Control over Charge Separation by Imine Structural Isomerization in Covalent Organic Frameworks with Implications on CO₂ Photoreduction

Abstract

Two-dimensional covalent organic frameworks (COFs) are an emerging class of photocatalytic materials for solar energy conversion. In this work, we report a pair of structurally isomeric COFs with reversed imine bond directions, which leads to drastic differences in their physical properties, photophysical behaviors, and photocatalytic CO 2 reduction performance after incorporating a Re(bpy)(CO) 3 Cl molecular catalyst through bipyridyl units on the COF backbone (Re-COF). Using the combination of ultrafast spectroscopy and theory, we attributed these differences to the polarized nature of the imine bond that imparts a preferential direction to intramolecular charge transfer (ICT) upon photoexcitation, where the bipyridyl unit acts as an electron acceptor in the forward imine case (f-COF) and as an electron donor in the reverse imine case (r-COF). These interactions ultimately lead the Re-f-COF isomer to function as an efficient CO 2 reduction photocatalyst, while the Re-r-COF isomer shows minimal photocatalytic activity. These findings not only reveal the essential role linker chemistry plays in COF photophysical and photocatalytic properties but also offer a unique opportunity to design photosensitizers that can selectively direct charges.

Reticular Ratchets for Directing Electrochemiluminescence

“…Benefiting from the tunability of COFs, we delicately designed 10 pairs of COF-based ratchets with mutually reversed orientations of polar CHN bonds between D–A units. We anticipated that the polarized CHN bonds could serve as ratchet teeth, which preferentially direct the electron transfer from the N-end units to the C-end units and impede the reversed electron migration during electron–hole combination (Figure a), in analogy to the recently reported “ICT Tesla Valve.” ,− Similar to the space charge in semiconductor heterojunctions, the CHN bond in COFs with reversed orientation could cause inverted band bending (Figures b and S2), where the C-end band bends upward, and the N-end band bends downward. − Consequently, the direction of CHN bonds not only affects the conduction band minimum (CBM) and valence band maximum (VBM) but also dictates the electron–hole combination and the resulting ECL performance (Figure c).…”

“…Moreover, periodically arranging molecular ratchets into extended network structures amplifies the ratchet effect, yielding unprecedented properties. , For example, Stoddart and co-workers developed MOF-based pumping cassettes with molecular ratchet structures for adsorption control and chemical energy storage . Furthermore, reticular materials utilizing molecular ratchets to control charge transfer directions within reticular structures hold promise in diverse areas, including organic (opto)­electronics, electrochemistry, photocatalysis, and electrochemiluminescence (ECL).…”

Progress of Covalent Organic Framework Photocatalysts: From Crystallinity–Stability Dilemma to Photocatalytic Performance Improvement

“…Direct harnessing and converting solar energy into electricity or fuels are essential for a net-zero carbon economy. To this end, solar-driven photocatalytic technology is capable of converting solar energy to renewable fuels and has long been regarded as a promising strategy to cope with the increasing challenges of the energy shortage and global warming. − As shown in Figure a, the solar spectrum consists of infrared light (52–55%), visible light (42–43%), and ultraviolet light (3–5%) . Apparently, considering the dominant portion of visible light in solar radiation, developing photocatalysts with visible-light responsiveness is highly desirable.…”