No abstract

No abstract

No abstract

We have measured the magnetic effects on the triplet exciton annihilation in naphthalene crystals at low and at high exciton densities. At high triplet densities the magnetic effects on delayed fluorescence intensity cannot be explained by the current theories on triplet exciton interactions in molecular solids. From the magnetic field anisotropy of the delayed fluorescence intensity at low triplet densities we conclude that in naphthalene the influence of spin-lattice relaxation on the kinematics of triplet exciton annihilation is not negligible. This implies that the exciton diffusion in this material is almost two dimensional, with the anisotropy of more than 1000 to 1.

We have solved the density matrix equation for mutual triplet annihilation in some special cases. The solution allows a simple calculation of the magnetic field dependence of the annihilation rate γs for homofusion at high and intermediate magnetic fields and for heterofusion in the high field limit. In our experiments we have measured the magnetic field anisotropy of the guest delayed fluorescence in doped anthracene and naphthalene crystals. By using the resonance condition for heterofusion, the positions of the resonance peaks have been calculated. A comparison of the measured and calculated resonance directions shows that tetracene enters the anthracene lattice substitutionally oriented, whereas the orientation of pyrene molecules in naphthalene differs slightly from the substitutional one. Furthermore, we have shown that in the latter case the host–guest annihilation gives rise almost exclusively to guest delayed fluorescence, although the excitation of the host singlet state is energetically possible.