Transition metal complexes have been exploited as light harvesters and luminophores in a number of applications such as artificial photosynthesis, 1 organic light-emitting diodes, photoredox catalysis, photodynamic therapy, and biological sensing. They have traditionally involved second-and third-row platinum group metals because of their strong metal-ligand bonds and long-lived photoexcited states after light absorption, typically involving metal-to-ligand charge transfer (MLCT) transitions. However, because of the scarce natural abundance, generally high cost, and typically higher toxicity of the heavier metals, there has been a growing interest in the development of photoactive first-row transition metal complexes that do not suffer from these issues. Due to their lower effective nuclear charges, photoactive first-row transition metal complexes have weaker metal-ligand bonds and weaker ligand fields, inducing the presence of many low-lying metal-centered (MC) excited states in complexes with partially filled d-orbitals. Such MC states typically foster unproductive nonradiative decay pathways following photoexcitation into the MLCT absorption band. To overcome these challenges, one approach adopted by Wärnmark and co-workers was to introduce strongly σ-donating and moderately π-accepting N-heterocyclic carbenes in nearly perfectly octahedral coordination around a lowspin Fe III center, which resulted in long-lived ligand-to-metal charge transfer (LMCT) excited states with lifetimes reaching up to the ns domain. Hanan, Zysman-Colman, and co-workers have adopted a similar strategy, using strongly -donating guanidylcontaining ligands about a Co III center that accessed ns-long LMCT excited states. Another approach by Yersin, Kato, Chen, Amaroli, Castellano, and others was to completely circumvent the ligand-field MC states by accessing Cu I 3d 10 systems. Since the Cu I center possesses a fully filled d subshell and thus cannot undergo d-d transitions, the emissive excited states have MLCT character and there are no low-lying MC states, since these perforce involve higher energy 4s and 4p orbitals, which results in longer-lived photoactive species. Nonetheless, owing to the lack of ligand-field stabilization and weak electrostatic interactions from the low oxidation state, most of the Cu I luminophores are trigonal planar or tetrahedral complexes, supported by bulky, soft phosphine, or oligopyridyl ligands. In these geometries, the highest occupied molecular orbital (HOMO) of d 10 complexes is often degenerate or nearly degenerate with the HOMO-1 and are metal-centered. Thus, when these complexes are photoexcited, the copper becomes formally d Cu II and the compounds usually undergo first-order Jahn-Teller distortion, which results in strong nonradiative decay due to the significant excited-state geometric distortion. One approach to control and reduce both Jahn-Teller distortion and nonradiative decay has been to introduce bulky groups on the ligands to impede the distortion away from a tetrahedral and trigonal planar geometries. Shaw et al. have performed timeresolved X-ray absorption spectroscopic studies to monitor the structural photodynamics of Cu I complexes coordinated by 1,10phenanthroline derivatives with methyl or phenyl substituents at the 2-and 9-positions and observed spectroscopic signatures consistent with a flattening of the tetrahedron and coordination of a solvent molecule after photoexcitation. McCusker et al. then developed a Cu I complex with substituted 1,10-phenanthrolines containing sec-butyl and methyl groups that has excitedstate lifetimes of up to 2.8 ms, which was later demonstrated to be an effective photosensitizer for photocatalytic H2 evolution in combination with a Co-based co-catalyst. However, the steric bulk cannot be indefinitely increased since doing so will promote ligand photodissociation from the Cu I center, which is an obviously detrimental nonradiative pathway.