Compact Self-Assembled Porphyrin Macrocycle: Synthesis, Cooperative Enhancement, and Ultrafast Response

In this study, we present the synthesis of a carbazole-bridged porphyrin dimer system that possesses a 90° change in orientation between porphyrin units and the single-product four dimer macrocycle, which is formed upon self-assembly of the dimer via imidazolyl-to-zinc complementary coordination. Subsequent characterization of the two-photon absorption and ultrafast emission lifetimes of these systems indicates a very strong coupling between constituent dimers in the assembled macrocycle structure. These interactions lead to a red-shifted two-photon response and a full order of magnitude increase in the two-photon absorption (TPA) cross section per building block with respect to the lone dimer. Excitonic coupling through the slipped cofacial arrangement created by imidazolyl-to-zinc interaction has been shown to play a critical role in the observed TPA enhancement. This points to the use of our small light-harvesting mimic for nonlinear optical applications in which aggregation effects have often stymied development.