Real-Time CASSCF Ehrenfest (TD-CASSCF) Modeling of Spintronics in a Tetrathiafulvalene-Based Nitronyl Nitroxide (BTBN) Model: The Competition between Charge Transfer and Spin Permutation

Using real-time all-electron TD-CASSCF/Ehrenfest simulations, we investigate the molecular origin of magnetoresistance in the organic material BTBN, a tetrathiafulvalene-nitronyl nitroxide diradical. The calculations reveal that charge transport and spin dynamics are strongly coupled. In the high-spin state, where localized spins are aligned in parallel, charge propagates efficiently through the π-stacked system via coherent wave-like migration. In contrast, in low-spin states containing antiparallel spin alignments, the charge migration competes with the spin-permutation processes between localized radical spins. These additional spin-exchange pathways do not transfer charge but divert population away from the charge transport channel, reducing charge-transfer efficiency by roughly 50%, in the example studied, relative to the high-spin state. The simulations therefore identify the competition between electron migration and spin permutation as the fundamental molecular mechanism responsible for magnetoresistance in BTBN, explaining why magnetic-field-induced spin alignment enhances charge transport and lowers electrical resistance.