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 (Δ V str ) and enthalpy (Δ H str ) changes associated with the formation of the triplet metal-to-ligand charge transfer ( 3 MLCT) 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 Δ V str vs Δ H str exhibited a linear relationship in the salt series, whereas for Ru(bpy) 3 2+ 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 3 MLCT state was unperturbed by the salts (as determined by the constancy of the absorption and emission spectra), the plots Δ H str vs Δ V str (the latter shown to be proportional to Δ S str ) yield the free energy for the formation of the 3 MLCT state, Δ 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 3 MLCT state of Ru(bpy)(CN) 4 2- is due to the larger entropic factor which originates in the high flexibility photoinduced in its 3 MLCT state by the loosening of four CN-bound water molecules, rather than only two in Ru(bpy) 2 (CN) 2 . The correlation Δ V str vs Δ S str (a consequence of the correlation between the environment reorganization parameters Δ V sol and Δ S sol ) finds support in the proportionality between Δ V str of the cyano complexes and the calculated water-structuring entropy of the salts, Δ S ° str (salt). Water structuring salts [negative Δ S ° str (salt) values] afforded a larger Δ V str, due to the extension of the water network, opposite to structure breaking salts [positive Δ S ° str (salt) values] which yielded smaller Δ 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.