Ligand substitution can significantly affect the photochemical properties of coordination compounds. These properties include absorption and emission spectra, excited-state lifetimes, and photochemical reactivity. The underlying principles involved in these changes are based on the electronic structure of the coordination compounds and the nature of the ligands.1. Absorption and emission spectra: The absorption and emission spectra of coordination compounds are determined by the electronic transitions between different energy levels. Ligand substitution can alter the energy levels and the nature of the transitions, leading to changes in the absorption and emission spectra. For example, replacing a weak-field ligand e.g., Cl- with a strong-field ligand e.g., CN- in an octahedral complex can cause a shift in the absorption spectrum to higher energy blue shift due to the increased ligand field splitting. This can also lead to changes in the color of the compound.2. Excited-state lifetimes: The lifetime of an excited state is influenced by the rate of radiative emission and non-radiative e.g., internal conversion, intersystem crossing processes. Ligand substitution can affect these rates by altering the energy gap between the excited state and the ground state or by changing the nature of the excited state e.g., metal-centered vs. ligand-centered . For example, replacing a ligand with a heavier one can enhance the spin-orbit coupling, leading to a faster intersystem crossing and a shorter excited-state lifetime.3. Photochemical reactivity: The photochemical reactivity of coordination compounds depends on the ability of the excited state to undergo various reactions, such as electron transfer, energy transfer, or bond cleavage. Ligand substitution can modulate the reactivity by changing the electronic properties of the excited state or by introducing new reactive sites. For example, replacing a non-photoactive ligand e.g., H2O with a photoactive one e.g., CO in a metal carbonyl complex can enable the photochemical release of CO upon light irradiation.Specific examples:a Ruthenium II polypyridyl complexes: The photochemical properties of these complexes can be tuned by varying the nature of the polypyridyl ligands. For instance, replacing 2,2'-bipyridine bpy with 1,10-phenanthroline phen in [Ru bpy 3]2+ results in a red shift in the absorption and emission spectra and an increase in the excited-state lifetime due to the stronger -acceptor ability of phen.b Chromium III hexaaqua complex: The photochemical substitution of water ligands in [Cr H2O 6]3+ can be induced by light irradiation, leading to the formation of aqua-hydroxo species. This process is facilitated by the metal-centered nature of the excited state, which weakens the Cr-O bonds and allows the water ligands to be replaced by hydroxide ions.In summary, ligand substitution can greatly influence the photochemical properties of coordination compounds by affecting their electronic structure and the nature of the excited states. Understanding these effects is crucial for designing coordination compounds with tailored photochemical properties for various applications, such as solar energy conversion, photocatalysis, and photodynamic therapy.