Photoactive Naphthalene Diimide Functionalized Titanium-Oxo Clusters with High Photoelectrochemical Responses

YANG Yu(杨 雨), ZHAO Qixin(赵启新), ZHENG Qi(郑 琦), XUAN Weimin(宣为民)*

1 College of Chemistry and Chemical Engineering, Donghua University, Shanghai 201620, China

2 College of Materials Science and Engineering, Donghua University, Shanghai 201620, China

3 State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Donghua University, Shanghai 201620, China

Abstract：Photoactive functionalized titanium-oxo clusters (TOCs) are regarded as an important model compound for dye-sensitized titanium dioxide solar cells. However, the dyes used for sensitizing TOCs are still limited. Herein, two cyclic TOCs are reported, namely, [Ti6(μ3-O)2(Oi-Pr)8)(LA)2]·i-PrOH (S1) and [Ti6(μ3-O)2(Oi-Pr)8)(LV)2]·i-PrOH (S2), which are functionalized by photoactive naphthalene diimide (NDI) chromophores. Their molecular structures and photophysical and photochemical properties were systematically studied. As shown by ultraviolet-visible (UV-vis) spectra and photocurrent study results, the band gap and the photocurrent response of S1 and S2 were derived from NDI ligands which extend the absorption edge of S1 and S2 approaching 500 nm and afford high photocurrent densities of 2.12 μA/cm2 and 1.95 μA/cm2 for S1 and S2, respectively, demonstrating the significance of the photoactive ligand in modulating photoresponse of TOCs. This work is expected to enrich the structural library of photoactive TOCs and provide insights into understanding the structure-property relationships of sensitized clusters.

Key words：titanium-oxo cluster; naphthalene diimide; photoactive; photoelectrochemical response

0 Introduction

Titanium-oxo clusters (TOCs) have aroused increasing interest owing to their critical role as structural and reactivity models to elucidate the structure-property relationship of nano TiO2at the atomic level[1-5]. The last two decades have witnessed the rapid development of TOCs regarding their structural diversity and promising applications in catalysis, dye-sensitized solar cells (DSSCs),etc.[6-22]. Along with the remarkable progress, rational tuning of the photophysical and photochemical properties of TOCs via structural design has been extensively studied since these are closely related to their employment as photoactive materials[23-24]. So far, functionalization with organic chromophores, hetero metal doping and Ti-O core tuning have emerged as the main strategies to endow TOCs with desirable photoresponsive properties[15,25-31]. This has led to the discovery of a number of functional TOCs which show high efficiency for photo-driven energy conversion and organic transformation[32-35], wide band gap covering the whole visible region and even the near-infrared (NIR) region[36-37], and excellent optical performance[38-39].

In general, TOCs are composed of central Ti-O cores encapsulated by organic ligands on the outer sphere. This inherent structure feature, coupled with the almost unlimited structure library of ligands, renders it quite facile and effective in modulating the photoresponsive properties with ligand functionalization[25]. Based on the milestone work initialized by Coppens’s group[40], usingp-nitrophenyl acetylacetone and coumarin 343 as photoactive ligands, Dai’s group[41-44]further extended this strategy and expanded the ligand scope to a vast variety, including anthracene, tetrathiafulvalene, ferrocene and triphenylamine derivatives. Later on, Zhang’s group[36]conducted the systematic tuning of the band gap by varying the electron-withdrawing effect of the organic ligands. Moreover, Zhang’s group[45]demonstrated the efficiency of using a catechol ligand to reduce the band gap, and this finally resulted in black TOCs with an ultralow band gap of 1.51 eV, a tremendous breakthrough reported very recently by Zhang’s group[39]. With the functionalization of photoactive ligands, the as-synthesized TOCs usually exhibit enhanced photocurrent response, broadened light absorption with modulated band gaps, and better photoluminescence as compared with the pristine TOCs.

Naphthalene diimide (NDI) derivatives are a class of electron-deficient and redox-active dyes that can be used for a variety of applications ranging from biomedicine to electronics[46]. Upon coordination with metal centers, NDI derivatives have been proven to be good organic chromophores for the construction of organic-inorganic hybrid materials such as metal-organic frameworks (MOFs) and metal-organic polyhedral[47-49], showing interesting optical properties[50-51]. However, NDI is rarely used to build metal-oxo clusters despite its excellent photophysical and photochemical properties. In this context, we envision that functionalization with NDI may result in photoactive TOCs showing high photoelectrochemical responses, which is expected to facilitate the discovery of functional TOCs as photoactive materials.

Herein, we report the synthesis of two cyclic TOCs, namely, [Ti6(μ3-O)2(Oi-Pr)8)(LA)2]·i-PrOH (S1) and [Ti6(μ3-O)2(Oi-Pr)8)(LV)2]·i-PrOH (S2), functionalized by photoactive NDI chromophores (Scheme 1). S1 and S2 are synthesized facilely via one-pot reaction and their structures are fully characterized by a variety of spectroscopies and techniques. Both of the compounds adopt the same architecture of [Ti6(μ3-O)2(Oi-Pr)8)] in which two triangular {Ti3(μ3-O)} units are connected by two NDI ligands. As revealed by solid-state ultraviolet-visible (UV-vis) absorption spectra, the introduction of NDI ligands extends the absorption edge of S1 and S2 to the visible-light region, with band gaps of 2.85 eV and 2.91 eV, respectively. Moreover, owing to the presence of NDI ligands, S1 and S2 exhibit excellent photocurrent density up to 2.12 μA/cm2and 1.95 μA/cm2, respectively.

*——indicating the structural frames that omit the isopropyl group.

1 Experiments

1.1 Materials and characterization methods

Reagents and solvents were commercially available and employed without further purification.

Powder X-ray diffraction (PXRD) results were recorded on a diffractometer (DX-2700B, Dandong Haoyuan Instrument Company Limited, China) with monochromatized Cu-Kα radiation (λ=0.154 0 nm) and a scanning rate of 0.02(°)/s. Thermogravimetric Analysis (TGA) was performed on a thermogravimetric analyzer (TG8000, Mettler Toledo Instruments (Shanghai) Company Limited, China) under nitrogen flow at a typical heating rate of 10 ℃/min. Element analyses for C, N and H mass fractions were determined by an elemental analyzer (VARIDEL III, Elementar Trading (shanghai) Company Limited, China). The samples were prepared as a KBr pellet and the Fourier transform infrared (FTIR) spectra were collected in the transmission mode in a range of 500 to 4 000 cm-1using a spectrometer (NEXUS-670, Thermo Electron Corporation, USA). All compounds were prepared into powders which were tested for solid-state UV-vis spectra in the wavelength mode by integrating sphere attachment in a UV-Vis spectrometer (UV3600, Shanghai Titan Technology Company Limited, China). All photoelectrochemical measurements (photocurrent, the Mott-Schottky plots and electrochemical impedance spectra (EIS)) were carried out by using an electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Company Limited, China) in a three-electrode system, with the sample-coated indium tin oxide (ITO) glass as the working electrode, a Pt wire as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. The electrolyte was Na2SO4(0.2 mol/L) aqueous solution. For the preparation of working photo-electrodes, the crystal sample (about 5 mg) was dispersed in a mixed solution of 1 mL ethanol and 100 μL Nafion, which was then dropped onto a precleared ITO glass (1 cm×2 cm). The working photoelectrode was obtained after evaporation. The Mott-Schottky plots were also measured over an alternating current frequency of 1 000, 1 500 and 2 000 Hz. Electrochemical impedance spectra measurements were recorded over a frequency range of 1 000 kHz to 0.1 Hz with an amplitude of 20 mV at 0 V.

1.2 Synthesis of ligands H2LA and H2LV

The ligands 2,2′-(1,3,6,8-tetraoxo-1,3,6,8-tetrahydrobenzo[lmn][3,8]phenanthroline-2,7-diyl)dipropionic acid(H2LA) and 2,2′-(1,3,6,8-tetraoxo-1,3,6,8-tetrahydrobenzo[lmn][3,8] phenanthroline-2,7-diyl)bis(3-methylbutanoic acid) (H2LV) were synthesized according to the reported procedure based on the condensation of naphthalene dianhydride and the related amino acids[49].

1.3 Synthesis of S1

H2LA(0.063 6 mmol, 26.1 mg) was dissolved in a mixed solution ofi-PrOH (1.1 mL) and dimethylformamide(DMF) (1.1 mL). The solution was stirred for 5 min, and then Ti(Oi-Pr)4(1.2 mmol, 0.368 mL) was added to the glass vial and mixed at room temperature. The resultant solution was heated at 80 ℃ for 2 d. After the solution cooled to room temperature, the pale-yellow crystal S1 was obtained (yield: 35.25 mg, 8.23% based on Ti(Oi-Pr)4). The calculated mass fractions of C, H and N were 51.04%, 6.78% and 2.62%, respectively. According to elemental analysis, the mass fractions of C, H and N were 51.32%, 6.96% and 2.53%, respectively.

1.4 Synthesis of S2

H2LV(0.063 6 mmol, 29.7 mg) was dissolved in a mixed solution ofi-PrOH (1.0 mL) and DMF (1.2 mL). The solution was stirred for 5 min, and then Ti(Oi-Pr)4(1.2 mmol, 0.368 mL) was added and mixed in the solution at room temperature. The resultant solution was heated at 80 ℃ for 2 d. After the solution being cooled to room temperature, the pale-yellow crystal S2 was obtained (yield: 33.75 mg, 7.48% based on Ti(Oi-Pr)4). The calculated mass fractions of C, H and N were 52.76%, 7.16% and 2.49%, respectively. According to elemental analysis, the mass fractions of C, H and N were 53.02%, 7.03% and 2.53%, respectively.

1.5 X-ray crystallography

The crystallographic data of S1 and S2 were collected using synchrotron radiation (λ=0.067 0 nm) on beamline 17B1 at National Facility for Protein Science Shanghai (NFPS) in Shanghai Synchrotron Radiation Facility, China. Then the diffraction data reduction and integration were performed by the APEX3 program, which converted the format into sfrm files. The empirical absorption correction was conducted by using the SADABS program. The structures were solved by intrinsic phasing with the software ShelXT and refined with a full-matrix least-squares technique of ShelXL interpreted by the software Olex2. Anisotropic thermal parameters were applied to all non-hydrogen atoms except the isolated guest molecules. The hydrogen atoms were generated by the riding mode. The X-ray crystallographic data for structures reported in this article were deposited at Cambridge Crystallographic Data Centre, under deposition number CCDC-2239929-2239930. These data can be obtained free of charge from Cambridge Crystallographic Data Center (www.ccdc.cam.ac.uk/data_request/cif).

2 Results and Discussion

2.1 Description of crystal structures

Single-crystal X-ray diffraction structural analysis reveals that S1 crystallizes in the orthorhombic Pnna space group. S1 features a cyclic [Ti6(μ3-O)2(Oi-Pr)8)(LA)2] structure in which two {Ti3(μ3-O)} units are connected by two LA. The outer surface of each {Ti3(μ3-O)} unit is surrounded by two carboxylate groups from two NDI ligands and eight isopropoxy groups. Two isopropoxy groups act as bridging ligands while the others as terminal ligands (Figs. 1(a) and 1(b)). Interestingly, one of the terminal isopropoxy is located within the cavity defined by the cyclic architecture, and the weak intra C—H…π interaction (0.304 9 nm) could be identified between hydrogen atoms on methyl groups and the NDI ring (Fig.1(c)). The {Ti3(μ3-O)} unit is a versatile building block that can be easily transferred to build cyclic, triangular and rectangular TOCs[52-55]. In contrast to reported cyclic PCT-71[52]where two cyclohexane groups on ligands adopt almost parallel orientation, two NDI rings in S1 twist towards each other owing to the increasing steric hindrance, with the dihedral angle of 68.57° (Figs. 1(b) and 2(a)). S2 shares the same framework as S1 except for the different substituents on NDI ligand H2LV(Fig.1(d)). The presence of bulky isopropyl exerts more steric hindrance, which pushes the NDI rings to twist outward, giving a larger dihedral angle of 71.19° for S2 (Fig.2(b)).

Fig.1 Structural representations of S1 and S2: (a) {Ti3(μ3-O)} unit; (b) and (d) ball-and-stick representations of molecular structures of S1 and S2; (c) C—H…π interaction between isopropoxy group and NDI in S1

Fig.2 Dihedral angles of two NDI rings in different compounds: (a) S1; (b) S2

In crystallography, crystal structure is described in terms of lattice and cell. The lattice is the periodic arrangement of atoms or molecules, which can be viewed as a three-dimensional(3D) coordinate system consisting of three mutually perpendicular coordinate axes, usually denoted bya,bandc. The neighboring NDI rings of S1 adopt two kinds of π-π interactions with centroid-centroid distances of 0.367 6 nm and 0.376 8 nm (Fig.3(a)), resulting in a one-dimensional (1D) superamolecular chain alongbaxis (Fig.3(b)). Moreover, weak hydrogen bonds are found between the methyl substituent on ligands and O atoms on the NDI rings (0.321 6 nm and 0.349 6 nm), which further assist the formation of 1D superamolecular chain (Fig.3). The introduction of peripheral isopropyl substituent in S2 diminishes the π-π interactions, and the shortest centroid-to-centroid distance between neighboring NDIs is 0.516 6 nm. Instead, C—H…π contacts of 0.271 0 nm and 0.276 0 nm arise from aromatic protons and the planar NDI rings. Also, the weak hydrogen bonds (0.312 7 nm) form among adjacent NDI moieties (Fig.4). It is well known that π-π stacking plays an important role in charge transport[56]. Indeed, a series of conductive MOFs operate on the π-π stacking between a diverse set of organic cores, such as anthracene, naphthalene and NDI[57-59]. Based on the packing modes of S1 and S2, it can be therefore reasonably concluded that S1 will in principle provide a more favorable pathway than S2 for charge transfer.

Fig.3 Structures of S1: (a) π-π interactions; (b) packing structure

Fig.4 Structures of S2: (a) C—H…π contacts and weak hydrogen bonds; (b) packing structure

2.2 Structural characterization of S1 and S2

The experimental PXRD patterns of S1 and S2 match well with the simulated ones (Fig.5), confirming their high phase purity. The TGA curves show that clusters S1 and S2 have good thermal stability with the onset temperature of thermal decomposition at about 150 ℃ (Fig.6). For S1 (Fig.6(a)), the first mass loss of 4.08% in the range of 30 to 120 ℃ is attributed to the removal of isopropanol guest molecules. Then the isopropoxy ligands are lost between 120 ℃ and 300 ℃, corresponding to a mass loss of 31.31%. Further mass loss (10.12%) at 300-460 ℃ is attributed to the decomposition of carboxylate ligands. Afterward, the whole framework of S1 collapses and transforms into TiO2. A similar mass loss profile for S2 is shown in Fig.6(b).

Fig.5 Experimental and simulated PXRD patterns: (a) S1; (b) S2

Fig.6 TGA curves: (a) S1; (b) S2

Fig.7 FTIR spectra of samples before and after photoelectrochemical experiments: (a) S1; (b) S2

As shown by the solid-state UV-vis spectra (Fig.8(a)), the absorption bands of S1 and S2 mainly derive from the π-π transition of the NDI ligand, with the absorption edge approaching 500 nm. Owing to the similar structure, the maximum absorption wavelengths of S1 and S2 are also close to each other,i.e., 385 nm for S1 and 383 nm for S2. The small shoulder peaks observed at 450-510 nm for S1 and 425-500 nm for S2 could be tentatively assigned to the charge transfer bands from NDI to Ti(IV) metal, which overlapped with the NDI absorption band. This phenomenon has been observed in photoactive ferrocene and triphenylamine-anchored TOCs[19,60-61]. According to the Kubelka-Munk function, the band gaps for S1 and S2 are 2.85 eV and 2.91 eV, respectively (Figs. 8(b) and 8(c)). In Figs.8(b) and 8(c),F(R) is the variable in the Kubelka-Munk formula. Compared with the nonphotoactive cyclohexanedicarboxylate-functionalized PCT-71[52], the π conjugation in the NDI ligands can effectively reduce the transition energy, broaden the light absorption, and narrow the band gap of S1 and S2.

Fig.9 Mott-Schottky plots of samples in 0.2 mol/L Na2SO4 aqueous solution: (a) S1; (b) S2

2.3 Photoelectrochemical properties

The Mott-Schottky measurement of the TOC-treated photoelectrodes was conducted at the frequencies of 1 000, 1 500 and 2 000 Hz to explore the lowest unoccupied molecular orbital (LUMO). Figure 9 shows the Mott-Schottky plots of the samples, whereCdenotes the capacitance. The positive slopes in the plots confirm that S1 and S2 are n-type semiconductor materials. According to the Mott-Schottky plots, the LUMO potential levels vs. the normal hydrogen electrode (NHE) of S1 and S2 are -0.14 V and -0.27 V, respectively. Based on the band gaps of S1 and S2, the highest occupied molecular orbital (HOMO) potential levels (vs. NHE) of S1 and S2 are evaluated as 2.71 V and 2.64 V, respectively.

The photoelectrochemical activities of S1 and S2 were investigated under cyclic irradiation with Xe light (300 W). The photoelectrodes treated with S1 and S2 show reversible transient short-circuit photocurrent responses (Fig.10(a)), indicating rapid photoinduced electron-hole separation in the photoelectrodes. It is worth noting that the photoelectrodes treated with S1 and S2 exhibit excellent photocurrent response, with values of 2.12 and 1.95 μA/cm2, respectively. These values are relatively high and comparable with those of other TOCs functionalized by photoactive ligands such as ferrocene[32,60-61], which could be ascribed to the effective charge transfer derived from NDI. Electrochemical impedance spectroscopy was also carried out. As impedance is a vector, it is represented in the plane in the form of a complex number, where the horizontal and vertical coordinatesZ′ andZ″ are the real and imaginary parts of the complex number, denoting resistance and reactance, respectively. It is clearly seen from Fig.10(b) that the electrochemical impedance of the photoelectrodes treated with S1 is slightly lower than that of the photoelectrode treated with S2, indicating that the surface charge transfer rate of S1 is faster than that of S2. Therefore, the charge separation efficiency of S1 is better than that of S2, which is also consistent with the photocurrent responses.

Fig.10 Photoelectrochemical measurements to investigate the charge separation efficiency: (a) photocurrent responses of S1 and S2; (b) electrochemical impedance spectroscopy of S1 and S2

3 Conclusions

In summary, two cyclic TOCs S1 and S2 are successfully synthesized and functionalized by photoactive NDI ligands bearing different substituents. S1 and S2 show quite similar cyclic structures consisting of two {Ti3(μ3-O)} units linked by NDI chromophores. The incorporation of NDI endows S1 and S2 the photophysical and photochemical properties. As reflected by solid-state UV-vis spectra and the Kubelka-Munk function, the band gaps of S1 and S2 exhibit n-type semiconductor characteristics of 2.85 eV and 2.91 eV, respectively. In particular, S1 and S2 exhibit excellent photocurrent responses with photocurrent densities of 2.12 and 1.95 μA/cm2, respectively. This work provides valuable models for investigating the relationship between the structure and photochemical properties of TOCs using photoactive ligands.