Conjugation length dependence on the electronic and optical properties of oligothiophene-F4TCNQ complexes*

To understand how the donor conjugation length affects the doping mechanisms in organic semiconductors [1], we investigate a series of charge transfer (CT) complexes formed by oligothiophene molecules of increasing length doped by the acceptor F4TCNQ. Using hybrid DFT as a starting point, we assess the electronic and optical properties of these systems from many-body perturbation theory (GW and the Bethe-Salpter equation). We find that the frontier orbitals (HOMO and LUMO) have CT character in all complexes, while deeper occupied and higher virtual states depend on the nT length. The first bright excitation is dominated by the HOMO-LUMO transition occurring approximately at the same energy in all systems. At increasing donor length, higher-energy peaks exhibit different character depending on the donor conjugation length. The rationale offered by our results contributes to clarify the excitation processes in organic donor/acceptor complexes.
[1] H. Mendez, et al., Nat. Comms. 6, 8560 (2015).