Fluorescence self quenching and triplet state generation of dyes in constrained media

SILVIA BRASLAVSKY; HERNÁN B. RODRÍGUEZ; YAIR LITMAN; ENRIQUE SAN ROMÁN

Bad Hofgastein

Congreso; Central European Conference on Photochemistry; 2014

Dye molecules incorporated into thin films or bulk solids are used, e.g., for photosensitization, photocatalysis, solar cells and fluorescent labelling. High concentrations of dyes are desirable to maximize light absorption. However, formation of aggregates and statistical traps dissipate the excitation energy and lower the excited states population. The design of highly absorbing systems, avoiding energy dissipation, requires the knowledge of the photophysical parameters, e.g., the quantum yields. The determination of these parameters is complicated by light scattering in microheterogeneous materials. Reflectance, fluorescence and laser-induced optoacoustic spectroscopy data can be used together with theoretical models for the quantitative evaluation of inner filter effects, fluorescence and triplet formation quantum yields and energy transfer efficiencies. Studies on the photophysics of xantenic dyes (Bengal Rose and Erythrosin B) adsorbed on microcrystalline cellulose have shown that, whereas the fluorescence shows an appreciable concentration quenching, the quantum yield for triplet formation remains practically unchanged [1]. A similar phenomenon was observed for Eosine Y and Floxine B adsorbed on the same support, whereas for Safranine O a simultaneous decrease of fluorescence and triplet quantum yield was observed. A possible mechanism explaining the results takes into account the formation of charge transfer (CT) states between nearby excited singlet states, leading to a triplet state by spin flip. (S...S) + h  1(S...S)  1(S?...S?)  3(S?...S?)  3(S...S) When the redox potentials of the singlet state are not enough for the CT process (such as in Safranine), then no CT state could be formed and quenching of the triplet state will be observed. [1] E. P. Tomasini, S. E. Braslavsky, E. San Román, Photochem. Photobiol. Sci. 2012, 11, 1010-1017.