Structural Volume Changes in Photoinduced Electron Transfer Reactions. Laser-Induced Optoacoustic Studies of Speciation during the Quenching Reaction of Excited Ru(bpy) 3 2+ by Fe(III) in Aqueous Solutions Claudio D. Borsarelli, ² Horacio Corti, ‡ Dario Goldfarb, ‡ and Silvia E. Braslavsky* ,² Max-Planck-Institut fu ¨ r Strahlenchemie, Postfach 10 13 65, D-45413 Mu ¨ lheim an der Ruhr, Germany, and Comisio ´ n Nacional de Energı ´a Ato ´ mica, Departamento Quı ´mica de Reactores, AV. del Libertador 8250, 1429 Buenos Aires, Argentina ReceiVed: April 16, 1997; In Final Form: July 10, 1997 X Laser-induced optoacoustic spectroscopy has been applied to the photoinduced electron-transfer reaction between Ru(bpy) 3 2+ and various salts of Fe(III) [Fe(ClO 4 ) 3 , Fe 2 (SO 4 ) 3 , and FeCl 3 ]. The reaction volume changes associated with photoinduced ligand exchange derived from these measurements allowed the determination of partial molar volumes at high dilutions of several species such as FeSO 4 , FeSO 4 + , Fe(SO 4 ) 2 - , Fe(OH) 2+ , FeCl + , FeCl 2+ , and FeCl 2 + , not attainable from other techniques. The volume change of oxidation of Ru(bpy) 3 2+ to Ru(bpy) 3 3+ at high dilution was also calculated. Introduction In a recent study by laser-induced optoacoustic spectroscopy (LIOAS 1 ), at different temperatures, we observed that the intermolecular photoinduced electron-transfer reaction A be- tween the MLCT state (largely of triplet character) of the ruthenium(II)-tris(bipyridyl) complex, Ru(bpy) 3 2+ , and Fe(III) in sulfuric acid media is accompanied by a volume contraction, ΔV R , of 11 cm 3 /mol 2 . This relatively large contraction could not be explained only in terms of simple bond changes in the reactants after light absorption, since the electron-transfer reaction reduces the quencher [Fe(III)/Fe(II)] leading to a small expansion and oxidizes Ru(bpy) 3 2+ to Ru(bpy) 3 3+ , probably leading to a contraction [smaller than the expansion Fe(III)/Fe(II)] of the ligands around the metal. As a result, the volume change observed was largely attributed to a difference in the medium reorganization around the system present after the time window of the LIOAS experiment (ca. 600 ns 1,2 ). The enthalpic change (ΔH R ) for reaction A obtained by LIOAS was in good agreement with the literature value of 112 kJ/mol obtained for the spontaneous back electron-transfer reaction between Ru(bpy) 3 3+ and Fe(II). 3 As pointed out by Pollmann et al., 4 the reaction volume change, ΔV R , may be composed of at least two contributions: (1) an internal variation (ΔV int ) due to differences in bond length and/or angles between products and reactants and (2) a variation (ΔV ext ) due to reorganization in the solvation shell of the solutes induced by the charge redistribution (ion-solvent interactions) (eq 1) Following an electron-transfer reaction such as A, water reorganization is expected as a result of changes in the ion sizes, although there is no change in the net charge in the reaction. In principle, the external volume change can be ascribed to electrostriction (organization of the solvent due to a change in charge) and can be calculated using the Drude-Nernst eq 2. 5 which describes the contraction of the solvent molecules due to the electric field of an ion of charge z and radius r. The model is relatively simple and assumes that the solvent is a continuum of dielectric constant ǫ interacting with hard sphere ions only through Coulombic forces. The volume change due to electrostriction, V (elec) , is thus related to the partial derivative of ln ǫ with respect to the pressure p. In water at 25 °C the theoretical value of constant B is 4.175. 6 However, experimental results have shown that higher B values are needed to explain the partial molar volume of ions in water. 6 The Drude-Nernst model is, however, qualitatively correct to describe the charge and size effects on the ionic volumes in solution. Taking into account that Fe 3+/2+ (aq) ions are smaller than the Ru(bpy) 3 2+/3+ complex ions, a qualitative analysis of eq 2 predicts also a Volume expansion by electrostriction effects after reaction A, and not a volume contraction as observed. 2 It appears that ΔV R for this reaction is dominated by additional processes, such as changes in the chemical nature of the species, since Fe 3+ (aq) , upon reduction to Fe 2+ (aq) in sulfuric acid undergoes ligand exchange reactions in its first coordination sphere (speciation). In fact, it has been shown that the rate constant for the back electron-transfer reaction between Ru(bpy) 3 3+ and Fe(II) cation is several orders of magnitude lower than the diffusional rate and strongly dependent on the composition of the solution, indicating specific effects of the counterions on the rate of the reaction. 7 To analyze in more detail the origin of the volume changes obtained for reaction A, we present in this report a LIOAS study of the reaction performed with different Fe(III) salts, i.e., perchlorate, chloride, and sulfate, in their respective acid solutions. LIOAS is the method of choice for studying volume changes of photoinduced processes producing either stable products or transient species in the nanosecond to microsecond time domain. 1,8,9 The experiment consists of measuring the pressure * To whom correspondence should be addressed. ² Max-Planck-Institut fu ¨r Strahlenchemie. ‡ Comisio ´n Nacional de Energı ´a Ato ´mica. X Abstract published in AdVance ACS Abstracts, September 1, 1997. Ru(bpy) 3 2+ + Fe 3+ (aq) 9 ' hν Ru(bpy) 3 3+ + Fe 2+ (aq) (A) ΔV R ) ΔV int + ΔV ext (1) V (elec) ) (ze) 2 2rǫ ∂(ln ǫ) ∂p )- Bz 2 r (2) 7718 J. Phys. Chem. A 1997, 101, 7718-7724 S1089-5639(97)01317-0 CCC: $14.00 © 1997 American Chemical Society