Electronic Structure Characterization of a Thiophene Benzo-Annulated Series of Common Building Blocks for Donor and Acceptor Compounds Studied by Gas Phase Photoelectron and Photoabsorption Synchrotron Spectroscopies

The near-edge x-ray-absorption fine-structure (NEXAFS) and Xray photoelectron spectroscopy (XPS) spectra of benzo[b]thiophene (BBT) and dibenzothiophene (DBT) in the gas phase have been measured at the carbon Kedge and sulfur LII,III-edge regions. The assignment of the spectral features has been provided by theoretical calculations based on density functional theory (DFT) and its time-dependent generalization (TDDFT) in the linear response regime. Observed trends in computed C 1s and S 2p ionization potentials (IPs) have been rationalized in terms of both the inductive effects due to the presence of S and the increased π-electrons delocalization arising from the benzoannulation process. The analysis of the NEXAFS carbon K-edge and sulfur LII,IIIedge regions provided information on both low-lying delocalized virtual π orbitals, and higher-lying localized σ*(C−S) states. The evolution of the NEXAFS carbon K-edge spectral features along the series thiophene (T) and derivatives, BBT and DBT, is informative of a stabilizing effect due to increased aromaticity. This effect is however more pronounced in going from T to BBT compared to the introduction of a second annulated phenyl ring in DBT. The nature of the most intense sulfur LII,IIIedge NEXAFS spectral features is instead conserved along the series reflecting thus the localized nature of the virtual states involved in the S 2p core-excitation process.