A supramolecular dimer strategy for enhancing the selective generation of sulfides and sulfoxides by visible-light induced photoredox thiol–ene cross-coupling reactions of anthraquinone

Abstract

Enhancing the photocatalytic efficiency of organic photocatalysts and increasing the generation capacity of reactive oxygen species (ROS) have consistently been substantial obstacles. In this study, we designed and synthesized a supramolecular dimer based on anthraquinone (Amvp-CB[8]) between the methylated vinylpyridinium substituted anthraquinone derivative (Amvp) and cucurbit[8]uril (CB[8]) through host–guest interactions in the aqueous solution. Compared to the monomer Amvp, the supramolecular dimer Amvp-CB[8] exhibited significantly enhanced fluorescence and demonstrated remarkable capabilities in generating singlet oxygen (¹O₂) and superoxide anion radicals (O₂˙⁻) in water. Importantly, Amvp-CB[8] displayed superior photocatalytic activity under visible light, facilitating efficient photoredox thiol–ene cross-coupling reactions between phenthiol and styrene, which selectively enabled rapid synthesis of sulfides within 0.5 h and sulfoxides within 12 h, showcasing high efficiency, low catalyst loading, and excellent functional group tolerance. This study introduces a novel supramolecular dimer strategy that enhances the design of efficient photocatalysts for organic conversions in photocatalysis.

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Introduction

Within the field of molecular architecture, supramolecular dimers arise as captivating creatures that exemplify the intricate interplay of atoms and molecules through complex interactions and forces, highlighting the delicate equilibrium between simplicity and complexity.^1–7 Supramolecular dimers are formed through reversible, non-covalent interactions such hydrogen bonding, π–π stacking, electrostatic forces, and van der Waals interactions,^8–30 unlike covalent bonds which create permanent links between atoms. These interactions facilitate the self-assembly of molecules into dimers, resulting in the formation of structures that possess distinct and distinctive features that differ from their individual components. Their intrinsic diversity and configurable qualities make them highly sought after for creating functional materials with precisely tailored properties and behaviors in several fields such as materials science, biology, and nanotechnology.^31–34

Anthraquinone is a highly adaptable chemical molecule that has garnered significant interest in the realm of photocatalysis, which possesses a distinct molecular arrangement consisting of a fused aromatic ring and a carbonyl group.^35–45 This arrangement allows it to efficiently absorb light across a wide range of wavelengths, particularly in the ultraviolet and visible areas. Anthraquinone, functioning as a photocatalyst, possesses notable photochemical characteristics such as exceptional photostability and prolonged excited states, which plays a vital role in the catalytic process.^46,47 Extensive research has been conducted on this topic for a range of purposes, including organic synthesis, pollutant degradation, and solar energy conversion.^48–50 Anthraquinone's capacity for photoinduced electron transfer processes renders it a very promising contender for the advancement of sustainable and eco-friendly photocatalytic devices. So far, photocatalytic systems utilizing anthraquinone have only been transformed into photocatalysts using a single anthraquinone molecule. The non-covalent interaction between anthraquinone molecules is crucial for creating a stronger supramolecular photocatalyst.

Anthraquinone is a very interesting building ingredient in supramolecular motifs because of its ability to trigger catalytic activities when exposed to light. Furthermore, the utilization of anthraquinone-based supramolecular dimers in conjunction with photocatalysis offers a collaborative method to tackle urgent social and environmental issues. In this study, we synthesized a supramolecular dimer (Amvp-CB[8]) by mixing a methylated vinylpyridine salt substituted anthraquinone derivative (Amvp) with cucurbit[8]uril (CB[8]) in an aqueous solution. The self-assembled dimers exhibit a substantial increase in fluorescence emission compared to the monomer, as well as a significant enhancement in their capacity to produce singlet oxygen (¹O₂) and superoxide anion radicals (O₂˙⁻) in the aqueous solution. The supramolecular dimer Amvp-CB[8] demonstrates excellent efficacy and favorable tolerance towards functional groups as a photocatalyst for photoredox thiol–ene cross-coupling reactions involving phenthiol and styrene. This enables the selective and rapid synthesis of sulfides within 0.5 h and sulfoxides within 12 h. These results highlight the significant potential of supramolecular dimers in the field of photocatalysis (Scheme 1).

Scheme 1 Illustration of the construction of supramolecular dimer by the assembly of Amvp and CB[8] for the photoredox thiol–ene cross-coupling reaction.

Results and discussion

The monomer building block of the anthraquinone derivative (Amvp) was synthesized through a two-step reaction, as detailed in Scheme S1,† and confirmed by ¹H NMR and ¹³C NMR spectroscopy (Fig. S1 and S2†). Our initial investigations focused on the formation of supramolecular dimers between Amvp and CB[8], characterized by UV-vis absorption and fluorescence emission spectra. Fig. 1a illustrates that the absorption peak of Amvp at 322 nm decreased upon incremental addition of CB[8] in the aqueous solution, accompanied by a notable redshift, while the absorption at 375 nm exhibited slight increase, indicating encapsulation of the methylvinylpyridine units within the CB[8] cavity to form supramolecular dimers. Meanwhile, the UV-vis absorption spectrum does not change at all upon reaching 0.5 equiv. of CB[8], suggesting a binding ratio of 2 : 1 between Amvp and CB[8]. As shown in Fig. 1b, the fluorescence emission peak at 542 nm increased gradually with the addition of CB[8], reaching maximum intensity at 0.5 equiv. CB[8], confirming the formation of supramolecular dimers between Amvp and CB[8]. Furthermore, the fluorescence quantum yields and lifetimes for Amvp and Amvp-CB[8] can be determined to be 3.7% and 9.1%, 0.66 ns and 1.43 ns, respectively (Fig. S3 and S4†). Additionally, the stoichiometry of the supramolecular dimers formed between Amvp and CB[8] was investigated using Job's plot (Fig. S5†), which indicated the maximum fluorescence intensity at a molar ratio of 0.67, confirming a binding stoichiometry of 2 : 1 between Amvp and CB[8].

Fig. 1 (a) UV-vis absorption spectra of Amvp (2.0 × 10⁻⁵ M) with addition of CB[8] in the aqueous solution; (b) fluorescence emission spectra of Amvp (2.0 × 10⁻⁵ M) with addition of CB[8] in the aqueous solution, inset: photographs of Amvp and Amvp-CB[8] (from left to right) under UV light; (c) DLS of Amvp-CB[8] (2.0 × 10⁻⁵ M), inset: photograph of Tyndall effect of Amvp-CB[8]; (d) zeta potential of Amvp-CB[8] (2.0 × 10⁻⁵ M); (e) ¹H NMR titration experiments of Amvp, Amvp-CB[8], and CB[8] in D₂O ([Amvp] = 2.0 × 10⁻³ M, [Amvp-CB[8]] = 2.0 × 10⁻³ M, [CB[8]] = 1.0 × 10⁻³ M).

Furthermore, the self-assembly properties of Amvp-CB[8] were comprehensively explored using zeta potential and dynamic light scattering (DLS). Initially, the particle size of Amvp alone was determined to be approximately 164 nm (Fig. S6†). Upon addition of CB[8] to the Amvp solution, the average particle size increased to about 255 nm (Fig. 1c), indicating the formation of supramolecular dimers via host–guest interactions. Zeta potential measurements conducted in the aqueous solution revealed that Amvp exhibited a relative zeta potential of +23.9 mV. However, upon addition of 0.5 equiv. of CB[8], the zeta potential of Amvp-CB[8] decreased to +9.9 mV (Fig. 1d). This decrease can be attributed to the shielding effect of CB[8] in the host–guest interaction between Amvp and CB[8], altering the surface charge characteristics of the supramolecular assembly. To further probe the interaction between Amvp and CB[8], ¹H NMR experiments were performed (Fig. 1e). Upon addition of 0.5 equiv. of CB[8] in D₂O, the proton chemical signals corresponding to Amvp exhibited significant broadening in the low field region, while the signals from the methylated vinylpyridine unit were nearly completely shielded. These observations provide direct evidence that Amvp and CB[8] indeed form supramolecular dimers through robust host–guest interactions, laying a solid foundation for their application as efficient photocatalysts in organic transformations under visible light.

Anthraquinone and its derivatives have long been recognized for their efficacy in generating reactive oxygen species (ROS), making them pivotal in photocatalytic applications. In our investigation into the applications of supramolecular dimers, we explored Amvp-CB[8] as a potential photocatalyst. Initially, we employed 2,7-dichlorodihydrofluorescein diacetate (DCFH-DA) as a fluorescent probe to monitor ROS generation by using fluorescence emission spectroscopy. Fig. S7† illustrates a distinct fluorescence emission peak at 525 nm upon irradiation with purple light following the addition of both Amvp and Amvp-CB[8]. However, the enhancement in fluorescence emission at 525 nm was markedly more pronounced with the supramolecular dimer Amvp-CB[8] than with the monomer Amvp, underscoring the significant enhancement in ROS production capacity facilitated by supramolecular dimerization. Next, we utilized 9,10-anthracenediylbis(methylene)dimalonic acid (ABDA) as a probe to assess singlet oxygen (¹O₂) generation by both Amvp and Amvp-CB[8]. Fig. S8† demonstrates a reduction in ABDA absorption intensity upon addition of Amvp under continuous purple light irradiation, indicating effective ¹O₂ generation by Amvp. Similarly, Fig. S9† shows sustained reduction in absorption intensity with Amvp-CB[8] addition, highlighting its enhanced ¹O₂ generation capacity compared to Amvp. To quantify the ¹O₂ generation capability, we compared the ¹O₂ quantum yields using Rose Bengal (RB) as a reference photosensitizer. Amvp-CB[8] exhibited a significantly higher quantum yield (94%) compared to Amvp (48%) (Fig. S10†), confirming the superior ¹O₂ production capability of the anthraquinone supramolecular dimer. Furthermore, we evaluated the superoxide anion radical (O₂˙⁻) generation ability using tetramethyl-phenylenediamine (TMPD) as a probe. Fig. 2b shows characteristic absorption peaks at 563 nm and 612 nm in solutions containing both Amvp and Amvp-CB[8], confirming their respective O₂˙⁻ generation capabilities. Notably, Amvp-CB[8] demonstrated enhanced O₂˙⁻ generation compared to Amvp. To validate the production of active species, electron paramagnetic resonance (EPR) studies were conducted by introducing scavengers such as 2,2,6,6-tetramethylpiperidine (TEMP) or 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) into Amvp-CB[8] solutions. Without irradiation, no ROS signals were detected. However, under purple light irradiation, signals corresponding to both ¹O₂ and O₂˙⁻ were observed (Fig. 2c and d), conclusively confirming the robust ROS generation capability of Amvp-CB[8].

Fig. 2 (a) UV-vis absorption spectral changes of ABDA in the presence of Amvp and Amvp-CB[8] under purple light irradiation; (b) UV-vis absorption spectra of the cationic radical of TMPD generated in the presence of Amvp and Amvp-CB[8]; (c) EPR spectra of the solution of Amvp-CB[8] and TEMP in the aqueous solution in the dark (black) and upon irradiation with purple light (red); (d) EPR spectra of the solution of Amvp-CB[8] and DMPO in the aqueous solution in the dark (black) and upon irradiation with purple light (red) ([Amvp] = 2.0 × 10⁻⁵ M, [Amvp-CB[8]] = 2.0 × 10⁻⁵ M).

Based on these promising discoveries, we conducted detailed studies to explore the potential application of the supramolecular dimer Amvp-CB[8] as a photocatalyst for photoredox thiol–ene cross-coupling reactions between phenthiol and styrene. These experiments were conducted under ambient conditions for 0.5 h using purple light as the excitation source (Fig. S11†). As shown in Table 1, we observed a remarkable 91% yield of product 3a when employing the 0.5 mol% Amvp-CB[8] system (Table 1, entry 1), indicating that the supramolecular dimer exerts a significant promotional effect on the coupling of 4-methylbenzenethiol and styrene. In contrast, the yield was negligible when using only CB[8] as the photocatalyst (Table 1, entry 2), and modest at 36% with Amvp alone as the catalyst (Table 1, entry 3). Furthermore, reducing the concentration of Amvp-CB[8] to 0.25 mol% resulted in a decreased catalytic yield to 64% (entry 4). Conversely, increasing the concentration to 1 mol% did not significantly enhance the yield, which only rose to 93% (entry 5). When there is no photocatalyst, the reaction hardly occurs (entry 6). Adjusting the reaction time showed a notable decrease in product yield by 57% (entry 7), while extending the time had no significant impact on the yield (entry 8). We also introduced the electron donor DIPEA to investigate the role of O₂˙⁻ in the reaction, but the catalytic yield dropped to 20% after the addition of DIPEA (entry 9), which can be attributed to the competitive relationship between DIPEA and the substrate. Notably, the catalysts maintained good activity in the 400–410 nm wavelength range, emphasizing their potential for green and sustainable applications (entry 10). Finally, critical control experiments revealed that the Amvp-CB[8] system failed to proceed in the absence of light and air, underscoring the essential roles of light and oxygen in facilitating the effective function of this catalytic mechanism (entries 11 and 12).

Table 1 Optimization of the reaction conditions^a,b

Entry	Variation from standard conditions	Yield^b (%)
a Reaction conditions: 1a (0.2 mmol, 20.8 mg), 2a (0.3 mmol, 37.2 mg), Amvp-CB[8] as photocatalyst, purple light (395 nm), room temperature (rt). b Isolated yield.
1	None	91
2	CB[8]	No reaction
3	Amvp	36
4	0.25 mol% Amvp-CB[8]	64
5	1 mol% Amvp-CB[8]	93
6	No Amvp-CB[8]	Trace
7	0.2 h	57
8	1 h	94
9	DIPEA	20
10	400 nm–410 nm	74
11	N2	No reaction
12	No light	No reaction

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Under optimized reaction conditions, we thoroughly explored the effectiveness of the novel photocatalyst Amvp-CB[8] across a range of phenthiol and styrene derivatives (Table 2). Amvp-CB[8] proved highly adept at facilitating the coupling of these derivatives with various substituents, consistently yielding the corresponding sulfides in high efficiency (82% for 3b, 83% for 3c, 93% for 3d, 78% for 3e, 76% for 3f, 74% for 3g, 87% for 3h, 80% for 3i, 86% for 3j, 88% for 3k, 84% for 3l, 94% for 3m, 93% for 3n, 66% for 3o, 73% for 3p, 72% for 3q, 71% for 3r, 70% for 3s, 77% for 3t, 74% for 3u, 68% for 3v, 69% for 3w, 67% for 3x, 76% for 3y, 59% for 3z, 60% for 3aa, and 62% for 3ab). This underscores the catalyst's strong promotional effect on the phenthiol-styrene coupling process. Notably, extending the reaction time revealed an intriguing capability of Amvp-CB[8] to selectively convert sulfides into sulfoxides with high selectivity, avoiding excessive oxidation to sulfones. By optimizing the reaction time, we achieved an impressive 86% yield of the sulfoxide (4a) after 12 h. Furthermore, we assessed the catalyst's applicability to diverse derivative substrates for further sulfide oxidation, demonstrating high yields across different substituted derivatives as shown in Table 3 (82% for 4b, 83% for 4c, 88% for 4d, 78% for 4e, 85% for 4f, 87% for 4g, 90% for 4h, 74% for 4i, 76% for 4j, 72% for 4k, 79% for 4l, 69% for 4m, 73% for 4n, and 71% for 4o). This broad applicability underscores the robust performance of Amvp-CB[8] in various chemical environments. To gauge the practicality of this method, we conducted a scale-up reaction. Remarkably, using 7.2 mmol of 4-methylbenzenethiol and 6.0 mmol of styrene under standard conditions, we achieved the product separation yields of 89% (1.22 g) and 82% (1.2 g), respectively. This outcome indicates minimal efficiency loss and highlights the potential scalability and efficiency of Amvp-CB[8] in practical synthetic applications (Scheme S2†). In addition, after separating the reaction products, we continue to add raw materials to the reaction system, and the reaction can still occur, which proves the stability of the photocatalytic system (Fig. S12†).

Table 2 Scope of the photocatalyst Amvp-CB[8] towards various derivatives of phenthiol and styrene for sulfides^a,b

a Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), Amvp-CB[8] as photocatalyst, purple light (395 nm), room temperature (rt). b Isolated yield.

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Table 3 Scope of the photocatalyst Amvp-CB[8] towards various derivatives of phenthiol and styrene for sulfoxides^a,b

a Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), Amvp-CB[8] as photocatalyst, purple light (395 nm), room temperature (rt). b Isolated yield.

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In our exploration of the photoredox thiol–ene cross-coupling reaction process (Scheme 2), we conducted control experiments by adding scavengers for various active species to elucidate the underlying reaction mechanism. Scheme 2a illustrates that the addition of TEMPO and butylated hydroxytoluene (BHT) notably inhibited the reaction, suggesting the involvement of a radical process. Conversely, the addition of potassium iodide (KI) as a hole (h⁺) scavenger, triethylamine (TEA) as a ˙OH scavenger, and sodium azide (NaN₃) as an ¹O₂ scavenger did not significantly alter the reaction yield. This indicates that the presence of h⁺, ˙OH, and ¹O₂ species has negligible impact on the photocatalytic activity. However, the addition of DMPO as a O₂˙⁻ scavenger resulted in a substantial decrease in yield, particularly affecting the formation of sulfides. This outcome underscores the pivotal role of O₂˙⁻ in mediating the coupling reaction between phenthiol and styrene.

Scheme 2 Mechanistic investigations: (a) control experiments; (b) Stern–Volmer quenching experiment; (c) UV-vis absorption spectra and fluorescence emission spectra of Amvp-CB[8]; (d) CV experiment of Amvp-CB[8]; (e) energy level diagram; (f) proposed mechanism.

In our continued investigation, we added scavengers of various active substances to the products obtained in the initial reaction step to further verify the mechanism. The results confirmed that ¹O₂ and O₂˙⁻ are crucial in the photooxidation process of sulfides (Scheme 2a). This finding underscores the persistent role of ¹O₂ and O₂˙⁻ in driving the photooxidation reactions mediated by Amvp-CB[8]. Additionally, we conducted fluorescence quenching inhibition experiments using the substrate for the photocatalyst in water. As depicted in Scheme 2b, minimal fluorescence quenching was observed when only the styrene model substrate was introduced to the catalytic system. In contrast, a significant fluorescence quenching effect was noted when the 4-methylbenzenethiol model substrate was added to the catalytic system (Scheme 2b). This observation suggests that phenthiol undergoes initial interaction with the catalyst during the photocatalytic process. According to the Stern–Volmer equation: I₀/I = 1 + k_sv [Q] = 1 + k_qτ₀ [Q], k_q can be calculated to be 8.44 × 10⁵ M⁻¹ s⁻¹ (Fig. S13†). Furthermore, as shown in the ground-state UV-vis absorption and fluorescence emission spectra of Amvp-CB[8] of Scheme 2c, the free energy E_0–0 stored by Amvp-CB[8] in the excited state could be estimated from the intersection and tangent of the absorption and fluorescence spectra, which was +2.67 V. Subsequently, we tested the redox properties of Amvp-CB[8] and the model substrate in water using cyclic voltammetry (CV) (Scheme 2c), and it was observed that Amvp-CB[8] possessed two reversible redox warps, and the redox potential of Amvp-CB[8]/Amvp-CB[8]˙⁻ was determined by the first oxidation reduction wave as E°(Amvp-CB[8]/Amvp-CB[8]˙⁻) = −0.28 V (Scheme 2d). Therefore, E°(Amvp-CB[8]*/Amvp-CB[8]˙⁻) = E°(Amvp-CB[8]/Amvp-CB[8]˙⁻) + E_0–0 = +2.39 V. To test whether the excited state of Amvp-CB[8] has the ability to oxidize phenthiol, we also performed CV experiments on the model substrate (Fig. S14†). The oxidation potential of the model substrate was +1.58 V, which was much lower than E°(Amvp-CB[8]*/Amvp-CB[8]˙⁻) (Scheme 2e), suggesting that the photocatalyst in the excited state could be able to oxidize the substrate.

On the basis of the above mechanism experiments and existing references,^51–55 we propose a possible reaction mechanism for the photoredox thiol–ene cross-coupling reaction (Scheme 2f). Under photoexcitation, the catalyst Amvp-CB[8] firstly became excited state Amvp-CB[8]*, then phenthiol is oxidized to cationic radical intermediate I by the excited state Amvp-CB[8]*, while Amvp-CB[8]* is reduced to Amvp-CB[8]˙⁻. Furthermore, Amvp-CB[8]˙⁻ and O₂ undergo electron transfer to form O₂˙⁻ and return to the ground state Amvp-CB[8]. O₂˙⁻ can be used as a hydrogen atom transfer (HAT) agent to interact with intermediate I to form intermediate II. On the one hand, intermediate II can form unstable intermediate III, on the other hand, intermediate II continues to interact with styrene to form IV, and finally IV undergoes HAT with HOO˙ to form the sulfide product. However, by prolonging the reaction time, the sulfide product can continue to undergo energy transfer or electron transfer with ¹O₂/O₂˙⁻, and pass through intermediate V to form the final sulfoxide product with another sulfide.

Conclusions

In summary, we successfully designed and synthesized the supramolecular dimer Amvp-CB[8] through host–guest interactions between the anthraquinone derivative Amvp and CB[8] in the aqueous solution. This supramolecular dimer not only exhibited significant enhancement in fluorescence emission but also demonstrated enhanced production of ¹O₂ and O₂˙⁻ in water. These properties make it an effective photocatalyst for the selective formation of sulfides and sulfoxides via the photoredox thiol–ene cross-coupling reactions, adjustable by reaction times. Moreover, the supramolecular dimer photocatalyst showed versatility in accommodating derivatives with various functional group substituents, underscoring its broad applicability in photocatalysis. Our findings represent a novel strategy for designing supramolecular catalysts that hold promise as a valuable tool for photocatalytic organic conversions.

Author contributions

Fa-Dong Wang: data curation, formal analysis, investigation, methodology. Kai-Kai Niu: data curation, formal analysis. Shengsheng Yu: methodology, resources. Hui Liu: data curation, formal analysis. Ling-Bao Xing: conceptualization, methodology, resources, supervision, writing – review & editing.

Data availability

All the data supporting this article have been included in the main text and the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (52205210), and the Natural Science Foundation of Shandong Province (ZR2022QE033 and ZR2021QB049).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qo01454e

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