Water-soluble porphyrin monomer—dimer systems: fluorescence dynamics and thermodynamic properties

Abstract

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°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 Δ H 0=−5000 cm −1 and Δ S 0= −8.7 cm −1 K −1 for the tetranegative porphyrin, but Δ H 0=+1150 cm −1 and Δ S 0= +13.4 cm −1 K −1 for the tetrapositive derivative, where dimerization seems to be entropy driven.