A Large Entropic Term Due to Water Rearrangement is Concomitant with the Photoproduction of Anionic Free-Base Porphyrin Triplet States in Aqueous Solutions † Gabriel O. Andre ´s, Franco M. Cabrerizo, Vı ´ctor Martı ´nez-Junza and Silvia E. Braslavsky* Max-Planck-Institut fu ¨ r Bioanorganische Chemie (formerly Strahlenchemie), Mu ¨ lheim an der Ruhr, Germany Received 2 September 2006; accepted 1 November 2006; published online 9 November 2006; DOI: 10.1562 ⁄ 2006-09-02-RA-1026 ABSTRACT The enthalpy change, D T H, and volume change, D T V, associated with triplet state formation upon excitation of free-base meso-tetra-(4-sulfonatophenyl)porphyrin, TSPP 4) , its Zn deriv- ative, ZnTSPP 4) , and meso-tetra-(4-carboxyphenyl)porphyrin, TCPP 4) , were obtained in aqueous solutions by the application of laser-induced optoacoustics spectroscopy in the presence of phosphate salts of various monovalent cations (Li + , Na + ,K + , NH 4 + and Cs + ). A linear correlation was found between D T H and D T V at different phosphate concentrations for the free-base porphyrins. The intercepts (132 ± 8 kJ mol )1 for TSPP 4) and 164 ± 23 kJ mol )1 for TCPP 4) ) of these plots correspond to the respective value of the triplet energy content obtained from phosphorescence at 77 K (140 and 149 kJ mol )1 ). This suggests that D T G for the triplet state formation is independent of the medium and an enthalpy–entropy compensation is responsible for the much smaller and salt-dependent D T H values obtained at room temperature. The Gibbs energy for triplet state formation of the free-base porphyrins at room temperature is thus mainly determined by the entropic term due to solvent rearrangement. The D T H values for 3 ZnTSPP 4) at different buffer concentra- tions and different cations are all between 130 and 150 kJ mol )1 , close to the triplet energy obtained from phosphorescence (E T = 155 kJ mol )1 ). The solvent structure and the nature of the counterion have a negligible influence on the 3 ZnTSPP 4) formation due to the blockage of the electron pairs on the central N atoms. Thus, the small D T V value should be due to intrinsic bond changes upon 3 ZnTSPP 4) formation and no correlation between D T H and D T V should be expected in this case. The enthalpy change determines the Gibbs energy for 3 ZnTSPP 4) formation at room temperature. INTRODUCTION The entropy change for the triplet-triplet energy transfer process has been analyzed in several fundamental papers (1–3). An important conclusion has been that the entropy change for the triplet energy transfer reaction in organic solvents is associated with intrinsic geometric changes in the molecular species. For example, entropy changes determined by direct measurement of the temperature dependence of the equilib- rium constant between triplet energy donors and acceptors by nanosecond laser flash photolysis were attributed to intrinsic changes in the flexible partner in the triplet-triplet energy transfer process (2). Thus, when a nonrigid energy acceptor, such as 4-methylbiphenyl, or donor, such as 4-methylbenzo- phenone, goes from the ground to the triplet state or from the triplet to the ground state, respectively, the so-called rigid partner, e.g., 10,10-dimethylanthrone as energy donor and 9,9- fluorene as energy acceptor, was assumed to have a negligible entropic change, and conclusions were drawn about the conformational changes in the flexible partner giving rise to the entropic changes (2). It is, however, difficult to measure directly the entropy change upon triplet state formation, without resourcing to an energy or electron transfer equilib- rium process. In a series of laser-induced optoacoustic spectroscopy (LIOAS) studies with water-soluble chromophores we have shown that, in systems in which the strong water–chromo- phore interactions determine the enthalpy and structural volume changes produced during the photoinduced reaction, there is a compensation between those changes when measured in aqueous solutions with different structurally perturbing cations and ⁄ or anions (depending on the systems). The variations observed in the thermodynamic parameters upon variations in the solvent properties were attributed to changes in specific chromophore-medium (water or even protein environment) interactions (4,5). When the Gibbs energy is the same irrespective of the medium-perturbing factor, this enthalpy-structural volume change compensation affords the entropy change for the particular reaction analyzed, e.g., for the formation of free radicals in the case of the quenching of the metal-to-ligand charge transfer state of Ru(bby) 3 2+ by the methyl viologen dication (6) as well as for the quenching of flavin triplet states by amines and amino acids, again in the presence of various monovalent cations (7,8). We have recently analyzed by LIOAS the triplet state formation upon excitation of meso-tetrakis-(4-sulfonatophe- nyl)porphyrin, TSPP 4) , in the presence of various monovalent cations (9). We found a linear correlation between the enthalpy change (D T H) and the volume change (D T V) for the triplet state formation, with an intercept (i.e., D T V = 0) similar to the triplet energy determined by phosphorescence at 77 K. The interpretation of these data is that in fluid media the entropic †This paper is part of a symposium-in-print dedicated to Professor Eduardo A. Lissi on the occasion of his 70th birthday. *Corresponding author email: braslavskys@mpi-muelheim.mpg.de (Silvia E. Braslavsky) Ó 2007 American Society for Photobiology 0031-8655/07 Photochemistry and Photobiology, 2007, 83: 503–510 503