Binding of Fluorescein Derivatives to Polyelectrolytes in Solution and Self-assembled Thin Films.

MARTÍN MIRENDA; LELIA E. DICELIO; ENRIQUE SAN ROMÁN

La Plata - Argentina

Jornada; Fronteras en fisicoquímica. Un enfoque interdisciplinario. 60 Aniversario de actividades en el campo de la fisicoquímica en el Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas.; 2008

INIFTA

Layer-by-layer self-assembly is currently used to obtain thin films on different types of substrates by the alternate deposition of polyelectrolytes or polyelectrolytes and dyes of opposite charge. Recently, we applied this technique to obtain films with photosensitizing properties based on poly(diallyldimethylammonium chloride) (PDDA) and Rose Bengal (RB), whose properties could be understood on the basis of solution experiments.[1] As the structure of thin films composed by fluorescein (FL) and PDDA are a matter of discussion in the literature, we decided to perform further studies using different xanthene dyes. We report here results on the interaction of RB, FL and dichlorofluorescein (DCFL) with PDDA, both in solution and in thin films. Steady-state absorption and emission spectra of FL/PDDA thin films are red shifted 34 and 40 nm, respectively, from those in aqueous solution. Nicol et al.[2] assign this shift to the formation of J-aggregates. In contrast, we demonstrate that this shift is a result of a change in environment on passing from water to PDDA. The incorporation of FL to a solution of PDDA is pH-dependent. In neutral medium, the dye, which is in water in the form of a mixture of its singly and doubly charged species, is incorporated into the polyelectrolyte in its dianionic form. Incorporation is more efficient than in alkaline solution, showing that OH- displaces the dye from the polymeric matrix. These properties explain the lability of FL/PDDA self-assembled thin films. On the other side, RB is incorporated efficiently into aqueous PDDA at a large range of pH values, is not displaced by OH- and remains in the aggregated state at low PDDA:RB ratios. The larger hydrophobicity of RB and different pKa values explain the behavior of both dyes. DCFL has a lower pKa than FL and remains in its dianionic form in solution at lower pH values. It is also more hydrophobic than FL and builds up more stable films though not as stable as those including RB. Solution absorption and fluorescence spectra of the different systems are rationalized in terms of the hydrophobicity and pKa values of the corresponding dyes. RB/PDDA solutions show a large dye aggregation even at very high polyelectrolyte to dye (P/D) ratios, while the dye is always monomeric in FL/PDDA solutions. Interestingly, the intermediate properties of DCFL lead to the coexistence of dye monomers and dimers at a relatively low P/D ratio and to the occurrence of monomer to dimer energy transfer, which lowers the apparent fluorescence quantum yield of the system. On the other hand, a highly hydrophobic dye as RB yields stable films with nearly 50 % dye aggregation, while films composed by FL monomers are less stable. Solution experiments yield invaluable results for the design of layer-by-layer self-assembled thin films with photosensitizing properties and show how system properties may be tuned to obtain stable films with controlled interactions and low dye aggregation to inhibit energy trapping. [1] M. Mirenda, L.E. Dicelio, E. San Román. J. Phys. Chem. B 112 (2008) 12201-12207 [2] E. Nicol, A. Moussa, J.-L. Habib-Jiwan, A.M Jonas. J. Photochem. Photobiol. A. 167 (2004) 31-35