coordinated Dye Ligands on Photocurrent Responses

Apr 16, 2018 - synergistic effect on the enhancement of photocurrent responses was found and discussed in view of the inter-dyes electron communicatio...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

A Titanium Oxo Cluster Model Study of Synergistic Effect of Cocoordinated Dye Ligands on Photocurrent Responses Jin-Le Hou,† Peng Huo,‡ Zheng-Zhen Tang,† Li-Na Cui,† Qin-Yu Zhu,*,† and Jie Dai*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China Department of Aeronautics and Astronautics, Fudan University, Shanghai 200433, P. R. China

‡

S Supporting Information *

ABSTRACT: The use of multiple sensitizers in dye sensitized solar cells has been attractive as a promising way to achieve highly efficient photovoltaic performance. However, except for the complementary absorption, synergistic effects among the dye components have not been well understood. Herein, using ferrocene-1-carboxylate (FcCO2) and catechol (Cat) as dye ligands, two titanium oxo clusters (TOCs), [Ti3O(OiPr)6(Cat)(FcCO2)2] (1) and [Ti7O4(OiPr)8(Cat)5(FcCO2)2] (2), were synthesized and structurally characterized. Another TOC, [Ti7O3(OiPr)12(Cat)4(o-BDC)] (3) (o-BDC = o-benzene dicarboxylate), was also prepared as a contrast. Electronic spectra and theoretical calculations showed that charge transfer occurs from ligands FcCO2 and Cat to the TiO cluster core and the contribution of redox active FcCO2 is greater than that of Cat. Using the clusters as TiO-dye pre-anchored precursors, multidye sensitized TiO2 electrodes were prepared. Although the two dyes FcCO2 and Cat do not complement each other in spectra, a synergistic effect on the enhancement of photocurrent responses was found and discussed in view of the inter-dyes electron communication.

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INTRODUCTION Dye sensitized solar cells (DSSCs) have drawn considerable attention due to their high photoelectric conversion efficiency and low production cost.1 The performance of DSSCs generally depends on the energy levels of the dyes which should match with those of the conductive bands of semiconductors,1,2 and the anchoring of the dye molecules which should be effectively bonded to the surface of the semiconductor.3 Besides optimization of the sensitizer’s structures, another regarded strategy to enhance the performance of DSSCs is the modification of TiO2 electrodes with multi-dye components. Using a combination of dyes that complement each other in their absorption properties can obtain a broad absorption that extends throughout the visible and near-infrared region.4 However, there might be other synergistic effects among the dye components, such as electron push−pull or charge transfer (CT) effects, spatial isolation effect, and so on. These aspects have not been well understood and are worth further studying. In recent years, titanium oxo clusters (TOCs) or polyoxotitanates (POTs) have been attractive as model compounds of nano TiO2 due to their accurate structural information that can be directly used in theoretical research.5,6 TOCs coordinated with some organic ligands have been used as model compounds for spectroscopic studies of light absorption of a photosensitizer on semiconductor surfaces.7 Recently, we reported a series of dye molecule coordinated TOCs for the research of photocurrent responses.8 Since most of the TOCs have alkoxide ligands, with the reactions of © XXXX American Chemical Society

dealkoxide and condensation, the dye-pre-anchored TiO clusters can be fused on the surface of the TiO2 substrate to give an effective dye-TiO2 anchoring, and consequently, the photocurrent properties improve.9 Coppens and Benedict have prepared a series of model TOCs with catechol (Cat) ligand as the photosensitizer.7a The electronic spectra and theoretical calculations indicated that charge transfer occurs from Cat to the Ti(IV) cluster core. We have studied the photocurrent response properties of TiO2 electrodes modified with Cat functionalized TOCs.10 The results showed that charge transfer from Cat to the TiO cluster core plays an effective role in enhancement of photocurrent conversion under visible light irradiation. Recently, ferrocene-1carboxylate (FcCO2) modified TOCs, FcCO2-TOCs, were reported by our group and the Fan group, and similar photoelectric behaviors as that of Cat-TOCs were found.11 To study the synergistic effects of multiple dyes on the photocurrent properties of the TiO2 electrodes, herein, two TOCs, [Ti3O(OiPr)6(Cat)(FcCO2)2] (1) and [Ti7O4(OiPr)8(Cat)5(FcCO2)2] (2), were synthesized and characterized, in which the TiO cluster core is coordinated by both FcCO2 and Cat ligands as photosensitizers. Another TOC, [Ti7O3(OiPr)10(Cat)4(o-BDC)] (3) (o-BDC = o-benzene dicarboxylate or phthalic acid), was also prepared as a contrast. Using the clusters as TiO-dye pre-anchored precursors, multi-dye Received: April 16, 2018

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DOI: 10.1021/acs.inorgchem.8b01050 Inorg. Chem. XXXX, XXX, XXX−XXX

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geometry and refined with fixed isotropic displacement parameters. Relevant crystal data, collection parameters, and refinement results can be found in Table S1. Electrode Preparation and Photocurrent Measurement. A uniform porous TiO2/ITO substrate (about 30 μm thickness) was prepared according to the literature12 except that the ITO substrate is 7 Ω/cm2. The dye sensitized TiO2 electrodes for photocurrent measurements were prepared by immersing the TiO2/ITO electrode in the solution of the TiO clusters or ligands (generally, in 1.0 × 10−3 mol·L−1, trichloromethane), respectively, for 20 h and then the electrodes were dripped and dried in air. Photocurrent measurement is carried out similar to the literature.12 Theoretical Calculations. Density functional theory calculations were carried out using the Gaussian 09 program package for 1 and 2 at the B3LYP level. The basis set used for C, O, and H atoms was 6-31G, while effective core potentials with a LanL2DZ basis set was employed for Ti and Fe atoms. The crystal structures were used as the initial structures and the OiPr groups were replaced by OH groups to cut the computational cost with no significant change in the electronic properties and optimized to the minimum energy configurations.

sensitized TiO2 electrodes were prepared. A synergistic effect on the enhancement of photocurrent responses was found in contrast with those of single sensitizer systems. An electron communication mechanism between the ligands is proposed.

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EXPERIMENTAL SECTION

General Remarks. All analytically pure reagents were purchased commercially and used without further purification. Elemental analyses of C, H, and N were performed using a VARIDEL III elemental analyzer. The FT-IR spectra were recorded as KBr pellets on a Nicolet Magna 550 FT-IR spectrometer. The Raman spectra were recorded on a RENISHAW inVia Raman Microscope. Solid-state room-temperature optical diffuse reflectance spectra were obtained with a Shimadzu UV-2600 spectrometer using BaSO4 as a standard reference. Roomtemperature X-ray diffraction data were collected on a D/MAX-3C diffractometer using a Cu tube source (Cu-Kα, λ = 1.5406 Å). X-ray photoelectron spectroscopic (XPS) measurements were recorded on an ESCALAB 250Xi spectrometer. The morphologies of the electrodes were observed with a JSM-5600LV scanning electron microscope (SEM). Cyclic voltammetry (CV) experiments were performed on a CHI660 electrochemistry workstation in a three-electrode system, a sample modified porous TiO2 working electrode or a surface-modified Pt-plate working electrode for crystals 1, 2, and 3, a saturated calomel electrode (SCE) as reference electrode, and a Pt plate as the auxiliary electrode. Synthesis of the Clusters. [Ti3O(OiPr)6(Cat)(FcCO2)2] (1). Analytically pure Ti(OiPr)4 (0.1 mL, 0.33 mmol), FcCO2H (0.0136 g, 0.06 mmol), and catechol (0.01 g, 0.1 mmol) were mixed in 0.2 mL of mixed solvent of acetonitrile−isopropanol (1:1 in volume). The mixture was sealed in a thick glass tube and degassed quickly with argon. The sealed tube was heated under autogenous pressure at 60 °C for 7 days, and then cooled to −18 °C to yield red crystals (47% yield based on Ti(OiPr)4). The crystals were rinsed with acetonitrile and dried for measurements. Anal. Calcd for C46H64Fe2O13Ti3 (MW 1080.37): C, 51.14; H, 5.97. Found: C, 50.82; H,5.70. Selected IR data (KBr, cm−1): 2972(w), 1528(m), 1476(s), 1391(m), 1360(m), 1256(m), 1118(s), 987(s), 930(m), 823(s), 777(w), 740(w), 626(vs). [Ti7O4(OiPr)8(Cat)5(FcCO2)2]·2CH3CN (2). Cluster 2 was prepared by following a similar procedure to that of 1 except for the mole ratio. Ti(OiPr)4 (0.2 mL, 0.66 mmol), FcCOOH (0.0136 g, 0.06 mmol), and catechol (0.040 g, 0.36 mmol) were mixed in 0.3 mL of mixed solvent of acetonitrile−isopropanol (2:1 in volume). The mixture was sealed in a thick glass tube and degassed quickly by argon. The sealed tube was heated under autogenous pressure at 100 °C for 7 days, and then cooled to −18 °C to yield dark-red crystals (55% yield based on Ti(OiPr)4). Anal. Calcd for C80H100Fe2O26N2Ti7 (MW 1952.53):C, 49.21; H, 5.16; N, 1.43. Found: C, 48.93; H, 5.26; N, 1.25. Selected IR data (KBr, cm−1): 2971(w), 1509(w), 1478(s), 1398(w), 1362(w), 1200(w), 1112(s), 1003(vs), 823 (s), 732(s), 698(w), 620(vs). [Ti7O3(OiPr)12(Cat)4(o-BDC)] (3). Analytically pure Ti(OiPr)4 (0.1 mL, 0.33 mmol), phthalic acid (0.0075 g, 0.045 mmol), and catechol (0.005 g, 0.045 mmol) were mixed in 0.25 mL of mixed solvent of isopropanol−acetone (4:1 in volume). The mixture was sealed in a thick glass tube and degassed quickly by argon. The sealed tube was heated under autogenous pressure at 60 °C for 7 days, and then cooled to room temperature to yield red crystals (38% yield based on Ti(OiPr)4). Anal. Calcd for C68H104O27Ti7 (MW 1688.81):C, 48.36; H, 6.21. Found: C, 48.09; H, 6.26. Selected IR data (KBr, cm−1): 2976(m), 1731(m), 1560(vs), 1479(m), 1430(s), 1258(m), 1127(s), 1005(m), 826(w), 740(m), 631(s). X-ray Crystallographic Study. The measurements of 1−3 were carried out on a Rigaku Mercury CCD diffractometer at room temperature with graphite monochromated MoKα (λ = 0.71075 Å) radiation. X-ray crystallographic data were collected and processed using CrystalClear (Rigaku 2004). The structure was solved by direct methods using SHELXS-97 for 1−3, and the refinement against all reflections of the compound was performed using SHELXL-16 for 1 and 2 and SHELXL-14 for 3. All the non-hydrogen atoms are refined anisotropically. The hydrogen atoms are positioned with idealized

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RESULTS AND DISCUSSION

Clusters 1−3 were prepared by one-step in situ solvothermal synthesis at a temperature of 60−100 °C (see the Experimental Section). All the bulk samples 1−3 were obtained as red or dark-red crystals (Figure S1 in the Supporting Information) and were isolated by carefully hand-picked under microscope. Their identity with the single crystal used in structure analysis was confirmed by comparing the experimental XRD pattern with the calculated pattern from the crystal data (Figure S2). The clusters are well soluble in trichloromethane, which is a desired property for solution treatment in electrode fabrication. Cyclic voltammogram (Figure S3) showed that clusters 1 and 2 are redox active, but no redox peak of cluster 3 was found. Therefore, the redox peak in 1 and 2 is contributed by the FcCO2 ligand. The ball-and-stick plots of 1 and 2 are shown in Figure 1a,b (the isopropyl groups are omitted for clarity). The structure of 1, [Ti3O(OiPr)6(Cat)(FcCO2)2], is a trinuclear oxo-cluster with a Ti3O core linked by a μ3-O oxo-bridge. All the three

Figure 1. Ball-and-stick plots of 1 (a) and 2 (b), showing the molecular structures. The isopropyl groups are omitted for clarity. Clusters of 1 (c) and 2 (d) (two co-crystallized CH3CN solvent molecules being omited for clarity), illustrating the coordination polyhedra of each Ti and the coordination of FcCO2 and Cats. B

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take octahedral geometry. The cluster is irregular and can be described as one saddle-shaped Ti4O16 co-sharing an edge with a Ti3O13 cluster (Figure 2b). Figure 3a shows the UV−vis absorption spectra of 1, 2, FcCO2H, and CatH2 in the solid state, which are calculated from the data of diffuse reflectance. In comparison with the absorption band of FcCO2H in 468 nm, the spectra of 1 and 2 show a broader band with higher intensity in the range of 350− 650 nm. Those lowest energy bands can be assigned to CT bands from both FcCO2 and Cat to the TiO cluster core based on theoretical DFT calculations (discussed below). The CT bands contributed by FcCO2 and Cat are around the similar absorption range, and therefore, the two dyes are not complement in absorption. Although the CatH2 shows no absorbance above 400 nm, compound 3, a Cat-TiO cluster, also shows a broad band centered at 465 nm (Figure 3b), which has been demonstrated as a CT band when ligand Cat is coordinated to the TiO cluster core.7a,10 Theoretical DFT calculations of compounds 1 and 2 based on the crystal data were performed. The molecular orbital analyses of 1 and 2 (Figure 4a,b), obtained from B3LYP/631G/LanL2DZ calculations, show that most coefficients of the highest occupied molecular orbitals (HOMO, HOMO−1, and HOMO−2) are located on the Cat and FcCO2 moiety, and the lowest unoccupied frontier molecular orbitals (LUMO, LUMO +1, and LUMO+2) are mainly located on the TiO cluster core. This is taken as evidence that the lowest energy bands for 1 and 2 are assigned to CT bands from FcCO2 and Cat to the TiO cluster core. The density of states (DOS) plots in Figure 4c,d clearly show that the conduction band is dominated by the TiO cluster core (blue line) and the valence band locates on FcCO2 (red line) and Cat (magenta line) moieties for 1 and 2. The calculated energy gap is 2.59 eV for 1 and 2.37 eV for 2, which agrees with the experimental data of the lowest energy band of 496 nm (2.50 eV) for 1 and 512 nm (2.42 eV) for 2. The DOS plots (Figure 4c,d) show that the valence band is contributed mainly by the FcCO2 group and only with a small fraction of Cat for 1. Even though the ratio of FcCO2 to Cat in cluster 2 is 2:5, the contribution of FcCO2 to the valence band is still greater than that of Cat. Therefore, the mutual contribution of FcCO2 and Cat results in the CT bands, but the contribution of FcCO2 is clearly greater than that of Cat. Clusters 1 and 2 were used as precursors to fabricate dye modified TiO2 electrodes. Photoelectrochemical properties of the clusters modified TiO2 electrodes were studied to understand the effects of the two co-coordinated dye ligands on the TiO2 electrodes. The crystals were quantitatively dissolved in trichloromethane, and then a pre-prepared porous

Ti(IV) atoms take octahedral geometry and are bridged by two carboxylate groups from two FcCO2 ligands in a μ2-η1,η1 mode and one Cat ligand in a μ2-η1,η2 mode. There are six isopropanol groups, two of which are in a μ2-O bridge mode and the other four are in a monocoordinated mode. The three TiO6 octahedra co-share a μ3-O oxygen, forming a Ti3O13 cluster (Figure 1c). The structure of 2, [Ti7O4(OiPr)8(Cat)5(FcCO2)2], is a Ti7 oxo-cluster with a quasi reflection plane at the center. All the Ti(IV) atoms take octahedral geometry and are bridged by two carboxylate groups from two FcCO2 ligands in a μ2-η1,η1 mode and five Cat ligands, four of which are in a μ2-η1,η2 mode and the central one in a μ2-η1,η1 mode. There are eight isopropanol groups, two of which are in a μ2-O bridge mode and the others are in a monocoordinated mode. The seven TiO6 octahedra co-share two μ4-O oxygen and two μ3-O oxygen, forming a Ti7O26 cluster (Figure 1d). Comparing the clusters in 1 and 2 (Figure 1c,d), the latter is formed by two Ti3O13 clusters connected by an additional TiO6 octahedron. The cluster can also be considered as two saddle-shaped Ti4O16 co-sharing one centered Ti atom. Cluster 3 was isolated as a contrast of 2. The two redox FcCO2 ligands in 2 were replaced by a nonredox benzene dicarboxylate (o-BDC) in 3 (Figure 2). Although the core

Figure 2. (a) Ball-and-stick plot of 3, showing the molecular structure. The isopropyl is omitted for clarity. (b) Cluster of 3, illustrating the coordination polyhedra of each Ti and the coordination of o-BDC and Cats.

structure of 3 is different from that of 2, the similar numbers in Ti, CO2−, and Cat make them have some similarity. In 3, the seven Ti(IV) atoms are coordinated by 1 μ4-O oxygen, 2 μ3-O oxygen, 1 o-BDC, 4 Cat, and 10 isopropanol groups forming a Ti7O24 cluster (Figure 2a). All the four cat ligands coordinated to Ti(IV) in a μ2-η1,η2 mode and the carboxylate groups of oBDC in a μ2-η1,η1 mode. One Ti(IV) atom takes a five coordinated triangular bipyramid, and the other Ti(IV) atoms

Figure 3. Solid-state absorption spectra of (a) 1 and 2 along with the ligands and (b) 3 and ligand CatH2. C

DOI: 10.1021/acs.inorgchem.8b01050 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Molecular orbital distribution of selected HOMOs and LUMOs for 1 (a) and 2 (b). Total and moiety separated DOS plots of 1 (c) and 2 (d) based on DFT calculations.

Figure 5. (a) Raman spectra of 1 and 1-TiO2 before and after measurement along with the TiO2 substrate. (b) XPS spectra of Fe in 1 and 1-TiO2 electrode.

The photocurrent responsive properties of the 1 and 2 treated TiO2 electrodes were examined using a three-electrode cell in an aqueous solution (see the Experimental Section). The blank TiO2 electrode and cluster 3 treated TiO2 electrode were also examined as the contrast. The results of photocurrent responses at 0 V bias with 20 s interval light on and off are displayed in Figure 6a, which showed that the photocurrent responses were increased for all the modified TiO2 electrodes comparing with that of the blank TiO2 electrode. Their photocurrent densities (J) are in the order of 2 > 1 > 3. The results revealed that the photosensitivity of the FcCO2 group is better than that of the Cat group (FcCO2:Cat, 2:1 in 1; 2:5 in 2; 0:5 in 3). The effect of the cluster core is not so obvious if the number of Ti atoms in the core is limited, because the TiO cluster cores are bonded or fused to the TiO2 substrate in the electrodes. To further understand the effect of the dye ligands, a series of FcCO 2 -TiO 2 electrodes (TiO 2 electrodes treated with FcCO2H) and Cat-TiO2 electrodes (TiO2 electrodes treated

TiO2/ITO glass substrate was immersed in the solution for 20 h, then dripped and dried for measurements (see the Experiment Section). The 1 and 2 treated electrodes (represented as 1-TiO2 and 2-TiO2, respectively) were characterized by Raman and XPS spectra before and after measurement. The Raman spectra showed a signal superposition of the TiO2 substrate and the clusters except for a disappeared peak at 1016 cm−1 that is assigned to the Ti-O-C of the TiOR moiety13 (Figure 5a and Figure S4a). The covalent bond of the carboxyl group to Ti(IV) is very strong. The results indicated that the clusters were maintained and only the isopropyl groups were lost when the clusters were condensed on the TiO2 surface. XPS (Fe signal) (Figure 5b and Figure S4b) results confirmed the existence of the FcCO2 groups on the electrodes before and after measurement. The SEM images show that the surface morphology of the sample treated TiO2 film maintains the nanograin surface and there is no change observed in comparison with that of the TiO2/ITO substrate (Figure S5). D

DOI: 10.1021/acs.inorgchem.8b01050 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Photocurrent densities of TiO2 electrodes treated with (a) 1−3 (1.0 × 10−3 mol·L−1) and the blank, (b) different concentrations of FcCO2H (the interval is 1.0 × 10−3 mol·L−1), (c) different concentrations of CatH2 (the interval is 1.0 × 10−3 mol·L−1), and (d) mixed solutions of FcCO2H and CatH2 at 2:1 and 2:5 ratios (a three-electrode system, 0 V bias, 0.1 mol·L−1 Na2SO4).

density using the electrodes treated with the mixed solution of FcCO2 and Cat with the calculated density based on experimental data using electrodes treated with separate FcCO2H or CatH2, an increase of about 10 μA cm−2 appeared for both 2:1 and 2:5 systems (Figure 7b). The difference between the experimental value and calculated value indicated that there should be a synergistic effect between the cocoordinated FcCO2 and Cat. There are several interesting findings needing to be discussed. (1) It should be noted that photocurrent densities of electrodes treated by the solutions of clusters 1 and 2 are all higher than those treated by the mixed solutions of FcCO2H and CatH2 at the same mole ratios and concentrations (Figure 7b). This should benefit from the TiO-dye pre-anchored structure of the clusters. The dealkoxide and condensation of the clusters on the TiO2 surface improve the anchoring property of dyes. The advantage of the TiO-dye pre-anchored clusters as a new type of photosensitizers in photocurrent conversion has been reported previously.9 (2) Although the energies of CT bands (spectra) and the highest occupied orbitals (theoretical calculations) contributed by FcCO2 and Cat are similar to each other, why is the photocurrent density of FcCO2-TiO2 about 2 times that of Cat-TiO2? The key to this problem should be owing to the better redox property of the ferrocene group. Figure 8a shows the reversible redox curves of 1-TiO2 and 2-TiO2 electrodes, while no obvious redox peak was recorded for the 3-TiO2 electrode in the same CV measurement. The reversible redox activity relates to the better electron-transfer kinetics of the electrode. The result shows that electron transfer via FcCO2-TiO is better than that via the CatTiO pathway. (3) A synergistic effect on the enhancement of photocurrent response clearly occurred for the electrodes treated with co-coordinated FcCO2 and Cat. Then what is the mechanism?

with CatH2) were prepared using the same method as that of the clusters and their photocurrent response properties were measured (Figure 6b,c). The current densities of the modified TiO2 electrodes were increased along with the increasing concentration of FcCO2H or CatH2 from 1.0 × 10−3 to 6.0 × 10−3 mol·L−1 (the solution concentration used in dipping the TiO2 electrode). When the concentration is increased by 1.0 × 10−3 mol·L−1, the average value of J of the FcCO2-TiO2 electrode increases by 8 μA cm−2 (ΔJ1), higher than 4 μA cm−2 (ΔJ2) of the Cat-TiO2 electrode. All the results were obtained from the parallel experiments done at the same time using the same techniques and from repeated experiments. Figure 6d recorded the photocurrent densities of the electrodes treated with the mixed solution of FcCO2H and CatH2 at the same molar ratios of 2:1 and 2:5 as those in the crystals of 1 and 2. The data (Figure 6a,d) showed that the photocurrent densities of the clusters modified electrodes were larger than those of the related electrodes treated with the mixed solution of FcCO2H and CatH2 at the same molar ratio. To be more clear, a histogram that summarized the relationship of the data is given in Figure 7, where A is the J of the blank TiO2 electrode (about 16 μA cm−2), B, about 16 μA cm−2, is the J contributed by the “basic sensing” of the FcCO2 (see Figure 6b, B = J(1.0 × 10−3) − J (blank TiO2) − ΔJ1), C (8 μA cm−2, Figure 6b) and D (4 μA cm−2, Figure 6c) correspond to the ΔJ1 and ΔJ2 when per unit concentration (1.0 × 10−3 mol L−1) of FcCO2 and Cat is increased, respectively. The calculated J value (JA + JB + 2JC + JD) for the 2:1 mixture (the ratio of FcCO2 to Cat being the same as that in cluster 1) is 52 μA cm−2, while the experimental J value for the electrode treated by the 2:1 mixture is 60 μA cm−2 (Figure 7a). When the ratio of FcCO2 to Cat is 2:5 (the same as that in cluster 2), the calculated J value (JA + JB + 2JC + 5JD) is 68 μA cm−2, while the experimental J value of the 2:5 mixture is 80 μA cm−2 (Figure 7b). Comparing the experimental photocurrent E

DOI: 10.1021/acs.inorgchem.8b01050 Inorg. Chem. XXXX, XXX, XXX−XXX

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Cat and FcCO2 ligands (Figure 4) and the energies of these orbitals are very close to each other. Although the electrontransfer kinetics of Cat is not so good as that of FcCO2, since the Cat and FcCO2 groups are located close to each other on the TiO core, the Cat can transfer an electron to the vacant HOMO of the FcCO2+ group due to the ligand-to-ligand electron communication (electron transfer or charge transfer).14 This should be the main reason for the synergistic effect of this type of photoelectric electrodes.

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CONCLUSIONS In summary, to understand the synergistic effect of multi-dye ligands on photocurrent response properties, two TOCs with both FcCO2 and Cat ligands were synthesized and characterized. They are a trinuclear Ti3 oxo-cluster and a Ti7 oxocluster with FcCO2 and Cat at 2:1 and 2:5 ratios, respectively. Using the two clusters as TiO-dye pre-anchored precursors, sensitized TiO2 electrodes were prepared and the effect of multicomponent dyes on photocurrent properties were studied. The conclusions are as follows: (1) The advantages of the TiOdye pre-anchored precursors over the original organic dye molecules as photosensitizers have been further confirmed. (2) The redox active FcCO2 showed outstanding photosensitivity in comparison with Cat, which is in agreement with their contribution to charge transfer. (3) A synergistic effect on the enhancement of photocurrent responses was first reported for the multicomponent dye pre-anchored TiO precursors, which should be due to the electron communication between the neighboring FcCO2 and Cat groups. The information obtained from this work reveals that, besides the complement in absorption of two co-sensitized dyes, there are other electron cooperations between the co-coordinated dyes, which will be helpful in the design of new electrodes for high efficiency DSSCs.

Figure 7. (a) A schematic representation of photocurrent densities estimated from the experimental data (Figure 6) (A, B, C, and D), the calculated and experimental densities of the TiO2 electrode treated with mixed ligands of FcCO2H and CatH2 at 2:1 ratio. (b) A schematic representation of the calculated and experimental photocurrent densities of the TiO2 electrodes treated with mixed ligands of FcCO2H and CatH2 at 2:1 and 2:5 ratios, and the experimental results of the TiO2 electrodes treated with clusters 1 and 2.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01050. Photos of the crystals, XRD and CV of 1−3, Raman and XPS of 2 and 2-TiO2, and SEM of the electrodes (PDF)

Figure 8b is a schematic energy diagram for the mixed system based on theoretical calculations and experimental results. The LUMOs are mainly located on the TiO cluster core, and since the TiO clusters have bonded to the TiO2 surface, their energy level should be close to that of the TiO2 matrix with only a small increase. For the same reason, there should be no big interval for the electron transfer from the TiO cluster to the TiO2 matrix. The HOMOs (HOMO, HOMO−1, and HOMO−2) of clusters 1 and 2 are centered on the coordinated

Accession Codes

CCDC 1811788−1811790 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The

Figure 8. (a) Cyclic voltammograms of 1-TiO2, 2-TiO2, and 3-TiO2 electrodes in CH3CN (0.1 mol·L−1 Bu4NClO4, 100 mV s−1, vs SEC). (b) A schematic representation of the photoinduced electron transfer mechanism based on theoretical calculations and experimental results. F

DOI: 10.1021/acs.inorgchem.8b01050 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.-Y.Z.). *E-mail: [email protected] (J.D.). ORCID

Qin-Yu Zhu: 0000-0003-1864-1175 Jie Dai: 0000-0002-3549-726X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the NSF of China (21571136 & 21771130), the Program of the Priority Academic Program Development of Jiangsu Higher Education Institutions, and by the project of scientific and technologic infrastructure of Suzhou (SZS201708).

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DOI: 10.1021/acs.inorgchem.8b01050 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry from carboxylic acid-modified titanium alkoxides. syntheses, characterizations, X-ray structures of Ti3(μ3-O)(O2CH)2(ONep)8, Ti3(μ3O)(O2CMe)2(ONep)8, Ti6(μ3-O)6(O2CCHMe2)6(ONep)6, [Ti(μO2CCMe 3)(ONep)3] 2, and Ti3(μ3-O)(O2CCH2CMe3)2(ONep)8 (ONep = OCH2CMe3). J. Am. Chem. Soc. 1999, 121, 12104−12112. (b) Wu, Y.-Y.; Luo, W.; Wang, Y.-H.; Pu, Y.-Y.; Zhang, X.; You, L.-S.; Zhu, Q.-Y.; Dai, J. Titanium−oxo−clusters with dicarboxylates: singlecrystal structure and photochromic effect. Inorg. Chem. 2012, 51, 8982−8988. (14) (a) Lazarides, T.; McCormick, T. M.; Wilson, K. C.; Lee, S.; McCamant, D. W.; Eisenberg, R. Sensitizing the sensitizer: the synthesis and photophysical study of bodipy-Pt(II)(diimine)(dithiolate) conjugates. J. Am. Chem. Soc. 2011, 133, 350−364. (b) Zheng, S.; Phillips, H.; Geva, E.; Dunietz, B. D. Ab initio study of the emissive charge-transfer states of solvated chromophore-functionalized silsesquioxanes. J. Am. Chem. Soc. 2012, 134, 6944−6947.

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DOI: 10.1021/acs.inorgchem.8b01050 Inorg. Chem. XXXX, XXX, XXX−XXX