Towards the conductivity-magnetism interplay in hydroxylated ethylene-coupled tetrathiofulvalenes and nitronyl nitroxides for magnetoresistance applications

BTBN, a covalently linked tetrathiafulvalene-nitronyl nitroxide (TTF-NN) dyad, represents a prototypical purely organic system exhibiting magnetoresistance (MR). In this material, hole transport along the TTF π-stacks is modulated by ferromagnetic (FM) exchange with the NN radicals, rendering the hopping probability spin-dependent; magnetic-field-induced spin alignment thus leads to reduced resistance. Building on this mechanism, we computationally investigate three in silico designed BTBN derivatives (E1-E3) in which the benzene coupler is replaced by a hydroxylated ethylene unit previously predicted to enhance FM coupling. Using density functional theory and multiconfigurational methods, we compute key charge-transport parameters, including reorganization energies, electronic couplings and rate constants. All three derivatives, E1-E3, exhibit higher hole mobility than BTBN along the π-stacks, a result validated by their larger valence-band dispersion, reaching the same conclusion from both hopping-based and band-structure analyses. Regarding hole injection, Fermi-level alignment with gold electrodes shows that E1 and E3 require voltages similar to BTBN, while E2 requires only a slightly higher bias. Magnetic exchange interactions, evaluated through multiple models, consistently reveal strong intramolecular FM coupling in all compounds. Intermolecular interactions vary, with weak FM or AFM character depending on the system, suggesting potential for MR even in weakly coupled systems. Under appropriate external magnetic fields, weak AFM interactions may convert to FM, further supporting the MR viability of E1-E3. Overall, the results highlight the potential of molecular design to enhance spin-dependent transport and identify E1-E3 as strong candidates for future organic MR materials.