Influence of the Radicaloid Character of Polyaromatic Hydrocarbon Couplers on Magnetic Exchange Interactions

Understanding the properties of the coupler influencing spin-spin interactions in organic diradicals are crucial for designing of molecular spintronic systems with long-range effects. In this quest, using density functional theory (DFT) based broken symmetry method, magnetic exchange coupling (2J) between localized radical centers at a given length connected via spacers with different radicaloid character (y) is investigated. The nitroxy radical sites are coupled through prototypical decacene coupler keeping them ~ 27 A apart. Couplers with different y values, modeled by addition of benzenoid rings in the spin confining region of decacene are used as different spacers between nitroxide diradicals. The strength of magnetic coupling between the localized spin centers is found to be correlated with the radicaloid character of polyaromatic hydrocarbon (PAH) spacers. The stronger exchange interaction is observed for the diradical with higher radicaloid character. It is further observed that the fractional occupied molecular orbitals (MOs) responsible for the open-shell nature of the coupler predominantly constitute the Frontier MOs (FMOs), while in diradicals SOMOs constitute the FMOs.