Temperature dependence of ESR lineshapes of triplet excitons in molecular pairs

The ESR lineshape of triplet excitons moving between two differently oriented molecules of a pair is calculated. Our model hamiltonian contains the electronic interaction matrix element for the coherent exciton transport, the Zeeman energy of the triplet spin in an external magnetic field, the fine structure of the differently oriented molecules, the phonons in the crystal which are described as a heat bath, and the microscopic interaction between excitons and phonons. For the naphthalene pair the lineshapes are discussed with respect to the temperature and a parameter characterizing the strength of the interaction between excitons and photons.