Cobalt-porphyrin-based covalent organic frameworks (Co-Por-COFs) were prepared with different donor building units. Their photophysical studies indicated that the Co-Por-COFs with higher charge-carrier density and longer conjugation length led to a more efficient charge separation with a reduced exciton binding energy, resulting in an enhanced photocatalytic activity of CO2 reduction into CO.
Abstract
Covalent organic frameworks (COFs) have emerged as a promising platform for photocatalysts. Their crystalline porous nature allows comprehensive mechanistic studies of photocatalysis, which have revealed that their general photophysical parameters, such as light absorption ability, electronic band structure, and charge separation efficiency, can be conveniently tailored by structural modifications. However, further understanding of the relationship between structure-property-activity is required from the viewpoint of charge-carrier transport, because the charge-carrier property is closely related to alleviation of the excitonic effect. In the present study, COFs composed of a fixed cobalt (Co) porphyrin (Por) centered tetraamine as an acceptor unit with differently conjugated di-carbaldehyde based donor units, such as benzodithiophene (BDT), thienothiophene (TT), or phenyl (TA), were synthesized to form Co-Por-BDT, Co-Por-TT, or Co-Por-TA, respectively. Their photocatalytic activity for reducing carbon dioxide into carbon monoxide was in the order of Co-Por-BDT>Co-Por-TT>Co-Por-TA. The results indicated that the excitonic effect, associated with their charge-carrier densities and π-conjugation lengths, was a significant factor in photocatalysis performance.