The photo-induced isomerization of [Ru bpy 3]2+ tris bipyridine ruthenium II involves the conversion between two isomers of the complex upon absorption of light. The mechanism behind this process can be described in terms of electronic transitions and structural changes in the complex.When [Ru bpy 3]2+ absorbs light, it undergoes a metal-to-ligand charge transfer MLCT transition, where an electron is promoted from a metal-based orbital d-orbital of Ru II to a ligand-based orbital *-orbital of the bipyridine ligand . This results in the formation of an excited state species, [Ru bpy 3]*2+, which has a different electronic configuration and geometry compared to the ground state complex.In the excited state, the coordination environment around the ruthenium center becomes more labile, allowing for isomerization to occur. The isomerization process involves the rotation of one or more bipyridine ligands around the Ru-N bond, leading to the formation of a new isomer. The excited state complex then relaxes back to the ground state by emitting light or undergoing non-radiative decay, and the new isomer is formed.The rate of photo-induced isomerization in [Ru bpy 3]2+ depends on the medium in which the complex is present. In solution, the process occurs rapidly due to the high degree of molecular motion and the presence of solvent molecules that can facilitate the isomerization process. The solvent molecules can interact with the complex, stabilizing the excited state species and lowering the energy barrier for isomerization. Additionally, the rapid molecular motion in solution allows for the complex to sample different conformations, increasing the probability of isomerization.In the solid state, however, the rate of photo-induced isomerization is much slower. This is because the complex is locked in a rigid lattice, which restricts molecular motion and prevents the complex from sampling different conformations. Moreover, the lack of solvent molecules in the solid state means that there are no stabilizing interactions to lower the energy barrier for isomerization. As a result, the process occurs much more slowly in the solid state compared to the solution phase.