Orbital determining spintronic properties of a π-conjugated system

Spintronic properties of cyclobutadiene (CBD) systems are investigated based on a qualitative frontier orbital analysis. CBD undergoes a Jahn-Teller distortion from the square triplet state to the rectangular singlet state. According to the qualitative Hückel molecular orbital analysis, the electron transport through the square triplet state is symmetry allowed, whereas that through the rectangular singlet state is symmetry forbidden. The magnetic triplet state is a possible coexisting system of conductivity and magnetism. Sophisticated first-principles quantum chemical calculations are performed by using a realistic molecular junction model. Obtained results are in good agreement with the prediction based on the qualitative orbital analysis. Interesting spin filtering properties are found in the square-shaped CBD system. The high- and low-spin states of the square-shaped CBD system produce the spin-α and spin-β polarized conductance, respectively. The qualitative orbital analysis is useful as a guiding principle for designing molecular spintronics.