Ultrafast Excited-state Dynamics in metaled 2, 2'-Bipyridine Covalent Organic Frameworks Photocatalysts

Semiconductor photocatalysis technology has attracted much attention in recent years, it can convert solar energy into chemical energy and store it in solar fuel, which is regarded as an effective way to resolve energy and environmental crises nowadays. Covalent organic frameworks (COFs) are a kind of very potential photocatalytic material. Despite the well-documented recent advances in material development and photocatalytic application of those COFs structures, the fundamental catalytic process, especially, excited-state dynamics that dominate the catalytic performance has not been fully understood. In particular, the photo-generated excitons or charge carriers can undergo transfer and recombination within a broad temporal regime, containing multiple charge separation mechanisms. Therefore, it is necessary to create a systematic profile to review such photophysical processes and consequently rationalize the high catalytic activity of COFs photocatalysts. In this thesis, the excited state kinetics of two covalent organic backbone materials modified by metal-complexes molecular catalysts, (i.e. Re-TpBpy and Ni-TpBpy), were explored experimentally by advanced spectroscopic measurements and theoretical by time-dependent density functional theory (TD-DFT) calculations. The results are utilized to explain the photocatalytic performance. The main contents are as follows:

1) TpBpy, a two-dimensional (2D) COFs with 2,2'-bipyridine, was successfully prepared by a one-step reversible Schiff base reaction and one-step irreversible enol keto tautomerism, then Re-TpBpy and Ni-TpBpy were obtained by modifying TpBpy with Re and Ni-based metal-complex catalysts using impregnation method. Various characterization techniques showed that the two materials possess high crystallinity, large specific surface area, good chemical stability, and perfect visible light response. Both Re-TpBpy and Ni-TpBpy exhibit excellent CO₂ reduction activity in photocatalytic measurement. However, the catalytic performance is excitation wavelength-dependent: CO₂ reduction can only be observed when excited with high-energy photons well above the band edge (i.e. 440 nm). We put our assumption to explain such phenomenon based on the electronic structure and charge carrier dynamics after excitation. In addition, the CO yield in Ni-TpBpy is higher than that of Re-TpBpy. We interpret such differences by comparing the morphology and intrinsic photophysics of the two structures. We concluded that: 1) the CO₂ adsorption capacity of Ni-TpBpy is higher than that of Re-TpBpy; 2) the electronic structure of Ni-TpBpy is more conducive to the separation and transfer of photogenerated charges.

2) We revealed the excited-state structure in TpBpy, Re-TpBpy, and Ni-TpBpy by TD-DFT calculation. We found that the light absorption of the three samples can be classified by low energy and high energy optical transitions. The low-energy optical transition of TpBpy, Re-TpBpy, and Ni-TpBpy are barely in Bpy moiety. The high energy optical transition of TpBpy and Re-TpBpy is evenly distributed at the whole COFs moiety, while the high energy optical transition of Ni-TpBpy is partially distributed throughout the COFs and partially in the Ni²⁺.

3) Through steady-state and time-resolved spectroscopy methods, such as ultraviolet-visible (Uv-Vis) absorption spectroscopy, time-resolved photoluminescence (TRPL) spectroscopy, femtosecond transient absorption (fs-TA) spectroscopy, and femtosecond time-resolved mid-infrared absorption (fs-TRIR) spectroscopy, the excited state dynamics of these three samples were explored. We demonstrate that the coupling of metal-complex catalysts can indeed facilitate the charge transfer in COFs, where an efficient electron transfer process from COFs to Re/Ni can be observed. Moreover, we found such an electron transfer process together with excited-state lifetime is highly dependent on excitation phonon energy. The photo-generated electrons and holes tend to separate in metal-center and COFs moiety after charge transfer under band-edge excitation. When excited with high energy photon (400 nm, 3.1 eV) much larger than the band-gap, the generated hot carriers will undergo different relaxation pathways depends on the initial orbital they locate. The photo-generated electrons can reside both in COFs moiety and metal center simultaneously in this case, which promotes the two-electron-mediated CO₂ reduction. In addition, such phenomenon is produced in Ni-TpBpy compared with Re-TpBpy due to its more diverse excited structure that allows the relaxation of excited electrons and holes highly depends on the initially excited orbitals.