A Large Entropic Term Due to Water Rearrangement is Concomitant with the Photoproduction of Anionic Free-Base Porphyrin Triplet States in Aqueous Solutions

• Autores: Andrés, Gabriel O.; Cabrerizo, Franco Martín; Martínez Junza, Víctor; Braslavsky, Silvia E.
• Año de publicación: 2007
• Idioma: inglés
• Tipo de recurso: artículo
• Materia: Rearrangement
• Institución: Consejo Nacional de Investigaciones Científicas y Técnicas

The enthalpy change, DTH, and volume change, DTV, 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 (LIOAS) in the presence of phosphate salts of various monovalent cations (Li+, Na+, K+, NH4 +, and Cs+).A linear correlation was found between DTH and DTV 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 thatDTG for the triplet state formation is independent of the medium and an enthalpy-entropy compensation is responsible for the much smaller and salt-dependent DTH 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 DTH values for 3ZnTSPP4- at different buffer concentrations and differentcations are all between 130 and 150 kJ mol-1, close to the triplet energy obtained from phosphorescence (ET = 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 DTV value should be due to intrinsic bond changes upon 3ZnTSPP4- formation and no correlation between DTH and DTV should be expected in this case. The enthalpy change determines the Gibbs energy for 3ZnTSPP4- formation at room temperature.
Fil: Andrés, Gabriel O.. Institut Max Planck fuer Bioanorganische Chemie; Alemania
Fil: Cabrerizo, Franco Martín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); Argentina
Fil: Martínez Junza, Víctor. Institut Max Planck fuer Bioanorganische Chemie; Alemania
Fil: Braslavsky, Silvia E.. Institut Max Planck fuer Bioanorganische Chemie; Alemania