PROTONATION OF THE GROUND-STATES OF RUTHENIUM(II) PHOTOSENSITIZERS

Ru(II) complexes that possess 2,2'-bipyrazine, 2,2'-bipyrimidine, and 2-(2'-pyridyl)pyrimidine ligands in combination with 2,2'-bipyridine u ndergo protonation at the basic nitrogen heteroatoms on the periphery of the aromatic rings as the solution is made more acidic, resulting i n changes in the absorption spectra. In this study, the acidity of the solution was controlled with concentrated H2SO4; titration curves for the first protonations were obtained from plots of absorbance versus acidity, from which the pK(a1) values for the monoprotonated species w ere extracted. As has been observed before for the acid-base propertie s of the excited states and the one-electron reduced forms of the comp lexes, the pK(a1) values of the protonated forms of the ground states (- 1.6 to - 5.0) correlate with the reduction potentials, and demonstr ate site selectivity of the protonation process. (C) 1998 Elsevier Sci ence S.A. All rights reserved.