Assembly of cyclic ferrocene-sensitized titanium-oxo clusters with excellent photoelectrochemical activity

Abstract

The development of crystalline titanium-oxo clusters has made great progress in recent years. However, the geometric assembly of titanium-oxo clusters is still very challenging. Herein, we report the assembly of two cyclic titanium-oxo clusters with ferrocene connectors, formulated as Ti₁₂(μ₃-O)₄(Fcdc)₄(OⁱPr)₃₂·0.5CH₂Cl₂ (Fcdc = 1,1′-ferrocenedicarboxylic acid; Ti₁₂Fcdc₄) and Ti₁₈(μ₂-O)₆(μ₃-O)₆(Fcdc)₂(L1)₂(L2)₄(OⁱPr)₃₄·2HOⁱPr (L1 = isonicotinic acid; L2 = salicylhydroxamic acid; Ti₁₈Fcdc₂). Single-crystal X-ray diffraction studies demonstrate that ferrocene with two carboxylic arms can adhere to the surface of {Ti₆} and/or {Ti₃} units, resulting in fascinating cyclic structures of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂. Notably, ferrocene ligands can not only serve as connectors to induce the geometric assembly of Ti-oxo clusters, but can also act as photosensitizers to greatly improve the light absorption behavior of the resulting compounds. Furthermore, the DFT calculation results suggest that the HOMO → LUMO transition mainly involves ferrocenyl group → Ti-oxo core charge transfer. Using the two ferrocene-containing clusters as electrode precursors, both electrodes exhibited excellent photocurrent responses. Our work opens a new way for the geometric assembly of titanium-oxo clusters and also contributes to the deep understanding of the structure–property relationships of sensitized Ti-based materials.

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Introduction

Recently, crystalline titanium-oxo cluster materials have attracted a significant amount of attention.^1–8 The main motivation not only comes from their potential applications in photocatalysis and photoelectric conversion, but also lies in their accurate atomic structure which is beneficial for theoretical calculations and mechanism studies.^9–14 In the past decade, crystalline titanium-oxo clusters have achieved rapid growth in their structural library.^15–25 Although some attractive titanium-oxo cluster architectures were assembly synthesized and characterized by X-ray diffraction,^26–35 such as cages, prisms, and cyclic structures, the rational design and geometric assembly of titanium-oxo clusters is still very challenging. Various synthesis parameters, including temperature, reactant ratio, solvent, and coordinating ligands, will directly affect the assembly of titanium-oxo clusters.^36–40 Among them, the coordinating ligands as one of the most important raw materials play crucial roles in the assembly of titanium-oxo clusters.^41–49

The metallocene-based molecule 1,1′-ferrocenedicarboxylic acid (FcdcH₂) is a potential coordinating ligand. The FcdcH₂ connector can display multiple conformations formed by the different angles between the carboxylate groups, which can be helpful for the assembly of complexes with fascinating geometries.^50–52 Additionally, ferrocene-containing complexes, which are a class of potential excellent photoelectric materials, usually show narrow band gaps and excellent photoelectric responses.^53–55 However, as far as we know, only a few titanium-oxo clusters with FcdcH₂ ligands have been reported to date, and they also show broad light absorption and efficient charge transfer.^56–58 Based on the above considerations, we hope to effectively regulate the structural geometry and light absorption of titanium-oxo clusters by employing FcdcH₂ as a bridging ligand, thereby realizing the controllable assembly and photoelectric applications of titanium-oxo clusters containing ferrocene connectors.

Herein, we successfully synthesized two cyclic titanium-oxo clusters with FcdcH₂ connectors, formulated as Ti₁₂(μ₃-O)₄(Fcdc)₄(OⁱPr)₃₂·0.5CH₂Cl₂ (Fcdc = 1,1′-ferrocenedicarboxylic acid; Ti₁₂Fcdc₄) and Ti₁₈(μ₂-O)₆(μ₃-O)₆(Fcdc)₂(L1)₂(L2)₄(OⁱPr)₃₄·2HOⁱPr (L1 = isonicotinic acid; L2 = salicylhydroxamic acid; Ti₁₈Fcdc₂). Single-crystal X-ray diffraction studies demonstrate that the {Ti₆} and/or {Ti₃} units are connected by bridging FcdcH₂ connectors to generate fascinating cyclic structures of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂. We found that ferrocene ligands can not only serve as connectors to induce the assembly of titanium-oxo clusters, but can also act as photosensitizers to greatly improve the light absorption behavior of the resulting compounds. The DFT calculation results suggest that the HOMO → LUMO transition mainly involves ferrocenyl group → Ti-oxo core charge transfer. Using the two compounds as electrode precursors, both electrodes exhibited excellent photocurrent responses.

Experimental

Synthesis of Ti₁₂(μ₃-O)₄(Fcdc)₄(OⁱPr)₃₂·0.5CH₂Cl₂ (1, Ti₁₂Fcdc₄)

To a 23 mL Teflon-lined autoclave were added 55 mg of FcdcH₂ (0.2 mmol) and 3 mL of CH₂Cl₂, and then 100 μL of Ti(OⁱPr)₄ (0.3 mmol) was added. After stirring for 10 min, the mixture was transferred to an oven at 100 °C for 72 h. After cooling, red crystals of Ti₁₂Fcdc₄ were obtained (76% yield based on Ti(OⁱPr)₄). IR data (cm⁻¹): 1633 (m), 1491 (s), 1394 (s), 1358 (m), 1194 (m), 1029 (w), 924 (w), 787 (m), 618 (w), 576 (w), 521 (w).

Synthesis of Ti₁₈(μ₂-O)₆(μ₃-O)₆(Fcdc)₂(L1)₂(L2)₄(OⁱPr)₃₄·2HOⁱPr (2, Ti₁₈Fcdc₂)

To a 23 mL Teflon-lined autoclave were added 55 mg of FcdcH₂ (0.2 mmol), 77 mg of salicylhydroxamic acid (0.5 mmol), 62 mg of isonicotinic acid (0.5 mmol), and 5 mL of isopropanol, and then 100 μL of Ti(OⁱPr)₄ (0.3 mmol) was added. After stirring for 10 min, the mixture was transferred to an oven at 100 °C for 72 h. After cooling, yellow crystals of Ti₁₈Fcdc₂ were obtained (68% yield based on Ti(OⁱPr)₄). IR data (cm⁻¹): 1600 (s), 1577 (m), 1509 (s), 1399 (s), 1358 (w), 1312 (w), 1267 (m), 1249 (w), 1194 (w), 1157 (w), 1102 (w), 1033 (m), 920 (w), 763 (m), 677 (m), 613 (m), 521 (w).

Results and discussion

Syntheses

Crystals of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ were obtained by a solvothermal reaction. Fig. 1 displays the crystal pictures of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂. During synthesis, we found that the solvent has an important effect on the assembly synthesis of Ti₁₂Fcdc₄. When dichloromethane was replaced by isopropanol, only a microcrystalline sample could be obtained. As for the synthesis of Ti₁₈Fcdc₂, salicylhydroxamic acid as an auxiliary ligand plays a significant role in the reaction system. If there is no salicylhydroxamic acid, only a solution sample could be obtained. For the discussion of the molecular structures, intrinsic characteristics, and photoelectric behaviors of these two Ti-oxo clusters containing ferrocene connectors, see the following sections.

Fig. 1 (a) Crystal photograph of Ti₁₂Fcdc₄; inset is the {Ti₃} unit of Ti₁₂Fcdc₄. (b) Ball–stick view of Ti₁₂Fcdc₄. (c) Polyhedral view of Ti₁₂Fcdc₄. (d) Crystal photograph of Ti₁₈Fcdc₂; insets are {Ti₃} and {Ti₆} units of Ti₁₈Fcdc₄. (e) Ball–stick view of Ti₁₈Fcdc₂. (f) Polyhedral view of Ti₁₈Fcdc₂.

Structures

X-ray single crystal diffraction analysis reveals that Ti₁₂Fcdc₄ crystallizes in the orthorhombic system, space group Pbcn, and the structure of Ti₁₂Fcdc₄ consists of twelve Ti⁴⁺ ions, four Fcdc²⁻ connectors, four μ₃-O²⁻, thirty-two –OⁱPr⁻ ions, and half a free CH₂Cl₂ molecule. In the structure of Ti₁₂Fcdc₄, three Ti atoms are linked by one μ₃-O atom to form a planar triangle [Ti₃(μ₃-O)]¹⁰⁺ unit, and four [Ti₃(μ₃-O)]¹⁰⁺ units are connected by four bridging Fcdc²⁻ linkers to form a closed-loop structure of Ti₁₂Fcdc₄ (Fig. 1b and S1†). Notably, the two carboxylate groups of each Fcdc²⁻ connector present an angle of 142.76°, and each carboxylate group was coordinated to two Ti atoms in the [Ti₃(μ₃-O)]¹⁰⁺ unit. Except for the Fcdc²⁻ groups, the outer surface of each [Ti₃(μ₃-O)]¹⁰⁺ unit is surrounded by two bridging isopropyl groups and six terminal isopropyl groups. In the [Ti₃(μ₃-O)]¹⁰⁺ unit, Ti1 and Ti2 (Ti4 and Ti5) atoms are six-coordinated in an octahedral environment, while the Ti3 (Ti6) atom is five-coordinated in a hexahedral environment (Fig. 1c). Furthermore, the space-filling structure and packing structure of Ti₁₂Fcdc₄ are shown in Fig. 2a and b. The distance between the two adjacent [Ti₃(μ₃-O)]¹⁰⁺ units in Ti₁₂Fcdc₄ is about 4.98 Å.

Fig. 2 Space-filling structure and packing structure of Ti₁₂Fcdc₄ (a and b) and Ti₁₈Fcdc₂ (c and d).

X-ray single crystal diffraction analysis revealed that Ti₁₈Fcdc₂ crystallizes in the triclinic system, space group P1̄. In the structure of Ti₁₈Fcdc₂, two [Ti₃(μ₃-O)]¹⁰⁺ units and two [Ti₆(μ₂-O)₃(μ₃-O)₂]¹⁴⁺ units are connected by two Fcdc²⁻ and two isonicotinic acid connectors, resulting in the rectangular structure of Ti₁₈Fcdc₂ (Fig. 1e and S2†). The two carboxylate groups of each Fcdc²⁻ connector present an angle of 135.47°, and four O atoms of the Fcdc²⁻ linker were coordinated with Ti1, Ti2 in the [Ti₃(μ₃-O)]¹⁰⁺ unit and Ti4, Ti5 in the [Ti₆(μ₂-O)₃(μ₃-O)₂]¹⁴⁺ unit, respectively. One N atom and two O atoms of isonicotinic acid were coordinated to Ti4 in the [Ti₆(μ₂-O)₃(μ₃-O)₂]¹⁴⁺ unit and Ti2, Ti3 in the [Ti₃(μ₃-O)]¹⁰⁺ unit, respectively. The two salicylhydroxamic acid ligands are attached to each [Ti₆(μ₂-O)₃(μ₃-O)₂]¹⁴⁺ unit. Moreover, the outer surface of each [Ti₆(μ₂-O)₃(μ₃-O)₂]¹⁴⁺ unit is surrounded by nine isopropyl groups, involving three bridging isopropyl groups and six terminal groups. In the [Ti₆(μ₂-O)₃(μ₃-O)₂]¹⁴⁺ unit, all the Ti atoms except for the Ti8 atom show octahedral TiO₆ coordination environments, while the Ti8 atom is five-coordinated in a hexahedral environment (Fig. 1f). Also, the space-filling structure and packing structure of Ti₁₈Fcdc₂ are shown in Fig. 2c and d. The length and width of the rectangles in Ti₁₈Fcdc₂ are about 4.96 and 4.29 Å, respectively.

The X-ray diffraction (XRD) patterns of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ are in good agreement with those simulated from the crystallographic data (Fig. S5 and S6†), indicating the phase purity of the samples. Thermogravimetric analysis (TGA) showed that Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ have similar thermal behaviour, and the two cluster frameworks remain stable up to approximately 200 °C (Fig. S7 and S8†). The infrared spectroscopy (IR) spectra of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ are shown in Fig. S9 and S10.† The vibrational bands in the range of 600–700 cm⁻¹ can be attributed to Ti–O–Ti vibrations, while the vibrational bands in the range of 1000–1100 cm⁻¹ correspond to Ti–O–C vibrations. The vibration observed at around 1500 cm⁻¹ belongs to the characteristic vibrations of the ferrocene groups. To identify the chemical state of the elements in Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂, X-ray photoelectron spectroscopy (XPS) analysis was carried out. Fig. S11† shows the Ti 2p and Fe 2p regions of the high resolution XPS spectra. There were two signal peaks located at 458.8 and 464.6 eV, corresponding to Ti 2p_3/2 and Ti 2p_1/2, respectively, which confirms the existence of a Ti⁴⁺ cation. The pair of binding energies existing at 708.1 and 720.7 eV correspond to Fe 2p_3/2 and Fe 2p_1/2, indicating the presence of an Fe²⁺ cation.

Light absorption behaviors

To evaluate the light absorption of these compounds, UV-vis absorption spectrum analysis was carried out. As shown in Fig. 3a, Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ display broad absorption bands with an absorption edge at about 600 nm, which is similar to previously reported ferrocene-sensitized Ti-oxo clusters.^55,57,58 The ultraviolet absorption is mainly ascribed to the O_2p → Ti_3d charge transfer transitions in titanium-oxo clusters.^2,4 The remarkable visible absorption bands of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ can be mainly attributed to the charge transfer transition from the ferrocenyl groups to the Ti-oxo cores. The band gaps of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ were calculated to be about 2.05 and 2.11 eV, respectively, from the absorption band edge (Fig. 3b). Actually, both Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ display lower band gaps than pure Ti-oxo clusters. The narrow band gap may be explained by the contribution from charge transfer between the Fcdc ligands and the Ti-oxo cores.

Fig. 3 (a) Solid-state UV/vis absorption spectra and (b) the band gaps of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂.

Density functional theory (DFT) calculations

Theoretical DFT calculations based on the crystal data were performed to further investigate the charge transfer transition process of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂. For Ti₁₂Fcdc₄, the electron densities of the highest occupied molecular orbitals (HOMO to HOMO−3) are mainly assigned to the Fe 3d orbitals of the ferrocenyl groups, and the lowest unoccupied molecular orbitals (LUMO to LUMO+3) are predominantly dominated by contributions from the Ti 3d orbitals of the Ti-oxo core (Fig. 4a). The DFT calculation results suggest that the HOMO → LUMO transition mainly involves the charge transfer from the ferrocenyl groups to the Ti-oxo cores. For Ti₁₈Fcdc₂, most coefficients of the HOMO, HOMO−2 and HOMO−3 are located on the salicylhydroxamic acid and FcdcH₂ ligands, and the LUMO, LUMO+2 and LUMO+3 are mainly located on the Ti-oxo core and isonicotinic acid (Fig. 4b). This result suggests that for Ti₁₈Fcdc₂, the HOMO → LUMO electron transition can mainly correspond to the charge transfer from the salicylhydroxamic acid and Fcdc ligands to the Ti-oxo cores.

Fig. 4 Some related frontier molecular orbitals of Ti₁₂Fcdc₄ (a) and Ti₁₈Fcdc₂ (b).

Photoelectrochemical (PEC) measurements

To investigate the carrier charge transfer efficiency, we measured transient photocurrent responses of these Ti-oxo clusters containing ferrocene connectors. As shown in Fig. 5a, Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ as electrode precursors exhibit clear photocurrent responses upon on–off cycling irradiation. This rapid rise and fall in photocurrent density indicates that the carriers transport in these cluster-based materials proceeds quickly. The incorporation of FcdcH₂ ligands into Ti-oxo structures may effectively promote the charge transfer transition between the ferrocenyl groups and Ti-oxo cores and greatly extend the visible light absorption band, resulting in the excellent photoelectric activity of these ferrocene-sensitized titanium-oxo clusters. The photocurrent densities are 0.53 and 0.46 μA cm⁻² for Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂, respectively. The enhanced photoelectric activity of Ti₁₂Fcdc₄ can be mainly attributed to the broader light absorption and narrower band gap in comparison with that of Ti₁₈Fcdc₂. Moreover, the intrinsic structure and coordination environment of Ti-oxo clusters also directly affect their photoelectric responses. The XRD patterns and IR spectra of the samples after photocurrent measurements were basically identical to those of the original compounds, indicating that these compounds are stable during the photoelectric experiments (Fig. S5 and S6†).

Fig. 5 (a) Photocurrent responses of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂. (b) Mott–Schottky plots of Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂.

Fig. 5b displays the Mott–Schottky plot. The slopes of the plots for these photoelectrodes were positive which demonstrated that Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂ exhibit typical characteristics of n-type semiconductors. A lower slope was observed for Ti₁₂Fcdc₄ compared to that of Ti₁₂Fcdc₄, suggesting a higher carrier density. The corresponding flat band potential can be obtained from the interception of the plots with the potential axis. They are −0.26 V (vs. NHE) for Ti₁₂Fcdc₄ and −0.20 V (vs. NHE) for Ti₁₈Fcdc₂. For an n-type semiconductor, the flat band potential is about 0.1 eV positive than the conduction band potential. Based on the band gap, the corresponding valence band potentials are calculated to be about 1.69 and 1.81 V (vs. NHE) for Ti₁₂Fcdc₄ and Ti₁₈Fcdc₂, respectively. Additionally, the Nyquist plots of electrochemical impedance spectroscopy (EIS) could illustrate the interfacial charge transfer process (Fig. S12†). The radius of the arc is related to the charge transfer resistance occurring on the surface of the electrode, and the smaller radius means the higher charge transfer efficiency. Furthermore, the size of the semicircle for the Ti₁₂Fcdc₄ electrode is smaller than that of the Ti₁₈Fcdc₂ electrode, suggesting the greatly improved charge transfer efficiency. These findings are also consistent with the results of the photocurrent measurements.

Conclusions

In summary, two Ti-oxo clusters containing ferrocene connectors were successfully synthesized using {Ti₆} and/or {Ti₃} units as building blocks and FcdcH₂ ligands as bridging connectors. The introduction of ferrocene ligands has positive effects on the light absorption behaviors and carrier charge transfer of the resulting clusters. Moreover, these two clusters as electrode precursors showed excellent photocurrent responses under visible light irradiation, suggesting that they may have promising applications in fields such as photocurrent sensors and dye-sensitized solar cells. Further studies on the assembly engineering and photoelectric behaviors of Ti-oxo clusters containing ferrocene ligands as photosensitizers are underway.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

C. Wang acknowledges the support from the Qilu University of Technology Science Research Initial Fund.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2164045(1) and 2164046(2). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2qi01007k

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