Steric Effects in the Ground and Excited States of Cu(NN)2+ Systems

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 ≈ n-butyl > 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 D2 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 (∼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.