The design and synthesis of new molecules with large macroscopic optical nonlinearities represent an active research area in modern chemistry and material science. Such materials have potential applications in optoelectronic devices, two photon photodynamic therapy and holographic data storage systems. Currently, inorganic crystals such as LiNbO3, KH2PO4, KTiPO4 and semiconductor materials like GaAs and InSb are being used for such applications. However, to overcome the problems of low response time, magnitude of optical nonlinearity and molecular tailorability in Inorganic materials, search is on for organic NLO materials. Organic molecules with large conjugated π-electrons exhibit bulk optical nonlinearity and are easy to process into thinfilms. In this talk, an overview of the design criteria for third order nonlinear response using organic molecules based on expanded porphyrin systems will be discussed. Specifically, the talk will address the following; (a) the design and synthesis of specific core-modified expanded porphyrins and fused porphyrins containing 22π, 26 π, 30 π, 34 π electrons in conjugation, (b) the third order nonlinear optical response measured by Two Photon Absorption (TPA) technique and (c) preliminary structure-function correlation.