A Large Entropic Term Due to Water Rearrangement is Concomitant with the Photoproduction of Anionic Free-Base Porphyrin Triplet States in Aqueous Solutions†
| Autor: | Víctor Martínez-Junza, Gabriel O. Andres, Franco M. Cabrerizo, Silvia E. Braslavsky |
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| Rok vydání: | 2007 |
| Předmět: |
chemistry.chemical_classification
Aqueous solution Chemistry Enthalpy Free base General Medicine Biochemistry Porphyrin Gibbs free energy symbols.namesake chemistry.chemical_compound Computational chemistry symbols Physical chemistry Physical and Theoretical Chemistry Counterion Triplet state Phosphorescence |
| Zdroj: | Photochemistry and Photobiology. 83:503-510 |
| ISSN: | 0031-8655 |
| DOI: | 10.1562/2006-09-02-ra-1026 |
| Popis: | The enthalpy change, Δ T H, and volume change, AT V, associated with triplet state formation upon excitation of free-base meso-tetra-(4-sulfonatophenyl)porphyrin, TSPP 4- , its Zn derivative, ZnTSPP 4- , and meso-terra(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 Δ T H and Δ 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 Δ 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 Δ 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 Δ T H values for 3 ZnTSPP 4- 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 3 ZnTSPP 4- formation due to the blockage of the electron pairs on the central N atoms. Thus, the small Δ T V value should be due to intrinsic bond changes upon 3 ZnTSPP 4- formation and no correlation between Δ T H and Δ T V should be expected in this case. The enthalpy change determines the Gibbs energy for 3 ZnTSPP 4- formation at room temperature. |
| Databáze: | OpenAIRE |
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