Temperature Dependence of Intersystem Crossing in Crystalline Anthracene

Blue‐light‐excited delayed fluorescence is used to study the temperature dependence of monomolecular intersystem crossing in crystalline anthracene. The associated rate constant $k$ is found to obey an equation of the form ${\mathrm{k\hspace{0.17em}=\hspace{0.17em}k}}_1{\mathrm{\hspace{0.17em}+\hspace{0.17em}k}}_2\exp \mathrm{(-\hspace{0.17em}E\hspace{0.17em}/\hspace{0.17em}kT)}$ , where $\text{E\hspace{0.17em}=\hspace{0.17em}900\hspace{0.17em}±\hspace{0.17em}100}{\mathrm{cm}}^{\text{−1}}$ and $k_2{\mathrm{\hspace{0.17em}/\hspace{0.17em}k}}_1\text{\hspace{0.17em}=\hspace{0.17em}800\hspace{0.17em}±\hspace{0.17em}200}$ . A calculation of the relevant Franck–Condon factors using the method developed by Siebrand gives good agreement with experiment. At temperatures less than 90°K, the delayed fluorescence appears to be dominated by trapping effects similar to those reported in red‐light‐excited measurements.