Fused porphyrins show NIR absorption because they have larger p-conjugation system than regular porphyrins. General fused porphyrins have been synthesized by the peripheral oxidative fusion reaction of the porphyrin skeleton. On the other hand, we successfully synthesized carbazole-based porphyrin via the annulation reaction of butadiyne-bridged carbazole dimer and the subsequent oxidation of the isophlorin. The carbazole-based porphyrin shows strong NIR absorption due to the effect of the quadruple benzo-fusion.

We have synthesized several carbazole-based porphyrins, and effective substituent effect was observed at the carbazole moieties. Therefore, we then tried to oligomerize the carbazole-based porphyrin at the carbazole moieties. Four kinds of carbazole-based diporphyrins were synthesized, and their photophysical properties were investigated. As a result, thiophene-bridged diporphyrin was found to show strong intramolecular electron coupling. Furthermore, cyclic porphyrin trimer and tetramer with thiophene linkages were synthesized.

All the molecules show NIR absorption. It is worth to note that the spectra of oligomers show broad and red-shifted NIR band as compared to the monomer, which indicates strong intramolecular electronic interaction.