Optical limiting of transition metal-phthalocyanine complexes: A photochromic effect involving the excited state of the conjugated molecule

The nonlinear optical effect of optical limiting which is produced upon high intensity irradiation in the visible spectrum of an optical system containing phthalocyanines is a photochromic event. The photoswitching process at the basis of the optical limiting effect in phthalocyanines is the increase of the optical absorption of the conjugated complex at the wavelength of irradiation following the formation of a highly absorbing excited electronic state of phthalocyanines. The intensity of the optical fields that provoke optical limiting effect in phthalocyanines can be as high as several megawatts per square centimeter. In the present contribution the optical limiting effect generated by phthalocyanines against nanosecond laser pulses in the visible range is reported and analyzed in terms of the structural features of the photoactive molecule. Phthalocyanines with varied structural elements, e.g., different ring-substituents, coordinating central atoms and axial ligands, could be prepared through various synthetic strategies. The analysis of the optical limiting properties of the various phthalocyanines has been carried out under conditions which could allow a meaningful comparison of their actual effectiveness.