Knowledge Bank

A method has been developed using the intensities of delayed fluorescence and of phosphorescence of coronene which permits calculation of the rate constant for crossing to $S1$ from isoenergetic triplet levels $(k6\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>})$ as well as the $S1-T1$ gap. For these calculations, measurements of the fluorescence yield, and the temperature dependence of both the phosphorescence lifetime and the ratio of delayed fluorescence to phosphorescence were made using polymethylmethaerylate specimens containing coronene-$h12$ and coronene-$d12$. The calculated $S1-T1$ gap of $3930cm-1$ agrees with the value obtained from the 0.0 bands of the fluorescence and phosphorescence spectra. Values of $k61$ of coronene-$d12$ and coronene-$d12$ are equal to within the experimental error and are significantly smaller than rate constant $k3$ for the reverse intersystem crossing from $S1$ to $T1$. The delayed fluorescence accounts for only a small fraction of the total radiationless deactivation of triplet coronene. The temperature-dependent part of the rate constant for deactivation of triplet coronene is larger for ordinary coronene than for deuterated coronene and is nearly the same for coronene-$h12$ in both polymethylmethaerylate and an epoxy resin, DER-332. These results are discussed in terms of possible mechanisms for temperature dependent deactivation of triplet coronene.