Porphyrins, known as "pigments of life", are distinct π-conjugated compounds exhibiting excellent chemical stability, characteristic optical, electrical, magnetic properties, various metal coordination abilities, and ease of chemical modification. To date, numerous porphyrin derivatives and analogs (i.e., porphyrinoids) have been thus synthesized and widely used in multidisciplinary research fields transcending the biochemical-related areas. By discovering the specific porphyrin isomer "N-confused porphyrin", Dr. Hiroyuki Furuta has proposed a unique modification method, namely, the "N-confusion approach", aiming at the artificial evolution of porphyrin-related compounds. He has contributed to creating and developing N-confused porphyrinoid chemistry based on this methodology. His main achievements are summarized below.

1. Discovery of N-confused porphyrin (NCP) and related metal complexes
In the primary stage of work, Dr. Furuta investigated the anion-templated cyclization reaction of oligopyrroles to prepare porphyrin derivatives. During the period, he serendipitously found the formation of a novel porphyrin isomer in the crude mixture, which is named "N-confused porphyrin". The uniqueness of the NCPs is that two independent NH-tautomeric structures are present, and their electronic states are differed significantly by metal ions, solvents, temperature, thus serving as a chameleon-like ligand. Unlike porphyrins, the outwarding nitrogen donor site in the NCP could provide a second metal coordination site and forms multinuclear metal complexes. Furthermore, it is noteworthy that the NCP metal complexes possessing a metal-carbon bond inside the core function as highly selective and efficient molecular catalysts by taking advantage of its organometallic coordination environment. Furthermore, his effort to synthesize doubly N-confused porphyrins containing two mislinked pyrrole rings in the porphyrin framework gave rise to the construction of unique supramolecular architectures.

2. Evaluation of aromatic ring interaction and anion-π interaction
In the view of physical organic chemistry, Dr. Furuta succeeded in evaluating the enthalpy of the independent benzene trimer interaction by utilizing the molecular platform of the C₃-symmetric NCP zinc complex trimer in which one phenyl group is substituted at the meso-position selectively. He also clarified the halide anion binding capability of the NCPs at the peripheral pyrrolic amine site. Based on the careful thermodynamic studies, it is noted that the binding constant is significantly enhanced through the specific anion-π interaction assisted by the adjacent meso-pentafluorophenyl group of NCPs, which leads to the potent sensing applications

3. Transformation to the N-fused porphyrin (NFP)
Inspired by the intrinsic structural flexibility of NCPs, Dr. Furuta has developed a new porphyrinoid, "N-Fused Porphyrin (NFP)", which was transformed from the NCP mentioned above through the ring inversion of the mislinked pyrrole and fusion with the adjacent pyrrole, forming a tricyclic ring-embedded tetrapyrrolic framework. The species is fully interconvertible to NCP through the ring-opening reaction. Interestingly, NFP has a unique coplanar, 18π-conjugated structure and exhibits near-infrared absorption and emission up to a wavelength of 1000 nm. Furthermore, he has succeeded in synthesizing a doubly N-fused porphyrin showing a remarkably shifted absorption band beyond the wavelength of 1600 nm, albeit the same 18π-conjugated structure as porphyrins. In addition, he has established the coordination chemistry of NFP; it serves as an N3 donating ligand at the inner metal coordination site, accommodating various transition metal ions to afford sitting-atop complexes. The unique out-of-plane coordination mode of the NFP complexes provided new structural motifs in the catalysis, such as oxygen atom transfer reaction.

4. Coordination chemistry of expanded and contracted N-confused porphyrinoids
Direct skeletal modification of the porphyrin frameworks has been widely explored to seek hitherto-unknown properties and functions of the porphyrins. Dr. Furuta developed the ring-expanded porphyrinoids composed of five or more pyrrole rings to accomplish the multi-numbered metal coordination and the ring-contracted analogs to stabilize the metal complexes with unusual oxidation states based on the N-confusion approach. The expanded analog, such as a doubly N-confused hexaphyrin (N₂CH), embedded with inverted mislinked pyrrole ring(s), enabled the construction of the various large π-conjugated bis-metal complexes. Among the families, he found that the N₂CH metal complexes showed distinct deep near-infrared characteristics caused by the orbital interaction between metal d-electrons and porphyrin π-electrons, which contributes to new opt material applications, such as photoacoustic imaging and photothermal therapies. On the other hand, he has demonstrated that the contracted analogs, N-confused corrole (NCC), served as a trianionic organometallic ligand. In the organocopper species of NCC, an unusual formal oxidation state of copper(III) has been established in the NCC complex. In analogy with NCC, he successfully synthesized the neo-confused corrole in which pyrrole nitrogen atoms are linked at the α-position of adjacent pyrrole rings and the N, N-linked analog possessing the direct N-N bond of the mislinked pyrrole rings. This new class of porphyrinoids can create a unique electronic structure, physical properties, and reactivity.

5. Biological application of N-confused porphyrinoids
Using the N-confused porphyrinoid platform mentioned above, Dr. Furuta has synthesized various water-soluble congeners that exhibit high telomerase-inhibitory activity (NCP) and capabilities in the near-infrared optical sensing for zinc ions (N₂CH) and glutathione (NFP). These results have contributed to and impacted the development of biocompatible porphyrin-related materials.

As described above, discovering N-confused porphyrin and subsequent development of a series of N-confused porphyrinoids done by Dr. Hiroyuki Furuta opened new avenues for advancing the frontier of coordination chemistry, catalytic chemistry, and photochemistry of the porphyrin-related compounds. His achievements have been thus recognized worldwide and are worthy of the Chemical Society of Japan Award.