The formation and the decay of the triplet metal to ligand charge transfer state (3MLCT) of ruthenium(II) bis(2,2′-bipyridine)(4,4′- dicarboxy-2,2′-bipyridine) (Ru(bpy)2(dcbpy)) were characterized using photoacoustic calorimetry. At pH 6 and 2, the 3MLCT state formation leads to a volume change of -8 mL mol-1 and enthalpy changes of 17 kcal mol-1 and 13 kcal mol-1, respectively. We attribute the volume contraction to structural changes and to solvent electrostriction. At pH 4, the photoexcitation of the complex leads to an expansion of 14 mL mol-1 and an enthalpy change of ∼119 kcal mol-1 due to protonation of the carboxyl group in the excited state.
