Phenyl-Bis-Naphthyl Derivative-Based Artificial Light-Harvesting System for Singlet Oxygen Oxidation

Abstract

A novel artificial light-harvesting system (LHS) for the photooxidation reaction was constructed by the phenyl-bis-naphthyl derivative (PBN) and water-soluble phosphate-pillar[5]arene (WPP5). After host–guest interaction, WPP5 integrated with PBN to form WPP5-PBN amphiphiles, which self-assembled to WPP5-PBN nanoparticles. Based on the aggregate state of PBN in WPP5-PBN nanoparticles, WPP5-PBN nanoparticles emitted a significant yellow fluorescence as energy donors. Due to the yellow fluorescence fully covering the absorption of sulforhodamine 101 (SR101), SR101 was used as energy acceptors and loaded in WPP5-PBN nanoparticles for constructing the WPP5-PBN-SR101 LHS, whose energy transfer efficiency and antenna effect were 66.32% and 22.34. Notably, after the energy of the WPP5-PBN antenna transferred to SR101, more singlet oxygen (¹O₂) production was observed in the WPP5-PBN-SR101 LHS, which was successfully used as a photocatalyst to catalyze the oxidation reaction of 4-methoxythioanisole to 1-methoxy-4-(methylsulfinyl)benzene, imitating the solar energy conversion to chemical energy.

1. Introduction

Photosynthesis is one of the most important light-harvesting and energy transfer processes in nature, which converts solar energy to chemical energy for life activities [1,2]. In a natural light-harvesting system (LHS), solar energy is absorbed by a pigment-protein complex and transferred to the reaction center for photocatalysis, which achieve solar energy conversion and storage [3,4]. Currently, inspired by the solar energy utilization of natural LHS, more artificial light-harvesting systems are being developed to imitate the solar energy transfer and conversion behavior by fluorescence resonance energy transfer (FRET) [5,6,7,8]. Initially, artificial LHSs were usually constructed by aggregation-caused quenching (ACQ) donors, which underwent multi-step covalent synthesis in an organic environment and performed lower emission efficiency in an aqueous environment. Fortunately, Tang et al. revealed a novel emission mechanism of aggregation-induced emission (AIE) in an aggregate state, which provided an approach to designing aggregate state donors and constructing artificial LHSs in an aqueous environment [9,10]. Recently, supramolecular strategy has been widely used in the construction of AIE-based artificial LHSs in an aqueous environment because of the higher assembly efficiency and the lower synthesis limit [11,12]. Moreover, supramolecular AIE-based artificial LHSs can not only actualize the energy transfer in an aqueous environment, but also finish the solar energy output for photocatalysis [13,14,15]. For example, Hu et al. reported a tetraphenylethylene-embedded pillar[5]arene-based artificial LHS, which significantly achieved the dehalogenation of bromoacetophenone in an aqueous environment [13]. Yi et al. developed a tetraphenylethylene-based artificial light-harvesting metallacycle system, which was successfully utilized to catalyze the C-H alkylation reaction in an aqueous solution [14]. Xing et al. designed a triphenylamine-based supramolecular artificial light-harvesting organic framework, which exhibited the significant photocatalytic ability of C-P bond coupling [15]. However, the photocatalysis applications of artificial LHSs always focused on the traditional chemical reactions and photocatalytic oxidation to sulfoxide was rarely reported. Therefore, it was meaningful to design a novel artificial LHS for the photocatalytic oxidation reaction in water.

Herein, a novel artificial LHS for the photooxidation reaction to sulfoxide was constructed by the host–guest interaction of phenyl-bis-naphthyl derivative (PBN) and water-soluble phosphate-pillar[5]arene (WPP5). As shown in Scheme 1, after WPP5 binding with PBN, the supramolecular amphiphilic WPP5-PBN self-assembled to WPP5-PBN nanoparticles, which emitted significant yellow fluorescence as donors. Due to the yellow emission of WPP5-PBN nanoparticles fully covering the absorption of sulforhodamine 101 (SR101), SR101 was loaded as acceptors in WPP5-PBN nanoparticles. After tightly stacking in an assembly process, a prominent fluorescence resonance energy transfer (FRET) was constructed in WPP5-PBN-SR101 nanoparticles and the harvested energy of WPP5-PBN antennas was transferred to SR101, whose fluorescence significantly changed to a red emission, confirming the construction of WPP5-PBN-SR101 LHS. Moreover, benefitting from the energy transfer behavior, more energy was transferred to excite SR101, which achieved a significant singlet oxygen (¹O₂) production. Notably, ¹O₂ could be used as photocatalysts to catalyze the oxidation reaction of 4-methoxythioanisole to 1-methoxy-4-(methylsulfinyl)benzene, which successfully imitated the energy conversion of the solar energy to chemical energy, suggesting WPP5-PBN-SR101 potential in artificial light-harvesting energy storage.

2. Results and Discussion

2.1. Construction of Supramolecular Nanoparticles

WPP5 was synthesized according to the reported procedures [16,17]. PBN was synthesized by using the 6-hydroxy-2-naphthaldehyde as starting materials, which suffered three-step reactions (Scheme 2 and Figures S1–S7). After obtaining WPP5 and PBN, their host–guest interaction was investigated by UV–Vis absorption spectrum [4,16]. As shown in Figure 1, the 320–500 nm absorption peak of PBN appeared a significant red shift after mixing with the WPP5 solution, confirming their host–guest interaction in water.

Moreover, after adding WPP5 to PBN solution for a while, a significant Tyndall effect light-path was generated in the WPP5-PBN solution by laser pen irradiation, revealing the formation of WPP5-PBN nanoparticles (Figure 2a). Fortunately, benefitting from the formation of WPP5-PBN nanoparticles, a special space structure was generated in the WPP5-PBN nanoparticle. Due to the rich C-H and π bonds in the chemical structures of PBN, WPP5, and SR101, when SR101 was added to the solution of WPP5 and PBN, SR101 could effectively load in the layers of WPP5-PBN via C-H···π and π···π stacking interactions [18,19]. For further exploring the details of these supramolecular nanoparticles, their nano-size distribution and micromorphology were investigated by dynamic light scattering (DLS) and transmission electron microscope (TEM). As shown in Figure 2, the DLS result of WPP5-PBN nanoparticles displayed a small-scale distribution of 190–260 nm, whose average diameter was around 200 nm. The TEM image indicated WPP5-PBN nanoparticles to have a sphere shape. After loading SR101, the distribution of WPP5-PBN-SR101 nanoparticles significantly changed to 160–380 nm, but the morphology of WPP5-PBN-SR101 nanoparticles also exhibited a spherical shape (Figure 2b,d). Furthermore, the nanostructure of WPP5-PBN-SR101 was investigated by X-ray photoelectron spectroscopy (XPS). As shown in Figure S8, the significant C-S, S=O, and S-O signals of SR101 were observed, confirming the encapsulation of SR101 in WPP5-PBN-SR101 nanoparticles.

2.2. Investigation of Energy Transfer Process

Based on the AIE feature of PBN guest, the aggregated PBN in WPP5-PBN nanoparticles emitted significant yellow fluorescence, which could conduct as energy donors (D) to construct an energy transfer process. As shown in Figure 3a, as WPP5-PBN nanoparticles’ yellow fluorescence entirely covered the absorption of SR101, SR101 was considered as energy acceptors (A) to receive the energy from the WPP5-PBN artificial light-harvesting antenna. After loading SR101 in WPP5-PBN nanoparticles, SR101 was tightly loaded in WPP5-PBN amphiphiles and the donor–acceptor distance was in the ideal state, arising a significant FRET process in WPP5-PBN-SR101 nanoparticles. With the molar ratio of SR101 loaded from 1000:1 to 100:1 (D:A), WPP5-PBN’s emission at 551 nm gradually decreased, but the characteristic emission of SR101 at 608 nm exhibited a rapid increase, displaying an efficient artificial energy transfer process in WPP5-PBN-SR101 nanoparticles (Figure 3b). In addition, according to the reported investigation in the energy transfer process [4,8], fluorescence color changes were significant signals for confirming the energy transfer process. The fluorescence spectra of the energy transfer process were often converted into fluorescence color coordinate points in a CIE chromaticity diagram, which clearly revealed the fluorescence color change trajectory, identifying the energy transfer process. Thus, when the fluorescence spectra of the above energy transfer process were converted into fluorescence color coordinate points in a CIE chromaticity diagram, the coordinate points in the CIE chromaticity diagram indicated varying amounts of additive and the energy transfer process was clearly observed according to the coordinate trajectory from yellow to red regions, corresponding to the fluorescence color changes from yellow emission in the WPP5-PBN solution to red emission in the WPP5-PBN-SR101 solution (Figure 3d). Moreover, the artificial light-harvesting behaviors were also verified by tracking the lifetime changes in donors (WPP5-PBN) in the energy transfer process [4,8,16]. According to the time-correlated single photon counting (TCSPC) curve of WPP5-PBN nanoparticles, the τ₁ and τ₂ of donors were determined to be 2.19 ns and 9.34 ns by the bis-exponential fitting calculation (Figure S9). However, after loading SR101 in WPP5-PBN nanoparticles, the τ₁ and τ₂ of the donors in WPP5-PBN-SR101 nanoparticles were decreased to 0.96 ns and 4.69 ns, attributing to WPP5-PBN’s energy transferred to SR101 (acceptors) (Figure S10). The τ of the donor was significantly changed from 6.48 ns in WPP5-PBN nanoparticles to 2.64 ns in WPP5-PBN-SR101 nanoparticles, suggesting a convincing energy transfer process in WPP5-PBN-SR101 nanoparticles. The energy transfer efficiency and antenna effect were calculated to be 66.32% and 22.34 in WPP5-PBN-SR101 nanoparticles, displaying the significant light-harvesting ability for energy transfer utilization (Figures S11 and S12).

2.3. Investigation of Photooxidation Reaction

Subsequently, the WPP5-PBN-SR101 LHS was used as a photocatalyst to investigate the harvested energy output for utilization. As shown in Figure S13, 9,10-anthracenediylbis(methylene)dimalonic acid (ABDA) was applied as a singlet oxygen (¹O₂) indicator to explore the ¹O₂ production ability in the WPP5-PBN-SR101 LHS according to the absorbance quench at 376 nm [20]. No significant absorbance quench was observed in WPP5-PBN and SR101 groups, displaying WPP5-PBN and SR101 to have weak ¹O₂ production ability (Figure 4a). But, after the light-harvesting process in WPP5-PBN-SR101, a significant absorbance quench at 376 nm was found, which displayed more ¹O₂ produced in the WPP5-PBN-SR101 LHS due to more energy of WPP5-PBN (donors) transferred to excite SR101 (Figure 4a and Figure S14). In addition, the ¹O₂ production ability of the WPP5-PBN-SR101 LHS was directly measured by electron paramagnetic resonance (EPR), which used 2,2,6,6-tetramethylpiperidine (TEMP) as the ¹O₂ trapping agent. As shown in Figure 4b,c, the ¹O₂ signals were weak in WPP5-PBN and SR101 groups under 365 nm irradiation for 30 min, in accordance with their ABDA experiment results. However, a significant ¹O₂ signal peak was found in the WPP5-PBN-SR101 LHS, attributing to more energy of WPP5-PBN being transferred to excite SR101 for ¹O₂ production (Figure 4d). According to reported studies [21,22], ¹O₂ is usually used as an oxidant for the photocatalytic generation of sulfoxide, which were significant for the chemical-pharmaceutical products. Due to the excellent ¹O₂ production ability, the WPP5-PBN-SR101 LHS was used as the photocatalyst to oxidize the 4-methoxythioanisole to 1-methoxy-4-(methylsulfinyl)benzene, imitating the solar energy conversion to chemical energy of natural LHS (Scheme S1, Figures S15 and S16). As shown in Figure 4e,f, only 3% and 2% yields of the photooxidation products were observed in WPP5-PBN and SR101 groups because of the poor production of ¹O₂. Notably, after the artificial light-harvesting process in WPP5-PBN-SR101, the harvested energy of the WPP5-PBN antenna was efficiently transferred to SR101 for more ¹O₂ production, whose yield was significantly improved to 65%, displaying the WPP5-PBN-SR101 LHS potential in artificial photosynthesis.

3. Materials and Methods

3.1. Synthesis of PBN

Compound 1 was synthesized according to our reported method [23].

Synthesis of Compound 2: Compound 1 (1.17 g, 3.00 mmol), 1,4-phenylenediacetonitrile (0.23 g, 1.47 mmol), and tetrabutylammonium hydroxide (8 drop, 50%, TBAH) were added in 40 mL ethanol and stirred for 24 h. The solid was separated and washed with ethyl ether to afford Compound 2 (1.16 g, 1.29 mmol, 86%). ¹H NMR (CDCl₃, 400 MHz) δ (ppm): 8.24 (s, 2H), 8.13 (d, J = 8.8 Hz, 2H), 7.83–7.78 (m, 8H), 7.72 (s, 2H), 7.22 (dd, J = 8.8, 2.4 Hz, 2H), 7.15 (d, J = 2.0 Hz, 2H), 4.12 (t, J = 6.4 Hz, 4H), 3.43 (t, J = 6.8 Hz, 4H), 1.90–1.83 (m, 8H), 1.53–1.33 (m, 24H). ¹³C NMR (CDCl₃, 100 MHz) δ (ppm): 158.95, 142.90, 135.94, 135.29, 130.95, 130.43, 128.80, 128.40, 127.55, 126.43, 125.77, 120.15, 118.27, 108.98, 106.57, 68.19, 34.12, 32.83, 29.47, 29.39, 29.36, 29.18, 28.77, 28.18, 26.09. HR-ESI-MS: m/z [M + H]⁺ calcd for [C₅₂H₅₉Br₂N₂O₂]⁺ 901.2938, 903.2917, 904.2951, found 901.2936, 903.2893, 904.2961.

Synthesis of Compound PBN: Compound 2 (0.90 g, 1.00 mmol) and trimethylamine (5 mL, 2 M in THF) were added in 20 mL CHCl₃ and refluxed overnight. The solvent was concentrated and washed with ethyl ether to afford Compound PBN (1.01 g, 0.99 mmol, 99%). ¹H NMR (DMSO-d₆, 400 MHz) δ (ppm): 8.40 (s, 2H), 8.28–8.26 (m, 2H), 8.16 (d, J = 8.8 Hz, 2H), 7.97–7.92 (m, 8H), 7.41 (s, 2H), 7.26 (d, J = 9.2 Hz, 2H), 4.15 (t, J = 6.4 Hz, 4H), 3.29–3.24 (m, 4H), 3.04 (s, 18H), 1.84–1.77 (m, 4H), 1.71–1.63 (m, 4H), 1.49–1.44 (m, 4H), 1.32–1.27 (m, 20H). ¹³C NMR (DMSO-d₆, 100 MHz) δ (ppm): 158.83, 143.95, 135.96, 134.96, 131.39, 130.87, 129.36, 128.33, 127.88, 126.80, 125.83, 120.35, 118.60, 108.42, 107.32, 68.22, 65.74, 52.59, 29.39, 29.24, 29.08, 28.98, 26.23, 26.02, 22.50. HR-ESI-MS: m/z [M − 2Br]²⁺ calcd for [C₅₈H₇₆N₄O₂]²⁺ 430.2979, found 430.2973.

3.2. Construction of Supramolecular Nanoparticles

Firstly, the PBN solution in DMSO (0.02 M), WPP5 solution in water (0.42 mM), and SR101 solution in DMSO (0.20 mM) were prepared, respectively. Then, 10 μL PBN solution and 72 μL WPP5 solution were mixed in water and diluted to 4 mL by ultrapure water for constructing WPP5-PBN nanoparticles. Next, 10 μL PBN solution, 72 μL WPP5 solution, and (0–10) μL SR101 solution were mixed in water and diluted to 4 mL by ultrapure water for constructing WPP5-PBN-SR101 nanoparticles. The nanoparticles solution was prepared by ultrapure water under room temperature. The molar ratio of [PBN]:[WPP5]:[SR101] in artificial LHS for photocatalysis was 100:15:1.

3.3. Photooxidation of 4-Methoxythioanisole

4-Methoxythioanisole (80 μL) was added in the freshly prepared LHS solution (4 mL) in a 10 mL glass vial. The mixture was bubbled by air, and then irradiated by 9 W 365 nm LED at room temperature to obtain the photooxidation product.

4. Conclusions

In summary, we have successfully constructed a new artificial light-harvesting system (LHS) by the supramolecular interaction of water-soluble phosphate-pillar[5]arene (WPP5) host molecule and phenyl-bis-naphthyl derivative (PBN) guest molecule. Based on the aggregate emission mechanism in the AIE effect, WPP5-PBN nanoparticles emitted a significant yellow fluorescence after the self-assembly process, displaying prospective donor ability in artificial LHSs. Due to WPP5-PBN’s yellow fluorescence fully covering the absorption of sulforhodamine 101 (SR101, acceptor), WPP5-PBN conducted as a light-harvesting antenna and transferred energy to SR101 after loading SR101 in WPP5-PBN nanoparticles, whose energy transfer efficiency and antenna effect were 66.32% and 22.34. Benefitting from the efficient energy transfer process in the WPP5-PBN-SR101 LHS, more energy was transferred to excite SR101 and produce a significant singlet oxygen (¹O₂), which had potential for the photooxidation reaction. Subsequently, the WPP5-PBN-SR101 LHS was used as a photocatalyst to oxidize the 4-methoxythioanisole to 1-methoxy-4-(methylsulfinyl)benzene, whose yield was significantly improved to 65%, imitating the solar energy conversion to chemical energy.