The magnetic field spin effect on photoluminescence of charge-transfer single crystal caused by annihilation of triplet excitons and controlled by Zeeman and zero-field splitting (ZFS) interactions is investigated Phase transition related to reorientation of dibromo-anthracene molecules in the (x-y) plane of ZFS tensor of anthracene molecule is observed at T=205 K. When the temperature approaches the phase transition point at orientation of ZFS axis z parallel to the magnetic field the rate of spin-lattice relaxation dramatically increases and hence the magnitude of the magnetic field spin effect sharply drops down.