Sapphyrins

Sapphyrins are a family of macrocyclic compounds closely related to porphyrins. They are expanded tetrapyrrolic macrocycles in which the normally small porphyrin ring is enlarged by incorporating additional linking units, such as extra pyrrolic segments and methine bridges. This expansion gives a larger, highly conjugated π-system and typically yields a planar, highly aromatic core that can support various substitutions and metal ions in the central cavity.

Electronic properties: The extended conjugation shifts electronic transitions to longer wavelengths, producing intense absorption bands in the visible region and, in many cases, near the near-infrared. This makes sapphyrins attractive as chromophores and potential photosensitizers. Substitution on the periphery and metalation of the core provide routes to tune absorption, redox behavior, fluorescence, and two-photon properties.

Synthesis and structure: Sapphyrins are prepared by macrocyclization routes that assemble pyrrolic fragments with appropriate linkers, followed by oxidation to establish the conjugated macrocycle. Variants include different numbers of pyrrolic subunits and bridging units, giving a family with variable sizes and shapes. Central metals can be coordinated to form metallosapphyrins, altering photophysical properties.

Applications and research: Owing to their strong visible and near-infrared absorption and potential for efficient intersystem crossing, sapphyrins have been explored as photosensitizers for photodynamic therapy, in light-harvesting assemblies, and as catalysts or sensing reagents. Ongoing work investigates their aromaticity, electronic structure, and the design of new derivatives with improved stability and functionality.