RYDMR spectra for reactions of triplet−triplet annihilation and photogeneration of triplet pairs in molecular crystals

Abstract

The contribution to the RYDMR spectrum from an elementary event of collision of two triplet excitons during their mutual annihilation is calculated, and the reverse process of photogeneration of a pair of triplet excitons in a molecular crystal are considered. The interaction between the excitons in the cell is assumed weak in comparison with the Zeeman splitting and the fine-structure parameters for the triplet state of each of the molecules. For both the annihilation of excitons and the photogeneration of a pair, the RYDMR signal from a single-crystal sample disappears at certain orientations of the crystal relative to the external magnetic field. This effect was previously experimentally observed for the anthracene−tetracyanobenzene crystal and was explained in the case of stationary optical excitation of the molecule. The results obtained in this paper make it possible to consider an arbitrary, not only stationary, mode of irradiation sample. An interval of angles between the static magnetic field and the crystallographic axes of the crystal within which the lines in the spectrum can disappear is determined. The possibility of using the RYDMR method for studying nanosized objects, for which additional peaks in the RYDMR spectrum may arise, is briefly discussed.