n → π* Interaction Promoted Charge Carrier Transfer between Helical SWNTs and a 4-(1-Pyrenyl)phenyl Group

Recently, an n → π* weak interaction has been found to be ubiquitous and significant in proteins, functional materials, medicines, and biological processes. To reveal the mechanism underlying the n → π* interaction, we performed density functional theory calculations for single-walled carbon nanotubes (SWNTs) covalently or noncovalently functionalized by the 4-(1-pyrenyl)phenyl (PP) group. The n → π* interaction can be formed between the nonbonding (n) radical electron of the PP group and the π-conjugation antibonding orbitals (π*) of SWNTs facilitated by the π–π stacking between them. The n → π* interaction is found to be dominantly responsible for a distinguishable optical response, leading to visible light absorption at ∼450 nm. Specifically, the photo-induced charge carriers are separated effectively through n → π* electron transition with the hole “locked” on the PP group. Our result is useful for the rational design of novel SWNT modification by aromatic groups in applications of (photo)electrochemical reactions, functional devices, or sensors.