Electronic and optical properties of hexathiapentacene in the gas and crystal phases

Using Density Functional Theory (DFT) and its time-dependent (TD) extension, the electronic, optical, and transport properties of the hexathiapentacene (HTP) molecule, a derivative of pentacene (PNT) obtained by symmetric substitution of the six central H with S atoms, are investigated for its molecular (gas phase) and crystal (solid phase) structures. For the molecular structure, all-electron calculations are performed using a Gaussian localized orbital basis set in conjunction with the hybrid exchange-correlation functional B3LYP. Electron affinities, ionization energies, quasi-particle energy-gaps, optical absorption spectra, exciton binding energies are calculated and compared with the corresponding results for PNT, as well as with the available experimental data. The DFT and TDDFT results are also validated by performing many-body perturbation theory calculations within the GW and Bethe-Salpeter equation formalisms. The functionalization with S atoms induces an increase of both ionization energies and electron affinities, a sizeable 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 PNT molecule. For the crystal structure, pseudopotential calculations are adopted using a plane-wave basis set together with the Perdew-Burke-Ernzerhof exchange-correlation functional empirically corrected in order to take dispersive interactions into account. The electronic excitations are also obtained within a perturbative B3LYP scheme. A comparative analysis is carried out between the ground-state and excited-state properties of crystalline HTP and PNT linking to the findings obtained for the isolated molecules