Exciton Interactions

Abstract

The F€ orster theory we considered in the last chapter applies to molecules that are far enough apart so that intermolecular interactions are very weak. Jumping of excitations from one molecule to the other is slow relative to the vibrational relaxation and dephasing that determine the homogeneous widths of the absorption bands, and it has little effect on the absorption spectra of the molecules. If the energy donor and acceptor are distinguishable we could examine the overall absorption or stimulated-emission spectrum of the system and, at least in principle, determine which molecule is excited at any given time. But suppose we move the molecules together so that the time required for energy to hop from one to the other becomes shorter and shorter. At some point, it will be impossible to say which molecule is excited. In this situation, we might expect that resonance between multiple excited states could cause the absorption spectrum of an oligomer to differ from the spectra of the individual molecules, and indeed this turns out to be the case.The term exciton means an excitation that is delocalized over more than one molecule, or that moves rapidly from molecule to molecule. Exciton interactions, the intermolecular interactions that cause the excitation to spread over several molecules, are physically just the same as the weak interactions that result in stochastic jumping of excitations by resonance energy transfer; they are just stronger because the molecules are closer together or have larger transition dipoles. As a result, the absorption, fluorescence, and circular dichroism of the system can be significantly different from those of the individual molecules. But we are not yet in the region where overlap of the molecular orbitals allows new bonds to form and the definition of the molecules themselves becomes blurred.Our discussion will concern mainly what are called Frenkel excitons, in which an electron that has been excited to a normally empty molecular orbital remains associated with a vacancy or "hole" in a normally filled orbital as they migrate from