee, S.-H. A.; Kobayashi, Y.; HernandezPagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E. Photoassisted Overall Water Splitting in a Visible LightAbsorbing Dye-Sensitized Photoelectrochemical Cell. J. Am. Chem. Soc. 2009, 131, 926−927. I



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−Ruthenium Complexes with Tertiary Phosphinesand Phosphites. J. Chem. Soc., Dalton Trans. 1974, 241−246. (26) Rogers, J. E.; Kelly, L. A. Nucleic Acid Oxidation Mediated by Naphthalene and Benzophenone Imide and Diimide Derivatives: Consequences for DNA Redox Chemistry. J. Am. Chem. Soc. 1999, 121, 3854−3861. (27) 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. Consequences of N,C,N′- and C,N,N′-Coordination Modes on Electronic and Photophysical Properties of Cyclometalated Aryl Ruthenium (II) Complexes. Inorg. Chem. 2009, 48, 1887−1900. (28) Wurthner, F.; Ahmed, S.; Thalacker, C.; Debaerdemaeker, T. Core-Substituted Naphthalene Bisimides: New Fluorophors with Tunable Emission Wavelength for FRET Studies. Chem.Eur. J. 2002, 8, 4742−4750. (29) Li, L.; Gibson, E. A.; Qin, P.; Boschloo, G.; Gorlov, M.; Hagfeldt, A.; Sun, L. Double-Layered NiO Photocathodes for p-Type DSSCs with Record IPCE. Adv. Mater. 2010, 22, 1759−1762. (30) Ji, Z.; Natu, G.; Huang, Z.; Wu, Y. Linker Effect in Organic Donor−Acceptor Dyes for p-Type NiO Dye Sensitized Solar Cells. Energy Environ. Sci. 2011, 4, 2818−2821. (31) Morandeira, A.; Boschloo, G.; Hagfeldt, A.; Hammarström, L. Photoinduced Ultrafast Dynamics of Coumarin 343 Sensitized pType-Nanostructured NiO Films. J. Phys. Chem. B 2005, 109, 19403− 10. (32) Caspar, J. V.; Meyer, T. J. Application of The Energy Gap Law to Nonradiative, Excited-State Decay. J. Phys. Chem. 1983, 87, 952− 957. (33) Hofmeier, H.; Schubert, U. S. Recent Developments in The Supramolecular Chemistry of Terpyridine−Metal Complexes. Chem. Soc. Rev. 2004, 33, 373−399. (34) Larson, S. C.; Elliott, L. F.; Kelley, C. M.; Charge, D. F. Separation in Donor−Chromophore−Acceptor Complexes: Inverted Region Behavior in Reverse Eectron Transfer Reactions. J. Am. Chem. Soc. 1992, 114, 9504−9509. (35) Thomas, K.; Biju, V.; Kamat, P.; George, M. V.; Guldi, D. Dynamics of Photoinduced Electron-Transfer Processes in FullereneBased Dyads: Effects of Varying the Donor Strength. ChemPhysChem 2003, 4, 1299−1307. (36) Boschloo, G.; Hagfeldt, A. Spectroelectrochemistry of Nanostructured NiO. J. Phys. Chem. B 2001, 105, 3039−3044. (37) Qin, P.; Wiberg, J.; Gibson, E. A.; Linder, M.; Li, L.; Brinck, T.; Hagfeldt, A.; Albinsson, B.; Sun, L. Synthesis and Mechanistic Studies of Organic Chromophores with Different Energy Levels for p-Type Dye-Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 4738−4748.

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Photoinduced Electron Transfer Dynamics of Cyclometalated

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