De novo synthesis and properties of analogues of the self-assembling chlorosomal bacteriochlorophylls

Natural photosynthetic pigments bacteriochlorophylls c, d and e in green bacteria undergo self-assembly to create an organized antenna system known as the chlorosome, which collects photons and funnels the resulting excitation energy toward the reaction centers. Mimicry of chlorosome function is a central problem in supramolecular chemistry and artificial photosynthesis, and may have relevance for the design of photosynthesis-inspired solar cells. The main challenge in preparing artificial chlorosomes remains the synthesis of the appropriate pigment (chlorin) equipped with a set of functional groups suitable to direct the assembly and assure efficient energy transfer. Prior approaches have entailed derivatization of porphyrins or semisynthesis beginning with chlorophylls. This paper reports a third approach, the de novo synthesis of macrocycles that contain the same hydrocarbon skeleton as chlorosomal bacteriochlorophylls. The synthesis here of Zn(ii) 3-(1-hydroxyethyl)-10-aryl- 13¹-oxophorbines (the aryl group consists of phenyl, mesityl, or pentafluorophenyl) entails selective bromination of a 3,13-diacetyl-10- arylchlorin, palladium-catalyzed 13¹-oxophorbine formation, and selective reduction of the 3-acetyl group using BH₃· ^tBuNH₂. Each macrocycle contains a geminal dimethyl group in the pyrroline ring to provide stability toward adventitious dehydrogenation. A Zn(ii) 7-(1-hydroxyethyl)-10-phenyl-17-oxochlorin also has been prepared. Altogether, 30 new hydroporphyrins were synthesized. The UV-Vis absorption spectra of the new chlorosomal bacteriochlorophyll mimics reveal a bathochromic shift of ∼1800 cm^-1 of the Q_y band in nonpolar solvent, indicating extensive assembly in solution. The Zn(ii) 3-(1-hydroxyethyl)-10-aryl-13¹-oxophorbines differ in the propensity to form assemblies based on the 10-substituent in the following order: mesityl<phenyl<pentafluorophenyl. Infrared spectra show that assemblies also can be formed in solid media and likely involve hydrogen-bonding (or other) interactions of the ring E keto group.