The donor acceptor pair in the external periodic electric field is under studying. The molecular system is excited by the short impulse of the polarized light, and the electronic excitation initially localized on the donor and interacts only with full symmetrical vibration of the ligand environments, thus the vibrational subsystem forms the package in the form of the coherent state. Donor and acceptor molecules are assumed as identical type of the molecules. The time dependence of the depolarization of the fluorescence is numerically calculated on the basis of the semi phenomenological model. Numerical estimates are given for cases of the strong and weak electron vibrational interaction. Transfer of the excitation in the system is defined by the non linear properties of the system, especially in the case of strong electron vibrational inter action. The depolarization of the fluorescence is consequence of the such transport and contains the interference addends which conduct to the quantum beats in the time dependence.