Radical pair formation from excited states in doped aromatic crystals. I. Epr studies of the guest-host system acridine-fluorene

Following light absorption in acridine doped fluorene single crystals formation of a triplet state complex (heteroexcimer) has been established earlier. More detailed EPR investigations presented here permit the clarification of the nature of this complex. Hyperfine structure has been resolved for three nuclear spins which could be assigned by selective deuteration experiments: the nitrogen and meso-proton spin located in the guest-constituent of the complex and one proton spin originating from the CH2-group of a fluorene host molecule. The complete hyperfine tensors could be evaluated. The anisotropy is found to be that of a planar aromatic CH-fragment for both observed proton spins. As a consequence of the CH-fragment assignment the central CH2-group of the fluorene constituent of the complex has to change from a sp3 to a planar sp2 configuration indicating a photoinduced radical pair formation involving hydrogen abstraction from a fluorene and hydrogen addition to an acridine molecule. The model is able to explain: (a) the high spin density located at the observed CH-fragments; (b) the geometry of the molecular partners involved in the complex, and (c) the fine structure tensor on the basis of a simple point charge dipole-dipole coupling of the two unpaired electron spins