Measurements of sensitive RSA phenonema in new porphyrin dyes are presented. Both the linear and non-linear optical properties are shown to depend strongly on the molecular structure. A detailed analysis of two of the porphyrins reveals that inter-systems crossing rate and the excited state absorption profile are strongly influenced by substitution of a heavy atom into the central atom position. It is shown that by appropriate choice of wavelength values in excess of 30 can be achieved for the material parameter σex / σgr. This value compares well with previously reported results and is achieved in conjunction with excited state lifetimes on the order of 300 ns.