Temperature dependent excimer luminescence of naphthalenes on α-alumina

When ethyl, methoxy and methyl substituted naphthalenes are vacuum deposited on a dielectric surface and optically pumped, the luminescence that is observed is excimer fluorescence, because the morphology of the adlayer is amorphous. As the surface is heated, the thermal energy states that are available to the molecules become more broadly populated. Distinct disorder-to-order transition is observed by the onset of two characteristic spectral features, the decrease in the excimer intensity and the concomitant increase in the trap fluorescence. Since the position of the substitution in the naphthalene molecule affects the extent to which ordering occurs, the choice of adsorbates reflects the range of ordering that was observed. By depositing another adlayer of molecules that does not emit and desorbs at a lower temperature than the fluorophores, the surface dynamics was monitored by the luminescent changes as the molecules mixed at temperatures below desorption. Molecular fluorescence was observed in the bilayers that indicated the formation of clusters as the percolation occurred.