English

• Sunlight has been regarded as the most promising renewable energy source to meet humanity's future energy demand due to its advantages in clean and inexhaustible [1–3]. Photosynthesis in plant represent the most successful example for solar energy utilization, which can massively transform the low energy density of sunlight into the available carbohydrate compounds [4]. In view of this, chemists have long been devoted to developing photocatalysts to mimic natural process. Thanks to their similar structural features with chlorophyll and the ease of their structure/property tuning, transition metal complexes have been widely used as molecular antenna for solar energy conversion [5–8]. For example, the typical [Ru(bpy)₃]²⁺ (bpy = 2, 2′-bipyridine) or [Ir(ppy)₂(bpy)]⁺ (ppy = 2-phenylpyridine) derivatives have been applied to a range of energy-storing reactions for the past half a century [9–14]. The work mechanism of these complexes was usually as follows: upon light irridiation, charge transfer from metal to ligand was triggered to afford the singlet state (¹MLCT) and then it converted into the triplet excited state (³MLCT) via intersystem crossing (ISC), which can further initial the intermolecular electron/energy tranfer process [6]. This process reveal that both visible light absorbing ability and excited state lifetime of complexes have a significant influence on their catalytic performance. Several attempts have made to couple the typcial Ru/Ir complexes with organic chromohores to enhance visible absorption and extend excited state lifetime, which have been confirmed as an efficient stratgy to improve the photosynthetic efficiency [15–18]. Nevertheless, these metal complexes are homogeneous in solution, which have some intrinsic drawbacks in humble catalytic stability, poor recyclable and excited state self-quenching. As a result, it's highly desirable but remains a great challenge to pursue the further development of strong sensitzing heterogeneous photocatalysts for efficient and sustainable photosynthesis.

  As a class of crystalline porous materials, metal-organic frameworks (MOFs) have provided an ideal molecular platform to meet different application scenarios by precisely regulating their organic ligands, inorganic second building units (SBUs) or channels [19–28]. Of particular interest is to develop MOF photocatalysts for solar energy conversion associated with a series of processes of light absorption, interface- and intra-framework electron/energy transfer [29–34]. However, the traditional MOFs with terephthalic acid ligands or its derivatives such as UiO-66 and UiO-67 can just harvest ultraviolet (UV) light, which severely limited their solar energy utilization [34]. In order to overcome this limitation, attempts have been made to replace the traditional ligands with polypyridinal Ru/Ir complexes in MOFs for enhancing their visible light absorbing ability [30,35–40]. In this field, Lin et al. doped [Ir(ppy)₂(dcbpy)]Cl and [Ru(bpy)₂(dcbpy)]Cl₂ into UiO-MOFs for visible light induced heterogeneous photosynthesis [41]. Chen et al. co-doped [Ru(dcbpy)(bpy)₂]²⁺ (dcbpy = 2, 2′-bipyridyl-5, 5′-dicarboxylic acid) photosensitizer and Pt(dcbpy)Cl₂ catalyst into UiO-MOFs for photochemical conversion of H₂O to H₂ [42]. Kong et al. explored a Ru(Phen)₃²⁺-based (Phen = phenanthroline) Eu-MOF for photocatalytic CO₂ reduction [43]. In addition, sorts of single sites and nano-clusters catalysts were introduced into the Ru/Ir complexes sensitized MOFs via ligand or channel modification for facilitating intra-framework electron transfer [44–48]. Despite this great progress on co-doping and engineering catalytic centers, the study on sensitizing center is still rare for photoactive MOFs [49–54]. The performance for MOF photocatalysts with the typical Ru/Ir complexes was limited by their poor visible light harvesting ability and relative short excited lifetime. Subsequently, we hypothesized that the catalytic performance of these MOFs is promising to be improved significantly by engineering the metal sensitizing center to enhance the visible absorption and impetus interface electron transfer.

  Bearing these aspects in mind, we proposed a strategy to significantly improve the sensitizing ability of the typical photoactive UiO-MOFs (UiO-Ir) by engineering the Ir coordination center with NBI (1, 8-naphthalenebenzimidizole) chromophore (UiO-Ir-NBI). The coordination environment of Ir center in UiO67-Ir-ppy was regulated by replacing ppy ligand with NBI, resulting in the transformation of excited state distribution from ³MLCT excited state to ³IL state (IL = intraligand). Remarkably, the conversion rate for the detoxification of a sulfur mustard simulant can reach 99% with UiO-Ir-NBI as photocatalyst, over 6 times higher than that with the typical UiO-Ir (16.4%). Systematic investigations reveal that the visible light absorption, excited state lifetime and electron-hole separation of UiO-Ir were significantly improved via NBI sensitization, which contributed to efficiently harvesting visible light and facilitating interface electron/energy transfer for boosting photosynthesis. This work opens up a new avenue to significantly efficient boost photosythesis by engineering their sensitizing centers with chromophores.

  Upon excitation, ³MLCT state was populated in the traditional Ru-/Ir-based photoactive MOFs, resulting in weak visible absorption and short excited state lifetime, which was harmful to the interface electron/energy transfer and sunlight utilization. To tackle these issues, we tried to improve photosensitization of the MOFs by engineering their metal sensitizing center with organic chromophores. The design concept of this strategy is to enhance visible harvesting ability and prolong the excited state lifetime of MOFs via altering their light absorbing channel from ¹MLCT to π-π* transition and switching their triplet state distribution from ³MLCT state to ³IL stae for boosting photosynthesis (Fig. 1a). The excited state distribution of photosensitizing units in UiO-Ir and UiO-Ir-NBI can be visualized by density function theory (DFT) calculation. As shown in Fig. 1, the spin density surface of L-1 was distributed on the Ir center, ppy and 2, 2′-bipyridine-5, 5′-dicarboxylic acid, representing the typical MLCT excited state of cyclometalated Ir(Ⅲ) complexes. After chromophore sensitization, the spin density surfaces distributed on NBI chromophore, Ir center and 2, 2′-bipyridine-5, 5′-dicarboxylic acid in L-2, assigned to the mixed state of ³MLCT /³IL excited state.

  Figure 1

  

  Figure 1.  (a) Schematic show of the energy level of MOFs with ³MLCT excited state, the structure of UiO-Ir and electron spin density of L-1. (b) Schematic show of the energy level of MOFs with ³MLCT/³IL excited state, the structure of UiO-Ir-NBI and electron spin density of L-2.

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  Under the guidance of design principle, NBI chromophore was used to sensitize the Ir sensitizing center of the typical UiO-Ir to afford UiO-Ir-NBI via bottom-up synthetic approach. As shown in Scheme S1 (Supporting information), the synthetic process of L-2 was similar to that of the typical L-1. Iridium dimer (6) was prepared by coordinating NBI chomophore with IrCl₃·3H₂O, which can further react with 5, 5′-dimethoxycarbonyl-2, 2′-bipyridine to generate complex 7. Subsequently, L-2 can be achieved via hydrolysis of complex 7. According to the typical synthesis route of UiO-MOFs, the mixture of L-2, ZrCl₄ and 4, 4'-biphenyldicarboxylic acid (8) was dissloved in N, N-dimethylformamide (DMF) and heated at 100 ℃ for 24 h to afford the photosensitizing MOF UiO-Ir-NBI. The structures of organic intermediates and ligands were characterized by ¹H nuclear magnetic resonance (NMR) and high resolution mass spectrometer (HRMS) (Figs. S1-S9 in Supporting information). Both UiO-Ir-NBI and UiO-Ir presented a similar powder X-ray diffraction (PXRD), which well matched with the simulated crystal data of UiO-67 (Fig. 2a). This result manifested that the introdution of L-2 did not significantly disturb the structure and crystallinity of UiO-67. A shape of close to octahedron was observed for UiO-Ir-NBI by scanning electron microscope (SEM) (Fig. S10 in Supporting information). Besides, elemental mapping images showed the adequate distribution of Zr, Ir, Cl, C, N and O elements in UiO-Ir-NBI, supporting that L-2 was successfully introduced into UiO-67 (Fig. 2b). X-ray photoelectron spectroscopy (XPS) was carried out on UiO-Ir-NBI to determine the valence state of Ir in MOFs. As shown in Fig. S11 (Supporting information), two peaks at around 62.3 and 65.3 eV were observed in Ir 4f region, which can be attributed to Ir 4f_7/2 and 4f_5/2 of Ir³⁺. As a result, the valence state of Ir in MOFs was estimated as +3, well matched with that in L-2. Furthermore, according to the results of inductively coupled plasma-mass spectrometry (ICP-MS) test, the Ir content in UiO-Ir-NBI and UiO-Ir was detemined to be 158 µmol/g and 157 µmol/g, respectively, excluding the potential influence of Ir loading amount in different MOFs on their photosynthetic performance.

  Figure 2

  

  Figure 2.  (a) PXRD patterns of UiO-Ir and UiO-Ir-NBI. (b) High angle annular dark field scanning transmission electron microscopy and elemental mapping images for UiO-Ir-NBI.

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  The photoelectrochemical properties of UiO-Ir-NBI and UiO-Ir were sysmetically investigated by steady, transient and photoelectrochemical spectra (Fig. 3). As shown in Fig. 3a, UiO-Ir-NBI presented a strong visible light absorption between 400 nm and 650 nm, significantly stronger than that of traditional UiO-Ir, indicating a more efficient solar energy utilization for UiO-Ir-NBI. The transient absorption of L-2 exhibited a bleaching between 420 nm and 480 nm, which could be attributed that is triplet state was localized on the NBI ligand (Fig. S12 in Supporting information). This result well matched with spin density distribution of L-2 (Fig. 1). In addition, the excited state lifetime of UiO-MOFs extended from 0.7 µs for UiO-Ir to 1.2 µs for UiO-Ir-NBI, which contributed to promoting its interface electron/energy transfer (Fig. 3b). As compared with UiO-Ir, UiO-Ir-NBI exhibited a strong and sharp phosphorescence, indicating its weak triplet-triplet annihilation and efficient triplet population, which contributed to facilitating interface electron transfer (Fig. S13 in Supporting information). Photocurrent measurements reveal that UiO-Ir-NBI exhibits a stronger photocurrent response than UiO-Ir, confirming a more efficient photogenerated electron-hole separation for the former (Fig. 3c). This viewpoint was also supported by electro-chemical impedance spectroscopy (EIS), where UiO-Ir-NBI showed a smaller radius and a lower resistance for charge transfer than UiO-Ir (Fig. 3d). Subsequently, the advantages of UiO-Ir-NBI in strong visible light absorbing ability, long excited state lifetime and efficient electron-hole seperation, higlighting its great potential on energy- and eletron transfer photoreaction.

  Figure 3

  

  Figure 3.  (a) UV–Vis absorption spectra of UiO-Ir-NBI and UiO-Ir. (b) Photoluminescence lifetime of UiO-Ir-NBI and UiO-Ir. (c) Photocurrent response and (d) Nyquist of UiO-Ir-NBI and UiO-Ir.

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  In view of the superior photoelectrochemical properties of UiO-Ir-NBI, it was used for photo-oxidation of sulfide to exam its sensitzing ability (Fig. 4 and Tables S1-S3 in Supporting information). 2-Chloroethyl ethyl sulfide is a chemical warfare agent simulant of mustard gas, which can be oxidized into nontoxic 2-chloroethyl ethyl sulfoxide by using light as driven force and O₂ as green oxidant. Notably, the 2-chloroethyl ethyl sulfoxide yield with UiO-Ir-NBI was as high as 99% within 30 min, over 6 and 24 times higher than that with UiO-Ir and the mixture of UiO-Ir/NBI, respectively. This result indicates that it is necessary to directly coordinate NBI chromophore to Ir sensitizing center for efficient synergism between NBI and Ir center. Besides, UiO-Ir-NBI exhibited an outstanding catalytic stability, which can be recycled for 5 times without abvious structrue change and activity loss (Figs. S14 and S15 in Supporting information). Furthermore, UiO-Ir-NBI can photo-oxidize various aromatic sulfides and alkyl sulfides into the corresponding sulfoxides with over 90% yield, indicating an excellent substrate tollerance (Table S3).

  Figure 4

  

  Figure 4.  (a) Catalytic activities of UiO-Ir-NBI, UiO-Ir and UiO-Ir/NBI for photo-oxidation of sulfide. (b) Recycling experiments with UiO-Ir-NBI. (c) The catalytic performance of UiO-Ir-NBI for different substrates.

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  In order to unveil the distinguished catalytic performance of UiO-Ir-NBI, the catalytic mechanism was comprehensively investigated by a series of control experiments and electron spin-resonance (ESR) tests (Fig. 5 and Table S4 in Supporting information). Almost no product was detected without MOF PS, light or O₂, indicating that all above factors are essential for efficient photo-oxidation. Both NaN₃ and 1, 4-diazabicyclooctane (DABCO) were the typical ¹O₂ trapping agent. After adding NaN₃ or DABCO, the yield of methyl phenyl sulfoxide significantly decreased to 3.8% or 8.3%. In the presence of p-benzoquinone as a ^•O₂⁻ scavenger, the catalytic yield also decreased to 6.5%. These results preliminarily confirmed that both ¹O₂ and ^•O₂⁻ were the important oxidant for efficient photosynthesis. In addition, the catalytic yield was detemined to be 35.9% with KI as a hole scavenger and 18.7% with DDQ as an electron scavenger, indicating that the electron transfer pathway played an important role in catalytic process. A yield of 99.0% was obtained in the prescence of isopropanol as an ^•OH radical scavenger, excluding the possibility that ^•OH participated in catalytic reaction. Furthermore, ESR measurements were performed to further discern the reactive oxygen species (ROS) [55]. 4-oxo-TMP and 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) were employed as ¹O₂ and ^•O₂⁻ trapping agents, respectively. Upon irridiation, the strong triple peaks were emerged in the presence of 4-oxo-TMP, which was the typical characteristic signal of adduct of ¹O₂–4-oxo-TMP (Fig. 5b). This characteristic signal significantly decreased after adding methyl phenyl sulfoxide. Meanwhile, the characteristic signal of the ^•O₂^–TEMPO complex was also observed, which decreased after the addition of substrate (Fig. 5c). These results further supported that both ¹O₂ and ^•O₂⁻ was indeed a ROS for efficient photo-oxidation of sulfide.

  Figure 5

  

  Figure 5.  (a) Control experiments of UiO-Ir-NBI for photocatalytic sulfoxidation reaction. (b, c) ESR spectra of UiO-Ir-NBI in the presence of 4-oxo-TMP and DMPO in CH₃OH. The black line represents under dark, the red line represents under light and the blue line represents with thioanisole under light. (d) The proposed mechanism for photo-oxidation of thioanisole.

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  Accordingly, the catalytic mechanism was proposed as follows (Fig. 5d): (1) Upon light excitation, UiO-Ir-NBI can efficient harvest visible light via dual absorbing channels of S_o→¹MLCT and S_o→¹IL to attain ¹MLCT/¹IL state. After a series of intramolecular photophysical processes, a long-lived ³IL state was achieved for UiO-Ir-NBI, which contributed to interface electron/energy transfer. (2) ³IL state of UiO-Ir-NBI can efficiently transfer energy/electron to O₂ to produce ¹O₂ and ^•O₂⁻, respectively. Besides, the HOMO of UiO-Ir-NBI can accept the electron from thioanisole to afford the reduced UiO-Ir-NBI, which can further deliver electron to O₂ to promote the generation of ^•O₂⁻. (3) Both ¹O₂ and ^•O₂⁻can efficiently photo-oxidize sulfides into the sulfoxides product. As a result, as compared with the typical UiO-Ir, the long-lived ³IL state and the new absorbing channel from S_o to ¹IL of UiO-Ir-NBI can facilitate interface electron/energy transfer and solar energy utilization, which greatly contributed to boosting photosynthesis.

  We have explored a strategy to improve the sensitzing ability of MOFs via engineering their metal sensitizing center for dramatically boosting photosynthesis. A novel strong sensitizing MOF UiO-Ir-NBI was prepared by replacing ppy ligand of the typical UiO67-Ir with NBI ligand, which established a strong visible absorbing channel from S_o to ¹IL and a long-lived ³IL excited state. Impressively, the catalytic yield of 2-chloroethyl ethyl sulfoxide with UiO-Ir-NBI photocatalyst can reach to 99%, over 6 times higher than that with the typical UiO-Ir (16.4%). In addition, UiO-Ir-NBI exhibited an excellent catalytic stability and a broad substrate tolerance, highlighting its great potential for practical application. Systematical investigations revealed that the superior properties of UiO-Ir-NBI in strong visible light absorption, long excited state lifetime and efficient electron-hole separation contributed to efficiently harvesting visible light and facilitating interface electron/energy transfer for boosting photosynthesis. This work not only develops a strong sensitizing MOF photocatalyst, but also provides a new horizon to significantly boost photosythesis by engineering their metal sensitizing centers with chromophores.

  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 supported by National Key R&D Program of China (No. 2019YFA0705201), National Natural Science Foundation of China (No. 22171209).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.108661.

  
  
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• 

  Figure 1  (a) Schematic show of the energy level of MOFs with ³MLCT excited state, the structure of UiO-Ir and electron spin density of L-1. (b) Schematic show of the energy level of MOFs with ³MLCT/³IL excited state, the structure of UiO-Ir-NBI and electron spin density of L-2.

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  Figure 2  (a) PXRD patterns of UiO-Ir and UiO-Ir-NBI. (b) High angle annular dark field scanning transmission electron microscopy and elemental mapping images for UiO-Ir-NBI.

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  Figure 3  (a) UV–Vis absorption spectra of UiO-Ir-NBI and UiO-Ir. (b) Photoluminescence lifetime of UiO-Ir-NBI and UiO-Ir. (c) Photocurrent response and (d) Nyquist of UiO-Ir-NBI and UiO-Ir.

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  Figure 4  (a) Catalytic activities of UiO-Ir-NBI, UiO-Ir and UiO-Ir/NBI for photo-oxidation of sulfide. (b) Recycling experiments with UiO-Ir-NBI. (c) The catalytic performance of UiO-Ir-NBI for different substrates.

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  Figure 5  (a) Control experiments of UiO-Ir-NBI for photocatalytic sulfoxidation reaction. (b, c) ESR spectra of UiO-Ir-NBI in the presence of 4-oxo-TMP and DMPO in CH₃OH. The black line represents under dark, the red line represents under light and the blue line represents with thioanisole under light. (d) The proposed mechanism for photo-oxidation of thioanisole.

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