[Cu(dmit)(2)](2-) formally consists of a magnetic Cu2+ ion (S = 1/2) and closed-shell pi-conjugated dmit(2-) ligands. However, it has been determined by electron spin resonance and molecular orbital calculations that the majority (91-100%) of the spin delocalizes over the ligands in the ground state, while the spin significantly (40-90%) localizes on Cu under UV irradiation. Considering the results of X-ray structural analyses under UV irradiation, the spin distribution varies depending on the wavelengths irradiated and retaining the square-planar coordination geometries.