Charge Transfer Dynamics and Molecular Orientation Probed by Core Electron Spectroscopies on thermal-annealed Polysilafluorene Derivative: Experimental and Theoretical Approaches

Ultrafast charge delocalization dynamics in excited states of internal donor–acceptor copolymer poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) was studied by resonant Auger spectroscopy (RAS) measured around silicon and sulfur K edges using the core-hole clock approach. The effect of the thermal annealing treatment at 100 and 200 °C on the charge transfer delocalization times and molecular orientation were probed. Exponential dependence of charge transfer times (τ CT) with photon energy was found at both absorption edges with a decreasing curve slope with the thermal treatment. Features characteristics of the Auger Resonant Raman effect were observed at sulfur KL2,3L2,3 Auger decay spectra. Edge-on and plane-on molecular orientations with respect to the substrate surface were measured for the thiophene and benzothiadiazole units, respectively, using angular-dependent NEXAFS spectra at the S K edge. Molecular orientation of silafluorene was also probed by NEXAFS at the Si K edge. The improvement of the polymer ordering with annealing was evaluated by NEXAFS. Differences in charge transfer times at Si and S K edges may be related to the localized–delocalized character of the molecular orbitals (MOs) involved in those excitation processes, which was corroborated by theoretical calculations at the Hartree–Fock (HF) level, with explicit relaxation of molecular orbitals due to the core-hole. The molecular orientation effect in the charge transfer process was also investigated through the nonadiabatic coupling matrix elements involving the first low-lying excited states in both K edge excitations.