Systematic studies of the nonlinear optical properties of metallo-organic materials have led to the development of promising new phthalocyanine materials for optical limiting. Several heavy metal substituted phthalocyanines exhibit a strong nonlinear absorption that is useful for optical limiters in the visible. In fast optical systems, other mechanisms, such as the thermal refraction, contribute to the limiting. The spectral window for limiting can by modified by altering the molecular structure.