Multi-edge NEXAFS study of non-fullerene acceptors: electronic structure and molecular orientation supported by...

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is a powerful tool to probe the electronic structure of organic semiconductors. However, the interpretation of core-level excitations in chemically complex systems remains challenging. Here, we present a combined experimental and theoretical study of the carbon, nitrogen, and oxygen K-edges NEXAFS spectra of two non-fullerene electron acceptors widely used in organic photovoltaics: ITIC and IDTBR. The simulated angular-dependent NEXAFS spectra consistently reproduce the experimental data for all edges, enabling a detailed assignment of the main spectral features in terms of specific core-level excitations, variation of peak intensities, and final-state symmetries. The experimental dichroic ratios for ITIC indicate a higher degree of face-on organization at the surface compared to the bulk. For IDTBR, differences between surface- and bulk-sensitive measurements reveal a more complex behavior, where the observed trends depend on the specific absorption edge, reflecting variations in the localization of the excited states and the degree of molecular torsion. Theoretical dichroic ratios become higher when the in-plane components of the transition dipole vector show a magnitude comparable to its perpendicular component. These results further support the tendency of ITIC toward a face-on orientation at the surface, while indicating a higher degree of structural disorder in IDTBR. Additionally, a comparative assessment of exchange-correlation functionals was performed by evaluating M06 against B3LYP and PBE, showing that M06 outperforms the others, especially in the carbon K-edge. By providing a unified interpretation of experimental NEXAFS spectra across multiple edges, this work offers new insights into the electronic structure of these acceptors and establishes a robust framework for analyzing core-level excitations in complex organic materials.