A Computational Study of Semiconducting Benzobisthiazoles: Analysis of the Substituent Effects on the Electronic Structure, Solid-State Interactions, and Charge Transport Properties Using DFT Methods

Abstract

In this study, using DFT methods, we analyze the electronic structures at both the molecular and solid-state level of the recently suggested three-dimensional semiconducting bis(hexylthieno)benzobisthiazole molecule (2), the unsubstituted benzobisthiazole ring (1), its 2,6-disubstituted derivatives (3 and 4), and its 2,6-dihexyl-4,8-dithiophene derivative (7). The uniqueness of 2, as per literature reports, is the close intermolecular contacts in the three directions (3D ordering) due to the crystal packing which should lead to more efficient charge transport. Gas-phase geometry optimization of the above molecules using the B3LYP functional is carried out and compared with the X-ray data. Reorganization energies to estimate the charge transfer, calculated using the same functional, do not indicate any particular bias toward hole transport or electron transport process. Transfer integrals (for estimation of intermolecular charge hopping pathways) between the HOMOs (for hole transport) are quite large in the π–π stacked direction in the 1–3 crystals, while in n-type 4 the transfer integrals between the LUMOs are large. In the case of 7, which has not been tested so far for its semiconducting properties, the calculated transfer integral is very small and reorganization energies are large, indicating that it may show poorer performance when compared to the other derivatives. Analysis of the intermolecular interactions (noncovalent) in the crystals has been carried out using dispersion-corrected functionals, namely B2PLYP-D, M06-2X, and B97-D. For the 2,6 derivatives, a maximum of ∼24 kcal/mol interaction energy (B2PLYP-D) is obtained in the π–π stacking direction but for the S–N contacts the binding energies are only around 5–6 kcal/mol. In 7, the binding energies obtained are much smaller and in the range of 2–9 kcal/mol only. Band structure calculations using the PBE functional of crystal 2 are also carried out. We conclude from this study that this fused heterocyclic ring at the molecular level by itself is not in the same class like semiconducting pentacene or rubrene but substituted benzobisthiazole rings pack in the crystal in some cases like 2, 3, and 4 to yield very large transfer integrals which can play a crucial role in charge transport. The present work will be helpful to understand and rationally design molecules which self-assemble in the solid state giving intermolecular close contacts in the crystal.