Interplay of Bridge Topology and Conformation in Phenylene-Bridged Trityl Diradicals

Tuning the exchange coupling constant J is a valuable tool in the design of molecular systems for applications in molecular magnetism and quantum information science. The phenylene group is an archetypal bridge to influence the sign and magnitude of J via topology and substitution. Motivated by this, we synthesized an m-phenylene-bridged trityl diradical (m-ben-(TAM)₂ ^••) and explored its magnetic coupling in comparison to the p-phenylene- and p-xylene-bridged analogues. We find a weak antiferromagnetic coupling of -4 GHz for m-ben-(TAM)₂ ^•• in the powder, which increases to -169 GHz in solution. The antiferromagnetic nature of the spin-spin coupling is rationalized by the large dihedral angle between the trityl and bridge aryl groups, which diminishes the ferromagnetic π-contribution. The increase in the magnitude of J by going from the powder to the solution state is rationalized by a large structural change, as investigated by density functional theory (DFT) calculations and molecular dynamics (MD) simulations. We speculate that these geometric changes lead to changes in the intramolecular through-bond and through-space contributions to J, but can't rule out that it is also connected to intermolecular contributions in the powder. These results highlight that the design of ferromagnetic coupling cannot be based only on the topological rule.