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Molecules with strong absorption in the near-infrared (NIR) spectral region are of interest for a variety of applications in the photosciences. Nature's NIR absorbers are bacteriochlorophylls, which contain a tetrahydroporphyrin chromophore accentuated by attached auxochromes, including a β,meso-bridging isocyclic ring. Modification of the isocyclic ring has provided a convenient means of wavelength tuning. In other tetrapyrroles, annulation at the β,meso-, β,β-, and β-meso-β-positions also has enabled wavelength tuning. Here, synthetic bacteriochlorins bearing a gem-dimethyl group in each pyrroline ring have provided the foundation for studies of annulation. Seven new free base bacteriochlorins have been prepared and seven others have been elaborated therefrom. Six distinct routes known among porphyrins were explored. Ultimately, annulation of phenaleno or benzo groups at the β,meso-sites (i.e., spanning the 3,5- and 13,15-positions) of bacteriochlorins was achieved by a new route that entails two successive Pd-mediated coupling reactions of a dibromobacteriochlorin with a bromoarylboronic acid. The corresponding Phen-BC or Benz-BC absorbs at 913 or 1033 nm, respectively. Examination by time-resolved spectroscopy revealed an excited-state lifetime of 150 ps or 7 ps, with commensurably low fluorescence quantum yield (Φf) values estimated to be ∼0.004 or 3 × 10−5, respectively. The origin of the short excited-state lifetimes due to β,meso-annulation is unknown, but is not simply a consequence of the energy-gap law for non-radiative decay.

Tetradehydrocorrins contain an A–D ring junction and a level of saturation intermediate between that of a corrin (characteristic of vitamin B12) and that of a corrole. Self-condensation of a p-tolyl-substituted dihydrodipyrrin-acetal (DHDPA-T) in CH2Cl2 containing Yb(OTf)3 at room temperature afforded the corresponding free base B,D-tetradehydrocorrin (TDC-T) as the sole macrocycle in 77% yield. The presence of a geminal dimethyl group in each of the reduced rings in TDC-T affords stability toward adventitious dehydrogenation. Treatment of TDC-T to acidic conditions results in rearrangement and loss of methanol to give a bacteriochlorin with or without a 5-methoxy substituent; the yield was 56% (5-methoxybacteriochlorin MeOBC-T), 35% (MeOBC-T) or 20% (5-unsubstituted bacteriochlorin HBC-T) with TMSOTf/CH2Cl2, BF3·OEt2/CH3CN or BF3·OEt2/CH2Cl2, respectively. TDC-T undergoes regioselective bromination with NBS to give the 10-bromotetradehydrocorrin in 22% yield. The conversion of the (non-aromatic) tetradehydrocorrin to the (aromatic) bacteriochlorin reflects the relative stability of the two macrocycles and indicates the former may be an intermediate—kinetically trapped or transient depending on catalysis conditions—upon dihydrodipyrrin-acetal self-condensation.

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.