Photophysics of Auramine O adsorbed on solid clays

The dye loading effect on the photophysical behavior of Auramine O adsorbed onto solid clays was studied. When the dye concentration is increased, solid samples of Auramine O incorporated in SYn-1, SAz-1 and SWy-1 clays show an enhancement of the β-band in the UV-vis-DR spectra and the band at 450 nm shifts to the blue. This behavior can be attributed to the formation of H-type dye aggregates. For SYn-1 and SAz-1 clays, which show higher charge density, the formation of H-aggregates of the dye is favored. The fluorescence intensity and lifetime values of AuO decrease with the increasing of dye loading in these clays, since H aggregates do not exhibit fluorescence. The basal spacing of SAz-1 and SYn-1 containing 5% of AuO remains the same as that for pure SAz-1 and SYn-1. The adsorption of the dye predominantly occurs on the external surface of the SAz-1 and SYn-1 clays. On the other hand, for SWy-1 clay, UV-vis results suggest the presence of H- and J- aggregates. The fluorescence emission and lifetimes increase with the AuO concentration. XRD measurements confirm the penetration of the Auramine O into interlayer regions of the SWy-1 clay. When the Auramine is in the interlamellar regions of clay, the rotation of its phenyl rings is restricted, diminishing the internal conversion rate, therefore increasing the emission. The adsorption of the dye occurs on the external surface and in the interlamellar layers of SWy-1.