Mono- and Bisquinoline-Annulated Porphyrins from Porphyrin β,β′‑Dione Oximes

An acid-induced reaction of meso-tetraphenyl-2-hydroxyimino-3-oxoporphyrin leads, with concomitant loss of water, to a formal electrophilic aromatic substitution of the ortho-position of the phenyl group adjacent to the oxime, forming a quinoline moiety. Owing in part to the presence of a π-extended chromophore, the resulting meso-triphenylmonoquinoline-annulated porphyrin (λ_max = 750 nm) possesses a much altered optical spectrum from that of the starting oxime (λ_max = 667 nm). An oxidative DDQ-induced ring-closure process is also possible, generating the corresponding meso-triphenylmonoquinoline-annulated porphyrin quinoline N-oxide, possessing a slightly shifted and sharpened UV–vis spectrum (λ_max = 737 nm). The connectivity of the chromophores was conclusively shown by NMR spectroscopy. Both ketone functionalities in meso-tetraphenyl-2,3-dioxoporphyrin can be converted, via the oxime and using the acid- or oxidant-induced reaction pathways, either in one step or in a stepwise fashion, to bisquinoline-annulated porphyrin (λ_max = 775 nm) and its N-oxide (λ_max = 779 nm), respectively. This process is complementary to a previously established pathway toward bisquinoline-annulated porphyrins. Their zinc­(II), nickel­(II), and palladium­(II) complexes are also described. Several examples of the quinoline-annulated porphyrins were crystallographically characterized, proving their connectivity and showing their conformations that are extremely distorted from planarity. The work presents a full account on the synthesis, structure, and spectroscopic properties of these classes of NIR-absorbing dyes.