Knowledge Bank

“Delayed fluorescence has been observed at $77\circ K$ from rigid glass solutions of some aromatic hydrocarbons and their halo-derivatives. The intensity of the delayed fluorescence is proportional to the square of the phosphorescence intensity. The decay of delayed fluorescence in all cases may be represented as the sum of two first-order processes; the lifetime of the longer-lived component varies between $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}/2⟷\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}/5$, that of phosphorescence while the lifetime of the shorter-lived component varies between $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}/10⟷\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}/20$ that of phosphorescence. The intensity of delayed fluorescence observed from a solution of a given compound varies as the ratio $\Phi f\Phi p2/K3\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$, where $\Phi f$ and $\Phi p$ are fluorescence and phosphorescence quantum yields, respectively and where $k\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}1$ is the emissive phosphorescence rate coanstant. Phosphorescence decay exhibits a slight non-exponentiality only in the case of those solutions where the delayed fluorescence is especially strong. Delayed fluorescence is spectrally identical, within the limits of spectrograph resolution available to us, with ordinary fluorescence. The above facts may only be interpreted by a kinetic mechanism which involves a triplet-triplet annihilation process, this mutual annihilation resulting in the eventual production of the singlet excited state of once of the partners. Some ancillary hypotheses, are required to interpret the shorter-lived component of the delayed fluorescence, and a kinetic flowsheet representative of the energy-transfer processes involved in the production of delayed fluorescence is proposed.”