On the radicalar properties of graphene fragments: double-hybrid DFT and perturbation theory approaches

The physical–chemical properties of polycyclic aromatic hydrocarbons (PAHs), molecules formed by fused carbon rings passivated by hydrogen atoms, make them attractive to several applications, including light-emitting diodes, photovoltaic cells, and transistors. They borrow some of the unique properties of graphene, nanotubes, and fullerenes. Additionally, radicals related to these structures may be involved in carcinogenic effects. In this work, electronic and energetic aspects of PAHs, including triangulenes, were analyzed using BLYP and B2PLYP density functionals as well as with unrestricted Hartree–Fock (UHF) and second-order Møller–Plesset perturbation (MP2) theories. The results show that DFT BLYP and B2PLYP functionals predict adiabatic singlet–triplet energy gap in better agreement to reference data when compared to MP2, with the double-hybrid B2PLYP producing better results than BLYP. On the other hand, for calculation of properties involving radical species, as homolytic bond dissociation energies, B2PLYP overestimates the binding energy, especially for larger PAHs. The erratic behavior of UHF and MP2 for open-shell species limits the B2PLYP performance regarding the stability analysis of triangulenes over different multiplicities.