Leveraging Intramolecular π-Stacking in Ru(II)-Pyridine Complexes to Induce Photoinduced Ligand Dissociation

[Ru(tpy)(L)(py)]²⁺ (tpy = 2,2;6'2″-terpyridine; py = pyridine), where L represents 1,10-phenanthroline (1; phen), 2-phenyl-1,10-phenanthroline (2; phenyl-phen), and 2-(1'-pyrenyl)-1,10-phenanthroline (3; pyrenyl-phen), were synthesized and characterized by 1D and 2D ¹H NMR and through X-ray crystallography. These methods show that 2 and 3 exhibit intramolecular, interligand π-stacking between the phenyl and pyrenyl groups of the substituted phen ligands in 2 and 3, respectively, and the tpy ligand in each complex. DFT calculations show that the deviation from octahedral geometry around the metal in 2 and 3 leads to a lower energy e_g-type orbital set with Ru-py(σ*) character, making the dissociative metal-centered (MC) excited state(s) more accessible from the metal-to-ligand charge transfer (MLCT) states in these complexes relative to 1. The lifetimes of the lowest energy Ru→tpy ³MLCT excited states in 2 and 3 are shorter than that of 1, consistent with the more facile ³MLCT deactivation through the ³MC state(s) in 2 and 3 as compared to 1. Complexes 2 and 3 undergo pyridine ligand exchange with a Cl^- ion in CH₂Cl₂ with quantum yields, Φ₄₅₀, of 0.024(2) and Φ₄₅₀ = 0.019(3), respectively (λ_irr = 450 nm), representing a > 100-fold increase as compared to that of 1, Φ₄₅₀ < 10^-4. The increase in photoinduced ligand exchange in 2 and 3 is attributed to the greater population of the dissociative ³MC state(s) in these complexes. This represents the first demonstration that intramolecular π-stacking in Ru(II) complexes can be used to enhance ligand release upon irradiation, highlighting a new method to achieve more efficient delivery of pyridine-containing drugs for photochemotherapy.