Tuning the optical and electronic properties of perylene diimides through transversal core extension

Abstract

Herein, we present a systematic study under the density functional theory on a series of perylene diimides (PDIs) to unravel the effect that transversal π-extension of the perylene core has on the optical and electronic properties. An unexpected increase in the HOMO–LUMO gap is predicted upon increasing the size of the polycyclic core going from the simplest PDI to the coronene-based CDI, whereas a systematic reduction in the gap is calculated upon further increase of the π-core in the D(X)CDI derivatives (X = B, N, A and T refer to benzene, naphthalene, anthracene and tetracene, respectively). This behaviour is explained in terms of orbital topology, geometry parameters and accumulated charges, evidencing an electronically isolated coronene moiety in CDI, whereas the π-extension for D(X)CDI disrupts the coronene conjugation pattern and creates electronically defined acene units. A hypsochromic shift of the lowest-lying singlet excited state is predicted going from PDI to CDI, in good accord with the experimental evidence. The low-lying experimental bands of CDI are unequivocally assigned to the vibrational structure of the first singlet electronic transition. Otherwise, moving to higher extended D(X)CDI derivatives, the expected bathochromic shift of the first singlet excited state was found along with an increase in the absorption intensity. Finally, appealing charge-transport properties are demonstrated for the family of perylene diimides. A gradual decrease in both hole and electron reorganization energies is calculated upon transversal core extension but for CDI. A hole reorganization energy of only 0.05 eV is calculated for the most π-extended DTCDI derivative, which competes with the best hole-transporting materials reported so far. These derivatives can therefore be viewed as appealing ambipolar systems, and especially as hole-transporters, to be exploited in next-generation photovoltaics.