Determination of pK(a) values of 4-phosphonato-2,2 ': 6 ',2 ''-terpyridineand its ruthenium(II)-based photosensitizer by NMR, potentiometric, and spectrophotometric methods

The dissociation constants of the 4-phosphonato-2,2':6',2 "-terpyridine lig and were measured by pH-NMR and potentiometric titration methods. The 4-pho sphonato-2,2':6',2 "-terpyridine ligand shows three measurable pK(a) values at 5.99, 4.65, and 3.55 using the latter method. The fourth pK(a) was esti mated 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 "-terpy ridine and Me(2)bpy = 4,4'-dimethyl-2,2'-bipyridine) exhibits two ground-st ate pK(a) values at 6.0 and below 4.0, which can be assigned to pK(a1) and pK(a2), respectively. The emission maximum of complex 1 shows a blue shift with decreasing pH, and apparent pH-dependent excited-state lifetimes. Comp lex 1 shows two excited-state pK(a)* values at 6.5 (pK(a1)*) and below 4.5 (pK(a2)*). Comparison of the ground- and excited-state pK(a) values of comp lex 1 demonstrates that the excited state has metal to 4-phosphonato-2,2':6 ',2 "-terpyridine ligand charge-transfer character.