Excimers in Crystals

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Abstract
The concept of the reaction cavity is used to indicate what types of reaction are allowed in the crystal state. Application is made to the formation of excimers in the crystal. The lattice determines whether or not the excimer is formed. There two types of excimer-forming structures, stack and pair ones; it appears that the former forms cis-excimers, the latter trans- ones. In the anthracenes, and probably generally, the excimer is an intermediate on the route to photodimer. The crystal constraints which determine the geometry of the excimer determine the stereochemistry of the product. However, the geometry of the product and of the precursor excimer may differ between bulk phase and structural defects. Reactions in chiral crystals may yield optically active products. In an example of photodimerization the chirality of the dominant product appears to be determined by the chirality of the dominant excimer precursor.