Core-modified analogues of expanded porphyrins containing rigid β,β′-linked o-phenylene bridges

Abstract

Structural modifications that lead to the creation of π-extended aromatic macrocyles involving a heterocyclic ring other than pyrrole and rigid β-β′ linkages have not been well studied up to date. The rigidity caused by the conformational restriction would change the spectroscopic properties of the system as compared with those of the normal congeners. With these considerations, we have synthesized and fully characterized π-extended, core modified expanded porphyrins bearing rigid bipyrrole units. Core-modified naphthorubyrins were synthesized by the Lewis acid-catalyzed condensation of naphthobipyrrole with thiophene/furan diols, whereas naphthosapphyrins were obtained by reacting 2,9-diformyl-naphthobipyrrole with 16-thia/oxatripyrranes under mild reaction conditions. The core-modified analogues of both naphthorubyrin and naphthosapphyrin displayed the aromatic character. The dithiarubyrin analogues showed a lack of conformational change as expected and displayed well-resolved 1H NMR resonances at room temperature. On the other hand, the oxasapphyrin analogue adopts a furan-inverted geometry, and the ring inversion is independent of the protonation state. The oxanaphthosapphyrin also exhibited a weak fluorescence emission at 613 nm. Rigid to the core: Core-modified analogues of expanded porphyrins bearing one or two rigid bipyrrole units were synthesized and spectroscopically characterized. The dithianaphthorubyrins showed a lack of conformational change due to the rigidity induced, whereas the monooxasapphyrin showed a protonation-independent inversion of the furan ring. Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.