Effect of Molecular Interactions on the Photophysics of Rose Bengal in Polyelectrolyte Solutions and Self-Assembled thin Films

MARTÍN MIRENDA; LELIA E. DICELIO; ENRIQUE SAN ROMÁN

JOURNAL OF PHYSICAL CHEMISTRY B

Año: 2008 vol. 112 p. 12201 - 12207

1089-5647

Abstract Solutions and layer-by-layer self-assembled thin films containing Rose Bengal and poly(diallyl-dimethyl-ammonium chloride) are studied with the aim of understanding the interactions controlling their structures and the photophysics of the dye in both media. A detailed spectroscopic and theoretical analysis shows 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 self-assembled thin films, providing an environment which reduces hydrophobic interactions and lowers the aggregation tendency. Self-assembled films with a roughly 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.