Fluorescence enhancement of a xanthene derivative upon adsorption on microcrystalline cellulose

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

Granada

Conferencia; International Soft Matter Conference 2010; 2010

Tokyo Green II (TG II: (9-[1-(2-methyl-4-metoxiphenyl)]-6-hydroxy-3H-xanthen-3-one) is a recently developed xanthene derivative [1]. Its photophysics in aqueous solution has been recently studied [2], and its characteristic behaviour as on-off fluorescent probe at physiological pH was confirmed (monoanion Ff = 0.86, and neutral Ff = 0.01). Herein we report on the photophysics of TG II adsorbed onto microcrystalline cellulose, both alone and coadsorbed with Dabcyl (DB), a non-fluorescent dye acting as acceptor for the excitation energy of TG II. TG II was adsorbed from ethanol at concentrations ranging from 0.03 to 0.86 mmol g-1. On cellulose, the absorption and the emission spectrum are similar to those found for the neutral form in water but the fluorescence quantum yield (Ff = 0.33±0.1) is more than thirty times higher than in aqueous solution. The absorption maximum increases linearly with dye loading, and the shape of the spectrum does not depend on concentration. Hence, the presence of dye aggregates can be disregarded. Both fluorescence spectrum and quantum yield are independent of the excitation wavelength. The fluorescence enhancement can be ascribed to several causes, in particular to the low polarity of cellulose, which may be responsible for a decrease in the rate of photoinduced intramolecular charge transfer leading to non-radiative deactivation[1]. Coadsorbed samples were prepared in two stages: First, TG II was adsorbed from ethanol and then DB was coadsorbed from the same solvent. TG II inhibits the aggregation of DB in the whole range. A similar effect has already been observed for other dye pairs [3,4]. Increasing DB loading, the fluorescence decay shortens due to the occurrence of non-radiative energy transfer from TG II to DB. FLIM images show that the lifetime distribution maximum shifts to shorter values increasing DB concentration, in accordance with results obtained by time resolved fluorimetry. Non-radiative energy transfer efficiencies were calculated by means of a model reported elsewhere[5]. For the most concentrated samples, non-radiative (Förster) energy transfer efficiencies are higher than 60% in spite of the low donor fluorescence quantum yield. [1] Y. Urano, M. Kamiya, K. Kanda, T. Ueno, K. Hirose, T. Nagano. J. Am. Chem. Soc. 127 (2005) 4888. [2] J. M. Paredes; L. Crovetto, R. Rios, A. Orte, J. M. Alvarez-Pez, E. M. Talavera. Phys. Chem. Chem. Phys. 11, (2009), 5400. [3 ] H. B. Rodríguez, A. Iriel, E. San Román. Photochem. Photobiol. 82 (2006) 200. [4] H. B. Rodríguez, E. San Román. Photochem. Photobiol. 83 (2007) 547. [5] S. G. López, G. Worringer, H. B. Rodríguez, E. San Román. Phys. Chem. Chem. Phys. 12 (2010) 2246.