Charge-induced spin alignment in diradical donor molecules: numerical calculations of correlated many-electron-spin systems

The mechanism of charge-induced high spin is studied in pi-conjugated molecules by means of a model-Hamiltonian approach. Intersite Coulomb interactions are taken into account in a pi-conjugated moiety, which is coupled with two localized spins through exchange interactions. We clarify spin alignment in neutral and oxidized states by exact numerical calculations including all the correlation effects. In thianthrene-based molecules, one-electron oxidation induces strong ferromagnetic correlation between the localized spins irrespective of the spin alignment in the neutral state. The localized spins are coupled to the delocalized hole spin ferromagnetically, leading to a high-spin state in the oxidized molecule. Our calculations on structural dependence and effective exchange interaction are consistent with the recent experiment of thianthrene bis(nitronyl nitroxide). By comparing the thianthrene-based molecule with the anthracene-based one, we clarify the role of superexchange interactions via the sulfur atoms.