Triplet-triplet annihilation and exciton-exciton interactions

A theoretical discussion is presented on the nature of direct and indirect interactions between two triplet excitons. It is shown that the direct exciton interaction through the coulombic interaction of electrons is important when there is a large change in the polarizability of the molecule during its excitation. The indirect exciton-exciton interaction is mainly due to the exciton-phonon interaction arising from the variation in the D-shift term. It is independent of temperature and spin substate. It is attractive in nature for excitons of narrow band-width, and if the exciton-acoustic phonon coupling is inducing the indirect interaction, it falls as R^-3. It is suggested that a kinematic theory of the triplet-triplet annihilation should incorporate the effects of such interactions.