Efficient energy transfer among dye molecules randomly distributed on surfaces

ANALÍA IRIEL; HERNÁN B. RODRÍGUEZ; ENRIQUE SAN ROMÁN

Lisboa, Portugal

Conferencia; Ninth International Conference on Methods and Applications of Fluorescence, Spectroscopy, Imaging and Probes; 2005

Through this work we demonstrate that very efficient energy transfer (E.T.) can be achieved in disordered systems composed of mixtures of dyes on solid surfaces at high local concentrations. Studied donor / acceptor systems were: A) coadsorbed pheo­phorbide-a / methylene blue; B) coadsorbed Dapoxyl® / 5-(and-6)-carboxynaphto­fluorescein; C) covalently linked rhodamine 101 / adsorbed methylene blue. Microcrystalline cellulose (20 mm particles) was used as supporting material. Overall dye concentrations ranged from 0.1 to more than 1 mmol / g cellulose. In spite of the high concentrations used neither dye aggregation nor exciplex formation were observed. Systems were characterized by reflectance spectroscopy and E.T. efficiencies were quantified by steady-state fluorescence spectroscopy, either minimizing inner filter effects by working with thin layers of particles or modeling reabsorption and reemission of fluorescence for optically thick samples. Up to 60 % of the excitation energy was transferred by resonance E.T. at the highest acceptor concentrations, whereas more than 80 % was transferred when radiational E.T. was allowed by working with thick samples. System (A) was too complex because the strong overlap between acceptor emission and donor absorption causes back E.T. For system (B), efficiencies were independent of the donor concentration because of the large Stokes shift of the donor and could be modeled by Förster theory with R0 » 53 Å. Conversely, system (C), R0 » 58 Å, was affected by energy migration among donor molecules, yielding higher resonance E.T. efficiencies at similar acceptor concentrations. In the figure, resonance E.T. efficiencies are plotted against acceptor concentration for system (B) at two different donor concentrations (open squares and triangles) and for system (C) (full circles). Lines 2D and 3D are fittings obtained using Förster theory in two and three dimensions. Departures found for system (C) at low acceptor concentrations are stressed through the straight line L