Vinylene-linked fully conjugated porous organic polymers based on difluoroboron β-diketonate complexes for green and efficient photocatalysis

Abstract

Nowadays, organic boron-containing dyes have garnered considerable interest as efficient photocatalysts owing to their advantages including large molar absorption coefficients, minimal bandgaps, and easy excitation. Incorporating various boron-containing dyes into porous organic polymers effectively addresses the issue of small boron-containing molecules, which are difficult to separate and prone to photobleaching. Nevertheless, the integration of boron-containing dyes into vinylene-linked conjugated porous organic polymers (POPs) remains unexplored. This study reports the synthesis of two distinct vinylene-linked fully conjugated POPs by reacting difluoroboron β-diketonate complexes with reactive methyl groups and a carbazole-bearing aldehyde. The formation of effective D–A interaction between the difluoroboron complexes and the carbazole subunit enhances charge transport and separation, thereby improving photocatalytic efficiency. CTBA-B2, with a higher content of the difluoroboron complex, exhibits superior catalytic activity in both oxidation and reduction reactions, and is capable of efficiently catalyzing sulfide oxidations as well as dehalogenation reactions in green ethanol solvents.

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Introduction

Photocatalysis is a growing area of research due to its sustainable and energy-saving synthetic processes.^1–3 It has wide application in various sectors, including water splitting, carbon dioxide transformation, pollution remediation, and eco-friendly organic synthesis.^4–10 Therefore, there is a persistent pursuit of green, efficient photocatalysts.^11,12 Recently, the development of organic dye photocatalysts has gained significant attention for overcoming the limitations of precious metals.^13–15 Organic boron-containing dyes are of particular interest due to their high quantum fluorescence yields, large molar absorption coefficients, minimal bandgaps, and easy excitation.^16,17 However, despite the advantages of homogeneous boron-bearing dyes, severe separation problems and easy photobleaching hinder their wider application.^18–23

Accordingly, the integration of small molecule boron-bearing dyes within porous organic polymers (POPs) via chemical bond polymerization, thus forming heterogeneous photocatalysts, provides a powerful solution to these challenges.^24–28 Numerous studies have successfully incorporated well-known BODIPY dyes into POPs, exhibiting outstanding catalytic performance.^29–32 Furthermore, non-BODIPY dyes have also been examined, yielding more compelling results that compensate for certain limitations of BODIPY dyes. For instance, Marta Liras et al. synthesized conjugated POPs using non-BODIPY dyes, known as BOPHY (bis(difluoroboron)-1,2-bis((1H-pyrrol-2-yl)-ethylidene) hydrazine).³³ These polymers exhibit unique photocatalytic potential compared with traditional BODIPY dyes. In our previous research, a novel Boranil-based POP was designed and prepared via Sonogashira coupling for the first time, subsequently serving as an efficient metal-free photocatalyst in two organic reactions.³⁴ All results proved that porous polymers based on boron-containing dyes are a very effective and promising class of non-metallic photocatalyst. However, previously reported boron-containing dye-based POPs were typically synthesized through metal catalyzed coupling reactions, which carried product separation and environmental concerns. Recently, vinylene-linked conjugated POPs have attracted considerable attention due to their unmatched characteristics, including continuous π-conjugation, unique optoelectronic properties, facile alkaline catalyzed condensation, and exceptional chemical stability.^35–40 However, only a few monomers with active methyl groups are available for synthesizing such materials, significantly limiting their application and development.^41,42 Therefore, there is an urgent need to develop novel active methyl monomers to construct vinylene-linked conjugated POPs with diverse structures and properties. Theoretically, owing to the electron-withdrawing property of difluoroboron complexes, the adjacent methyl group exhibits a certain extent of acidity, bearing the potential for efficient Knoevenagel condensation. However, to the best of our knowledge, vinylene-linked conjugated POPs with boron-containing dyes have not been explored to date.

This work resulted in the successful design and synthesis of two vinylene-linked conjugated POPs (CTBA-B1 and CTBA-B2) by polymerizing two difluoroboron β-diketonate complexes, which contain reactive methyl groups, with carbazole-based tetrabenzaldehyde. The combination of the electron-withdrawing property of a difluoroboron complex and the electron-donating carbazole moiety formed an efficient donor–acceptor (D–A) effect to enhance photocatalytic performance with wide light adsorption, good durability, and excellent photoelectric properties. These polymers not only reduce oxygen to superoxide radicals, thereby catalyzing sulfide reactions, but also facilitate dehalogenation reactions. The photocatalytic reaction rate could be controlled by modulating the boron content in the polymers. Photocatalytic performance was significantly enhanced by increased boron content. These results not only demonstrate the ability of boron-containing complexes with reactive methyl groups to polymerize into novel vinylene-linked POPs, but also highlight the fact that boron-containing dyes can be used as environmentally friendly and efficient non-metallic heterophase photocatalysts.

Results and discussion

Synthesis and characterization

Two novel vinylene-linked polymers, CTBA-B1 and CTBA-B2, were conveniently prepared by alkali-catalyzed polymerization of 4,4′,4′,4′-(9H-carbazole-1,3,6,8-tetrayl) tetrabenzaldehyde (CTBA) with two difluoroboron β-diketonate complexes bearing reactive methyl groups, using DMF as the solvent and piperidine as the catalyst. These polymers were observed to be insoluble in a variety of organic solvents, including methanol, ethanol, acetone, toluene, dichloromethane, tetrahydrofuran, N,N-dimethylformamide, and water, indicating the successful completion of the polymerization process (Scheme 1).

Scheme 1 Synthetic routes of CTBA-B1 and CTBA-B2.

The chemical structures of both polymers were initially demonstrated by Fourier transform infrared (FT-IR) spectroscopy. The peak at 1626 cm⁻¹ corresponded to the C=C bond stretching of an olefinic bond, whereas the aldehyde peak at 1687 cm⁻¹ was nearly absent in the polymers, suggesting aldehyde group polymerization with the acidic methyl group (Fig. 1a). A broad peak with several close shoulders from 115 to 140 ppm can be assigned to the aromatic carbon and ethylene carbon of CTBA-B2 in the ¹³C CP-MAS NMR spectrum (Fig. S5†). The permanent porosity of CTBA-B1 and CTBA-B2 was measured by nitrogen adsorption–desorption at 77 K (Fig. 1b). The specific surface areas of both polymers were found to be 6.26 m² g⁻¹ and 64.54 m² g⁻¹, respectively. The relatively small specific surface areas may be due to the coordination of boron difluoride in the main chain. The adsorption isotherms of both materials conform to Type I according to the IUPAC classification, indicating a microporous structure. Notably, CTBA-B2 shows slightly higher adsorption capacities across the entire pressure range compared to CTBA-B1, which may be attributed to its pore structure or higher specific surface area. The amorphous structure of both polymers was confirmed by the absence of any diffraction signals in the powder X-ray diffraction (PXRD) analysis (Fig. 1c). Scanning electron microscopy (SEM) showed that the samples consisted of irregular particles and they had a smaller pore size (Fig. S7†). Thermogravimetric analysis (TGA) under a nitrogen atmosphere demonstrated that CTBA-B2 and CTBA-B1 exhibited a weight loss of about 10% below 400 °C, indicating their strong thermal stability (Fig. 1d).

Fig. 1 (a) FT-IR spectra of CTBA-B1 and CTBA-B2; (b) nitrogen adsorption–desorption isotherms of CTBA-B1 and CTBA-B2; (c) XRD spectra of CTBA-B1 and CTBA-B2; (d) thermogravimetric analysis of CTBA-B1 and CTBA-B2.

The UV-vis-DRS data, as shown in Fig. 2a, indicate broad light absorption for both CTBA-B1 and CTBA-B2. Increased boron content leads to a decrease in the band gap of the polymer. Bandgap values were derived via the Kubelka–Munk function at 2.08 eV for CTBA-B1 and 1.74 eV for CTBA-B2 (Fig. 2b). To better determine the valence band (VB) and conduction band (CB) positions, electrochemical Mott–Schottky curves were obtained (Fig. S8†). The positive slope indicates that the synthesized vinylene-linked polymers are both n-type semiconductors. The LUMO levels of CTBA-B1 and CTBA-B2 are −1.43 eV and −1.50 eV for SCE, respectively, suggesting that they may facilitate O₂ formation from O₂˙⁻.^43–45 Consequently, the HOMO levels of CTBA-B1 and CTBA-B2 were calculated and found to be 0.65 eV and 0.24 eV relative to SCE, respectively (Fig. 2c).

Fig. 2 (a) Normalized UV/vis absorption spectra of CTBA-B1 and CTBA-B2 with the corresponding monomers; (b) Kubelka–Munk-transformed reflectance spectra; (c) schematic energy band structures of CTBA-B1 and CTBA-B2; (d) UV–vis absorption spectra and photographs of the cationic radical of TMPD generated by CTBA-B1 and CTBA-B2 in the presence of light and oxygen.

Reactive oxygen species (ROS) are integral in photocatalytic reactions. Initially, the ability of the two polymers to generate superoxide radicals was assessed. As depicted in Fig. 2d, both photocatalysts effectively promoted the generation of 1,4-bis(dimethylamino)benzene cation radicals and O₂˙⁻. The darker solution color and stronger absorption indicate the superior ability of CTBA-B2 to produce O₂˙⁻. Besides superoxide radicals, singlet oxygen (¹O₂) is also a significant type of ROS. Electron paramagnetic resonance (EPR) tests were conducted using 2,2,6,6-tetramethylpiperidine (TEMP) as a spin trapping agent to investigate the ability of the two polymers to generate singlet oxygen. Under light irradiation, both polymers exhibited ideal 1 : 1 : 1 triplet signals, indicating efficient formation of ¹O₂. The EPR signal intensity revealed CTBA-B2's superior ¹O₂ production ability (Fig. 3a).

Fig. 3 (a) The electron paramagnetic resonance (EPR) spectra of CTBA-B1 and CTBA-B2 in the presence of air and TEMP; (b) transient photocurrent response of CTBA-B1 and CTBA-B2 under visible-light irradiation; (c) EIS curves of CTBA-B1 and CTBA-B2; (d) photoluminescence spectroscopy of CTBA-B1 and CTBA-B2.

To investigate the charge separation and transport properties of both polymers, photocurrent tests were performed. Recording transient photocurrent changes under intermittent illumination can demonstrate the charge–hole separation capability of polymers (Fig. 3b). The higher photocurrent response observed in CTBA-B2 suggests the modulation of photophysical properties through controlling the boron complex content. We then evaluated the carrier mobility of both polymers using electrochemical impedance spectroscopy. The Nyquist plots reveal that the arc radius of CTBA-B2 is significantly smaller than that of CTBA-B1 (Fig. 3c), indicating lower charge transfer resistance and enhanced carrier transfer capability in the former. Subsequently, the fluorescence emission intensity of both polymers was recorded using a fluorescence spectrophotometer at an excitation wavelength of 365 nm. The significantly reduced fluorescence emission intensity of CTBA-B2 indicates its ability to suppress charge–hole recombination (Fig. 3d). These findings confirm that CTBA-B2 possesses superior photophysical properties and enhanced charge transport and separation capabilities.

Photocatalytic oxidation of sulfides to sulfoxides

Organosulfones play a pivotal role in pharmaceutical and industrial chemistry. The use of visible light photochemistry with porous organic polymers as heterogeneous catalysts for sulfide oxidation into sulfoxides presents an efficient methodology.^46–52 To confirm the catalytic efficacy of these two polymers, sulfide photocatalytic oxidation was performed under light radiation. Using relatively eco-efficient ethanol as the reaction medium, we achieved 91% conversion in just 6 hours with CTBA-B2 as the photocatalyst. Conversely, the conversion rate with CTBA-B1 as the photocatalyst was only 49% for the same duration (Table 1, entries 1 and 2). This enhancement is attributed to the elevated difluoroboron complex content in the polymer, highlighting the crucial role of the boron-containing dye in boosting photocatalytic efficiency. The absence of a catalyst, light, or oxygen resulted in no reaction products, indicating their necessity for the reaction to occur (Table 1, entries 3–5). Conversion efficiency was also investigated in various solvents using CTBA-B2 as the catalyst. The reaction failed to progress in DMF, but reached 48% in methanol (Table 1, entries 6 and 7). Acetonitrile, THF, and ethanol emerged as superior reaction media, with ethanol being the most efficient (Table 1, entries 1, 8, and 9). In addition, a series of substrate expansion experiments was carried out, demonstrating that various sulfides with varying substituents could be transformed into the corresponding sulfoxides. However, conversion rates varied due to steric hindrance and other factors introduced by substituents (Table 2).

Table 1 Photoselective oxidation of sulfides to sulfoxides^a

Entry	Substrate	Catalyst	Solvents	Con. (%)	Yield^b^,^c (%)
a Reaction conditions: 2 mmol% catalyst, 0.5 mmol sulfide, a 10 W blue LED lamp (λ = 460 nm) at room temperature for 6 h. b Conversion and selectivity were determined by ^1H NMR. c The ratio between sulfoxide and sulfone. d Without light. e Vacuum. f 2.0 equiv. of KI. g 2.0 equiv. of NaN3. h 2.0 equiv. of benzoquinone. i 7 hours of reaction.
1	1A	CTBA-B2	EtOH	91	91
2	1A	CTBA-B1	EtOH	51	49
3	1A	None	EtOH	NR	NR
4^d	1A	CTBA-B2	EtOH	NR	NR
5^e	1A	CTBA-B2	EtOH	Trace	Trace
6	1A	CTBA-B2	MeOH	48	48
7	1A	CTBA-B2	DMF	Trace	Trace
8	1A	CTBA-B2	MeCN	87	86
9	1A	CTBA-B2	THF	74	74
10^f	1A	CTBA-B2	EtOH	4	4
11^g	1A	CTBA-B2	EtOH	30	30
12^h	1A	CTBA-B2	EtOH	23	23
13^i	1A	CTBA-B2	EtOH	99	99

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Table 2 Scope of sulfides oxidation

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A series of trapping experiments was carried out under optimized conditions to investigate the plausible mechanism of sulfide conversion to sulfoxides. The introduction of KI as a hole scavenger yielded only 4% sulfides, suggesting the importance of efficient photoelectron–hole separation and holes in the transformation. In addition, reactive oxygen species (ROS) such as superoxide radicals (O₂˙⁻) and singlet oxygen (¹O₂) play crucial roles in photocatalytic aerobic oxidation. Using NaN₃ as an ¹O₂ scavenger reduced the sulfoxide yield to 30%, while benzoquinone as an O₂˙⁻ scavenger reduced it to 23%. These results highlight the significance of both ¹O₂ and O₂˙⁻ as critical reactive intermediates in this reaction (Table 1, entries 10–12).

Therefore, the possible reaction mechanism for the conversion of sulfide to sulfoxide can be hypothesized (Scheme 2). Under visible light irradiation, CTBA-B2 is excited to the excited state CTBA-B2* containing electron–hole pairs. Separated electrons and holes reduce O₂ to O₂˙⁻, while CTBA-B2 can convert from the triplet state of oxygen (³O₂) to ¹O₂ through energy transfer. There are two possible photocatalytic pathways for sulfides. In one pathway, the sulfide is oxidized to a cationic radical, which then interacts with O₂˙⁻ to form a sulfoxide radical, then generates sulfoxide with another sulfide. In the other pathway, ¹O₂ reacts with the sulfide molecule to form a thiodioxane intermediate, which then reacts with the sulfide to form the sulfoxide product.

Scheme 2 Photocatalytic mechanism of sulfide oxidation and dehalogenation reaction.

Visible light-induced reductive dehalogenation reactions

A lower LUMO band indicates that the polymers have larger electron affinity and higher reduction potentials and are more easily oxidized.^53,54 This means that they have the potential to catalyze reduction reactions. Dehalogenation of α-bromoacetophenone was employed to evaluate the photocatalyst's reduction capabilities. Environmentally friendly ethanol was chosen as the solvent, and this solution was illuminated with 520 nm green light under a nitrogen atmosphere. An impressive 88% yield was achieved within 9 hours using CTBA-B2 as the catalyst (Table 3, entry 1). In contrast, only 63% yield was achieved under the same conditions as CTBA-B1 (Table 3, entry 2). In the absence of a catalyst, the conversion level was only 17%, highlighting the catalytic potency of the polymer (Table 3, entry 8). It is noteworthy that the reaction hardly progressed in the absence of light, indicating that light is indispensable for the photocatalytic process (Table 3, entry 10). Evaluation of conversion efficiency across various solvents revealed CTBA-B2's robust performance in MeCN, MeOH, THF, and DMF (Table 3, entries 3–6). However, when exposed to air, the yield of the CTBA-B2-catalyzed reaction diminished to 48%, decreasing to 6% in the absence of a catalyst (Table 3, entries 7 and 9). These findings suggest that O₂ may impede the reaction.

Table 3 Visible light-induced reductive dehalogenation reactions^a

Entry	Substrate	Catalyst	Solvents	Con. (%)	Yield^b^,^c (%)
a Reaction conditions: 2 mmol% catalyst, 0.2 mmol α-bromoacetophenone, 0.22 mmol Hantzsch ester, 0.4 mmol DIPEA, a 10 W green LED lamp (λ = 520 nm) at room temperature for 9 h. b Yields determined by ^1H NMR using dibromomethane as an internal standard. c In air. d Without light. e No DIPEA. f No Hantzsch ester.
1	1A	CTBA-B2	EtOH	99	88
2	1A	CTBA-B1	EtOH	87	63
3	1A	CTBA-B2	DMF	96	79
4	1A	CTBA-B2	MeCN	99	91
5	1A	CTBA-B2	MeOH	98	86
6	1A	CTBA-B2	THF	97	82
7^c	1A	CTBA-B2	EtOH	50	48
8	1A	None	EtOH	17	17
9^c	1A	None	EtOH	9	6
10^d	1A	CTBA-B2	EtOH	Trace	Trace
11^e	1A	CTBA-B2	EtOH	75	49
12^f	1A	CTBA-B2	EtOH	50	28

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A detailed mechanistic exploration of the dehalogenation reaction of α-bromoacetophenone was carried out (Scheme 2). The catalytic reaction yields decreased to 49% in the absence of DIPEA and 28% in the absence of Hantzsch ester (Table 3, entries 11 and 12). This observation indicates that both Hantzsch ester and DIPEA can provide hydrogen atoms for the reaction, with Hantzsch ester as the primary hydrogen source. DIPEA can also function as an electron donor. Based on previous studies^55–59 and our experimental findings, a plausible mechanism for the photocatalytic reduction reaction of α-bromoacetophenone can be proposed: under light irradiation, CTBA-B2 is excited to produce photogenerated holes and electrons via charge separation. The hole can extract electrons from the electron sacrificial agent (DIPEA), and the electrons are subsequently transferred from the conduction band of CTBA-B2 to α-bromoacetophenone (E_1/2 = −0.49 V), resulting in the formation of the α-carbonyl radical and bromide ion. Finally, the α-carbonyl radical combines with the H⁺ provided by the hydrogen source to form acetophenone. In addition, a series of substrate expansion experiments was conducted, demonstrating that the photocatalyst exhibited good catalytic performance for various substrates (Table 4).

Table 4 Reaction time of 9 h and scope of dehalogenation reactions

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Recyclability and stability

Finally, the reusability and stability of CTBA-B2 were explored. After completing the reaction, the catalyst was filtered out, washed with dichloromethane and ethanol, and then dried. Recyclability experiments showed that the catalyst could be reused at least five times for both oxidation and reduction reactions (Fig. S12†). FT-IR spectra confirmed that there was no significant difference between the catalyst after five cycles and the fresh catalyst. These results demonstrate the stability of the photocatalyst (Fig. S13†).

Conclusions

To conclude, two novel vinylene-conjugated POPs, CTBA-B1 and CTBA-B2, derived from difluoroboron β-diketonate complexes with reactive methyl groups, have been successfully synthesized for the first time. This combination of conjugated vinylene-linkage with the intermolecular D–A effect endows these POPs with the potential to serve as environmentally friendly and efficient metal-free heterogeneous photocatalysts. Remarkably, CTBA-B2, enriched with a higher content of the boron complex, exhibits exceptional photocatalytic performance in both oxidation and reduction reactions, such as the transformation of sulfide to sulfoxide and dehalogenation processes. This study highlights that boron-containing complexes with reactive methyl groups could serve as efficient monomers for the synthesis of new vinylene-conjugated POPs, and confirms the active role of boron-containing dyes in metal-free heterogeneous photocatalysis.

Author contributions

W. T. Gong conceived and designed the experiments. W. S. Xu completed the experiment and analyzed the data to write the original draft. Z. H. Zhao, Y. Q. Liu and D. X. Yang participated in the discussion of the results and also proofread the manuscript. All authors have given approval to the final version of the manuscript.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author upon reasonable request.

All data generated or analysed during this study are included in this published article and its ESI.†

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors acknowledge the financial support from the Natural Science Foundation of China [No. 22478601] and the assistance of the DUT Instrumental Analysis Center.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4py00947a

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