Excitonic processes in molecular crystalline materials

This article provides an overview of the basic aspects of the structure and dynamics of excitons in molecular crystals that give rise to their unique spectroscopic behavior. The two different types of optically accessible excitons, charge-transfer and Frenkel, are described and their different properties discussed. Particular attention is paid to the spin properties of Frenkel excitons (i.e., singlet and triplet) and also to their coupling to intramolecular vibrations. Experimental challenges in the study of molecular crystal optical properties are also reviewed, including their high optical density, complex refractive index behavior, and issues with sample crystallinity and chemical purity. Once created, excitons in molecular crystals can exhibit interesting dynamical behavior, including diffusion over large length scales and ionization into electron hole pairs. Exciton-exciton interactions are also important, ranging from fusion or annihilation (two excitons combine into one exciton) to fission (one exciton splits into two excitons). The long-range diffusion and exciton fission effects have particular relevance for the design of organic photovoltaic materials.