De novo synthesis and photophysical characterization of annulated bacteriochlorins. Mimicking and extending the properties of bacteriochlorophylls

Bacteriochlorophylls contain the bacteriochlorin chromophore and a fifth, five-membered oxopentano ring that encompasses positions 13-15 known as the "isocyclic" ring E. Such bacterio-13¹-oxophorbines have heretofore only been available in the naturally occurring compounds, and analogues bearing six-membered rings have only been available by derivatization of bacteriochlorophylls. A de novo route to synthetic bacteriochlorins, which bear a geminal dimethyl group in each pyrroline ring, has been extended to gain access to a bacterio-13¹-oxophorbine and bacteriochlorin-13,15- dicarboximides. The route relies on acid-catalyzed condensation of a dihydrodipyrrin-acetal to form the bacteriochlorin, which then is subjected to regioselective 15-bromination. Pd-mediated cyclization of the 15-bromobacteriochlorin bearing a 13-acetyl group (intramolecular α-arylation) or 13-ethoxycarbonyl group (carbamoylation and intramolecular imidation) gives the bacterio-13¹-oxophorbine or bacteriochlorin-13,15-dicarboximide, respectively. The resulting macrocycles exhibit absorption in the near-infrared spectral region (733-818 nm), which extends the spectral coverage beyond that obtained previously with synthetic bacteriochlorins that lack a fifth ring. The macrocycles also exhibit excited singlet-state lifetimes (1.9-4.6 ns) comparable to or longer than those of natural photosynthetic pigments. Density functional theory calculations predict that the bathochromically shifted absorption is primarily due to lowering of the energy of the lowest unoccupied molecular orbital. The new route complements existing semisynthetic routes and should enable fundamental spectroscopic studies and diverse photochemical applications.