Boosting exciton dissociation by regulating dielectric constant in covalent organic framework for photocatalysis

Polymer semiconductor photocatalysts usually suffer from the high exciton binding energy because of the low dielectric constant. Herein, two crystalline polymer photocatalyst prototypes, neutral covalent organic framework (COF) and ionic covalent organic framework (iCOF), were employed to investigate the exciton effect. After polarization by the ionic sites with high dipole moment, the iCOF endows a greatly increased dielectric constant to reach the low exciton binding energy of 23 meV, below the thermal energy under room temperature (k_BT, 26 meV). Thus, enhanced generation, separation, and transport of photogenerated charge carriers over the iCOF catalyst resulted in a hydrogen peroxide production rate of 10.01 mmol g^-1 h^-1 in the photocatalytic oxygen reduction coupling with water oxidation reaction, which is almost 7 times higher than the neutral COF under identical conditions. This work demonstrates a promising way to tune the dielectric properties of polymer photocatalysts in order to regulate the exciton effect and related photocatalytic behavior.