The photochemical behavior of [Fe CN 6]4- complex in aqueous solution under visible light irradiation can be described as follows:Upon visible light irradiation, the [Fe CN 6]4- complex can undergo a metal-to-ligand charge transfer MLCT process, where an electron is transferred from the metal center Fe to the ligand CN . This process generates an excited state species, which can participate in various photochemical reactions, such as electron transfer, energy transfer, or ligand substitution.However, the [Fe CN 6]4- complex has a relatively low absorption in the visible light region, which limits its photocatalytic activity. To enhance the photocatalytic activity of this complex, ligand substitution can be employed.Ligand substitution involves replacing one or more of the cyanide CN ligands with other ligands that have stronger absorption in the visible light region. This can increase the overall absorption of the complex, leading to a higher probability of MLCT and, consequently, enhanced photocatalytic activity.For example, replacing one or more CN ligands with N-heterocyclic ligands, such as bipyridine or phenanthroline, can significantly improve the visible light absorption of the complex. These substituted complexes can then be more effective in photocatalytic reactions, such as water splitting, organic pollutant degradation, or CO2 reduction.In summary, the photochemical behavior of [Fe CN 6]4- complex in aqueous solution under visible light irradiation involves MLCT processes, which can be enhanced by ligand substitution to improve the complex's visible light absorption and photocatalytic activity.