Photoreaction in aromatic crystals: EPR studies of anthracene doped fluorene

Light absorption in fluorene single crystals doped with anthracene induces a photoreaction. One hydrogen atom from a fluorene host gets transferred to an anthracene guest molecule over a distance of several angstroms. The resulting triplet state radical pair is investigated by EPR. Fine‐ and hyperfine structure analyses are given. The photoprocess is similar to that found earlier in fluorene crystals doped with acridine or phenazine. Where the nitrogen lone pair orbitals of acridine or phenazine were suitable acceptors for the transferred hydrogen atoms with anthracene, the hydrogen has to enter a tetrahedral sp3 configuration at the guest molecule. Somewhat higher crystal temperatures are required for comparable intensity EPR‐lines of the photoproduct.