Excitation energy transfer and trapping in dye-loaded solid particles

HERNÁN RODRÍGUEZ; ENRIQUE SAN ROMÁN

Salzburgo, Viena

Congreso; 10th International Conference on Methods and Applications of Fluorescence: Spectroscopy, Imaging and Probes; 2007

Physical or chemical attachment of dye molecules to solid particles allows the attainment of high local dye concentrations. Surface densities near 4 ´ 10-4 dye molecules / Å2, resulting in average inter-molecular distances of about 5 nm, are easily reached. To attain this proximity in solution, concentrations in the order of 10-2 M would be required. In these conditions and assuming random distribution, ca. 25 % of the molecules have neighbors at less than 15 Å. Therefore, the effect of  interactions among dye molecules and singlet-singlet energy migration and transfer cannot be disregarded. Indeed, in systems containing two different dyes efficient energy transfer was observed [1]. For single dyes, on the other side, fluorescence quenching was found as the surface concentration increases [2-3]. An explanation of this behavior is assayed on grounds of suitable models. Results obtained for a rhodamine on micro-crystalline cellulose [3] show that fluorescence quantum yields – corrected for inner filter effects – decrease somewhat more rapidly than fluorescence lifetimes on increasing the dye concentration. These effects may be attributed to energy trapping by a) dimers or quasi dimers or b) statistical traps. Energy migration and transfer should be responsible for the decrease in lifetimes [4].   Application of model (a) shows that nearly 20 % of the dye molecules should be in the dimeric state for the highest dye concentration. On the other hand, if model (b) is applied assuming a Poisson distribution of dye molecules, a quenching radius of nearly 15 Å is found. As no conclusive evidence on changes of the absorption spectrum with concentration was found, the nature of the traps cannot be ascertained. On the other hand, the trapping effect of dimers could be demonstrated for methylene blue adsorbed on the same support, where dimerization could be quantified. The application of model (a) explained quantitatively in this case the fluorescence quantum yield decrease with concentration. Irrespective of the nature of the traps, it is clear that concentration quenching is the result of both static (trap absorption) and dynamic (energy migration and transfer) nature. No excimer fluorescence has been detected in the so far studied systems.   Another common observation is the occurrence of concentration dependent Stokes shifts, resulting in a displacement of the fluorescence spectrum – again after correction for inner filter effects – to higher wavelengths as the dye concentration increases, while the absorption spectrum remains unchanged. This effect is noticed even at average intermolecular distances in excess of 10 nm.   The aim of these studies is the development of solid energy or charge transfer photosensitizers, exploiting the occurrence of high dye concentrations to afford substantial absorption of incident light and energy transfer among different dyes to broaden the excitation spectrum. The unraveling of energy trapping mechanisms in systems composed by a single dye is a key to the development of efficient systems.       References: [1] H.B. Rodríguez et al., Photochem. Photobiol., 82 (2006) 200. [2] M.G. Lagorio et al. Phys. Chem. Chem. Phys., 3 (2001) 1524. [3] H.B. Rodríguez et al., to be published. [4] P. Bojarski et al., Chem. Phys. 210 (1996) 485-499