(bipy)

The spin crossover phenomenona is a temperature-induced transition of diamagnetic low spin (LS) state to paramagnetic high spin (HS) state. This transition can also be triggered by pressure, light or magnetic field. The Fe(II) spin crossover molecules are known to exhibit spin-crossover transition and we have shown that [Fe{H₂B(pz)₂}₂(bipy)] exhibits a temperature dependent spin-crossover transition from its LS state to the HS state, if the temperature is increased above approximately 160 K. The temperature dependence of the spin state occupancy Fe(II) complex can be affected by addition of polar molecules. The highly polar p-benzoquinonemonoimine zwitterion mixed with [Fe{H₂B(pz)₂}₂(bipy)] resulted in locking [Fe{H₂B(pz)₂}₂(bipy)] largely into a low spin state while addition of the di-ethyl derivative of p-benzoquinonemonoimine zwitterion mixed with [Fe{H₂B(pz)₂}₂(bipy)] did not appear to perturb the spin crossover transition. Addition of different polar molecule, benzimidazole with small dipole moment results in a re-entrant spin crossover transition.