Magnetic Exchange Interactions: Mechanistic Insights and Understanding Orbital Influences in Organic Diradicals

Frontier molecular orbitals play a crucial role in determining the magnetic behavior and exchange interactions in organic radicals. In this study, we investigate the underlying mechanism influencing the need for orbital planarity and the role of frontier orbital overlap in magnetic exchange interactions. To study this, we designed a series of 12 polyacene-coupled triarylmethyl diradicals, systematically increasing in length of polyacene. We have used nine different DFT functionals for the calculation of the magnetic exchange coupling constant (J). The calculation of magnetic exchange coupling reveals that the GGA functionals define a more accurate spin state, hence more correct magnetic behavior than the meta-GGA and hybrid functionals. We have studied the effect of orbital orientation and their energy gap to understand the high magnetic exchange coupling in the higher polyacene-coupled diradicals. Our calculations revealed that the planarity and overlap of the frontier molecular orbitals are one of the key factors in influencing the strength and behavior of the magnetic exchange interactions in diradicals. Specifically, the overlap between SOMOs and LUMO influences the strength of the magnetic exchange interaction.