Abstract
To date, alternating co-polymers based on electron-rich and electron-poor units are the most attractive materials to control functionality of organic semiconductor layers in which ultrafast excited-state processes play a key role. We present a computational study of the photoinduced excited-state dynamics of the 4-(2-thienyl)-2,1,3-benzothiadiazole (BT-1T) molecule, which is a common building block in the backbone of π-conjugated polymers used for organic electronics. In contrast to homo-polymer materials, such as oligothiophene, BT-1T has two non-identical units, namely, thiophene and benzothiadiazole, making it attractive for intramolecular charge transfer studies. To gain a thorough understanding of the coupling of excited-state dynamics with nuclear motion, we consider a scenario based on femtosecond time-resolved x-ray absorption spectroscopy using an x-ray free-electron laser in combination with a synchronized ultraviolet femtosecond laser. Using Tully's fewest switches surface hopping approach in combination with excited-state calculations at the level of configuration interaction singles, we calculate the gas-phase x-ray absorption spectrum at the carbon and nitrogen K edges as a function of time after excitation to the lowest electronically excited state. The results of our time-resolved calculations exhibit the charge transfer driven by non-Born-Oppenheimer physics from the benzothiadiazole to thiophene units during relaxation to the ground state. Furthermore, our ab initio molecular dynamics simulations indicate that the excited-state relaxation processes involve bond elongation in the benzothiadiazole unit as well as thiophene ring puckering at a time scale of 100 fs. We show that these dynamical trends can be identified from the time-dependent x-ray absorption spectrum.
Highlights
The sub-femtosecond x-ray pulses provided by x-ray free-electron lasers enable investigations of ultrafast, non-equilibrium dynamics of functional materials as well as their electronic and geometric structure
With respect to the necessity of a deeper understanding of photophysical properties of conjugated oligomers/polymers for a successful design of new photovoltaic materials and architectures, we have systematically investigated the excited-state dynamics of the 4–(2thienyl)-2,1,3-benzothiadiazole (BT-1T) molecule
We have verified that the employed level of electronic-structure modeling, configuration interaction singles (CIS), is able to qualitatively reproduce low-lying absorption-band features reasonably well compared to experiments
Summary
The sub-femtosecond x-ray pulses provided by x-ray free-electron lasers enable investigations of ultrafast, non-equilibrium dynamics of functional materials as well as their electronic and geometric structure. The same dTBT molecule has been considered as a model compound to study the charge-transfer relaxation in D–A type conjugated materials by Scarongella et al.[24] Using time-resolved femtosecond transient-absorption spectroscopy in solution and in the solid state as well as density functional theory (DFT) calculations, they reported a planar backbone in both the ground and excited states and weak conformational relaxations including marginal contraction of the central benzene ring and elongation of the molecule along the short axis. We do not consider relaxation mechanisms due to intersystem crossing, which are expected to play a role on time scales from tens to hundreds of picoseconds[40–43] and, potentially could play a relevant role for the ultrafast relaxation of BT-1T
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