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Summary form only given. Reverse saturable absorption/thermal hybrid optical limiter materials can be used to make devices where the energy transmitted through the limiter is kept to approximately the MPE (Maximum Permissible Exposure according to the ANSI standard) over a range of input energies. However, to achieve this level of protection requires an optical system where there is an intermediate focus with an optical gain on the order of 10/sup 6/ to 10/sup 7/. It is possible to reduce the threshold and increase the opacity at high fluence by using nonlinear materials at a high concentration within a Rayleigh range of the focus. However, at higher concentrations, intermolecular interactions often play a substantial role in determining the nonlinear optical properties of the material. This paper reports a study of the structure, the nonlinear optics and the underlying photophysics of molecular aggregates of some organic materials that are known to be good limiter materials. Specifically, we compare intermolecular interactions in tetrakis-cumylphenoxy phthalocyanines with those in the axially substituted chloro-indium phthalocyanines and naphthalocyanines. The supramolecular structures in these materials differ and this is reflected in both the threshold and the high fluence optical properties in these materials.