β-Functionalized trans -A2B2 push–pull tetrabenzoporphyrins
A series of β-functionalized trans-A2B2 tetrabenzoporphyrins have been successfully designed and synthesized through newly developed regioselective bromination chemistry of porphyrins and a Pd(0) catalyzed three-step-one pot reaction. These benzoporphyrins exhibit unique patterns of UV-Vis absorptions and emissions. Tetrabenzoporphyrins possess a unique combination of photophysical, optoelectronic, and physicochemical properties due to their largely extended π-conjugation and associated dynamics of ring deformation which make them distinctive from their parent porphyrins. As compared with their parent porphyrins, tetrabenzoporphyrins often display significantly red-shifted absorption bands, a much-desired feature for various applications ranging from biomedicine to organic electronics. However, due to limited synthetic methods, only a small number of simple symmetrical tetrabenzoporphyrins have been available for study. Unsymmetrical tetrabenzoporphyrins, in particular those functionalized at the porphyrin β-positions, are very difficult to access. A2B2 porphyrins, on the other hand, represent an attractive class of materials for non-linear optical applications as well as for other applications such as those involving dye-sensitized solar cells. meso-Functionalized push–pull trans-A2B2 porphyrins have previously been made available through the Suzuki coupling reaction, the Heck reaction and the Sonogashira coupling reaction of 5,10-dibromoporphyrin with the corresponding alkyl boronic acids, olefins and alkynes, respectively and have demonstrated interesting non-linear optical properties. In contrast, β-functionalized push–pull A2B2 porphyrins of the type shown in Fig. 1 have until now remained unknown. Furthermore, we present the synthesis and characterization for the first examples of β-functionalized A2B2 push–pull tetrabenzoporphyrins.