Switching Between Singlet and Triplet Excitation in Covalent Organic Frameworks for Highly Efficient Photocatalysis

Singlet (S₁) and triplet (T₁) excitation serve as the two primary and competing pathways, playing crucial yet entirely distinct roles in the photocatalytic process. Achieving flexible switching between S₁ and T₁ excitation energies has remained a challenge. Herein, three 2D covalent organic frameworks (COFs) with offset stacking angles of 90°, 105°, and 128° were successfully synthesized by integrating folding building blocks within the skeleton. The results show that the strategic offset stacking can harness efficient π-σ attraction, thereby inducing intersystem crossing from S₁ to T₁ state. The face-to-face stacked BDT-HHTP-COF tends to follow the electron transfer pathway, thereby generating ·O₂ ^-. In contrast, BDT-CTC-COF with the most optimal offset stacking distance produces high concentrations of ¹O₂, primarily attributing to the energy transfer pathway. Theoretical calculations prove that the BDT-CTC-COF can boost Coulomb interaction, trigger intersystem crossing, and accelerate the transfer of the T₁ exciton to the adsorbed O₂ throughout the matrix of the framework. This switch in the mechanistic pathway is critically important, as the highly electrophilic ¹O₂ exhibits superior efficacy in attacking the electron-rich aromatic ring of toluene, initiating a selective oxidation process that rapidly achieves over 98% degradation and 80% CO₂ mineralization, representing a 1.5-fold enhancement compared to the electron transfer-dominated pathway.