Charge transfer within excited states of boron/nitrogen doped polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) have attracted significant attention in scientific research due to their unique electronic properties and potential applications in various fields such as photovoltaics and photocatalysis. In this study, the excited states and intramolecular charge transfer mechanisms within boron/nitrogen (B/N) doped PAHs using a 5-armchair,5-zig-zag periacene as model were investigated. Starting with a pristine periacene sheet, twelve chemically modified structures were explored, with different topologies of boron and nitrogen doping. Geometry optimization calculations in the ground state were performed at the ωB97XD/def2-SV(P) level, followed by single-point calculations of the low-lying singlet excited states using multireference MR-CISD and SC-NEVPT2 methods and single reference (SR) ADC(2) and TD-DFT theories for comparison. The analysis of energy spectra and charge transfer (CT) character were conducted using the one particle density matrices, analyzing the involved natural transition orbitals (NTOs) and through the decomposition of the states into contributions of local excitations (LE), charge transfer (CT) or double excitations (2-el.) A S₁ CT state was characterized for three of the suggested doped PAHs. Interestingly, in one of these cases, the CT state was dominated by a double excitation character. Beyond this case, it turned out that most of the other excitations also have a strong double excitation component. This fact makes MR calculations highly desirable for accurate investigations, as SR methods such as ADC(2) and TD-DFT become questionable in many cases.