ACID-BASE PROPERTIES OF THE GROUND AND EXCITED-STATES OF RUTHENIUM(II) TRIS(4'-METHYL-2,2'-BIPYRIDINE-4-CARBOXYLIC ACID)

The acid dissociation constant, pR(a), for the ground and excited stat es of ruthenium tris(4'-methyl-2,2'-bipyridine-4-carboxylic acid) comp lex have been measured. The ground state pK(a) obtained from the pH ti tration curve of the complex absorption at 454 nm was 2.5. The lifetim es of the excited-state for deprotonated and protonated ruthenium comp lexes are 595 and 150 n,c,, respectively. The excited-state pK(a) is obtained from the emission titration curve at 630 nm and corrected for the excited-state lifetime to be 4.2. The increase of 1.7 pH units in the acid dissociation constant in the excited-state indicates that th e ligand is much more basic in the excited-state. This result confirms the MLCT assignment for the lowest electronic transition of [Ru(mbpyC OOH)(3)](2+).