Conjugated porphyrin arrays that possess delocalised electronic networks have, for the most part, been assembled by using alkene or alkyne type bridging units or by directly connecting individual porphyrin chromophores with multiple bonds to form fused porphyrin arrays.

Various aromatic segments have been fused onto the porphyrin periphery to create porphyrinoids that have expanded π-conjugated networks and thus exhibit red-shifted absorption spectra. Fused coplanar porphyrin oligomers, represented by meso–meso, β–β, β–β triply linked porphyrin arrays (porphyrin tapes), are endued with more red-shifted absorption spectra and better nonlinear optical properties. These fused porphyrinoids have emerged as promising near-infrared (NIR) absorbing dyes, pointing to future applications such as conducting wire, NIR emitter, photovoltaics, and nonlinear optical materials. Developments of the fused porphyrinoid chemistry are reviewed in this feature article with focus on the synthesis, the relationships between their absorption spectra and molecular structures, and the applications in materials science.

After a brief survey of our efforts in the development of novel porphyrinoids that include meso–meso-linked porphyrin arrays, meso-aryl expanded porphyrins, and transition-metal-catalyzed functionalizations of porphyrins, a particular focus in this account is placed on the chemistry of subporphyrins that has been explored in our group. Subporphyrin is a legitimate ring-contracted porphyrin consisting of three pyrrolic subunits domed in a C₃ symmetric bowl shape. While subporphyrins are simple and small macrocycles and possess a key position in porphyrin chemistry, they had been elusive until our first synthesis of tribenzosubporphines in 2006. Shortly after, synthetic protocols of meso-aryl-substituted subporphyrins were developed to produce various subporphyrins with versatile electronic properties that can be widely tuned by meso-aryl substituents. Raney nickel reduction was used to prepare meso-alkyl-substituted subporphyrins from meso-thienyl-substituted subporphyrins. Subchlorins and subbacteriochlorins were prepared respectively by the reduction of subporphyrins with p-tosylhydrazide and Raney nickel. While subporphyrins and subchlorins share conjugated 14π-electronic circuits, subbacteriochlorins have a rare [13]diazaannulene circuit maintained through the lone-pair electrons of the nitrogen atom. The aromaticity decreases in the order of subporphyrin > subchlorin > subbacteriochlorin, as indicated from ¹H- and ¹¹B NMR spectra and nuclear independent chemical shift (NICS) calculations. Despite these progresses, the chemistry of subporphyrins is still in the infant stage with many untouched aspects and further improvements in synthetic yields are highly desirable for the developments of the chemistry of subporphyrins as well as their applications in diverse fields.