This investigation focuses on a series of pseudotetrahedral complexes of the form Cu(NN)(2)(+), where NN denotes a 1,10-phenanthroline ligand with alkyl substituents in the 2 and 9 positions and the counterion is PF6-. In these copper(I) systems, steric effects are of considerable interest because the electronic configuration predisposes the reactive charge-transfer excited state to undergo a flattening distortion or to add a fifth ligand. Both effects lead to emission quenching and a shorter excited-state lifetime. Bulky substituents inhibit these processes, but the spatial distribution of the atoms involved is more important than the total molecular volume In determining the influence of a substituent. According to the results of this study, the effective size decreases in the following order : sec-butyl >neopentyl>n-octyl approximate to n-buty>methyl. In conjunction with the electrochemical data, the absorption and the emission spectra reveal three kinds of steric effects : (1) Clashes between substituents on opposite phenanthroline ligands hinder D-2 flattening distortions in the oxidized form of the complex and in the charge-transfer excited state of the Cu(NN)(2)(+) system itself. (2) Steric interactions connected with a highly branched substituent, like the neopentyl group, destabilize the Cu(NN)(2)(+) ground state. (3) Finally, the presence of bulky groups disfavors expansion of the coordination number. The complex with sec-butyl substituents is noteworthy because it exhibits the longest excited-state lifetime (similar to 400 ns in CH2Cl2) ever measured for a Cu(NN)(2)(+) system in fluid solution. In addition, it exhibits a luminescence lifetime of 130 ns in acetonitrile which is ordinarily a potent quencher of photoexcited Cu(NN)(2)(+) systems.