Phthalocyanines and related compounds as switchable materials upon strong irradiation: the molecular engineering behind the optical limiting effect

The possibility of varying the optical properties of materials in a controlled fashion constitutes an appealing objective, which represents the necessary starting point for the development of those advanced technologies designated to exploit the energy of the luminous radiation into a desirable way. Usually, in electrochromic, thermochromic or photochromic processes, the reversible switching of the material absorbance or reflectance in the UV–vis–NIR spectrum is accompanied by reversible variations of the material chemical composition. This is followed by mostly redox, dimerization or isomerization reactions which are induced by temperature changes or slowly and rapidly variable electric fields. Other means of varying the optical properties of materials are offered by nonlinear optical (NLO) processes in which the intensity of the light modifies absorptive and refractive properties of the irradiated material within the limits of its photochemical stability (photochromic effect). Among various nonlinear optical phenomena, the optical limiting (OL) effect represents an important one for its possible exploitation in the protection of light-sensitive elements which must operate below a safety threshold of light intensity. In the present contribution, the optical limiting effect generated by different classes of phthalocyanine-based materials is described and analyzed in terms of those molecular features which are associated with the better optical limiting performances.