Vinylene-Linked Covalent Organic Frameworks for Visible-Light-Promoted Selective Oxidation of Styrene in Water.

Abstract

Vinylene-linked COFs, as an emerging class of crystalline porous polymers, have been regarded as ideal heterogenous photocatalysts due to their ordered structure, tailored pore size, outstanding stability and fully π-conjugated structure. Unfortunately, their photocatalytic performances are usually impeded by high exciton binding energy and unsatisfactory exciton dissociation efficiency. Herein, the authorsbroke through this dilemma by arrangement of complementary donor-acceptor (D-A) pairs within the COF skeleton to improve charge transfer/separation. Two vinylene-linked COFs (TMT-BT-COF and TMT-TT-COF) are synthesized by Aldol condensation using highly photoactive thienothiophene and benzothiazole groups as donor and electron-deficient triazine units as acceptor. Photochemical/electrochemical studies as well as DFT calculation suggest that these D-A type vinylene-linked COFs endow high charge transfer efficiency and low charge recombination. As a result, both of them demonstrate remarkably catalytic activity in the oxidation of styrene to benzaldehyde with molecular oxygen, with an exceptionally high conversion rate (≥92%) and selectivity (≥90%). Intriguingly, in the presence of NaHCO3, the above COFs could photocatalyze epoxidation styrene in water, and the styrene oxide selectivity reached 53%. This work elucidates the prominent capability of vinylene-linked COFs in the photocatalytic transformation of organic compounds in aqueous media, which may pave a new avenue for their future development.