Molecular conformation: a key factor underlying the performances of heterojunction photocatalysts

Abstract

Organic semiconductors (OSCs) have been emerging as attractive photocatalysts in recent years. However, the low dielectric constant of OSCs suggests a high binding energy of photogenerated excitons, leading to severe charge recombination. Organic heterojunctions are always adopted to facilitate the charge transfer process. Efficient organic heterojunctions require rational energy level alignment, which is generally suggested to be determined from the molecular structure. In addition, according to Dexter theory, the charge transfer process is also directly related to the intermolecular distance. Herein, with rational molecular design, two copper porphyrin (CuPy) derivatives containing different terminal functional groups but similar energy levels are synthesized. Carbon nitride (CN)/CuPy van der Waals heterojunctions are fabricated and their photocatalytic hydrogen evolution (PHE) performances are characterized. Experiments and theoretical calculations reveal that, besides photoelectrical properties defined by the molecular structure, molecular conformation (i.e., planarity) plays an equally important role in the charge transfer process by regulating the intermolecular distance. Efficient charge transfer can still occur when the intermolecular distance is reasonable, even under conditions where the energy level alignment is not optimized. These findings may contribute to a deeper understanding of OSC heterojunction photocatalysts.