Near-edge X-ray absorption fine-structure spectroscopy of benzothienobenzothiophene at carbon and sulfur K-edges.

Alkylated benzothienobenzothiophenes are an important class of organic semiconductor which exhibit high performance in organic field-effect transistors (OFETs). In this work, we perform near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiophene (C10-BTBT). In particular, angle-resolved NEXFAS spectra of C10-BTBT are measured at both the carbon and sulfur K-edges with the results compared to theoretical simulations performed using the density functional theory-based Many-Body X-ray Absorption Spectroscopy (MBXAS) method. Interestingly, opposite dichroism is observed for the dominant, lowest energy peak at carbon and sulfur K-edges. MBAXS calculations enable the assignment of the peaks observed at both edges to specific molecular orbitals. While the lowest energy peak at the carbon K-edge is assigned to carbon 1s ® p* transitions, with transition dipole moment (TDM) perpendicular to the planar BTBT core, the lowest energy peak at the sulfur K-edge is assigned to sulfur 1s ® s* transitions with TDM oriented along the long axis of the BTBT core. This work highlights differences in the NEXAFS spectra of organic semiconductors at carbon and sulfur K-edges and provides new insight into peak assignment and X-ray dichroism.