A Comparative Study on the Photophysics and Photochemistry of Xanthene Dyes in the Presence of Polyamidoamine (PAMAM) Dendrimers.

The photophysical and photochemical properties of the xanthene dyes Eosin Y, Erythrosin B, and Rose Bengal are evaluated in the presence of amino-terminated polyamidoamine (PAMAM) dendrimers of relatively high generation (G3-G5) in alkaline aqueous solution. UV/Vis absorption and fluorescence spectra of the dyes show bathochromic shifts, which correlate with the size of the dendrimer. Binding constants (Kbind ) are calculated from absorption data. The resulting high Kbind values indicate strong interactions between both molecules. Triplet-triplet absorption spectra of the dyes are recorded by laser flash photolysis, and a decrease in the triplet lifetimes is observed in the presence of dendrimers. At the same time, an increase in the absorption of the semireduced form of the dyes is observed. Rate constants for triplet quenching (3 kq ) and radical quantum yields (ΦR ) are obtained. The results are explained by a very efficient electron-transfer process from PAMAM to xanthene dyes for all of the dye/dendrimer couples that are evaluated.

Photophysics and reductive quenching reactivity of gadusol in solution.

The photostability and photophysics of gadusol in aqueous solution has been studied. The photodecomposition quantum yields (ca. 4 × 10(-2) and 1 × 10(-4) at acidic and neutral pH, respectively) confirm the high photostability of the metabolite, independently of the presence of oxygen, under physiological conditions. The nature of the electronic transition of gadusol has been assigned as π→π* on the basis of the solvatochromic shifts of the UV absorption spectrum and the time-dependent density functional theory calculation of the vertical transition energies. The results from the photoacoustic calorimetry point to the rapid non-radiative decay as the dominant relaxation pathway of the excited species at pH 7, which is consistent with the proposed UV-sunscreening role of the molecule in the early atmosphere. Laser flash photolysis experiments probed that the ground state of the enolate form (gadusolate) undergoes electron transfer reactions with some triplet sensitizers in water or methanol solution. A rate constant of 2 × 10(8) M(-1) s(-1) has been determined for the quenching of rose bengal triplet state in water at pH 7. This reductive quenching reactivity may be considered as one of the underlying mechanisms that support the antioxidant capacity of gadusol in biological environments.

Sensitized photo-oxidation of gadusol species mediated by singlet oxygen.

Gadusols are efficient nature UV sunscreens with antioxidant capacity. The kinetics of the quenching reactions of singlet oxygen O2(1∆g) by gadusol species was evaluated in aqueous solution as well as in the presence of direct charged micelles. Time-resolved phosphorescence detection of O2(1∆g) indicated that gadusolate, the main species under biological pH, is a more efficient quencher than the enol form with a rate constant of ca. 1.3 × 108 L mol-1 s-1. The deactivation proceeds via a collisional mechanism with clear dominance of chemical pathways, according to the rates of gadusol and oxygen consumptions, and typical photooxidation quantum yields of ca. 7%. The relative contributions of the chemical and physical quenching steps were not affected by the presence of anionic or cationic micelles emulating simple pseudo-biological environments. The products of the photo-oxidative quenching support a type II mechanism initiated by the addition of O2(1∆g) to the C-C double bond of gadusolate. These results point to the relevance of considering the role of sacrifice antioxidant along with the UV-screening function for gadusol, particularly in the context of potential biotechnological applications of this natural molecule.