A theoretical study on the isomers of the B5TB heteroacene for improved semiconductor properties in organic electronics

Dibenzodithienothiophene (DBDTT) is a high-performing fused-ring heterocyclic organic semiconductor, with charge-transfer mobilities up to 1.8 cm2 V−1 s−1, yet attempts to develop this heteroacene with the synthesis of higher homologues such as benzopentathienobenzene (B5TB) have not yielded superior properties (0.04 cm2 V−1 s−1). In this study, we re-worked the structure of B5TB through isomerisation of the terminal benzothiophene rings, which gave a small pool of six isomers. These were theoretically screened to (1) probe their fundamental electronic and optical properties of these new structures, and (2) determine if an isomer of the same chemical formula could impart improved properties over the parent B5TB. In the present study, Density Functional Theory (DFT), Koopmans’ Theorem and two electron propagator theories (EPTs) such as the Outer Valence Green’s Function (OVGF) and the third-order pole (P3) were applied to calculate properties such as the Frontier Molecular Orbitals (FMOs), the vertical ionization energies and the vertical electron affinities. Marcus Theory was then deployed to determine their hole and electron internal reorganization energies, the transfer integrals at intermolecular separations between d = 3.0–5.0 Å via the “splitting-in-dimer” method, and the rates of charge transfer for all structures. The six isomers were found to have a wider HOMO-LUMO gap (ca. Eg = 3.48–3.71 eV) than that of B5TB (2.98 eV) and four of which have a greater vertical ionization energy, suggesting excellent air-stability. Two isomers have exceptional hole reorganization energies, as low as λh = 84–101 meV and relatively high rates of hole charge transfer (0.602 × 1014 s−1) at d = 4.0 Å, which exceed those of B5TB (λh = 240 meV, KCTh = 0.095 × 1014 s−1). Interestingly, all isomers including the parent B5TB showed relatively low electron affinities (ca. <1 eV), but one has a greater rate of electron charge transfer (1.613 × 1014 s−1), than that of B5TB (1.393 × 1014 s−1), suggesting possible ambipolar OTFT applications for these structures. These findings show that the theoretical consideration of structural isomers can significantly improve the physical properties of a given chemical formula and aids the development of the next-generation of organic electronic candidates.