Molecular excitons in small aggregates

The molecular exciton model, which deals with the excited state resonance interaction in weakly coupled electronic systems, is described as an interpretative tool for the study of the spectra and photochemistry of composite molecules. Under composite molecules are grouped loosely bound groups of light-absorbing units, held together by hydrogen bonds or by van der Waals forces. Another group of composite molecules included in the study consists of covalently bound light-absorbing units. A skeletal outline of the simplest quantum mechanical framework for the description of the model is presented. Dimers of various geometries, cyclical higher aggregates, linear chain polymers, helical polymers, and molecular lamellar arrays are reviewed. The exciton splitting diagrams and electric dipole selection rules are discussed quantum mechanically and by means of a transition dipole vector model. Applications to absorption and luminescence spectroscopy of molecular aggregates are cited. Photochemical sensitization and photobiological applications are suggested, and areas of new research are enumerated