A voltage-induced transition of hemin in BIODE (biomolecular light-emitting diode)

We fabricated biomolecular light-emitting diodes (BIODE) using hemin, and measured the electroluminescence (EL) spectra, current-voltage and EL intensity-voltage characteristics. The current-voltage and EL intensity-voltage characteristics revealed an irreversible transition of hemin for the applied voltage of V ≈ 4.5 V, above which drastic changes of the EL spectra occur. In order to clarify the origin of this voltage-induced transition, we determined the spin state of several compounds containing heme (hemin, myoglobin, and cytochrome c) using magnetic susceptibility and Raman spectra. We clarified the features of the EL spectra of heme compounds both in the oxidized low-spin state and in the oxidized high-spin state. Based on the spectral features thus obtained, we concluded that the voltage-induced transition in hemin is associated with the change of iron from the oxidized high-spin state (S = 5/2) to the oxidized low-spin state (S = 1/2).