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)₂a and (2F)₂b. In contrast, oxidation of the corresponding Ni(II) complex 2Ni generated dimer (2Ni)₂. Subsequent demetalation resulted in the formation of bipyrrole-linked freebase dimer (2)₂, which could chelate Ni(II) and Cu(II) ions to furnish complexes (2Ni)₂ and (2Cu)₂, respectively. In comparison to the fused dimeric species (2F)₂a and (2F)₂b, the nonfused dimer (2)₂ and its complexes (2Ni)₂ and (2Cu)₂ 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.