BIOMIMETIC PHOTOCATALYSIS FOR EFFICIENT CO2 FIXATION USING COVALENT ORGANIC POLYMERIC PHOTOCATALYST (COP)

Abstract

Solar-driven photocatalysis provides an efficient pathway for synthesizing high-value chemicals by converting solar energy into chemical energy. Porous heterogeneous materials incorporating organic chromophores with strong light absorption have been widely utilized as photocatalysts in artificial photosynthetic processes. This study presents a novel covalent organic polymeric photocatalyst (COP) composed of dual light-active units, enabling directed photoinduced charge transfer similar to natural systems. The COP, developed as a reusable thin-film platform, supports multi-functional photocatalytic applications, including CO2 reduction and Biginelli condensation under solar light. Catalytic tests indicate that the COP-coated film achieves NADH regeneration efficiency of 67.96%, facilitating photoinduced electron transport to the catalytic center, thereby enabling effective CO2 conversion with a methanol yield of 19.27 µmol. Additionally, the COP-coated film demonstrates high conversion efficiency and turnover frequency in producing diverse 3,4-dihydropyrimidine compounds via the Biginelli reaction. Mechanistic investigations using time-resolved spectroscopy and density functional theory (DFT) calculations reveal that the catalyst’s long-lived charge separation state is vital to efficient electron transport. This study highlights the potential of COPs for multifunctional catalysis and provides insights into photoinduced electron dynamics and their impact on catalytic efficiency.

Specification

DESC:FIELD OF THE INVENTION
The present invention relates to a biomimetic photocatalysis for efficient CO2 fixation using covalent organic polymeric photocatalyst (COP).
BACKGROUND OF THE INVENTION
References which are cited in the present disclosure are not necessarily prior art and therefore their citation does not constitute an admission that such references are prior art in any jurisdiction. All publications, patents and patent applications herein are incorporated by reference to the same extent as if each individual or patent application was specifically and individually indicated to be incorporated by reference.
Solar-driven photocatalytic reaction has been attracting to produce value-added chemicals such as H2 or CH3OH by converting renewable solar energy into chemical energy.1-4 Since the microscopic mechanism in the photocatalyst reaction directly affects the macroscopic catalytic efficiency, many experimental efforts have been focused on the development of highly-efficient catalytic reaction by mimicking natural approach operated in plants or algae.5-7 Natural photosynthetic modules adopt the interfacial structure by incorporating large protein complexes acting as catalytic centers and small soluble proteins acting as electron mediators in order to induce the chain of efficient electron transport shown in Scheme 1. Based on such a characteristic structure in nature, photons absorbed in the light-harvesting moiety initiate multiple photo-induced electron transfers toward catalytic center, which is utilized for the oxidation reaction of water and the assimilation reaction of carbon dioxide. The main challenges to emulate the natural photosynthesis in an artificial manner are to develop the reaction chain based on the photo-induced sequential electron transfers as well as the light-harvesting module for the efficient photon absorption.5
Organic polymeric materials have been emerged as a heterogeneous photocatalyst in the artificial photosynthesis due to its high surface-to-volume ratio with microporous structure.8-15 Covalent organic polymeric photocataylsts (COPs) composed of covalent networks between light atoms provide practical opportunities in the photocatalytic production of value-added chemicals.16,17 In COPs, the photo-sensitizing properties associated to the light absorption ability can be systematically altered by varying the building units.18-20 The porous reticular structure, which is tunable from the chemical modification in the organic linker moiety, can facilitate the efficient charge separation and also accelerate the charge transport following the chemically defined reaction pathways. In this regard, COPs have been widely applied as heterogeneous photocatalysts for the CO2 photo-reduction21-26 and organic conversion processes such as oxidation of sulphides and alcohols or cross-coupling reactions.27,28 To realize the potential of COPs, some studies have been tried for the multi-functional photocatalytic reactions to produce value-added chemicals used in the field of industrial chemistry.29,30 Nevertheless, such applications are still in the early stage. Especially, the improvement of photocatalytic efficiency has been hindered by the lack of exploration for highly photoactive component and the absence of mechanistic insights for those reactions. To solve those issues, the systematic development of heterogeneous COPs in terms of photo-induced electronic dynamics and its actual applications should be required. For this reason, we propose that the efficient electron transfers boosted by the high absorption ability of solar light around 500 nm should be embedded into heterogeneous COPs and its photocatalytic reaction. In this study, we chose the chemical species of perylene tetra-anhydride (PTA) and ethidium bromide as building units to construct the multi-functional heterogeneous COPs. Perylene tetra-anhydride belongs to a superfamily of perylene derivative.31 It has been actively used as a chemical component of photocatalyst owing to a characteristic light-harvesting property and high chemical stability.32,33 Ethidium bromide, which is a well-known fluorescent dye, has an absorption ability around 475 nm.34 Based on the excellent visible-light absorption properties of PTA and ethidium bromide, the heterogeneous perylene-based COPs was synthesized from a direct poly-condensation between those building units.19,35-37
As a proof-of-concept, we aim to two photocatalytic applications as follows; (i) visible light-driven CO2 reduction reaction to produce methanol, and (ii) photocatalytic reaction for multi- component Biginelli condensation to synthesize 3,4-dihydropyramidine derivatives. The former reaction has been considered as a complementary strategy for acquiring the CO2-neutral sustainable energy sources as well as reducing the CO2 emission in order to resolve the global climate crisis arising from the massive CO2 emission.20 The latter reaction is one of key reactions for the synthetic production of 3,4-dihydropyramidines, which belongs to a class of heterocyclic quinazolinone derivatives vastly used in the field of bio-medical chemistry.38,39 To do so, we developed the artificial photosynthetic module based on the heterojunction of COPs and bio- catalytic proteins for the CO2 reduction and also the light-driven Biginelli condensation for the production of 3,4-dihydropyramidines as shown in Scheme 2. As a first step, we characterized the chemical properties of the as-synthesized COPs by employing FT-IR, PXRD, NMR, TGA, and SEM combined to the elemental mapping. To comprehensively understand the photo-physical properties associated to the photocatalytic efficiency, we systematically performed the time- resolved spectroscopic studies combined to the density functional theory (DFT) calculations. From this result, it was revealed that the covalent linkage of PTA and ethidium bromide in COPs promotes the photo-induced charge transfers along the interface between the building units, resulting in the formation of the long-lived charge separated (CS) state. For the photocatalytic application, we prepared the COPs-coated flexible thin film which was used as a reusable photocatalyst platform. According to the catalytic performance based on the artificial photosynthesis, the COPs-coated film shows an excellent production of methanol with 19.27 µmol assisted from the efficient regeneration of NADH cofactor with 67.96 %. Meanwhile, the light- driven Biginelli reaction based on the COPs-coated film was tested without any co-factors commonly used in artificial photosynthesis. The catalytic test clearly demonstrated that the COPs-coated thin film offers high production yield and TOF numbers for the various substances of benzaldehyde, acetylacetone, and urea derivatives. Considering the mechanistic insights obtained from the time-resolved spectroscopic studies, the long-lived CS state in COPs would facilitate the photo-induced electron transport from COPs to NADH cofactor, which is similar to the sequential electron transfer in the natural photosynthetic module.5,6 In the light- driven Biginelli reaction, such long-lived CS state might induce the through-space electron transport from COPs to the benzaldehyde intermediate, which is presumed as a key intermediate.38 The current study will be a cornerstone for the practical photocatalytic applications based on the multi-functional heterogeneous COPs with the aid of the underlying electronic dynamics.
Several patents issued for photocatalysts but none of these are related to the present invention. Patent US11724253B2 concerns a method for the photocatalytic reduction of carbon dioxide carried out in the liquid phase and/or in the gas phase under irradiation using a photocatalyst comprising a support made from alumina or silica or silica-alumina and nanoparticles of molybdenum sulfide or tungsten sulfide having a band gap greater than 2.3 eV, said method comprising the following steps: a) bringing a feedstock containing carbon dioxide and at least one sacrificial compound into contact with said photocatalyst, b) irradiating the photocatalyst with at least one source of irradiation producing at least one wavelength smaller than the width of the band gap of said photocatalyst so as to reduce the carbon dioxide and oxidise the sacrificial compound in the presence of said photocatalyst activated by said source of irradiation, in such a way as to produce an effluent containing, at least in part, C1 or above carbon-containing molecules, different from CO2.
Another patent US10818703B2 belongs to method for manufacturing a semiconductor device includes: forming a photocatalytic layer and an organic compound layer in contact with the photocatalytic layer over a substrate having a light transmitting property; forming an element forming layer over the substrate having the light transmitting property with the photocatalytic layer and the organic compound layer in contact with the photocatalytic layer interposed therebetween; and separating the element forming layer from the substrate having the light transmitting property after the photocatalytic layer is irradiated with light through the substrate having the light transmitting property.
Another patent CN111389458B discloses a carboxyl-containing perylene bisimide/oxygen-doped carbon nitride nanosheet heterojunction photocatalyst and a preparation method and application thereof, wherein the photocatalyst is formed by compounding carboxyl-containing perylene bisimide and oxygen-doped carbon nitride nanosheets through electrostatic interaction and pi-pi interaction, wherein the mass ratio of the oxygen-doped carbon nitride nanosheets to the carboxyl-containing perylene bisimide is 1: 0.001-0.8; the carboxyl perylene bisimide is modified on the oxygen-doped carbon nitride nanosheet through an in-situ method. Compared with the prior art, the invention has the following advantages: (1) compared with O-CN and PDI in the prior art, the photocatalyst has more excellent performances of photocatalytic degradation of pollutants, killing of pathogenic bacteria and photolysis of water to generate oxygen; (2) the method has the advantages of low raw material cost and simple process, effectively reduces the product cost, is suitable for industrial mass production, and has very high application prospect and practical value.
Another patent US20220395821A1 describes a covalent organic framework. The covalent organic frameworks have unique structural and physical properties, which lends them to be versatile in a number of different applications and uses. In one aspect, the covalent organic frameworks are composed of a plurality of fused aromatic groups and electron-deficient chromophores. The covalent organic frameworks are useful as photocatalysts in a number of different applications.
Another patent CN104525258B discloses a kind of covalent triazine organic polymer visible light catalyst and its preparation method and application, belong to material preparation and photocatalysis technology field. The present invention adopts room temperature liquid polymerization, with para-Phthalonitrile for monomer, carries out trimerization reaction under trifluoromethayl sulfonic acid catalytic action, prepares covalent triazine organic polymer visible light catalyst. This catalyst has visible light-responded, it is possible to realize photodissociation Aquatic product hydrogen and without obvious deactivation phenomenom, simultaneously can efficient degradation Organic Pollutants in Wastewater. Preparation condition of the present invention is gentle, and production cost is low, and productivity is higher, meets needs of production, has bigger application potential.
OBJECTS OF THE INVENTION
Main object of the present invention is a biomimetic photocatalysis for efficient CO2 fixation using covalent organic polymeric photocatalyst (COP).
Another object of the present invention is covalent organic polymeric photocatalyst to show the impressive performance for the production of methanol and light-driven Biginelli reaction.
Another object of the present invention is to use of two building units boosts the charge separation process associated to the catalytic efficiency.
Another object of the present invention is to use organic polymeric photocatalysts for various artificial photosynthetic approaches.
SUMMARY OF THE INVENTION
This summary is provided to introduce a selection of concepts, in a simplified format, that are further described in the detailed description of the invention.
This summary is neither intended to identify key or essential inventive concepts of the invention and nor is it intended for determining the scope of the invention.
To further clarify advantages and features of the present invention, a more particular description of the invention will be rendered by reference to specific embodiments thereof, which is illustrated in the appended drawings.
The invention involves the synthesis of a perylene-based covalent organic polymeric photocatalyst through a two-step process: Synthesis of Perylene Tetra-Anhydride (PTA): Perylene dianhydride, maleic anhydride, and p-chloranil are dissolved in anhydrous benzene. The mixture is ultra-sonicated to ensure even dispersion, then reacted in a nitrogen-purged environment. Following the reaction, the mixture is cooled, filtered using water and n-hexane, and purified through Soxhlet extraction to yield PTA.
Formation of the Perylene-Based Photocatalyst: Imidazole, PTA, and ethidium bromide are combined in a Pyrex tube and heated at 120 °C under nitrogen. After the reaction, the mixture is cooled, filtered with water and ethanol, and dried, yielding the photocatalyst in powder form. This process produces a perylene-based polymeric photocatalyst with potential applications in fields like photochemistry or environmental remediation.
Herein enclosed a covalent organic polymeric photocatalyst (COP) comprising of: perylene dianhydride (500 mg), maleic anhydride (249 mg), p-chloranil (156 mg), anhydrous benzene (15 mL), imidazole (5g), perylene tetra-anhydride (PTA) (250 mg), ethidium bromide (EB) (335 mg).
A method of the photocatalyst as claimed in claim 1, wherein the said method comprising the steps of:
dissolving perylene dianhydride, maleic anhydride, and p-chloranil in anhydrous benzene;
ultra-sonicating the reaction mixture for even dispersion;
performing the reaction under a nitrogen-purged environment;
taking the reaction mixture from the oil bath after finishing the reaction
cooling the reaction mixture to room temperature post-reaction;
filtering the resultant product using a large amount of water and n-hexane;
purifying the product through Soxhlet extraction to obtain perylene tetra-anhydride;
dissolving imidazole, perylene tetra-anhydride (PTA), and ethidium bromide (EB) in a Pyrex tube;
heating the reaction mixture to 120 ºC in a nitrogen-purged environment;
cooling the reaction mixture to room temperature post-reaction;
filtering the resultant product with water and ethanol; and
drying the product to obtain a powdered perylene-based covalent organic polymeric photocatalyst.
The proportions of reactants are 500 mg of perylene dianhydride, 249 mg of maleic anhydride, and 156 mg of p-chloranil in 15 mL of anhydrous benzene.
The proportions of reactants are 5 g of imidazole, 250 mg of perylene tetra-anhydride, and 335 mg of ethidium bromide.
The photocatalyst exhibits enhanced stability and photocatalytic efficiency suitable for photochemical applications.
BRIEF DESCRIPTION OF THE DRAWINGS
The illustrated embodiments of the subject matter will be understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are consistent with the subject matter as claimed herein, wherein:
Figure 1. Molecular structure and characterization of COP photocatalyst. (a) Modelled structure of COP. For clarity, two building units of perylene tetra-anhydride (PTA) and ethidium bromide are highlighted. (b) FT-IR spectra of COP and its building units. The coloured bars show the characteristic IR bands corresponding to the functional groups. In the COP, the IR peak around 1330 cm-1 corresponding to the vibrational mode of diimide group (green bar) was newly formed, while the IR peaks associated to -NH3 in ethidium bromide monomer (blue bar) and di-anhydride in PTCDA monomer (yellow bar) disappeared. (c) Powder X-ray diffraction pattern. The Bragg peak observed at 25° is assigned to the (001) plane, which corresponds to the d-spacing of 3.53 Å between the stacked layers in COP as displayed in the inset. (d) TGA thermographs of of COP and its building units. In COP, the formation of the covalent bonds between the building units increased the thermal stability compared to that in the monomeric units. (e) Linear absorption and emission spectra. COP shows the broad absorption band up to 800 nm compared to that of monomeric units. The inset shows the emission spectrum of COP, which mainly originates from the radiative decay in the perylene units.
Figure 2. HR-SEM images and XPS spectra of COP. (a) Magnified SEM image and (b) SEM image with wide-view mode. The globular nanoparticles are agglomerated each other. EDX images measured at (c) multiple elements, (d) C element, (e) N element, and (f) Br elements. XPS spectra measured at (g) C 1s, (h) N 1s, (i) O 1s, and (j) Br 3d.
Figure 3. Time-resolved optical spectroscopic data for as-synthesized photocatalyst. (a) Wavelength-resolved nanosecond transient absorption (ns-TA) spectra collected at the representative time delays. (b) Decay profiles of ns-TA measured at the probe wavelengths of 450 nm and 700 nm. The exponential fitting with the shared time constants of 8.56 ns, 61.5 ns and 3.72 µs (solid lines) show good agreement with the experimental data. (c) Decay profiles of time- resolved photoluminescence (TR-PL) measured at the wavelengths of 535 nm and 700 nm. The exponential fitting based on the convolution of the instrumental response function (IRF) were implemented to estimate the decay constants. From this way, the time constant of ~3.43 ns was determined. (d) Schematic illustration for the plausible excited charge carrier dynamics in the photocatalyst. The right figure shows the change in the electron density of COP moiety at the optical transition of 2.78 eV. Considering the results from ns-TA and TR-PL, it is inferred that the formation of charge-transferred state originating from the conjugation of the building units is formed with the time constant of 8.56 ns and slowly decays toward the ground state via the long- lived charge separated (CS) state.
Figure 4. Photocatalytic reactions for the regeneration of NADH and the production of methanol.
(a) Catalytic performance of COP for the regeneration of enzymatically active 1,4-NADH compared to that of PTCDA and ethidium bromide monomer. (b) Production yield of methanol from CO2 in the COP-biocatalyst integrated artificial photosynthetic module. The control experiments using the dark-incubated condition without the light irradiation exhibited no yield of products in the artificial photosynthesis.
DETAILED DESCRIPTION OF THE INVENTION
The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the present disclosure as defined by the appended claims.
It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms "a"," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises," "comprising," "includes" and/or "including," when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
In addition, the descriptions of "first", "second", "third", and the like in the present invention are used for the purpose of description only, and are not to be construed as indicating or implying their relative importance or implicitly indicating the number of technical features indicated. Thus, features defining "first" and "second" may include at least one of the features, either explicitly or implicitly.
Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
In some embodiments of the present invention, 500 mg of perylene dianhydride, 249 mg of maleic anhydride, and 156 mg of p-chloranil were dissolved in the anhydrous benzene (15 mL). The reaction mixture was ultra-sonicated to make the evenly dispersed medium and then the dissolved mixture was reacted under a nitrogen-purged environment.
In some embodiments of the present invention, after the reaction was completed, the reaction mixture was taken from the oil bath and was cooled down at the room temperature. The resultant product was filtered with copious amounts of water and ethanol and was dried to get the powder product.
In some embodiments of the present invention, in order to reduce the Rh, the light-harvesting eosin Y (acting as e- donor) gathers photons that are incident due to electronic transition between a localized orbital around it (HOMO to LUMO) and conducts via selenium bridge (acting as multi e- acceptor).
Herein enclosed a covalent organic polymeric photocatalyst (COP) comprising of: perylene dianhydride (500 mg), maleic anhydride (249 mg), p-chloranil (156 mg), anhydrous benzene (15 mL), imidazole (5g), perylene tetra-anhydride (PTA) (250 mg), ethidium bromide (EB) (335 mg).
A method of the photocatalyst as claimed in claim 1, wherein the said method comprising the steps of:
dissolving perylene dianhydride, maleic anhydride, and p-chloranil in anhydrous benzene;
ultra-sonicating the reaction mixture for even dispersion;
performing the reaction under a nitrogen-purged environment;
taking the reaction mixture from the oil bath after finishing the reaction
cooling the reaction mixture to room temperature post-reaction;
filtering the resultant product using a large amount of water and n-hexane;
purifying the product through Soxhlet extraction to obtain perylene tetra-anhydride;
dissolving imidazole, perylene tetra-anhydride (PTA), and ethidium bromide (EB) in a Pyrex tube;
heating the reaction mixture to 120 ºC in a nitrogen-purged environment;
cooling the reaction mixture to room temperature post-reaction;
filtering the resultant product with water and ethanol; and
drying the product to obtain a powdered perylene-based covalent organic polymeric photocatalyst.
The proportions of reactants are 500 mg of perylene dianhydride, 249 mg of maleic anhydride, and 156 mg of p-chloranil in 15 mL of anhydrous benzene.
The proportions of reactants are 5 g of imidazole, 250 mg of perylene tetra-anhydride, and 335 mg of ethidium bromide.
The photocatalyst exhibits enhanced stability and photocatalytic efficiency suitable for photochemical applications.
EXAMPLE 1
EXPERIMENTAL SECTION
General information for chemical reagents
All the chemical reagents were purchased from Sigma Aldrich and TCI and were used without further purification.
Synthesis of perylene tetra-anhydride
500 mg of perylene dianhydride, 249 mg of maleic anhydride, and 156 mg of p-chloranil were dissolved in the anhydrous benzene (15 mL). The reaction mixture was ultra-sonicated to make the evenly dispersed medium and then the dissolved mixture was reacted under a nitrogen-purged environment. After finishing the reaction, the reaction mixture was taken from the oil bath and was cooled at room temperature. The resultant product was filtered by employing a large amount of water and n-hexane solvents and further was purified using the Soxhlet method.31
Perylene-based covalent organic polymeric photocatalyst
5 g of imidazole, 250 mg of perylene tetra-anhydride (PTA), and 335 mg of ethidium bromide (EB) were dissolved in the Pyrex tube. The reaction mixture was heated at 120 ºC in a nitrogen-purged environment. After the reaction was completed, the reaction mixture was taken from the oil bath and was cooled down at the room temperature. The resultant product was filtered with copious amounts of water and ethanol and was dried to get the powder product.
EXAMPLE 2
Characterizations of as-synthesized heterogeneous photocatalyst
Solid-state cross-polarized magic angle spinning (CP-MAS) 13C NMR was implemented by using a 400 MHz solid-state NMR spectrometer (Avance III HD Bruker) at KBSI Western Seoul center. For the NMR measurement, the following parameters were used: a delay time of 3 seconds, and the basic frequency of 100.653 MHz the samples were packed in 4 mm zirconia rotors and were subjected to a spinning speed of 14 kHz. The morphologies of the as-synthesized photocatalyst were examined by using a high-resolution scanning electron microscope (HR-SEM; TESCAN CLARA). The HR-SEM measurement was coupled with energy-dispersive X-ray spectroscopy (EDX; Ultim Max, Oxford) in order to check the chemical elements. The elements in the as- synthesized photocatalyst were also examined by employing X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific Inc., NEXSA G2 system). The thermogravimetric analysis (TGA; SDT 650 analyzer, TA Instruments) was performed for the temperature range covering from 26 °C to 782 °C with a heating rate of 5 °C/min under the nitrogen-purged atmosphere. A powder X-ray diffractometer (PXRD; Bruker, D8 Advance Eco) was employed to investigate the crystalline property. Fourier transform infrared (FT-IR) spectra was recorded on a Bruker ALPHA-T FT-IR spectrometer. Linear absorption spectra of the photocatalyst sample were measured by using a UV- visible spectrophotometer (UV-2600i, Shimadzu). Based on the absorption spectrum, the fluorescence character of the sample was checked by measuring the fluorescence spectrum covering from 500 nm to 800 nm with excitation at 490 nm using a spectrophotometer (RF-6000, Shimadzu).
Computational details
All the quantum mechanical calculations were implemented by Gaussian 16 package.40 The molecular geometry of partial moiety of perylene-based COP was optimized and then the optimized structure was used for the time-dependent density functional theory (TD-DFT) calculation with the ?B97XD functional and 6-31G(d) basis set. For the clarity of electron density, the natural transition orbitals (NTOs) were reconstructed from the TD-DFT results.41
Photochemical reaction for the regeneration of NADH and the production of methanol
NADH was photochemically regenerated in a quartz reactor at room temperature with an inert environment by employing external light. The reaction mixture involved 2.48 mol of NAD+, 1.24 mol of rhodium (Rh) complex, 1.24 mmol of ascorbic acid (AsA), and 0.4 mg of photocatalyst dissolved in 3.1 ml of buffer solution (100 mM, pH 7.0). To estimate the yield of the regenerated NADH, a UV-visible spectrometer was used. For the production of methanol from CO2 using the COF photocatalyst, a quartz reactor was used under an inert atmosphere at room temperature. This photocatalytic reaction was based on mimicking the artificial photosynthetic approach by following the well-established protocol.42,43 The reaction mixture included three different types of enzymes with formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase, photocatalyst, NAD (1.24 mol), Rh complex (0.62 mol), and AsA (1.24 mmol) dissolved in the buffer (100 mM, pH 7.0). To make the CO2-saturated reaction medium, CO2 gas flowed for one hour under the dark-incubated condition prior to starting the photocatalytic reaction with an irradiation of external light. The quantity of the generated methanol from the photocatalytic reaction was determined from the HPLC system (LC-20AP, Shimadzu).
EXAMPLE 3
Results and Discussion
Characterization of as-synthesized photocatalyst
We implemented the Fourier transform infrared (FT-IR) spectroscopic measurements in order to investigate the molecular structure of as-synthesized COP. As shown in Figure 1, the FT-IR spectrum for the analog species of perylene tetra-anhydride shows the characteristic peaks at 1755 cm-1 and 1017 cm-1 which correspond to the C=O stretching and CO-O-CO stretching modes in the anhydride ring of perylene, respectively. The species of ethidium bromide moiety exhibits characteristic peaks at 3314 cm-1 and 3194 cm-1 corresponding to the -NH2 stretching mode. Those IR peaks observed in the building units of perylene tetra-anhydride and ethidium bromide disappeared in COP, while the new peak around 1330 cm-1 corresponding to the C-N-C stretching mode in the imide ring was formed. Such spectral change in the FT-IR spectrum provides the direct evidence for the formation of covalent bonds between two different building units from the condensation reaction, which is linked to the previous studies about the perylene-based and the ethidium bromide-based covalent organic frameworks (COFs).19,44 Furthermore, CP-MAS 13C NMR spectroscopic measurement was implemented to confirm the existence of the covalent bond between ethidium bromide and perylene tetra-anhydride. As shown in Figure S4, the NMR signal around 162.18 ppm was observed and was assigned to the carbon atom in the anhydride group. The two prominent peaks around 125 cm-1 can be attributed to the carbon atoms in the benzene rings of perylene core and ethidium bromide. The small peak around 14 cm-1 originated from the ethyl carbon atoms in the ethidium bromide moieties, implying that the structure of the ethidium bromide is well-conserved in COP.19 Powder X-ray diffraction pattern for the COP material was also collected to check the crystalline property. As shown in Figure 1c, a broad peak was observed at 25.2°, corresponding to the d-spacing of 0.353?nm. According to the previous studies for the two-dimensional (2D) COFs,10,18,19,45 it was reported that the diffraction peak around 25° ~ 28° was assigned to the 0.353 nm line spacing along with the (001) plane. Based on this result, it can be expected that the two-dimensional sheets in COP are aligned with the d-spacing of 0.353?nm.
To check the thermal stability of COP, thermogravimetric analyses (TGA) were performed by comparing that of its building units. As shown in Figure 1d, the TGA curve of perylene moiety showed drastic thermal decomposition around 570 °C. In contrast, the TGA curves of ethidium bromide and COP showed the gradual weight loss up to 780 °C. Especially, the thermal decomposition process in COP seems to be the multi-step reaction involving the elimination of physically adsorbed waters in the micro-pore sites and the breakages of the imide bonds around 700 °C. Such multi-step thermal process was similarly observed in the ethidium bromide, while The melting temperature was much lower than that in COP. This result means that the existences of covalent bonds between the building units in COP caused the improvement of the thermal stability relative to those in the building units.
The surface morphology of COP material was characterized by the HR-SEM measurement. As shown in Figure 2a, the COP material has a globular shape with the crystalline property confirmed from the PXRD. Such particles, which are several hundred nanometers in diameter, are agglomerated with each other. To check the chemical elements in the SEM image, the energy-dispersive X-ray spectroscopic (EDX) mapping was implemented shown in Figure 2c-f. The EDX image measured at Br energy shows a quite low intensity, indicating that the low chemical content of Br relative to that of C and N atoms. To investigate the chemical elements and its chemical network, we also performed the X-ray photoelectron spectroscopic (XPS) measurements. As shown in Figure 2g-j, the high-resolution XPS spectrum at C 1s clearly shows three peaks at 284.8 eV, 286.2 eV, and 288.0 eV, which correspond to binding energies of the C-C sp3 hybridized, C-N, N-C=O groups, respectively.44 The XPS spectrum at N 1s has two peaks with the binding energies of 398.3 eV and 400.5 eV, which correspond to the N-C sp2 in the ethidium bromide and the N atoms in the imide moieties, respectively. In addition, the XPS spectrum at O 1s clearly exhibits two peaks at 533 eV and 531.3 eV, assigned to the oxygen atoms in the imide group, supported from the result in the previous study about the microporous polyimide framework.44 Meanwhile, the small peak around 68.5 eV was observed in the XPS spectrum measured at Br 3d and was likely to be assigned as the Br atom in the ethidium bromide moiety. We note that the result from the Br 3d XPS show a good agreement with the very weak signal in the EDX image measured from the HR- SEM. Photo-physical properties of perylene-based COP To investigate the photo-physical properties of COP material, we measured the linear absorption and emission spectra. As shown in Figure 1e, the absorption spectrum of COP dispersed in N,N- dimethylformamide (DMF) shows a broad absorption feature in the entire wavelength region, including the prominent absorption peak around the wavelength of 530 nm. According to the absorption spectra of perylene dimide and ethidium bromide, the absorption around 530?nm with a feature of vibrational progression is mainly attributed to the electronic transitions of the perylene core, mainly dominant at 528 nm, and the ethidium bromide moiety, mainly dominant at 482 nm. The absorption band in the spectral region above 550 nm can be assigned to the characteristic absorption originating from the electronic communication between the perylene tetra-anhydride and ethidium bromide moieties. In the emission spectrum, we observed the characteristic emission signal in the spectral region from 500 nm to 750 nm, which is linked to the fluorescence from the perylene tetra-anhydride moiety. To interpret the electronic transitions in terms of molecular orbital, we performed the time-dependent density functional theory (TD-DFT) calculations based on the optimized structure for the repeating unit of the COP. As shown in Figure 3d, we displayed the natural transition orbitals (NTOs) reconstructed from the TD-DFT results. The vertical transition at 2.78 eV, equal to 446 nm, shows that the charge density in a hole state is dominantly located on the side-chain moiety of ethidium bromide, while the charge density in a particle state is dominantly located on the conjugated backbone. The lowest transitions around 1.7 eV, equal to 747 nm, involve the change of the electronic density around the ethidium bromide, originating from the high electronegativity of Br atom. The results from the TD-DFT calculations support the existence of efficient charge-transferred state linked to the electronic interaction between the two building units in COP, which are commonly observed in the two-dimensional COFs.18,45

To unveil the photo-induced electronic dynamics of COP material, we performed the nanosecond transient absorption spectroscopic (ns-TA) and time-resolved photoluminescence (TR-PL) experiments. A laser pulse centered at 490 nm was employed as a pump pulse to initiate the photoexcitation in COP. The broadband white light with the spectral window covering from 370 nm to 720 nm was subsequently irradiated to probe the absorption change, denoted as ?A, induced by the pump excitation. The condition of photoexcitation is likely to be predominantly resonant to the electronic transition of the charge-transferred state, as supported by the results from the TD-DFT calculations. Figure 3a shows the wavelength-resolved ns-TA spectra of COP in the wide time window covering from 0 ns to 2000 ns. The ns-TA spectra involve the positive absorption feature around 450 nm as well as the huge negative feature around the region of 530 ~ 700 nm. Considering the stead-state emission spectrum of COP shown in Figure 1e, the spectral position for the negative TA signal at 0 ns can be assigned to the laser-induced emission from the singly excited state, also known as stimulated emission (SE) feature. Since such SE signal in ns- TA shows a fast decay, we additionally measured the TR-PL signal at the wavelengths of 535 nm and 700 nm. As shown in Figure 3c, the kinetic profiles measured from the TR-PL showed the bi- exponential decays within ~3.43 ns. Meanwhile, the positive absorption feature around 450 nm, corresponding to the excited state absorption (ESA) feature, shows a quite slow decay in the measured time window. To systematically extract the kinetic components from the ns-TA data, we measured the decay profile of ns-TA at the representative probing wavelengths of 450 nm and 700 nm shown in Figure 3b. From the global fitting analysis for the measured data, it was confirmed that the ESA signal at 450 nm rises with the time constant of 8.56 ns and subsequently decays with the time constant of 61.5 ns. Notably, the ESA signal at 450 nm shows an exponential decay within ~200 ns and is changed to the negative signal in the late time region from 200 ns to 3500 ns, which is similar observed in the decay profile at 700 nm. Considering the fastest decay in the TR-PL data within 20 ns, it can be inferred that the negative signal observed in the late time delay originates from the ground state bleach (GSB) signal associated to the depletion of the electronic population in the ground state of COP. It implies that the photo-induced charge carriers in COP undergoes the additional dynamics in the excited state, which is highly associated to the long-lived charge separated (CS) state. Such a long-lived CS state with the slow recovery time was reported from the previous study for COFs based on a donor-acceptor conjugated scheme.46,47 Based on those results, we speculate that the formation of the CS state in the photoexcited COP mainly comes from the electronic interaction between two building units, which might affect the macroscopic photocatalytic ability.
COP-biocatalyst integrated artificial photosynthesis for the production of CH3OH from CO2
Based on the characteristic photo-physical properties of COP, we designed the COP-biocatalyst integrated artificial photosynthetic reaction in order to produce the methanol from CO2 under the irradiation of solar light. The as-synthesized COP was cast on the polymer film by following the experimental procedure. The details about the preparation of the thin film are described in the SI. Scheme 2 depicts a graphical illustration for the photocatalyst-biocatalyst integrated photosynthetic module including the COPs-coated thin film, Rh-based electron mediator, reduced nicotinamide adenine dinucleotide (NADH) cofactor, and enzyme-type biocatalysts.42,43 This reaction consists of two parts; i) the regeneration process of NADH from the photo-activated COP and ii) the production process of methanol from CO2 at a reaction center of the biocatalyst by consuming the reduced NADH. As a first step, we tested the photocatalytic ability of COP for the photo-induced regeneration of NADH from NAD+ acting as an oxidized form. The amounts of the regenerated NADH during the artificial photosynthesis was determined from the measurement of UV- visible absorption spectrum based on the well-established method.42,43 Figure 4 shows the catalytic efficiency of COP with 67.96 % during the reaction time of 90 minutes which is much higher than those in perylene (18.23 %) and in ethidium bromide (8.79 %) monomers. As summarized in Table S1, the regeneration yield of COP for NADH is much higher than that of graphene-based photocatalyst,21 while COP shows the lower regeneration yield relative to COF-based photocatalyst.26 Based on the regeneration yield of NADH in COP, we conducted the artificial photosynthetic reaction to produce methanol (CH3OH) from CO2 by employing the enzyme-type biocatalysts. The amounts of the produced CH3OH during the reaction were determined from the HPLC trace. Upon the light irradiation into the artificial photosynthetic module, the yield of the produced CH3OH linearly increased and showed the yield of 19.27 µM during the reaction time of 90 minutes as shown in Figure 4b. We performed separate experiments without Rh-complex and/or NADH cofactor in the artificial photosynthetic module, but the control experiments did not show any catalytic efficiency for the production of CH3OH from CO2. Such experiments indicate that the Rh-based mediator and/or NADH, acting as electron mediator, plays a critical role in transferring the photo-excited electron from COP toward the biocatalysts. To check the long-time stability of COPs-coated thin film, we implemented multiple experiments by re-using the thin film for the regeneration of NADH. As shown in Figure S5, the catalytic efficiency of the cast thin film was well kept during the 5 times cycles.
Photo-induced multi-component Biginelli reaction
To evaluate the photocatalytic ability of COP, we employed the COPs-coated thin film as a photocatalyst for the multi-component Biginelli reaction. The target reaction was based on the condensation of benzaldehyde, acetylacetone, and urea derivatives in order to synthesize 3,4- dihydropyramidine compounds. First, we implemented the preliminary experiments to find the optimal condition of the target reaction. From the experiments, we found that the reaction medium of C2H5OH is an optimal solvent under the light irradiation, while the absences of COPs-coated thin film and/or light irradiation resulted in no yield of product. Notably, the production yields in the reaction mediums of DCM, THF, DMF were quite lower than those in methanol and ethanol solvents, indicating that the production yield is highly associated to the solvent polarity. To expand the scope of the synthetic products in the photo-induced Biginelli reaction, we employed the various types of benzaldehyde (1a), urea derivatives (1b), and acetylacetone (1c) as starting reagents. At the optimal condition, the COP material showed the excellent photocatalytic ability for the synthesise of 3, 4-dihydropyrimidin-2-(1H)-one/thiones/seleons, denoted from 2a to 2h, as summarized in Table 1. All the products were characterized from the NMR measurements shown in Figure S6-S15. Among the various types of 3,4-dihydropyramidine products, the COP showed the superb catalytic yield for 2a and 2g compounds with ~98 % as well as the higher turnover frequency (TOF) more than 90 min-1 compared to the other products as summarized in Table S2. The catalytic efficiency and TOF value of COP were slightly dropped in the reagents of thiourea (S=C(NH2)2) and selenourea (Se=C(NH2)2) relative to urea (O=C(NH2)2). Considering the electronegativity of oxygen (3.44), sulphur (2.58), selenium (2.55) atoms, we speculate that the highest electronegativity in oxygen atom of urea leads to the high yield in the Biginelli reaction due to the efficient charge transport from COP.
Plausible mechanisms for COP-based photocatalytic reactions
The plausible mechanisms for the artificial photosynthetic reactions to produce methanol from CO2 and to synthesize 3,4-dihydropyramidine compounds were suggested by considering the results from the time-resolved spectroscopic experiments combined to those from the TD-DFT calculations. The irradiation of external light into the COP material brings about the electronic transition corresponding to the the formation of photo-induced charge transfer (CT) state across the conjugated backbone in COP, supported from the behavior of the electron density in the NTOs. Sequentially, the photo-induced CT state can induce the long-lived charge separation (CS) state predicted from the ns-TA and TR-PL, which would facilitate the electron transport from COP to Rh-complex, [Cp*Rh(bpy)H2O]2+. The reduced Rh complex will take a proton from the aqueous reaction media and transfer a hydride ion to the oxidized NAD+, which forms the reduced NADH. At the protein-based biocatalysts, such reduced NADH cofactors are eventually consumed to convert CO2 to methanol via the enzymatic reaction. In this regard, the formation of the long-lived CS state in COP would enable efficient electron transport from COP to Rh-complex and thus the efficient generation of reduced NADH from the Rh-complex ultimately boosts the production of methanol. In the catalytic test, the absences of Rh-complex and/or NADH cofactor in the artificial photosynthetic module caused no yield in the methanol production, indicating that the electron mediators are necessary in the photo-induced electron transport from COP to the biocatalyst.
In the Biginelli reaction, the photo-excited COP is likely to induce proton-coupled electron transfer (PCET) which produces a radical anion of COP state with the concurrent generation of cationic acetylacetone species. The efficient electron transport from the radical anion form of COP to benzaldehyde is operational to express a highly active iminium intermediate based on the nucleophilic addition with urea/thiourea/selenourea. As summarized in Scheme S1, the cationic acetylacetone species originating from the PCET process can highly interact with the iminium intermediate, resulting in the intramolecular cyclization to form the final product. Based on the results from the time-resolved spectroscopic experiments, it can be inferred that the long-lived CS state in COP facilitate the through-space electron transport from COP to benzaldehyde and would assist the formation of highly-active iminium intermediate reported in the previous study.38
Conclusions
In this study, we developed the perylene-based COP with the crystalline structure in order to apply the as-synthesized material into the various catalytic reactions. By employing the FT-IR, PXRD, NMR, XPS, and TGA measurements, we performed the systematic characterization of structural property for the COP material. The existence of the covalent conjugation between perylene and ethidium bromide moieties in COP is highly associated to the superb chemical stability and the photo-physical properties linked to the broad absorption ability in the visible range. To provide insights for the photo-induced electronic dynamics, we implemented the nanosecond transient absorption (ns-TA) and time-resolved photoluminescence (TR-PL) spectroscopic measurements combined to the TD-DFT calculations. According to the results from the ns-TA and TR-PL, the expression of the long-lived CS state in COP was observed in the late time window and was presumed to be highly associated to the photocatalytic efficiency. By employing the COP- biocatalyst integrated photosynthetic module, it was revealed that the COP material showed the excellent production yield of methanol with 19.27 µM via the efficient photo-induced electron transfer from COP to the biocatalyst reaction center. Furthermore, the application for the light-driven multi-component Biginelli reaction was tested by employing the COP-coated thin film under the polar solvent medium. It showed the high conversion efficiency and TOF for the productions of various 3,4-dihydropyramidine compounds. We believe that the characteristics of excited electronic dynamics in COP plays an essential role in view of macroscopic catalytic performance and thus such organic polymeric photocatalysts will be actively used for the single/multi-component artificial photosynthetic approach.
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,CLAIMS:1. A covalent organic polymeric photocatalyst (COP) comprising of:
perylene dianhydride (500 mg), maleic anhydride (249 mg), p-chloranil (156 mg), anhydrous benzene (15 mL), imidazole (5g), perylene tetra-anhydride (PTA) (250 mg), ethidium bromide (EB) (335 mg).
2. A method of the photocatalyst as claimed in claim 1, wherein the said method comprising the steps of:
• dissolving perylene dianhydride, maleic anhydride, and p-chloranil in anhydrous benzene;
• ultra-sonicating the reaction mixture for even dispersion;
• performing the reaction under a nitrogen-purged environment;
• taking the reaction mixture from the oil bath after finishing the reaction
• cooling the reaction mixture to room temperature post-reaction;
• filtering the resultant product using a large amount of water and n-hexane;
• purifying the product through Soxhlet extraction to obtain perylene tetra-anhydride;
• dissolving imidazole, perylene tetra-anhydride (PTA), and ethidium bromide (EB) in a Pyrex tube;
• heating the reaction mixture to 120 ºC in a nitrogen-purged environment;
• cooling the reaction mixture to room temperature post-reaction;
• filtering the resultant product with water and ethanol; and
• drying the product to obtain a powdered perylene-based covalent organic polymeric photocatalyst.
3. The method as claimed in claim 2, wherein the proportions of reactants are 500 mg of perylene dianhydride, 249 mg of maleic anhydride, and 156 mg of p-chloranil in 15 mL of anhydrous benzene.
4. The method as claimed in claim 2, wherein the proportions of reactants are 5 g of imidazole, 250 mg of perylene tetra-anhydride, and 335 mg of ethidium bromide.
5. The method as claimed in claim 2, wherein the photocatalyst exhibits enhanced stability and photocatalytic efficiency suitable for photochemical applications.

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