Effect of molecular interactions on the photophysics of xanthene dyes in polyelectrolyte solutions and self-assembled thin films.

MARTÍN MIRENDA; LELIA E. DICELIO; ENRIQUE SAN ROMÁN

Gothenburg, Suecia.

Simposio; XXII IUPAC – Symposium on Photochemistry.; 2008

IUPAC

Effect of Molecular Interactions on the Photophysics of XANTHENE DYES in Polyelectrolyte Solutions and Self-Assembled thin Films   Martín Mirenda, Lelia E. Dicelio, Enrique San Román INQUIMAE / DQIAyQF, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pab. II, C1428EAH Buenos Aires, Argentina. E-mail:  esr@qi.fcen.uba.ar             The photophysics of dyes at high local concentrations in constrained environments depends strongly on spatial ordering. Photosynthetic organisms developed quite complex structures to afford substantial absorption of light and canalization of the excitation energy into specific sites, avoiding molecular aggregation and excitation energy trapping, which lead ultimately to the shortening of excited state lifetimes and energy wasting. To prevent radiationless deactivation, a strict control of the spatial architecture at the nanometer scale is therefore essential. Self-assembling of alternate layers of a polyelectrolyte and dye molecules of opposite sign may yield, in principle, the environment needed to attain simultaneously molecular order and high local dye concentrations. Furthermore, reducing the inter-dye distance to the order of the Förster radius may open the way to vectorial energy transfer. This might be an interesting approach in order to make the excitation energy available at the surface of the material. In this work, solutions and layer-by-layer self-assembled thin films containing xanthene dyes and polyelectrolytes are studied with the aim of understanding the interactions controlling their structures and photophysics. Several dyes with different photophysical properties and acid-base behavior were considered. A detailed spectroscopic and theoretical analysis has been performed for Rose Bengal into poly(diallyl-dimethyl-ammonium chloride). Results show that hydrophobic interactions among dye molecules contribute to the coiling of the polyelectrolyte chain in solution at low Polyelectrolyte/Dye ratios, whereas extensive aggregation of the dye takes place even at ratios as high as 104 (expressed in monomeric units). A polyelectrolyte elongated form prevails in thin films self-assembled on glass plates, providing an environment which keeps molecules at larger distances than in solution, reduces hydrophobic interactions and lowers the aggregation tendency. Self-assembled films with an estimated overall dye concentration around 1 M at a P/D ratio in the order of seven are fluorescent and photogenerate singlet molecular oxygen. This contrasts with the behavior of polyelectrolyte solutions, which are almost non-fluorescent and do not evidence triplet state generation at the same P/D ratio.             Strongly fluorescing xanthene dyes were incorporated to the same polyelectrolyte with the aim of studying energy transfer to Rose Bengal. Fluorescein, being less hydrophobic, is incorporated to a lower extent than Rose Bengal in both media and is easily displaced from the films by the last dye. Its complex acid-base behavior renders difficult its speciation analysis. In solution, 2’,7’-dichlorofluorescein shows similar properties as Rose Bengal, though it behaves as fluorescein regarding self-assembly. Experiments with fluorescein attached to polyallylamine are presently underway. Acknowledgement: this work has been supported by the University of Buenos Aires, CONICET and the Agencia de Promoción Científica y Tecnológica of Argentina.