Hyperfine interactions in open-shell planar sp2-carbon nanostructures

We investigate hyperfine interaction (HFI) using density-functional theory for several open-shell planar sp2carbon nanostructures displaying pi magnetism. Our prototype structures include both benzenoid ([n]triangulenes and a graphene nanoribbon) as well as nonbenzenoid (indene, fluorene, and indene[2,1-b]fluorene) molecules. Our results obtained with ORCA indicate that isotropic Fermi contact and anisotropic dipolar terms contribute in comparable strength, rendering the HFI markedly anisotropic. We find that the magnitude of HFI in these molecules can reach more than 100 MHz, thereby opening up the possibility of experimental detection via methods such as electron spin resonance-scanning tunneling microscopy (ESR-STM). Using these results, we obtain empirical models based on pi-spin polarizations at carbon sites. These are defined by generic sp2 HFI fit parameters which are derived by matching the computed HFI couplings to pi-spin polarizations computed with methods such as ORCA, SIESTA, or mean-field Hubbard (MFH) models. This approach successfully describes the Fermi contact and dipolar contributions for 13C and 1H nuclei. These fit parameters allow to obtain hyperfine tensors for large systems where existing methodology is not suitable or computationally too expensive. As an example, we show how HFI scales with system size in [n]triangulenes for large n using MFH. We also discuss some implications of HFI for electron-spin decoherence and for coherent nuclear dynamics.