Synergetic effect between spin crossover and luminescence in the [Fe(bpp)2][BF4]2 (bpp = 2,6-bis(pyrazol-1-yl)pyridine) complex

A simple spin crossover (SCO) complex, [Fe(bpp)2][BF4]2 (bpp = 2,6-bis(pyrazol-1-yl)pyridine), was found to exhibit luminesce with a broad band from 425 to 565 nm in the solid state at ambient temperature, although it was non-luminescent at <250 K. In addition, an abrupt enhancement in luminescence was found at ∼260 K, which is exactly the temperature for a transition from the low spin (LS) to the high spin (HS) state of the complex. This implied that the luminescence was triggered by the thermal spin transition from the LS to HS state, which was different from the emission quenching effect of a normal paramagnetic metal center. A theoretical study via DFT calculations suggested that the excitation efficiency of the complex from the ground state to the lowest excited state for the HS state was higher than that for the LS state, which was proposed to be the reason for the HS-triggered luminescence. A hole/electron excitation analysis and orbital component analysis demonstrated that metal to ligand charge transfer (MLCT) upon excitation was responsible for the luminescence of this complex. This is the first report about the MLCT luminescence of SCO Fe(II) complex tuned by the thermal spin transition. Although the HS-triggered luminescence of solid [Fe(bpp)2][BF4]2 was very weak and not suitable for practical applications, molecular materials for both the multiple readout application and magnetism-regulated optical signaling could be anticipated by improving the luminescence efficiency of this type of SCO complexes in the future.