Interactions in noncovalent PAMAM/TMPyP systems studied by fluorescence spectroscopy

Steady-state absorption and emission spectroscopy and time-resolved fluorescence measurements were employed in the study of meso-tetrakis(4-N-methylpyridinium)porphine (TMPyP) interactions with half-generation carboxyl-terminated poly(amidoamine) (PAMAM) dendrimers in water. TMPyP experiences a less polar environment and a strong fluorescence quenching effect upon dendrimer association. The tertiary amine functional groups in PAMAM dendrimers are likely to be responsible for the fluorescence quenching of TMPyP through an electron-transfer mechanism. The Stern-Volmer plots achieve a plateau at high dendrimer concentrations that was attributed to full porphyrin-dendrimer association, and an average fluorescence quantum yield of 15-20% relative to aqueous TMPyP was estimated. The association constant for the 1:1 complex with generation 2.5 at dendrimer-porphyrin ratio D/P = 1 is 5.75 x 10(7) M(-1), indicating a strong binding affinity. The dissociation of the complex with increasing ionic strength reinforces the role of electrostatic forces in porphyrin-dendrimer association. Comparison of Stern-Volmer plots obtained from quantum yields or lifetimes showed the importance of a static effect in these systems. The fluorescence decays of the porphyrin-dendrimer complex were fitted with a dispersed kinetics model. At intermediate dendrimer-porphyrin ratios (D/P approximately 1), diffusional quenching processes between free porphyrin and dendrimer were modeled with the Sano-Tachiya pair survival probability equation. Transient diffusional effects were dismissed as a possible explanation for the static effect detected.