Fluorescence and Triplet Quantum Yields of Arenes on Surfaces

The photophysics of pyrene, perylene, coronene, and several other polyaromatic adsorbates on silica gel and related oxide surfaces have been examined using their excited singlet and triplet states. It is found that the fluorescence and triplet quantum yields for the arenes are lower on silica gel compared to simple solution. This is related to the adsorption of the molecules on SiO2 sites where charge-transfer (CT) complexes are formed. The CT complex is identified by its spectral absorption and emission, which are red-shifted with respect to pyrene, and by a much shorter lifetime of 20 ns compared to greater than 100 ns for pyrene. The extent of the CT state can be reduced by addition of polar coadsorbates such as nitromethane. The latter adsorb strongly at the active surface sites and replace pyrene, which then adsorbs at nonactive sites.