WATER-SOLUBLE PORPHYRIN MONOMER-DIMER SYSTEMS - FLUORESCENCE DYNAMICS AND THERMODYNAMIC PROPERTIES

Fluorescent dimers allow the direct determination of their thermodynamic properties from kinetic analysis of the temperature-dependent-two-exponential fluorescence decay of the monomer-dimer system. Monomers and dimers of tetra(carboxyphenyl)porphine in water at pH = 8 have lifetimes of 8.9 +/- 0.2 and 5.1 +/- 0.5 ns, respectively, at 23-degrees-C. In contrast, the monomer lifetime for tetra (N-methylpyridyl) porphine in water is 1.3 +/- 0.2 ns and much shorter than for the dimer, which is 4.9 +/- 0.4 ns. Global analysis yields temperature-dependent preexponential factors from which dimer equilibrium constants, and enthalpies and entropies of dimerization can be derived. We obtained DELTA-H0 = -5000 cm-1 and DELTA-S0 = -8.7 cm-1 K-1 for the tetranegative porphyrin, but DELTA-H0 = +1150 cm-1 and DELTA-S0 = +13.4 cm-1 K-1 for the tetrapositive derivative, where dimerization seems to be entropy