Molecular Orbital Engineering of a Panchromatic Cyclometalated Ru(II

Article pubs.acs.org/JPCC

Molecular Orbital Engineering of a Panchromatic Cyclometalated Ru(II) Dye for p‑Type Dye-Sensitized Solar Cells Mingfu He,† Zhiqiang Ji,†,‡ Zhongjie Huang,† and Yiying Wu*,† †

Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States S Supporting Information *

ABSTRACT: We report a panchromatic cyclometalated Ru(II) complex, denoted as O18, which shows intense metal-to-ligand charge transfer transitions in the visible to near-IR region with the absorption tail extending to 800 nm. The panchromatic spectra response and enhanced molar extinction coefficient (ε = 1.9 × 104 M−1 cm−1 at 593 nm in solution) of O18 are attributed to the stabilization of the lowest unoccupied molecular orbital and the increased absorption cross section via rationally extending the π-conjugated system of 2,2′-bipyridyl (bpy) ligands. As a result, NiO solar cells sensitized with O18 show short-circuit currents up to 3.43 mA cm−2 and efficiencies up to 0.104%, which are the best among all cyclometalated-Ru(II)-sensitized p-type dyesensitized solar cells. Femtosecond transient absorption spectroscopy reveals decreased geminate recombination across the O18−NiO interface by 1 order of magnitude compared to our previous report.

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INTRODUCTION In recent years, p-type dye-sensitized solar cells (p-DSCs) have attracted increasing research interest.1 In a p-DSC, the photocurrent is generated from the hole injection from photoexcited dyes into a p-type semiconductor, commonly NiO. This mechanism is the reverse of that in the conventional n-type dye-sensitized solar cells (n-DSCs). p-DSCs can be integrated with n-DSCs to form tandem solar cells,2 which can exceed the Shockley−Queisser limit to achieve higher efficiencies. Moreover, recent studies from us3 and others4 have demonstrated that a dye-sensitized photocathode can be used for hydrogen evolution in artificial photosynthesis. In this photocathode, a hydrogen evolution catalyst such as cobaloxime is incorporated onto dye-sensitized NiO nanoparticles via direct deposition4 or supramolecular assembly with dye molecules.3 To date, most of the high-efficiency p-DSCs are made of NiO, and the maxima efficiency of p-DSC is only 1.3%,5 which is much lower than that of n-DSCs.6 To improve the performances of p-DSCs, recent efforts focus on attaining higher open-circuit voltages. Alternative p-type semiconductors with lower-lying valence bands such as CuAlO27 and CuGaO28 have been used to replace NiO. Novel cobalt electrolytes, [Co(en)3]2+/3+, with an electrochemical potential 340 mV more negative than that of I−/I3−, has also been introduced.5 In contrast, little improvement in the short-circuit currents in p-DSCs has been achieved, mainly because of the lack of dyes with intense and broad absorption. Some donor-π-acceptor organic dyes2,9−12 and perylene monoimide-based dyes13 have been reported, most of which yield short-circuit currents below 3.5 mA cm−2. Only PMI-6TTPA2-based p-DSCs can achieve current higher than 5 mA cm−2 under 1 Sun conditions. However, the structure © XXXX American Chemical Society

complexity of PMI-6T-TPA, with regioregular hexakis(3hexylthiophenyl) as the π linker, renders its synthesis tedious and limits its wide application. Cyclometalated Ru(II) complexes were first applied in nDSCs by van Koten and co-workers14 and Grätzel and coworkers.15 By the optimization of structures and tuning of the electronic properties, these novel Ru(II) complexes have been developed into efficient thiocyanate-free sensitizers in n-DSCs in recent years.16−18 Among these complexes, cyclometalated Ru(II) complexes, denoted as [Ru(C∧N)(N∧N)2]+ in which N∧N is 2,2′-bipyridyl (bpy) and C∧N is anionic 2-phenylpyridyl bidendate ligand, feature visible and near-IR absorption profile because of the π-donor ability of the anionic C∧N ligand and tunable electrochemical potential via facile ligand modifications.19,20 Recently, we have reported cyclometalated Ru(II) complexes as sensitizers for p-DSCs. The representative O12 and O3 dyes are shown in Chart 1.21 We have found that by increasing the distance between the Ru(II) center and carboxylic anchoring group in O12, the interfacial charge recombination rate between holes in NiO and reduced sensitizer decreases, and the short-circuit current density of the devices increases.21 In addition, by incorporation of triphenylamino group as the linker between Ru(II) center and carboxylic anchoring group, we have achieved improved device performance with the largest short circuit current density of 3.04 mA cm−2 for O3.22 Special Issue: Michael Grätzel Festschrift Received: November 30, 2013 Revised: February 12, 2014

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dx.doi.org/10.1021/jp4117694 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Chart 1. Structure of Cyclometalated Ru(II) Complex (O12, O3, and O18)

Scheme 1. Synthetic Route of O18 Complex

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Because of the parasitic light absorption by NiO,23 only thin NiO films (less than 3 μm) can be applied in p-DSCs. Therefore, the p-DSC device performance is limited by the small absorption coefficients of our Ru(II) dyes. To further enhance their light harvesting ability, a promising approach is extension of the conjugated length of the bpy ligands. This approace is based on two reasons: (1) both our21 and others’24 results have shown that the lowest unoccupied molecular orbital (LUMO) of [Ru(C∧N)(N∧N)2]+ mainly consists of the π antibonding characteristics of bpy ligands, based on density functional theory (DFT) calculations. Thus, extending the πconjugated system could stabilize the LUMO and thus further red-shift the absorption profile; (2) previous reports25 also showed that the extension of bpy ligands can increase the absorption cross section, rendering dye molecules with enhanced molar extinction coefficients. In this work, we build on the success of the O3 dye and adopt the approach as discussed above by rationally extending the π-conjugated system of the bpy ligands. We report the facile synthesis of a new cyclometalated Ru(II) complex, denoted as O18, which shows intense and broad MLCT transition in the visible to near-IR region. We studied the device performance by side-by-side comparison with our previous best dye O3 and demonstrated that the O18-sensitized NiO p-DSCs exhibit about 25% enhancement of the short-circuit currents. Femtosecond transient absorption spectroscopy was carried out to investigate the interfacial charge-transfer dynamics and correlate it to the device performance.

RESULTS AND DISCUSSION The synthetic route of O18 is outlined in Scheme 1, and the detailed synthetic procedures are shown in the Experimental Section. 4,4′-distyryl-2,2′-bipyridine was synthesized via the Horner−Wadsworth−Emmons reaction from benzaldehyde and 4,4′-bis(diethylmethylphosphonate)-2,2′-bipyridine26 in 83% yield. The functionalized 2-phenylpyridyl ligand, TPAPhen-Pyr, was obtained via the Suzuki coupling reaction between 2-(3-bromophenyl)pyridine27 and tert-butyl 4,4′-(4(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenylazanediyl)dibenzoate28 in 90% yield. At room temperature, TPA-PhenPyr reacted with [(η6-p-cymene)RuCl2]2 dimer in the presence of KOAc to form the key intermediate, Ru(TPA-Phen-Pyr)(pcymene)Cl, which has been characterized by 1H NMR and electrospray ionization (ESI) mass spectroscopy. Ru(TPAPhen-Pyr)(p-cymene)Cl then reacted with 2 equiv 4,4′distyryl-2,2′-bipyridine ligands, and the tert-butyl carboxylate ester was hydrolyzed in LiOH ethanolic solution to yield O18 in the overall yield of 58%. O18 was characterized by 1H NMR, ESI mass spectroscopy, and elemental analysis. The synthesis of O3 was reported previously.22 The UV−vis absorption spectra of O3 and O18 in DMF solutions are shown in Figure 1a. O18 features broad absorption in the visible region with the absorption tail extending to 800 nm. Two intense metal-to-ligand charge transfer (MLCT) transition peaks are found at 451 nm (ε = 2.4 × 104 M−1 cm−1) and 593 nm (ε = 1.9 × 104 M−1 cm−1). Compared to O3, the lowest-energy MLCT band of O18 B



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Figure 1. UV−vis absorption spectra of (a) O18 and O3 in DMF solution, (b) O18- and O3-sensitized NiO films with thickness of 2.3 μm, and (c) normalized room-temperature absorption and 77 K emission spectra of O3 and O18 in ethanol/methanol (v/v = 4/1) (λex = 543 nm and 593 nm for O3 and O18, respectively.).

Table 1. Photophysical and Electrochemical Data of O18 and O3 dye

λ (nm) /ε ( × 104 M−1 cm−1)

λem, max (nm)

E0ox (V vs NHE)

E0red (V vs NHE)

E0−0a (eV)

E(S*/S−)b (V vs NHE)

E(S/S−)c (V vs NHE)

O18 O3

593/1.9, 451/2.4 543/1.1, 490/1.1

783 732

0.66, 1.25 0.59, 1.13

−1.18, −1.40 −1.49, −1.78

1.67 1.79

0.49 0.30

−1.18 −1.49

E0−0 is calculated from the intersection of the normalized room-temperature absorption and 77 K emission spectra. bE(S*/S−) = E(S/S−) − E0−0. E(S/S−) = E0red.

a c

shows red-shift by 50 nm and the molar extinction coefficient increases by about 2 fold. The superior spectral response of O18 is attributed to the extension of the π-conjugated length of bpy ligands, which can both lower the π* of bpy ligands and enhance the oscillator strength of MLCT transitions.25 This is confirmed by the DFT calculations showing that the LUMO energy level of O18 is about 0.4 eV lower than that of O3 (vide infra). The absorption spectra of O18- and O3-sensitized films and bare NiO film are shown in Figure 1b. The O18-sensitized NiO film shows panchromatic light-harvesting capability with broad and intense absorption in the whole visible and near-IR region, while the O3-sensitized film exhibits an absorption cut off at around 600 nm. As an example, at 600 nm, the O18- and O3-sensitized films can absorb 74% and 50% of the incident light, respectively. The emission spectra of O3 and O18 are shown in Figure 1c. They were recorded at 77 K in ethanol/ methanol glass because room-temperature emission was not observed. O3 and O18 have emission peaks at 732 nm and 783 nm, respectively, with similar Stokes shift of about 90 nm. The electrochemical properties of O18 and O3 are summarized in Table 1 and Figure 2. Both O3 and O18 show two reversible oxidation peaks, which are the oxidation of Ru(II) center and triphenylamino moiety, respectively, and two

Figure 2. Cyclic voltammograms of O18 in solution using 0.1 M TBAPF6 as the supporting electrolyte. Pt disk and Pt mesh were used as the working and counter electrodes, respectively.

reversible reduction peaks, corresponding to the subsequent reduction process of two bpy ligands. As shown in Table 1, the oxidation potentials of O18 at 0.66 and 1.25 V versus normal hydrogen electrode (NHE) are similar to those of O3 at 0.59 and 1.13 V versus NHE, which indicates that the functionalized C



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bpy ligands have negligible influence on the oxidation potential. This result is consistent with our DFT calculations showing that the electron density of the highest occupied molecular orbital (HOMO) is mainly delocalized on the Ru metal center and the TPA ligand,24 with no contribution from the bpy ligands (Figure 3). The first reduction potential of O18 at

Figure 3. Energy levels of the frontier orbitals and isodensity plots of the HOMO and LUMO of O3 and O18. Figure 4. (a) Characteristic J−V curves of O18-sensitized NiO p-DSC and O3-sensitized NiO p-DSC under optimized NiO thickness. (b) Dependence of short-circuit currents of O18-sensitized NiO p-DSC and O3-sensitized NiO p-DSC on the NiO film thickness under 1 Sun conditions.

−1.18 V is the one-electron reduction of 4,4′-distyryl-2,2′bipyridine ligands, which positively shifts by 0.31 V compared to the first reduction potential of O3. B3LYP/LanL2DZ DFT calculations were performed on O3 and O18 to optimize their ground-state geometry and determine their electronic structures, while time-dependent DFT (TD-DFT) calculations were carried out to simulate electronic transitions of O3 and O18. Figure 3 summarizes the energy levels of frontier orbitals and isodensity plots of HOMO and LUMO of O3 and O18. It can be observed that for both O18 and O3, the HOMOs are mainly localized on the Ru center and partially delocalized onto the anionic 2-phenylpyridyl ligands. The HOMO energy level of O18 is only about 0.07 eV smaller than that of O3, which is consistent with our electrochemical results. The LUMOs of O3 and O18 mainly consist of the π* character of bpy ligands, and their energy levels show a dramatic difference: the LUMO energy level of O18 decreases by about 0.4 eV in comparison to that of O3. This dramatic difference can be rationalized by the extension of π-conjugated system in 4,4′-distyryl-2,2′-bipyridine ligands in O18. The decrease in the LUMO energy level of O18 results in the intense near-IR MLCT transitions centering ca. 588 nm, as shown in Figure S1 of Supporting Information. NiO films with different thickness were prepared and soaked in 0.14 mM O18 or O3 dye solution for 16 h. The electrolyte consisting of 0.1 M I2/1 M LiI in CH3CN was used. Detailed procedures of NiO film preparation and solar cell assembly can be found in Experimental Section. As shown in Figure 4b, at the same film thickness, O18-sensitized NiO solar cells can achieve much higher short-circuit currents than O3-sensitized NiO solar cells. For example, when 0.6 μm thick NiO films were used, the short-circuit currents of O18- and O3-sensitized NiO solar cells were 1.77 and 0.78 mA cm−2, respectively. Increasing the film thickness results in the increased short-circuit currents, but when the film is thicker than 2.5 μm, further increase in the film thickness reduces the short-circuit currents. This can be rationalized by the increased parasitic absorption of thicker NiO films. Our optimized NiO thickness is around 2.3 μm.

For both dyes, three 2.3 μm thick NiO films were used to fabricate solar cells, and the average device performances are summarized in Table 2. The characteristic J−V curves of these Table 2. Device Performance of O18-Based p-DSCs and O3Based p-DSCs under 1 Sun Conditionsa device

Jsc (mA/cm2)

Voc(mV)

FF

η (%)

O18−NiO O3−NiO

3.43 ± 0.13 2.75 ± 0.11

93 ± 3 100 ± 10

0.33 ± 0.01 0.36 ± 0.01

0.104 ± 0.005 0.100 ± 0.002

a Three NiO films were used for each dye. Average film thickness was around 2.3 μm. Film area: 0.283 cm2. Electrolyte: 0.1 M I2/1 M LiI in MeCN.

optimized devices are shown in Figure 4a. Under simulated AM 1.5 irradiation at 100 mW cm−2, the short-circuit current of O18-based p-DSC is 3.43 mA cm−2, which is 0.68 mA cm−2 larger than that of O3-based p-DSC, and the open-circuit voltage of O18-based p-DSC is 93 mV, slightly smaller than that of O3-based p-DSC. Thus, O18-based p-DSC has an efficiency of 0.104%, slightly larger than the 0.100% efficiency of the O3-based p-DSC. The short-circuit current of the O18based p-DSC is the highest among our Ruthenium(II)complex-based p-DSCs and, to the best of our knowledge, also superior to most p-DSCs using organic dyes. We attribute the higher photocurrent in O18-p-DSC to the intense and panchromatic absorption profile of O18 that leads to better light-harvesting ability. One of the main differences of NiO DSCs from the typical TiO2 DSCs is the fast geminate recombination. In our previous report,21 we observed approximately picosecond time domain charge recombination between the injected holes in NiO and electrons at the reduced sensitizer across the dye−NiO D



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O18 and a transient absorption extending to the near-IR region. The ground state recovers, whereas the transient absorption decays, with an isosbestic point at ΔA = 0. As we discussed in our previous work, the fast formation of the transient species (∼180 fs) within our laser pulse width is due to the fast hole injection forming the interfacial chargeseparated state, and the recovery of the ground states is attributed to the electron−hole pair recombination. The ground-state recovery kinetics and the global fitting kinetic curves for O18−NiO and O3−NiO are shown in Figure 6c,d. The kinetics can be adequately fitted by eq 1 with two lifetimes and a residue. The fitted kinetic curves are in good agreement with the experimental data. The fitted lifetimes and the amplitudes are shown in Table 3. The average lifetimes (trec) are calculated by eq 2. The geminate recombination time constants, trec, average, are 1.7 and 1.1 ns for O18 and O3, respectively.

interface, which has been a major limitation of device performance. Here we performed femtosecond transient absorption (TA) spectroscopy on the O3−NiO and O18− NiO interfaces to rationalize the differences in their shortcircuit currents. We first studied the femtosecond TA spectra of O3 and O18 in dilute solutions to achieve understanding of the spectra and kinetics of their excited state. The pump wavelength of 500 nm was employed to selectively excite the lowest-energy MLCT band. The excitation power was 500 μW. In our previous studies, we found that the experiment is in the first-order process as long as the laser intensity is lower than 900 μW. Figure 5 shows the difference absorption spectra of O3 and

ΔOD = A 0 + A1e−t / t1 + A 2 e−t / t2

(1)

trec,average = (A1t12 + A 2 t 2 2)/(A1t1 + A 2 t 2)

(2)

The geminate recombination time constants for O3−NiO and O18−NiO are much larger than that of our previous O12− NiO (Chart 1), which is in line with the obtained Jsc value, presumably because of the incorporation of triphenylamino that increases electron transfer distance for geminate recombination. Longer lifetime of the interfacial charge-separated state provides more probabilities for the reduced dye to be regenerated by the electrolyte. In a p-DSC, the regeneration reaction is the reduction of the triiodide in solution, and the kinetics of this reaction has not been fully understood. According to the kinetics of regeneration in a TiO2 DSC,29 this reaction occurs in the nanosecond to microsecond time domain. The slow dye regeneration applies a kinetic obstacle for the device performance. The recombination lifetime of 1.7 ns of O18−NiO is about 1−2 orders magnitude larger than those in some donor−π-acceptor organic dyes10 and perylene monoimide dyes,13 which are in the range of 10−100 ps.

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CONCLUSIONS We report the facile synthesis of a novel cyclometalated Ru(II) complex, the O18 dye, which shows panchromatic absorption and high molar extinction coefficients. We obtained a photocurrent of 3.43 mA cm−2 on O18-sensitized NiO pDSCs, which is superior to most reported organic dyes and the highest one in our Ru(II)-based p-DSCs. The improved solar cell performance is due to the enhanced light-harvesting capability and the decreased geminate charge recombination kinetics revealed by femtosecond TA studies.

Figure 5. Transient difference absorption spectra of (a) O3 and (b) O18 in MeCN solution.

O18 in MeCN. Upon excitation, both O3 and O18 feature ground-state bleaching in the visible region and broad transient absorption bands in the near IR region with isosbestic point at 615 and 674 nm for O3 and O18, respectively. The negative peaks are 547 and 586 nm for O3 and O18, respectively, which roughly mirrors the ground-state absorption peaks for O3 and O18 in solution. The bleaching of O18 shows red-shift compared to O3, consistent with their absorption spectra. The excited-state absorption of both dyes persist within our experimental window (∼3 ns). The transient absorptions are assigned to long-lived 3MLCT states. Figure 6a,b displays the transient difference absorption spectra of O18- and O3-sensitized NiO films at different delay times. Both samples show similar absorption spectra. The difference absorption spectra feature the ground-state bleaching from 500 to ca. 610 nm for O3 and from 500 to ca. 680 nm for

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EXPERIMENTAL SECTION General Information. Benzaldehyde, potassium tertbutoxide, tetrakis(triphenylphosphine)palladium(0), dichloro(p-cymene)ruthenium(II) dimer ([Ru(p-cymene)2Cl 2]2 ), K 3 PO 4 , 4,4′-dimethyl-2,2′-bipyridine, potassium acetate (KOAc), silver triflate, ammonium hexafluorophosphate (NH 4 PF 6 ), concentrated H 2 SO 4 , sodium borohydride (NaBH4), ammonium chloride (NH4Cl), magnesium sulfate (MgSO4), 48% HBr, sodium hydroxide (NaOH), triethylphosphite, lithium hydroxide (LiOH), and hexafluorophosphoric acid (HPF6) were purchased from Sigma-Aldrich or Fisher Scientific. E



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Figure 6. Transient absorption spectra of (a) O18-sensitized and (b) O3-sensitized NiO films at different delay times with λpump= 500 nm. Kinetic and the global fitting traces of (c) O18-sensitized and (d) O3-sensitized NiO films probing at four different wavelength.

Table 3. Fitting Time Constants and Amplitudes of O18- and O3-Sensitized NiO Films t1 (ps), A1 t2 (ps), A2 trec, averagea (ps) a

O18

O3

129 ± 9, −(2.95 ± 0.01) × 10−3 1830 ± 112, −(2.47 ± 0.01) × 10−3 1698

105 ± 18, −(0.91 ± 0.11) × 10−3 1157 ± 291, −(1.01 ± 0.01) × 10−3 1077

trec, average = (A1t12 + A2t22)/(A1t1 + A2t2)

tert-Butyl 4,4′-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)phenylazanediyl)dibenzoate (Bor-TPA),28 2,2′-bipyridine4,4′-dicarboxylic acid,26 and 2-(3-bromophenyl)pyridine27 were synthesized according to published procedures. 1, 2, 3, and P-Bpy were synthesized with modified procedures.26 1H NMR was carried out on a Bruker DPX 250 or Avance III 400 instrument. ESI-MS was carried out on a BrukerMicroTOF (ESI) equipped with an Agilent 1200 LC. Synthesis of O18. 4,4′-Distyryl-2,2′-bipyridine (PhenBpy). P-Bpy (228 mg, 0.5 mmol), benzaldehyde (133 mg, 1.25 mmol), and potassium tert-butoxide (140 mg, 1.25 mmol) were dissolved into 10 mL of anhydrous THF. The solution was refluxed for 6 h. After the solution was cooled to room temperature, the white precipitate was filtered, washed with ethyl acetate and water, and dried in an oven. Yield: 150 mg (83%). 1H NMR (400 MHz, d6-DMSO, δ): 8.70 (d, J = 5.1 Hz, 2H), 8.58 (s, 2H), 7.74 (d, J = 7.3 Hz, 4H), 7.69 (dd, J = 5.1, 1.5 Hz, 2H), 7.64 (d, J = 16.5 Hz, 2H), 7.49−7.39 (m, 6H), 7.37 (d, J = 7.3 Hz, 2H). TPA-Phen-Pyr. 2-(3-bromophenyl)pyridine (117 mg, 0.5 mmol), Bor-TPA (314 mg, 0.55 mmol), and 2.2 mL of 2 M K3PO4 aqueous solution were added into 10 mL of ethylene

glycol dimethyl ether. The mixture was degassed and refilled with argon three time before and after tetrakis(triphenylphosphine)palladium(0) (6 mg, 0.005 mmol) was added into the solution. After the resulting solution was refluxed for 25 h, it was poured into water and extracted with ethyl acetate. The organic phase was collected and dried over anhydrous MgSO4. After the solvent was removed, the crude product was loaded onto silica gel and eluted with hexane/ethyl acetate (8/1). Yield: 270 mg (90%). 1H NMR (400 MHz, CDCl3, δ): 8.73 (d, J = 4.8 Hz, 1H), 8.24 (t, J = 1.6 Hz, 1H), 7.99−7.93 (m, 1H), 7.91−7.85 (m, 4H), 7.81 (d, J = 3.4 Hz, 2H), 7.67−7.60 (m, 3H), 7.55 (dd, J = 15.2, 7.5 Hz, 1H), 7.33− 7.27 (m, 1H), 7.23−7.16 (m, 2H), 7.16−7.07 (m, 4H), 1.59 (s, 18H). ESI-MS: m/z 599.4 [M+H+], 621.3 [M+Na+]. Ru(TPA-Phen-Pyr)(p-cymene)Cl. The procedure was modified from the synthesis of an analogous compound.30 TPAPhen-Pyr (270 mg, 0.45 mmol), [Ru(p-cymene)2Cl2]2 (138 mg, 0.0225 mmol) and KOAc (66 mg, 0.675 mmol) were dissolved into 8 mL of anhydrous acetonitrile. The solution was stirred at room temperature for 48 h under argon. After the solvent was removed, the crude product was loaded onto a basic Al2O3 column and eluted with dichloromethane and F



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DFT and TD-DFT Calculations. All computations were carried out using Gaussian 0931 software package in the Ohio Supercomputer Center. The geometries of both dyes were optimized using B3LYP/LanL2DZ DFT calculation (charge, 0; multiplicity, singlet), and further frequency analysis confirmed that the optimized structures were at the ground state. The UV−vis absorption spectra in MeCN were obtained via TDDFT calculation of the optimized structures using B3LYP/ LanL2DZ and the solvent model of IEFPCM. The symmetry was ignored, and only singlet excitations were considered. The simulated absorption spectra were obtained by setting the peak half-width at half height to be 1600 cm−1 in GaussView 5.0.8. Solar Cell Fabrication and Measurement. NiO films were prepared with a modified reported sol−gel method.32,33 NiCl2 solution was prepared as follows: 0.2 g of anhydrous NiCl2 and 0.18 g of copolymer F-108 were grinded and dissolved into 0.5 g of distilled water/1 g of anhydrous ethanol mixed solvent. The clear green solution was heated at 30 °C for 3 days. The solution was deposited onto the FTO glass via the doctorblading method and heated at 450 °C for 30 min. NiO films with different thickness were obtained via several doctorblading and heating cycles. The counter electrodes were prepared via the thermal deposition of H2PtCl6 solution at 385 °C for 20 min. The thickness of NiO films was determined using an AlphaStep D-100 profilemeter from KLA-Tencor corporation. NiO films were immersed in 0.14 mM dye solution in anhydrous acetonitrile for 16 h. After being rinsed thoroughly with acetonitrile and dried in the air, the NiO working and Pt counter electrodes were assembled by heating with a Surlyn60 film. The electrolyte solution (0.1 M I2/1 M LiI in MeCN) was filled from a predrilled hole in the counter electrode by applying a vacuum. The J−V curves were recorded under 100 mW cm−2 AM 1.5 illumination generated by a solar simulator (Small-Area Class B, Solar Simulator, PV Measurements, Inc.).

methanol. The yellow band was collected. After the solvent was removed, the product was dissolved into a small amount of dichloromethane and precipitated out by hexane to afford a yellow product. Yield: 203 mg (52%). 1H NMR (400 MHz, CD3CN, δ): 9.35 (d, J = 5.7 Hz, 1H), 8.27 (d, J = 7.9 Hz, 1H), 7.99 (dd, J = 11.0, 5.0 Hz, 2H), 7.93−7.86 (m, 4H), 7.85−7.79 (m, 1H), 7.74 (d, J = 8.6 Hz, 2H), 7.45 (dd, J = 7.9, 2.0 Hz, 1H), 7.23 (m, 3H), 7.16−7.05 (m, 4H), 5.72 (dd, J = 5.9, 1.8 Hz, 2H), 5.36 (d, J = 6.0 Hz, 1H), 5.08 (d, J = 6.0 Hz, 1H), 2.38 (m, 1H), 2.03 (s, 3H), 1.58 (s, 18H), 0.96 (d, J = 6.9 Hz, 3H), 0.85 (d, J = 6.9 Hz, 3H). ESI-MS m/z: 833.4 [M-Cl]+. O18-Ester. Ru(TPA-Phen-Pyr)(p-cymene)Cl (60 mg, 0.07 mmol), Phen-Bpy (50 mg, 0.14 mmol), and silver triflate (18 mg, 0.07 mmol) were added into 10 mL of ethanol and brought to reflux for 3 h. After the solvent was removed, the crude product was loaded onto a basic alumina column and eluted with dichloromethane and methanol. The dark green band was collected and removed of solvent. The black solid was dissolved into a small amount of methanol and precipitated out by adding saturated NH4PF6 methanol solution to afford a black solid. Yield: 80 mg (73%). ESI-MS m/z: 1419.7 [M-PF6+H+]. The compound was used without further characterization. O18. O18-ester (80 mg, 0.05 mmol) was added into 10 mL of anhydrous methanol/2 M LiOH methanol solution (v/v = 1/1). The solution was refluxed for 2 h until all O18-ester was converted into O18. The solution was concentrated and added into distilled water. After the pH was adjusted to be around 1 via several drops of HPF6 solution, the black precipitate was filtered and washed with water and diethyl ether. The solid was then dissolved into a small amount of DMF and precipitated out by adding diethyl ether. The solid was filtered and dried in an oven overnight. Yield: 58 mg (80%). 1H NMR (400 MHz, CD3CN, δ): 8.78 (s, 1H), 8.71 (s, 1H), 8.60 (d, 2H), 8.20 (d, J = 8.4 Hz, 1H), 8.14 (s, 1H), 8.02 (d, J = 5.4 Hz, 1H), 7.88− 7.82 (m, 5H), 7.81−7.59 (m, 14H), 7.58−7.31 (m, 12H), 7.31−7.14 (m, 7H), 7.18 (d, J = 8.5 Hz, 3H), 7.07 (d, J = 8.7 Hz, 3H), 7.01 (m, 2H), 6.68 (d, J = 7.8 Hz, 1H). ESI-MS m/z: 1307.3 [M-PF6+H]+. Anal. Calcd. for C83H61F6N6O4PRu· 2DMF·2H 2O: C, 65.39; H, 4.87; N, 6.85. Found: C, 62.06; H, 4.51; N, 7.02 Photophysical Characterization. UV−vis absorption spectra of dye and dye-sensitized NiO films were recorded using a Perkin-Elmer Lambda 950 instrument. The dye solutions were prepared in anhydrous DMF solution with the concentration of 3 × 10−5 M for both dyes. The emission spectra of dye solution (3 × 10−5 M in ethanol/methanol, 4/1) were recorded at 77 K using a Fluoromax-4 from Horiba Scientific. Electrochemical Measurement. Cyclic voltammetry was recorded using a Gamry potentiostat. The working, counter, and reference electrodes were Pt dish, Pt mesh, and Ag/0.01 M AgNO3 electrode, respectively. The electrolyte solution was 1 mM O18/0.1 M tetrabutyl ammonium hexafluorophosphate in anhydrous acetonitrile or DMF. The potential of the reference electrode was calibrated using the ferrocene electrolyte as the standard (E° (Fc+/Fc) = 0.640 V vs NHE). The electrolyte was purged with argon for 20 min before each measurement. Femtosecond Transient Absorption Spectroscopy. Experiments were carried out at the Center of Nanoscale Materials (CNM) of Argonne National Laboratory. The pump wavelength is 500 nm with the power of ∼500 μW. Detailed experiment setups and data analysis are contained in our previous report.21

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ASSOCIATED CONTENT

S Supporting Information *

The energy level of the frontier orbitals of O18 and O3, simulated UV−vis absorption profile and transitions of O18 in MeCN, and isodensity plots of representative orbitals of O18. 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]. Present Address ‡

(Z.J.) Chemistry Division, C-IIAC, Los Alamos National Laboratory, Los Alamos, NM 87545 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 DE-AC02-06CH11357. G



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Article

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Molecular Orbital Engineering of a Panchromatic Cyclometalated Ru(II

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