Covalent organic frameworks (COFs) emerging as a novel kind of visible light-responsive organic semiconductor have attracted extensive research attention in the field of photocatalytic organic transformations. However, the key parameters affecting their photocatalytic properties are still not clear. In this work, a series of [3 + 3] COFs with similar two-dimensional hexagonal structure but different compositions are synthesized and employed as model materials for investigating the key factors affecting the photocatalytic properties in the visible-light-driven reductive dehalogenation reaction and the aerobic cross-dehydrogenative coupling reaction. In comparison with -H and -CF3, the -OH substituent in the aromatic ring could narrow the band gap of the COFs. The COFs with a triazine skeleton in the framework usually boost the photocatalytic activity, possibly because of the enhanced charge separation efficiency by the formation of a donor-acceptor domain. As a combined result of the narrow band gap, efficient charge separation, and high conductivity, the COF possessing both a -OH group and triazine skeleton shows the highest activity in the photocatalytic reductive dehalogenation reaction. Notably, COFs could be easily recovered and reused several times without the loss of crystallinity. Our primary results may shed light on the design of efficient COF-based semiconductors for photocatalytic organic transformations.
Keywords: aerobic cross-dehydrogenative coupling reaction; covalent organic frameworks; photocatalysis; reductive dehalogenation reaction; structural engineering.