Room temperature phosphorescence and delayed fluorescence of organic molecules trapped in silica sol-gel glasses

Room temperature phosphorescence (RTP) is obtained from a wide variety of organic molecules when trapped in silica matrices prepared by the "sol-gel" polymerization process of tetramethoxysilane. Examples include polycyclic hydrocarbons, aromatic acids, an aromatic base (quinine) and an organic dye (eosin-y). Lifetimes of up to several seconds are observed. Conditions for observing RTP vary. In some cases co-trapping of a heavy atom is needed, whereas in others RTP is observed even in wet gels without a heavy atom. A detailed study of the phenomenon was performed with 4-biphenylcarboxylic acid. It RTP and delayed fluorescence properties were studied as a function of base concentration, reaction time and temperature (Arrhenius analysis), revealing a multitude of emitting species (the acid and its anion, either adsorbed on the silica cage surface or "dissolved" in solvent-rich cages).