Tuning Photoexcited Charge Transfer in Imine-Linked Two-Dimensional Covalent Organic Frameworks

The generation of a long-lived charge-separated state in versatile pi-conjugated two-dimensional covalent organic frameworks (2D COFs), a process essential to extending their great potentials in advanced semiconducting applications, is yet fully elucidated. Herein, we report a systematic investigation of the photophysical properties of three highly crystalline imine-linked 2D COFs using steady-state and transient absorption spectroscopy accompanied by time-dependent density functional theory (TDDFT) calculations. The different electron affinity between 5,5 ',5 ''-(1,3,5-benzenetriyl)tris (2-pyridine-carboxaldehyde) (BTPA) and three tunable electron-donating/accept-ing triamine monomers dominated the formation of the excited-state, charge-transfer direction, and lifetime. A prominent charge transfer from electron-rich 4,4 ',4 ''-triaminotriphenylamine (TAPA) to BTPA in COFTAPA-BTPA led to the long-lived charge-separated state, which was attributed to a greater degree of delocalization compared to the two other COFs. These results provide fundamental insight into the importance of structure-property correlation for designing advanced photoactive 2D COF materials with the efficient charge transfer and long-lived charge-separated state.

Automated summary

In this study, the photophysical properties of three highly crystalline imine-linked 2D COFs were systematically investigated using experimental and computational methods. It was found that the different electron affinities in the materials can influence the excited state, charge transfer direction, and lifetime, thus affecting the performance of the materials. One of the materials exhibited a greater degree of delocalization, resulting in a long-lived charge-separated state.