On-surface synthesis of indigo-based porphyrinoids and quadruply fused porphyrinoid oligomers with aromatic eight-membered rings

Deep within the microscopic world of materials science, researchers are constantly searching for new ways to build structures atom by atom. Imagine trying to construct a complex machine not by welding large parts together, but by arranging individual atoms with the precision of a jeweler. This field, known as on-surface synthesis, allows scientists to take simple molecules, place them on a flat metal surface, and heat them until they rearrange themselves into intricate, custom-designed shapes. These new shapes are often based on porphyrins, a family of ring-shaped molecules that nature uses for essential tasks like carrying oxygen in blood or capturing sunlight in plants. Scientists are fascinated by these molecules because they can conduct electricity, interact with light in unique ways, and hold metal atoms in their centers like a cage. The challenge has always been to create versions of these rings that are larger, more connected, and possess electronic properties that nature never intended, opening doors to new types of tiny electronic devices.

A team of researchers from institutions across Europe and Asia has now taken a significant step forward in this quest by creating a novel type of molecular structure using a common blue dye called indigo. Indigo, the pigment that gives denim jeans their color, is usually just a stable, flat molecule. However, when the researchers placed indigo molecules on a gold surface along with iron atoms and heated them, something unexpected happened. Instead of simply burning away or staying as they were, the molecules began to transform. The iron atoms acted as a guide, grabbing onto specific parts of the indigo and forcing the molecules to change their shape. This process stripped away hydrogen atoms and rearranged the carbon skeleton, turning the flat dye molecules into a new, ring-shaped structure that resembles a porphyrin but is built from a different blueprint.

The researchers observed this transformation using powerful microscopes that can see individual atoms. At a moderate temperature of 573 Kelvin, the indigo molecules linked up with the iron to form long chains. As the heat was increased to 633 Kelvin, these chains rearranged themselves into distinct, rectangular units. Each of these units is a macrocycle, a large ring made of four smaller nitrogen-containing rings fused together, with an iron atom sitting right in the middle. The team confirmed the exact shape of these rings by using a specialized microscope tip that could map the bonds between atoms, revealing that the structure was a new kind of porphyrinoid. This specific arrangement, which the researchers call Fe-Por, is structurally similar to a known molecule called porphycene but is formed here through a completely different, surface-driven process.

The story did not end with these single rings. When the researchers raised the temperature even further to 663 Kelvin, the single rings began to fuse together. Two of these rectangular units joined side-by-side, and then three, forming longer chains. What made this fusion remarkable was the nature of the connection. Usually, molecules link at one or two points, but here, the units fused at four distinct locations. This created a continuous, flat ribbon where the rings shared not just six-membered loops, but also new, stable eight-membered rings in the middle of the connection. The formation of these eight-membered rings required the breaking of strong carbon-oxygen bonds, a difficult step that the iron atoms helped to facilitate. The iron acted as a template, holding the pieces in the perfect position so that they could snap together in this specific, four-point fusion.

To understand what was happening inside these new structures, the team looked at how they conducted electricity. They measured the flow of electrons through the single rings and the fused chains. The results showed that as the molecules fused together, the gap between their energy levels shrank, making the fused chains better at conducting electricity than the single rings. The measurements revealed specific energy states where electrons could move freely, suggesting that these new fused structures could serve as tiny wires or components in future electronic circuits. The iron atoms at the center of the rings also showed magnetic properties, with their magnetic spins aligning in a specific pattern along the chain, adding another layer of potential functionality.

The researchers also explored why iron was so crucial to this process. When they tried the experiment with indigo alone, the molecules simply evaporated when heated. When they used a different metal, holmium, the molecules formed different shapes entirely, proving that the size and chemical personality of the metal atom dictate the final result. The iron atoms were essential not just for holding the structure together, but for guiding the chemical reactions, stripping away oxygen and hydrogen at the right moments to allow the carbon atoms to bond in this unique, four-way fusion. This work demonstrates that by carefully choosing the starting materials and the metal catalyst, scientists can direct chemical reactions on a surface to build complex, fused architectures that were previously difficult to imagine, let alone create.