Volume changes correlate with enthalpy changes during the photoinduced formation of the 3MLCT state of ruthenium(II) bipyridine cyano complexes in the presence of salts.: A case of the entropy-enthalpy compensation effect

The total structural volume (Delta V-str) and enthalpy (Delta H-str) changes associated with the formation of the triplet metal-to-ligand charge transfer ((MLCT)-M-3) state were determined by laser-induced optoacoustic spectroscopy for the complexes Ru(bpy)(3)(2+), Ru(bpy)(2)(CN)(2), and Ru(bpy)(CN)(4)(2-) in aqueous solutions of 0.1 M monovalent salts. For the cyano complexes (bound to water through hydrogen bonds) the values of Delta V-str vs Delta H-str exhibited a linear relationship in the salt series, whereas for Ru(bpy)32+ the values were independent of the salt. This linear correlation is interpreted as arising from an enthalpy-entropy compensation effect of the water structure perturbed by the 0.1 M salts. Since in all cases the intrinsic energy of the 3MLCT state was unperturbed by the salts las determined by the constancy of the absorption and emission spectral, the plots Delta H-str, vs Delta V-str (the latter shown to be proportional to Delta S-str) yield the free energy for the formation of the (MLCT)-M-3 state, Delta G(MLCT) = (123 +/- 8) kJ/mol for Ru(bpy)(2)(CN)(2) and (67 +/- 25) kJ/mol for Ru(bpy)(CN)(4)(2-). The higher stability of the 3MLCT state of Ru(bpy)(CN)(4)(2-) is due to the larger entropic factor which originates in the high flexibility photoinduced in its (MLCT)-M-3 state by the loosening of four CN-bound water molecules, rather than only two in Ru(bpy)(2)(CN)(2). The correlation Delta V-str vs Delta S-str (a consequence of the correlation between the environment reorganization parameters Delta V-sol and Delta S-sol) finds support in the proportionality between Delta V-str of the cyano complexes and the calculated water-structuring entropy of the salts, Delta S degrees(str)(salt). Water structuring salts [negative Delta S degrees(str)(salt) values] afforded a larger Delta V-str, due to the extension of the water network, opposite to structure breaking salts [positive Delta S degrees(str)(salt) values] which yielded smaller Delta V-str values. For Ru(bpy)(CN)(4)(2-) the linear dependence had twice as large a slope as for Ru(bpy)(2)(CN)(2), reflecting the difference in the number of hydrogen-bound CN groups.