Energy transfer and FLIM experiments involving rhodamines and

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

Cubatao, Sao Paulo, Brasil

Congreso; IX Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2008

Centro de Capacitación e Investigación en Medio Ambiente. Universidad de Sao Paulo.

Energy transfer and FLIM experiments involving rhodamines and coadsorbed energy acceptors on microcrystalline cellulose Sergio López,a Luis Crovetto,b Eva M. Talavera,b Enrique San Románaa Luis Crovetto,b Eva M. Talavera,b Enrique San Romána aINQUIMAE / DQIAyQF, Facultad de Ciencias Exactas y Naturales, UBA, Ciudad Universitaria, Pab. II, C1428EHA Buenos Aires, Argentina, e-mail: sgl@qi.fcen.uba.ar C1428EHA Buenos Aires, Argentina, e-mail: sgl@qi.fcen.uba.ar C1428EHA Buenos Aires, Argentina, e-mail: sgl@qi.fcen.uba.ar C1428EHA Buenos Aires, Argentina, e-mail: sgl@qi.fcen.uba.ar C1428EHA Buenos Aires, Argentina, e-mail: sgl@qi.fcen.uba.ar bDepartamento de Química Física. Campus de Cartuja. Universidad de Granada. Granada 18071, España.Departamento de Química Física. Campus de Cartuja. Universidad de Granada. Granada 18071, España. Rhodamines are excellent energy donors when attached or adsorbed on microparticles jointly with suitable acceptors because they have large fluorescence quantum yields, reasonably high singlet lifetimes and a small aggregation tendency.[1-3] In this work, we report on a) the photophysics of rhodamine 6G (R6G) coadsorbed with Rose Bengal (RB) on microcrystalline cellulose particles and b) on additional measurements performed on a previously studied system, rhodamine 101 (R101) chemically linked to the same support, to which methylene blue (MB) is adsorbed.2 A completely different behavior was observed for both systems. R6G was adsorbed from ethanol at around 3 × 10-7 mol g-1 and, at a second stage, RB was coadsorbed from the same solvent at concentrations ranging from 2 × 10-8 to 4 × 10-7 mol g-1. As a first approximation, remission function spectra may be decomposed into pure R6G and RB components showing that dye aggregation is almost negligible, in contrast to the large aggregation tendency of RB, when adsorbed alone on cellulose. Observed fluorescence quantum yields, in the order of 0.70 ± 0.05 for pure R6G, range from 0.55 to 0.35 as the concentration of RB increases, showing that strong fluorescence quenching takes place. However, average fluorescence lifetimes change only slightly, from 3.3 ns for pure R6G to somewhat less than 3 ns as the RB concentration increases. Fluorescence decays, monoexponential for R6G, become complex  - a shorter lifetime component becomes noticeable - but changes are not as large as predicted by Förster theory. Consistently, once corrected by reabsorption, fluorescence spectra are only slightly dependent on RB concentration. Results can be rationalized by a static quenching mechanism, whereas R6G fluorescence is lowered by RB, essentially without excitation of the acceptor. A model yielding the dependence of the unquenched fraction of R6G excited states, 1 - q, as a function of the RB concentration, C, yields the results shown in the figure. The unquenched fraction attains a limiting value of about 75 %. It is postulated that a fraction of positively charged R6G is complexed with negatively charged RB but most R6G molecules cannot be accessed by the quencher, pointing to an inhomogeneous distribution of dye molecules into the cellulose matrix. These results are quite different from those observed for attached R101 and MB, both positively charged, where a distinct change in the lifetime distribution was found. In this case, Förster energy transfer, suggested as the quenching mechanism,2 is confirmed by the present study.In this work, we report on a) the photophysics of rhodamine 6G (R6G) coadsorbed with Rose Bengal (RB) on microcrystalline cellulose particles and b) on additional measurements performed on a previously studied system, rhodamine 101 (R101) chemically linked to the same support, to which methylene blue (MB) is adsorbed.2 A completely different behavior was observed for both systems. R6G was adsorbed from ethanol at around 3 × 10-7  mol g-1 and, at a second stage, RB was coadsorbed from the same solvent at concentrations ranging from 2 × 10-8 to 4 × 10-7 mol g-1. As a first approximation, remission function spectra may be decomposed into pure R6G and RB components showing that dye aggregation is almost negligible, in contrast to the large aggregation tendency of RB, when adsorbed alone on cellulose. Observed fluorescence quantum yields, in the order of 0.70 ± 0.05 for pure R6G, range from 0.55 to 0.35 as the concentration of RB increases, showing that strong fluorescence quenching takes place. However, average fluorescence lifetimes change only slightly, from 3.3 ns for pure R6G to somewhat less than 3 ns as the RB concentration increases. Fluorescence decays, monoexponential for R6G, become complex  - a shorter lifetime component becomes noticeable - but changes are not as large as predicted by Förster theory. Consistently, once corrected by reabsorption, fluorescence spectra are only slightly dependent on RB concentration. Results can be rationalized by a static quenching mechanism, whereas R6G fluorescence is lowered by RB, essentially without excitation of the acceptor. A model yielding the dependence of the unquenched fraction of R6G excited states, 1 - q, as a function of the RB concentration, C, yields the results shown in the figure. The unquenched fraction attains a limiting value of about 75 %. It is postulated that a fraction of positively charged R6G is complexed with negatively charged RB but most R6G molecules cannot be accessed by the quencher, pointing to an inhomogeneous distribution of dye molecules into the cellulose matrix. These results are quite different from those observed for attached R101 and MB, both positively charged, where a distinct change in the lifetime distribution was found. In this case, Förster energy transfer, suggested as the quenching mechanism,2 is confirmed by the present study. This work was supported by grant A/01270607 from the Agencia Española de Cooperación Internacional (AECI). 1 H.B. Rodríguez, A. Iriel, E. San Román, Photochem. Photobiol. 82 (2006) 200–207H.B. Rodríguez, A. Iriel, E. San Román, Photochem. Photobiol. 82 (2006) 200–207 2 H.B. Rodríguez, E. San Román, Photochem. Photobiol. 83 (2007) 547–555H.B. Rodríguez, E. San Román, Photochem. Photobiol. 83 (2007) 547–555 3 H.B. Rodríguez, E.San Román, Annals New York Acad. Sci. 1130 (2008) 247-252H.B. Rodríguez, E.San Román, Annals New York Acad. Sci. 1130 (2008) 247-252