Electronic Energy Transfer in Molecular Crystals

In the last ten years, a number of reviews of properties of molecular crystals have appeared in this series (1, 2, 3). The spectral properties of insulating molecular crystals which are often described within the framework of the Frenkel theory of excitons (4, 5) have been reviewed by Hochstrasser (I) and by Robinson (2). The properties of charge transfer crystals have been reviewed recently by SODS (3). In addition, the review by El-Sayed (6) on double resonance techniques applied to triplet states of organic molecules concerned itself with organic molecular crystals. In the present review, I focus on a different aspect of molecular crystals: the transport of electronic excitation from molecule to molecule. This process, and its effect on optical spectra, electron paramagnetic resonance spectra, and fluorescence, has been of great interest in the last few years; I present a critical review of the theoretical developments in this field. Specific attention is paid to the theory of exciton transport in molecular crystals, the theory of exciton-molecular vibration and exciton-phonon interactions, and finally, to a quite different topic, the theory of excitation transport across surfaces. Those works dealing with the optical properties of molecular crystals within the coupled oscillator model are not dealt with. It will be assumed that the interaction of light with the system is weak enough so that retardation effects and the formation of polariton states may be neglected. Only work dealing with excitation transport is dealt with herein. We begin by discussing the simplest type of system in which excitation transfer takes p lace in order to define the terms necessary to treat excitation transfer in more complex systems.