The recently synthesized sterically constrained copper(I) complex [Cu(dtbp)[sub2]][sup+] (1), where dtbp is 2,9-di-tert-butyl-1,10-phenanthroline, exhibits unique photophysical and reactivity properties. Complex 1 (λ[subabs], 425 nm; ϵ, 3100 L M[sup-1] cm[sup-1]; λ[subemission], 599 nm) has the longest metal-to-ligand charge-transfer (MLCT) emission lifetime (τ, 3260 ns) and largest quantum yield (ϕ, 5.6%) of all [Cu(R[sub2]phen)[sub2]][sup+] complexes. Complex 1 also exhibits a large positive reduction potential for the [Cu[sup2+](dtbp)[sub2]]l[Cu[sup+](dtbp)[sub2]] couple (E[sub1/2] = 0.70 V vs Fc[sup+/0]) and a large negative excited-state reduction potential for the [Cu[sup2+](dtbp)(dtbp[sup—∙])]l[Cu[sup2+](dtbp)[sub2]] couple (E[sub1/2] = -1.66 V vs Fc[sup+/0]), indicating that this complex is a potent photoreductant in the excited state. The steric constraint imposed by the t-butyl substituents in 1 enables unusual ligand replacement reactivity. Either CH[sub3]CN ot CO replaces one of the dtbp ligands, a reaction that is readily followed by loss of the unique emission signature of 1. Monodentate CH[sub3]CN binds to the copper(I) center with an affinity 2 orders of magnitude greater than that of the displaced dtbp, despite the fact that the displaced ligand is bidentate. CO-induced displacement of dtbp from 1 is reversible, but only in the presence of 1 equiv of unbound dtbp. The exceptionally strong donor ligand CH[sub3]NC displaces both dtbp ligands from 1. In contrast to the facile displacement reactivity with good donor ligands, 1 does not react readily with O[sub2], by either a ligand displacement or an oxidative pathway. Rather, O[sub2] induces partial quenching of emission via an outer-sphere interaction with 1. [ABSTRACT FROM AUTHOR]