Knowledge Bank

The photoexcited EPR spectrum of single crystals of anthracene-tetracyanobenzene charge-transfer complexes was examined at temperatures between $300\circ K$ and $5\circ K$. The results show that the spectrum is due to triplet excitons at high temperatures, and to immobilized triplets at low temperatures. The exciton motion is primarily among translationally equivalent complexes, but it also occurs among translationally inequivalent complexes. The fine structure parameters, D and E, are roughly constant throughout the temperature range, and are characteristic of uncompleted anthracene. The resolved hyperfine structure observed at low temperatures is also very similar to that of anthracene alone. Very large spin polarization is observed at all temperatures. The $suggestion1$ that self-trapping of charge-transfer $excitons2$ is the cause of the observed temperature dependence of exciton motion will be discussed.