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Article Cite This: Inorg. Chem. 2017, 56, 12775-12782

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Phosphonate-Stabilized Titanium-Oxo Clusters with Ferrocene Photosensitizer: Structures, Photophysical and Photoelectrochemical Properties, and DFT/TDDFT Calculations Yang Fan,*,† Hua-Min Li,† Rui-Huan Duan,‡ Hai-Ting Lu,† Jun-Tao Cao,† Guo-Dong Zou,*,† and Qiang-Shan Jing†

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Henan Province Key Laboratory of Utilization of Non-metallic Mineral in the South of Henan, College of Chemistry and Chemical Engineering, Xinyang Normal University, Xinyang 464000, China ‡ Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China S Supporting Information *

ABSTRACT: The metal-to-core charge transfer (MCCT) transition in sensitized titanium-oxo clusters is an important process for photoinduced electron injection in photovoltaic conversion. This process resembles most closely the Type II photoinjection in dye-sensitized solar cells. Herein we report the synthesis and photophysical and photoelectrochemical (PEC) properties of the phosphonate-stabilized titanium-oxo clusters containing the ferrocenecarboxylate ligands. These ferrocene-containing clusters exhibit intense visible absorption extended up to 600 nm along with low optical band gaps of ∼2.2 eV. The low-energy transitions of these clusters were systematically investigated by UV−vis spectroscopy and DFT/ TDDFT calculations. The combined experimental and computational studies suggest that the ferrocenecarboxylate-substituted titanium-oxo clusters form a donor−acceptor (D−A) system. The low-energy transition of these clusters primarily involves the MCCT from the iron center to TiO cluster core. The TiO core structure and phosphonate ligands both have great influence on the PEC properties of the clusters. This work provides valuable examples for the sensitized titanium-oxo clusters in which electron injection takes place via MCCT transition.

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DSSCs, there are mainly two pathways for electron injection.6 Type I injection occurs via the excited dye injecting electron into TiO2 conduction band. In contrast, for Type II injection, the electron of dye is directly injected from ground state that leads to instantaneous charge separation without excited state of dye molecules. This one-step charge separation taking place via direct charge transfer transition is expected to be more efficient for photon-to-electron conversion. However, for both DSSCs and sensitized TOCs, the Type II injection has been less studied. In the pioneering work by Coppens and coworkers, the ligand-to-cluster charge transfer transitions in catechol-functionalized titanium-oxo clusters were investigated. The location of the catechol-based energy levels within the band gap of TiO cluster was observed.5h Recently, in their report on the cluster Ti17O28(OiPr)16(FeIIPhen)2 (phen = 1,10phenanthroline), it is revealed that the metal-to-core charge transfer (MCCT) transition from the iron center to TiO core is responsible for the visible light induced electron injection. In addition, other metal−phen substituted TOCs, including [Ti17O28(OiPr)16(CoIIphen)2], [Ti17O28(OiPr)18(CdIIphen)2],

INTRODUCTION Titanium-oxo clusters (TOCs) can be regarded as molecular analogues of TiO2 semiconductors.1 For TOCs, precise atomiclevel structure information could be obtained from X-ray crystallographic studies. On the basis of this information, the indepth structure−property relationship can be investigated and understood by experimental and theoretical studies.2 For instance, Wright and co-workers elucidated the novel band gap reduction induced by dipole moment in a series of Co(II)doped polyoxotitanate cages.3 Piotrowiak and co-workers revealed the fast hole hopping in the Ti17(μ4-O)4(μ3-O)16(μ2O)4(cat)4(OPri)16 (cat = catecholate) cluster by transient absorption spectroscopy and computational studies.2d Very recently, Zhang and co-workers reported the band gap modulation of a series of phosphonate-stabilized {Ti6} clusters.4a These results give general guidelines to modulate physicochemical properties of TOCs toward potential photocatalytic and photovoltaic applications.4 Considerable research efforts have been devoted to the sensitized titanium-oxo clusters, because they resemble most closely the dye-sensitized solar cells (DSSCs).5 As known, photoinduced electron injection is a key process for DSSCs and of fundamental interest for solar energy conversion. For © 2017 American Chemical Society

Received: June 15, 2017 Published: October 13, 2017 12775

DOI: 10.1021/acs.inorgchem.7b01527 Inorg. Chem. 2017, 56, 12775−12782

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Inorganic Chemistry

Crystal Structure Determination. Crystallographic data of the clusters [Ti6(μ3-O)2(μ2-O)2(μ2-OiPr)4(OiPr)6(O3PC6H5)2(O2CFc)2] (C1), [Ti6(μ3-O)2(μ2-O)2(μ2-OiPr)4(OiPr)6(O 3 PCH 2 C 6 H 5 ) 2 (O 2 CFc) 2 ] (C2), [Ti 5 (μ 3 -O) 2 (μ 2 -O)(μ 2 OiPr)2(OiPr)4(O3PCH2C6H5)2(O2CFc)4] (C3), and [Ti6(μ3-O)2(μ2O)2(μ2-OiPr)4(OiPr)6(O3PC6H5)2(O2C-C6H4-COOiPr)2] (C4) were collected at 295 (2) K on an Xcalibur Eos Gemini CCD diffractometer using graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. The structures were solved by direct methods using the SHELXS-2013 and refined by full-matrix least-squares on F2 using the SHELXL2016.9 The non-H atoms were refined with anisotropic thermal parameters. All H atoms were in calculated positions and refined as riding on their parent atoms. For the benzylphosphonate and isopropyl groups, some geometry restraints and displacement parameter restraints such as DFIX, SADI, DELU, and SIMU were applied. Unit-cell and refinement parameters are listed in Table S1. CCDC 1555926−1555928 (C1−C3) and 1570837 (C4) contain additional crystallographic details for these clusters. Computational Methods. DFT calculations were performed with the Gaussian 09 packages. The B3LYP functional was used with the 631G** basis set.3b,4f The crystal structure of the clusters was used for geometry optimization. To reduce the structure complexity, the OiPr groups were replaced with the OCH3 groups in calculations. TDDFT calculations at the B3LYP/6-31G** level were performed on the optimized structures. The first 50 excited states were considered in TDDFT calculations. The solvent (CH2Cl2) effects were taken into account in TDDFT calculations by the polarized continuum model (PCM). Photoelectrochemistry. For the preparation of photoelectrodes, ∼5 mg of crystal sample was dissolved in 2 mL of anhydrous dichloromethane, which was then dropped onto a selected area of precleaned indium tin oxide (ITO) glass and left to evaporate in an inert atmosphere. The PEC tests were performed with a CHI 660E electrochemistry analyzer. In the PEC cell, the sample-coated ITO glass was used as the working electrode. A Pt wire was used as the counter electrode. A Ag/AgCl (3 M KCl) electrode was used as the reference electrode. NaPF6 in dry CH3CN (0.05 M) was employed as the electrolyte. The ITO electrode was irradiated with a 300 W xenon lamp, which was positioned 20 cm from the electrode. Synthesis of [Ti6(μ3-O)2(μ2-O)2(μ2-OiPr)4(OiPr)6(O3PC6H5)2(O2CFc)2] (C1). A mixture of ferrocenecarboxylic acid (23 mg, 0.1 mmol), phenylphosphonic acid (8 mg, 0.05 mmol), Ti(OiPr)4 (92 μL, 0.3 mmol), and isopropanol (5 mL) was sealed in a 25 mL Teflonlined autoclave and heated at 80 °C for 3 d. After it cooled, orange crystals of ∼40 mg were obtained (93% yield based on phenylphosphonic acid). Anal. Calcd for C64H98O24P2Ti6Fe2: C, 44.89; H, 5.76. Found: C, 44.97; H, 5.81%. IR (KBr, cm−1): 2969 (m), 1539 (s), 1474 (s), 1389 (m), 1128 (s), 1082 (s), 1015 (s), 776 (m), 677 (w). Synthesis of [Ti6(μ3-O)2(μ2-O)2(μ2-OiPr)4(OiPr)6(O3PCH2C6H5)2(O2CFc)2] (C2). A mixture of ferrocenecarboxylic acid (12 mg, 0.05 mmol), benzylphosphonic acid (17 mg, 0.1 mmol), Ti(OiPr)4 (153 μL, 0.5 mmol), and isopropanol (5 mL) was sealed in a 25 mL Teflon-lined autoclave and heated at 80 °C for 3 d. After it cooled, orange crystals of ∼36 mg were obtained (83% yield b a s e d o n fe r r o c e n e ca r b ox yl i c a ci d ). A n a l . Ca l c d f or C66H102O24P2Ti6Fe2: C, 45.55; H, 5.91. Found: C, 45.83; H, 5.95. IR (KBr, cm−1): 2973 (w), 1554 (s), 1480 (s), 1391 (s), 1126 (m), 1073 (m), 1003 (s), 733 (m), 614 (w). Synthesis of [Ti5(μ3-O)2(μ2-O)(μ2-OiPr)2(OiPr)4(O3PCH2C6H5)2(O2CFc)4] (C3). A mixture of ferrocenecarboxylic acid (35 mg, 0.15 mmol), benzylphosphonic acid (9 mg, 0.05 mmol), Ti(OiPr)4 (92 μL, 0.3 mmol), and isopropanol (5 mL) was sealed in a 25 mL Teflon-lined autoclave and heated at 80 °C for 3 d. After it cooled, red crystals of ∼26 mg were obtained (55% yield based on benzylphosphonic acid). Anal. Calcd for C76H92O23P2Ti5Fe4: C, 48.09; H, 4.88. Found: C, 48.26; H, 4.51%. IR (KBr, cm−1): 2969 (m), 1537 (s), 1480 (s), 1391 (m), 1126 (s), 1073 (s), 1010 (s), 667 (w). Synthesis of [Ti6(μ3-O)2(μ2-O)2(μ2-OiPr)4(OiPr)6(O3PC6H5)2(O2C-C6H4-COOiPr)2] (C4). A mixture of phthalic acid (100 mg, 0.6

and [Ti18MnO30(OEt)20(MnPhen)3] have also been wellsynthesized and characterized by Dai and Wright and coworkers.5b,e These titanium-oxo clusters incorporated with metal complex photosensitizer provide valuable structure models to study the photoinduced electron injection and charge separation via the charge transfer transition pathway. Ferrocene exhibits good electron-donating capability in various electroactive and photoactive molecular systems. However, only several examples of ferrocene-containing titanium-oxo cluster have been reported until now.7 Very recently, Dai and co-workers, and our lab at almost the same time, reported the ferrocenecarboxylate-functionalized titanium-oxo cluster [Ti6(μ3-O)6(OiPr)6(O2CFc)6] (Fc = ferrocenyl) with Fc units as photosensitizer.7c,d The experimental results and DFT calculations suggest the charge transfer transition from ferrocene to {Ti6} cluster core. As known, ferrocene only exhibits very weak visible absorption in 400− 550 nm, which arise from the Laporte-forbidden ligand-field d− d transitions. However, in some ferrocene-containing donor− acceptor (D−A) systems, it was found that the metal-centered d−d transitions are mixed with considerable charge-transfer character, resulting in much enhanced and extended visible absorption.8 In the present study, the ferrocenecarboxylate ligands were introduced into the phosphonate-stabilized titanium-oxo clusters. Three ferrocene-containing titaniumoxo clusters stabilized by the phenylphosphonate ligand and benzylphosphonate ligand, respectively, were synthesized. In addition, a phenylphosphonate-stabilized titanium-oxo cluster containing the in situ monoesterified phthalate ligands was synthesized for comparison. The low-energy charge-transfer transitions of the ferrocene-containing clusters were systematically investigated by solution-state UV−vis spectroscopy and density functional theory (DFT)/time-dependent density functional theory (TDDFT) calculations. The experimental and computational results suggest that the low-energy absorption of the ferrocene-containing clusters primarily involves the MCCT from the iron center to TiO cluster core. Photoelectrochemical experiments show that the ferrocenecarboxylate ligands have substantial photosensitizing effect on titanium-oxo clusters. Moreover, the TiO core structure also has great influence on the photoelectrochemical (PEC) performance of the clusters.

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EXPERIMENTAL SECTION

Reagents. Ti(OiPr)4 (97%), ferrocenecarboxylic acid (98%), phenylphosphonic acid (98%), phthalic acid (99.5%), isopropanol (99.5%), ferrocene (99%), TiO2 (99.8%, 25 nm), benzylphosphonic acid (98%), and NOBF4 (98%) were purchased from commercial suppliers. These reagents were used as received. Instrumentation. X-ray diffraction (XRD) measurements were performed on a Rigaku SmartLab diffractometer with Cu Kα (λ = 1.5406 Å) radiation. Microanalyses were determined by a Vario MICRO instrument. IR measurements were performed on a Nicolet is50 FT-IR spectrometer. The diffuse reflectance spectra of the powder samples were measured using a spectrophotometer (PerkinElmer Lamda-950) with BaSO4 as standard. Thermogravimetric analysis (TGA) was performed with a TA Q600 Instrument. The UV−vis spectra of the cluster samples in CH2Cl2 solution were obtained on HITACHI U-3900H UV−vis spectrometer. Cyclic voltammetry (CV) measurements were conducted on a CHI 660E electrochemistry analyzer at scan rate of 100 mV s−1. The three-electrode setup consisted of a glassy-carbon working electrode, a platinum wire counter electrode, and a Ag/AgCl (3 M KCl) reference electrode. A 0.1 M solution of n-Bu4NPF6 in CH2Cl2 was employed as the supporting electrolyte. 12776

DOI: 10.1021/acs.inorgchem.7b01527 Inorg. Chem. 2017, 56, 12775−12782

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Figure 1. Crystal structures of (a) C1, (b) C2, (c) C3, and (d) C4. Color codes: Ti-blue; Fe-dark yellow; O-red; P-magenta; C-gray. H atoms are omitted for clarity.

Figure 2. (a) Diffuse reflectance spectra. (b) The transformed Kubelka−Munk function vs photon energy. mmol), phenylphosphonic acid (100 mg, 0.63 mmol), Ti(OiPr)4 (920 μL, 3 mmol), and isopropanol (5 mL) was sealed in a 25 mL Teflonlined autoclave and heated at 80 °C for 2 d. After it cooled, colorless crystals of ∼258 mg were obtained (51% yield based on phthalic acid). Anal. Calcd for C64H102O28P2Ti6: C, 46.07; H, 6.16. Found: C, 46.21; H, 6.17%. IR (KBr, cm−1): 2971 (m), 1718 (s), 1556 (s), 1412 (s), 1276 (m), 1132 (s), 1071 (s), 1006 (s), 667 (w).

possesses the {Ti6} core structure with the same bonding mode as that for the phenylphosphonate-stabilized clsuter C1. In contrast, cluster C3 forms a {Ti5} cluster containing four ferrocenecarboxylate ligands. The {Ti5} cluster core of C3 consists of two corner-sharing Ti3(μ3-O) subunits tilted by 58.9°, which are connected by one μ 2 -oxo and two benzylphosphonate ligands. This {Ti5} cluster has a noncrystallographic C2 axis passing through the μ2-oxo and Ti3 atoms, rendering the two Ti3(μ3-O) subunits chemically equivalent. A similar {Ti5} cluster [Ti5(μ3-O)2(μ2-O)(μ2OiPr)2(OiPr)4(OAc)4(O3P-xylyl)2] stabilized by the bis(trimethylsilyl) 3,5-dimethylphenylphosphonate ligand was reported in the pioneering work by Schubert and coworkers.10b For C3, the four ferrocenecarboxylate ligands in this {Ti5} cluster are symmetrically oriented around the Ti−O C2 axis. The Fe1···Fe1A distance is ∼6.5 Å, and the Fe2···Fe2A distance is ∼12.0 Å. The cluster C4 containing two phthalate monoester ligands possess the same {Ti6} core structure to that of C1 and C2 (Figures 1 and S1), in which the monoesterified phthalate ligands are in situ formed during the synthetic reaction.11 The well-matched XRD patterns between the experimental results and the simulated data confirm phase purity of the cluster samples (Figure S2). In the infrared spectra (Figure S3),

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RESULTS AND DISCUSSION The ferrocenecarboxylate ligands were introduced into the phenylphosphonate-stabilized titanium-oxo clusters through the reaction involving phenylphosphonic acid and ferrocenecarboxylic acid with Ti(OiPr)4 in isopropanol. As shown in Figure S1, the {Ti6} cluster core of C1 consists of two Ti3(μ3-O) subunits that are connected by two μ2-oxo and two bridging phenylphosphonate ligands, in agreement with the coordination mode of previously reported [Ti6O4(OiPr)10(O3PC6H5)2(O2CR)2] clusters.4a,10a In this centrosymmetric cluster, the ferrocenecarboxylate ligands occupy the acetate sites bridging the Ti1 and Ti3 atoms (Figure 1). The Fe···Fe distance between the iron centers of two Fc units is ∼1.5 nm. When benzylphosphonic acid was applied in place of phenylphosphonic acid, the clusters C2 and C3 were obtained by varying the reagent stoichiometry. The cluster C2 also 12777

DOI: 10.1021/acs.inorgchem.7b01527 Inorg. Chem. 2017, 56, 12775−12782

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Inorganic Chemistry the carboxylate groups are detected by the υas(COO) at 1539 cm−1 (C1), 1554 cm−1 (C2), 1537 cm−1 (C3), and 1556 cm−1 (C4).12 The OiPr groups are characterized by the C−H vibration bands (2970−2850 cm−1) and Ti−O−C vibration band (1015 cm−1 for C1, 1003 cm−1 for C2, 1010 cm−1 for C3, and 1006 cm−1 for C4).12 For C4, the ester group of phthalate monoester ligands is characterized by the CO and C−O−C vibration bands at 1718 and 1276 cm−1, respectively.11a The TGAs show that the ferrocene-containing clusters have favorable thermal stability with the onset temperature of thermal decomposition (Td) above ∼230 °C, while it is ∼150 °C for C4 (Figure S4). Because of the removal of OiPr groups, the samples exhibit a mass loss of ∼40% before 300 °C for C1, C2, and C4 and 440 °C for C3. The ligands then decompose before 600 °C, and TiO2 finally forms at ∼800 °C. In Figure 2a, the diffuse reflectance spectra (DRS) of the phenylphosphonate-stabilized cluster C4 with the phthalate monoester ligands is characterized by sharp absorption edge at ∼360 nm, without absorption in visible region. In contrast, the ferrocene-containing clusters show intense visible absorption with absorption edges at ∼580, 600, and 630 nm for C1, C2, and C3, respectively, consistent with their orange and red color crystals (Figure S5). In comparison to the {Ti6} cluster C1 and C2, the {Ti5} cluster C3 shows obvious bathochromic shifts in the low-energy band. This red shift suggests the enhanced charge-transfer interaction between ferrocenecarboxylate substituents and TiO core in cluster C3.7d The DRS of ferrocene and ferrocenecarboxylic acid were also characterized, in which the absorption band in 400−600 nm region is characteristic of d−d transitions (Figure S6).4a,f Figure 2b shows the absorption data in terms of Kubelka−Munk function: F(R) = (1 − R)2/2R, where R is the reflection. The band gap energy can be estimated by extrapolation of the linear portion of the absorption edge to the energy axis, which is 2.2, 2.2, 2.1, and 3.6 eV for the clusters C1, C2, C3, and C4, respectively.3b As known, for titanium-oxo cluster, the O→Ti charge transfer transition within the TiO core can only induce UV-light absorption as observed for cluster C4.2e The remarkably extended visible absorption band and reduced band gaps for the ferrocene-containing clusters indicate the generation of new energy levels due to the functionalization with ferrocenecarboxylate ligands. To characterize the low-energy transitions in visible region, the solution-state absorption spectra of the clusters C1, C2 and C3 were investigated. In comparison to ferrocene and ferrocenecarboxylic acid, these ferrocene-containing clusters exhibit greatly enhanced visible absorption in 400−600 nm (Figure 3). As known, for ferrocene, the weak absorption centered at ∼440 nm with small extinction coefficient of ∼80 M−1 cm−1 arises from the Laporte-forbidden ligand field d−d transitions.8a For ferrocenecarboxylic acid, carboxylation of the cyclopentadienyl (Cp) ring induces the ligand-field splitting that leads to slightly increased intensity of d−d transitions (ε = 273 M−1 cm−1). In contrast, intensity of the low-energy transitions was greatly enhanced to ε = 1589, 1496, and 3441 M−1 cm−1, along with red-shifted absorption maxima at λmax = 464, 461, and 483 nm, for C1, C2, and C3, respectively. The spectral changes suggest that, for the ferrocene-containing clusters, the low-energy transitions are mixed with appreciable charge-transfer character. The charge-transfer transitions could relax the Laporte selection rule that results in the substantially increased absorption intensity.8a Moreover, the π-conjugation between the Cp ring and carboxylate linker allows the charge delocalization from the Fc group to TiO cluster core, which

Figure 3. UV−vis spectra in CH2Cl2. (inset) Enlarged view for ferrocene.

would stabilize the resulting excited states and lower the transition energy. Therefore, the red-shifted low-energy transition for C3 can be explained by its unique {Ti5} structure in which each Ti3(μ3-O) subunit is coordinated with three Fc units. This allows extended delocalization and stronger electronic interaction between the Fc units and TiO core. Chemical oxidation of these ferrocene-containing clusters was further investigated by using NOBF4 as the oxidizing agent. As shown in Figure 4a, with stepwise addition of NOBF4 to a CH2Cl2 solution of C1, the initial band at 465 nm gradually decreased. Meanwhile, a new band centered at 633 nm occurred and increased progressively in intensity, indicating the formation of ferrocenium moiety upon chemical oxidation. The distinct isosbestic point at 529 nm indicates that the two species coexist in equilibrium during the oxidation process. Similar spectra changes under the stepwise chemical oxidation were also observed for C2 and C3 (Figure 4b,c). As known, for unsubstituted ferrocenium ions, the low-energy absorption band at 617 nm originates from the ligand-to-metal charge transfer transition (e1u→e2g).13 In this experiment, the chemical oxidation of ferrocene by NOBF4 also induced the gradually developed absorption band centered at 621 nm that confirms the ferrocenium formation (Figure 4d). However, the initial d− d transition band at 445 nm remains constant during the oxidation process that could be ascribed to similar d−d transition energy for Fc and Fc+.13a,b Therefore, in comparison, the remarkably decreased initial low-energy band induced by chemical oxidation of the ferrocene-containing clusters suggest that the low-energy transition of these clusters has a dominant charge-transfer nature. This low-energy transition can be tentatively assigned to the MCCT transition from Fe(II)→ TiO core. In agreement with this assignment, the oxidation of Fe(II) to Fe(III) leads to a considerable decrease in the MCCT band. The electrochemistry behavior of the ferrocene-containing clusters was investigated by cyclic voltammetry (CV) measurements in dichloromethane solution with n-Bu4NPF6 as the supporting electrolyte. All these clusters exhibit a ferrocenebased quasi-reversible oxidation process. The Fc/Fc+ redox couple was found at E1/2 = 0.64 V (C1), 0.62 V (C2), and 0.67 V (C3) (vs Ag/AgCl; Figure 5). The single redox peak suggests that the Fc units in the clusters are oxidized simultaneously and electrochemically independent of one another. DFT and TDDFT calculations were performed to understand the electronic transition nature of the clusters.3b,4f For C1, the levels ofhighest occupied molecular orbital (HOMO) 12778

DOI: 10.1021/acs.inorgchem.7b01527 Inorg. Chem. 2017, 56, 12775−12782

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Figure 4. UV−vis spectra for stepwise oxidation of (a) C1, (b) C2, (c) C3 and (d) ferrocene by NOBF4 in CH2Cl2.

ligands and O 2p orbitals of the ligands. The lowest-unoccupied molecular orbitals (LUMOs) from LUMO to LUMO+13 of C1 are primarily located on the Ti 3d orbitals in the TiO core (Figure 6). In addition, these unoccupied orbitals contain O 2p orbitals from the oxygen atoms in the TiO core and peripheral ligands. As revealed by DFT calculations, C2 and C3 exhibit similar orbital features in the ferrocene-based HOMOs and TiO core-based LUMOs to that of C1 (Figures S7 and S8). On the basis of the above frontier orbital analysis, the HOMO− LUMO transitions for these ferrocene-containing clusters have the same charge transfer nature that is the MCCT transition from Fe 3d orbitals in Fc groups to Ti 3d orbitals in TiO core. Density of states (DOS) plots further demonstrated that, for these ferrocene-containing clusters, the valence band maxima (VBM) are predominantly made by Fe 3d orbitals along with Cp-based π orbitals. The conduction band minimum (CBM) mainly consists of Ti 3d orbitals (Figure 7 and Figure S9). TDDFT calculations of the singlet electronic transitions for these clusters were performed at their optimized ground-state geometries. The TDDFT-predicted absorption spectra of C1 and the experimental data are shown in Figure 8. The TDDFT calculations suggest that the main low-energy transition at 437

Figure 5. CV curves of C1, C2, and C3.

to HOMO−5 have dominant Fe 3d orbitals contribution, along with some coefficients of π orbitals on Cp rings (Figure 6). Moreover, the orbitals from HOMO to HOMO−3 are degenerate orbitals at the same energy level. The orbitals from HOMO−6 to HOMO−9 are two degenerate pairs formed by combination of π orbitals on the phosphonate

Figure 6. Frontier molecular orbitals of C1. 12779

DOI: 10.1021/acs.inorgchem.7b01527 Inorg. Chem. 2017, 56, 12775−12782

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current response with intermittent xenon light irradiation. The anodic photocurrent response indicates these clusters behave as n-type semiconductor.14 The photocurrent density is in the order of C1 > C2 > C3 > C4. For the {Ti6} clusters, compared to C4, the significantly enhanced photocurrent density of the ferrocene-containing clusters C1 and C2 suggests the photosensitizing effect of ferrocenecarboxylate substituents to titanium-oxo clusters. This photosensitizing effect was also verified by ferrocene and ferrocenecarboxylic acid to the nanocrystalline TiO2. As shown in Figure S11, the TiO2 nanopowder mixed with ferrocene and ferrocenecarboxylic acid, respectively, exhibits remarkably enhanced photocurrent response compared to the bare TiO2. For C1 and C2, the different PEC activity may be ascribed to the structure difference in the phosphonate ligands. As revealed by DFT calculations (Figure S12), the phenylphosphonate-based π* orbitals have better electronic coupling with the Ti d-orbitals compared to that of the benzylphosphonate ligands, which is a character associated with efficient electron transfer. Interestingly, the {Ti5} cluster C3 possessing four Fc sensitizer units displays much lower photocurrent density. As discussed above, in the {Ti5} cluster, the TiO core forms in an open framework structure, and each of the two Ti3(μ3-O) subunits is coordinated with three Fc units. This structure would promote the electronic interaction between the Fc units and TiO core and hence lead to strong electron−hole coupling in excited state. For the {Ti6} cluster with cagelike TiO core structure, the DFT-calculated frontier orbitals show that the LUMOs on Ti 3d and O 2p orbitals form a confined space, which may benefit the stabilization of the excited-state electron density and thus the electron−hole separation. This is in agreement with our previous reported {Ti6} cluster [Ti6(μ3-O)6(OiPr)6(O2CFc)6], in which the cagelike Ti6O6 core was substituted by six ferrocenecarboxylate ligands.7d Its much enhanced photocurrent response in comparison with C4 further shows that the cagelike TiO core structure is beneficial to the PEC performance. Stability of the clusters after PEC experiments was verified by the similar IR spectra with the parent crystal samples (Figure S13). In Figure 9b, the linear region of Mott− Schottky plots exhibit a positive slope, confirming n-type character of the clusters.13b For the C1 photoelectrode, the lower slope of the linear part on Mott−Schottky plots suggests its higher charge carrier density compared to that of C2 and C3.7d,15 These results show that the cluster C1 possesses more enhanced PEC activity than the other clusters.

Figure 7. DOS plots for C1.

Figure 8. Experimental (black line) and TDDFT calculated (blue vertical line) absorption spectra of C1 in CH2Cl2.

nm is dominated by the HOMO→LUMO+1 single-electron transition (Table S2). This transition can be assigned as the MCCT transition from Fe(II)→TiO core. Other noticeable transitions in visible region also have mainly MCCT character. The TDDFT calculations revealed the similar electronic transition characters for C2 and C3 relative to that of C1, in which the MCCT transitions are responsible for the low-energy absorption in the UV−vis spectra (Figure S10). The PEC tests were performed to investigate the PEC activity of the as-synthesized titanium-oxo clusters. As shown in Figure 9a, the photoelectrode of C1 exhibits stable photo-

Figure 9. (a) Photocurrent density vs time measurements at a potential of 0 V in dry CH3CN containing 0.05 M NaPF6 as the supporting electrolyte. (b) Mott−Schottky plots measured at 1 kHz in dry CH3CN containing 0.05 M NaPF6. 12780

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CONCLUSIONS Three phosphonate-stabilized titanium-oxo clusters containing ferrocenecarboxylate ligands were synthesized and characterized. For comparison, the phenylphosphonate-stabilized {Ti6} cluster containing phthalate monoester ligands was also synthesized. The DRS show that these ferrocene-containing clusters exhibit intense visible absorption extended up to 600 nm along with low optical band gaps of ∼2.2 eV, while the phthalate monoester substituted cluster only exhibit UV-light absorption with a large band gap of 3.6 eV. The low-energy transitions of these clusters were systematically investigated by UV−vis spectroscopy and DFT/TDDFT calculations. The experimental and computational results suggest that the ferrocenecarboxylate-substituted titanium-oxo clusters form a donor−acceptor (D−A) system. The electronic interaction between donor and acceptor results in strong low-energy charge-transfer band. The low-energy transition of these clusters primarily involves Fe(II)→TiO MCCT. The {Ti6} clusters with cagelike TiO core structure possess substantially more enhanced PEC activity than that of the {Ti5} cluster with an open framework TiO core. It is suggested that confined space of the TiO core in {Ti6} clusters may benefit the stabilization of the excited-state electron density and the electron−hole separation. This work provides valuable examples for the sensitized TOCs in which electron injection takes place via the MCCT transition.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01527. Crystal data and structure refinements summary; TDDFT calculated main orbital contributions; coordination mode of the TiO core; XRD patterns; IR spectra; TGA curves; photo of the crystals; DRS; selected frontier orbital plots, DOS plots; experimental and TDDFT calculated absorption spectra (PDF) Accession Codes

CCDC 1555926−1555928 and 1570837 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 Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Article

AUTHOR INFORMATION

Corresponding Authors

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

Yang Fan: 0000-0003-4858-4036 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support by the National Natural Science Foundation of China (Nos. 21002082 and 21405129), the Foundation of Henan Educational Committee (No. 15A150078), and the Nanhu Scholars Program for Young Scholars of XYNU. 12781

DOI: 10.1021/acs.inorgchem.7b01527 Inorg. Chem. 2017, 56, 12775−12782

Article

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DOI: 10.1021/acs.inorgchem.7b01527 Inorg. Chem. 2017, 56, 12775−12782