Rhodamine 6G adsorbed on microcrystalline cellulose: Concentration self-quenching by excitation energy migration and trapping

GREGOR WORRINGER; HERNÁN B. RODRÍGUEZ; ENRIQUE SAN ROMÁN

Salvador, Bahia, Brasil

Conferencia; 17th I-APS Winter Conference; 2006

Inter-American Photochemical Society

On studying the photophysical behavior of dyes adsorbed on microparticles, we have found for a number of systems that fluorescence quantum yields corrected by inner filter effects decrease steadily with dye concentration, even when no evidence on dye aggregation exists. This means that some kind of self-quenching process becomes operative as the dye concentration increases. In order to understand the nature of this quenching, we have studied the photophysical properties of Rhodamine 6G (R6G) adsorbed on microcrystalline cellulose at dye concentrations ranging from 10-8 to 10-6 mol g-1. Samples were prepared by evaporation of the solvent from suspensions of weighed masses of cellulose microparticles in ethanol containing known amounts of the dye. True reflectance and remission function spectra, corrected for the emission of the dye, and true fluorescence quantum yields (devoid of inner filter effects) were calculated using previously developed methods and models. No evidence on dye aggregation was observed, as both shape and amplitude of the absorption spectrum (λmax = 536 ± 2 nm) are independent of concentration. Emission spectra have also the same shape, except that maxima are shifted from 561 to 572 nm as concentration increases. This effect was observed for different dyes and solid supports and was attributed to interactions between excited molecules and neighbor molecules in the ground state. No excimer formation was noticed as well. The shapes of the absorption and emission spectra and the fluorescence quantum yield and decay time for the most dilute samples, ФF = 0.92 ± 0.02 and τF = 4.3 ± 0.3 ns, are quite similar to those obtained in ethanolic solution. This means that the solid surface does not interact specifically with the dye and does not enhance significantly its rigidity. Even though, also in this case a steady decrease of ФF and τF with dye concentration was observed. Results show that concentration quenching is of a dynamical nature. As dyes have restricted motions on the surface, at least during the excited state lifetime, and inner filter effects were taken into account, a non-radiative energy transfer mechanism from R6G monomers to statistical traps is proposed. A model based on Förster theory with a two-dimensional random distribution of quenchers was used to fit the experimental data. Results are consistent with the model. Other theoretical approaches are presented and the nature of the statistical traps is discussed. According to the results, the number of traps may be very high at the highest concentrations. The interaction among ground state dyes within trapping centers must be low enough to maintain undisturbed the absorption spectrum but should be relevant between excited and ground state molecules outside traps, leading to the observed concentration dependent Stokes shift.