Intramolecular Magnetic Exchange Interaction in Dichalcogenide Substituted Organic Diradical Dications.

Organic diradical dications, due to reduced intermolecular interactions, exhibit a greater tendency to adopt high spin states in the solid phase compared to their neutral diradical counterparts. This characteristic makes them promising candidates for applications involving organic electronics. We present a theoretical study of a recently synthesized sulfur-based diradical dication, a unique system exhibiting a robust triplet ground state. Using a number of density functional theory (DFT)-based methods (e.g., standard broken-symmetry DFT, constrained DFT, spin-flip TDDFT) and wave function-based multireference CASSCF+NEVPT2 methods, we investigate its magnetic properties and explore the influence of chalcogen substitution on magnetic exchange coupling. An active space scanning method was adopted to overcome the difficulties in choosing the correct active space for multireference calculation. Our findings highlight the critical role of multireference methods in accurately capturing the magnetic behavior of highly π-conjugated systems. The study reveals a surprising variation in magnetic properties among sulfur, selenium, and tellurium-based diradical dications despite being elements of the same group. These results offer valuable insights into the design and tuning of magnetic properties in organic diradical dications.