Vinylene-linked covalent organic frameworks with fully pyridine-patterned iso-π-conjugation for photoredox catalysis

Abstract

Vinylene-linked covalent organic frameworks (COFs) featuring robust π-conjugated backbones and abundant periodic open nanochannels are potential candidates to act as photocatalysts. Besides their intrinsic semiconducting properties, their porous structures also play a crucial role in promoting their photocatalytic activity, which is associated with exposed active sites, rapid mass transport, and pore-confined selectivity. However, it remains unclear which parameter holds the dominant role among the multiple parameters for a two-dimensional (2D) COF structure. Herein, we present the construction of two vinylene-linked covalent organic frameworks (COFs) via the Knoevenagel condensation of pseudo-C₃-symmetrical trimethylpyridine and D_2h-symmetrical tetramethylbipyridine, respectively, as vertices with C₂-symmetrical diformylbipyridine. The resultant two-dimensional COFs were highly crystallized with hexagonal and orthorhombic topologies, respectively, showing distinct porous structures. Meanwhile, they possess similar in-plane π-delocalization patterns through the vinylene-linking of pyridine units at the ortho- or para-positions, but this delocalization is interrupted at the meta-positions of pyridine moieties. Accordingly, they showed almost the same semiconducting properties, including visible-light harvesting, energy levels and band gaps, as well as transient photocurrent responses. Owing to the matched energy band gaps, the as-prepared COFs can efficiently catalyze the conversion of 2-aminothiophenol and aldehydes to benzothiazole derivatives upon visible-light stimulation. Both COFs exhibited similar catalytic efficiency, selectivity, and recyclability toward substrates with different sizes and functional substituents. These results showed that the photocatalytic behaviors of such COFs are dominated by their π-conjugated structures, and not by their pore sizes or shapes.