Picosecond Z-scan and optical Kerr gate in metalloporphyrin derivatives: origin of the nonlinearity and its use as an optical probe to determine pK

In the present work, we report on Z-scan measurements of the nonlinear refractive index, of two water soluble porphyrins (PPh): a negatively charged meso-tetrakis(p-sulfonatophenyl)porphyrin and a positively charged meso-tetrakis(4-N-methylpyridiniumyl)porphyrin in their free-base forms and their Fe(III) and Mn(III) complexes, and the dependence of these characteristics on pH. Nonlinear absorption coefficients, proportional to the imaginary part of the nonlinear susceptibility, have also been measured. The data, together with time resolved measurements in the picosecond regime, led to the understanding of the origin of optical nonlinearities in the central atom out-of-plane porphyrin complexes. Furthermore, using the Z-scan technique, we were able to devise a method to determine pK points for these materials, as the pH was varied. The determination of pK is usually performed with linear optical absorption or EPR techniques.