Photophysics of Fluorescein Derivatives in Polyelectrolyte Solutions and Self-assembled Thin Films.

MARTÍN MIRENDA; LELIA E. DICELIO; ENRIQUE SAN ROMÁN

Cubatao, San Pablo, Brasil

Congreso; IX Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2008

Photophysics of fluorescein derivatives in polyelectrolyte solutions and self-assembled thin films   Martín Mirenda, Lelia E. Dicelio y Enrique San román   INQUIMAE / DQIAyQF, Facultad de Ciencias Exactas y Naturales, UBA, Ciudad Universitaria, Pab. II, C1428EHA Buenos Aires, Argentina. e-mail: martinm@qi.fcen.uba.ar   Since more than ten years, layer-by-layer self-assembly is used to produce thin films on different types of substrates by the alternate deposition of polyions or polyions 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,[2] we decided to perform further studies using different xanthenic dyes. We report here results on the interaction of FL and dichlorofluorescein (DCFL) with PDDA, both in solution and 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. 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 a mixture of the single and the 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 away 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 pK values explain the behavior of both dyes. DCFL has a lower pKa than FL and remains in its dianionic form at lower pH values. It is also more hydrophobic than FL but these properties do not assure the formation of stable thin films. In spite of that, its properties are more suited for that purpose than those of FL. Absorption and fluorescence spectra of DCFL into aqueous PDDA are rationalized in terms of these properties, showing that energy transfer processes are relevant in this case. [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