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Titanium Oxo Cluster with Six Peripheral Ferrocene Units and Its Photocurrent Response Properties for Saccharides Jin-Le Hou,† Wen Luo,†,‡ Yao Guo,† Ping Zhang,† Shen Yang,† Qin-Yu Zhu,*,† and Jie Dai*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China The Key Laboratory of Rare Earth Functional Materials and Applications, Zhoukou Normal University, Zhoukou 466001, P. R. China

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S Supporting Information *

ABSTRACT: A unique titanium oxo cluster with a ferrocene ligand was synthesized and characterized by single crystal X-ray analysis. Six ferrocene carboxylates coordinate to a D3d Ti6O6 core to be a redox active cluster 1, [Ti6O6(OiPr)6(O2CFc)6]. An analogue 2, [Ti6O6(OiPr)6(O2CiBu)6], where the redox active ferrocene group is replaced by isobutyrate, is also reported as a contrast. The six ferrocene moieties in 1 are structurally identical to give a main redox wave at E1/2 = 0.62 V in dichloromethane investigated by cyclic voltammetry. Photocurrent responses using electrodes of clusters 1 and 2 were studied, and the response properties of 1 are better than those of 2. The electronic spectra and theoretical calculations indicate that charge transfer occurs from ferrocene to Ti(IV) in 1, and the presence of the ferrocene moiety gives efficient electron excitation and charge separation. Cluster 1 is a cooperative system of TiO cluster and redox active ferrocene. Photocurrent response properties of an electrode of 1 for four saccharides, glucose, fructose, maltose, and sucrose, were tested, and only reducing sugars were responsive. The electrode of 2 is also photocurrent responsive to saccharides, but the current densities are lower than those of redox active 1.

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INTRODUCTION

Taking advantage of the unique and stable electrochemical properties, ferrocene (Fc) derivatives as smart materials have been applied in information storage devices, molecular switches, chemical detection, and so on.6 Recently, ferrocenium/ferrocene (Fc+/Fc) has been shown to be efficient as a redox coupled electrolyte for DSSCs involving a simple chargeregeneration process (no intermediates, and solely a one electron-transfer reaction) and has been used to replace the I2/ I3− redox system.7 The ferrocene unit can also act as a donor unit in photosensitizers. Dyes coupled with ferrocene units have been synthesized, and such dye anchored TiO2 electrodes show quite high efficiencies in DSSCs.8 Ferrocene derivatives with varying anchoring groups have been prepared, and their light-harvesting properties have been investigated. It has been found that they may effectively replace ruthenium compounds in DSSCs.8b However, ferrocene derivatives incorporated with TOCs as functional compounds for the applications of

Titanium oxo clusters (TOCs) have attracted great attention because they can be considered as the model compounds helping to understand the bonding structures and energy states of nano-titanium oxide materials at the atomic level.1 A number of nanoscale TOCs with the number of Ti atoms being up to 52 have been reported, and the TOCs can be also applied to photocatalytic hydrogen evolution.2 The most attractive progress is using TOCs modified with chromophores as model compounds for theoretical studies of dye-sensitized solar cells (DSSCs).3 We have recently focused on incorporating various functional ligands onto TOCs and using them as the single molecular source in fabrication of nano and film materials. Our aim is to bring some insights into the potential applications of functionalized TOCs.4,5 We designed and synthesized dye-modified TOCs to improve the photocurrent conversion efficiency, fluorescent groups modified TOCs to be luminescent materials, and TOCs with redox active moieties to be redox or electronic active materials. © XXXX American Chemical Society

Received: February 28, 2017

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DOI: 10.1021/acs.inorgchem.7b00522 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Molecular structures of (a) [Ti6O6(OiPr)6(O2CFc)6] (1) and (b) [Ti6O6(OiPr)6(O2CiBu)6] (2), showing the polyhedron views of the central cluster structures. Hydrogen atoms are omitted for clarity. autogenous pressure and then cooled to room temperature. Orange crystals are obtained (26% yield based on Ti(OiPr)4). The crystals are rinsed with ethanol and dried. These crystals are preserved under sealed and dry environment. Anal. Calcd for C84H96Fe6O24Ti6 (MW 2112.03): C, 47.77; H, 4.58. Found: C, 47.59; H, 4.65. IR data (KBr, cm−1): 2971(w), 2930(w), 2861(w), 1575(s), 1520(m), 1480(vs), 1396(s), 1357(s), 1195(m), 1125(s), 1000(s), 921(w), 857(w), 815(w), 776(w), 710(s), 638(w). [Ti6O6(OiPr)6(O2CiBu)6] (2). Compound 2 was prepared using a similar method to that for 1, except that the FcCOOH was replaced by isobutyric acid in anhydrous isopropanol. A thick Pyrex tube (0.7 cm dia., 18 cm length) is used to place the mixture and quickly degassed by argon. The tube is sealed and heated at 40 °C for 3 days under autogenous pressure, and then cooled to room temperature. Colorless crystals are obtained (30% yield based on Ti(OiPr)4). The crystals are rinsed with ethanol and dried. These crystals are preserved under sealed and dry environment. Anal. Calcd for C42H84O24Ti6 (MW 1260.49): C, 40.02; H, 6.72. Found: C,39.88; H, 6.85. IR data (KBr, cm−1): 2972(w), 2928(w), 2867(w), 1592(s), 1536(m), 1472(s), 1437(s), 1382(w), 1361(w), 1327(w), 1300(m), 1125(s), 1004(s), 853(w), 719(s), 658(w), 624(w). X-ray Crystallographic Study. A Rigaku Mercury CCD diffractometer with graphite monochromated Mo Kα (λ = 0.71075 Å) radiation were used to carry out the measurements at 293 K. X-ray crystallographic data for 1 and 2 were collected and processed using CrystalClear (Rigaku).12 The structures were solved by direct methods using SHELXS-16 for 1 SHELXS-13 for 2 program, and the refinements were performed against F2 using SHELXL-16 for 1 SHELXL-13 for 2.13 All the non-hydrogen atoms are refined anisotropically. The hydrogen atoms are positioned with idealized geometry and refined with fixed isotropic displacement parameters. Detailed crystal data and structural refinement parameters for 1 and 2 are listed in Table S1. CCDC 1527244 and 1527245 contain the supplementary crystallographic data for 1 and 2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Electrode Preparation and Photocurrent Measurement. A solution coating method is used to prepare the photocurrent measurement electrodes of compounds 1 and 2. The new prepared 1 (10.0 mg) was dissolved in 10.0 mL of chloroform (0.47 mmol·L−1). The solutions were dropped on the precleaned ITO plate (1.5 × 4.0 cm2, 50 Ω per square cm) and then spin coated at 1000/min. The coating film was obtained after repeated treatment five times. The electrode of 2 was prepared in the same manner. A light source is a 150-W high pressure xenon lamp, positioned 15 cm far from the surface of the ITO electrode. The photocurrent experiments were parallel performed on a CHI650 electrochemistry workstation using a quartz cell equipped with three-electrodes, the sample coated ITO glass as the working electrode, a Pt plate auxiliary electrode, and a saturated calomel reference electrode (SCE). An aqueous solution of Na2SO4 (0.1 mmol·L−1, 100 mL) was used as the supporting electrolyte.

photosensitivity or photocurrent conversion have been received few attentions. Therefore, it is of interest to obtain ferroceneTOCs (Fc-TOCs) and to understand the synergistic effect of the attached ferrocene moieties and TiO structure in photoexcitation and photoelectric conversion. Until now very few TOCs functionalized with ferrocene have been known, and their applications have not been considered yet. A trinuclear {(Fc2Bob)Ti(μ-O)}3 (Fc2BobH2 = bis(ferrocenyldiketonate))9 and a tetranuclear Ti4O2(OiPr)6(FcCOO)6 (FcCOO = ferrocene-1-carboxylate)10 compound were synthesized and structurally characterized. Recently, Stoddart et al. reported a Ti4O17 cluster that consists of two ferrocene units as bridges, and as a result, an unusual squareplanar tetracoordinate oxygen is generated.11 Thus, more novel ferrocene-based TOCs, and their applications are expected. Herein we report a new multiferrocene modified TOC in which six ferrocene carboxylates (FcCO2H) are coordinated to a Ti6O6 core to be a [Ti6O6(OiPr)6(O2CFc)6] (1). An analogue [Ti6O6(OiPr)6(O2CiBu)6] (2) where the redox active ferrocene group is replaced by isobutyrate is also reported as a contrast. Photoactive electrodes are prepared using these compounds by a solution coating method, and their photoelectric response properties for saccharides are examined. It is found that TOCFc is a potential sensor material for reducing sugars due to the good photocurrent response properties.

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EXPERIMENTAL SECTION

General Remarks. All analytically pure reagents were purchased commercially and used without further purification. Fourier transform infrared (FT-IR) spectra were recorded as KBr pellets on a Nicolet Magna 550 FT-IR spectrometer. Elemental analyses of C and H were carried out using a VARIDEL III elemental analyzer. Solid-state roomtemperature optical diffuse reflectance spectra of the micro crystal samples were gained with a Shimadzu UV-3150 spectrometer using BaSO4 as a standard reference. Powder X-ray diffraction (PXRD) of the compounds were carried out on a D/MAX-3C X-ray diffraction meter with CuKα (λ = 1.5406 Å) radiation. The morphologies of the resulted films were observed with a JSM-5600LV scanning electron microscope (SEM). Thermal analysis was conducted on a TGA-DCS 6300 microanalyzer. The samples were heated under a nitrogen stream of 100 mL·min−1 with a heating rate of 20 °C·min−1. Cyclic voltammetry (CV) experiments were performed on a CHI650 electrochemistry workstation in a three-electrode system, a Pt plate working electrode, a saturated calomel electrode (SCE) as reference electrode, and Pt wire as the auxiliary electrode. Synthesis of the Clusters. [Ti6O6(OiPr)6(O2CFc)6] (1). FcCOOH (4.6 mg, 0.002 mmol) and analytically pure Ti(OiPr)4 (0.1 mL, 0.26 mmol) were mixed in 0.5 mL of toluene. A thick Pyrex tube (0.7 cm dia., 18 cm length) is used to place the mixture and quickly degassed by argon. The tube is sealed and heated at 140 °C for 1 h under B


Article

Inorganic Chemistry

Figure 2. (a) The experimental XRD pattern and the crystal data simulated pattern of compound 1. (b) Solid-state UV−vis absorption spectra of 1, 2, and FcOOH.

Figure 3. (a) Cyclic voltammogram of 1 in CH2Cl2 (0.1 mmol·L−1, 10 mL) under repeated cycles. (b) Cyclic voltammogram of 1 in CH2Cl2 (0.1 mmol·L−1, 10 mL), after 2 mg of glucose powder was added; then repeated cycles were measured in 1 min intervals (0.1 mol·L−1 Bu4NClO4, 100 mV·s−1, vs SEC). Theoretical Calculations. Density functional theory calculations were carried out using GAUSSIAN 09 program package for 1 and 2 at the B3LYP level.14a The basis set used for C, O, and H atoms was 631G, while effective core potentials with a LanL2DZ basis set was employed for Ti and Fe atoms.14b,c 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.

organometallic compound, a tin oxo cluster [Sn6O6(Bu)6(O2CFc)6], was reported by Chandrasekhar,16 where the TiOiPr is replaced by SnBu. Cluster 2 consists of the same Ti6O6 unit, but the six FcCO2− ligands are replaced by six iBuCO2− ligands. Therefore, 2 is a non-redox active analogue of 1 and can be used as a contrast. The X-ray diffraction (XRD) patterns of the bulky microcrystal samples of 1 and 2 are in agreement with those simulated from the data of single-crystal analysis (Figure 2a, Figure S2), showing the purity of the bulky samples. Solid-state UV−vis absorption spectra of the clusters, calculated from the diffuse-reflectance spectra are shown in Figure 2b. An intense absorption band around 462 nm for 1 is close to the representative band of the ferrocene (452 nm) but with obvious higher intensity. The intense band should arise for both the ferrocene core and charge transfer. The charge transfer occurs from FcCO2− ligand to Ti(IV) due to the strong electron-donating ability of ferrocene and the high positive charge of Ti(IV) ion, which has been confirmed by theoretical calculations (see below). The on-set energy of 1 is 2.2 eV (orange, 564 nm), while that of 2 is 3.5 eV (colorless, 354 nm). The FT-IR spectra of clusters 1 and 2 are shown in Figure S3. The vibrations, 1575 cm−1 for 1, and 1573 cm−1 for 2, indicate the chelating coordination of the carboxyl group. Isopropoxy groups are detected by the νC−H (between 2980 and 2850 cm−1) and νTi−O−C (1000 and 1003 cm−1) vibrations. The bands around 710 to 720 cm−1 are assigned to the Ti−O vibrations. The band about 1480 cm−1 for 1 is assigned to the characteristic bands of the ferrocene moiety. Thermogravi-

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RESULTS AND DISCUSSION Synthesis and Characterization. Crystals of clusters 1 and 2 were prepared directly by one step in situ synthesis in anhydrous toluene or isopropanol (see Experimental Section). The structures of 1 and 2 were characterized by single crystal X-ray analysis. Ball−stick plots along with the polyhedron views of the Ti6O24 cluster of the structures are shown in Figure 1. The structures of 1 and 2 with thermal ellipsoid plots are provided in the Supporting Information (Figure S1). The structure of 1 is assembled by bridging ferrocene carboxylate, terminal alkoxide ligands, and μ3-bridging oxo atom. Each titanium atom is located at the center of an approximately octahedral array of oxygen atoms to form a titanium oxo cluster with D3d Ti6O6 core. A number of the Ti6O6 core as in 1 with various carboxylates have been reported, [Ti6O6(OR)6(O2CR′)6].15 Although the carboxylate bridging D3d Ti6O6 core has been documented, integration of the Ti6O6 core with redox active ferrocene ligand is unprecedented. The six large antenna groups of ferrocene are around the Ti6O6 core and direct in the same rotating orientation. A similar C


Article

Inorganic Chemistry

Figure 4. (a, b) SEM images of 1 film on ITO substrate prepared by solution coating method in five layers in different field views.

Figure 5. Photocurrent densities of 1 treated electrode (a) and 2 treated electrode (b) under different bias 0.4 V (green), 0.6 V (blue), and 0.8 V (red) (three electrodes cell with 0.10 mol·L−1 Na2SO4).

1 min intervals. The intensity of current is time-related and a significant current decrease is observed along with the scan time. The time-related decreasing in current should be attributed to the factor of reaction dynamics (a slow reaction between adsorbed 1 and glucose). These phenomena are disadvantageous for quantitative detection. Photocurrent Response Properties. The photocurrent response properties of 1 and 2 are examined using a three electrode cell with the cluster coated ITO working electrodes. The morphology of 1 coated electrode was characterized by SEM, and a film with unique fused crystals was recorded (Figure 4). The XRD patterns of the 1 and 2 coated electrodes after soaking in water are the same as those of the crystals (Figures S2 and S5). Therefore, unlike most of polyoxotitanates, compounds 1 and 2 are stable even treated and measured as the electrode materials. All the photocurrent experiments were carried out in a 0.10 mol·L−1 Na2SO4 electrolyte solution under illumination with a 150-W high pressure xenon lamp (a more detailed description is given in the Experimental Section). Upon repetitive irradiation, a photocurrent response was observed (Figure 5). Unlike the solution system in CV measurements, the solid state electrode system showed a steady photocurrent generation. As shown in Figure 5a,b, the photocurrent densities of cluster 1 treated electrode at different bias potentials are about three times those of cluster 2 treated electrode. However, the photocurrent of 1 treated electrode cannot reach the maximum immediately (Figure 5a) in comparison with that of 2 (Figure 5b) when the electrode is irradiated, which might be because the ferrocene moiety slows down the electron mobility of the electrode. As their photocurrent densities are in the same order of magnitude

metric analysis (Figure S4) indicates no cocrystallized solvent exists, because the temperature of the first inflection of the curve is over 200 °C. Clusters 1 and 2 decompose at 250−425 °C with the removal of isopropanol and decomposition of FcCO2 moiety, and finally the oxides formed. Electrochemical Behaviors. To explore the redox behavior, cyclic voltammetry (CV) of the ferrocene derivative TOC was conducted in CH2Cl2 (Figure 3a). The CV behaviors of 1 showed main reversible redox waves at E1/2 = 0.62 V, attributed to the uncoupled one electron redox of the Fc groups. The six ferrocene moieties are structurally identical so that the oxidation occurred at the nearly same time as that reported for [Sn6O6(Bu)6(O2CFc)6].16 The E1/2 potential is larger than that of the free ferrocene under the same experimental conditions, E1/2 (Fc+/Fc) = 0.558 V (vs SCE) due to the electron withdrawn property of the Ti(IV) atom. It was found that the current of E1/2 wave increased gradually when the cycle scan repeated. This behavior can be explained by electro accumulation (deposition) of the clusters on the electrode. At the beginning, besides the main redox wave, a weak wave at a more positive potential, E1/2′ = 0.730 V, can be observed. Then, it is covered when the main peak increased. This weak wave should be assigned to the compound in the solution state. Generally, the intensity of a deposition peak is much more intense than that of the peak of molecule in solution. Several ferrocene-based sensors have been reported for the recognition of saccharides based on electrochemical technique, mainly on CV experiments.17 The current intensity of 1 was also sensitive to the added glucose, and the result is shown in Figure 3b. After repeated cycles, 2 mg of glucose powder was added, and then repeated cycles were measured in D


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Inorganic Chemistry

Figure 6. DFT calculated frontier molecular orbitals of 1 (first line) and 2 (second line).

moiety, the electron hole pairs are generated easily and separated more effectively for 1 due to the following two reasons (1) the CT band of 1 increasing the absorption range and (2) the HOMO−LUMO separate occupation enhancing the charge separation and preventing the combination of electrons and holes.18 Photocurrent Response Properties for Saccharides. Saccharides are important biological molecules as they play essential roles in many biological processes, such as nutrition, metabolism, immunological protection, and cell structure formation.19 A lot of ferrocene-based electrochemical sensors for saccharides have been reported;17 however, photoelectrochemically response systems have acquired very little attention. In the case of compound 1, because the current intensity is time-related in the CV experiment and is constant in the photocurrent measurement, the latter will be a useful technique for sensors. Figure 8a shows photocurrent responses of cluster 1 treated electrode to the concentration of glucose. The current density is increased along with the increasing concentration of glucose (Figure 8b). The linearly response range is from 2.0 g/L to 5.0 g/L with the linear correlation coefficient R = 0.996 and the saturated concentration is 6.3 g/L. Although the detection limit and range have no practical significance in chemical analysis, the properties are noteworthy. The sensitivities of electrode of 1 for four saccharides, glucose, fructose, maltose, and sucrose, were tested and only reducing sugars, glucose, fructose, and maltose, are responsive, where fructose is most sensitive to the test (Figure 9a). The mechanism should be that at a constant bias potential, 0.4 V, the Fc group is photoexcited and transfers electrons from the excited state to Ti(IV), and then the oxidized Fc+ group obtains electrons from glucose to return to the ground state. A schematic view is given in Figure 9b. Electrode of non redox active 2 also showed a response to the concentration of glucose, but the current density is lower than that of electrode of 1. The current density of 2 is only about 1/2−1/3 of that of 1 (Figure S6) because the electron transfer for 2 involves only the Ti(IV) and glucose.

and 2 is an analogue of 1 except that the redox active ferrocene group is replaced by isobutyrate, the Ti6O6 cluster must be the photoactive center in both compounds. The increase of photocurrent densities of 1 treated electrode in comparison with that of 2 treated electrode should benefit from the redox character of the ferrocene moiety. The ferrocene moiety can be considered as a photosensitive antenna that has a lower excited energy gap than that of the TiO core, and then the light harvesting efficiency is increased. Theoretical Calculations. The results of theoretical calculations of 1 and 2 show that most coefficients of the highest occupied molecular orbitals (HOMO, HOMO-1, and HOMO-2) of compound 1 are all located on the ferrocene moiety (donor), and the lowest unoccupied frontier molecular orbitals (LUMO, LUMO+1, and LUMO+2) are mainly located on the TiO cluster (acceptor) (Figure 6 first line). The results strongly support that 1 is an intramolecular CT compounds and support the discussion of photoexcited electron transfer mentioned above. In comparison with the separated distribution of HOMOs and LUMOs in 1, it can be clearly observed that the HOMOs and LUMOs of 2 are not separated and are mainly located on TiO cluster center (Figure 6 second line). Calculated density of states (DOS) of 1 and 2 are plotted in Figure 7. There is a clear change of DOS between 1 and 2,

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Figure 7. Calculated density of states (DOS) of 1 and 2.

CONCLUSIONS In summary, we combine TOC with ferrocene in a unique structure where six ferrocene carboxylates coordinate to a Ti6O6 core to be a redox active TOC cluster 1. An analogue 2 is also reported as a contrast, where the redox active ferrocene group is replaced by isobutyrate. Their structures are characterized by single crystal X-ray analysis. The six ferrocene moieties in 1 are structurally identical to give a one-electron redox wave, and the potential of E1/2 of 1 is 0.62 V in dichloromethane. The CV of 1 is time-related due to the

though they are isostructural compounds. The energy gap of 1 between the unoccupied bands (>−2.5 eV) and the occupied bands (