Determination of p K a Values of 4-Phosphonato-2,2‘:6‘,2‘ ‘-Terpyridine and Its Ruthenium(II)-Based Photosensitizer by NMR, Potentiometric, and Spectrophotometric Methods

The dissociation constants of the 4-phosphonato-2,2‘:6‘,2‘ ‘-terpyridine ligand were measured by pH−NMR and potentiometric titration methods. The 4-phosphonato-2,2‘:6‘,2‘ ‘-terpyridine ligand shows three measurable p K a values at 5.99, 4.65, and 3.55 using the latter method. The fourth p K a was estimated to be below 2. Spectrophotometric titration of the ruthenium complex [Ru(P-terpy)(Me 2 bpy)(NCS)] ( 1 ) (P-terpy = 4-phosphonato-2,2‘:6‘,2‘ ‘-terpyridine and Me 2 bpy = 4,4‘-dimethyl-2,2‘-bipyridine) exhibits two ground-state p K a values at 6.0 and below 4.0, which can be assigned to p K a1 and p K a2, respectively. The emission maximum of complex 1 shows a blue shift with decreasing pH, and apparent pH-dependent excited-state lifetimes. Complex 1 shows two excited-state p K * a values at 6.5 (p K * a1 ) and below 4.5 (p K * a2 ). Comparison of the ground- and excited-state p K a values of complex 1 demonstrates that the excited state has metal to 4-phosphonato-2,2‘:6‘,2‘ ‘-terpyridine ligand charge-transfer character.