The interaction of the excited states of safranine-O with low generation carboxyl terminated PAMAM dendrimers in an aqueous medium.

The interaction of the singlet and triplet excited states of the synthetic dye safranine-O with carboxyl-terminated poly(amidoamine) (PAMAM) dendrimers was investigated in a buffer solution at pH 8. Low half-generation PAMAM dendrimers (G -0.5; G +0.5: G 1.5) were employed. The UV-vis absorption spectrum of the dye presents only a very small red shift in the presence of dendrimers. Fluorescence quenching was detected and it was interpreted by a static mechanism in terms of the association of the dye with the dendrimer. Laser flash photolysis experiments were carried out and transient absorption spectra of the triplet and radicals were obtained. The triplet state is quenched by the dendrimers with rate constants well below the diffusional limit. The quenching process was characterized as an electron transfer process and the quantum yield of radicals was estimated. It was found that radicals are formed with a high efficiency in the triplet quenching reaction.

A Comparative Study on the Photophysical and Photochemical Properties of Dyes in the Presence of Low Generation Amino-terminated Polyamidoamine Dendrimers.

The photophysical and photochemical properties of the xanthene dyes mercurochrome (MCr) and eosin-Y (Eos); and the phenazine dye safranine-O (SF) are evaluated in the presence of amino-terminated polyamidoamine (PAMAM) dendrimers of low generations. The dendrimers produce a red shift in the UV-vis absorption spectra of the dyes, which increases with concentration and the size of the PAMAM molecule. The Stern-Volmer plots of fluorescence quenching for xanthenic dyes present a downward curvature. It is ascribed to a static mechanism involving a dye-dendrimer binding. A non-linear fitting of the SV plots allows the calculation of the binding constants. For SF, the fluorescence is only slightly quenched by PAMAMs and the SV plots are linear. The binding constants are in the order Kbind (SF) ≪ Kbind (Eos) < Kbind (MCr). The difference must be due to important specific structural effects. A decrease in the triplet lifetime and an increase in the absorption of the semireduced form of the dyes are observed in the presence of dendrimers. While for the two xanthene dyes, the rate constants reach the diffusional limit for G2 and G3, for SF they are one order of magnitude lower. This is explained by a different quenching mechanism of the two types of dyes.

Proton-transfer mediated quenching of pyrene/indole charge-transfer states in isooctane solutions.

The fluorescence quenching of pyrene (Py) by a series of N-methyl and N-H substituted indoles was studied in isooctane at 298 K. The fluorescence quenching rate constants were evaluated by mean of steady-state and time-resolved measurements. In all cases, the quenching process involves a charge-transfer (CT) mechanism. The I(o)/I and tau(o)/tau Stern-Volmer plots obtained for the N-H indoles show a very unusual upward deviation with increasing concentration of the quenchers. This behavior is attributed to the self-quenching of the CT intermediates by the free indoles in solution. The efficiency of quenching of the polyaromatic by the N-H indoles increases abruptly in the presence of small amount of added pyridine (or propanol). A detailed analysis of the experimental data obtained in the presence of pyridine provides unambiguous evidence that the self-quenching process involves proton transfer from the CT states to indoles.