Impacts of the location and number of [Cu(bpy)₂]²⁺ cross-links on the emission photodynamics of [Ru(bpy)₃]²⁺ with pendant oligo(aminoethylglycine) chains

Multifunctional aminoethylglycine (aeg) derivatized [Ru(bpy) ₃]²⁺ complexes with pendant bipyridine (bpy) ligands coordinate Cu²⁺ to form coordinative chain cross-links in a "hairpin loop" motif. In this paper, we report the synthesis and characterization of a series of Ru aeg hairpins in which the relative aeg chain length and number of pendant bpy ligands is varied. Reaction of each of these with Cu²⁺ is monitored using spectrophotometric emission titrations to determine the binding stoichiometry. Coordination of Cu²⁺ causes quenching of the emissive excited state Ru species; the degree of quenching efficiency depends on the location and number of coordinated Cu ions. The heterometallic structures are fully characterized, and using the quantum yields and time-resolved emission following excitation of the Ru complexes in deoxygenated solutions, the radiative (k_r) and nonradiative (k _nr) relaxation rates are compared. These data reveal only a shallow decrease in k_nr with increasing distance between the Ru and Cu complexes. Activation energies, determined from temperature dependent studies of the time-resolved emission, also increase as the Ru-Cu separation increases, resulting in the smaller nonradiative rates. Together, these data are suggestive of excited state electron transfer as the quenching mechanism and demonstrate that metal coordination self-assembles structures made from modular artificial amino acids can provide controlled arrangements of chromophores, electron donors, and electron acceptors to shuttle electrons in a new approach for mimicking photosynthesis.