Precise Manipulation on Effects of Framework Microenvironment in Covalent Triazine Frameworks for Efficient Photocatalytic C3–H Thiocyanation of Indoles

Owing to their structural stability, diversity, and easy synthesis, characterization, and modification, covalent triazine frameworks (CTFs) are promising heterogeneous photocatalysts for different types of organic transformations. Herein, we prepared CTFs modified with different chromophore groups (nitro group: CTF-1, thiadiazole group: CTF-2) by changing the building unit structure and compared their properties with an unmodified CTF (CTF-3). The introduction of chromophores resulted in bandgap narrowing and facilitated charge separation/transport and light collection, which showed that the photoelectric properties of CTFs can be regulated by tuning their framework microenvironment. The photocatalytic activity of these CTFs for the C3–H thiocyanation of indole followed the order of CTF-3 < CTF-1 (×1.24 that of CTF-3) < CTF-2 (×1.44 that of CTF-3) and was consistent with our expectations. In addition, CTF-2 has demonstrated excellent catalytic activity in the photocatalytic C3–H thiocyanation of imidazopyridines. Thus, this study presents a green synthetic approach for functionalized indole and imidazopyridine derivatives, while establishing a rational design paradigm for CTFs as photocatalysts in diverse organic transformations.