Photocurrent, Photodegradation, and Proton Conductivity of the Stable Dipyridyl and Thiophene-Functionalized Cu^II₂ Supramolecular Compound

Abstract

Due to its excellent visible light absorption characteristics, the photocurrent, photodegradation, and proton conductivity of the stable dipyridyl and thiophene-functionalized supramolecular compound [Cu₂(TAA)₄(4,4′-bpy)]_n (Cu^II₂ for short, HTAA = 2-thiopheneacetic acid, 4,4′-bpy = 4,4′-bipyridine) have been studied in detail. The current density of photocurrent of Cu^II₂ is 1.87 μA·cm⁻², and Cu^II₂ degrades methylene blue (MB) with a degradation efficiency of 68.0% under xenon lamp. In addition, Cu^II₂ shows remarkable proton conductivity of 1.79 × 10⁻³ S·cm⁻¹ (at 75 °C and 98% relative humidity), superior to most copper(II)-based coordination polymers (CPs), and is expected to become a potential proton conductor in the future.

1. Introduction

Coordination polymers (CPs), metal-organic frameworks (MOFs), and covalent organic frameworks (COFs) are attracting interest, owing to their excellent optical and electrical performance, and are widely used in fields such as photothermal therapy [1,2,3], photothermal water vaporization [4], photothermal catalysis [5,6], fuel cells [7], dye degradation [8,9], and proton conduction [10,11,12]. CPs, as a class of crystalline materials, have also been widely studied for their well-defined structures and fascinating properties, which can be structurally categorized into one-dimensional (1D) chains, and two-dimensional (2D) and three-dimensional (3D) framework structures [13,14]. Simultaneously, 1D linear chains can display 2D layers and even 3D supramolecular structure, owing to their noncovalent interactions, including hydrogen bonding and π···π stacking interactions [15]. Copper, an Earth-abundant and relatively low-cost material, is one of the most studied transition metals in coordination polymers due to its flexible coordination modes [16,17,18]. Furthermore, the introduction of acidic functional group organic ligands, such as -COOH, -PO₃H, -SO₃H, enhances their acidity and hydrophilicity to obtain CPs with high proton conductivity, contributing to a better understanding of the link between proton conductivity, structure, and the acidity of functional groups [19,20,21,22,23]. Copper(II) CPs, based on sulfonic/carboxylic ligands, display a higher proton conductivity [24,25,26,27,28,29]. Therefore, the design and synthesis of new coordination compounds is extremely important for creating high-performance proton conductivity materials.

The 3D supramolecular compound [Cu₂(TAA)₄(4,4′-bpy)]_n (1) (Cu^II₂ for short, HTAA = 2-thiopheneacetic acid, 4,4′-bpy = 4,4′-bipyridine) reported by us in 2024 [30] contains two Cu^II ions, four thiophene-functionalized TAA⁻ ligands, and one dipyridyl-based 4,4′-bpy ligand in an asymmetric unit, as shown in Figure 1a. Cu^II₂ contains the classic binuclear [Cu₂O₈] unit, which further forms a 1D linear chain by bridging 4,4′-bpy ligands, consolidated by intramolecular hydrogen bonds. A 2D layer is formed through the intermolecular π···π interactions between the 4,4′-bpy and TAA⁻ ligands beyond the 1D chains, resulting in a final 3D supramolecular structure via additional intermolecular π···π interactions between the TAA⁻, as shown in Figure 1b.

The introduction of hydrophilic functional groups into the compound structure can form an effective hydrogen-bonding network, along which protons jump from a proton donor to an acceptor, reducing the proton conduction activation energy of the compound [31,32,33,34]. In Cu^II₂, the 3D supramolecular structure, constructed by π···π stacking and hydrogen-bonding interactions and benefitting from the 4,4′-bpy and hydrophilic TAA⁻ ligands, may exhibit high proton conductivity. Furthermore, Cu^II₂ shows strong and broad absorption spectrum, even showing characteristics of near-infrared absorption. Therefore, in this paper, the photocurrent and photodegradation of MB, and the proton conductivity of Cu^II₂ were investigated.

2. Results and Discussion

2.1. Photocurrent Response

Because of the strong absorption of Cu^II₂ in the UV-vis region [30], we have developed a strong interest in the photocurrent response characteristics of Cu^II₂. Therefore, the photocurrent responses of Cu^II₂ and blank FTO glass were investigated. As shown in Figure 2a, Cu^II₂ exhibits stable and clear photocurrent, with a density of 1.87 μA·cm⁻², which suggests that Cu^II₂ has high generation and separation efficiency of photoinduced electrons–hole pairs in FTO electrodes [18,35]. We separately conducted Fourier transform infrared spectroscopy (FT–IR) tests on Cu^II₂ before and after the photocurrent test; the FT–IR of the tested photoelectrochemical properties were equivalent to the original sample, as shown in Figure 2b. This indicates that the sample did not undergo hydrolysis during the electrode preparation process, and the structure of Cu^II₂ did not undergo significant changes [36,37,38].

2.2. Photodegradation

The smaller band gap value means that the material can absorb light in a wider wavelength range and can show better photocatalytic performance [30,39,40,41]. The photocatalytic activity of Cu^II₂ was determined by the degradation of methylene blue (MB). Blank experiments demonstrated that the presence of Cu^II₂ increased the photocatalytic decomposition rate of MB. As shown in Figure 3a, the irradiation time varied with changes in MB concentration. The maximum absorption peaks of the MB solution decreased significantly in the presence of Cu^II₂; notably, the maximal absorbance at 664 nm decreased from 2.38 to 0.77 with prolonged irradiation time. Cu^II₂ reduced the MB concentration by 68.0% after irradiation for 420 min.

Under irradiation, the photocatalyst Cu^II₂ absorbs photons and excites electrons from the valence band (VB) to the conduction band (CB) of the catalyst. Because Cu^II₂ contains an energy band structure, when the energy of the incident light is greater than the band gap during xenon lamp irradiation, electrons are excited from the VB to the CB, generating photogenerated electrons and holes simultaneously. These excited electrons rapidly migrate to the catalyst surface and participate in the degradation reaction. Then these photogenerated electrons and holes react with O₂, OH, and H₂O and produce ·OH, ·O₂, etc. These strong radical species degrade the dye into small molecules of CO₂ and H₂O [41]. During the process of photocatalytic degradation of MB by the photocatalyst Cu^II₂, H₂O is available; the catalyst interacts with the H₂O molecules, generating hydroxyl radicals as part of the photocatalytic process. These radicals subsequently contribute to the photo-oxidation of MB [42]. In addition, as shown in Figure 3b, the FT–IR spectra of Cu^II₂ were essentially unchanged before and after MB degradation, indicating the stability and reusability of Cu^II₂ [43], which is expected to be applied to the treatment of organic pollutants in sewage.

2.3. Proton Conductivity

The PXRD pattern of Cu^II₂ immersed in the water for one week (Figure 4, blue line) suggests outstanding structural integrity and water stability, laying a good foundation for electrochemical applications [44].

We were curious about the proton conduction performance of the 3D supramolecular Cu^II₂ constructed by the hydrophilic functional TAA⁻ ligands. Therefore, proton conductivity was measured by alternating current (AC) impedance at different temperatures and relative humidity (44%, 58%, 67%, 76%, 86%, and 98% RH) (

Table 1. Proton conduction test value of Cu^II₂ (σ/S·cm⁻¹).

T/°C	44% RH	58% RH	67% RH	76% RH	86% RH	98% RH
30		2.27 × 10−6	4.61 × 10−5	2.96 × 10−4	7.58 × 10−4	1.06 × 10−3
35		3.64 × 10−6	7.31 × 10−5	3.90 × 10−4	9.11 × 10−4	1.11 × 10−3
40		8.46 × 10−6	1.10 × 10−4	5.31 × 10−4	1.01 × 10−3	1.15 × 10−3
45		2.48 × 10−5	2.16 × 10−4	6.92 × 10−4	1.14 × 10−3	1.22 × 10−3
50		7.61 × 10−5	3.23 × 10−4	7.20 × 10−4	1.26 × 10−3	1.26 × 10−3
55		1.36 × 10−4	4.35 × 10−4	8.14 × 10−4	1.33 × 10−3	1.34 × 10−3
60		2.27 × 10−4	6.65 × 10−4	9.71 × 10−4	1.41 × 10−3	1.41 × 10−3
65		3.88 × 10−4	9.31 × 10−4	1.20 × 10−3	1.47 × 10−3	1.48 × 10−3
70	4.52 × 10−8	4.78 × 10−4	1.13 × 10−3	1.29 × 10−3	1.51 × 10−3	1.67 × 10−3
75	9.32 × 10−8	6.17 × 10−4	1.21 × 10−3	1.47 × 10−3	1.66 × 10−3	1.79 × 10−3

and Figure 5).

Under low relative humidity conditions of 44%, Cu^II₂ did not exhibit proton conduction properties within the range of 30–65 °C. When the temperature rose to 70 °C, Cu^II₂ began to exhibit proton conductivity properties, with a conductivity value of 4.52 × 10⁻⁸ S·cm⁻¹. Keeping the humidity constant, and when the temperature rose to 75 °C, the conductivity value reached 9.32 × 10⁻⁸ S·cm⁻¹, as shown in Figure 5a. Keeping the temperature constant and increasing the relative humidity, the conductivity value of Cu^II₂ also increased. At 75 °C, the conductivity values of Cu^II₂ under 44%, 58%, 67%, 76%, and 86% RH conditions were 9.32 × 10⁻⁸ S·cm⁻¹, 6.17 × 10⁻⁴ S·cm⁻¹, 1.21 × 10⁻³ S·cm⁻¹, 1.47 × 10⁻³ S·cm⁻¹, and 1.66 × 10⁻³ S·cm⁻¹, respectively, as shown in Figure 5b–e. The conductivity value of Cu^II₂ reached its maximum value at 98% RH (σ = 1.79 × 10⁻³ S·cm⁻¹), the conductivity value at 98% RH is five orders of magnitude higher than that at 44% RH, as shown in Figure 5f, superior to most CPs [25,26,27] and close to copper(II)-based CPs [28,29].

With the increase in temperature, the hydrophilic functional TAA⁻ ligands in Cu^II₂ increased hydrophilicity and promoted proton transfer. Increased humidity also allowed Cu^II₂ to adsorb water molecules from the high-humidity environment, construct dense proton transport channels, and promote an increase in conductivity values of the compound [45,46,47], as shown in Figure 6.

To comprehend the proton conduction mechanism of Cu^II₂, Arrhenius plots were generated at different humidity levels [48], as shown in Figure 7. In low-humidity environments, the activation energy values of the proton conduction of Cu^II₂ were 1.52 eV (44% RH), 1.25 eV (58% RH), and 0.72 eV (67% RH), which were greater than 0.4 eV, suggesting that the proton conduction behavior of Cu^II₂ follows the vehicular mechanism, that is, proton movement is driven through the carrier. Under high-humidity conditions, the activation energy values of Cu^II₂ were 0.33 eV (76% RH), 0.17 eV (86% RH), and 0.12 eV (98% RH), respectively, following the Grotthus mechanism.

Therefore, the activation energy values of proton conduction of Cu^II₂ decreases with increasing relative humidity. When the relative humidity is low, the small amount of adsorbed water molecules in the pore channels of Cu^II₂ act as carriers and promote proton transfer through self-diffusion, thereby generating an effective transport pathway. Under high relative humidity, a large number of adsorbed water molecules enter the frameworks of Cu^II₂. Adsorbed water molecules can form a hydrogen bond network, which can provide efficient conducting pathways [44]. To further test the stability of the sample, we conducted PXRD analysis on Cu^II₂ after electrochemical testing. Even under these conditions, the structure of Cu^II₂ did not change [49,50], as shown in Figure 4 (red line), indicating that Cu^II₂ has good structural stability. In addition, FT–IR tests indicated that Cu^II₂ could maintain its own structure before and after the impedance measurement, as shown in Figure 8.

3. Materials and Methods

3.1. Material and Instruments

All reagents were purchased commercially and were not further purified. Cu^II₂ can be synthesized according to reference [30]. Powder X-ray diffraction (PXRD) data were tested on microcrystalline powders using a Bruker D8 Advance diffractometer (purchased from Bruker, Karlsruhe, Germany) with Cu radiation (λ = 1.54184 Å). A PLS-SXE300/300UV xenon lamp source (purchased from Beijing PerfectLight Technology Co., Ltd., Beijing, China) was used to carry out the photocurrent measurement and photodegradation experiments. A PerkinElmer Spectrum GX spectrometer (purchased from Agilent Technologies Inc., Santa Clara, CA, USA) was used to perform the FT–IR from KBr pellets. An Agilent Cary5000 (purchased from Agilent Technologies Inc., Santa Clara, CA, USA) was used to measure ultraviolet-visible (UV-vis) spectra. A CHI660E electrochemistry workstation (purchased from Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) was used for the photocurrent testing and proton conductivity.

3.2. Photocurrent Measurement

The photocurrent responses of Cu^II₂ and blank FTO glass were investigated in a typical three-electrode system using fluorine-doped tin oxide (FTO) glass as the working electrode, Pt as the assisting electrode, Ag/AgCl as the reference electrode, and keeping the voltage at 1.2 V. In general, 2 mg of Cu^II₂ was dispersed into 400 μL of ethanol solution, followed by the addition of 10 μL of naphthol (0.5 wt%), and ultrasonic treatment was performed until Cu^II₂ was completely dispersed. Then the suspension of Cu^II₂ was evenly dripped onto the conductive surface of the FTO glass. The photocurrent experiment was carried out in 0.2 M Na₂SO₄ solution with a xenon lamp as light source (λ = 320 nm, 300 W, intervals of 20 s). The whole experiment proceeded in a continuous and repeated on/off cycle.

3.3. Photodegradation Measurement

The degradation of organic dye-methylene blue (MB) at room temperature was used to measure Cu^II₂’s photocatalytic activity. The test procedure was as follows: To begin, 2 mg MB was weighed and diluted in 200 mL H₂O to make a 12 mg·L⁻¹ MB solution. Second, 12 mg of Cu^II₂ was distributed in MB solution and agitated for 30 min in a dark setting. Under the irradiation of a xenon lamp, 3 mL of reaction solution was taken every 1 h, and the resultant solution was analyzed by an ultraviolet visible spectrophotometer to indirectly study the degradation process of Cu^II₂.

3.4. Proton Conductivity Measurement

We performed proton conduction tests on Cu^II₂. The process was as follows: First, the sample Cu^II₂ was ground into a powder using a mortar. Then, an appropriate amount of the powdered sample was added to a tablet press for the pressing process. The diameter of the sheet sample was 7.00 mm and the thickness was 1.06 mm. Finally, both sides of the sheet sample were coated with conductive silver paste, dried at 50 °C, and fixed to the test bottle using copper wires. Before testing, the sample piece was placed in a humidity vial and stabilized for 48 h.

4. Conclusions

In summary, the 3D supramolecular compound Cu^II₂ reported was composed of the classic binuclear [Cu₂O₈] unit, bridging 4,4′-bpy ligands, and TAA⁻ ligands containing carboxyl groups. The photocurrent, photodegradation of MB, and proton conductivity of Cu^II₂ were investigated. The photocurrent density of Cu^II₂ was 1.87 μA·cm⁻², and Cu^II₂ degraded MB with a degradation efficiency of 68.0% under xenon lamp. Furthermore, Cu^II₂ showed high proton-conductive properties, with a conductivity of approximately 1.79×10⁻³ S·cm⁻¹ at 75 °C and 98% RH. Therefore, copper(II) coordination polymers have excellent proton conduction stability and are expected to become potential proton conductors in the future.