EPR evidence of the paramagnetism of a long-living metastable excited state of a sodium nitroprusside single crystal

An EPR signal characteristic of low-spin Fe(III) (S = 12) was observed in a Na2[Fe(CN)5NO] · 2H2O single crystal after about 12 h of continuous irradiation with blue-green light at 77 K. This is a result of charge transfers within the nitroprusside anions forming a long-living metastable excited state. Then a slow 6 h evolution of the EPR spectrum is observed, which seems to be due to a continuous deformation of the [Fe(CN)5NO]2− structure. This deformation forms when thermal equilibrium in the multilevel orbital system and in the crystal lattice is established.