Computational investigation on the electronic, optical and transport properties of hexathiapentacene in the molecular and solid phases

The hexathiapentacene molecule (HTP), a derivative of pentacene (PNT) obtained by symmetric substitution of the six central hydrogens with sulfur atoms, has recently received increasing attention for its potential applications in organic electronic devices. We present a Density Functional Theory (DFT) investigation of the electronic, optical and transport properties of HTP molecule, in its molecular and solid phases. For the first part of the work we performed all-electron calculations using the hybrid exchange-correlation functional B3LYP in conjunction with a Gaussian localized orbital basis set to expand the molecular orbitals. Electron affinities, ionization energies, fundamental energy-gaps, optical absorption spectra, exciton binding energies, and reorganization energies for holes and electrons have been calculated and compared with the corresponding observables of PNT and with the available experimental data. Following sulfur functionalization we found an increase of both ionization energies and electron affinities, a sensible reduction of the fundamental electronic gap, and a redshift of the optical absorption onset. Notably, the intensity of the first absorption peak of HTP falling in the visible region is found to be nearly tripled with respect to the pure pentacene molecule. As to the corresponding HTP molecular solid, we performed our simulations within a plane-wave+pseudopotential framework using the PBE exchange-correlation functional, with the inclusion of an empirical correction taking into account dispersive interactions. The comparative analysis between the ground-state and excited-sate properties of solid-state HTP and PNT confirmed the general findings obtained for the isolated molecules.