Acid-base properties of the ground and excited states of ruthenium(II) bis(2,2'-bipyridine) 3-carboxyl-2,2'-bipyridine

Acid-base properties for ruthenium(II) bis(2,2'-bipyridine) 3-carboxyl-2,2'-bipyridine reveal a ground state pK(a) of 0.82 +/- 0.07 and an excited state pK(a)* of 2.31 +/- 0.05, a 1.5 pH unit increase from the ground state. The excited state pK(a)* is temperature independent while the ground state pK(a)(0) increases with temperature and has Delta H-0 and Delta S-0 values of -990 +/- 149 cm(-1) and -4.57 +/- 0.48 cm(-1) K-1, respectively. The acidic form of the complex emits at lower energy than the basic form at both 296 and 77 K. The emission energy maxima are solvent dependent and decrease in energy when the solvent changes from 4:1 (v/v) 2-MeTHF-CH2Cl2 to water and when the pH decreases. Changes in excited state lifetimes with emission energy follow the energy gap law with an intercept of 49 +/- 1 and a slope of (2.11 +/- 0.09) x 10(-3). Emission quantum yields for protonated and deprotonated species in 4:1 (v/v) 2-MeTHF-CH2Cl2 are 0.023 +/- 0.001 and 0.110 +/- 0.002, respectively. The temperature dependence of the emission lifetimes gives energy barriers of 270 cm(-1) for the complex in aqueous solution at pH -0.5, and 990 cm(-1) in aqueous solution at pH 4.5, and 1920 cm(-1) in 4:1 (v/v) 2-MeTHF-CH2Cl2.