Tokyo Green on cellulose: fluorescence quantum yield and energy transfer towards a non-fluorescent acceptor

SERGIO G. LOPEZ, LUIS CROVETTO, EVA M. TALAVERA, E. SAN ROMÁN

La Serena

Congreso; X Latin-American Meeting on Photochemistry and Photobiology; 2010

Tokyo Green II (TGII, 9-[1-(2-methyl-4-methoxyphenyl)]-6-hydroxy-3H-xanthen-3-one) is a recently developed fluorescein derivative with pKa = 6.2, whose fluorescence quantum yield and lifetime are high in basic solutions (F = 0.84, t = 3.7 ns) and very low in acid media (F = 0.01, t = 0.20 ns). , Due to this pH-dependence TGII is a promising on-off fluorescent probe. We report here the main photophysical parameters of TGII adsorbed on microcrystalline cellulose. In addition, we characterize the same dye as energy donor to Dabcyl (DB), a non-fluorescent dye, on the same support. TGII was adsorbed from ethanol at 2 × 10–8 to 2 × 10–6 mol g–1. Remission function spectra resemble the absorption spectrum of the acidic TGII specie in solution. As the shape of the remission function spectrum does not depend on dye loading and its amplitude grows linearly with concentration, aggregation can be disregarded up to 8 × 10–7 mol g–1. Emission spectra are independent of the excitation wavelength and match the spectrum of the acidic (neutral) form. However, the observed fluorescence quantum yield, obtained against adsorbed rhodamine 6G as the reference, is calculated as Fobs = 0.26 ± 0.1. Once corrected for inner filter effects, a constant true fluorescence quantum yield, F = 0.31 ± 0.1, is obtained up to 8 × 10–7 mol g–1. The distribution lifetimes on the cellulose surface, explored by fluorescence lifetime imaging microscopy (FLIM), peaks around 3.6 ns. Mixed TGII-DB samples were prepared in two steps: 1) TGII was adsorbed from ethanol at 1 × 10–7 mol g–1 and 2) DB was adsorbed from ethanol at 8 × 10–8 to 3 × 10–6 mol g–1. No evidence of DB aggregation is found in the whole range. Non-radiative energy transfer efficiencies, calculated by means of a previously developed model, are as high as 0.9. Fluorescence decays become shorter and non-exponential as the DB concentration increases, suggesting a Förster type energy transfer mechanism. The origin of the high F and t values found for the apparently acidic form is analyzed in terms of the dye structure and the interaction with the cellulose surface.