Structures and electronic states of trimer radical cations of coronene: DFT–ESR simulation study

Abstract

Coronene (C₂₄H₁₂), a charge transfer complex with low-cost and high-performance energy storage, has recently attracted attention as a model molecule of graphene nano-flakes (GNFs). The stacking structures of the trimer radical cation correlate strongly with the conduction states of the GNFs. In the present paper, the structures and electronic states of the monomer, dimer and trimer radical cations of coronene were investigated by means of density functional theory calculations. In particular, the proton hyperfine coupling constants of these species were determined. The radical cation of coronene⁺ (monomer) showed two structures corresponding to the ²A_u and ²B_3u states due to the Jahn–Teller effect. The ²A_u state was more stable than the ²B_3u state, although the energy difference between the two states was only 0.03 kcal mol⁻¹. The dimer and trimer radical cations took stacking structures distorted from a full overlap structure. The intermolecular distances of the molecular planes were 3.602 Å (dimer) and 3.564 and 3.600 Å (trimer). The binding energies of the dimer and trimer were calculated to be 8.7 and 13.3 kcal mol⁻¹, respectively. The spin density was equivalently distributed on both coronene planes in the dimer cation. In contrast, the central plane in the trimer cation had a larger spin density, ρ = 0.72, than the upper and lower planes, both with ρ = 0.14. The proton hyperfine coupling constants calculated from these structures and the electronic states of the monomer, dimer, and trimer radical cations of coronene were in excellent agreement with previous ESR spectra of coronene radical cations. The structures and electronic states of (coronene)_n⁺ (n = 1–3) were discussed on the basis of the theoretical results.