Robust chromophore-integrated MOFs as highly visible-white-light active and tunable size-selective photocatalysts towards benzothiazoles

Hu Liu, Qunqun Li, Penghui Pn, Li Zhou, Bing Deng, Shuy Zho, Ping Liu,*,Yoyu Wng, Jinli Li,*

a College of Chemistry & Materials Science, Northwest University, Xi’an 710127, China

b College of Urban and Environmental Sciences, Northwest University, Xi’an 710127, China

Keywordss:Metal-organic framework Heterogeneous photocatalyst Tetrazine Benzothiazole Substrates selectivity

ABSTRACT Visible-light heterogeneous photocatalyst with high activity and selectivity is crucial for the development of organic transformations, but remains a formidable challenge.Herein, a simple and effective strategy was developed to integrate tetrazine moiety, a visible light active unit, into robust metal-organic frameworks (2D MOF-1(M), M=Co, Mn, Zn, and 3D MOF-2(Co)).MOF-1 series are isomorphous 2D porous frameworks, and MOF-2(Co) displays 3D porous framework.Interestingly, benefiting from the oxidative active species of O2·-, these MOFs all exhibit obviously highly enhanced photocatalytic activities toward the straightforward condensation of o-aminothiophenol and aromatic aldehydes at room temperature in EtOH under visible-white-light irradiation.Notably, compared to 3D MOF, the 2D layered MOF-1(Co) exhibited more excellent catalytic activity with a wide range of substrates possessing preeminent tolerance of steric hindrance.Most impressively, MOF-1(Co) can be recycled at least five times without significant loss of catalytic activity or crystallinity, exhibiting excellent stability and reusability.This study sheds light on the wide-ranging prospects of visible light active 2D MOFs as green photocatalysts for the preparation of fine chemicals.

Compared to traditional synthetic reactions, visible light driven organic transformations offer a green and sustainable route for high value-added compounds [1–7].In this aspect, heterogeneous visible-light photocatalysts are especially promising for their great recyclability and environment friendly nature [8–13].However,the development of highly active heterogeneous photocatalysts remains a formidable challenge [14,15].To this end, metal-organic frameworks (MOFs) are recently emerging as a kind of photocatalysts for the combination of well-defined crystalline structures,large surface areas, fast charge separation and tunable photon absorption [16–21].Combining the merits of organic and inorganic chemistry, both metal central and organic linkers, as the basic components of MOFs, can be easily tailored to improve photocatalytic performance at the molecular level [22–28].In addition, it has been found that the structure dimension of MOFs can greatly influence the catalytic performances for their degree of access to the active sites [29–35].Nevertheless, to the best of our knowledge, it remains an unexplored area that the systematic effects of the metal central, and structural dimension in MOFs as photocatalysts on the activity and size selectivity under the visible-whitelight irradiation.The starting point is to design and synthesize suitable MOF platform as a model to study the structure–activity relationships, which can provide valuable insights for the further development of MOFs with highly efficient photocatalytic performance.

Fig.1.(a) Schematic illustration of synthesis of 2D MOF-1(M) (M=Co, Zn, Mn)and 3D MOF-2(Co).(b) UV-visible diffuse reflectance spectrum and Tauc plots (inset) of MOF-1(Co).(c) Mott-Schottky plots and band structure diagram (inset) of MOF-1(Co).(d) Photocurrent responses of dptz and MOF-1(Co).(e) EIS Nyquist plots for dptz and MOF-1(Co).

Compared to the metal central, integrating the organic linkers with well photoredox activities into MOF frameworks would be a relatively efficient strategy to enhance the photocatalytic performance owing to the easily modified nature [36–39].Nevertheless,the attractive visible-light active ligands are limited, and currently are typically expensive Ir- and Ru-complexes based linkers, or porphyrins and metalloporphyrins based chromophore linkers[36,40-43].Other visible light active ligands are thus encouraged to develop for ameliorating the current photocatalytic application.1,2,4,5-Tetrazines, as a representative electron-deficient structure,are well-known for their unique optical and photophysical properties [44–47].It possesses high electron affinity and is easily reduced by accepting an electron to form an anion radical.This reduction is even more pronounced for its first excited state, thus exhibiting a relatively strong oxidizing capacity.Also,1,2,4,5-tetrazines often display bright colors, exhibiting noteworthy visible light absorption.Therefore, developing the related 1,2,4,5-tetrazine-based porous MOFs as photocatalysts would be a crucial travel direction for the enhanced activity and tunable size selectivity.Herein, with a 1,2,4,5-tetrazine linker (3,6-di(pyridin-4-yl)-1,2,4,5-tetrazine, dptz) as a visible-light active unit, we have constructed a series of the structure- and property-comparative robust MOFs, 2D MOF-1(M) (M=Co, Zn, Mn) and 3D MOF-2(Co)as the visible-white-light driven heterogeneous photocatalysts towards benzothiazoles.Compared to the homogeneous dptz, its immobilized MOFs presented more outstanding photocatalytic activity.Notably, for the small sized substrates, these four MOFs displayed excellent photocatalytic activity with the yield up to 90%, while for the sterically hindered substrates, two-dimensional(2D) MOF-1(M) series displayed similar catalytic performance,superior to three-dimensional (3D) MOF-2(Co), thus exhibiting tunable substrate size-selectivity.Furthermore, MOF-1(Co) also demonstrated good recyclability.

Four dptz-based MOFs, MOF-1(M) (M=Co, Mn, Zn) and MOF-2(Co) were synthesized through the solvothermal reactions(Fig.1a).Single-crystal X-ray diffraction studies revealed that 2D MOF-1(Co), MOF-1(Zn) and MOF-1(Mn) are isostructural, and crystallize in theP1 space group of triclinic crystal system.Taking MOF-1(Co) structure as an example, it exhibits a two-dimension network consisting of [M2(COO)4N4] secondary building unit(SBU), dptz, and partially protonated 1,3,5-benzenetricarboxylic acid (HBTC2-), which are assembled through the hydrogen bonding interactions andπ···πstacking interactions to generate a 3D supramolecular structure bearing open 1D channels with the pore dimension being 11.073×8.432 ˚A2along theb-axis.In the structure, there is an uncoordinated carboxyl group in the HBTC2-ligand (Fig.S4 in Supporting information), which can act as acidbase buffer sites to protect the MOF-1(Co) from aqueous solution [34,48].In the case of MOF-2(Co), it displays a 3D pillarlayered framework constituted by [Co2(COO)4]nchain, dptz, and completely protonated 1,2,4,5-benzenetetracarboxylic acid (TTC4-)ligand, with the open 1D channels of 12.207×8.371 ˚A2, showing slightly larger than that of MOF-1(Co) (Figs.S2–S8 in Supporting information).Notably, the presence ofπ···πstacking interactions between the benzene/tetrazine rings in MOF-1(Co)/MOF-2(Co) might favour the transfer of charges (Figs.S5 and S8), suggesting the potential photocatalytic performances [49–51].

Solvent-stability tests showed that the framework of assynthesized MOF-1(M) and MOF-2(Co) remain intact after immersing them in various solvents for one month, demonstrating the excellent solvents resistance (Figs.S11 and S12 in Supporting information).Impressively, the crystallinity of MOF-1(Co) can be retained in water for one year, in water with pH from 2 to 12 for two weeks, or under white LED irradiation for 48 h (Figs.S13–S15 in Supporting information).Furthermore, thermogravimetric analysis(TGA) showed that MOF-1(M) and MOF-2(Co) are thermally stable up to 548 K (Figs.S16–S19 in Supporting information).The good stabilities of these MOFs render them to be excellent candidates for photocatalytic application.

To evaluate the photocatalytic performance, the optical and electrochemical properties were initially studied.As shown in Fig.1b and Fig.S20 (Supporting information), the ultraviolet–visible (UV–vis) diffuse reflectance spectrum of MOF-1(Co) as a representative and MOF-2(Co) displayed a broad absorption range from 200 nm to 600 nm.By the Kubelka-Munk (KM) method from Tauc plots, the band gap (Eg) of MOF-1(Co) and MOF-2(Co) was estimated to be about 1.99 eV and 2.01 eV, respectively, demonstrating their application prospects as semiconducting visible light active catalysts.To assess the conduction band (CB) and the valence band (VB) levels, Mott-Schottky electrochemical experiments were performed, and the CB positions were determined to be -0.81 V for MOF-1(Co) and -0.82 V for MOF-2(Co) (Fig.1c and Fig.S21 in Supporting information).The VB was thus estimated to be 1.18 V and 1.19 V, respectively.

Given that the charge separation efficiency of photocatalysts is an important factor for photocatalytic processes, the measurements of photocurrent responses and electrochemical impedance spectroscopy (EIS) were then performed.As shown in Fig.1d and Fig.S22 (Supporting information), MOF-1(Co) and MOF-2(Co) show significant stronger photocurrent responses than dptz linker, implying an effective separation of photogenerated electron-hole pairs and an enhanced catalytic activity.Furthermore, this result was further demonstrated by their smaller radius and lower chargetransfer resistance (Fig.1e and Fig.S23 in Supporting information).Overall, optical and electrical studies suggested that building the dptz linker into MOF can improve the charge separation efficiency,prompting our great interest in examining the photocatalytic performances of these synthesized MOFs.

Benzothiazoles have received increasing attention for their interesting applications in dyes, chemosensing, as well as advanced materials such as nonlinear optics, and organic light-emitting diodes [52–54].However, the traditional synthetic methods commonly suffer from low yield, poor selectivity, or harsh reaction conditions (high temperature or acid conditions).Thus, it is urgent to develop green, sustainable and efficient synthetic methods.In view of this, the photocatalytic performances of MOF-1 series and MOF-2(Co) are evaluated in the synthesis of benzothiazoles.

Fig.2.Photocatalytic synthesis of benzothiazoles with different sizes of aromatic aldehyde substrates catalyzed by MOF-1(Co), MOF-2(Co) and dptz, and the assumed structures and the molecular size were calculated by using the program Chem3D.

MOF-1(Co) was initially selected and assessed for the condensation cyclization ofo-aminothiophenol and benzaldehyde as the model reaction.Then, the reaction conditions were optimized under 10 W white light emitting diode (LED) irradiation at room temperature (Table S2 in Supporting information).Various solvents were examined, and the desired product3awas obtained in 88%yield in ethanol (EtOH), while low yields with other solvents such as acetonitrile (MeCN), methanol (MeOH), tetrahydrofuran (THF),dimethyl sulfoxide (DMSO) orN,N-dimethylformamide (DMF) (Table S2, entries 1–6).It is noteworthy that good yields were observed at 0.5 mol% catalyst loading (Table S2, entries 2 and 7–9),and further increase in catalyst loading led to a decrease in yield because the excess MOF particles in the suspension may reduce photon utilization.Then, the reaction times were evaluated, and we found the optimum reaction for one hour (Table S2, entries 7,10 and 11).Therefore, the optimized reaction conditions can be 0.5 mol% MOF-1(Co), EtOH, and 1 h.

To examine the role of metal central, we conceived the comparable utilization of 2D MOF-1(M) series with different metal central as photocatalysts under the optimal condition.As shown in Table S2, all 2D MOF catalysts displayed similar and excellent yields of3a(91%–94%), and significantly outperformed that of the dptz ligand (28%), indicating metal central is not a pivotal factor for the photocatalytic activity.

Fig.3.Scope of photocatalytic synthesis of benzothiazoles catalyzed by MOF-1(Co).Reaction conditions: 1 (0.3 mmol), 2 (0.3 mmol), 10 W white LED, room temperature, air.Isolated yields.

To gain an insight into dimension-activity relationships, several aldehydes with increasing sizes, such as benzaldehyde (2a),1-naphthaldehyde (2b), 1-pyrenecarboxaldehyde (2c), 4-(N,N-dip henylamino)benzaldehyde (2d) and 4-(1,2,2-triphenylvinyl)benza ldehyde (2e) were selected to examine their product yields catalyzed by 2D MOF-1(Co) and 3D MOF-2(Co) (Fig.2).The dptz linker performed poorly in the synthesis of benzothiazole and showed comparable catalytic activity for all aldehydes (28%, 33%,27%, 30%, and 29% for2a–2e).With 3D MOF-2(Co) as a catalyst,the yields of benzothiazole products from2ato2edecreased in the order of 88%, 84%, 33%, 26%, and 20%.In contrast, 2D MOF-1(Co) showed excellent activity with benzothiazole product yields of 93%, 95%, 91%, 93%, and 96% for2a–2e, indicating that 2D MOF-1(Co) exhibits excellent catalytic activity with a wide range of substrates possessing preeminent tolerance of steric hindrance.Despite the pore size of 3D MOF-2(Co) is slightly larger than that of 2D MOF-1(Co), the latter presents better catalytic activity in catalyzing sterically hindered substrates, which can be ascribed to the relatively limited channels of 3D MOF-2(Co) that causes the large reactants to barely transport through pores and access to the active sites [55].In this case, photocatalytic reactions occur on the outer surface of MOF-2(Co), thus exhibiting comparable or even lower catalytic efficiency with dptz.By contrast, the 2D network of MOF-1(Co) cannot only reduce diffusion barriers, facilitate the contact of substrates with the active sites, and afford rapid mass transport and electron transfer, but also easily dissociated intermediates in photocatalysis in comparison with 3D MOFs [11].Overall, for the tetrazine-based MOFs with the different dimension, 2D MOF-1(Co)demonstrates superior photocatalytic performance than that of 3D MOF-2(Co).

Fig.4.(a) Time-dependent curves of the synthesis for benzothiazoles.(Red: standard conditions.Blue: the catalyst was filtered after 20 min).(b) Recycling experiments with MOF-1(Co) as the photocatalyst for the synthesis of benzothiazoles.(c)PXRD patterns of MOF-1(Co) after five cycles for the photocatalytic reaction.(d) EPR spectra of MOF-1(Co) sample in the presence of DMPO under air atmosphere under dark and visible-light conditions for 0.5 and 2 min.(e) Proposed reaction mechanisms for the synthesis of benzothiazoles over MOF-1(Co) powered by white LED light.

Based on the above results, the substrate scope of aromatic aldehydes was expanded with the optimized protocol using 2D MOF-1(Co) as photocatalyst (Fig.3).First of all, for model benzaldehyde2a, the 93% yield of the product3awas observed.When benzaldehyde bears the groups of -Cl, -OH, -NO2and -CH3at theo-position, it is observed that the presence of electron-donating and -withdrawing groups has little effect on the reaction efficiency(3f–3m).When substitution at them- andp-positions, the desired products can also be given in good yields (3nand3o).We were pleased to find that furaldehyde or thenaldehyde as the substrates also afforded the target products in good yields (3pand3q).In general, regardless of the electronic properties and substitution modes, its extensive functional group compatibility showed that MOF-1(Co) can be a powerful photocatalyst for the condensation cyclization of 2-substituted benzothiazoles upon visible-white-light irradiation.

Forward, control experiments were conducted, and no product was obtained under dark conditions or without the catalyst, revealing that photocatalyst and light were both necessary for this organic transformation (Table S2, entries 16 and 17).In addition,comparative experiments were also implemented.As indicated,when a powder mixture of Co(NO3)2·6H2O, dptz, and H3BTC was used as the photocatalyst, lower yield of products3awas observed(25%) (Table S2, entry 20), suggesting that the presence of framework of MOF-1(Co) is critical.This result is in good agreement with the leaching experiments (Fig.4a) that nearly no further conversion is observed after the removal of the MOF catalyst after 20 min of reaction.And after filtrating MOF-1(Co), no obvious dptz, H3BTC,or Co signal in the filtrate was observed in the inductively coupled plasma-mass spectrometry (ICP-MS), mass spectrometry (MS) or UV–vis spectra (Figs.S28–S30 in Supporting information), demonstrating the heterogeneous nature and stability of MOF-1(Co).

The recyclability and stability are critical factors for heterogeneous photocatalysts.Therefore, recycling experiments to synthesize3awere carried out to examine the photocatalytic durability.MOF-1(Co) can be quickly recovered from the reaction system by centrifugation, and then used in the next cycle without additional treatment or activation.As revealed in Fig.4b, MOF-1(Co)can maintain the high photocatalytic activity toward the above condensation cyclization after five runs of experiments.The powder X-ray diffraction (PXRD) patterns of MOF-1(Co) after the recyclability tests remain unchanged (Fig.4c), demonstrating the structural integrity of MOF-1(Co) after the organic transformation and the enough stability to recycle and reuse.

To determine the key active species in the photocatalytic reaction.Radical capture by (2,2,6,6-tetramethylpiperidin-1-yl)oxyl(TEMPO) was performed.As shown in Fig.S31 (Supporting information), the TEMPO completely shut down the condensation reaction.Then, we performed the reaction in the presence of a known superoxide radical anion (O2·-) quencher, 1,4-benzoquinone(BQ).The significant reduced yield of3aindicates that O2·-is key intermediate in this reaction.In addition, electron paramagnetic resonance (EPR) experiments were performed to confirm its ability to activate oxygen upon visible-light irradiation.The signals observed upon 400–470 nm visible-light irradiation of MOF-1(Co) in air indicated the generation of O2·-in the presence of a radical trapping reagent, 5,5-dimethyl-1-pyrrolineN-oxide (DMPO)(Fig.4d).

On the basis of the above experimental results and reported literature [56], a mechanistic hypothesis for the synthesis of benzothiazole by visible-light assisted catalysis is depicted in Fig.4e.Upon light irradiation, MOF-1(Co) was first photoexcited to the MOF-1(Co)*species.O-aminothiophenol and benzaldehyde were quickly cyclized and reduced to intermediate I, and meanwhile,the MOF-1(Co)*species accepted one electron to produce MOF-1(Co)*-, which was oxidized to the ground state by O2in air, producing superoxide radicals.The desired product was then obtained by oxidative dehydrogenation and hydrogen abstraction.

In summary, we have successfully developed four robust heterogenous MOFs catalysts (MOF-1(M), M=Co, Mn, Zn, and MOF-2(Co)), with a 1,2,4,5-tetrazine visible light-active unit as the organic linker.MOF-1(M) series exhibited 2D network, while MOF-2(Co) displayed 3D porous structure.All these four MOFs significantly outperformed homogeneous dptz ligands towards benzothiazoles under 10 W white LED irradiation.For 2D MOFs,MOF-1(M) series with various metal centers displayed similar catalytic performance.For Co(II)-based MOFs presenting different dimensionalities, 2D MOF-1(Co) significantly outperformed the 3D MOF-2(Co) in catalyzing sterically hindered substrates with the nearly free substrate accessibility.MOF-1(Co) as an excellent photocatalyst shows superb catalytic activity in the condensation cyclization to afford a broad scope of benzothiazoles in EtOH at room temperature.The heterogeneity and structural integrity of MOF-1(Co) were also confirmed by the recycling experiments.This work successfully illustrates that incorporating the visible-light active 1,2,4,5-tetrazine linker into 2D MOF platform would be as an efficient strategy to construct promising robust photoactive catalysts.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos.22171223, 22077099 and 21531007), the Innovation Capability Support Program of Shaanxi(Nos.2023-CX-TD-75 and 2022KJXX-32), the Natural Science Foundation of Shaanxi Province of China (Nos.2020TG-031, 2022JQ-125, 2023-JC-YB-141, 2022JQ-151 and 2021JQ-440), the special fund of Shaanxi Key Laboratory of Special Fuel Chemistry and Material(No.SPCF-SKL-2021-0011), and Young Talent Fund of Association for Science and Technology in Shaanxi, China (No.SWYY202206).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108562.