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

Abstract

The total structural volume (ΔV str) and enthalpy (ΔH str) changes associated with the formation of the triplet metal-to-ligand charge transfer (3MLCT) 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 ΔV str vs ΔH 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 3MLCT state was unperturbed by the salts (as determined by the constancy of the absorption and emission spectra), the plots ΔH str vs ΔV str (the latter shown to be proportional to ΔS str) yield the free energy for the formation of the 3MLCT state, ΔG 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 3MLCT state by the loosening of four CN-bound water molecules, rather than only two in Ru(bpy)2(CN)2. The correlation ΔV str vs ΔS str (a consequence of the correlation between the environment reorganization parameters ΔV sol and ΔS sol) finds support in the proportionality between ΔV str of the cyano complexes and the calculated water-structuring entropy of the salts, ΔS°str(salt). Water structuring salts [negative ΔS°str(salt) values] afforded a larger ΔV str, due to the extension of the water network, opposite to structure breaking salts [positive ΔS°str(salt) values] which yielded smaller ΔV 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.

Photoinduced Electron Transfer from Tetrasulfonated Porphyrin to Benzoquinone Revisited. The Structural Volume-Normalized Entropy Change Correlates with Marcus Reorganization Energy

“…This compensation leads to the determination of the entropy changes for the particular step analyzed, in general with a decay time of hundreds of nanoseconds. [9][10][11][12][13] This information is not attainable from any other technique.…”

“…70%) to a contraction of the solvent shell around the center of the macrocycle upon formation of the triple state. 16 In view of the importance of the water structure in determining the magnitude of the volume changes, [9][10][11][12][13] and the possible correlation of these changes with the Marcus reorganization energy, 6 we decided to study the ET form 3 TSPP 4-to benzoquinone (BQ) in water and analyze the effect of addition of structure-organizing (e.g., Li + ) and -perturbing (e.g., Cs + ) monovalent cations. We report in this paper the enthalpy and structural volume changes for 3 TSPP 4-formation (∆ T H and ∆ T V, respectively) and for the ET quenching step affording free radicals (∆ R H and ∆ R V) in the presence of BQ, in various cations phosphate salts solutions and analyze their possible correlation with the Marcus reorganization energy (λ).…”

Time-Resolved Thermodynamic Profile upon Photoexcitation of a Nitrospiropyran in Cycloalkanes and of the Corresponding Merocyanine in Aqueous Solutions

“…In fact, the structural volume changes measured with LIOAS for an intramolecular electron transfer reaction in nonpolar solvents were quantitatively related with the entropy changes through one of the Maxwell relations [10]. For intra-and intermolecular electron transfer reactions in aqueous media, however, an empirical relation was found between both quantities [5] [7]. It is evident that while reactions in nonpolar solvents can be treated with equations that handle the medium as a continuum experiencing only an expansion or a contraction due to induced-dipole interactions, those of molecules forming specific bonds with the solvent, such as H-bonds with H 2 O that break and reform during the reaction, cannot be treated in a similar manner.…”

Photoinduced formation of colloidal Au by a near-infrared femtosecond laser

“…The effect of focus position was investigated by putting the focus at a series of positions (3,8,13,18,23,28, and 33 mm) below the solution surface. The laser power used was 0.36 mJ, and the irradiation time was 20 min.…”

“…9 This is the unique merit for a near-IR irradiation, although ultraviolet irradiation can be used to photoinduce formation for other colloids or materials than metal colloids. [10][11][12][13] In this study, a Au colloid was prepared with a focused near-IR femtosecond laser at 800 nm. In addition, the mechanism for the precipitate of the Au particles was investigated, so this technique can be widely used for the formation of other metal nanoparticles or nonmetal nanoparticles.…”