The photochemical properties of transition metal complexes can be significantly modified by changing the ligands coordinated to the metal center. Ligands play a crucial role in determining the electronic structure, stability, and reactivity of the metal complex. They can influence the absorption and emission properties, as well as the excited state lifetimes and reactivity patterns of the complexes.To illustrate this, let's consider two different transition metal complexes with different ligands:1. [Ru bpy 3] bpy = 2,2'-bipyridine 2. [Ru phen 3] phen = 1,10-phenanthroline Both complexes contain the same metal center Ru but have different ligands. The bpy ligand is a bidentate ligand with two nitrogen donor atoms, while the phen ligand is also bidentate but has a more extended -system due to the presence of additional fused aromatic rings.The photochemistry of these two complexes can be compared in terms of their absorption and emission properties, excited state lifetimes, and reactivity patterns.Absorption and emission properties:[Ru bpy 3] exhibits strong absorption in the UV-visible region, with a maximum at around 450 nm, and a broad, structureless emission band centered at around 620 nm. The complex has a relatively high quantum yield for emission 0.1 . In contrast, [Ru phen 3] has a similar absorption profile but with a slightly red-shifted maximum around 480 nm and a more intense and red-shifted emission band around 650 nm . The quantum yield for emission is also higher for [Ru phen 3] 0.2 .Excited state lifetimes:The excited state lifetimes of these complexes are influenced by the nature of the ligands. [Ru bpy 3] has a relatively short excited state lifetime 1 s , while [Ru phen 3] has a longer excited state lifetime 5 s . The longer lifetime of [Ru phen 3] can be attributed to the more extended -system of the phen ligand, which can stabilize the excited state through increased delocalization of the excited electron.Reactivity patterns:The photochemical reactivity of these complexes can be influenced by the ligands in several ways. For example, the excited state of [Ru bpy 3] is known to undergo electron transfer reactions with various substrates, leading to the formation of reactive intermediates that can participate in subsequent reactions. The relatively short excited state lifetime of [Ru bpy 3] can limit the efficiency of these reactions, as the complex can undergo non-radiative decay back to the ground state before the electron transfer process occurs. In contrast, the longer excited state lifetime of [Ru phen 3] can allow for more efficient electron transfer reactions, leading to higher overall reactivity.In summary, the photochemical properties of transition metal complexes can be significantly influenced by the choice of ligands. By changing the ligands coordinated to the metal center, it is possible to modify the absorption and emission properties, excited state lifetimes, and reactivity patterns of the complexes. In the case of [Ru bpy 3] and [Ru phen 3], the differences in the ligands lead to distinct photochemical behaviors, with [Ru phen 3] exhibiting red-shifted absorption and emission bands, longer excited state lifetimes, and potentially higher reactivity due to more efficient electron transfer processes.