Pyrrole βC‐B‐N Fused Porphyrins: Molecular Structures and Opto‐Electrochemical Studies

Herein, we report the design, synthesis, structure, and electrochemical study of doubly βC‐B‐N fused Ni(II) porphyrins (1‐trans, 1‐cis, 2‐trans, and 2‐cis). These compounds were synthesized from A2B2 type dipyridyl Ni(II) porphyrins (Ar = Ph for 1a; Ar = C6F5 for 2a) via Lewis base‐directed electrophilic aromatic borylation reactions. The solution state structures of these compounds were established using 1H NMR, 11B NMR, 1H‐1H COSY, 1H‐13C HSQC, and 19F‐13C HSQC NMR techniques. Single crystal X‐ray analysis revealed that 1‐trans, 1‐cis, and 2‐trans adopt ruffled conformations, with alternate meso‐carbons on the opposite sides of the mean porphyrin plane. The Soret bands in the absorption spectra of the B‐N fused molecules are ~40 nm redshifted compared to unfused Ni(II) porphyrin precursors. The B‐N fusion diminishes the redox potential of fused porphyrins. Although 1‐trans and 1‐cis, show four oxidation processes, 2‐trans and 2‐cis showed only three oxidation processes. DFT studies revealed that the tetrahedral geometry of the boron induces a twist in the π‐conjugation, which destabilizes the HOMO and stabilizes the LUMO in 1‐trans, 1‐cis, 2‐trans, and 2‐cis.