Modelling energy transfer among dye molecules on particles

A. IRIEL, H.B. RODRÍGUEZ AND E. SAN ROMÁN

Otawa, Canadá

Congreso; Reactive Intermediates in Photochemistry, an International Symposium Celebrating Tito Scaiano Achivement´s; 2005

Mutual adsorption or chemical linkage of dyes on particulate materials is interesting from both the theoretical and practical points of view. Several years ago we developed a model to account for the effects of dye aggregation and reabsorption and reemission of fluorescence in light scattering systems composed of dyes at high local concentration attached to micrometer size particles(1). More recently, we devised a method to obtain absolute fluorescence quantum yields in this kind of systems(2). The next step was to consider Förster resonance in addition to radiational energy transfer and to evaluate their individual contributions. For that sake, we considered various systems composed of mixtures of dyes randomly distributed 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 0.01 to more than 1 mmol / g cellulose. Neither dye aggregation nor exciplex formation was observed within a broad range of dye concentrations. Moreover, aggregation of pure methylene blue was strongly reduced in the mixed systems. Up to 60 % of the excitation energy was transferred by the resonance mechanism at the highest acceptor concentrations, whereas around 80 % was transferred when radiational energy transfer was allowed by working with thick samples. In this work, the model used to account for both kinds of energy transfer will be discussed and exemplified through system (A), together with some results obtained for the other two systems, and the role of energy migration among donor molecules will be stressed. The next task will be the introduction of molecular ordering to achieve vectorial resonance energy transfer with the aim of designing solid photoactive materials, which may act as efficient energy or charge transfer photosensitizers, while broadening at the same time the spectral response.