Photochemistry of potassium ferrocyanide and its reaction with uridine 5^′-monophosphate in aqueous solution under ultraviolet irradiation

The photochemical reaction of potassium ferrocyanide (K₄Fe(CN)₆) exhibits excitation wavelength dependence and non-Kasha rule behavior. In this study, the excited-state dynamics of K₄Fe(CN)₆ were studied by transient absorption spectroscopy. Excited state electron detachment (ESED) and photoaquation reactions were clarified by comparing the results of 260, 320, 340, and 350 nm excitations. ESED is the path to generate a hydrated electron (⁠$eaq−$⁠). ESED energy barrier varies with the excited state, and it occurs even at the first singlet excited state (¹T_1g). The ¹T_1g state shows ∼0.2 ps lifetime and converts into triplet [Fe(CN)₆]⁴⁻ by intersystem crossing. Subsequently, ³[Fe(CN)₅]³⁻ appears after one CN⁻ ligand is ejected. In sequence, H₂O attacks [Fe(CN)₅]³⁻ to generate [Fe(CN)₅H₂O]³⁻ with a time constant of approximately 20 ps. The ¹T_1g state and $eaq−$ exhibit strong reducing power. The addition of uridine 5^′-monophosphate (UMP) to the K₄Fe(CN)₆ solution decrease the yield of $eaq−$ and reduce the lifetimes of the $eaq−$ and ¹T_1g state. The obtained reaction rate constant of ¹T_1g state and UMP is 1.7×10¹⁴ (mol/L)⁻¹·s⁻¹, and the $eaq−$ attachment to UMP is ∼8×10⁹ (mol/L)⁻¹·s⁻¹. Our results indicate that the reductive damage of K₄Fe(CN)₆ solution to nucleic acids under ultraviolet irradiation cannot be neglected.