Dimerization of Hexaphyrin with an Appendant Pyrrole Possessing a Reactive Site to Alleviate the Steric Hindrance

Oxidative dimerization of π-conjugated molecules is a straightforward approach for effectively extending π-conjugation and absorption features. However, it is challenging to construct dimeric species of bulky π-conjugated frameworks because of the steric hindrances and/or poor regioselectivity. To address these issues, a pyrrole unit has been regioselectively appended to the α position of N-confused hexaphyrin (1.1.1.1.1.0) 1 by a facile acid-catalyzed condensation reaction, leading to the formation of pyrrole-appendant 2. Subsequent oxidation of 2 yielded an inner-fused monomer 2F and two fused dimeric species, namely, (2F)a and (2F)b. In contrast, oxidation of the corresponding Ni(II) complex 2Ni generated dimer (2Ni). Subsequent demetalation resulted in the formation of bipyrrole-linked freebase dimer (2), which could chelate Ni(II) and Cu(II) ions to furnish complexes (2Ni) and (2Cu), respectively. In comparison to the fused dimeric species (2F)a and (2F)b, the nonfused dimer (2) and its complexes (2Ni) and (2Cu) exhibit diminished local aromaticity, narrowed HOMO-LUMO gaps, and a red-shifted absorption profile that extends up to 2200 nm. These findings underscore a potent strategy for creating expanded porphyrin dimers, wherein the aromaticity and near-infrared absorption can be fine-tuned by incorporating an appendant pyrrole unit.