Various strategies for highly-efficient two-photon absorption in porphyrin arrays

In this review article, we have described various strategies to increase the efficiency of two-photon absorption process of porphyrin related molecular systems. While 18-π electronic aromatic porphyrin monomers exhibit relatively small two-photon absorption cross-section values, the perturbation of electronic nature of porphyrins by substituting electron donating-accepting moieties, π-conjugated peripheral susbtituents, linking the porphyrins in π-conjugated manner using π-bond linkages, increasing the electronic coupling by controlling the dihedral angles between the neighboring porphyrin planes, and forming completely fused structures between the adjacent porphyrins leads to the significant enhancement in the two-photon absorption cross-section values. We have demonstrated that the charge transfer character as well as the elongation of π-conjugatin pathway in a unidirectional way is the key factor to increase the overall efficiency of the two-photon absorption processes. At the same time, it is equally important to maintain the rigid scaffold structures of porphyrin assemblies to ensure the efficient π-conjugation pathway throughout the whole molecular framework. In this regard, this review will be a guideline for future directions in the investigations of the two-photon absorption properties of porphyrin related molecular systems and their practical applications.