Revisiting fluorescein and layered double hydroxide using a synergistic approach: A complete optical study

The luminescence of Fluorescein, an inexpensive dye, ought to be interesting for lighting application devices such as commercial LEDs. However, the molecule suffers from severe instability, making it inappropriate for such applications. As previously studied, fluorescein molecules may be hosted in a layered double hydroxide (LDH), and the role of such an inorganic material in stabilizing the dye has been underlined. However, a deep understanding of the resulting optical properties is still required. Using extremely small amounts of fluorescein, it is demonstrated here that Zn2Al cation-based LDH tethering acts as a "solid solvent" for the dye, enabling its luminescence even in powder form, whereas fluorescein molecules on their own emit no luminescence at all in the solid state. However, aggregations of the dye occur in the interlamellar space, leading to non-radiative emission. Emission and excitation spectra, as well as fluorescence decays, are studied to explain the difference between the hybrid LDH-Fluorescein material and an aqueous solution of fluorescein. Additionally, LDH platelets are found to help the dispersion of the intercalated fluorescein in silicone to obtain a homogeneous composite film, which exhibits luminescent properties. Finally, an accelerated photoaging study demonstrates that the LDH-fluorescein filler has a pro-degradant effect on polymer aging under UV radiation.