“Stillness is always on its way to movement.” In nature, plants exemplify this principle through their dynamic behaviors, facilitating the optimal conversion of solar energy into chemical energy. For instance, plants exhibit directional movement in response to the position of the sun, thereby maximizing their exposure to sunlight throughout the day. Similarly, photoproteins undergo dynamic structural changes to stabilize excited-state charges for effective charge transfer.^[1] In heterogeneous catalysis, dynamic behavior is ubiquitous and plays crucial roles in achieving highly efficient catalysis.^[2] However, such dynamic behavior is difficult to control in conventional heterogeneous catalysts.

    In a recent work in Nature Chemistry, we present the discovery of an excited-state dynamic structure twist in a metal-organic framework, denoted as CFA-Zn. Analogous to the dynamic structural changes observed in photoproteins, this behavior in the MOF significantly prolongs the lifetime of excited-state charge carriers, thereby achieving photocatalytic overall water splitting (OWS).

    CFA-Zn is constructed from Zn nodes and two crystallographically independent linkers. The unique asymmetric structure and closed-shell metal nodes ensure a chemically separated band edges without spatial overlap (Figure 1a,b), resulting in the conduction band minimum (CBM) linker fully accepting the energy of the excited electrons. Subsequently, a structural rearrangement occurs to achieve optimal system energy (Figure 1c,d), aligning both CBM and valence band maximum (VBM) wavefunctions on the same linker (Figure 1e,f). This prevents the excited electrons from recombining with holes remaining in the ground-state VBM linker, thereby significantly suppressing the radiative relaxation process. As a result, CFA-Zn effectively drive OWS under visible-light irradiation. The photocatalytic activity of CFA-Zn, in the presence of co-catalysts, ranks among the highest reported for one-step excitation MOF photocatalysts, with activity and stability comparing very favorably with other visible-light-responsive OWS photocatalysts.

    Furthermore, experimental support for the mechanism of dynamic excited-state structural twist to suppress charge recombination is provided based on the high tailorability of MOFs. Initially, -CH₃ groups were grafted onto the linkers in the MOF skeleton to inhibit linker twisting in the excited state. Spectral experiments indicate that the recombination of electrons and holes accelerates with increased -CH₃ content, accompanied by a decrease in OWS activity. This underscores the significance of structural twist for long-lived charge separation. Additionally, the Zn nodes in MOF were partially replaced with open-shell Co nodes, inducing an orbital overlap. This modification causes the energy of excited electrons to be shared among the linkers and Co nodes, resulting in insufficient energy fluctuations to drive structural change. Similarly, rapid charge recombination and reduced photocatalytic activity were also observed through systematic experiments. These control experiments explicitly demonstrate the roles of the excited-state structural twist in suppressing charge recombination.

    In addition, the unique asymmetric crystal structure of CFA-Zn facilitates the selective deposition of Pt cocatalysts on different facets. This promotes the directed migration of photogenerated electrons, as demonstrated by photon-irradiated Kelvin probe force microscopy, further enhancing the photocatalytic performance. More details about this work can be found in Nature Chemistry, titled “Dynamic structural twist in metal-organic frameworks enhances solar overall water splitting”, DOI: 10.1038/s41557-024-01599-6.

    Our group have engaged in MOF-based catalysis for over 10 years (website: http://mof.ustc.edu.cn). Based on MOFs that integrate the advantages of heterogeneous and molecular catalysis, providing a platform to investigate the structural regulation of catalysts at the molecular and atomic levels, we proposed the concept of biomimetic microenvironment modulation (MEM) around catalytic sites.^[3] We aim to improve catalysis by precisely creating specific microenvironment surrounding catalytic centers, inspired by natural enzymes. Achieving efficient photocatalytic reactions require directing more electrons to catalytic sites rather than allowing them to recombine. With this objective, our work leverages the flexible feature of MOFs to create dynamic microenvironment, thereby forbidding the radiative relaxation process and enabling more excited charges to participate in water splitting. In the future, we aim to optimize catalysis through MEM and extend this methodology beyond MOFs, exploring new frontiers in catalysis.

References:

[1] Dods, R. et al. Ultrafast structural changes within a photosynthetic reaction centre. Nature 589, 310-314 (2021).

[2] Chavez, S. et al. Studying, Promoting, Exploiting, and Predicting Catalyst Dynamics: the Next Frontier in Heterogeneous Catalysis. J. Phys. Chem. C 127, 2127-2146 (2023).

[3] Jiao, L., Wang, J. & Jiang, H.-L. Microenvironment modulation in metal–organic framework-based catalysis. Acc. Mater. Res. 2, 327-339 (2021).