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 (λmax = 750 nm) possesses a much altered optical spectrum from that of
the starting oxime (λmax = 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 (λmax = 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 (λmax = 775 nm) and its N-oxide (λmax = 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.