Symmetry-Selective Ultrafast Charge Transfer via Cyano End Groups at the PDIF-CN₂-Au(111) Interface.

Abstract

Cyano-functionalized perylene diimides provide a platform for engineering orbital-specific electronic transparency at organic-metal interfaces. Here we resolve the adsorption geometry, orbital symmetry, and ultrafast charge-transfer dynamics of PDIF-CN₂ on Au(111) using polarization-dependent NEXAFS, XPS, resonant Auger spectroscopy with core-hole-clock analysis, and STM. Molecules in the monolayer lie flat with out-of-molecular plane polarized cyano-derived π* orbitals aligned toward Au(111). Although the overall electronic structure remains largely molecular, the nearly degenerate cyano σ*(π)/π*(π) doublet becomes distinctly split at the interface, enabling symmetry-selective coupling of cyano orbitals. We use Auger spectator shifts to quantify bidirectional charge exchange within the N 1s core-hole lifetime, yielding injection of ∼0.43 e^- from Au into the cyano π*(π) orbital and the transfer of ∼0.17 e^- in the opposite direction. These findings establish cyano end groups as symmetry-selective gateways for femtosecond charge transfer at molecule-metal interfaces.