Knowledge Bank

At low temperatures in weakly interacting solids, many molecules exhibit solvent induced phosphorescence. The intensity of phosphorescence and the ratio of fluorescence to phosphorescence, as well as the ratio of radiative to radiationless decay, change with temperature. This effect is observed even at low temperatures where delayed thermal fluorescence does not occur. We will discuss cases where the temperature dependence is due to intramolecular energy transfer due to changes in intersystem crossing $S1$ to $T1$ and $T1$ to $Sa$. We have measured the temperature dependence of fluorescence yield, phosphorescence yield and phosphorescence lifetime of $SO2$, thiazole, naphthalene, naphthalene-$d8$ and phenanthrene in solid rare gases, solid $SF4$ and solid samples of hydrocarbons, perdeutero carbons and perfluoro carbons in the temperature range of $4\circ K$ to 0.7 of the solvent melting point.