Interplay between the Conformational Flexibility and Photoluminescent Properties of Mononuclear...

The macrocyclic ligand conformational behavior in solution, solid-state structures and the photophysical properties of copper(I) cationic and neutral mononuclear complexes supported by tetradentate N, N'-dialkyl-2,11-diaza[3.3](2,6)-pyridinophane ligands ^RN4 (R = H, Me, ⁱBu, ^secBu, ^neoPent, ⁱPr, Ts) were investigated in detail. Steric properties of the alkyl group at the axial amine in the ^RN4 ligand were found to strongly affect the conformational preferences and dynamic behavior in solution. Several types of conformational exchange processes were revealed by variable-temperature NMR and 2D exchange spectroscopy, including degenerative exchange in a pseudotetrahedral species as well as exchange between two isomers with different conformers of tri- and tetracoordinate ^RN4 ligands. These exchange processes are slower for the complexes containing bulky alkyl groups at the amine compared to less sterically demanding analogues. A clear correlation is also observed between the steric bulk of the alkyl substituents and the photoluminescent properties of the derived complexes, with less dynamic complexes bearing bulkier alkyl substituents exhibiting higher absolute photoluminescence quantum yield (PLQY) in solution and the solid state: PLQY in solution increases in the order Me < ^neoPent < ⁱBu < ^secBu ≈ ⁱPr < ^tBu. The electrochemical properties of the cationic complexes [(^RN4)Cu^I(MeCN)]X (X = BF₄, PF₆) were also dependent on the steric properties of the amine substituent.