Energy migration and transfer in dye-polyelectrolyte layer-by-layer self-asembled thin films

MARTÍN MIRENDA; LELIA E. DICELIO; ENRIQUE SAN ROMÁN

Toledo, Espa~a

Congreso; XXIV International Conference on Photochemistry; 2009

Thin films composed by xanthene dyes bearing two negative electric charges and suitable polyelectrolytes were obtained by layer-by-layer self-assembly onto glass slides. Together with films containing a single dye, following composite films were synthesized: 1) (PDDA/PMA)2 / (PAH-FITC/PMA)9 / (PDDA/RB)9 + PDDA 2) (PDDA/PMA)2 / (PAH-FITC/PMA)9 / (PAH-EITC/PMA)9. where PDDA is poly(diallyl-dimethyl-ammonium) chloride; PMA, sodium poly(methacrylate); PAH, poly(allylamine) chlorhydrate; RB, Rose Bengal; FICT and EICT, fluorescein and eosin isothiocyanate, respectively. Subscripts refer to the number of consecutive bilayers. The dash means chemical binding: in average one FICT or EICT molecule was bound to 24 or 28 PAH monomeric units, respectively. PDDA/RB bilayers had one RB molecule every 7 PDDA units, yielding a volumetric concentration around 1 M. Films were fluorescent in all cases, showing that monomeric dyes are present even at the highest concentrations. Particularly for (PAH-EICT/PMA)9 no evidence on dye aggregation was found. The purpose of the composite films was the search of evidence about energy transfer (E.T.) from FICT to RB or EICT, respectively. For comparison, experiments were performed on the interaction of PDDA with RB,1 fluorescein (FL), dichlorofluorescein (DCFL), eosin (EO), and DCFL+EO in aqueous solution. Systems were studied by absorption and linear dichroism spectroscopy, steady-state and time-resolved fluorescence, and fluorescence anisotropy.             In experiments carried out in solution at different dye:polyelectrolyte ratios, E.T. to dimeric traps was demonstrated for EO and DCFL. E.T. was modeled using 1-D Burshtein’s hoping-quenching theory,2 showing that energy migration is relevant. In addition, the system (DCFL+EO)/PDDA showed clear evidence on E.T. from DCFL to EO, though, owing to the complexity of the system, a quantitative analysis could not be performed.             Composite film (1) was excited at wavelengths at which RB does not absorb essentially, showing a reduction of FICT fluorescence by 60 % and noticeable emission of RB. Similar results on film (2) showed a reduction of FICT fluorescence by 55 % and a noticeable effect on EICT fluorescence as well. Though energy trapping by FICT aggregates is also possible, these results demonstrate clearly that E.T. to the acceptor molecule is present in both systems. Diffusion of RB molecules in film (1) to the FICT domain enhances RB fluorescence in spite of trapping by RB aggregates. Though certain degree of disorder is possible on sequential bilayer deposition, as demonstrated for RB/PDDA by linear dichroism experiments, the absence of diffusion of the chemically linked dyes in film (2) prevents direct contact of donor and acceptor molecules.   Acknowledgements: Funds were obtained from ANPCyT,  CONICET and UBA. References [1]. M. Mirenda, L.E. Dicelio, E. San Román, J. Phys. Chem. B, 2008, 112, 12201. [2]. A. I. Burshtein, J. Luminescence 1985, 34, 201.