title: A large entropic term due to water rearrangement is concomitant with the photoproduction of anionic free-base porphyrin triplet states in aqueous solutions.
abstract: The enthalpy change, DeltaTH, and volume change, DeltaTV, associated with triplet state formation upon excitation of free-base meso-tetra-(4-sulfonatophenyl)porphyrin, TSPP4-, its Zn derivative, ZnTSPP4-, and meso-tetra-(4-carboxyphenyl)porphyrin, TCPP4-, 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+, NH4+ and Cs+). A linear correlation was found between DeltaTH and DeltaTV at different phosphate concentrations for the free-base porphyrins. The intercepts (132 +/- 8 kJ mol(-1) for TSPP4- and 164 +/- 23 kJ mol(-1) for TCPP4-) 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 DeltaTG for the triplet state formation is independent of the medium and an enthalpy-entropy compensation is responsible for the much smaller and salt-dependent DeltaTH 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 DeltaTH values for 3ZnTSPP4- at different buffer concentrations 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 3ZnTSPP4-formation due to the blockage of the electron pairs on the central N atoms. Thus, the small DeltaTV value should be due to intrinsic bond changes upon 3ZnTSPP4- formation and no correlation between DeltaTH and DeltaTV should be expected in this case. The enthalpy change determines the Gibbs energy for 3ZnTSPP4-formation at room temperature.
authors.affiliation: Max-Planck-Institut für Bioanorganische Chemie (formerly Strahlenchemie), Mülheim an der Ruhr, Germany.
authors.forename: Gabriel O
authors.initials: GO
authors.lastname: Andrés
authors.forename: Franco M
authors.initials: FM
authors.lastname: Cabrerizo
authors.forename: Víctor
authors.initials: V
authors.lastname: Martínez-Junza
authors.forename: Silvia E
authors.initials: SE
authors.lastname: Braslavsky
mesh_terms: D004583:Electrons; D019277:Entropy; D010777:Photochemistry; D011166:Porphyrins / Q000528:radiation effects*; D012996:Solutions; D014867:Water / Q000737:chemistry*
chemical_list: D011166:Porphyrins; D012996:Solutions; D014867:Water