Spectroscopic Approach to Triplet Exciton Dynamics in Anthracene

Abstract

The singlet-triplet delayed fluorescence excitation spectrum for anthracene crystals in polarized light has been investigated at room temperature. For the first peak (0-0 transition), the observed Davydov splitting $\ensuremath{\Delta}$ is 17\ifmmode\pm\else\textpm\fi{}6 ${\mathrm{cm}}^{\ensuremath{-}1}$. The half-width at quarter-maximum $\ensuremath{\Gamma}$ of this peak is 70\ifmmode\pm\else\textpm\fi{}10 ${\mathrm{cm}}^{\ensuremath{-}1}$. The value of $\ensuremath{\Delta}$ implies a value of 3.4\ifmmode\times\else\texttimes\fi{}${10}^{4}$ cm ${\mathrm{sec}}^{\ensuremath{-}1}$ for the rms exciton velocity in the a crystal direction, when reasonable assumptions are made regarding the importance of various transfer matrix elements, or, equivalently, on the exciton band shape. With $\frac{\ensuremath{\hbar}}{2\ensuremath{\Gamma}}$ interpreted as the exciton scattering time (4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ sec), the component ${D}_{\mathrm{aa}}$ of the diffusion tensor is then estimated as about 0.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$, in reasonable agreement with the directly measured value of 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{2}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. The calculation of ${D}_{\mathrm{aa}}$ from the room-temperature data does not involve the necessity of making a choice between a band or a hopping model for the motion of the excitons. However, the data make possible the assertion that any theoretical evaluations of quantities measured in this experiment would best be carried out within a hopping model.