The role of ligands in modulating the luminescence and charge transfer dynamics of copper photocatalysts is substantial. The introduction of electron-donating substituents on ligands exerts a significant influence on the charge transfer processes occurring within the excited state of the complex. This alteration induces intra-ligand charge transfer (ILCT) transitions at lower energies, characterized by enhanced absorption coefficients, thereby significantly enhancing the visible light absorption of the complex. Additionally, it prolongs the excited state lifetimes to the order of several tens of microseconds, attributed to the absence of pseudo-Jahn-Teller (PJT) distortion, as the hole resides not on the copper ion but on the ligand moiety. Likewise, in copper complexes where both ligands possess HOMO ( or non-bonding orbital) and LUMO (* orbital) localized on the respective ligands, a distinct charge transfer mechanism emerges in the excited state, termed ligand-to-ligand charge transfer (LLCT). This phenomenon is evident in various complexes such as heteroleptic copper(I) diimine diphosphine and heteroleptic copper(I) diphosphine with bridged halogen ligands (Figure ).