The identification and comprehensive understanding of the photophysical properties of any photocatalyst entails a diverse set of experiments such as X-ray diffraction studies, UV-VIS spectroscopy, cyclic voltammetry, time correlated-single photon counting (TC-SPC), and density functional theory (DFT). The advancement in these techniques has helped establish a general photophysical scheme for the tetra-coordinated copper(I) complexes (Figure ). However, it is noteworthy that both the photophysical and electrochemical properties of these complexes are significantly contingent upon the choice of ligand. The selection of the ligand is a crucial determinant influencing the overall flexibility of the complex and the electronic environment at the copper center. Copper (I) exhibits a filled d orbital (d 10 configuration) and a symmetrical electron density surrounding the copper center. This electronic arrangement favors a pseudo-tetrahedral geometry by the complexes in the ground state. This category of complexes manifests two prominent absorption bands; one located in the UV part of the spectrum is associated with the entirely ligand-centered (LC) intense  -* transition, and the other one situated in the visible part of the spectrum corresponding to the transition from metal-centered HOMO to ligand-centered LUMO (d -*) (Figure and Figure ). This phenomenon is recognized as metal-to-ligand charge transfer (MLCT) and holds considerable importance for photochemistry. In this MLCT step, the copper (I) ion undergoes oxidation to copper (II), concurrently reducing one of the ligands. This excited copper complex has a d 9 configuration and can either relax to the ground state through fluorescence or undergo a flattening distortion through pseudo-Jahn Teller distortion (PJT) to a flattened state identified as 1 MLCTflat. The singlet MLCT state of copper is also susceptible to undergoing inter-system crossing (ISC) to the triplet MLCT state, facilitated by the spin-orbit coupling. However, the spin-orbit coupling not being sufficiently high for the relatively flattened copper (II) (300 cm -1 for tetrahedral and 30 cm -1 for square planar in [Cu(dmp)2] + ) , the singlet to triplet relaxation ( 1 MLCT to 3 MLCT) remains in competition with pseudo-Jahn Teller distortion (PJT) ( 1 MLCT to 1 MLCTflat). The 3 MLCT state can also undergo relaxation to a relatively flattened state MLCTflat, which lies below the 1 MLCTflat. If the energy gap between the 1 MLCTflat and MLCTflat states is small enough, viz. ES-T < 0.37 eV, then luminescence decay mostly occurs from MLCTflat through thermally activated delayed fluorescence (TADF). Otherwise, the decay is primarily attributed to the phosphorescence of the complex from 3 MLCTflat. Both the 1 MLCTflat and MLCTflat states exhibit potential interaction with nucleophiles such as acetonitrile leading to a reduction of the excited state lifetime of the complex and the luminescence quantum yield. The quenching of the MLCTflat state of copper (I) complexes can occur through interaction with an electron acceptor or electron donor substrate, facilitated by electron transfer (a) from the ligand (oxidative quenching) or, (b) to the metal center (reductive quenching) respectively (Figure ). For example, oxidative quenching of excited copper complex forms oxidized copper and reduced substrate, initiating subsequent synthetic transformations. This approach enables the utilization of first-row transition metals as photocatalysts, offering a viable alternative to expensive and scarce rare-earth metals such as Ru or Ir. Consequently, over the last decade, this avenue of investigation has been receiving significant attention and as a result, a wide variety of synthetic transformations using copper (I) catalysts have surfaced. Ligand to Metal Charge Transfer (LMCT)