Photophysics of fluorescein and rose bengal in sequentially deposited polyelectrolyte-dye thin films

MARTÍN MIRENDA; LELIA E. DICELIO; ENRIQUE SAN ROMÁN

Salvador, Bahía, Brasil.

Conferencia; 17 Inter-American Photochemical Society Winter Conference; 2006

Inter-American Photochemical Society

Photophysics of fluorescein and rose bengal in sequentially deposited polyelectrolyte-dye thin films Martín Mirenda, Lelia E. Dicelio and 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 Xanthene dyes fluorescein (FL) and rose bengal (RB) were incorporated into poly(diallyl-dimethyl-ammonium) chloride (PDDA) layers on activated glass surfaces by layer-by-layer self-assembling. In some experiments the same dyes were incorporated into preformed bilayers of PDDA and poly(metacrylic acid) sodium salt (PMA). FL has a high fluorescence quantum yield in basic medium and, due to a suitable spectral overlap, its excitation energy may be transferred to RB, a well known singlet oxygen photosensitizer. In a typical experiment, activated glass was dipped alternatively into PDDA and PMA solutions and, after the formation of two bilayers, successive PDDA and dye layers were deposited in the same way. Systems were composed of various layers containing pure RB, pure FL or FL followed by RB, with or without separating PDDA-PMA bilayers. The amount of dyes incorporated into PDDA increases with the polymer chain length, thus in most of the experiments the polymer molecular weight was about 500 kDa. Shapes of absorption and emission spectra of pure RB films are similar to those found for the RB dianionic monomer in solution. However, for FL the shape of the absorption spectrum of films matches that of the dianionic form in solution but the emission spectrum is similar to that of the monoanionic species. Energy transfer between FL and RB in mixed thin films was followed by steady-state and time-resolved fluorescence. RB quenches efficiently the fluorescence of FL but no effect of intermediate PDDA-PMA bilayers is observed. Most probably, dyes diffuse into the polymer matrix[1]. Furthermore, FL is partially detached by immersion of the film in the PMA solution. Singlet molecular oxygen quantum yields were determined in pure RB and mixed systems using a special setup, immersing films in dichloromethane and following the decline of the absorbance of added diphenylisobenzofuran (DPBF) at 415 nm. Separate experiments were performed in solution by addition of increasing quantities of PDDA to a fixed amount of the dye. Dianionic aqueous RB is incorporated initially as dimers into the polymer chain at a ratio of two PDDA monomer units per RB molecule and undergoes deaggregation as the concentration of PDDA increases. Dimers are only slightly fluorescent and do not yield triplets significantly. In contrast, for FL an equilibrium between the aqueous monomer and the dye attached in its monomeric form is established in a very broad PDDA concentration range. Experimental results on thin films are analyzed in the light of the spectroscopic behavior of aqueous solutions and the acid-base properties of FL in the ground and excited states[2]. [1] C. Tedeschi, F. Caruso, H. Möhwald and S. Kirstein, J. Am. Chem. Soc. 2000, 122, 5841-5848 [2] R. Sjöback, J. Nygren and M. Kubista, Spectrochimica Axta Part A. 1995, 51, L7-L21